The current state of the art electrolytes for LiBs are based on alkyl carbonate mixtures containing ethylene carbonate (EC). However, this base electrolyte limits the temperature window as well as the potential window of operation of LiBs. Designing new EC-free solvents is not always straightforward since most solvents that have low freezing point or high oxidation stability do not provide a good passivation at the graphite electrode without the presence of EC. Over the past few years, we have designed new electrolyte systems that can operate over a wide temperature window or over a wide potential window. This talk will present two main routes for the design of new EC-free electrolyte systems. This will hopefully help the scientific community in designing new electrolyte systems that will meet the needs of advanced LiBs.
Discussion
Designing new electrolytes compatible with graphite-based negative electrodes can be achieved using two main routes. The first route consists of using small amounts of passivating additives such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC) in combination with solvents having good properties such as low melting point or high oxidation potential. The amount of these passivating additives must be small since they often impart high impedance or high gas evolution during high voltage and/or high temperature cycling. Using this method, three new electrolyte systems were developed. These consist of Ester:VC blends [1], Sulfolane:linear carbonate:VC blends [2], as well as FEC:ditrifluoroethylcarbonate (TFEC) blends.
Figure 1a shows that electrolyte blends consisting of 1M LiPF6 methyl propanoate:VC have similar capacity retention to 1M LiPF6 EC:EMC (3:7) + 2% VC electrolyte in a full cell configuration (220 mAh Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cell) when cycled at 40°C between 2.8 V and 4.2 V. This is surprising since electrolytes containing esters undergo heavy reduction at the graphite surface without the formation of a passivating layer [1]. Figure 1b also shows that this ester-based electrolyte provides superior low-temperature rate performance to EC-based electrolytes. This shows that using small amounts of passivating additives can allow the use of atypical solvents such as esters, without the use of EC.
Similarly, Figure 2 shows that sulfolane-based electrolytes and fluorinated solvent-based electrolytes provide superior capacity retention to 220 mAh Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells operated to high voltage (4.5 V) and relatively high temperature (40°C). Once again, the use of VC or FEC allows the use of solvents with high oxidation potential such as sulfolane or fluorinated carbonates.
The second route for the design of EC-free electrolytes is the use of the peculiar properties of highly concentrated (3 – 5M) electrolytes. Yamada et al. [3] and Jeong et al. [6] showed that high salt concentration allows for a large array of solvents to be stable against lithiated graphite. Following the same method an ester-based and additive-free electrolyte, kinetically stable against lithiated graphite and high potential, (4.78 V) was developed [7,8]. Figure 3a shows that electrolytes composed of EA:LiFSi:LiPF6 (1:0.5:0.05, molar ratio) provide better capacity retention in Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells than EC-based electrolyte containing VC when cycled up to 4.4 V. Figure 3b also shows that these highly concentrated electrolytes provide reasonable capacity retention in Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells cycled up to 4.7 V and 40°C.
Conclusion
Following two simple design principles, a large array of new electrolyte systems can be developed. These two principles consist of the use of small amounts of passivating additives such as FEC or VC, or the peculiar effect of high salt concentration. These two simple principles allow the use of atypical solvents that are normally not used due to their instability against lithiated graphite. The design of new EC-free electrolyte systems can play a crucial role in developing advanced Li-ion batteries.
Acknowledgments
The authors would like to thank 3M Canada and NSERC for the funding of this work. The authors thank Dr. Jing Li of BASF for providing some of the solvents, salts and additives used in his work. The authors also thank Xiodong Cao of HSC Corporation for providing LiFSI. Remi Petibon thanks NSERC and the Walter C. Sumner Foundation for Scholarship support.
References
[1] R. Petibon et al., Electrochimica Acta 154 (2015) 227–234.
[2] J. Xia et al., J. Electrochem. Soc. 162 (2015) A1424–A1431.
[3] Y. Yamada et al., J. Am. Chem. Soc. 136 (2014) 5039–5046.
[6] S.-K. Jeong et al., Electrochem. Solid-State Lett. 6 (2003) A13–A15.
[7] R. Petibon et al., Electrochimica Acta 154 (2015) 287–293.
[8] R. Petibon et al., J.R. Dahn, Electrochimica Acta 174 (2015) 417.