batteries

Review Critical Review of the Use of Reference Electrodes in Li-Ion Batteries: A Diagnostic Perspective

Rinaldo Raccichini * , Marco Amores and Gareth Hinds *

National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK; [email protected] * Correspondence: [email protected] (R.R.); [email protected] (G.H.); Tel.: +44-(0)20-8943-6861 (R.R.); +44-(0)20-8943-7147 (G.H.)


Received: 26 November 2018; Accepted: 14 January 2019; Published: 18 January 2019


Abstract: Use of a reference electrode (RE) in Li-ion batteries (LIBs) aims to enable quantitative evaluation of various electrochemical aspects of operation such as: (i) the distinct contribution of each cell component to the overall battery performance, (ii) correct interpretation of current and voltage data with respect to the components, and (iii) the study of reaction mechanisms of individual electrodes. However, care needs to be taken to ensure the presence of the RE does not perturb the normal operation of the cell. Furthermore, if not properly controlled, geometrical and chemical features of the RE can have a significant influence on the measured response. Here, we present a comprehensive review of the range of RE types and configurations reported in the literature, with a focus on critical aspects such as electrochemical methods of analysis, cell geometry, and chemical composition of the RE and influence of the electrolyte. Some of the more controversial issues reported in the literature are highlighted and the benefits and drawbacks of the use of REs as an in situ diagnostic tool in LIBs are discussed.

Keywords: reference electrode; lithium-ion battery; batteries; impedance; EIS; two-electrode; three-electrode; geometry; composition; electrolyte

1. Introduction The increasing demand for Li-ion battery (LIB) energy storage systems, from portable electronics to electric vehicles, and their potential to accelerate the transition from a finite fuel-based to a more sustainable and renewable energy production model have intensified research into the development of improved components for high energy density LIBs [1–5]. The key components of a LIB, apart from the non-aqueous electrolyte [6,7] and the separator [8], are the two electrodes: (i) a negative electrode [9,10] (also called the anode) where the Li+ ions are stored during battery charging and released during discharge and (ii) a positive electrode [11] (also called the cathode), which acts as a solid reservoir for Li+ ions when the battery is discharged and as a source of Li+ ions during charging (Figure1)[ 12]. In commercial cells the negative electrode is typically graphite, while a wide range of positive electrode materials have been developed over the years, based on lithium salts containing transition metals such as nickel, cobalt, or iron.

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Figure 1. SchematicSchematic representation representation of a Li Li-ion‐ion battery (LIB) during during the the discharge discharge process. Reproduced Reproduced with permission from [[1].1].

The specific specific capacity (i.e. (i.e.,, the total amount of charge that can be stored per unit of volume or mass) of a commercial battery, which together with the cell voltage determines its practical specificspecific energy [[13],13], isis routinely routinely measured measured in in a two-electrodea two‐electrode configuration configuration during during discharge discharge of the of battery the battery from fromthe fully the charged fully charged state (Figure state 1(Figure). However, 1). However, this configuration this configuration does not distinguish does not thedistinguish contribution the contributionof each individual of each electrode individual to electrode the overall to the battery overall performance battery performance [14,15]. Incorporation [14,15]. Incorporation of a third of aelectrode third electrode into the into cell the to actcell as to aact reference as a reference electrode electrode (RE) is (RE) one is obvious one obvious means means of separating of separating these thesecontributions contributions [16,17 [16,17].]. Although Although several several patents patents report report the the implementation implementation of of an an RE RE inin practical rechargeable batteriesbatteries [ 18[18–23],–23], to to the the best best of our of knowledgeour knowledge no commercially no commercially available available LIB is currently LIB is currentlyequipped equipped with a dedicated with a dedicated RE and therefore RE and therefore no information no information related to related the identification to the identification of limiting of processeslimiting processes and individual and individual cell components cell components can be directly can be directly acquired acquired [24]. On [24]. the otherOn the hand, other the hand, use theof REs use inof batteryREs in battery research research and development and development is relatively is relatively widespread, widespread, providing providing a useful a useful in situ in situdiagnostic diagnostic tool tool that that can can enable enable characterization characterization of of the the various various processes processes occurring occurring in in aa LIBLIB under operando conditions. In the context of LIBs, the term “electrode” describes the solid, conducting components of the cell consisting of either a purely electron-conductingelectron‐conducting phase (e.g., graphite) or an ion-conductingion‐conducting phase (e.g., a Li-ionLi‐ion hosting active material) in close contact with an electron-conductingelectron‐conducting additive (e.g., disordered conductive carbon). The electron conductors are in electrical contact with a current collector (e.g., Al and Cu foil for positive and negative electrodes, respectively); all electrode phases are in contact with an electrolyte (e.g., a non non-aqueous‐aqueous liquid solution comprising a Li Li-containing‐containing salt and an organic organic-based‐based solvent) [25]. [25]. In In electrochemical electrochemical testing, the electrode under study is known by convention as the “working” electrode (WE) and the other electrode is termed termed the the “counter” “counter” electrode electrode (CE) [[25].25]. When When an an electrode electrode provides provides a constant, a constant, stable stable reference reference potential potential against whichagainst the which potential the potentialof the WE of can the beWE measured, can be measured, it is defined it is defined as a “reference” as a “reference” electrode electrode [26]. [26]. In a In two-electrode a two‐electrode cell cellconfiguration configuration (Figure (Figure2a), the 2a), CE the also CE acts also as acts RE. as However, RE. However, this configuration this configuration provides provides reliable reliable results resultsonly if theonly passage if the passage of current of current does not does significantly not significantly affect the affect potential the potential of the CE. of the For CE. this For reason, this reason,a three-electrode a three‐electrode cell configuration, cell configuration, where the where CE and the RE CE are and physically RE are physically different electrodes,different electrodes, is usually preferred.is usually preferred. In this case, In the this current case, the is passed current between is passed WE between and CE andWE asand a resultCE and the as RE a result is not polarizedthe RE is notby the polarized flow of by current the flow so long of current as it is so located long as outside it is located the main outside current the path main (Figure current2b) path [26 ].(Figure 2b) [26]. Batteries 2019, 5, 12 3 of 24

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Figure 2. Schematic representation ofof ((aa)) two-electrodetwo‐electrode and and ( b(b)) three-electrode three‐electrode cell cell configurations configurations for for a aLi-ion Li‐ion battery battery during during the the charging charging process. process.

In general, an RE should exhibit a reversible,reversible, ideallyideally non-polarizable,non‐polarizable, stable, and reproducible potential under all conditions experienced during electrochemical measurements [[17,24,27].17,24,27]. Selection and design of REs for LIBsLIBs areare veryvery sensitivesensitive toto experimentalexperimental conditions,conditions, including chemical composition of of the the electrolyte electrolyte and and cell cell geometry geometry [25]. [25]. While While REs REs have have been been successfully successfully applied applied to otherto other electrochemical electrochemical technologies technologies such such as as fuel fuel cells cells [28,29] [28,29 ]and and electrolyzers electrolyzers [30], [30], their their application to LIBs is somewhat more challenging due to safety considerations associated with the risk of short circuits and the combustible nature of the electrolyte. The The aim aim of of this review paper is to report the state of the art in in the the application application of REs REs to to LIBs, LIBs, addressing addressing in in particular particular the the most most controversial controversial points reported in the literature and clearly indicating the benefits benefits and drawbacks of the use of REs as an in situ diagnostic tool for practical LIB applications.

2. Electrochemical Methods of Analysis 2.1. Controlled-Potential and Controlled-Current Techniques 2.1. Controlled‐Potential and Controlled‐Current Techniques Various electrochemical techniques are routinely applied to investigate the behavior of LIBs. Various electrochemical techniques are routinely applied to investigate the behavior of LIBs. Controlled-potential (e.g., cyclic voltammetry) and controlled-current (e.g., galvanostatic cycling) Controlled‐potential (e.g., cyclic voltammetry) and controlled‐current (e.g., galvanostatic cycling) techniques enable the study of reactions at battery electrodes and measurement of battery capacity techniques enable the study of reactions at battery electrodes and measurement of battery capacity by applying large perturbations that drive the electrodes far from equilibrium [26]. In the former by applying large perturbations that drive the electrodes far from equilibrium [26]. In the former technique, by making use of a potentiostat, the current between WE and CE is adjusted in order to technique, by making use of a potentiostat, the current between WE and CE is adjusted in order to achieve the desired potential difference between WE and RE; the resulting current flowing between WE achieve the desired potential difference between WE and RE; the resulting current flowing between and CE is recorded as a function of time or potential (Figure3a). In the latter technique, by making use WE and CE is recorded as a function of time or potential (Figure 3a). In the latter technique, by of a galvanostat, a constant current is applied between WE and CE; the resulting potential difference making use of a galvanostat, a constant current is applied between WE and CE; the resulting potential between WE and RE is recorded as a function of time or current (Figure3b) [26]. difference between WE and RE is recorded as a function of time or current (Figure 3b) [26].

Figure 3. SchematicSchematic representation representation of ( a)) controlled controlled-potential‐potential and ( (b)) controlled controlled-current‐current experiments. Batteries 2019, 5, 12 4 of 24 Batteries 2019, 5, x FOR PEER REVIEW 4 of 25

ControlledControlled-potential‐potential experiments can be an extremely useful and versatile tool for fundamental electrochemical studies,studies, e.g., e.g., reaction reaction kinetics kinetics at single at single electrodes electrodes or electrolyte or electrolyte electrochemical electrochemical stability. Thesestability. experiments These experiments rely on the rely use on of athe well-characterized use of a well‐characterized RE; however, RE; since however, such techniques since such are techniquesnot generally are employednot generally in employed practical LIB in practical testing LIB we testing will not we discuss will not them discuss in morethem in depth more in depth this inreview this review [13,26]. [13,26]. A widelywidely used used controlled-current controlled‐current technique technique in the in LIBthe field LIB isfield galvanostatic is galvanostatic cycling withcycling potential with potentiallimits. With limits. this technique,With this thetechnique, use of an the RE use enables of an the RE simultaneous enables the acquisition simultaneous of potential acquisition profiles of potentialof both positive profiles and of both negative positive electrodes, and negative in addition electrodes, to that in of addition the full to cell, that which of the is full not cell, possible which in is a nottwo-electrode possible in cell a two configuration‐electrode cell (Figure configuration4)[24,31– 42(Figure]. 4) [24,31–42].

Figure 4. TypicalTypical examples of potential potential profiles profiles obtained from galvanostatic cycling in three three-electrode‐electrode cell configuration. configuration.

The three-electrodethree‐electrode configurationconfiguration also also facilitates facilitates tailoring tailoring of of specific specific test test protocols protocols to to avoid avoid the the Li platingLi plating effect effect on on the the commonly commonly used used graphite graphite electrode, electrode, by monitoringby monitoring under under charging charging conditions conditions the + potentialthe potential at which at which the the negative negative electrode electrode potential potential falls falls below below 0 V 0 vs V Li vs /Li.Li+/Li. Operando Operando monitoring monitoring of ofindividual individual electrode electrode potentials potentials allows allows for the for analysis the analysis of different of different operational operational effects on effects each electrode, on each electrode,such as temperature such as temperature and rate of charge/discharge,and rate of charge/discharge, as well as balancing as well of as the balancing specific capacity of the specific of each capacityelectrode of for each optimum electrode performance for optimum of the performance battery [23 of,32 the,40 ,battery43–45]. [23,32,40,43–45]. 2.2. Electrochemical Impedance-Based Techniques 2.2. Electrochemical Impedance‐Based Techniques Another useful and widely adopted technique is electrochemical impedance spectroscopy Another useful and widely adopted technique is electrochemical impedance spectroscopy (EIS) (EIS) [26], which is based on the concept of the opposition to flow of current produced by the [26], which is based on the concept of the opposition to flow of current produced by the various various battery components when the cell is operated under an alternating current (ac) signal. The battery components when the cell is operated under an alternating current (ac) signal. The impedance impedance is measured as a function of the frequency of the ac signal, which facilitates separation of is measured as a function of the frequency of the ac signal, which facilitates separation of processes processes occurring with different time constants within the cell. In contrast to controlled-current and occurring with different time constants within the cell. In contrast to controlled‐current and controlled-potential techniques, EIS generally makes use of a perturbation signal of small magnitude controlled‐potential techniques, EIS generally makes use of a perturbation signal of small magnitude to avoid any non-linear response from the system under study [16]. Two-electrode configurations are to avoid any non‐linear response from the system under study [16]. Two‐electrode configurations are most common in the literature, although three-electrode configurations are often employed to isolate most common in the literature, although three‐electrode configurations are often employed to isolate impedance spectra for individual electrodes. Although EIS is a non-destructive technique that provides impedance spectra for individual electrodes. Although EIS is a non‐destructive technique that useful time-dependent information on material properties and electrochemical processes within a provides useful time‐dependent information on material properties and electrochemical processes battery, its analysis is rather complex and extraction of quantitative data is generally a time-consuming within a battery, its analysis is rather complex and extraction of quantitative data is generally a time‐ and challenging task [46–49]. It is generally used to extract empirical parameters indirectly related to consuming and challenging task [46–49]. It is generally used to extract empirical parameters battery performance using equivalent circuit models comprising electrical elements such as resistors indirectly related to battery performance using equivalent circuit models comprising electrical and capacitors [26]. elements such as resistors and capacitors [26]. In 1981, Goodenough et al. [50] used a Li metal RE to demonstrate for the first time the In 1981, Goodenough et al. [50] used a Li metal RE to demonstrate for the first time the topotactic topotactic electrochemical extraction of Li from a defined ternary transition-metal oxide. By applying electrochemical extraction of Li from a defined ternary transition‐metal oxide. By applying galvanostatic cycling with potential limits to a three‐electrode cell, in which LiCoO2 and Li0.1V2O5 Batteries 2019, 5, 12 5 of 24

Batteries 2019, 5, x FOR PEER REVIEW 5 of 25 galvanostatic cycling with potential limits to a three-electrode cell, in which LiCoO2 and Li0.1V2O5 were thewere active the materialsactive materials for the positive for the and positive negative and electrodes, negative respectively,electrodes, therespectively, authors demonstrated the authors thedemonstrated reversible Li-ionthe reversible storage behaviorLi‐ion storage of LiCoO behavior2 at room of LiCoO temperature.2 at room A temperature. few years later, A few the sameyears researchlater, the group same appliedresearch a three-electrodegroup applied cella three configuration,‐electrode usingcell configuration, impedance measurements using impedance and equivalent-circuitmeasurements and models equivalent (see Figure‐circuit5) models to characterize (see Figure the various 5) to characterize processes occurring the various at the processes LiCoO 2 electrodeoccurring such at the as LiCoO ionic diffusion,2 electrode charge such as transfer ionic diffusion, and surface charge adsorption transfer [and51]. surface adsorption [51].

Figure 5. Experimental and fitted impedance response for a Co-containing positive LIB electrode (left). Figure 5. Experimental and fitted impedance response for a Co‐containing positive LIB electrode Equivalent circuit used to fit the experimental data (right). Figure adapted with permission from [51]. (left). Equivalent circuit used to fit the experimental data (right). Figure adapted with permission Interpretationfrom [51]. of impedance measurements in terms of equivalent circuits for Li+ diffusion in WO3 thin films had been investigated previously by Huggins et al. [52] Moreover, during the same Interpretation of impedance measurements in terms of equivalent circuits for Li+ diffusion in period, Owen et al. [53] reported the development of a model describing the performance of composite WO3 thin films had been investigated previously by Huggins et al. [52] Moreover, during the same electrodes in solid-state Li batteries by means of galvanostatic cycling and impedance measurements. period, Owen et al. [53] reported the development of a model describing the performance of In general, EIS measurements in three-electrode cell configuration are carried out under steady composite electrodes in solid‐state Li batteries by means of galvanostatic cycling and impedance state conditions at open circuit potential (to avoid any complications related to electrode polarization) measurements. at intermittent stages during galvanostatic cycling tests. As mentioned above, EIS is the method of In general, EIS measurements in three‐electrode cell configuration are carried out under steady choice to study the electrochemical properties of various battery electrodes as a function of cycling state conditions at open circuit potential (to avoid any complications related to electrode polarization) or state of charge [54]. Positive electrode active materials including V6O13 [14,55], LiMn2O4 [56,57], at intermittent stages during galvanostatic cycling tests. As mentioned above, EIS is the method of LiCoO2 [16,17,31,37,58–64], LiNi0.8Co0.2O2 [58,65], LiNi0.8Co0.15Al0.05O2 [35,66,67], LiNi0.85Co0.1Al0.05O2 [68], choice to study the electrochemical properties of various battery electrodes as a function of cycling Li(Li0.1Ni0.3Co0.3Mn0.3)O2 [68], Li(Ni0.3Mn0.3Co0.3)O2 [15,38,69–71], LiFePO4 [72,73], LiNi0.5Mn1.5O4 [74], or state of charge [54]. Positive electrode active materials including V6O13 [14,55], LiMn2O4 [56,57], LiNi0.5Mn0.3Co0.2O2 [36,75], and LiNi0.6Mn0.6Co0.2O2 [76], as well as negative electrode active materials, LiCoO2 [16,17,31,37,58–64], LiNi0.8Co0.2O2 [58,65], LiNi0.8Co0.15Al0.05O2 [35,66,67], LiNi0.85Co0.1Al0.05O2 such as carbons [35,60,68,77–80], graphite [16,31,36–38,57,62–75,81,82], Sn-containing composites [82,83], [68], Li(Li0.1Ni0.3Co0.3Mn0.3)O2 [68], Li(Ni0.3Mn0.3Co0.3)O2 [15,38,69–71], LiFePO4 [72,73], LiNi0.5Mn1.5O4 LiTi2(PO4)3 [15], Li4Ti5O12 [61], and Si-containing composites [34,84,85] have been extensively investigated [74], LiNi0.5Mn0.3Co0.2O2 [36,75], and LiNi0.6Mn0.6Co0.2O2 [76], as well as negative electrode active in the literature. materials, such as carbons [35,60,68,77–80], graphite [16,31,36–38,57,62–75,81,82], Sn‐containing Various research works have reported the application of EIS using either a potential composites [82,83], LiTi2(PO4)3 [15], Li4Ti5O12 [61], and Si‐containing composites [34,84,85] have been perturbation (i.e., potentiostatic EIS or PEIS) or a current perturbation (i.e., galvanostatic EIS or extensively investigated in the literature. GEIS) [17,33,42,73,75,77,78,81,86–94]. Novák et al. [17] compared these two EIS techniques and Various research works have reported the application of EIS using either a potential perturbation concluded that stability against a potential perturbation does not necessarily indicate the suitability (i.e., potentiostatic EIS or PEIS) or a current perturbation (i.e., galvanostatic EIS or GEIS) of a material as an RE. Interestingly, the authors reported that hysteresis between the lithiation and [17,33,42,73,75,77,78,81,86–94]. Novák et al. [17] compared these two EIS techniques and concluded delithiation of a battery material (e.g., magnitudes of 2 mV for Li and 17.5 mV for Li Bi) is always that stability against a potential perturbation does not necessarily indicate the suitability2 of a material present even at low current densities and therefore an activation overpotential needs to be overcome as an RE. Interestingly, the authors reported that hysteresis between the lithiation and delithiation of for the electrochemical reaction to occur. Thus, if the RE potential is forced to change when a PEIS a battery material (e.g., magnitudes of 2 mV for Li and 17.5 mV for Li2Bi) is always present even at measurement is carried out, the current response will be zero until the overpotential of the RE is low current densities and therefore an activation overpotential needs to be overcome for the reached. This effect delays the response of the RE and adds a phase shift to the EIS measurement if the electrochemical reaction to occur. Thus, if the RE potential is forced to change when a PEIS magnitude of the potential perturbation is too low (see Figure6a). In contrast, the current perturbation measurement is carried out, the current response will be zero until the overpotential of the RE is applied during GEIS leads to an instantaneous potential response that can overcome this overpotential, reached. This effect delays the response of the RE and adds a phase shift to the EIS measurement if yielding a more accurate measurement (see Figure6b) [17]. the magnitude of the potential perturbation is too low (see Figure 6a). In contrast, the current perturbation applied during GEIS leads to an instantaneous potential response that can overcome this overpotential, yielding a more accurate measurement (see Figure 6b) [17]. Batteries 2019, 5, 12 6 of 24 BatteriesBatteries 2019 2019, ,5 5, ,x x FOR FOR PEER PEER REVIEW REVIEW 66 ofof 2525

+ + FigureFigureFigure 6. 6.6. Electrochemical Electrochemical impedance impedance spectra spectra measuredmeasured measured using using using a a a LiCoO LiCoO LiCoO222 electrodeelectrode electrode at at at 4 4 4 V V V vs. vs. vs. Li Li Li+/Li/Li with with (((aa))) sinusoidal sinusoidalsinusoidal potentialpotential perturbationperturbation withwith amplitude amplitudeamplitude of ofof 5 55 mV mVmV and andand ( ((bbb))) sinusoidal sinusoidalsinusoidal current currentcurrent perturbation perturbationperturbation withwith amplitude amplitude of of 300 300 μ μµA.A. Figure Figure adapted adapted with with permission permission from from [[17]. 17[17].].

However,However,However, Aurbach Aurbach et etet al. al.al. [ [78]78[78]] pointed pointedpointed out outout that thatthat PEIS PEISPEIS measurements measurementsmeasurements have havehave the thethe intrinsic intrinsicintrinsic advantage advantageadvantage of ofallowingof allowingallowing direct directdirect control controlcontrol of the of potentialof thethe potentialpotential of the WE, ofof which thethe WE,WE, is of which criticalwhich importanceisis ofof criticalcritical for importance electrochemicalimportance forfor electrochemicalstudieselectrochemical in terms studiesstudies of elucidation inin termsterms of of theof elucidationelucidation phenomena ofof occurring thethe phenomenaphenomena at the negative occurringoccurring and atat positive thethe negativenegative electrodes. andand positiveGasteigerpositive electrodes.electrodes. et al. [73 Gasteiger],Gasteiger by implementing etet al.al. [73],[73], byby a implementing three-electrodeimplementing aa lab-scalethreethree‐‐electrodeelectrode LIB with lablab‐‐scalescale a Li-Au LIBLIB with micro-RE,with aa LiLi‐‐ AudemonstratedAu micromicro‐‐RE,RE, demonstrateddemonstrated conclusively that conclusivelyconclusively correct use thatthat of correctcorrect the RE useuse is essential ofof thethe RERE for isis the essentialessential acquisition forfor thethe of acquisitionacquisition reliable and ofof reliableunbiasedreliable andand impedance unbiasedunbiased spectra. impedanceimpedance Furthermore, spectra.spectra. Furthermore,Furthermore, in this work inin the thisthis authors workwork the obtainedthe authorsauthors identical obtainedobtained impedance identicalidentical impedanceresultsimpedance with resultsresults PEIS and withwith GEIS PEISPEIS measurements andand GEISGEIS measurementsmeasurements (Figure7). (Figure(Figure 7).7). Recently,Recently,Recently, NottenNotten Notten et etet al.al. al. [ [75] 75[75]] proposed proposed proposed a a a method methodmethod to toto compensate compensatecompensate EIS EISEIS artefacts artefactsartefacts when whenwhen using usingusing a micro-RE a a micromicro‐‐ REbyRE by averagingby averagingaveraging two twotwo individual individualindividual three-electrode threethree‐‐electrodeelectrode measurements. measurements.measurements. They TheyThey claimed claimedclaimed that, that,that, by byby applying applying this this method,method,method, both bothboth PEIS PEISPEIS and and GEIS GEIS measurements measurements on onon three three-electrodethree‐‐electrodeelectrode battery battery systems systems could couldcould be be accurately accurately analyzed.analyzed.analyzed. TheThe The disadvantagesdisadvantages disadvantages ofof of thisthis this methodmethod method areare areits its time itstime time-consuming‐‐consumingconsuming naturenature nature and and potential andpotential potential issuesissues issues withwith reproducibility.withreproducibility. reproducibility.

FigureFigureFigure 7. 7. Comparison Comparison of of the the impedance impedance response response (100 (100(100 kHz–0.1 kHz–0.1kHz–0.1 Hz) Hz)Hz) of ofof graphite graphitegraphite and andand LiFePO LiFePOLiFePO44 electrodes electrodes usingusingusing potentiostatic potentiostaticpotentiostatic electrochemical electrochemicalelectrochemical impedance impedanceimpedance spectroscopy spectroscopy (PEIS) (PEIS) (5 (5 mV mV amplitude, amplitude, straight straight lines) lines) andandand galvanostatic galvanostaticgalvanostatic electrochemical electrochemicalelectrochemical impedance impedance spectroscopy spectroscopy (GEIS) (GEIS) (0.5 (0.5 mA mA amplitude, amplitude, dotted dotted lines). lines). ReproducedReproducedReproduced with with permission permission from from [73] [[73]73].. .

AnAnAn alternativealternative alternative techniquetechnique technique toto to EISEIS EIS isis thethe is measurement themeasurement measurement ofof areaarea of‐‐specificspecific area-specific impedanceimpedance impedance (ASI)(ASI) byby (ASI) hybridhybrid by pulsehybridpulse powerpower pulse power characterizationcharacterization characterization [66,68,95–97][66,68,95–97] [66,68,95 –oror97 ]galvanostaticgalvanostatic or galvanostatic cyclingcycling cycling withwith with intermittentintermittent intermittent current currentcurrent interruptioninterruptioninterruption [98]. [[98].98]. The The ASI ASI is isis time time-dependenttime‐‐dependentdependent and and is isis affected affected by byby the thethe same samesame processes processesprocesses that thatthat affect affectaffect the thethe EIS EISEIS measurementmeasurementmeasurement (e.g., (e.g.,(e.g., ohmic ohmicohmic drop, drop,drop, Li-ion LiLi‐‐ionion diffusion diffusiondiffusion through throughthrough the electrolyte, thethe electrolyte,electrolyte, and solid-state andand solidsolid diffusion‐‐statestate diffusiondiffusion within withinthewithin electrode), thethe electrode),electrode), although althoughalthough they cannot theythey cannot becannot individually bebe individuallyindividually analyzed. analyzed.analyzed. However, However,However, since ASI sincesince measures ASIASI measuresmeasures the cell thewiththe cellcell instantaneous withwith instantaneousinstantaneous non-uniformity nonnon‐‐uniformityuniformity of concentration ofof concentrationconcentration and electric andand field, electricelectric it could field,field, be more itit couldcould representative bebe moremore representativethanrepresentative data from thanthan EIS in datadata terms fromfrom of evaluationEISEIS inin termsterms of of theof evaluationevaluation total cell resistance ofof thethe totaltotal during cellcell resistanceresistance cycling, without duringduring the cycling,cycling, need withouttowithout reach the equilibriumthe needneed toto reach conditionsreach equilibriumequilibrium as in the conditionsconditions case of EIS. asas The inin thethe ASI casecase value ofof is EIS.EIS. obtained TheThe ASIASI by valuevalue dividing isis obtainedobtained the change byby dividingdividing the the change change in in voltage voltage measured measured during during the the relaxation relaxation period period by by the the current current density density applied applied beforebefore thethe currentcurrent interruptioninterruption (Figure(Figure 8).8). Batteries 2019, 5, 12 7 of 24 in voltage measured during the relaxation period by the current density applied before the current interruptionBatteries 2019 (Figure, 5, x FOR8). PEER REVIEW 7 of 25

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FigureFigure 8. Example 8. Example of of area-specific area‐specific impedance impedance (ASI) plot plot for for a aLi Li-ion‐ion cell cell equipped equipped with with an RE. an RE. ReproducedReproduced with with permission permission from from [95 [95].]. Figure 8. Example of area‐specific impedance (ASI) plot for a Li‐ion cell equipped with an RE. In general,In Reproducedgeneral, the the use with use of permission anof an RE RE for for from EIS EIS [95]. measurements measurements enables enables separation separation of the of the various various contributions contributions fromfrom the individual the individual cell componentscell components (i.e., (i.e. separator,, separator, electrolyte, electrolyte, current current collectors, collectors, and and electrodes)electrodes) [99]. In general, the use of an RE for EIS measurements enables separation of the various contributions Usually,[99]. EIS Usually, measurements EIS measurements in commercial in commercial cells (i.e., cells two-electrode (i.e., two‐electrode cell configuration) cell configuration) are are carried carried out by outfrom by applying the individual perturbations cell components in the frequency (i.e., separator, range 100electrolyte, kHz–100 current mHz. collectors,Various published and electrodes) research applying perturbations in the frequency range 100 kHz–100 mHz. Various published research studies studies[99]. Usually,have proposed EIS measurements that: (i) inat commercialhigh frequency cells (i.e. the, two ionic‐electrode resistance cell configuration) of the electrolyte are carried can be have proposed that: (i) at high frequency the ionic resistance of the electrolyte can be identified, (ii) identified,out by applying (ii) in perturbationsthe mid‐frequency in the frequency range various range 100 contributions kHz–100 mHz. of Various the positive published and research negative studies have proposed that: (i) at high frequency the ionic resistance of the electrolyte can be in theelectrodes mid-frequency appear, rangeand (iii) various at low contributionsfrequencies the of diffusion the positive of Li‐ions and into negative the porous electrodes electrode appear, identified, (ii) in the mid‐frequency range various contributions of the positive and negative and (iii)structures at low can frequencies be studied [46,100,101]. the diffusion Outside of Li-ions of this frequency into the range, porous distortions electrode related structures to the cell can be electrodes appear, and (iii) at low frequencies the diffusion of Li‐ions into the porous electrode studiedgeometry[46,100 and,101 the]. Outside inductive of effect this of frequency connecting range, cables distortions can impair the related measurements to the cell (Figure geometry 9) [78]. and the structures can be studied [46,100,101]. Outside of this frequency range, distortions related to the cell inductivegeometry effect ofand connecting the inductive cables effect canof connecting impair the cables measurements can impair the (Figure measurements9)[78]. (Figure 9) [78].

FigureFigure 9.FigureExample 9. 9.Example Example of impedanceof of impedance impedance spectrum spectrum spectrum and andand fittedfitted equivalentequivalent circuit circuit circuit of of commercial ofcommercial commercial LIB. LIB. Circuit LIB. Circuit Circuit elementselements are are as as follows: follows: L, L, R RI, Iinductance, inductance andand resistanceresistance ofof batterybattery leads leads and and connector connector cables; cables; RS ,R RSF, , RF, elements are as follows: L, RI, inductance and resistance of battery leads and connector cables; RS, resistanceresistance of ofelectrolyte, electrolyte, and and SEI;SEI; CPECPEFF,, constantconstant phase elementelement ofof SEI; SEI; R AR, AR, CR, Ccharge, charge transfer transfer RF, resistance of electrolyte, and SEI; CPEF, constant phase element of SEI; RA,RC, charge transfer resistanceresistance of ofanode anode and and cathode; cathode; CPE CPEAA, , CPECPECC,, constant phasephase element element of of anode anode and and cathode cathode surface; surface; resistance of anode and cathode; CPEA, CPEC, constant phase element of anode and cathode surface; andand ZW Z, WWarburg, Warburg diffusion diffusion through through porousporous electrode structures.structures. Figure Figure adapted adapted with with permission permission and Z , Warburg diffusion through porous electrode structures. Figure adapted with permission fromWfrom [54] [54]. . from [54]. Batteries 2019, 5, 12 8 of 24 Batteries 2019, 5, x FOR PEER REVIEW 8 of 25

However,However, a a moremore recentrecent studystudy byby Tatara etet al.al. [[102]102] investigated the effect of electrode electrode-electrolyte‐electrolyte interfaceinterface on on the the electrochemical electrochemical impedance impedance spectra spectra for a LIB for positive a LIB electrode positive in electrode half-cell configuration in half‐cell using a mesh Li Ti O reference electrode placed between the positive and negative electrode. configuration using4 5 a 12mesh Li4Ti5O12 reference electrode placed between the positive and negative Inelectrode. contrast In to contrast what is to generally what is generally reported reported for two-electrode for two‐electrode cells or cells in the or in case the ofcase artefactual of artefactual EIS responseEIS response from from three-electrode three‐electrode cells cells with with asymmetric asymmetric cell cell configurations, configurations, the the authors authors claimed claimed that: that: (i) at(i) high at high frequencies, frequencies, the the impedance impedance response response is is related related to to adsorption/desorption adsorption/desorption of of lithium lithium ionsions onon thethe surfacesurface ofof thethe compositecomposite porepore structurestructure coupledcoupled withwith lithiumlithium ionion migrationmigration inin thethe pore,pore, (ii)(ii) thethe low-frequencylow‐frequency impedanceimpedance responseresponse reflectsreflects thethe combinationcombination ofof chargecharge transfertransfer resistanceresistance ofof lithiumlithium de-solvation/solvation,de‐solvation/solvation, lithium lithium intercalation/deintercalation intercalation/deintercalation to/from to/from the the active active particle particle surface surface of the of theelectrode, electrode, and and(iii) lithium (iii) lithium migration migration through through the electrode the electrode-electrolyte‐electrolyte interface interface layer during layer duringcharge chargetransfer. transfer. ForFor EISEIS measurements,measurements, thethe amplitudeamplitude of thethe perturbationperturbation of the appliedapplied potential (or current) mustmust bebe highhigh enoughenough toto ensureensure anan adequateadequate signal-to-noisesignal‐to‐noise ratio.ratio. However, it should also be lowlow enoughenough thatthat thethe inducedinduced currentcurrent (or(or potential)potential) showsshows aa linearlinear responseresponse (i.e.,(i.e., thethe currentcurrent andand potentialpotential amplitudesamplitudes mustmust bebe proportionalproportional toto oneone another).another). In the case of PEIS,PEIS, aa strongstrong linearitylinearity generallygenerally requiresrequires aa potentialpotential perturbationperturbation ofof thethe orderorder of of 5 5 mV–10 mV–10 mV mV [ 25[25].]. InIn LIBs,LIBs, thethe impedanceimpedance contributionscontributions ofof thethe positivepositive andand negative electrodeselectrodes cancan bebe summedsummed andand matchedmatched withwith thethe fullfull cell impedance (Figure 1010))[ [16,31,35,60,66,71,72,76–78,81,103,104].16,31,35,60,66,71,72,76–78,81,103,104]. This criterion isis widelywidely appliedapplied withwith thethe intentionintention of validating the cell set-upset‐up used for the EIS measurements. However,However, it it cannot cannot yield yield new new information information about about the the quality quality of theof the cell cell set-up set‐up because because by definitionby definition the twothe two single single electrode electrode impedances impedances sum tosum the to full the cell full impedance, cell impedance, since they since are they measured are measured with respect with torespect the same to the reference same reference electrode electrode potential. potential.

FigureFigure 10.10. Example of three-electrodethree‐electrode EIS measurements for positive and negative electrodes (a); comparisoncomparison between two-electrodetwo‐electrode measurement and the sum of three-electrodethree‐electrode measurements ( b). ReproducedReproduced withwith permissionpermission fromfrom [[16].16].

InIn general,general, three-electrode three‐electrode measurements measurements have have shown shown that the that main the contribution main contribution to the impedance to the ofimpedance a LIB cell of originates a LIB cell originates from the positive from the electrode positive electrode [37,63,71 ,[37,63,71,73,81].73,81]. The negative The negative electrode, electrode, which generallywhich generally comprises comprises a graphitic a graphitic active material, active commonlymaterial, commonly exhibits a stable exhibits impedance a stable contribution impedance independentcontribution independent of the state of of charge the state of theof charge battery of [ 63the,71 battery,73]. Its [63,71,73]. magnitude Its ismagnitude generally is lower generally than thatlower of than the positivethat of the electrode positive and electrode mainly and represents mainly therepresents ohmic contributionthe ohmic contribution due to the due formation to the offormation a stable of solid a stable electrolyte solid electrolyte interphase interphase (SEI) derived (SEI) derived from partial from partial electrolyte electrolyte degradation degradation at low at potentialslow potentials [105 ].[105]. It has It has also also been been reported reported that that overcharge overcharge conditions conditions produce produce a a high high impedanceimpedance at the positive electrode, while over discharge of the battery results in a higher impedance at the negative electrode [72]. Interestingly, when metallic Li is used as the negative electrode instead of a Batteries 2019, 5, 12 9 of 24

Batteries 2019, 5, x FOR PEER REVIEW 9 of 25 atBatteries the positive2019, 5, x FOR electrode, PEER REVIEW while over discharge of the battery results in a higher impedance9 at of the 25 negativecarbon‐based electrode electrode [72]. Interestingly,(as in the case when of Li‐ metallicmetal batteries), Li is used the as main the negative source of electrode impedance instead appears of a carbon-basedcarbonto be the‐based anode electrode [56,103]. (as This in the could case be of due Li-metalLi‐ metaleither batteries), to the slower the mainkinetics source of Li of stripping impedance and appears plating tocompared be the anode to lithiation/delithiation [[56,103].56,103]. This could atbe a due carbon either‐based to the electrode slower or kinetics the formation of Li stripping of a more and resistant plating comparedSEI layer on to Li lithiation/delithiation [14,32,106]. at at a a carbon carbon-based‐based electrode electrode or or the the formation formation of of a more resistant SEI layer on Li [[14,32,106].14,32,106]. 3. Cell Configuration and Geometry 3. Cell Where Configuration Configuration practical, andthe use Geometry of a three ‐electrode configuration (i.e., WE, CE, and RE) for the study of electrochemicalWhere practical, energy the use storage of a three-electrodethreedevices‐electrode is always configurationconfiguration recommended. (i.e. (i.e., ,As WE, mentioned CE, and RE) previously, for the study when ofonly electrochemical two electrodes energy are physically storage devices available, is always the CE recommended. can also be used As as mentioned RE. However, previously, although when this onlypractice two is electrodes widely used are physicallyin the LIB researchavailable, field, the CE it does can also not guarantee be used as the RE. stability However, of the although RE when this a practicelarge perturbation is widely used (in terms in the of LIB either research current field,field, or potential) it does not is guarantee applied to the the stability cell. of the RE when a large Various perturbation types (in of termsterms electrochemical of either currentcurrent cell ororconfiguration potential)potential) isis are appliedapplied available toto thethe for cell.cell. battery assembly and testing.Various The typesmosttypes ofcommon of electrochemical electrochemical laboratory cell ‐scalecell configuration configuration cells used are in availableareresearch available forlaboratories battery for battery assembly are coinassembly and cells testing. [107]and Thetesting.(generally most The common two most‐electrode common laboratory-scale cells) laboratory and cellsSwagelok‐scale used cells incells research used [73] in (generally laboratoriesresearch laboratoriesthree are‐electrode coin cells are cells) [coin107] (generallycellswhere [107] the two-electrode(generallyelectrodes twoare cells)‐singleelectrode and‐side Swagelokcells) coated and and cellsSwagelok the [ 73cells] (generally cellshave [73]a plane three-electrode(generally‐parallel three electrode cells)‐electrode wheregeometry cells) the (Figurewhere electrodes the11). areelectrodesThese single-side are typicallyare single coated used‐side and to coated the investigate cells and have the the acells electrochemical plane-parallel have a plane electrode performance‐parallel geometry electrode of component (Figuregeometry 11 materials. ).(Figure These 11). are typicallyThese are used typically to investigate used to investigate the electrochemical the electrochemical performance performance of component of component materials. materials.

Figure 11. Schematic representation of a two‐electrode coin cell (a) and three‐electrode T‐shape FigureSwagelok 11. cellSchematicSchematic (b). Figure representation representation adapted with of of permission a a two two-electrode‐electrode from [73,107]. coin coin cell ( a) and three-electrodethree‐electrode T-shapeT‐shape Swagelok cell (b). Figure adapted with permission from [[73,107].73,107]. Larger electrochemical cells, such as cylindrical, prismatic, and pouch cells, where the electrodes are usuallyLarger electrochemical double‐side coated cells, and such rolled as cylindrical, or folded, prismatic, are produced and pouch as two cells, ‐electrode where the cells electrodes and are aremostly usually used double-sidedouble for practical‐side coated applications and rolled (Figure or 12) folded, [108]. are Specific produced lab‐scale as two-electrodetwo cells,‐electrode designed cells to improve,and are mostlyfor example, used for the practical reproducibility applications of EIS(Figure measurements 12)12)[ [108].108]. Specific Specific [109] labor lab-scale ‐toscale perform cells, designedoperando to to studies improve, improve, of forelectrode example,example, expansion the the reproducibility reproducibility during cycling of EISof [110],EIS measurements measurements have been [109 implemented] [109] or to performor to inperform operando recent operandoyears studies by of commercialstudies electrode of expansionelectrodecompanies expansion during [111]. cycling during [110 cycling], have been[110], implemented have been implemented in recent years in by recent commercial years companies by commercial [111]. companies [111].

Figure 12. Schematic representation of cylindrical (a), prismatic (b), and pouch (c) cell configurations. Figure 12. Schematic representation of cylindrical (a), prismatic (b), and pouch (c) cell configurations. Reproduced with permission from [108]. FigureReproduced 12. Schematic with permission representation from [108].of cylindrical (a), prismatic (b), and pouch (c) cell configurations. Reproduced with permission from [108]. Various research works have described modification of cells by adding an internal RE [24,35,37,38,64,73,78,95,99]Various research works and haveoptimization described of themodification RE location of within cells theby cell adding to avoid an measurementinternal RE [24,35,37,38,64,73,78,95,99] and optimization of the RE location within the cell to avoid measurement Batteries 2019, 5, 12 10 of 24

Various research works have described modification of cells by adding an internal BatteriesRE [24 2019,35,,37 5, ,x38 FOR,64 PEER,73,78 REVIEW,95,99] and optimization of the RE location within the cell to10 avoid of 25 measurement artefacts [61,62,76,78,96,99,112–115]. In particular, good alignment of the electrodes artefactsis critical [61,62,76,78,96,99,112–115]. to avoid any distortion of theIn particular, electric field good probed alignment by the of RE the near electrodes the edges is critical of the to CE avoid and anyWE (Figuredistortion 13 )[of78 the]. Errorselectric due field to probed the size by and the surface RE near resistance the edges of of reference the CE and electrode WE (Figure wires 13) can [78]. also Errorsbe quantified due to the [116 size]. and surface resistance of reference electrode wires can also be quantified [116].

Figure 13. Example of misalignment, d, d, between the positions of WE and CE. When d is comparable with the separator thickness, l, this results in a frequency (ω))-dependent‐dependent change in in the the position of the the equipotential surface probed by the RE. Reproduced with permission from [[78].78].

Although implementation and and optimization optimization of of the the RE RE in in small lab lab-scale‐scale cells cells is relatively straightforward, this is not the case for larger commercially available cells. Indeed, although several papers [[24,35,38,72,95,112]24,35,38,72,95,112] andand patents patents [ 18[18–23]–23] have have reported reported the the implementation implementation of REsof REs in pouch in pouch and andcylindrical cylindrical cells, cells, they havethey yet have to beyet applied to be inapplied commercial in commercial cells, which cells, is mainly which due is tomainly performance due to performanceand safety constraints. and safety The constraints. procedure The for procedure RE insertion for requiresRE insertion an alteration requires of an the alteration battery designof the batterythat could design eventually that could impact eventually on cell lifetime,impact on due cell to lifetime, the presence due to of the gas presence into the of cell gas or into to the the amount cell or toand the concentration amount and ofconcentration electrolyte available of electrolyte for battery available cycling. for battery Furthermore, cycling. ifFurthermore, these modifications if these modificationsare performed are after performed the fabrication after the process, fabrication even process, if carried even out if in carried an inert out atmosphere, in an inert atmosphere, they can be theyhazardous can be in hazardous terms of cell in short-circuiting terms of cell short which‐circuiting could eventually which could lead to eventually thermal runaway lead to [ 117thermal–119] runawayconditions [117–119] that could conditions result in athat battery could explosion result in [a24 battery]. explosion [24].

4. RE RE Materials Materials and and Geometry

4.1. RE Active Materials 4.1. RE Active Materials Conventional REs operate on the principle of maintaining a stable equilibrium potential by Conventional REs operate on the principle of maintaining a stable equilibrium potential by controlling the concentration of the various reacting chemical species at the electrode/electrolyte controlling the concentration of the various reacting chemical species at the electrode/electrolyte interface [25,26]. For laboratory REs based on sparingly soluble salts, such as the silver/silver chloride interface [25,26]. For laboratory REs based on sparingly soluble salts, such as the silver/silver chloride (Ag/AgCl) electrode [26,27,120], this is achieved by maintaining a constant activity of chloride ions, (Ag/AgCl) electrode [26,27,120], this is achieved by maintaining a constant activity of chloride ions, aCl− , in the RE electrolyte, since the potential, E, of the Ag/AgCl RE is given by the Nernst equation: 𝑎, in the RE electrolyte, since the potential, E, of the Ag/AgCl RE is given by the Nernst equation:

0 RT𝑅𝑇 E𝐸= E 𝐸− lnlna 𝑎 − (1)(1) F𝐹 Cl where E𝐸0 isis the the standard standard thermodynamic thermodynamic electrode electrode potentialpotential versusversus thethe hydrogen electrode, R is the gas constant, andand FFis is the the Faraday Faraday constant constant [25 [25].]. However, However, the the electrochemistry electrochemistry of REof RE materials materials that that are arecompatible compatible with with common common Li-ion Li battery‐ion battery chemistries chemistries is somewhat is somewhat more complex more complex and generally and generally exhibits exhibitsnon-Nernstian non‐Nernstian kinetic behaviorkinetic behavior [26]. A [26]. wide A range wide ofrange electrochemically of electrochemically active active materials materials have have been beenconsidered considered for application for application as REs as for REs LIBs. for Although LIBs. Although Li metal Li has metal been has the mostbeen commonlythe most commonly employed employedmaterial [ 14,16,24,31,35,36,38,39,41,46,50,51,55,56,61,62,67,69,70,72,82,91,95,100,112,121,122],material it is far [14,16,24,31,35,36,38,39,41,46,50,51,55,56,61from ideal as an RE due to the irreversibility,62,67,69,70,72,82,91,95,100,112,121,122], of its reactions [16,123] (Figure 14) andit is the far varying from idealinfluence as an of RE different due to the electrolytes irreversibility on the of natureits reactions of the [16,123] passivation (Figure layer 14) onand the the Li varying surface influence [15,124]. ofMoreover, different theelectrolytes relatively on low the meltingnature of point the passivation of Li metal layer (i.e., on 180 the◦C) Li issurface also a [15,124]. limiting Moreover, factor for thehigh-temperature relatively low melting applications point [of125 Li]. metal Indeed, (i.e., despite 180 °C) theis also critical a limiting effect factor of the for electrolyte high‐temperature in terms applicationsof high volatility, [125]. flammability,Indeed, despite and the ambient critical temperatureeffect of the flashelectrolyte point [in126 terms], research of high works volatility, have flammability, and ambient temperature flash point [126], research works have focused on using more stable materials for studies above 100 °C (e.g., LiFePO4) [127]. Use of conventional pseudo‐reference electrode materials [26], such as Ag or Pt wire, is not recommended due to irreproducibility and drift of steady state potential, while alternative pseudo‐reference electrodes such as Na metal have been investigated with only limited success [128,129]. Batteries 2019, 5, 12 11 of 24

◦ focused on using more stable materials for studies above 100 C (e.g., LiFePO4)[127]. Use of Batteriesconventional 2019, 5, x pseudo-reference FOR PEER REVIEW electrode materials [26], such as Ag or Pt wire, is not recommended11 of due 25 to irreproducibility and drift of steady state potential, while alternative pseudo-reference electrodes suchBatteries as Na 2019 metal, 5, x have FOR PEER been REVIEW investigated with only limited success [128,129]. 11 of 25

FigureFigureFigure 14. 14. ComparisonComparison 14. Comparison of of typicalof typical typical polarization polarization polarization curves curvescurves for for variousvarious various materials materials materials used used used in REsin REs for LIBs.for LIBs.

AA range rangeA range of of alloy of alloy alloy materials materialsmaterials such such such as as Li as Li‐Sn‐Sn Li-Sn [65,66,68,96],[65,66,68,96], [65,66,68 ,Li96Li‐Al],‐Al [63,130], Li-Al [63,130], [ 63Li ,‐130LiBi ‐Bi[17]], [17] Li-Bi and andLi [17‐Au Li] ‐ andAu [73,74,103,131]Li-Au[73,74,103,131][73,74,103 have,131 have been] have been investigated beeninvestigated investigated as as alternative alternative as alternative RE materials RE (Figure (Figure materials 14). 14). Although (Figure Although these14). these alloys Although alloys demonstrate relatively stable potentials (which vary from 0.1 V to 0.8 V vs Li/Li+) for reasonable + demonstratethese alloys demonstraterelatively stable relatively potentials stable (which potentials vary from (which 0.1 varyV to from0.8 V 0.1 vs VLi/Li to+ 0.8) for V reasonable vs Li/Li ) lengths of time (i.e., in the order of weeks), they generally require additional lithiation processes in lengthsfor reasonable of time lengths (i.e., in ofthe time order (i.e., of in weeks), the order they of generally weeks), they require generally additional require lithiation additional processes lithiation in order to maintain a stable chemical composition (and therefore a constant potential) over longer processes in order to maintain a stable chemical composition (and therefore a constant potential) over orderperiods to maintain (i.e., in athe stable order chemicalof months) composition [66,73]. Moreover, (and thetherefore quality ofa theconstant alloy (in potential) terms of chemicalover longer longer periods (i.e., in the order of months) [66,73]. Moreover, the quality of the alloy (in terms of periodscomposition (i.e., in the and order surface of morphology)months) [66,73]. is also Moreover, strongly influenced the quality by temperatureof the alloy and(in termsthe magnitude of chemical compositionchemicaland duration composition and surfaceof the andapplied morphology) surface current morphology) during is also the strongly alloy is also formation influenced strongly process influencedby temperature [66,73]. Other by temperature and research the magnitude works and the andmagnitude durationhave reported and of the duration the applied use of of currentintercalation the applied during compounds current the alloy during such formation as the LiFePO alloy process4 formation(LFP) [66,73]. [15,78] process Otheror Li4Ti research5 [O6612, 73(LTO)]. works Other haveresearch [15,16,37,59,76,93,94,102,132–134]reported works the have use reported of intercalation the use as REcompounds of materials intercalation (Figure such compounds as 14). LiFePO These 4compounds such (LFP) as [15,78] LiFePO are idealor4 Li(LFP) 4forTi 5useO [1512 in(LTO),78 ] or + + [15,16,37,59,76,93,94,102,132–134]Li4Ti5REs,O12 exhibiting(LTO) [15 ,distinctive16,37,59,76 plateaus,93 as,94 RE,102 at materials~3.4,132 –V134 vs ] Li/Li(Figure as RE for materials 14).LFP These [15,135,136] (Figure compounds and14). ~1.5 These are V vs compoundsideal Li/Li for for use are in + REs,ideal exhibitingLTO for use [16] in (Figure REs,distinctive exhibiting 15). plateaus distinctive at ~3.4 plateaus V vs Li/Li at ~3.4+ for V LFP vs Li/Li [15,135,136]for LFP and [15 ,~1.5135, 136V vs] and Li/Li ~1.5+ for V + LTOvs Li/Li [16] (Figurefor LTO 15). [16 ] (Figure 15).

Figure 15. Charge/discharge profile of Li4Ti5O12 (left) and LiFePO4 (right) vs. Li+/Li during RE preparation. Figure adapted with permission from [15].

Figure 15. Charge/discharge profile of Li Ti O (left) and LiFePO (right) vs. Li+/Li during RE FigureThe 15. presence Charge/discharge of a flat plateau profile region, of Li associated4Ti5O1212 (left with) and a two LiFePO‐phase4 (mechanismright) vs. Li during+/Li during the Li ‐ionRE preparation.preparation.intercalation/deintercalation Figure Figure adapted with processes, permission acts as from a buffer [15]. [15]. against the effect of small changes in chemical composition of the RE during battery operation. In this way, the partially‐lithiated material behaves Theas an presence ideal non of‐polarizable a flat plateau electrode, region, whereby associated the potentialwith a two does‐phase not vary mechanism even if a during large current the Li ‐ion perturbation is applied (Figure 14) [15]. The practical drawback of these materials is related to the intercalation/deintercalation processes, acts as a buffer against the effect of small changes in chemical composition of the RE during battery operation. In this way, the partially‐lithiated material behaves as an ideal non‐polarizable electrode, whereby the potential does not vary even if a large current perturbation is applied (Figure 14) [15]. The practical drawback of these materials is related to the Batteries 2019, 5, 12 12 of 24

The presence of a flat plateau region, associated with a two-phase mechanism during the Li-ion intercalation/deintercalation processes, acts as a buffer against the effect of small changes in chemical composition of the RE during battery operation. In this way, the partially-lithiated material behaves asBatteries an ideal 2019, 5 non-polarizable, x FOR PEER REVIEW electrode, whereby the potential does not vary even if a large current12 of 25 perturbation is applied (Figure 14)[15]. The practical drawback of these materials is related to the fact thatfact that they they need need to be to first be first delithiated delithiated and and then then partially-lithiated partially‐lithiated in order in order to obtain to obtain phases phases which which lie inlie thein the middle middle of of the the plateau plateau region region (Figure (Figure 15 15))[15 [15].]. In In addition, addition, the the material material and and potentialpotential shouldshould be chosen carefully as the RE can be poisoned by reduction or oxidation of metal dissolved from the WE [[134].134].

4.2. RE Geometry Independent of the type of active material used, various geometries and locations of the RE have been explored for the three-electrodethree‐electrode LIB cell configuration.configuration. These aspects are particularly important for EIS measurements where the presence of non-uniformnon‐uniform current densities can result in sampling of various equipotential surfaces between the CE and WE for a large surface area RE, which eventually leads to distortion of the impedance spectrum [[78,137].78,137]. The coaxial cell geometry (Figure 1616a),a), where the CE and WE discs are surrounded by a concentric, ring ring-shaped‐shaped RE RE placed placed at at the the separator separator border, border, has beenbeen proposedproposed to to mitigate mitigate impedance impedance artefacts artefacts due due to, e.g.,to, e.g., improper improper electrode electrode positioning positioning or stray or capacitancesstray capacitances [87,138 [87,138].].

Figure 16. CoaxialCoaxial (a ()a and) and reversed reversed coaxial coaxial (b) ( cellb) cell configurations. configurations. Reproduced Reproduced with withpermission permission from from[87,115]. [87, 115].

A reversed coaxial coaxial cell cell geometry, geometry, where where a a small small circular circular RE RE is isplaced placed at atthe the center center of WE of WE and and CE CEring ring electrodes, electrodes, has has been been explored explored via via finite finite element element modelling modelling (FEM), (FEM), to determine to determine the the influence influence of ofRE RE position position and and asymmetry asymmetry [139]. [139 ].This This latter latter geometry geometry was was also also experimentally experimentally investigated by several research groups (Figure(Figure 1616b)b) [[61,115].61,115]. Although these geometries were shown toto minimizeminimize the presence of artefacts in EIS measurements, their construction is not straightforward, particularly regarding precisionprecision inin electrodeelectrode alignment.alignment. Coin cells have also been implemented using micro-REsmicro‐REs where a Li annulus or a Li-containingLi‐containing alloy is placed on a metallic wire (e.g., copper) and inserted into into the the cell cell [68,78,89,99,140,141]. [68,78,89,99,140,141]. Researchers atat Argonne Argonne National National Laboratory Laboratory have have systematically systematically exploited exploited this this type type of cell of design cell design with thewith use the of use a Li-Sn of a alloy Li‐Sn RE alloy for theRE studyfor the of study commercially of commercially available available and next generationand next generation LIB electrodes LIB viaelectrodes EIS measurements. via EIS measurements. They also reported, They also for reported, the same for cell the geometry, same cell the geometry, use of a secondthe use Liof metala second RE forLi metal the determination RE for the determination of individual of electrode individual potentials electrode [65 ,potentials84,133,142 [65,84,133,142–151].–151]. However, the However, micro-RE canthe micro probe‐RE only can a specificprobe only and a limited specific area and of limited the cell. area Moreover, of the cell. its Moreover, construction its andconstruction placement and is notplacement an easy is task not andan easy disruption task and of disruption the wire structure of the wire is notstructure an isolated is not an occurrence isolated occurrence (Figure 17)[ (Figure37,57]. 17) [37,57]. Electrode misalignment can easily occur, thereby generating artefacts during the EIS measurements, and dealloying reactions can also shift the RE potential [68]. Batteries 2019, 5, 12 13 of 24

Electrode misalignment can easily occur, thereby generating artefacts during the EIS measurements, Batteriesand dealloying 2019, 5, x FOR reactions PEER REVIEW can also shift the RE potential [68]. 13 of 25

FigureFigure 17. CoinCoin cell cell equipped equipped with with a a third third RE RE ( (aa).). Enlargement Enlargement of of the the coin coin cell cell area area containing containing the the RE wire connection ( b).). Two Two different shapes of RE used in coin cell configurations configurations ( c,d). Position of of the the RERE inside the three three-electrode‐electrode coin cell ( e). Reproduced with permission from [[37].37].

SwagelokSwagelok “T “T-shape”‐shape” cells, widely widely used in battery research laboratories, generally comprise a a planeplane-parallel‐parallel WE,WE, CE,CE, and and a a perpendicular perpendicular RE RE generally generally a Li a metalLi metal disc, disc, which which is placed is placed in contact in contact with withthe other the other electrodes electrodes through through a separator a separator containing containing the electrolyte the electrolyte (Figure 11 (Figureb) [78,152 11b),153 [78,152,153].]. Although Althoughthis cell configuration this cell configuration has been implemented, has been implemented, e.g., applying e.g., aapplying probe-like a probe micro-RE‐like micro [73,74‐],RE Swagelok [73,74], “T-shape”Swagelok “T cells‐shape” are not cells artefact-free. are not artefact Nevertheless,‐free. Nevertheless, their use for EIStheir measurements use for EIS measurements is preferred over is preferredcoin cells equippedover coin withcells wireequipped REs, becausewith wire of REs, the location because of of the the latter location within of the electricallatter within current the electricalpath [78]. current path [78]. SignificantSignificant efforts efforts have have also also been been made made to implement to implement cylindrical cylindrical [24,35,38,43,72,154] [24,35,38,43,72 and,154 pouch] and [31,36,63,64,70,71,75,94,98,155–157]pouch [31,36,63,64,70,71,75,94,98,155 cells–157 with] cells an withRE (Figure an RE 18a,b). (Figure Besides 18a,b). the Besides practical the difficulties practical associateddifficulties with associated positioning with the positioning RE, particularly the RE, in particularly cylindrical cells in cylindrical where significant cells where modification significant of themodification cell is required, of the cellneither is required, the location neither nor the the location geometry nor of the the geometry RE is optimal of the REto obtain is optimal artefact to obtain‐free EISartefact-free measurements EIS measurements [36,72]. [36,72]. Batteries 2019, 5, 12 14 of 24 Batteries 2019, 5, x FOR PEER REVIEW 14 of 25

FigureFigure 18. Schematic18. Schematic representation representation of of RE RE implementationimplementation for for cylindrical cylindrical (a) (anda) and pouch pouch (b,c) (cells.b,c) cells. ReproducedReproduced with with permission permission from from [36 [36,38,112].,38,112].

Nevertheless,Nevertheless, minimally minimally invasive invasive insertion insertion ofof an RE RE into into commercial commercial Li‐ Li-ionion pouch pouch cells cells has been has been reportedreported in the in the literature literature (Figure (Figure 18 18c)c) [ 112[112].]. ThisThis procedure facilitates facilitates measurement measurement of WE of WEand CE and CE potentialpotential profiles profiles without without impacting impacting battery battery performance, performance, at at leastleast forfor the initial 20 20 cycles, cycles, although although no EIS measurementsno EIS measurements were reported. were reported. Other Other configurations configurations have have also also been been explored, explored, such such as theas the Conflat cell (whichConflat iscell similar (which to is a similar coin cell to a and coin where cell and a ring-shapedwhere a ring‐ REshaped is employed), RE is employed), where where an electrode an alignmentelectrode chosen alignment to enhance chosen to measurement enhance measurement reliability reliability has been has been reported reported [158 [158].]. Some Some studies studies have have also reported analyses of cells with cylindrical geometry, where the body of the cell is immersed also reported analyses of cells with cylindrical geometry, where the body of the cell is immersed in an in an electrolyte‐containing vessel together with the RE, all in an inert atmosphere [97,159,160]. electrolyte-containing vessel together with the RE, all in an inert atmosphere [97,159,160]. 4.3. Effect of Electrolyte 4.3. Effect of Electrolyte Non‐aqueous electrolytes are used in LIBs to facilitate ionic conduction of Li+ between the two + Non-aqueouselectrodes. The volume electrolytes of liquid are electrolyte used in LIBsemployed to facilitate within the ionic porous conduction separator structure of Li between plays a the two electrodes.critical role Thein the volume battery of performance liquid electrolyte and can employed have a significant within the impact porous on separatorelectrochemical structure playsmeasurements a critical role [161]. in the The battery thickness performance and porosity and of canthe separator have a significant are two of the impact key parameters on electrochemical that measurementsdetermine the [161 volume]. The of thickness electrolyte and used, porosity which can of thelead separator to problems are when two comparing of the key lab parameters‐scale cells that determinewith commercial the volume cells. of electrolyte For example, used, the glass which fiber can separators lead to problems generally when used in comparing small lab‐scale lab-scale cells cells withtypically commercial have cells. thicknesses For example, up to several the glass hundred fiber microns separators and are generally highly porous used in [48], small which lab-scale means cells that a relatively large amount of electrolyte is required. Such cells are generally described as “flooded typically have thicknesses up to several hundred microns and are highly porous [48], which means that cells”. Conversely, separators in commercial cells (e.g., pouch cells with thin polyolefin‐based a relativelyseparators) large are amount only a of few electrolyte tens of microns is required. in thickness Such cells and are therefore generally a describedmuch lower as amount “flooded of cells”. Conversely,electrolyte separators is added to in the commercial separator. cellsThese (e.g., cells are pouch described cells with as “electrolyte thin polyolefin-based‐starved” cells [78]. separators) In are onlyboth acases, few the tens electrolyte of microns strongly in thickness influences andthe capability therefore of a the much RE to lower accurately amount probe of the electrolyte electric is addedfield to between the separator. the electrodes These and cells artefacts are described can therefore as “electrolyte-starved” arise, particularly during cells EIS [measurements.78]. In both cases, the electrolyteIn flooded strongly cells, the influences experimental the capability conditions of theunder RE which to accurately the electrodes probe the are electric tested fieldare betweennot the electrodes and artefacts can therefore arise, particularly during EIS measurements. In flooded cells, the experimental conditions under which the electrodes are tested are not representative of those in practical applications, which are better represented by electrolyte-starved cells. Furthermore, because of the low ionic conductivity of the electrolyte used in LIBs, there is an additional requirement to Batteries 2019, 5, x FOR PEER REVIEW 15 of 25 Batteries 2019, 5, 12 15 of 24 representative of those in practical applications, which are better represented by electrolyte‐starved cells. Furthermore, because of the low ionic conductivity of the electrolyte used in LIBs, there is an additionalminimize therequirement distance betweento minimize WE the and distance CE. Satisfying between this WE requirement and CE. Satisfying is not anthis easy requirement task, since is notplacement an easy of task, the since RE is placement often challenging of the RE because is often ofchallenging space constraints because [of78 ].space For constraints all of these [78]. reasons, For allit is of good these practice reasons, to it reportis good the practice ratio of to the report volume the ratio of electrolyte of the volume to the of mass electrolyte of active to the material mass of in activethe electrode. material in the electrode. Some researchers have advised that in order to accurately study and verify the individual impedance of of a aspecific specific electrode, electrode, a symmetrical a symmetrical cell set cell‐up set-up with two with identical two identical electrodes electrodes [48] (which [48] should(which be should completely be completely artefact‐free) artefact-free) should be should adopted be [78]. adopted However, [78]. implementation However, implementation of this set‐up of isthis limited set-up to is uncycled limited to electrodes uncycled since electrodes the assembly since the of assembly symmetrical of symmetrical cells comprising cells comprising cycled electrodes cycled implieselectrodes disassembling implies disassembling and reconstruction and reconstruction procedures procedures which whichcould couldbe detrimental be detrimental in terms in terms of representativenessof representativeness and and reproducibility reproducibility of ofthe the real real electrochemical electrochemical behavior behavior of of the the electrode electrode in in the battery [121,162–164]. [121,162–164]. Room temperature ionic liquids (RTILs), i.e., salts having a melting point at or below room temperature [165], [165], are being explored because they provide a safer alternative to the commonly employed carbonate carbonate-based‐based electrolytes. Although Although their their high high costs costs of of production [166] [166] and relatively low ionic conductivity [167] [167] hinder the use of RTILs in practical applications, recent work has highlighted the the possibility possibility to to use use partially partially-lithiated‐lithiated LFP LFP as as an an RE RE active active material material when when an an RTIL RTIL is usedis used as electrolyte as electrolyte [168]. [168 The]. The authors authors reported reported that thata very a verystable stable and reproducible and reproducible LFP potential LFP potential could becould obtained be obtained even in even the inpresence the presence of oxidizing of oxidizing and reducing and reducing gases gases such as such oxygen as oxygen and hydrogen. and hydrogen. The developmentThe development of this of LFP this‐based LFP-based RE is particularly RE is particularly important important for research for researchactivities activitiesrelated to relatedthe study to ofthe innovative study of innovative RTIL‐based RTIL-based electrolytes electrolytes for LIBs since for most LIBs of since the mostelectrochemical of the electrochemical studies in this studies field relyin this on fieldthe use rely of on pseudo the use‐REs of (i.e., pseudo-REs electrodes (i.e., that electrodes maintain that a given, maintain but generally a given, butnot generallywell‐defined, not well-defined,potential during potential the course during of electrochemical the course of electrochemical experiments) experiments)[25] such as Ag [25 or] such Pt wires. as Ag or Pt wires. SolidSolid-state‐state electrolytes, either either polymeric polymeric or or inorganic, inorganic, are are also also of of great great interest interest in in the the LIB LIB field. field. They act bothboth asas solid solid ionic ionic conductors conductors and and separators separators and and were were widely widely investigated—particularly investigated—particularly the thepolymer polymer varieties—during varieties—during the the late late 70s 70s and and early early 80s 80s [169 [169].]. However, However, they they never never reached reached the the stage stage of oflarge-scale large‐scale commercial commercial production, production, mainly mainly due due to to concerns concerns over over the the use use of of Li Li metalmetal asas thethe negative electrode. On On the the other other hand, hand, polymer polymer electrolytes electrolytes are are now now in in practical use in Li-metalLi‐metal polymer batteries such as those produced by BollorBolloréé [[170,171].170,171]. The use of Li metal as RE RE in in solid solid-state‐state batteries batteries was reported in the early 1990s [14,172]. [14,172]. However, However, more recently recently Janek and co co-workers‐workers [103] [103] reported reported the use of a Li-AuLi‐Au alloy deposited on a W wire as a stable micro-REmicro‐RE for a pouch cell comprising a polymerpolymer-based‐based electrolyte, which they claimed was capable of providing reliable measurement free from artefacts (Figure 19a).19a). With With respect to solid solid-state‐state LIBs with inorganic inorganic electrolytes, a recent study reports the use of Li0.5InIn as as RE RE material material (Figure (Figure 19b)19b) [173]. [173]. The The same same research research work also investigated the effect of RE geometry and position with respect toto thethe thicknessthickness ofof thethe solidsolid electrolyteelectrolyte [[173].173].

Figure 19. Schematic representation of a lithium/lithium symmetric symmetric cell cell ( (aa)) all all-solid-state‐solid‐state Li Li-ion‐ion cell, cell, (b) each equipped with an RE. Reproduced with permission fromfrom [[103,173].103,173]. Batteries 2019, 5, 12 16 of 24

5. Summary and Outlook The use of reference electrodes in LIBs provides a number of benefits that are intended to enable: (i) correct interpretation of current and voltage data obtained during charge/discharge, (ii) identification of the contribution of each cell component to the overall battery performance, in particular the impedance response, and (iii) study of reaction mechanisms at the positive or negative electrode. As summarized in this review paper, research scientists have proposed various cell configurations and geometries to obtain reliable, reproducible, and representative experimental results. From our analysis of the literature, we can conclude that particular attention needs to be paid to the chemical composition of the RE, as well as its location and geometry with respect to the other cell components. Although Li metal is the material of choice for most RE studies in the literature, its potential is not stable over long periods and at high current densities, and also varies significantly with nature of the electrolyte. Various Li alloys have been investigated as potential alternatives. However, partially delithiated LiFePO4 and partially lithiated Li4Ti5O12 show the most promise to date. Chemical composition, volume and concentration of the electrolyte also have a major influence on reliable application of the RE. Furthermore, emerging battery chemistries, such as Li-S [174], Na-ion [129,175–177], Mg-ion [177,178], Ca-ion [177,179], and Na-O2 [180], have also considered the use of REs for enhanced electrochemical characterization. Despite much progress in the field over several decades, a reliable, user-friendly, and fully artefact-free cell configuration containing a chemically stable RE is far from being widely available. The electrochemical energy storage research field should focus on further development and exploration of novel architectures, cell geometries, and chemistries to achieve the ultimate goal of a practical RE that behaves ideally under the widest possible range of operating conditions. The implementation of REs in commercial LIBs appears to be even further from realization since: (i) the required change in cell geometry is not cost-effective and would require sacrifice of part of the cell volume for the reference electrode placement and (ii) additional costs related to the fabrication and placement of one or more reference electrodes are commercially prohibitive.

Author Contributions: Conceptualization, R.R. and M.A. (equal contribution); methodology R.R. and M.A. (equal contribution); investigation, R.R. and M.A. (equal contribution); writing—original draft preparation, R.R. and M.A.; writing—review and editing, R.R., M.A., and G.H.; visualization, R.R. and M.A.; supervision, G.H.; funding acquisition, G.H. Acknowledgments: The authors gratefully acknowledge financial support from the National Measurement System of the UK Department of Business, Energy and Industrial Strategy. The authors thank Edmund Dickinson for the productive scientific and linguistic discussions. Conflicts of Interest: The authors declare no conflict of interest.

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