Research On Nano Conductive Materials Used For Surface Modification Of LiFePO₄

Due to its high theoretical capacity, potentially low-cost, and environmentally benign, LiFePO4 battery has been extensively applied in the market of power lithium-ion battery in recent years. However, the poor intrinsic electronic conductivity and low solid-diffusion coefficient of lithium ion significantly limit its further improvement of electrochemical properties, which are urgent technical bottlenecks to be broken at present. Focusing on above issues, this thesis has studied in the surface modification of Li FePO4 to improve the electronic conductivity and solid-diffusion coefficient of lithium ion. The specific research contents are as follows:A two-step chemical method was proposed to deposite nano sized copper particles on the surface of LiFePO4, and finally synthesized LFP/Cu composite. The effects of different process parameters on nucleation rate, deposition process of nano sized cooper particles, and the electrochemical properties of LFP/Cu composite were also investigated. Optimized parameters of the chemical process were 16 mL/L HCHO at 40?C, solution pH = 11.5 and reaction time of 12 minutes. The optimized LFP/Cu delivered higher initial discharge capacity of 146.7 mAh/g at 0.1 C, and the capacities at 0.5 C, 1 C, 2 C, 5 C were 141.5, 128.5, 112.1 and 88.3 mAh/g, respectively. Distributed nano-sized copper particles could reduce the Rct, enhance the electronic conductivity, and decrease the polarization of LiFePO4, thus improve the rate property of LiFePO4, but the cycle stability remained to be improved.Based on the Mo-doped Li Fe0.85Mo0.05PO4 cathode materials, we prepared LiFe0.85Mo0.05PO4/Cu composites which achieved nano-sized copper particles used for surface modification of LiFe0.85Mo0.05PO4. Uniformly distributed nano-sized copper particles formed a conductive net on the surface of Li Fe0.85Mo0.05PO4, which can help to improve the electronic conductivity. The initial discharge capacity of LiFe0.85Mo0.05PO4/Cu composite at 0.1 C was 154.3 mAh/g. Metal ion doping and surface coating compound modification can both improve the electronic conductivity and ionic conductivity of Li FePO4. In addition, Mo6+ doping can effectively decrease the polarization of LiFePO4. LiFe0.85Mo0.05PO4/Cu composite exhibited better rate performances, the discharge capacities under different rate had improved, and the retention rate after 90 cycles at different rates was 89.5%.Furthermore,we prepared carbon nanoscrolls(CNSs) using monolayer graphene as raw material,and then recombined with LiFePO4 to obtain LFP/CNSs composite. The effect of solution with different surface tension on CNSs production and rolling extent was discussed. In LFP/CNSs composite, a few CNSs with small diameter dispersed among LiFePO4 particles, which had little effect on tap density. The charge and discharge capacity of LFP/CNSs composite under 0.1C were 156.2 and 153.9 mAh/g, which attained approximately 91% of the theoretical value. The discharge capacity under 0.5C, 1C, 2C and 5C reached 148.5, 139.2, 131.6 and 119.0 mAh/g, respectively. The retention rate after 90 cycles under different rate was 96.2%, indicating outstanding rate performances and cycling stability.