Microstructure analysis of Si/graphite composite anode during charge-discharge cycle for lithium-ion battery with tetraglyme- and sulfolane-based less volatile electrolyte

The microstructure of a composite anode of Si-alloy and graphite during charge-discharge tests was analyzed for a lithium-ion battery (LIB) with a less volatile electrolyte in order to clarify its degradation mechanism. An equimolar complex composed of tetraethylene glycol dimethyl ether and lithium bis(trifluoromethanesulfonyl)amide (Li(G4)TFSI) and a highly concentrated solution of sulfolane and LiTFSI (Li(SL)3TFSI) were used as the main Li conductors of the less volatile electrolyte. Also, low-viscosity solvents (propylene carbonate (PC), butylene carbonate (BC), and ethylene carbonate (EC)) were added to enhance the conductivity and increase the rate performance. The Li(SL)3TFSI-based electrolyte showed a sufficient rate performance with less amount of low-viscosity solvent compared to the Li(G4)TFSI-based electrolyte thanks to the high Li transportation number of Li(SL)3TFSI, which is a favorable feature for highly durable and less volatile electrolytes. The Li(G4)TFSI-45 wt.% PC and Li(SL)3TFSI-20 wt.% BC, which had respective volatilization temperatures of 378 K and 413 K, were incorporated into laminated cells consisting of a Si alloy/graphite anode and LiNixMnyCozO2 cathode. The cycle tests revealed that the discharge capacity of the cell with Li(SL)3TFSI-20 wt.% BC dropped rapidly during charge-discharge. The results of thickness measurement and scanning electron microscopy of the anodes after the cycle test indicated that the agglomerate structure of the Si-alloy, graphite, and poly(amide-imide) (PAI) binder was deteriorated by the volume change during the cycle test. Also, a solubility test of the PAI film and analysis of the Hansen solubility parameters of the film and solvent in the electrolyte suggested that the binder was likely to dissolve or swell in the Li(SL)3TFSI-20 wt.% BC solution, which would weaken the mechanical strength of the anode microstructure. Hence, to suppress the microstructure deterioration, we modified the chemical composition of the electrolyte to control the solubility of the binder. The replacement of BC with EC was effective to suppress the microstructural deterioration and then enhance the capacity retention during the cycle test.