Study On Preparation And Chemical Doping Modification Of LiFe_0.5Mn_0.5PO₄/C Cathode Materials

The olivine type cathode material LiFe PO₄ is favoured for its good stability,safety and low cost.However,the low operating voltage and low energy density of LiFe PO₄limit its wide application.In response,LiFe_1-xMn_xPO₄ cathode materials is formed by replacing Fe with Mn,which take advantage of the high potential properties of the Mn element as a means of increasing energy density,making LiFe_1-xMn_xPO₄ cathode materials one of the most promising cathode materials available.In this paper,LiFe_0.5Mn_0.5PO₄/C cathode materials were prepared by the carbonthermal reduction solid-phase method.The preparation process conditions such as iron phosphate nanosizing,iron-manganese ratio,roasting temperature,roasting time,carbon amount and lithium excess,as well as the effects of chemical doping of elements such as nitrogen,yttrium and titanium on their effects on their structure,morphology and electrochemical properties were mainly investigated,and good results were obtained.The specific research content and related results were as follows:（1）The effect of particle size on the electrochemical properties of LiFe_0.5Mn_0.5PO₄/C materials was investigated by nano-sanding iron phosphate.SEM test showed that the initial particle size of the LiFe_0.5Mn_0.5PO₄/C material,which was synthesised from abraded iron phosphate,was reduced from 1μm―3μm to 0.1μm―0.3μm.The electrochemical test results show that the material with smaller particles has better electrochemical performance,and it has a larger discharge specific capacity of 144.97 m Ah/g at 0.1 C.（2）LiFe_1-xMn_xPO₄/C was prepared by carbothermal reduction method using sand-milled Fe PO₄,Mn CO₃,Li₂CO₃,NH₄H₂PO₄ and glucose as raw materials.It was shown that the Li_1.05Fe_0.5Mn_0.5PO₄/C material obtained when the Fe:Mn ratio was 1:1,the calcination temperature was 700°C,the calcination time was 10h,the glucose addition was 12.5%wt,and the excess lithium content was 5%had optimal electrochemical properties:its discharge specific capacity at 0.1C is 160.97 m Ah/g,close to the theoretical specific capacity,and still has a capacity retention rate of 97.11%after 100cycles.（3）Using melamine as the nitrogen source,the effect of non-metallic elemental N-doped covered carbon on the electrochemical properties of Li_1.05Fe_0.5Mn_0.5PO₄/C materials was investigated by high temperature solid phase method.It was shown that a suitable amount of N proportional doping of the carbon layer could be used to improve the conductivity of the carbon layer by using N as a carrier of electrons,resulting in a first discharge specific capacity of 165.57 m Ah/g at 0.1C and a capacity retention of86.91%after 300 cycles at 1C for the material synthesised at 10%N proportional doping.（4）The effects of Y doping and Ti doping on the structure,morphology and electrochemical properties of LiFe_0.5Mn_0.5PO₄/C materials were investigated by using Y₂CO₃ as yttrium source and Ti O₂ as titanium source.Physical characterisation tests such as XRD,XPS,TEM and SEM showed that both Y³⁺and Ti⁴⁺were successfully doped into the olivine structure and reduced the degree of agglomeration between the sample particles.Electrochemical tests such as constant current charge/discharge,CV,EIS and GITT show that both appropriate amounts of Y³⁺and Ti⁴⁺doping can significantly reduce the potential polarisation of the material and improve the Li⁺diffusion coefficient.Therefore,the rate performance and cycle performance of the material are improved.