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

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Critical Review of the Use of Reference Electrodes in Li-Ion Batteries: a Diagnostic Perspective 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. Batteries 2019, 5, 12; doi:10.3390/batteries5010012 www.mdpi.com/journal/batteries Batteries 2019, 5, 12 2 of 24 Batteries 2019, 5, x FOR PEER REVIEW 2 of 25 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 Batteries 2019, 5, x FOR PEER REVIEW 3 of 25 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
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