Evaluating LiBOB/Lactone Electrolytes in Large-Format Lithium-Ion Cells Based on Nickelate and Iron Phosphate

The electrolyte compositions based on lithium bis(oxalato)borate (LiBOB) and lactones were tested in large-format lithium-ion cells designed for hybrid electric vehicle applications using either LiNi0.85Co0.10Al0.05O2 or LiFePO4 cathode chemistry. It was found that the impurity in LiBOB has a significant impact on the cell cycle life, especially at elevated temperatures, where the anodic decomposition of the oxalate impurities and carboxylate remnants accelerates and results in pressure to vent the cell. Recrystallization can essentially remove those detrimental impurities and eliminates the internal gassing. As compared to its LiPF6 counterparts, purified LiBOB imparts the cells with cycling stability at high temperatures and enhanced safety under abuses of nail penetration and overcharging; however, the more protective interfaces formed by BOB also render lower power densities and poorer performances at subzero temperatures. Considering the trade-off, LiBOB electrolytes may find an application niche with high-temperature and low-to-medium power requirements. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2990708] All rights reserved.