Aluminum-doping induced micro-porous structure and improved anion redox reversibility in cobalt-free lithium-rich cathode materials for its enhanced electrochemical performance

Cobalt-free lithium-rich layered manganese-based oxides (LLMOs) are expected to become the next generation cathode materials for lithium-ion batteries due to its high capacity and low cost [1]. In this study, a series of cobalt-free high-nickel lithium-rich layered cathode materials Li1.5-3xAlxMn0.55Ni0.45O2.5 (LLMO-Al x, x = 0, 0.025, 0.075 and 0.1) with excellent electrochemical performance were synthesized through a simple calcination process of LLMO precursor with nano-alumina particles and non-stoichiometric amounts of lithium salts. The cross-sectional SEM measurement of as-prepared samples shows that a unique micro-porous structure is formed in Al-dopped LLMOs and maintained even after 200 cycles of charge-discharge process at 1C rate, whereas the original LLMO with compact structure breaks into particles at the same cyclability test condition. The stability and reversibility of the lattice oxygen and oxygen vacancies in Al-dopped LLMOs which attributed to the stronger Al-O bonding are indicated by the ex-situ XPS spectrum analysis of O 1s spectrum in the different states of charge/discharge test, and first principles calculations show that Al doping significantly affects the local transition metals coordination circumstance. As a result of enhanced mechanical and electrochemical stability, the optimized LLMO-Al 0.075 cathode material has a discharge capacity of 353.8 mAh g−1 at 0.1C, a capacity retention of more than 92.9% after 200 cycles at 1C, and improved cycling and voltage stability. This work provides a promising inspiration for the rational design of porous structure in LLMOs to enhance the mechanical stability and electrochemical performance of cobalt-free lithium-rich layered cathode materials.