Study On Preparation And Properties Of Phosphate Solid Solution Cathode Materials For Lithium Ion Batteries

Lithium-ion batteries occupy a large proportion in the new energy system,and the phosphate cathode material of LiMPO₄?M = Co,Mn,Fe?with high theoretical capacity and cycle stability has been widely studied.However,the intrinsic shortcomings of those materials limit their application in practice.Forming solid solution materials through substituting partial ions can effectively alleviate the defects of monobasic phosphate materials.In this paper,a variety of solid solution materials were synthesized by high temperature solid-state reaction and solvothermal method,and their electrochemical properties were studied.The major contents are as follows:1.The effects of synthesis conditions?calcination temperature,carbon sources and ion doping?on the electrochemical properties of LiCo_1-xFe_xPO₄ solid solution prepared by high temperature solid state method were studied.The results show that the initial discharge capacity of LiCo0.9Fe0.1PO₄/C composite cathode material prepared at 650 oC is 139 mAh g^-1 at 0.1 C.The capacity retention of Li1.025Co_0.8Fe0.15V0.05?PO₄?1.025/C is up to 70% after 30 cycles.While the initial discharge capacity of LiCoPO₄/C is only 111 mAh g^-1 at 0.1 C.The LiCoPO₄/C has the best cycling performance when sintering at 800 oC.2.The effects of Mn content and carbon sources on the morphology,particle size and electrochemical properties of the LiCo_1-xMn_xPO₄ solid solution were studied.The LiCo_1-xMn_xPO₄ particles with rod-like morphology were synthesized by solvothermal method,and the particle size decreases gradually with the increase of Mn content.The first discharge capacity of LiCo_0.5Mn_0.5PO₄/C with particle size of 70 ¹50 nm is 137 mAh g^-1 at 0.1 C,and the discharge capacity is up to 136 mAh g^-1 at 1 C rate and at 50 oC.The LiCo_0.5Mn_0.5PO₄/C synthesized by high temperature solid state method using ?-cyclodextrin as carbon source delivers a discharge capacity of 119 mAh g^-1 at 0.1 C rate.When using Tween40 and Tween80 as mixed carbon source,the particle size of LiCo_0.5Mn_0.5PO₄/C is 90²00 nm with a good dispersion.The LiCo_0.5Mn_0.5PO₄/C delivers an initial discharge capacity of 114 mAh g^-1 at 0.1 C with capacity retention of 73% after 50 cycles.3.The effects of Fe content and carbon sources on the morphology,particle size and electrochemical properties of the LiMn_1-xFe_xPO₄ solid solution were studied.The LiMn_1-xFe_xPO₄ synthesized by solvothermal method shows plate morphology with particle size of 100²50 nm.The first discharge capacity of LiMn0.7Fe0.3PO₄/C is 161 mAh g^-1 at 0.1 C,and the capacity retention is 88% after 200 cycles at 1 C.It also exhibits better cycle stability at 50 oC.The LiCo_0.8Mn_0.2PO₄/C synthesized by high temperature solid state method using soluble starch as carbon source delivers an initial discharge capacity of 135 mAh g^-1 and also shows good cycling performance at 0.1 C rate.Furthermore,using Tween40 and Tween80 as mixed carbon source,the LiCo_0.8Mn_0.2PO₄/C has the particle size of 140²50 nm and gives discharge capacity of 144 mAh g^-1 at 0.1 C.4.The LiCo_1/3Mn_1/3Fe_1/3PO₄ solid solution was synthesized by solvothermal method and high temperature solid state method.The effects of solvent ratio and carbon source on the morphology and electrochemical properties of LiCo_1/3Mn_1/3Fe_1/3PO₄ were studied.The results show that the particle size of the material prepared by solvothermal method is 140²00 nm and the discharge capacity is 137 mAh g^-1 at 0.1 C.The first discharge capacities of the material synthesized by solid state reaction are 147 and 151 mAh g^-1 at 0.1 C using chitosan and Tween80 as carbon source,respectively.The LiCo_1/3Mn_1/3Fe_1/3PO₄/C prepared by evaporation-assisted solid-state reaction shows the particle size of 170²00 nm,and delivers an initial discharge capacity of 150 mAh g^-1 at 0.1 C with capacity retention of 85% after 50 cycles.