Tungsten-doping Enables Excellent Kinetics and High Stability of Cobalt-free Ultrahigh-nickel Single-crystal Cathode

Abstract

Cobalt-free ultra-high nickel (LiNi_xMn_1-xO₂, NM, x≥0.9) single crystal cathode material possesses great potential application due to its low cost and high structure stability, but it demonstrates poor rate performance and low capacity, suppressing its practical application progress. Doping high-valent ions (such as tungsten, W) is suggested to be a promising solution to address the above problems, however, the doping intrinsic role of which is still unclear since non-doping effects coexist. In this work, only W bulk-doping in single crystal NM cathode is achieved by high-temperature two-step sintering method to explore the W-doping effects, which can enhance Li⁺ diffusion and electronic conductivity regardless of the Co deficiency and long Li⁺ diffusion channel, thereby increasing the available specific capacity and rate capability of the cathode material. It shows that the initial Coulombic efficiency increases by about 4%, corresponding to a discharge specific capacity increase of more than 10 mAh g⁻¹ after doping W. Besides, the specific capacity of W-doped cathode can reach 133 mAh g⁻¹ at a high current of 5 C, which is much higher than 107 mAh g⁻¹ of the pristine cathode. Moreover, the introduction of strong W-O bonds can bind lattice oxygen, inhibiting oxygen release and harmful phase transitions, improving structural and thermal stability as a result. This work provides an effective strategy for developing cobalt-free cathode materials and a new perspective for understanding the electrochemical performance enhancement by doping high-valence ions.