Preparation And Modification Of LiFePO₄ As Cathode Moterial Of Lithium-ion Batteries

In this study, Li Fe PO₄ cathode material for lithium ion batteries was synthesized by solid state reaction. The optimum reaction conditions are determined based on the synthetic conditions of experiment. Then the materials were modified by doping metal elements and coating metal oxide.Through the investigation of mixed iron source（Fe₂O₃+Fe C2O₄, Fe PO₄+Fe C2O₄,Fe PO₄+Fe₂O₃, Fe+Fe₂O₃, and Fe+Fe PO₄） and the comparative experiment of mixed iron source and single iron source, mixed iron（Fe₂O₃+Fe C2O₄） was identified as iron source; Through the investigation of carbon source（polyvinyl alcohol-1788, polyethylene glycol-2000, citric acid, glucose and acetylene black）, citric acid was identified as carbon source. Through the investigation of dispersant（ethanol,ethanol/water and ethanol）, calcination temperature（600, 650 and 700℃） and calcination time（8, 10, 12 and 14h）, the optimum conditions are determined as follow: ethanol was as dispersant, 650℃ was as calcination temperature, and 10 h was as calcination time. After 10 cycles at 0.1, 0.2, 0.5, 1.0 and 2.0C, the maximum specific capacity of the optimum material are 146.9, 131.5, 115.5, 100.0, 72.1m Ah/g, respectively.Li Fe_1-x Tix PO₄ was synthesized by Ti doping（x=0.00, 0.01, 0.02, 0.03）.Ti doping can improve the low-rate discharge capacity and cycle performance, the initial discharge capacities of Li Fe0.98Ti0.02PO₄ at 0.1C is 150.2m Ah/g, the discharge capacities is154.0m Ah/g after 10 cycles, and the capacity retention is 102.5%.Li Fe_1-x Nix PO₄ was synthesized by Ni doping（x=0.00, 0.01, 0.02, 0.03, 0.04）.; Ni doping can improve the high-rate discharge capacity and cycle performance, the initial discharge capacities of Li Fe0.98Ni0.02PO₄ at 0.1C, 0.2, 0.5, 1.0 and 2.0C are 131.5, 123.9,109.3, 95.5 and 80.4m Ah/g, respectively. After 10 cycles at 0.1, 0.2, 0.5, 1.0 and 2.0C, the capacity retention is 100.6%、100.7%、99.8%、98.8% and 100.5%, respectively.Li Fe_1-x Vx PO₄ was synthesized by V doping（x=0.00, 0.01, 0.02, 0.03, 0.04）. V doping can improve discharge capacity and rate-cycle performance, after 10 cycles at 0.1, 0.2,0.5, 1.0 and 2.0C, the maximum specific capacity of Li Fe0.98V0.02PO₄ are 152, 140.9, 130.3,119.8 and 105.9m Ah/g, respectively. The capacity retention is 98.2%, 100.9%, 99.3%,100.3% and 98.8%, respectively.Li Fe_1-x Mnx PO₄ was synthesized by Mn doping（x=0.00, 0.01, 0.02, 0.03, 0.04） Mn doping reduces the discharge capacity, but it can improve the cycle performance in some degree, the initial discharge capacities of Li Fe0.99Mn0.02PO₄ and Li Fe0.97Mn0.03PO₄ at 0.2C are 66.7 and 68.1m Ah/g, respectively. After 10 cycles at 0.2C, the discharge capacities ofLi Fe0.99Mn0.02PO₄ and Li Fe0.97Mn0.03PO₄ are 65.2 and 65.9m Ah/g, the capacity retention is98.2% and100.9%.Coated 1wt%-Mg O, 2wt%-Mg O, 3wt%-Mg O and 4wt%-Mg O materials were synthesized. Mg O-coating can improve the low-rate cycle performance. Especially at 0.1C,the discharge capacities of coated 1wt%-Mg O, 2wt%-Mg O, 3wt%-Mg O and 4wt%materials are 82.8, 90.3, 83.7 and 74.9m Ah/g after 10 cycles, the capacity retention is105.2%, 96.8%, 98.2%, and 100.5%.Coated 1wt%-Al₂O₃, 2wt%-Al₂O₃, 3wt%-Al₂O₃ and 4wt%-Al₂O₃ materials were synthesized. Al₂O₃-coating can improve the high-rate cycle performance. Especially coated3wt%-Al₂O₃ material, the first discharge capacities at 0.5, 1.0 and 2.0C are 71.7, 58.5 and44.8m Ah/g, the capacity retention is 97.4%, 92.1% and 97.5%, respectively.