To overcome the problems faced by the preparation of high-performance LiFe 0.5 Mn 0.5 PO 4 /C cathode material for commercial application in large scale, such as complicated procedures in the synthesis process and/or low volumetric energy density of nanometer-sized sample, herein, micro-agglomerated LiFe 0.5 Mn 0.5 PO 4 /C is prepared by a facile surfactant-assisted solid-state route. A composite carbon source and controllable pre-calcining process are used for achieving the homogenous carbon coating, interconnected pores and uniform particle size distribution . X -ray powder diffraction, Raman spectrum , N 2 -adsorpotion-desorpotion, scanning/transmission electron microscopy and elemental analysis confirm the microstructure and accurate composition of the as-prepared samples. It is found that the composite source not only increases the homogeneity of the carbon coating layer and the pore-forming ability, but also contributes to inhibiting the size growth of the primary particles. Meanwhile, the suitable pre-calcining time can improve the specific surface area and optimize the carbon content and pore structure , and thus enhance the tap density of the as-prepared material. The typical LiFe 0.5 Mn 0.5 PO 4 /C sample with carbon content of 3.50 wt% displays high reversible capacities and good rate capability, with discharge capacities of 155.0, 141.5 and 120.1 mA h g −1 at 0.1, 1.0 and 5.0 C (1 C = 170 mA g −1 ), respectively. Furthermore, the sample exhibits superior cycling performance, with a capacity retention above 98% after 200 cycles at 1.0 and 5.0 C. As a result, this LiFe 0.5 Mn 0.5 PO 4 /C exhibits great potential as a cathode material for high power/energy lithium ion batteries because of its easy synthesis, high specific capacity, good rate capability and cycle stability.
Olivine-type LiFe x Mn 1- x PO 4 /C cathode materials are of great interest for lithium-ion batteries because of their higher lithium ion intercalation potential compared with that of LiFePO 4 . Two types of LiFe 0.5 Mn 0.5 PO 4 /C nanorods with different sizes and crystal structures were synthesized through a facile glycol-based solvothermal process with different precursor feeding sequences. The microstructural investigation revealed that the size and structure of the LiFe 0.5 Mn 0.5 PO 4 nanorods can be tuned by nucleation and crystal growth rates of the intermediate precipitates, which can be controlled by precursor feeding sequence. The LiFe 0.5 Mn 0.5 PO 4 nanorods obtained through the first feeding sequence (a solution of transition metal salts was added dropwise into the solution mixture of LiOH and H 3 PO 4 ) exhibited less exposure of the (010) crystal faces and had bigger sizes compared with those of the LiFe 0.5 Mn 0.5 PO 4 nanorods obtained through the second feeding sequence (a LiOH solution was added dropwise into the solution mixture of H 3 PO 4 and transition metal salts). Electrochemical investigations indicated that the LiFe 0.5 Mn 0.5 PO 4 nanorods obtained through the second feeding sequence showed substantially improved electrochemical performance, in which the discharge capacities reached 157 and 119 mAh g −1 at 0.2 and 5 C, respectively. Furthermore, a capacity retention of 89% was obtained after 500 cycles at 1 C, demonstrating excellent cyclic stability.
The rate performance of LiMnPO4-based materials is further improved via synergistic strategies including a surfactant-assisted solid state method, Fe-substitution and carbon-coating. The surfactant-assisted solid state strategy effectively decreases the primary particle size of the cathode material, which can greatly shorten the diffusion distance of lithium ions. The Fe-substitution improves the effectiveness of Li+ insertion/extraction reactions in the solid phase. The uniform carbon coating layer and the conductive networks provided by the carbon between the nanoparticles ensure the continuous conductivity by the nanoparticles. As a consequence of the synergistic effects, the as prepared LiFe0.5Mn0.5PO4 sample with 6.10 wt% carbon exhibits high specific capacities and superior rate performance with discharge capacities of 155.0, 140.9 and 121 mA h g−1 at 0.1, 1 and 5 C (1 C = 170 mA g−1), respectively. Meanwhile, it shows stable cycling stability at both room temperature (25 °C, 94.8% and 90.8% capacity retention after 500 cycles at 1 and 5 C rates, respectively) and elevated temperature (55 °C, 89.2% capacity retention after 300 cycles at 5 C rate). This material may have great potential application in advanced Li-ion batteries.
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