Synthesis And Performance Research Of Lithium-rich Ternary Li_1+x?NiCoMn?_yO₂ Cathode Material For Lithium-ion Batteries

Lithium-ion batteries?LIBs?have the performance of high voltage,high specific capacity and high specific energy,becoming one of the most promising energy storage devices for portable electronic equipment,electric vehicle,space technology and national defense industry in recent years.Although a variety of lithium-ion cathode materials have been applied in commercialization,all these materials show discharge capacities just below 160 m Ah/g,which is far lower than anode capacity?>350m Ah/g?.So in general,cathode material has become the key to restrict the battery capacity of LIBs.In recent years,the appearance of lithium-rich cathode materials brings hope to the development of lithium ion cathode materials due to the fact that their capacities can reach higher than 200 m Ah/g.However,lithium-rich materials have disadvantages too,such as the large irreversible capacity at the first cycle,poor high-rate capability and bad cycling stability.In addition,the composition of the material varies greatly,and the crystal structure is very complex,so at present there is no consistent recognition about the material structure and its change process in the cycle.But it is very important to understand these problems for the structural design of lithium-rich materials.Therefore,this paper made some exploratory researches in view of the above problems.In theory,it is committed to make further understanding about the crystal structure of different lithium-rich materials and make further exploration on the structural changes in the cycle process.In application,it is committed to make some modifications by using the relevant properties of graphene to improve the rate performance and cycle performance of the materials,and to explore the practical modification methods.The generation process of the co-precipitation precursor was studied.The composition of the precursor and its changes during the heat treatment were studied.It is found that the precursor particles have a solid soulution structure,and both Co CO3 and Ni CO3 were dissolved in Mn CO3 matrix.In the temperature rising process of heat treatment,The XRD diffraction peaks of lithium-rich materials have been initially formed after the pre-sintering at 500?.The crystal structure changed obviously from 700? to 800?,and lithium-rich material had been formed at 800?.From 800? to 900? and the subsequent heat preservation process,it was the process of lattice optimization and grain growth,in which process the crystal structure of Li2 Mn O3 was gradually perfected and the layered structure of Li MO₂ was gradually extended.It is found that the I110/I018 intensity ratio of XRD pattern is an explicit indicator for the layered structure of lithium-rich material.The increase of I110/I018 means the improvement of layered structure integrity.Three kinds of materials,layer structural LiLi_1.2NiLi_0.133CoLi_0.133Mn_0.533O₂,spinel containing structural Li_1.067NiLi_0.133CoLi_0.133Mn0.667O₂ and Li_1.1NiLi_0.133CoLi_0.133MnLi_0.633O₂,were designed and synthesized by changing the ratio of Li and Mn and keeping the content of Ni and Co unchanged.Through battery performance tests,it is found that the discharge specific capacity,the discharge voltage platform and the cycling stability are increased with the increase of lithium content.Through the study of crystal structure,it is found that the chemical state of Mn element mainly exists as the form of Mn4+ in LiLi_1.2NiLi_0.133CoLi_0.133Mn_0.533O₂,and no spinel phase is contained in this material;but the chemical state of Mn element parallelly exists as the form of both Mn3+ and Mn4+ in the other two materials,and Li4+x Mn5O12?0<x<2?spinel phase is contained in these two materials.It is found that the structural change potential of Li2 Mn O3 is affected by the spinel phase content,the structural change potential is increased with the increase of spinel phase content.Furthermore,the structural change is incomplete.According to the content of Mn3+ and spinel phase,it was inferred that the content of Mn3+ and the existence of the spinel phase has a positive correlation.In addition,the relationship between discharge curve characteristics and spinel phase content of the three kinds of materials was researched during the reaction mechanism analysis.The structural changes in cycle process of Li_1.1NiLi_0.133CoLi_0.133MnLi_0.633O₂ and LiLi_1.2NiLi_0.133CoLi_0.133Mn_0.533O₂ had been studied.The lattice transition mechanism of Li2 Mn O3 is different in the first charge process for the two kinds of materials.For Li_1.1NiLi_0.133CoLi_0.133MnLi_0.633O₂,the Mn ions migrate from the Li Mn2 layer to the Li layer,which leads to the phase transformation to spinel-like structure.For LiLi_1.2NiLi_0.133CoLi_0.133Mn_0.533O₂,the Mn ions migrate within the Li Mn2 layer inside and the crystal structure still keeps layered structure.It is found that most of the Li2 Mn O3 in Li_1.1NiLi_0.133CoLi_0.133MnLi_0.633O₂ turns into spinel structure of Li4+x Mn5O12?0<x<2?after the first cycle,and the spinel structure gradually turns into ?-Mn O₂ in the following charge process,then some of the ?-Mn O₂ gradually loses activity in the continuous circulation.The Li2 Mn O3 in LiLi_1.2NiLi_0.133CoLi_0.133Mn_0.533O₂ turns into layered structure of Li?Ni Co Mn?O₂ mixed together with Li Ni1/3Co1/3Mn1/3O₂ after the first cycle.For this material,the layered structure can be kept in the following cycles,but the structural integrity is gradually destroyed.In the cycle process,the layered structure is more favorable than the spinel structure for the lithium ion intercalation.The sol-gel process of synthesizing lithium-rich materials was improved by adding graphene oxide?GO?and reducing agent glucose in the sol solution to prevent the agglomeration of sol particles.It is found that the grain size and particle aggregation degree of the modified materials are decreased,and high rate discharge performance and voltage characteristics are significantly improved.Research shows that the particle size reduction of modified materials is advantageous to reduce the diffusion distance of lithium ions and increase the intercalation/deintercalation rate of lithium ions,so that Rct is decreased and diffusion coefficient is increased.The best modification effects are obtained when the amount of GO added in the sol solution is 5% of the theoretical quality of lithium rich material.Solution reduction method and hydrothermal reduction method were proposed to form graphene coating on the Li_1.067NiLi_0.133CoLi_0.133Mn0.667O₂ sample surface.The results show that the two methods were both effective to form the lithium-rich material/graphene composites.The electronic conductivity of the two kinds of coating materials was greatly enhanced and the ohmic polarization was reduced to about half of that of the original material.Discharge specific capacity was improved about 20³0m Ah/g than the original material,and high rate discharge specific capacity increase was particularly evident.Their discharge characteristic was better than that of the original material,too.Ethanol solution reduction and ethanol thermal reduction have been conducted to form graphene coating on the LiLi_1.2NiLi_0.133CoLi_0.133Mn_0.533O₂ sample surface.For the ethanol solution reduction method,the electronic conductivity was enhanced and the ohmic polarization was reduced when the material was coated with grapheme,and the particle aggregation degree was decreased at the same time.As a result,discharge specific capacity was increased by about 40 m Ah/g or 20 m Ah/g and voltage platform was increased by about 100 m V or 200 m V at low discharge rates or high rates respectively.In contrast,poor modification effect was obtained by the ethanol thermal reduction method.