Mitigating Performance Degradation of High-Capacity Lithium-Ion Cells with Boronate-Based Electrolyte Additives

Boronate-based electrolyte additives are tested in high-capacity lithium-ion cells containing graphite-based negative and Li1.2Ni0.15Mn0.55Co0.1O2-based positive electrodes. Cells containing small amounts (0.25 wt%) of phenyl boronic acid ethylene glycol ester (PBE) in an EC:EMC (3:7 by wt) 1.2M LiPF6 (Gen2) electrolyte show improved capacity retention and reduced impedance rise compared to cells without the additive. Adding a perfluorooctyl chain to PBE creates the phenyl boronic acid perfluorooctyl ethylene glycol ester (PFO-PBE) compound. Cells with 0.25 wt% PFO-PBE additive display the lowest capacity fade (21%) compared to the PBE (34%) and Gen2 (65%) cells after more than 200, 2.2-4.6 V cycles, at 30 degrees C. These data validate our modular-electrolyte additive concept (PBE head and perfluoroalkyl tail) for high-capacity lithium-ion cells. Impedance rise is reduced further by addition of 2 wt% LiDFOB (LiF2BC2O4) to cells with 0.25 wt% PFO-PBE, thus demonstrating that additive combinations can enhance performance during extended cycling. (C) 2014 The Electrochemical Society. All rights reserved.