Tuning Li-Ion Diffusion in α-LiMnFePO Nanocrystals by Antisite Defects and Embedded β-Phase for Advanced Li-Ion Batteries.

Olivine-structured LiMnFePO has become a promising candidate for cathode materials owing to its higher working voltage of 4.1 V and thus larger energy density than that of LiFePO, which has been used for electric vehicles batteries with the advantage of high safety but disadvantage of low energy density due to its lower working voltage of 3.4 V. One drawback of LiMnFePO electrode is its relatively low electronic and Li-ionic conductivity with Li-ion one-dimensional diffusion. Herein, olivine-structured α-LiMnFePO nanocrystals were synthesized with optimized Li-ion diffusion channels in LiMnFePO nanocrystals by inducing high concentrations of Fe-Li antisite defects, which showed impressive capacity improvements of approaching 162, 127, 73, and 55 mAh g at 0.1, 10, 50, and 100 C, respectively, and a long-term cycling stability of maintaining about 74% capacity after 1000 cycles at 10 C. By using high-resolution transmission electron microscopy imaging and joint refinement of hard X-ray and neutron powder diffraction patterns, we revealed that the extraordinary high-rate performance could be achieved by suppressing the formation of electrochemically inactive phase (β-LiMnFePO, which is first reported in this work) embedded in α-LiMnFePO. Because of the coherent orientation relationship between β- and α-phases, the β-phase embedded would impede the Li diffusion along the [100] and/or [001] directions that was activated by the high density of Fe-Li antisite (4.24%) in α-phase. Thus, by optimizing concentrations of Fe-Li antisite defects and suppressing β-phase-embedded olivine structure, Li-ion diffusion properties in LiMnFePO nanocrystals can be tuned by generating new Li tunneling. These findings may provide insights into the design and generation of other advanced electrode materials with improved rate performance.