Dual-functional boron-modification on a cobalt-free single-crystal layered cathode for high-voltage lithium-ion batteries

Abstract

A Ni-rich cobalt-free layered cathode material (LiNi_xMn_1−xO₂) is promising due to its low cost, excellent structural stability, and thermal stability. However, low Li-ion diffusion kinetics, highly reactive Ni⁴⁺, and stress-induced intragranular micro-cracking restrict its further application. Here, we report dual-functional boron-modification (doping and coating) on a cobalt-free single-crystal layered cathode (B-LiNi_0.75Mn_0.25O₂) via a simple solid-state method. The boron atoms prefer to occupy the tetrahedral interstices in the Li layer during the sintering process, which enlarges the c-axis for fast Li-ion diffusion kinetics and can also serve as a pillar to achieve an ultra-low (1.62%) c-axis lattice contraction at 80% state of charge (SOC). And boron doping passivates the lattice O, inhibits the irreversible phase transition and provides excellent thermal stability at a high cut-off voltage of 4.5 V. On the other hand, the byproduct LiBO₂ can improve Li-ion diffusion and alleviate side reactions at the electrode/electrolyte interface by serving as a Li-ion conductive coating layer. As a result, B–LiNi_0.75Mn_0.25O₂ is almost free of inter- and intragranular micro-cracking with faster Li-ion diffusion kinetics. It delivers 176.6 mA h g⁻¹ at 1C and achieves 82.9% capacity retention after 200 cycles. This work not only provides a low-cost and scalable method to remove Co from conventional Ni-rich layered cathodes, but also reveals the commercial feasibility of highly safe single-crystal layered cathode materials.