Simultaneous structure and surface engineering of metal-organic framework derived LiNi_0.5-xFe_2xMn_1.5-xO₄ cathode material for improving high voltage performance of lithium-ion batteries

This study presents a novel material synthesis approach utilizing a terephthalic acid–based metal–organic framework as a precursor and using an ethanol-assisted hydrothermal treatment to produce a high-performance Fe-doped LiNi_0.5Mn_1.5O₄ (LNMO) cathode material. This strategy yields a well-crystallized spinel structure with uniformly distributed transition-metal ions. Additionally, the appropriate quantity of Fe dopant can achieve the desired Mn³⁺/Mn⁴⁺ ratio, level of structural disordering, and crystal-phase purity for Fe-doped LNMO. The synthesis process also aids in forming an amorphous Li₂CO₃ surface layer, approximately 1-nm thick, which protects the active material from excessive reaction with electrolyte. The typical Fe-doped LNMO cathode (S-05) exhibits superior performance compared to a commercialized LNMO counterpart. It delivers an initial discharge capacity of 135/mAhg at 1C with exceptional cycling stability over 500 cycles (capacity retention of 90 %) under high voltage (5.0 V vs. Li⁺/Li). Furthermore, its rate capability significantly improves, with a capacity retention of 85% (5C/0.2C). This enhanced performance aligns with evidence of activated Ni²⁺/Ni³⁺/Ni⁴⁺ redox reactions and suppressed cathode electrolyte interphase resistance, leading to promoted Li⁺ diffusion. Moreover, it is found that the cathode helps alleviate electrolyte degradation at high voltages by inhibiting the formation of singlet oxygen in the electrolyte.