Surface Modifications of LiNi_0.96Co_0.02Mn_0.02O₂ with Tungsten Oxide and Phosphotungstic Acid

Abstract

With the rapid development of electric vehicles, enormous demands are made for higher energy density, better cycling performance and lower cost of lithium-ion batteries (LIBs). As an important high capacity cathode material for LIBs, the high nickel layered oxide material LiNi_0.8Co_0.1Mn_0.1O₂(NCM811) can reach an energy density of 760 Wh·kg^-1. The ultra-high nickel ternary positive electrode material (LiNi_1-x-yCo_xMn_yO₂, x ≥ 0.90) has a specific capacity of more than 210 mAh·g^-1, and can realize higher energy density. Besides, an ultra-high nickel material uses lower cobalt content, and reduces material cost. Tungsten oxide coating has been reported to effectively improve the electrochemical performance of ternary materials, but no reports can be found for tungsten oxide coating modified ultra-high nickel cathode materials. On the other hand, phosphate coating has been widely used in surface coating modification of high nickel cathode materials to improve their electrochemical performance, but it is difficult to achieve uniform coating. Phosphotungstic acid (PTA) can function as a double coating with tungsten oxide (WO₃) and phosphate at the same time, which is expected to achieve better electrochemical performance than single coating. In this work, LiNi_0.96Co_0.02Mn_0.02O₂ (NCM96) was selected. The NCM96 precursor and PTA/WO₃ were dispersed in ethanol for mixing. After drying, the product was mixed with lithium source and sintered, so as to achieve tungsten oxide and phosphotungstic acid coating. The structures, morphologies and electrochemical performances of the PTA modified and WO₃ modified NCM96 materials are compared. The results showed that, in the process of either PTA or WO₃ coating modification, W and P elements were not doped into the lattice of NCM96 material, forming a relatively uniform coating structure, in which the WO₃ coating modification led to single element coating structure, while the PTA coating modification led to P/W double elements coating structure. Electrochemical test and analysis revealed that the two types of the surface modification methods had no effects on the first cycle discharge capacity of the NCM96 material, while had effectively improved the long-term cycling performances. By comparing the high temperature electrochemical performance of the WO₃ and PTA coated samples, the PTA coated sample NCM96@1wt%PTA material exhibited superior cycling stability at 60 °C indicating that the P/W double elemental surface modification with PTA is superior to the W single elemental modification with WO₃.

Keywords

Lithium ion battery / LiNi_0.96Co_0.02Mn_0.02O₂ positive electrode material / Surface modification / Tungsten oxide coating / Phosphotungstic acid coating

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