Synthesis And Modification Research Of LiMnPO₄ Cathode Material For Lithium Ion Batteries

LiMnPO₄ with olivine structure,is one of the most promising cathode materials for lithium-ion batteries.It has the greatest advantages of high voltage platform and high theoretical energy density,as well as its excellent thermal stability,high safety performance,easy availability and low cost of synthetic materials,which makes it suitable for the large-scale commercial use.However,pure phase LiMnPO₄ has extremely low electronic conductivity and lithium ion diffusion coefficient,and the Jahn-Teller effect caused during charging and discharging process.These will decrease its electrochemical performance.In this work,LiMn_0.8Fe_0.2PO₄/C cathode material was prepared by a simple solvothermal method,modified by graphene oxide,nitrogen doped assisted by PECVD technology and by melamine as the nitrogen source to improve the electrochemical performance of the cathode material.The effects of the above modifications on the structure,morphology and electrochemical performance of the materials were also investigated.The main results are given as follows:（1）The effect of different amount of graphene oxide on the structure,morphology and electrochemical performance of LiMn_0.8Fe_0.2PO₄/C cathode material are investigated by adjusting the ratio of added graphene oxide using sucrose as the carbon source.It is found that the morphology and electrochemical performance of the material are the best when the addition ratio of graphene oxide to the mass of the active material is 1%.The morphology of sample is smooth surface nanoparticle,and better three-dimensional structure is constructed together with the reduced graphene oxide flakes formed by sintering.The sample has a specific discharge capacity of 145.7 mAh·g^-1 at 0.1 C,and it can still deliver capacity of 103.7 mAh·g^-1 at the rate of 5 C.（2）The synthesized LiMn_0.8Fe_0.2PO₄/C material was doped by nitrogen assisted by PECVD technology,in which sucrose is used as the carbon source and nitrogen is used as the treatment atmosphere,And the effect of different radio frequency powers on the structure and electrochemical performance of LiMn_0.8Fe_0.2PO₄/C was also investigated.Results show that the active defect sites are introduced when the radio frequency power is adjusted to 100 W,which is favorable to the diffusion migration of lithium ions and the promotion of electrochemical performance of the samples.The specific discharge capacities of samples at 0.1 C and 5 C are 144.4 and 102.3 mAh·g^-1.（3）N-doped carbon coated LiMn_0.8Fe_0.2PO₄ nanocrystal was prepared by a facile solvothermal method using melamine as nitrogen source.Result shows that the introduction of N atoms does not change the crystal structure of LiMn_0.8Fe_0.2PO₄/C.The-NH₂ functional groups in melamine can react with the-OH groups in pyrolytic carbon generated by the pre-sintering of sucrose.Then,multiple nanosheets are combined together and the morphology of LiMn_0.8Fe_0.2PO₄/C is transformed to nano particle-like in LMFP-7.The doped nitrogen in the forms of pyridinic,pyrrolic and graphitic N are derived from the combination of pyrolytic carbon and melamine,which can generate active defective sites and improve the electronic conductivity and diffusion rate of lithium ions.Sample LMFP-7 delivers the best electrochemical performance with capacity of 154.7,144.2 and 110.0 mAh·g^-1 at 0.1,1 and 5 C,respectively.It also exhibits good electrochemical reversibility,low charge transfer resistance（46.9Ω）and high diffusion coefficient(1.35×10^-13 cm²·s^-1),as well as excellent cyclic performance,structural stability and chemical stability.