energies

Communication Effect of Salt Concentration, Donor Number and Coordination Structure on the Variation of the Li/Li+ Potential in Aprotic Electrolytes

P. K. R. Kottam , S. Dongmo, M. Wohlfahrt-Mehrens and M. Marinaro *

ZSW—Zentrum für Sonnenenergie und Wasserstoff-Forschung Baden-Württemberg, 89081 Ulm, Germany; [email protected] (P.K.R.K.); [email protected] (S.D.); [email protected] (M.W.-M.) * Correspondence: [email protected]

 Received: 20 February 2020; Accepted: 17 March 2020; Published: 20 March 2020 

Abstract: The use of concentrated aprotic electrolytes in lithium batteries provides numerous potential applications, including the use of high-voltage cathodes and Li-metal anodes. In this paper, we aim at understanding the effect of salt concentration on the variation of the Li/Li+ Quasi-Reference Electrode (QRE) potential in Tetraglyme (TG)-based electrolytes. Comparing the obtained results to those achieved using DMSO-based electrolytes, we are now able to take a step forward and understand how the effect of solvent coordination and its donor number (DN) is attributed to the Li-QRE potential shift. Using a revised Nernst equation, the alteration of the Li redox potential with salt concentration was determined accurately. It is found that, in TG, the Li-QRE shift follows a different trend than in DMSO owing to the lower DN and expected shorter lifespan of the solvated cation complex.

Keywords: lithium; reference electrode; high concentrated electrolytes; ferrocene; Nernst equation; cyclic voltammetry; donor number

1. Introduction Li- batteries (LIBs) are undeniably the current world leader in solving energy storage problems. For LIB cells, electrolytes based on carbonates are state-of-the-art materials. However, these electrolytes possess a relatively low electrochemical stability window that limits their use for both next generation 5V cathodes and Li metal anodes [1]. Recent reports on the use of concentrated solutions as electrolytes for future Li batteries have been on the rise [2,3]. The presence of a near zero free solvent concentration at these high concentrations favours its stability against solvent decomposition. We previously investigated the effect of Li-salt concentration on the variation of the Li-QRE potential in DMSO-based electrolytes [4]. Based on our previous results, the trend in the potential shift is more pronounced at high salt concentrations owing to the change in the coordination structure of Li+ . Such a similar shift in the potential trend has also been reported by Ernould et. al., in ethylene carbonate (EC): diethyl carbonate (DEC): Tri-ethyl phosphate (TEP) mixtures, wherein they observed a maximum shift of -500 mV with increasing organic phosphate concentration [5]. In addition, a variation in solvent also affects the shift of the Li-redox potential, revealing the solvent-dependence of the Li- free energy [6]. For example, a relatively high donor number (DN) solvent more readily solvates the Li+ ions than solvents with lower DN. However, there are only a few studies reporting on the variation of the Li- Quasi-Reference Electrode (QRE) potential shift as a function of both salt concentration and solvent DN. In this prospect, we chose Tetraglyme (TG) solvent, which possesses a low DN (12 kcal/mol) [7], and compared it to the previously investigated Dimethyl sulfoxide (DMSO; DN = 29.9 kcal/mol) [8] to

Energies 2020, 13, 1470; doi:10.3390/en13061470 www.mdpi.com/journal/energies Energies 2020, 13, 1470 2 of 9 specifically address the effect of the DN. These two aforementioned solvents are quite commonly used in post Li-ion battery cells (i.e., Li-O2)[3,9–16]. Using a concentration cell setup, the electromotive force (EMF) is measured with respect to various salt concentrations. Furthermore, the obtained results are validated using a standard internal reference system, namely ferrocene (Fc), as the Fc/Fc+ couple is reportedly stable under salt and solvent variation [4,17]. We believe that the dependency of Li-QRE potential on (1) solvent DN and (2) Li-ion concentration in aprotic electrolytes is scarcely investigated and little-understood. Our study aims to shed some light on such phenomena. Moreover, in agreement with Mozhzhukhina and Calvo [17], our current study stresses the necessity of specifying the solvent, electrolyte salt and Li+ concentration when performing an electrochemical measurement using lithium metal as the reference electrode.

2. Materials and Methods Lithium-Bis(trifluoromethane)sulfonimide (LiTFSI) purchased from Solvionic (99.9% pure) was dried in a Büchi oven at 140 ◦C for 72 h under vacuum. TG purchased from Sigma-Aldrich (>99.9% pure) was dried with pre-treated molecular sieves (4 Å) in accordance with our previously reported studies [4]. Additionally, ferrocene (Fc) purchased from Sigma-Aldrich (99% pure) was dried overnight in the Büchi oven at 150 ◦C. Finally, 0.05, 0.2, 1,2 and 3 M LiTFSI/TG solutions were prepared with 7.5 mM Fc and stored in a glovebox with O2 and H2O levels below 0.1 ppm. A three-electrode beaker cell setup was used to collect the cyclic voltammograms for different concentrations. An Au disk (PINE E6 standard) was used as working electrode and polished before and after each measurement with fine grinding papers. The polished and cleaned electrode was then dried at 120 ◦C under vacuum in a Büchi oven for 24 h before the next experiment. Lithium metal strips (Alfa Aesar; 99.9% pure; 0.03-inch thickness) acted as counter and reference electrodes. Raman spectroscopy measurements 1 ® (wavenumber range of 780–900 cm− ) were performed using a BRUKER Senterra with a 532 nm laser source. The electrolyte samples were handled under an argon environment and only exposed to air before the measurement.

3. Results Figure1 provides the Raman spectra corresponding to di fferent concentrations of LiTFSI in TG 1 1 solution. From the figure, the region between 800 cm− and 900 cm− can be seen to be characteristic of mixture of modes for ether COC symmetric stretching (vsCOC) and CH2 rocking(rCH2) modes of glymes [18]. The spectral region can be differentiated into two sections: (a) a series of three bands 1 characteristic of various modes of vibration in glymes chains at 809, 828 and 851 cm− , which in general decreases in intensity as the salt concentration increases, (b) an increase in intensity at a 1 wavenumber if ~873 cm− with concentration (breathing mode from the complex formation with metal ions; i.e., M–On(A1g)) which refers to more coordinated TG solvent concentration [18,19]. Since the 3 M solution is reported to possess a fully coordinated solvent state [20], the normalization of the solvent coordination peak is performed by considering the boundary conditions (i.e., a zero coordination state in pure solvent and fully coordinated state in 3 M solution) and applying them to the peak 1 intensity at 873 cm− . Such a similar Raman peak normalization was performed by Tatara et al. [21] in DMSO-based electrolytes. From the normalized data, solvent parameters such as the molar ratio of solvent to salt ([TG]/[LiTFSI]), concentrations of coordinated ([TG]coor) and free TG molecules ([TG]free) in the electrolyte and coordination number (n) were extrapolated and are shown in Table1. The solvent parameters for the xM LiTFSI/DMSO electrolytes (0.05 x 3) can be found from our ≤ ≤ previous works [3,4]. It is to be noted that DMSO is a monodentate system where the only available oxygen molecule acts as a solvation centre. This corresponds that for a full solvation of Li+ ions, ~4 DMSO molecules are involved [3]. However, in case of a multidendate chain molecule such as TG, all the 5 coordination centres are involved in complete solvation of Li+ ion [20,22]. EnergiesEnergies2020 2020, ,13 13,, 1470 x FOR PEER REVIEW 33 ofof 98

pure solvent 0.05M LiTFSI/TG 0.2M LiTFSI/TG 1M LiTFSI/TG 2M LiTFSI/TG 3M LiTFSI/TG Intensity(a.u)

900 880 860 840 820 800 780 Wavenumber(cm-1)

Figure 1. Raman spectra recorded for different concentrated electrolytes of LiTFSI/TG (Tetraglyme). Figure 1. Raman spectra recorded for different concentrated electrolytes of LiTFSI/TG From(Tetraglyme). the solvent parameters in Table1, at low concentrations (0.05 M, 0.2 M), the coordination number is found to be in a similar range due to the availability of excess solvent (n [TG]/[LiTFSI] = 87.1 89 and 21.1From for the 0.05 solvent M and parameters 0.2 M respectively). in Table 1, However, at low concentrations the value increases (0.05 M, when 0.2 M), going the from coordination 0.2 M to 90 1number M and fromis found 1 M to to be 2 M, in unlikea similar in DMSO.range due Such to anthe increase availability in coordination of excess solvent number (n with[TG]/[LiTFSI] increasing = 91 salt87.1 concentration and 21.1 for 0.05M is scarcely and 0.2M reported respectively). for TG-based However, solutions. the value Recent increases work by when Mandai going et.al. from suggests 0.2 M 92 thatto 1 Li M+ transportand from proceeds 1 M to via2 M, continuous unlike in ligandDMSO. exchange Such an in increase dilute solution in coordination of TG due number to the higher with 93 + + solventincreasing/Li saltion diconcentrationffusivity ratio is (Dscarcelysol/DLi reported)[23]. This for leads TG-based to a short solutions. lifetime Recent of the work solvated-cation by Mandai 94 complexet.al. suggests in solutions that Li with+ transport excess glymes. proceeds In thisvia scenario,continuous the ligand Lewis basicityexchange of in anions dilute is highersolution than, of TG or 95 comparabledue to the higher to, that solvent/ of the electron-donating Li+ ion diffusivity group ratio of (D glymessol/DLi+ (ethyl)[23]. This oxide leads (EO) to chain),a short and lifetime the weaker of the 96 Lisolvated-cation+-EO interaction complex leads to in poor solutions complex with formation excess glymes. [24]. In In contrast, this scenario, in concentrated the Lewis solutions, basicity of because anions 97 + + ofis thehigher strong than, Li -solventor comparable interaction, to, that Dsol of/D Lithe approacheselectron-donating unity [23 group]; i.e., of the glymes residence (ethyl time oxide of solvent (EO) 98 increases.chain), and This the might weaker suggest Li+-EO that interaction Li+ cations leads and glymes to poor di ffcomplexuse together, formation leading[24] to. In the contrast, formation in + + + [23] 99 ofconcentrated a stable [Li(TG) solutions,n] complex. because Asof the a result strong of Li stable-solvent complex interaction, formation Dsol/D inLi concentratedapproaches electrolytesunity ; i.e., 100 withthe residence increased time residence of solvent time, Liincreases.+ ions might This dimightffuse suggest with a coordinationthat Li+ cations shell and [20 ].glymes The e ffdiffuseect of 101 co-ditogether,ffusion leading in concentrated to the formation solutions of might a stable be responsible[Li(TG)n]+ complex. for the increase As a result in coordination of stable complex number 102 withformation the increase in concentrated in salt concentration. electrolytes Inwith 3 M increased solution, residence however, time, due to Li the+ ions availability might diffuse of a limited with a 103 amountcoordination of solvent, shell [20] the.The coordination effect of co-diffusion number decreases in concentrated again. solutions might be responsible for the 104 increase in coordination number with the increase in salt concentration. In 3 M solution, however, 105 due to the availability ofTable a limited 1. Solution amount properties of solvent, analyzed the fromcoordination Raman spectra. number decreases again.

106 Solution nTable [TG]/ [LiTFSI]1. Solution properties ([TG]coor analyz) mol/edL from ([TG]Ramanfree spectra.) mol/L n Free solvent - - 4.53 - 0.05 M LiTFSI/TG 87.14 0.043 4.313 0.81 0.2 M LiTFSISolution/ TG n 21.15 [TG]/[LiTFSI] 0.152([TG]coor) mol/L 4.077([TG]free) mol/L 0.76 n 1 MFree LiTFSI solvent/ TG 3.79 - 0.995 - 2.796 4.53 0.99 - 20.05 M LiTFSI M LiTFSI/TG/ TG 1.58 87.14 2.217 0.043 0.937 4.313 1.11 0.81 3 M LiTFSI/ TG 0.83 2.502 - 0.83 0.2 M LiTFSI/ TG 21.15 0.152 4.077 0.76 1 M LiTFSI/ TG 3.79 0.995 2.796 0.99 However,2 M LiTFSI/ this TG cannot be the 1.58case for DMSO, as the 2.217 coordination number 0.937 (n) decreases with1.11 the increase3 M inLiTFSI/ Li-salt TG concentration. 0.83 The high DN of DMSO 2.502 determines the high - Li salt dissociation 0.83 (preferential solvent-separated ion pairs) with the formation of stable Li+-DMSO complexes. A 107 common example can be deduced from the work of Rocio et al. [25], in which they used Li-containing DMSO:ACN mixtures. They observed the effect of the preferential solvation of Li-salt by DMSO over 108 ACNHowever, due to the this relative cannot stabilization be the case of for ionic DMSO, species as the and coordination increased solvent–solute number (n) decreases energy coupling with the 109 increase in Li-salt concentration. The high DN of DMSO determines the high Li salt dissociation 110 (preferential solvent-separated ion pairs) with the formation of stable Li+-DMSO complexes. A

Energies 2020, 13, 1470 4 of 9 in DMSO-based electrolytes. The relative stabilization of the Li+-DMSO complex prevents Li+-anion interaction, affecting the solvent coordination number. In order to experimentally determine the effect of salt concentration on the Li-redox potential, a concentration cell has been employed. Further information regarding the experimental setup can be obtained from our previous work [4]. Equation (1b) provides the Nernstian potential (E), which is + + dependent on the activity of Li (αLi ) according to Equation (1a) [4].

+ Li + e− Li (1a) ↔ RT E = E + 2.303 log α + (1b) ◦ F Li

αLi+ (the activity of lithium species) can be defined as the product of the activity coefficient and + concentration of Li species, (γ + ) and (C + ), respectively; i.e., α + = C + γ + . Equation (1b) can Li Li Li Li × Li be rewritten as RT E = E + 2.303 (log γ + + log C + ) (2a) ◦ F Li Li Based on our previously published results [4], for Equation (2b), a revised version of the + Nernst equation including the concentration (C) of lithium-TG solvated ion complex Li(TG)n (C + = C + ) and free TG molecules (C ) is shown in Equation (3). Li[TG]n Li [TG] f ree

+ [Li[TG] ] + e− Li + n[TG] (2b) n ↔ f ree

C + RT RT [Li[TG]n] E = E◦ + 2.303 logγLi+ + 2.303 log (3) F F C n [[TG] f ree] Here, n is the number of TG molecules solvating Li+. For simplicity, the activity coefficient of Li+ (γLi+ ) value is assumed to be equal to 1. Measurements using the concentration cell were carried out on five different concentrated solutions ranging from 0.05 M up to 3 M. For all the measurements, the left compartment of the cell was filled with 0.2 M LiTFSI/TG electrolyte, which acted as a measurement reference point. The right compartment was instead filled with the electrolyte under investigation (x= 0.05, 1, 2 and 3 M). The electromotive force (EMF) is therefore described by Equation (4):    CLi(TG) +(in xM) CLi(TG) +(in 0.2M)  RT  n n  EMFELi/Li+( in xM) ELi/Li+( in 0.2M) = 2.303 log log  (4) − F  C n − C n  (TG) f ree (in xM) (TG) f ree (in 0.2M)

Based on the above equation, the EMF is obtained for all the concentrations and provided in Figure2. The calculated values are in close agreement with those experimentally measured using the concentration cell. In the case of the 3 M solution, the theoretical EMF between 0.2 and 3 M could not be calculated, as the C in the concentrated solution (3 M) is zero (Table1) and the calculated (TG) f ree EMF value in Equation (4) reaches infinity. Energies 2020, 13, x FOR PEER REVIEW 5 of 8 Energies 2020, 13, 1470 5 of 9

0.6 Nernst Equation based on Li++ e- <-> Li Experimental- DMSO + - Nernst Equation based on [Li(DMSOn] + e <-> Li + n(DMSO)free Experimental- TG + - Nernst Equation based on [Li(TGn] + e <-> Li + n(TG)free 0.4

+ - Li(DMSO)n + e ↔ Li + n(DMSO)

EMF/V 0.2

+ - Li(TG)n + e ↔ Li + n(TG) + - Li + e ↔ Li

0.0

0.1 1 Conc. (mol/L)

Figure 2. Reference potential measurement compared to the theoretical calculations for different concentratedFigure 2. Reference solutions potential of DMSO measurement and TG. compared to the theoretical calculations for different concentrated solutions of DMSO and TG. Based on our previous knowledge on the use of Fc as a standard internal reference system [4], the measured values of the potential shift in the TG-based solution are further validated using a Fc/Fc+ 138 redoxBased couple. on our Figure previous3a provides knowledge the cyclic on the voltammogram use of Fc as a standard (CV) measurements internal reference of the system reversible[4], the 139 measuredredox reaction values of of Fc the under potential varying shift Li-salt in the concentrations. TG-based solution In Figure are further3a, the xvalidated value denotes using variousa Fc/Fc+ 140 redoxconcentrations; couple. Figure i.e., 0.2 3a M,provides 1 M and the 2 cyclic M. For voltammogram this work, a three-compartment(CV) measurements beaker of the cellreversible with a 141 redoxcompartment reaction dedicatedof Fc under to varying the reference Li-salt electrode concentrations. is chosen. In Figure An Li 3a, strip the dipped x value in denotes the electrolyte various 142 concentrations;under investigation i.e., in0.2 the M, reference 1 M and compartment 2 M. For this served work, as a the three-compartment QRE. By eliminating beaker the iR cellohmic withdrop a 143 compartment(Rohmic= 767 Ω dedicated, 277 Ω and to 492 theΩ referencefor 0.2 M, electrode 1 M and 2 is M, chosen. respectively), An Li thestrip peak dipped separation in the between electrolyte the 144 underoxidation investigation and reduction in the of reference Fc was observed compartmen to be int served the range as the of 60–64 QRE. mV By foreliminating all the electrolytes the iRohmic under drop 145 (Rinvestigation,ohmic= 767 Ω, in 277 agreement Ω and 492 with Ω for theoretical 0.2 M, 1 value M and of 592 M, mV respectively), for the one-electron the peak reaction separation [26]. between It can be 146 theseen oxidation that the peak and positionreduction shifts of Fc cathodically was observed as the to be Li saltin the concentration range of 60–64 in the mV electrolyte for all the increases. electrolytes As 147 underthe Fc/ Fcinvestigation,+ potential is in reportedly agreement independent with theoretical from thevalue salt of concentration 59 mV for the in one-electron the supporting reaction electrolyte,[26]. It 148 canthe extentbe seen of that the the observed peak position shift should shifts only cathodica dependlly onas the Li variation salt concentration of the Li-QRE in the potential electrolyte as a 149 increases.function of As the the salt Fc/Fc concentration.+ potential The is reportedly data reported independent in Figure2 forfrom the the TG salt electrolytes concentration are therefore in the 150 supportingused to correct electrolyte, the potential the extent scale ofof thethe Fcobserved/Fc+ CVs. sh Theift should potential only correction depend on for the Fc/ variationFc+ redox of has the been Li- 151 QREmade potential based on as Equation a function (5). Figureof the3 bsalt shows concentration. the CVs for The Fc / Fcdata+ in reported all three investigatedin Figure 2 electrolytes,for the TG 152 electrolyteswhere the potential are therefore is now used referred to correct against the the potential Li/Li+ couple scale of in the 0.2 Fc/Fc M LiTFSI+ CVs./TG The solution. potential Moreover, correction in 153 forcomparison Fc/Fc+ redox to our has previous been made work, based a notable on Equation cathodic [5]. shift Figure in Fc 3b/Fc shows+ redox the potential CVs for isFc/Fc observed+ in all whenthree 154 investigatedthe solvent is electrolytes, changed from where high-DN the DMSOpotential to low-DN is now TG.referred As suggested against bythe Calvo Li/Li et.al+ couple [17], assumingin 0.2 M 155 LiTFSI/TGthe potential solution. of ferrocene Moreover, is not in influenced comparison by to the our solvent previous variation work, a due notable to the cathodic small contribution shift in Fc/Fc of+ 156 redoxsolvation, potential such variationis observed arises when due the to thesolvent shift is of changed the Li-QRE from potential high-DN in eachDMSO solvent. to low-DN TG. As 157 suggested by Calvo et.al[17], assuming the potential of ferrocene is not influenced by the solvent 158 variation due to the small contributionEFc/Fc+corrected of solvation,= EFc/Fc such+measured variation+ EMF arises due to the shift of the Li-(5) 159 QRE potential in each solvent.

EnergiesEnergies 20202020, ,1313, ,x 1470 FOR PEER REVIEW 66 of of 8 9

0.2M LiTFSI/TG+ 7.5mM Fc 0.08 0.2M LiTFSI/TG+ 7.5mM Fc 0.08 1M LiTFSI/TG+ 7.5mM Fc 1M LiTFSI/TG+ 7.5mM Fc 2M LiTFSI/TG+ 7.5mM Fc 0.06 2M LiTFSI/TG+ 7.5mM Fc 0.06

0.04 0.04

0.02 0.02

I / mA I / 0.00 I / mA 0.00

-0.02 -0.02

-0.04 -0.04

-0.06 -0.06

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 E / V vs. Li/Li+ in xM LiTFSI/TG E / V vs. Li/Li+ in 0.2M LiTFSI/TG

(a) (b)

FigureFigure 3.3.( aa)) CyclicCyclic voltammograms voltammograms (CVs) (CVs) recorded recorded for threefor three different different concentrations, concentrations, (b) CVs b) rescaled CVs accordingrescaled according to the reference to the potentialreference of potential 0.2 M LiTFSI of 0.2/TG: M WE:LiTFSI/TG Au disk, : WE: CE/ RE:Au Lidisk, strip CE/RE: immersed Li strip in the respectiveimmersed electrolytein the respective under electrolyte study, sweep under rate: study, 10 mV sweep/s. Here, rate: x =100.2, mV/s. 1, 2 Here, M. x = 0.2, 1 , 2 M.

To understand the effect of the solvent DN on the potential shift trend, we also show in Figure2 the variation of the EMF of Li| Li symmetrical cells as a function of the Li-salt concentration in DMSO electrolytes [4]. In the case of DMSO, a linear shift in accordance with Equation (1a) is witnessed up / =/ + [5] to concentration range of 0.2 M. With elevated concentrations, an exponential deviation of Li-QRE is observed [4], which can be explained by considering the solvent activity in Equation (2b). However, in 160 the caseTo understand of TG, the linear the effect shift of the Li-QRE solvent potential DN on extends the potential to 1 M asshift the trend, exponential we also deviation show in can Figure only 161 2be the witnessed variation after of the at EMF higher of concentrations. Li| Li symmetrical Considering cells as a that function Equation of the (1a) Li-salt is valid concentration only for dilute in 162 DMSOsolutions, electrolytes in the case [4] of. In TG, the 1 Mcase still of acts DMSO, as a dilute a linear solution. shift in A similaraccordance trend with is witnessed equilibrium by Ueno1 is 163 witnessedet.al. [18] whenup to concentration the corresponding range anion of 0.2 isM. changed With elevated from a concentrations, highly associated an [OTF]exponential- to a dissociative deviation [4] 164 ofone Li-QRE [TFSI] is-. observed In the case, which of an associated can be explained salt, due by to considering the strong the Li+ -anionsolvent interactions activity in equilibrium (compared 2. to 165 However,Li+-solvent in interaction) the case of thereTG, the is a linear resulting shift presence of Li-QRE of more potential uncoordinated extends to solvent 1 M as concentration. the exponential This 166 deviationleads in turn can only to an be extended witnessed linear after shift at higher trend concentrations. of the Li-redox Considering potential with that increased equilibrium concentration. 1 is valid 167 onlyAs discussed for dilute above, solutions, the weakin the (Li case+ -solvent) of TG, interactions1 M still acts and as varieda dilute Li +solution.-solvent A complex similar residencetrend is [18] - 168 witnessedtime makes by the Ueno availability et.al. when of free the solvent corresponding in 1 M LiTFSI anion/TG is abundant,changed from thus a possiblyhighly associated acting as a[OTF] dilute - + 169 tosolution. a dissociative Owing one to its [TFSI] high. DN, In the a strong case of Li +an-DMSO associated interaction salt, due exists, to the which strong might Li -anion increase interactions the solvent 170 (compared to Li+-solvent interaction) there is a resulting presence of more uncoordinated+ solvent residence time. The elapsed residence time and possible formation of stable Li - [DMSO]n complex 171 concentration.presumably makes This theleads Li +inions turn di toffuse an inextended a differentiable linear shift way trend in DMSO of the in comparisonLi-redox potential to that inwith TG + + 172 increasedsolution [27concentration.,28]. As discussed above, the weak (Li -solvent) interactions and varied Li - 173 solventTo complex understand residence the eff ecttime of themakes solvent the availabi DN on thelity potentialof free solvent shift trend, in 1 M we LiTFSI/TG also show abundant, in Figure2 + 174 thusthe variationpossibly acting of the EMFas a dilute of Li| Lisolution. symmetrical Owing cells to its as high a function DN, a ofstrong the Li-salt Li -DMSO concentration interaction in exists, DMSO 175 whichelectrolytes might [increase4]. In the the case solvent of DMSO, residence a linear time. shift The in elapsed accordance residence with Equationtime and (1a)possible is witnessed formation up + + 176 ofto stable concentration Li - [DMSO] rangen ofcomplex 0.2 M. Withpresumably elevated makes concentrations, the Li ions an diffuse exponential in a deviationdifferentiable of Li-QRE way in is [27,28] 177 DMSOobserved in comparison [4], which can to bethat explained in TG solution by considering. the solvent activity in Equation (2b). However, in the case of TG, the linear shift of Li-QRE potential extends to 1 M as the exponential deviation can only 178 4. Conclusions be witnessed after at higher concentrations. Considering that Equation (1a) is valid only for dilute 179 solutions,The effect in the of casethe Li-salt of TG, concentration 1 M still acts as on a the dilute variation solution. of Athe similar Li/Li+ trendQuasi-Reference is witnessed Electrode by Ueno 180 potentialet.al. [18 ]has when been the investigated. corresponding For anionthis work, is changed we chose from LiTFSI/TG a highly associatedwith five different [OTF]- to concentrations a dissociative 181 (0.05one [TFSI]M, 0.2 -M,. In 1 theM, 2 case M and of an 3 M), associated and a concentration salt, due to thecell strongwas implemented Li+-anion interactions for measurement. (compared Using to 182 theLi+ -solventrevised interaction)Nernst equation there is from a resulting our previous presence ofknowledge, more uncoordinated the potential solvent shift concentration. was calculated This 183 theoretically,leads in turn and to an a extendedgood agreement linear shift was trend obtained of the with Li-redox the measured potential withvalues. increased The obtained concentration. values 184 wereAs discussed further validated above, the using weak an (Li Fc/Fc+ -solvent)+ internal interactions reference couple, and varied which Li +matches-solvent with complex the measured residence 185 EMFtime makesvalues. theOne availability important of phenomenon free solvent in observed 1 M LiTFSI is/ TGthat abundant, the onset thus of deviation possibly acting in the as Li-QRE a dilute 186 potential from the trend of the linear (ELi/Li+ vs. log CLi+) shift in TG was further extended to higher 187 concentration in comparison to that in DMSO-based electrolytes. The increased solvent residence

Energies 2020, 13, 1470 7 of 9 solution. Owing to its high DN, a strong Li+-DMSO interaction exists, which might increase the solvent + residence time. The elapsed residence time and possible formation of stable Li - [DMSO]n complex presumably makes the Li+ ions diffuse in a differentiable way in DMSO in comparison to that in TG solution [27,28].

4. Conclusions The effect of the Li-salt concentration on the variation of the Li/Li+ Quasi-Reference Electrode potential has been investigated. For this work, we chose LiTFSI/TG with five different concentrations (0.05 M, 0.2 M, 1 M, 2 M and 3 M), and a concentration cell was implemented for measurement. Using the revised Nernst equation from our previous knowledge, the potential shift was calculated theoretically, and a good agreement was obtained with the measured values. The obtained values were further validated using an Fc/Fc+ internal reference couple, which matches with the measured EMF values. One important phenomenon observed is that the onset of deviation in the Li-QRE + + potential from the trend of the linear (ELi/Li vs. log CLi ) shift in TG was further extended to higher concentration in comparison to that in DMSO-based electrolytes. The increased solvent residence time and formation of a stable solvent- Li+ complex in DMSO-based solvent owing to its high DN is presumed to be responsible for such a deviation in the potential shift.

Author Contributions: Conceptualization, P.K.R.K., M.M. and M.W.-M.; methodology, P.K.R.K. and M.M.; validation, P.K.R.K., M.M. and S.D.; formal analysis, P.K.R.K.and M.M.; investigation, P.K.R.K.; writing—original draft preparation, P.K.R.K.; writing—review and editing, M.M and S.D.; supervision, S.D., M.W.-M. and M.M.; project administration, M.M.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Bundesministerium für Bildung und Forschung (BMBF) in the framework of LiBaLu project (FKz 03XP0029D). Acknowledgments: Financial support from Bundesministerium für Bildung und Forschung (BMBF) is gratefully acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

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