Encouraging Voltage Stability upon Long Cycling of Li-Rich Mn-Based Cathode Materials by Ta–Mo Dual Doping

The Li-rich and Mn-based material xLi₂MnO₃·(1-x)LiMO₂ (M = Ni, Co, and Mn) is regarded as one of the new generations of cathode materials for Li-ion batteries due to its high energy density, low cost, and less toxicity. However, there still exist some drawbacks such as its high initial irreversible capacity, capacity/voltage fading, poor rate capability, and so forth, which seriously limit its large-scale commercial applications. In this paper, the Ta-Mo codoped Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ with high energy density is prepared via a coprecipitation method, followed by a solid-state reaction. The synthetic mechanism and technology, the effect of charge-discharge methods, the bulk doping and the surface structure design on the structure, morphology, and electrochemical performances of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode are systematically investigated. The results show that Ta⁵⁺ and Mo⁶⁺ mainly occupy the Li site and transition-metal site, respectively. Both the appropriate Ta and Ta-Mo doping are conductive to increase the Mn³⁺ concentration and suppress the generation of Li/Ni mixing and the oxygen defects. The Ta-Mo codoped cathode sample can deliver 243.2 mA h·g^-1 at 1 C under 2.0-4.8 V, retaining 80% capacity retention after 240 cycles, and decay 1.584 mV per cycle in 250 cycles. The capacity retention can be still maintained to 80% after 410 cycles over 2.0-4.4 V, and the average voltage fading rate is 0.714 mV per cycle in 500 cycles. Compared with the pristine, the capacity and voltage fading of Ta-Mo codoped materials are effectively suppressed, which are mainly ascribed to the fact that the highly valence Ta⁵⁺ and Mo⁶⁺ that entered into the crystal lattice are favorable for maintaining the charge balance, and the strong bond energies of Ta-O and Mo-O can help to maintain the crystal structure and relieve the corrosion from the electrolyte during the charging/discharging process.