Insights into the Dual Role of Lithium Difluoro(oxalato)borate Additive in Improving the Electrochemical Performance of NMC811||Graphite Cells

Ni-rich layered oxides (LiNi x Mn y Co z O2, x ≥ 0.6, x + y + z = 1) are promising positive electrode materials for high energy density lithium-ion batteries thanks to their high specific capacity. However, large-scale application of Ni-rich layered oxides is hindered by its poor structural and interfacial stability, especially during cycling at a high cutoff potential (i.e., ≥ 4.3 V, versus Li+/Li). Herein, we demonstrate that lithium difluoro­(oxalato)­borate (LiDFOB) as a film-forming additive plays a dual role on the electrode|electrolyte interphase formation in a LiNi0.83Mn0.05Co0.12O2||graphite cell, meaning that it can not only be reduced on the graphite negative electrode but also oxidized on the nickel-rich oxide LiNi0.83Mn0.05Co0.12O2 positive electrode cycled at a high cutoff potential (4.4 V, versus Li+/Li) prior to typical carbonate-based electrolyte constituents. As a result, the addition of 1.5 wt % LiDFOB greatly reduces the polarization and improves the cycling stability of the LiNi0.83Mn0.05Co0.12O2||graphite cell, which shows a high discharge capacity of 198 mA h g–1, and more than 83.1% of the initial capacity was retained after 200 cycles at C/3 (the capacity retention obtained at the same cycling condition is only 59.9% for the cell without LiDFOB additive). Furthermore, the employ of LiDFOB additive also significantly suppresses the self-discharge of the LiNi0.83Mn0.05Co0.12O2||Li cell during high-temperature and long-term room-temperature storage at 4.4 V. These electrochemical performance enhancements could be attributed to the participation of LiDFOB in forming a stable and Li+ transfer favorable protective layer that is rich in inorganic boron, fluorine, and carbonate compounds on both the surface of the LiNi0.83Mn0.05Co0.12O2 positive electrode and the graphite negative electrode, thus suppressing the electrolyte decomposition on the positive electrode and negative electrode surfaces and decreasing the dissolution of transition-metal ions from the positive electrode bulk.