Synthesis And Performance Of LiFe_1-xMn_xPO₄/C Cathode Materialsfor Lithium Ion Batteries

The positive electrode material is one of the key factors limiting the further commercialization of lithium ion batteries.As the most widely used positive electrode material,Li Fe PO₄ is restricted for further development due to its low electronic conductivity and lithium ion diffusion coefficient.Lithium iron manganese phosphate anode materials formed by doping manganese has the advantages of lithium iron phosphate and lithium manganese phosphate,and is considered to be one of the most promising cathode material.In this thesis,Li Fe_1-xMn_xPO₄/C composites were prepared by replacing part of Mn with part of Fe to improve the electrochemical performance of the materials.Firstly,the conditions for the synthesis of Li Fe_1-xMn_xPO₄/C composites by the sol-gel methodwere explored.Then,in order to solve the problems of poor material rate performance and low electronic conductivity,the modification of Na,Mg,and Cu elements doping in orderand then nitrogen-doped carbon coating with polyvinyl pyrrolidone(PVP)as the nitrogen source and doping modification of Ni element under this conditionwere carried out.Using FeCl₂�4H₂O,Mn(CH₃COO)₂�4H₂O,NH₄H₂PO₄,CH₃COOLi�2H₂O,and citric acid as raw materials,a series of Li Fe_1-xMn_xPO₄/C composites were prepared by sol-gel method.The results show that when x is 0.35,calcination temperature is 750 ^oC,calcination time is 10 h,and carbon coating content is 30%,the Li Fe_0.65Mn_0.35PO₄/C composite has the best electrochemical performance.The first discharge specific capacity at 0.1 C is 138.5 m Ah/gin the voltage range of 2.5-4.5 V,and the coulomb efficiency is89.0%.After returning to 0.1 C through multiple different rates of charge and discharge,the capacity can still reach 132.3 m Ah/g,and the capacity recovery rate is 95.5%.Using CH₃COONa�3H₂O as the Na source and Mg(CH₃COO)₂�4H₂O as the Mg source,the Li_0.98Na_x(Fe_0.65Mn_0.35)_1-xPO₄/C and Li(Fe_0.65Mn_0.35)_1-xMg_xPO₄/C composite material were modified by Na-doped Li site and Mg-doped Fe and Mn sites.The results show that:the first charge-discharge specific capacities of the synthesized Li_0.98Na_0.02Fe_0.65Mn_0.35PO₄/C composite materials were 162.4 m Ah/g and 136.3 m Ah/g,respectively.The capacity retention rate of 1.0 C was 85.5%.When the rate is returned to0.1 C after 35 cycles,the capacity retention rate is 105.1%.Similarly,the synthesized Li(Fe_0.65Mn_0.35)_0.98Mg_0.02PO₄/C composite has the first specific discharge capacity of 167.7m Ah/g,139.0 m Ah/g,and the capacity retention rate of 1.0 C is 84.3%.After 35 cycles of multiple time charge/discharge at 0.1 C,the capacity retention rate is 99.4%.Since proper Na doping at the lithium site can increase the Li⁺one-dimensional diffusion channel and increase the conductivity of the material,while Mg doping at the Fe and Mn sites caused lattice defects and enhanced the material's conductivity.When the Li-site doped Na is 0.02,the synthesized Li_0.98Na_0.02(Fe_0.65Mn_0.35)_0.97Mg_0.03PO₄/C has an initial discharge capacity of147.7 m Ah/g.When charging and discharging at different rates and finally returning to 0.1C,the capacity retention rate was 96.2%.Cu(CH₃COO)₂�H₂O was used as the copper source,and Cu were used to modify the Fe and Mn sites.The synthesized Li(Fe_0.65Mn_0.35)_0.98Cu_0.02PO₄/C composite has the best electrochemical performance,the first discharge specific capacity at 0.1 C is 144.9 m Ah/g,the specific capacity at 1.0 C discharge is 111.0 m Ah/g,relative to 0.1 C,1.0 C capacity retention rate is 76.6%.Using PVP,melamine,and urea as the nitrogen source for doping modification of the coated carbon.The first discharge specific capacities of the three samples were 145.1m Ah/g,133.9 m Ah/g,and 133.6 m Ah/g,respectively,and the coulomb efficiency was91.1%,86.7%,and 82.1%,respectively.When the charge and discharge return to 0.1 C,the capacity retention rates are 85.3%,80.0%,and 77.2%,respectively.Since the addition of PVP increased the unit cell volume,at the same time,the material formed a porous structure,which was more conducive to ion migration and accelerated the extraction and insertion of lithium ions.This improved the electrochemical performance of the material.And its different contents were investigated.when the mass ratio of PVP to composite material is 1:5,the material obtained was an olivine porous structure.when the mass ratio of PVP to composite material is 1:5,the material obtained by sol-gel method was an olivine porous structure.The first discharge at 0.1 C reached 148.0 m Ah/g,and the coulomb efficiency was 84.9%.The discharge specific capacity is as high as 130.0 m Ah/g even at1.0 C.When returned to 0.1 C,the capacity retention rate reached 99.2%.Based on the optimal addition of PVP at 20%,Ni doping was performed to obtain the first discharge specific capacity of Li(Fe_0.65Mn_0.35)_0.97Ni_0.03PO₄/C composite material was 138.4 m Ah/g,and the coulomb efficiency was 93.5%.The capacity retention rate(relative to 0.1 C)at 1.0C also reached 78.3%.However,it can be seen that no better electrochemical performance is obtained than doping PVP alone through comparison.