Surface iron concentration gradient: A strategy to suppress Mn3+ Jahn-Teller effect in lithium manganese iron phosphate

Abstract

To address the low energy density of LiFePO₄ (LFP) for electric vehicles and high-voltage energy storage, LiMn_0.5Fe_0.5PO₄ (LMFP) provides a potential solution but faces performance degradation due to Mn³⁺-induced Jahn-Teller distortion and Mn ion dissolution during cycling. This study proposes a surface engineering strategy to enhance LMFP’s electrochemical performance by increasing surface iron concentration and reducing manganese content, based on the electronic differences between Mn³⁺ and Fe³⁺ in MO₆ octahedra. Density Functional Theory (DFT) calculations confirmed the viability of this approach by analyzing volume changes and binding energies with HF during charging. Guided by DFT, an LMFP@LFP/C material was synthesized with a high-iron-concentration surface layer (∼2 nm), as observed through AC-STEM. Post-cycling TEM analysis and corrosion simulations demonstrated that LMFP@LFP/C suppresses Mn ion dissolution and stabilizes the crystal lattice compared to unmodified LMFP/C. Electrochemical tests showed that LMFP@LFP/C has superior electronic conductivity and faster lithium-ion diffusion. It delivered an initial discharge capacity of 150.82 mAh g⁻¹ at 0.1C, surpassing LMFP/C (147.65 mAh g⁻¹). At 1C, LMFP@LFP/C retained 95.85 % of its capacity after 500 cycles, significantly outperforming LMFP/C (74.18 %). This surface modification strategy advances phosphate-based cathode materials for electric vehicles and renewable energy applications.