Tuning glycolide as an SEI-forming additive for thermally robust Li-ion batteries

A series of glycolide derivatives, glycolide (GL00), 3-methyl glycolide (GL01), 3,6-dimethyl glycolide (GL11) and 3,3,6,6-tetramethyl glycolide (GL22), are examined as SEI-forming electrolyte additives for LiCoO₂/graphite lithium-ion batteries (LIBs). The novel additives are also compared with vinylene carbonate (VC), the currently most popular SEI-forming additive in LIBs. Through both density functional calculations and electrochemical experiments, it is confirmed that the number of methyl groups systematically affects anodic stability and the SEI-forming reaction of the glycolide additives. The cyclability of the LiCoO₂/graphite cell is improved in the following order: VC ≥ GL00 ≥ GL01 > GL11 > GL22. With elevated temperature (90 °C) storage, however, the Al-pouch cells with VC and GL00 suffer from severe swelling, whereas the cells with GL01 and GL11 show reasonable dimensional stabilities. Both the good cyclability and the excellent thermal stability of GL01 make it the most appropriate SEI additive for the present LiCoO₂/graphite chemistry.