Dual optimization of LiFePO4 cathode performance using manganese substitution and a hybrid lithiated Nafion-modified PEDOT:PSS coating layer for lithium-ion batteries

Abstract

The energy storage market necessitates an increase in the specific capacity of battery materials, particularly cathodes, to meet growing demands. Manganese (Mn) substitution of lithium iron phosphate (LFP) cathodes presents a promising avenue, offering high specific capacity and operability at elevated voltages. However, during cycling, the dissolution of Fe/Mn ions into the electrolyte leads to capacity fading. In this study, we enhance the specific capacity of LFP by Mn substitution in the iron position at various ratios (0.1, 0.2, 0.3, and 0.4). LiFe0.6Mn0.4PO4 exhibits the highest capacity of 159.1 mAh g-1 at 0.1C with an initial Coulombic efficiency of 97.9 %. This improvement stems from an enhanced lithium diffusion coefficient, increasing from 1.86 × 10–14 cm2 s-1 for pristine LFP to 2.46 × 10–12 cm2 s-1 for LiFe0.6Mn0.4PO4. However, LiFe0.6Mn0.4PO4 demonstrates the poorest capacity retention among the substituted samples, reaching 78.4 % over 100 cycles due to severe Fe/Mn ion dissolution. To address this issue, we coat Mn-substituted LFP with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and lithiated Nafion (LN) polymer, which offer high electronic and lithium-ion conductivity, respectively. This coating layer enhances the capacity retention of LiFe0.6Mn0.4PO4 to 90.1 % at 0.2C after 100 cycles, effectively mitigating active material loss during charge-discharge processes. This study demonstrates that PEDOT:PSS-LN can improve the electronic and ionic conductivity of cathode materials and maintain high capacity retention during cycling.

Keywords Cathode materials, Lithiated Nafion, Lithium conductivity, Mn-substituted LiFePO₄, Protective layer