The developments of advanced Li-ion batteries (LIBs) not only depend on the advances in electrode chemistries but also rely on the improvement of the functional electrolyte system (salt, solvent, and additive) for high ionic conductivity and controlled solid electrolyte interphase (SEI) formation capability ¹ To increase the energy density of LIBs, high voltage cathode materials (>4.5 V vs. Li/Li⁺) such as the Li-rich compounds, olivine-type LiNiPO₄, and LiCoPO₄, as well as spinel-type LiCoMnO₄, and LiNi_0.5Mn_1.5O₄ (LNMO) ² have been intensively investigated. Particularly, LNMO has attracted more attention because of its fast lithium conduction through the three-dimensional channels and its low cost and environmentally benign. Unfortunately, it causes extensive oxidation of the electrolyte because of its high operation voltage, resulting in thickening of the passivation layer and large irreversible capacity loss. ³ To mitigate severe electrolyte oxidation and improve long cycling performance of LNMO based batteries, different additives have been evaluated. ⁴  In addition to organic molecules, lithium salts such as lithium bis(oxalato)borate (LiBOB), ⁵ and lithium difluoro(oxalato)-borate (LiDFOB) ⁶ have also been evaluated and showed improved long cycling stability. Even though LiBOB and LiDFOB have been extensively studied as additives, the resulting SEI layer and passivation layer generally suffers from high resistance, possibly due to two carbonyl groups being too close to each other. In an effort to reduce the resistance of the SEI layers by increasing the flexibility and yet at the same time enhance the electron delocalization of the anions, we have synthesized a series of fluorinated lithium bis(malonato) borate (LiBMB) salts (Scheme 1), ^7-9 the close analog of LiBOB.¹⁰ Besides structurally more flexible, the two hydrogens on the C-2 position of the BMB anion can be substituted by different functional groups, and the electrochemical properties of the salts and their SEI and passivation characteristics can be tuned and optimized. Indeed, improvement has been observed by replacing the acidic hydrogen in lithium bis(2-fluoromalonatoborate) (LiBFMB) with a methyl group in lithium bis(2-methyl-2-fluoromalonato) borate (LiBMFMB) (Scheme 1). ^{7, 11} Unfortunately, the ionic conductivities of both LiBFMB and LiBMFMB based electrolytes are lower than that of LiPF₆. To improve the ionic conductivity of these malonatoborate salts, we have further synthesized lithium difluoro-2-alkyl-2-fluoromalonatoborate salts (LiDFMFMB, LiDFEFMB, and LiDFPFMB where alkyl group is methyl, ethyl and propyl, respectively) whose ionic conductivities are close to that of LiPF₆ in the carbonate mixture of EC-ethylene carbonate (EMC) (1-2, by wt.). ⁹ Electrochemical floating test showed that these half salts are less stable than LiPF₆ above 4.9 V vs Li/Li⁺, ⁹ which inspired us to use them as additives in LNMO based batteries. Indeed, these additives exhibited high coulombic efficiency and improved long cycle stability in LNMO||NG full cells, thus providing a cost-effective way to improve the performance of high voltage LNMO batteries. In addition, the additives also exhibit better passivation of the aluminum current collector.

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