A Novel Electrolyte Salt Additive for High Voltage Lithium Ion Batteries
Authors: Sun Xiao-Guang , Li Yunchao , Wan Shun , Veith Gabriel M , Unocic Raymond R , Paranthaman Mariappan Parans , Dai Sheng 
Published: February 2020

Current battery technology is limited by the decomposition of electrolytes at high voltages (> 4.3 V vs. Li/Li+),[1] which prevents the use of high voltage cathodes having higher power than current cathode technology. Among various high voltage cathode materials, LiNi0.5Mn1.5O4 (LNMO) is one of the most promising candidates due to its high energy and power densities as well as being inexpensive and environmental benign.[2] The high working potential (about 4.7V vs. Li) of Ni2+/Ni3+ and Ni3+/Ni4+ redox couples delivers an energy density equivalent to ~650 W h kg-1, which is the highest among commercially available cathode materials.[3] Furthermore, the three-dimensional channels in the spinel lattice enhance lithium diffusion rates during intercalation-deintercalation process. While the high operation voltage of LNMO increases power and energy density, it also causes extensive oxidation of the conventional carbonate electrolytes, resulting in large irreversible capacity loss, low coulombic efficiency, and considerable thickening of the solid-electrolyte interphase (SEI) layer. [4] So far, different additives such as phosphite derivatives,[5] lithium aryl trimethyl borates,[6] lithium difluoro(oxalate)-borate[7] and lithium bis(oxalate)borate (LiBOB) [8] have been used to improve the capacity retention and cycling stability of the LNMO based batteries. [9] Among those additives, LiBOB has been intensively studied because of its distinct thermal stability and beneficial effect of SEI formation.
[8, 10] Considering the benefits of LiBOB, we have synthesized a close analog lithium borate salt, lithium bis(2-fluoromalonato)borate (LiBFMB), with higher oxidation stability than LiBOB and solubility in carbonate mixtures.[11] However, LiBFMB has poor stability against both reduction and oxidation because the C-2 hydrogen adjacent to both fluorine and carbonyl groups is acidic and is believed to deterimentally react. To mitigate this issue, we have synthesized a new lithium salt by replacing the acidic hydrogen with a methyl group, forming lithium bis(2-methyl-2-fluoromalonato) borate (LiBMFMB). [12] LiBMFMB based electrolyte showed good cycling performance in both LNMO and graphite based half cells, although its ionic conductivity was still lower than that of LiPF6. [12] In this talk, we report the use of LiBMFMB as an additive in conventional carbonate electrolyte for LNMO based lithium ion batteries, taking advantage of its good SEI formation ability while mitigating its conductivity issue. The LNMO based half-cells with LiBMFMB as an additive exhibited significantly improved cycling performance under a high current rate of 1C, due to a decrease in the decomposition of the LiPF6salt and electrolyte solvents and reduction of the SEI layer thickness.

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Figure 1