Constructing a 3D Interconnected Carbon Network for Mg-Doped Porous LiMn0.85Fe0.15PO4/C Cathode Materials

Abstract

Economical and high-safety LiMn_0.85Fe_0.15PO₄/C cathode materials have gained significant attention recently due to their theoretical specific energy advantage of 18% compared to LiFePO₄. However, their low electronic conductivity and sluggish diffusion kinetics limit the practical applications of LiMn_0.85Fe_0.15PO₄/C. This paper presents a simple solid-state synthesis of porous LMFM_0.01P-2C4P, which is doped with Mg and coated with composite carbon. Mg substitution for Mn shortens the transport path of lithium ions while increasing intrinsic conductivity and structural stability. Additionally, a 3D conductive network structure generated by the composite carbon source (citric acid and polyethylene glycol 400) improves the electronic conductivity and effectively minimizes the internal resistance of the battery. LMFM_0.01P-2C4P consists of secondary particles aggregated from primary particles smaller than 100 nm, each of which is coated with a uniform carbon layer. The electronic conductivity and lithium-ion diffusion coefficient greatly exceed those of unmodified LMFP-4C, measuring 7.22 × 10^–3 S cm^–1 and ∼10^–12 cm² s^–1, respectively. Electrochemical studies demonstrate that LMFM_0.01P-2C4P delivers a superior specific capacity of 152.1 m Ah g^–1 and 124.9 m Ah g^–1 at 0.1C and 1C, respectively, along with a capacity retention of 80.8% after 500 cycles at 1C. However, the initial capacity of LMFP-4C is merely 104.1 mAh g^–1 at 1C, with a capacity retention of only 65.7% after 500 cycles. This work presents a useful way to enhance the conductivity of phosphate cathode materials for lithium/sodium-ion batteries.