Study On Solvothermal Synthesis Of Phosphates Cathode Materials For Lithium-ion Batteries

Lithium ion batteries have been widely used in all aspects of industrial production and daily life owing to their advantages with regard to high energy density, high working voltage, long working life and green environmental protection. Cathode materials play an important role in the performance and cost of lithium ion batteries. Olivine LiMPO₄（M = Fe, Mn, Co） with higher specific energy is considered as the ideal cathode material for lithium ion batteries, however, its poor electrical conductivity and lithium ion diffusion rate limit the electrochemical properties of these materials. In this paper, LiMPO₄（M = Fe, Mn, Co） were synthesized via a solvothermal approach and the electrochemical properties were investigated. The major research contents are as follows:1. The solvothermal approach was used to prepare Li Mn PO₄ with single phase, uniformity and good crystallinity. The solvothermal synthesis conditions of Li Mn PO₄ were studied. Moreover, the effects of synthesis conditions（e.g. the solvent system, solvent components, raw material ratio, temperature, time, reactant mixing sequence） on the morphology, structure and the electrochemical properties were discussed in detail. The nanosheet-like Li Mn PO₄ was synthesized at 180 °C for 10 h using LiOH·H₂O, H₃PO₄ and Mn SO4·H₂O（the mole ratio is 3:1.1:1） as the start materials in ethylene glycol-water（volume ratio is 15:1） solvent. After carbon coating, the LiMnPO₄/C composite delivered an initial discharge capacity of 140.9 m Ah g-1 at 0.05 C rate with a capacity retention of 86.5% after 50 cycles at 0.1 C rate.2. LiFePO₄ nanoparticles with the size of 100 nm and good dispersion were synthesized under the optimized synthesis conditions. The effects of heat treatment temperature and carbon content on the electrochemical properties of LiFePO₄ were studied. The test results showed that the LiFePO₄/C composite including 9% carbon exhibited the best electrochemical performance. The material delivered the first discharge capacities of 164 m Ah g-1 at 0.5 C and 140 m Ah g-1 at 5 C. The discharge capacity remained 100 m Ah g-1 after 2000 cycles at 10 C. Furthermore, the LiFePO₄/C showed good cycling stability at high temperature.3. In order to further improve the energy density of phosphate materials, LiCo_0.5Mn_0.5PO₄, LiCo_0.8Mn_0.2PO₄, LiMn_1-xFe_xPO₄（0≦x≦0.3） and LiFe_1/3Mn_1/3Co_1/3PO₄ solid soluble materials were synthesized. The results of electrochemical tests showed that the LiCo_0.5Mn_0.5PO₄/C and LiCo_0.8Mn_0.2PO₄/C delivered discharge capacities of 142.5 m Ah g-1 and 141.7 m Ah g-1 at 0.05 C rate, respectively. In the case of LiMn_1-xFe_xPO₄, the sample displayed the best performance when x = 0.3. The LiMn_0.7Fe_0.3PO₄/C gave a discharge capacity of 156.1 m Ah g-1 with the capacity retention of 87.6% after 50 cycles at 0.1 C. When Fe, Mn and Co were coexisted, the LiFe_1/3Mn_1/3Co_1/3PO₄/C exhibited excellent electrochemical performance both at room temperature and high temperature. The material displayed the initial discharge capacity of 155.5 m Ah g-1 and the capacity retention of 90% after 50 cycles at 0.1 C at room temperature. When the test temperature increased to 50 °C, the composite delivered the initial discharge capacity of 146 m Ah g-1 with the capacity retention of 79.2% after 100 cycles at 1 C.