Synergetic Fe substitution and carbon connection in LiMn1−xFexPO4/C cathode materials for enhanced electrochemical performances

To enhance the rate and cyclic performances of LiMnPO4 cathode material for lithium-ion batteries, Mn is partially substituted with Fe, and LiMn1−xFexPO4 (x = 0.2, 0.3, 0.4, 0.5) solid solutions are synthesized and investigated. Amphiphilic carbonaceous material (ACM) forms well carbon coating and connects the LiMn1−xFexPO4 crystallites by a three-dimensional (3D) carbon network. The synergetic Fe substitution and carbon connection obviously improve the samples' rate capacities and cyclic stability. The optimized LiMn0.6Fe0.4PO4/C sample delivers discharge capacities of 160 mA h g−1 at 0.05 C, 148 mA h g−1 at 1 C, and 115 mA h g−1 at 20 C. All samples have well capacity retention (>92%) after 50 charge/discharge cycles at 1 C. The enhanced electrochemical properties are mainly attributed to the improvement of Li ion and electron transport in the LiMn1−xFexPO4/C samples, respectively mainly resulting from their modified crystal structures caused by Fe substitution and the 3D carbon coating/connection originating from ACM carbonization. LiMn1−xFexPO4 materials exhibit two discharge plateaus at ∼4.0 and ∼3.5 V (vs. Li+/Li), whose heights respectively reflect the redox potentials of Mn3+/Mn2+ and Fe3+/Fe2+ couples. The plateaus' lengths correspond to the Mn/Fe ratio in LiMn1−xFexPO4 and are affected by the kinetic behavior of samples. Though the ∼4.0 V plateau shrinks with increasing discharge rate, the ∼3.5 V plateau may slightly elongate. Moreover, the Fe substituted in the partial Mn sites could significantly improve the Li ion diffusion, thus enhance the kinetic behaviors of LiMn1−xFexPO4.