Revealing Li-ion diffusion kinetic limitations in micron-sized Li-rich layered oxides

Micron-sized cathode materials can usually alleviate the surface/interface side reaction to enhance cycling stability and endure high pressure to achieve a higher compacted density. However, the poor Li+ transport kinetic properties in micron-sized Li-rich layered oxides impede their practical applications. Here, we find a faster Li+ diffusion coefficient with the order of magnitudes of about 10−12 cm2 s−1 in micron-sized grains (compared with nano-sized grains about 10−14 cm2 s−1). By contrast, much lower initial capacity can be achieved in micron-sized grains. Simulated by finite element analysis, the increased diffusion distance would yet result in an extremely uneven Li+ concentration distribution during the charge-discharge process between surface and bulk. The undesired Li+ gradient distribution results in an accumulation of strain stress inside the grains during cycling, which also consequently contributes to an unsatisfied capacity retention of 78.4% as well as a rapid voltage decay of 738 mV under 0.2 C after 200 cycles in micron-sized grains. This work provides a research direction to construct a novel structure with Li+ homogeneous distribution in micron-sized Li-rich layered oxides.