Study On Synthesis And Modification Of AMnPO₄/C（A=Li,Na） As Cathode Materials For Lithium-ion Batteries

In recent years,the polyanionic material AMnPO₄/C（A=Li,Na）has been widelystudied.The main reason is that Mn is rich and the thermal stability of AMnPO₄ is high due to its structure.Li MnPO₄ is considered as a potential industrial resource owing to its high voltage platform and high energy density.However,the disadvantages of low ion mobility and poor electronic conductivity exist in Li MnPO₄,so it is necessary to find effective ways to solve these problems.The research of maricite type Na MnPO₄is still in the infancy,it is essential to improve its poor electrochemical performance on the basis of sound synthesis method of pure phase.Therefore,in view of these two problems,this paper firstly used traditional solid-phase and liquid-phase methods to synthesize Li MnPO₄ and maricite type Na MnPO₄,respectively,so as to find the optimal results of temperature,time and synthesis method.On the basis of the optimal results,cation doping and incorporation were carried out,improving the electrochemical properties of the materials.（1）The cathode material Li MnPO₄ coated with carbon was successfully synthesized bymechanical ball milling and sol-gel method,and the effects of synthesis conditions for different temperatures and time on the electrochemical properties of the materials were also explored in the two synthetic methods.The results show that the discharge capacity of mechanical ball milling is 83.8 m Ah·g^-1in the first cycle after calcining at 350℃for 2 h and then calcining at700℃for 16 h,and the capacity retention rate is 85.3%after 50 cycles.When the precursor was precalcined at 350℃for 2h,and then annealed at 700℃for 6h,the specific capacity was the highest,which was 121.2 m Ah·g^-1.After 50 cycles,the capacity retention was 90%.The electrochemical performance of Li MnPO₄/C synthesized by sol-gel method is obviously better than that of mechanical ball milling.（2）Maricite Na MnPO₄ cathode material coated with carbon was successfullysynthesized by mechanical ball milling and sol-gel method,and the effects of synthesis conditions for different temperatures and time on the electrochemical properties of the materials were also investigated.The results show that discharge capacity of materials calcined at 350℃for 2 h and then calcined at 700℃for 16 h is the highest in the first cycle by mechanical ball milling,which is 21 m Ah·g^-1.After 50 cycles,the capacity retention is 78.5%.When the precursor was precalcined at 350℃for 2h,the specific capacity of material calcined at 700℃for 16 h was the highest,which was 25.4 m Ah·g^-1.After 50 cycles,the capacity retention was78.5%.The electrochemical performance of Na MnPO₄/C synthesized by sol-gel method is superior to that of mechanical ball milling.However,the structure of the material blocks the ion channel and leads to poor electrochemical performance.Therefore,it is necessary to modify it under the optimum conditions.（3）The NaMn_1-xZr_xPO₄/C and NaMn_1-xTi_xPO₄/C cathode materials doped by highvalence cation were synthesized by sol-gel method.It was found that when the amount of Zr⁴⁺was 0.02,the discharge capacity and capacity retention were 42.1 m Ah·g^-1 and 98.3%respectively.When the amount of Ti⁴⁺was 0.03,the discharge capacity and capacity retention were 38.9 m Ah·g^-1and 97.4%,respectively.Compared with the pure phase,the electrochemical properties were improved.Secondly,composite materials with different ratios of Li_1-yNa_yMnPO₄/C were synthesized,and the lithium-sodium ratio of the composite material with the best electrochemical performance was determined to be n（Li）:n（Na）=9:1.Based on the synthesis of all the above materials,the composite material Li_1-yNa_yMn_1-xM_xPO₄/C（M=Zr,Ti）was synthesized.When n（Li）:n（Na）=9:1 and the doping amount of Zr is 0.02,the first week discharge specific capacity of Li_0.9Na_0.1Mn_0.98Zr_0.02PO₄ reaches 159.4 m Ah·g^-1;when（Li）:n（Na）=9:1 and the doping amount of Ti is 0.03,the first week discharge specific capacity of Li_0.9Na_0.1Mn_0.97Ti_0.03PO₄ reaches 152.6 m Ah·g^-1in the first cycle,respectively,which are close to the theoretical capacity(169 m Ah·g^-1),and they show good rate performance.Compared with pure phase,the electrochemical performance is greatly improved.