Preparation And Coating Modification Of Spinel-type Lithium Nickel Manganese Oxide Cathode Material

In order to satisfy the requirements of electric vehicles on excellent cycling and rate performance of lithium-ion battery,it is of great significance to develop highperformance lithium-ion battery cathode materials.Currently,LiCoO2 materials are the widely used and maturer commercially cathode materials for lithium-ion materials.However,the Co is in high price with toxicity,and has low working voltage and energy density,which cannot meet the needs of electric vehicles and large-scale energy storage equipments.Among the new cathode materials,the spinel nickel manganate(LiNi0.5Mn1.5O4)material has been the most promising lithium ion cathode material,due to the theoretical specific capacity of 147 mAh g-1,higher working voltage of 4.7 V(vs Li+/Li and energy density of 650 Wh kg-1.However,there are also problems with LiNi0.5Mn1.5O4 materials.It will undergo a disproportionation reaction and dissolution of manganese,owing to the presence of Mn3+ ions,which leads to the loss of active materials and reduce battery capacity.Moreover,Mn3+ will also exacerbate the JahnTeller effect,resulting in damages in the structure and poor electrochemical performance.At the same time,when working at high voltage,the side reactions of the electrolyte and electrode materials will intensify,leading to rapid decay of capacity.The current researches focus on improving the preparation methods,surface coating and ion doping.In this work,spinel LiNi0.5Mn1.5O4 materials are investigated from the following aspects to solve existing problems,and the main conclusions are as follows:(1)LNMO cathode materials were successfully prepared by sol-gel method and combined annealing method.The morphology and structure of the material were characterized by XRD,SEM,and FTIR.Based on this,the electrochemical performance of the prepared material as a lithium ion cathode material was tested.The analysis results show that the synthesized spinel LNMO materials belong to the Fd-3m space group and have disordered spinel structure.Increasing annealing time,the impurity phase LixNi1-xO impurity phasein the material decreases,and the degree of order of the material increases.Electrochemical charge-discharge cycle tests show that annealing treatment increases the specific discharge capacity of the electrode material,due to less amounts of purities in the material,and Mn3+ ion content in the electrode material after the annealing treatment is also significantly reduced,which is useful to reduce the Jahn-Teller effect and improve the stability of the material.Particularly,the cathode material that has been annealed for 6 h have prominent cycling performance,which deliver 125 mAh g-1 after 300 cycles at 1C with capacity retention of 96.9%.At 5C,the discharge capacity will still be 85.6 mAh g-1 after cycling for 1,000 cycles,the capacity retention rate is 77%.After rate and electrochemical impedance test,the results show that all the prepared cathode materials have good rate performance.The unannealed electrode material can release more capacity at 10C,and its resistance value before and after the 50 cycles is small,which indicate that the material without annealing contains more Mn3+ ions,which is conducive to electron conduction during charge and discharge,and reduces electric charge transfer resistance.(2)The LNMO precursor material was synthesized by the sol-gel method,and the surface of the LNMO material was coated with lithium borate and lithium borosilicate materials through a wet chemical coating method,respectively.The prepared materials belong to the Fd-3m space group by the characterization of the morphology,structure and electrochemical testing.The surface of the precursor material can be successfully coated with lithium borate and lithium borosilicate materials by wet chemical methods.The coating materials do not change the morphology and structure of the precursor material.The high temperature treatment process effectively reduces the Mn3+ions in the material,which is beneficial to stabilize the structure of LNMO.The electrochemical performance tests were performed on samples with different coating amounts.The results showed that the LBO2%@LNMO samples had excellent electrochemical performance.The initial discharge capacity was 127 mAh g-1 and the capacity retention rate is 92%after 500 cycles at 1 C rate,Even at 10 C,the initial discharge specific capacity can be 111 mAh g-1,and the discharge capacity can be maintained at 85 mAh g-1 after 1000 cycles.Compared to the Bare LNMO electrode,the capacity decreases sharply after 300 cycles.The LBO coating can not only be used as a protective layer,but also the chemical interaction between the coating and the positive electrode active material can be induced by high-temperature treatment,which will have a profound impact on the chemical composition and electrochemical performance of the material A new type of lithium borosilicate material was coated on the surface of the LNMO material by the same wet chemical coating method.After 1000 cycles at 1C rate,the discharge capacity of the LBO-LSiO2%@LNMO electrode is 105 mAhg-1,and the capacity retention rate is 87.5%,while the capacity of the Bare LNMO electrode material decays sharply.At 5 C and 10 C retes,the LBOLSiO2%@LNMO electrode deliver a high specific capacity of 89 mAh g-1 and 73 mAh g-1,respectively.The discharge capacity of Bare LIMO at the same discharge rate is 65 mAh g-1 and 3 mAh g-1,respectively.The results show that the lithium borate and lithium borosilicate coating materials can suppress the side reactions between the electrode material and the electrolyte,and improve the structural stability and thus enhance electrochemical performance of the materials.According to the analysis of the relevant mechanism,it shows that the amount of Mn3+ ions in the material can profoundly affect the electrochemical performance of the material.In this paper,the problems of lithium spinel nickel manganate are tackled by improving the synthesis method and surface modification.Optimizing the synthesis conditions canimprove cycling performance of the material and surface coating can stabilize the structure of the material and thus improve the cycling and rate performance of the material.