Delineating the kinetic limitations of Mn2+/3+ redox in LiMnxFe1−xPO4 cathodes for lithium-ion batteries

Abstract

LiMn_xFe_1−xPO₄ (LMFP) cathodes offer higher energy density than LiFePO₄ due to a higher operating voltage. However, the kinetic limitations of the Mn^2+/3+ redox couple in LMFP restrict its practically achievable capacity and hinder the ability to achieve high energy density. While the shortcomings of Mn^2+/3+ redox are recognized, its behavior and the factors that impact it remain to be fully understood. We present here an in-depth kinetics study of LMFP by utilizing novel techniques, such as chronoamperometry and galvanostatic electrochemical impedance spectroscopy. Compared to moderate-Mn LMFP (40–60% Mn), high-Mn LMFP (80% Mn) experiences reduced capacity, poor rate capability, and heightened impedance during Mn^2+/3+ redox. The oxidation of Mn²⁺ to Mn³⁺ becomes more resistive during charge due to the formation of Jahn–Teller active Mn³⁺ and the increasing charge-transfer insulating behavior of Mn_xFe_1−xPO₄. However, the impedance associated with Mn³⁺ to Mn²⁺ reduction remains constant during discharge in the entire region. The protracted Mn²⁺ oxidation in high-Mn LMFP results in excessive impedance compared to that in moderate-Mn LMFP, even at low C-rates and high cycling temperatures. Despite the promise of high energy density, the extreme kinetic limitations inherent to high-Mn LMFP present a barrier.