The Preparation And Modification Of LiMnPO₄/C Cathode Materials

Compared with ternary cathode material with poor thermal stability and with olivine structure LiFePO₄ with low platform and energy density,LiMnPO₄ cathode has a good application prospect in lithium ion batteries.It is attributed to the advantages of high voltage platform?4.1 V?,high specific capacity(170 mAh?g^-1),high energy density(701 Wh?kg^-1)and good thermal stability.In addition,the resource of manganese is abundant and the price is very low,therefore,it is suitable for the large-scale application.However,the disadvantages of LiMnPO₄,such as low electrical conductivity,low diffusion coefficient of lithium ion and the Jahn-Teller effect of Mn³⁺,hinder its application in which it is partially caused by its high diffusion activation energy.The method for the improvement of the rate performance of LiMnPO₄ is mainly focus on surface modification,grain refinement,morphology controlling and doping of ions.In this paper,the LiMn_0.8Fe_0.2PO₄/C cathode was prepared by hydrothermal method.The effects of raw materials and their proportions,heat treatment technology such as sintering temperature and holding time were studied.Then,the influence on the effects of structure,morphology and electrochemical performance were investigated.On this basis,Na⁺and Fe²⁺were co-doped in LiMnPO₄/C cathode by a simple one-step solvothermal method.The effects of doping on structure,morphology and electro-chemical performance were discussed.The formation mechanism of nanocapsule cathode was discussed.The main results are given as follows:?1?LiMn_0.8Fe_0.2PO₄/C cathode was prepared by a one-step hydrothermal method.The cathode with optimum performance is obtained at the appropriate heat treatment technology of heating the sample 700°C for 4 h.The best specific capacity of sample at 0.05 C is 83.5 mAh?g^-1.The capacity retention is satisfied.The specific capacity of the sample at 0.05 C is increased to 86.3 mAh?g^-1 when Li⁺:PO₄^3-is 2.6:1.The charge-discharge performance and rate performance of the sample are greatly improved when LiOH and H₃PO₄ are selected as reactants.The specific capacity at 0.05 C and 5C are 112.1 mAh?g^-1 and 58.2 mAh?g^-1,which is much higher than other samples.?2?Na⁺and Fe²⁺co-doped LiMnPO₄/C cathode was prepared by a simple one-step solvothermal method.The doping of Na⁺does not transform the crystal structure,however,improve the structural stability of cathode material.The nanocapsule cathode are synthesized and the formation mechanism of capsule has been analyzed.The Na⁺and Fe²⁺co-doped improve the rate performance and cycle performance of cathode.The sample LN-3 exhibits excellent electrochemical performance.Its specific capacities at 0.05 C and 5 C are 141.7 mAh?g^-1 and 89.5 mAh?g^-1,respectively.The capacity retention ratio after 200 cycles at 0.5 C is as high as 96.65%.The ion co-doping improves significantly the lithium ion diffusion coefficient,structural stability and chemical stability.The XRD patterns of sample LN-3 after 200 cycles do not change significantly.And the dissolution of Mn²⁺and Fe²⁺in electrolyte is also significantly inhibited.?3?The Li_0.97Na_0.03Mn_0.8Fe_0.2PO₄/C cathode was modified by adjusting the amount of lithium in solvothermal synthesis process.The pyrolytic carbon is coated on the surface of cathode through the past coating method,in which the precursor is immersed in the solution containing sucrose with different ratio.It is found that these modifications do not affect the structure and morphology of cathode material.The rate performance and cycle performance of the sample reach the optimum level when the molar ratio of LiOH:H₃PO₄ is 2.6:1 and the mass ratio of sucrose:LMFP is 1:5.The specific discharge capacity at 0.05 C and 5 C reaches 149.1 mAh?g^-1 and 125.3mAh?g^-1,respectively.The capacity retention ratio at 0.5 C was 97.12%after 200 cycles.Therefore,the electrochemical performance of sample reaches an excellent level.