The Role of Electrolyte Upon the SEi Formation Characteristics and Low Temperature Performance of Lithium-Ion Cells with Graphite Anodes M. C. Smart, B. V. Ratnakumar, S. Greenbaum and S. Surampudi Jet Propulsion Laboratory. California Institute of Technology 4800 Oak Grove Drive Pasadena, CA 91109 Abstract Quarternary lithium-ion battery electrolyte solutions containing ester co-solvents in mixtures of carbonates have been demonstrated to have high conductivity at low temperatures (< -20°C). However, in some cases the presence of such co-solvents does not dire.c0y translate into improved low temperature cell performance, presumably due to the formation of ionically resistive surface films on carbonaceous anodes. In order to understand this behavior, a number of lithium-graphite cells have been studied containing electrolytes with various ester co-solvents, including methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), and ethyl butyrate (EB). The charge/discharge characterization of these cells indicates that the higher molecular weight esters result in electrolytes which possess superior low temperature performance in'contrast to the lower molecular weight ester-containing solutions, even though these solutions display lower conductivity values. Introduction Both NASA and the Air Force have interest in lithium-ion batteries with improved low temperature performance for a number of future applications. Under sponsorship by the Mars Exploration Program, we have focused upon developing advanced electrolyte systems with improved low temperature properties. This has led to the identification of a carbonate-based electrolyte, consisting of 1.0 M LiPF_ in EC + DEC + DMC (33:33:34), that has been shown to have excellent performance at -20°C. (1,2) However, many applications, including future Mars Rovers, might be required to operate at temperatures as low as -40°(2 and high discharge rates (C/2). For this reason, we have investigated ethylene carbonate-based electrolytes to which appropriate co-solvents axe added to sufficiently lower the viscosity and melting point of the solution. Similar approaches have been adopted using formates (i.e, methyl formate (MF)), acetates (i.e, MA, EA, and EP), as well as, other carbonates (i.e., ethyl methyl carbonate and methyl propyl carbonate) as co-solvents in lithium-ion electrolytes. (3-8) Most of these studies involving the use of such solvents for Li-battcry electrolyte formulations have consL,;ted of binary (some ternary) solutions in which the low viscosity component is in large proportion (>30 vol%), magnifying any undesirable properties"(Le., t'dm-formation characteristics). In the present study, we have investigated a number of electrolyte solutions based on an optimized ternary mixture of carbonates (EC+DEC+DMC) to which potential co- solvents are added. The formulations studied include the following: 0.75M LiPF6 in EC+DEC+DMC+X (1: l: 1:1) (where x= methyl acetate (MA), ethyl acetate (EA), ethyl proprionate (EP), ethyl butyrate (EB), methyl formate (MF), and ethyl methyl carbonate (EMC). The selection of the electrolytes reported in this study is based upon the beneficial properties of adding low viscosity, low melting point solvent additives to base formuhtions of carbonate mixtures, which have been observed to have the desirable stability and passivating qualities. The use of multi-component quartemary solutions allows the physical properties of the electroytes to be improved (higher conductivity and low freezing point), while still maintaining the desirable film-forming properties provided by mixtures of carbonates. The selection of the ester-based solvents is founded upon their favorable physical properties, as shown in Fig. 1, and the demonstrated high conductivity when mixed into carbonate formulations. Although it has been determined that methyl acetate and ethyl acetate (lower molecular derivatives) form insulating films on the carbon electrodes preventing facile kinetics at low temperature, the higher molecular weight counterparts were studied with the expectation of decreased reactivity. The electrolyte formulations were investigated in lithium-graphite (KS-O) half-cells, equipped with lithium reference electrode, to determine their compatibility with graphite electrodes. In addition to studying the charge/discharge characteristics of these cells at various temperatures, a.c. impedance and D.C. micropolarization techniques have been employed to probe the nature of the passive fLlm-formation process. To augment these measurements, high resolution solid state 7Li NMR spectroscopy was used to investigate the solid electrolyte interface (SEI) formation in the graphite electrodes.(9) Specifically, the NMR method allows the direct quantitative determination of Li associated with the SEI as well as intercalated lithium. Some of the aspects that contribute to the formation of the SEI layer, its impact upon cell performance, and possible methods of characterization are illustrated in Fig. 2. Results and Discussion Conductivity Measurements A number of carbonate-based electrolytes, containing low viscosity and low melting aliphatic ester additives, have been prepared and their conductivity measured over a temperature range of -60°C to 25°C. Of the electrolytes investigated, the formulations that displayed the highest conductivity at low temperatures (Fig. 1) were the ones containing the lower molecular weight esters. The conductivity varies with the l\)llowing trend: I.OM LiPF_ EC + DEC + DMC + MA (l:l:l:i) > I.OM LiPF_ EC + DEC + DMC + EA (l:l:l:l) > I.OM LiPF 6 EC + DEC + DMC + EP (l:l:l:l) > i.0M LiPF_ EC + DEC + DMC + EB ( 1:1: I:l). _l.I , i./1_ILlPqqlEC*(I_:*4N¢*4IP0:I:I:I) • t._qSt_Ul_; E¢,,_i¢-,O_.¢.411(1:_:1:11 l_.e * ii._IiidLIIIR_ |C,Ol_ ,llA 0:i:i:i) } •20 "C t_ _m'_ et¢*4_t J IlLS U 4, ....... "- &il &tl Ltm LaIN _ &_141 Le t a.eMm _ m,m_r_ CoN_t lake) 0.0 S.0 10.0 11.0 20.0 N.O z'(o_m) Fig. 7. Discharge curves of graphite electrode in Fig.I0.AC impedance (Nyquist)plotsof contact with EB-co_taining electrolyte at lithium-gc_phiucseilspossessingester- dtffaeat temperatures (25aed-20"C). _g electrolytes at low _l_mt_ (-:_C). •• Od.,_}1Id1tJ._lJIFilif-_ [_'C..*d__iPIC.*.*O__C.d*[_Pll((ll::ll::ll::ll)} J:7 1 l_,'_ [ ,cd,._. _ I °$I ",. ,:;.._,_, r...__ ., .,_ -,. Db_,q,e capadlyt, Ju,) .:: : : r<o,-) Fig.& Charge curveof a graphiteelectrodein Fig. 11. AC impedance (Nyquist) plots of contactwithan EB-containingelectrolytaetlow lithium-graphite cells possessing EP- and EB- temperature (-20°C). containing electrolytes before and after cycling. / I.I IZ.OI _ * ¢I._$ML_r-_$_r'¢*0¢'C*_ *EA(I:t :1:1) I,B • 0._ IdLJlFtl LI(_*0_'_* _ *1_, (1:1:1:1) "f_m1_ttum .ZSC _" 1,00 U ! ._ | AR_r Cyc_in9 , • • J • • .& A 6 ILl_L2• _s s • ama tt $_ Z"(Ohms) z'(Ohms) Fig. 12. AC impedance (Nyquist) plots of lithium-graphite cells p_sessing MA- and EA- Fig,9. AC impedance (Nyquist) plots of containing electrolytes before and after cycling. lithium-graphite cells in ctmlact with various electrolyteast_erformation cycles.