Understanding the Behavior of LiCoO₂ Cathodes at Extended Potentials in Ionic Liquid-Alkyl Carbonate Hybrid Electrolytes

The current electrolyte compositions makes it hard to achieve a high energy density lithium-ion battery based on LiCoO₂ chemistry due to the destabilization of the LiCoO₂ crystal structure beyond 4.2 V vs Li/Li⁺ leading to oxygen evolution and electrolyte decomposition. Therefore, electrolyte developments may hold promise for improved performance, for example, if some of the advantageous properties of ionic liquids can be introduced into a carbonate electrolyte system. Here, we report the use of a hybrid electrolyte (HE) system with a LiCoO₂ cathode and have observed an excellent electrochemical performance when cycled to 4.4 V vs Li/Li⁺. This extended potential range produces higher capacity via greater ion insertion/extraction and better structural stability. A discharge capacity of 161 mAh/g (0.7 lithium extraction) was observed in the HE as compared to 128 mAh/g in conventional electrolyte, after 60 cycles. The charge-discharge studies at extended potentials also indicate better capacity retention in the HE as compared to the conventional electrolyte (LP30). Investigations to confirm the origin of such behavior establish that surface film formation is protecting or delaying the phase transition for LiCoO₂ at extended potentials. In situ XRD studies suggest that the electrolyte combination helps to delay the potential for monoclinic phase formation in LiCoO₂, and ex situ XRD studies suggest less structural degradation takes place in the HE than the conventional electrolyte at the end of 60 cycles. Therefore, we believe that the future tailoring of the HE will provide a significant step toward high energy density lithium batteries. (Graph Presented).