Lithium tetramethyl borate (LTMB, LiB(OCH 3 ) 4 ) has been prepared and investigated as a novel cathode film forming additive to improve the performance of LiNi 0.5 Mn 1.5 O 4 cathodes cycled to high potential (4. 25-4.8 V). Addition of LTMB to 1.2 M LiPF 6 in EC/EMC (3/7, v/v) improves the capacity retention of graphite/LiNi 0.5 Mn 1.5 O 4 cells cycled at 55 • C. The added LTMB is sacrificially oxidized on the surface of the cathode during the first charging cycle. Ex-situ surface analysis of the LiNi 0.5 Mn 1.5 O 4 by X-ray photoelectron spectroscopy (XPS) reveals the presence of a borate based passivating layer which appears to inhibit electrolyte oxidation on the cathode surface. Lithium ion batteries are widely used for portable electronics and are currently being incorporated into electric vehicles due to the high energy density.1,2 However, there is significant interest in increasing the energy density of lithium ion batteries. One method to achieve higher energy density is increasing the operating potential of the cathode material. Most commercial lithium ion batteries contain a lithiated transition metal oxide cathode which typically operate at ∼4.0 V vs. Li/Li + .3 Cathode materials with a potential over 4.2 V have been developed, including LiMnPO 4 , 4-6 LiNiPO 4 5,7-9 and LiCoPO 4 , 10-12 and LiNi 0.5 Mn 1.5 O 4 . [13][14][15][16] Among these promising new cathodes, LiNi 0.5 Mn 1.5 O 4 has attracted much attention in recent years because of the high intercalation/deintercalation potential of 4.8 V and excellent rate performance. However, a major difficulty in using these high-voltage materials is the instability of the standard electrolyte, LiPF 6 , in organic carbonate solvents at operating potentials over 4.5 V. 3,[17][18][19] The failure mechanisms of LiNi 0.5 Mn 1.5 O 4 cells at high voltage and elevated temperature has been recently investigated. 20,21 Electrolyte decomposition, electrode/electrolyte interface degradation, and transition metal dissolution are the leading factors reported for performance fade.Various methods have been proposed to inhibit the detrimental reactions on high voltage cathode materials. Inert surface coatings, such as Al 2 O 3 , ZnO, and Bi 2 O 3 , on the cathode materials have been reported to improve performance at high potential. 13,14,22,23 Alternatively there have been several investigations of the incorporation of cathode film forming additives which are sacrificially oxidized on the cathode surface to generate a cathode passivation layer similar in nature to the anode SEI.24-32 Among these additives, lithium bis(oxalato)borate (LiBOB) has been extensively investigated and provides multiple benefits in the battery system. However, the oxidation of LiBOB on the charged LiNi 0.5 Mn 1.5 O 4 surface at high voltages (>4.5 V, vs. Li/Li + ) is accompanied by CO 2 gas evolution, 33 which limits application. Thus it is important to develop novel cathode film forming additives to improve the performance of high voltage LiNi 0.5 Mn 1.5 O 4 cathodes.In the present report, a...

The novel electrolyte additive lithium dimethyl phosphate (LiDMP) has been synthesized and characterized. Incorporation of LiDMP (0.1% wt) into LiPF 6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (3:7 wt) results in improved rate performance and reduced impedance for graphite / LiNi 1/3 Mn 1/3 Co 1/3 O 2 cells. Ex-situ surface analysis of the electrodes suggests that incorporation of LiDMP results in a modification of the solid electrolyte interphase (SEI) on the anode. A decrease in the concentration of lithium alkyl carbonates and an increase in the concentration of lithium fluoro phosphates are observed. The change in the anode SEI structure is responsible for the increased rate performance and decreased cell impedance. Lithium ion batteries (LIB) are currently the preferred source of power for consumer electronics such as mobile phones, computers, and cameras and are of interest for large-scale high-powered battery markets including aerospace, military, and electric vehicles. The reaction of non-aqueous electrolytes on the surface of the anode during the first few charging cycles results in the generation of a solid electrolyte interphase (SEI) which is critical to the performance of LIB.1 While the structure and function of the anode SEI is still poorly understood, lithium ion intercalation through the SEI and into the anode is one of the largest limitations for high rate performance. 2-5Electrolyte additives have been used to modify the structure of the SEI and improve the performance of LIB via decreasing the irreversible capacity during formation, lowering SEI resistance, or stabilizing cells against extreme conditions such as high temperature and high rate cycling.1,6-8 Vinylene carbonate (VC) is one of the most frequently investigated additives and has been used to generate a more stable SEI on graphite, but unfortunately the films are typically more resistive.9 Improving the kinetics of lithium ion batteries has been investigated via incorporation of alternative co-solvents to improve electrolyte conductivity 10 or incorporation of electrolyte additives, such as propane sultone (PS), to reduce the impedance of the SEI.11 Organophosphorus additives such as trimethyl phospshate and dimethylmethyl phosphonate have also been investigated as novel flame retarding additives.12-14 Recently, a novel phosphorus additive, lithium difluoro phosphate (F 2 PO 2 Li), has been reported to improve the interfacial kinetics of the anode SEI. 15 In this manuscript, we report on the development of a structurally related novel organophosphorous additive, lithium dimethyl phosphate (LiDMP), which has been found to function as an anode film-forming additive, which decreases cell impedance. ExperimentalMaterials.-All of the materials for the synthesis of LiDMP were purchased from Sigma Aldrich or Acros and used without further purification. Battery-grade ethylene carbonate (EC), ethyl methyl carbonate (EMC), and lithium hexafluorophosphate (LiPF 6 ) were provided by BASF, Germany, and used as received. LiDMP was...