Novel Electrolytes for High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode Material
Authors: Xu Mengqing , Xing Lidan , Li Weishan 
Published: February 2020

Lithium ion batteries are widely used for portable electronics and are currently being incorporated into electric vehicles due to the high gravimetric and volumetric energy density. However, cost, safety, cycle and calendar life, energy, and power density are some of the major obstacles to the widespread use of lithium-ion batteries for vehicle applications. Among these problems, energy and power density are some of the most significant. To achieve higher energy and power density, many efforts have been addressed to improve the specific capacity and discharge voltage of the cathode materials. For example, cathode materials which could transport two Li ions such as Li2FeSiO4 and Li2FePO4F have been proposed and investigated as a high capacity cathode material, in which two electrons-exchange are involved upon charge-discharge process thus offering higher capacity, 323 and 292 mAh g-1, respectively. Another effective way to improve the energy density of lithium ion batteries is to expand the operating potential of the cathode material. Most commercial lithium ion batteries contain layered lithium transition metal oxide such as LiCoO2 or LiNi1/3Mn1/3Co1/3O2 which typically operates at approximately 4.1 V vs. Li/Li+. Cathode materials with an operating potential over 4.5 V (vs Li/Li+) have been developed, including LiMnPO4, LiNiPO4 and LiCoPO4, and LiNi0.5Mn1.5O4. Among these promising new cathodes, LiNi0.5Mn1.5O4 has attracted much attention in recent years because of the high intercalation/deintercalaition potential of 4.8 (vs. Li/Li+) and excellent rate performance. However, a major difficulty in using these high-voltage materials is the instability of the standard electrolyte, LiPF6, in organic carbonate solvents, in contact with the cathode surface at operating potentials over 4.5 V.
Various methods have been proposed to inhibit the detrimental reactions on high voltage cathode materials. One of them is adopting inert surface coatings, such as Al2O3, ZnO, and Bi2O3 to prevent the oxidation of the electrolyte. Surface coated cathodes have cyclability superior to that of uncoated material; however, the surface coating method has a negative effect on the discharge capacity of the material and may be difficult to scale for commercial applications. Since the oxidative stability of current LiPF6/carbonate electrolyte is considered to be limited by the oxidative stability of the organic solvents, there has been significant interest in development of novel organic solvents with high anodic stability. Sulfone, lactone, organic nitriles, and fluorinated carbonates were reported to be stable over 5 V (vs. Li/Li+), but most of them have high viscosity or do not favor the formation of a protective solid electrolyte interface (SEI) on graphite anode. 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.
Some additives have been investigated which improve the performance of Li/LiNi0.5Mn1.5O4 cells, including dimethyl methyl phosphonate and Lithium bis(oxalato)borate (LiBOB). Most recently, the extension research work of LiBOB on Graphite/LiNi0.5Mn1.5O4 full cells was also reported. The incorporation of LiBOB significantly improves the cycling performance of graphite/LiNi0.5Mn1.5O4 cells when cycled at elevated temperature (55 oC) and high voltage (4.8 V vs Li/Li+). However, huge gas generation was observed upon cycling at high voltage and elevated temperature, which is not acceptable in practical applications. Therefore, novel electrolyte system is still highly need to drive the applications of high voltage cathode materials in lithium ion batteries.
Several types of novel additives are developing in our laboratory to meet the requirements of high voltage cathode materials upon cycling at over 4.5 V, vs. Li/Li+.
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Acknowledgment

This research work is supported by the National Natural Science
Foundation of China (21373092, 21273084), the Joint Project of National Natural Science Foundation of China and Natural Science Foundation of Guangdong (No. U1134002).