Synthesis of New Lithium Salts for Application in Lithium Ion Batteries
Authors: Sun Xiao-Guang , Wan Shun , Dai Sheng 
Published: February 2020

Seeking new lithium salt has been an indispensable part in the development of lithium ion batteries (LIBs), as it plays a critical role in the battery performance. The lithium salt is not only the primary source of ionic species, especially free conducting lithium ion, but also mediates the electrochemical window of the electrolyte and the formation of solid electrolyte interphase (SEI) at the electrodes. Stable lithium salts are even more important for developing 5.0V lithium ion batteries, which are current actively being pursued in order to further increase the power and energy density of the cells.1 So far, LiPF6 has been the most common salt in carbonate mixtures for commercial LIBs, mainly due to its optimum combination of ionic conductivity, ion dissociation, electrochemical window, and electrode interfacial properties, even though it is seldom outstanding with respect to any single parameter.2 The preparation of LIBs as power sources for large scale electric vehicles (EVs) applications has raised safety concerns of LiPF6 related to its low chemical and thermal stability. Consequently, researchers have focused on investigating new lithium salts to replace LiPF6.3-6 Once such salt is lithium bis(oxalato)borate, which shows significantly improved thermal stability over LiPF6 at an elevated temperature of 70 oC. 7 A claimed unique feature of LiBOB is the participation in SEI formation by the BOB- anion, which allows the use of PC and graphite electrode together without suffering from solvent co-intercalation and graphite exfoliation.7,8 The reduction process of LiBOB at ca. 1.7 V vs. Li/Li+ has prompted several studies on whether it is related to its unique SEI formation ability on the graphite anode.
8,9 It is now generally recognized that the reduction process is closely related to the oxalate moiety and directly affects the initial irreversible capacity; however, it is still difficult to determine whether the oxalate originated from the BOB anion or from an independent oxalate impurity in the LiBOB electrolyte.9

As compared to LiBOB, its close analog salt, lithium bis(malonato)borate (LiBMB) has rarely been studied, mainly due to its insoluble in common carbonate solvents.6 Previously we have synthesized a new C-2 modified LiBMB, i.e. lithium bis(fluoromalonato)borate (LiBFMB) 10, which was soluble in carbpnate mixture and exhibited good performance on 5.0V cathode of LiNi0.5Mn1.5O4, but its performance on graphite electrode was relatively poor. In this presentation we will report on the synthesis and characterization of three new C-2 modified LiBMB salts, which showed good performance on both LiNi0.5Mn1.5O4 and graphite electrode, as well as those 4.0 V electrodes of LiCoO2 and LiMn2O4.

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