Tuning Li-Ion Diffusion in α-LiMn_1-xFe_xPO₄ Nanocrystals by Antisite Defects and Embedded β-Phase for Advanced Li-Ion...

Olivine-structured LiMn_1-xFe_xPO₄ 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 LiFePO₄, 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 LiMn_1-xFe_xPO₄ electrode is its relatively low electronic and Li-ionic conductivity with Li-ion one-dimensional diffusion. Herein, olivine-structured α-LiMn_0.5Fe_0.5PO₄ nanocrystals were synthesized with optimized Li-ion diffusion channels in LiMn_1-xFe_xPO₄ nanocrystals by inducing high concentrations of Fe²⁺-Li⁺ antisite defects, which showed impressive capacity improvements of approaching 162, 127, 73, and 55 mAh g^-1 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-LiMn_1-xFe_xPO₄, which is first reported in this work) embedded in α-LiMn_0.5Fe_0.5PO₄. 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 Fe²⁺-Li⁺ antisite (4.24%) in α-phase. Thus, by optimizing concentrations of Fe²⁺-Li⁺ antisite defects and suppressing β-phase-embedded olivine structure, Li-ion diffusion properties in LiMn_1-xFe_xPO₄ 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.