Effect of Lithium Bis(oxalato)borate Additive on Electrochemical Performance of Li1.17Ni0.17Mn0.5Co0.17O2 Cathode for Lithium-Ion Batteries

Abstract

Lithium ion batteries have been used for many devices such as cellular phone, digital camera, laptop computer, and tablet PC. Recently, the demands for high energy density and capacity has increased but commercialized lithium ion batteries have difficulties. The commercialized lithium cobalt oxide material has practical capacity is approximately 145 mAh g-1. It is relative low capacity compared to graphite anode. So cathode electrode need high loading density, but the process for high loading is not easy. Lithium nickel cobalt manganese oxide was investigated to get a high capacity and reduce the use of cobalt component. Reversible specific capacity of Lithium nickel cobalt manganese oxide can increase in 200 mAh g-1 but lithium nickel cobalt manganese oxide have to be charged up to high voltage with concomitant electrolyte decomposition. Even though difficulty of voltage fading and capacity decay exist, over-lithiated layered oxide cathode materials (OLO, Lithium-rich cathode) are investigated as prospective candidates. Over-lithiated layered oxide cathode materials can deliver a discharge capacity above 200 mAh g-1 under the conditions of charging above 4.5 V vs. Li/Li+. but oxidative decomposition of organic electrolytes happening above 4.5 V vs. Li/Li+ is inevitable. Many efforts have been researched to improve limitation of over-lithiated layered oxide cathode. Surface modification with Al2O3 have been studied for many cathode active materials to scavenge a trace of acidic HF species in electrolyte. However, it was confirmed that Al2O3 coating layer was covert to AlF3 and unstable. Because of the unstable Al2O3 coating layer on the cathode, AlF3 coating was attempted to stabilize surface of cathode. AlF3 coating layer act as inactive site and reduce unstable surface on the cathode. Inactive AlF3 coating layer inhibit electrolyte decomposition on the interface of cathode and electrolyte. Surface modification with reduced graphene oxide and chemical activation with hydrazine was proposed to improve electrochemical performance of over-lithiated layered oxide cathode. This method reduced excessive initial charge capacity for activation of the Li2MnO3 component and initial irreversible capacity. Also, stabilizing the Li2MnO3 surface suppresses transformation from layered to spinel structure. Tris(pentafluorophenyl)borane (TPFPB) was used as additive for over-lithiated layered oxide cathode. Because TPFPB additive dissolve insulating salts LiF/Li2O formed by electrolyte decomposition, thin SEI layer on the cathode is maintained during the cycle. Also, lithium bis(oxalato)borate (LiBOB) additive was reported to effectively stabilize surface layer on 5 V LiNi0.5Mn1.5O4 spinel cathode. Therefore, in this study, LiBOB was used as additive for over-lithiated layered oxide cathode material.