Preparation And Electrochemical Properties Of Rice Husks Based Li₂MnSiO₄ Cathode Materials

With the energy and environmental issues gradually becoming the focus of attention all over the world,it is imminent to develop a new type of green energy.Lithium-ion batteries?LIBs?are widely used in various portable electronic devices and new energy vehicle market owing to their advantages of large energy density,long cycle life,high operating voltage and environmental pollution-free.However,the lower capacity of positive electrode becomes the key factor,which restricts its development.At present,low-cost and pollution-free materials lithium manganese silicate?Li₂MnSiO₄?has become one of the most promising cathode materials for lithium-ion batteries because of its high theoretical capacity(333 mAh g^-1)and good thermal stability.However,the preparation of pure Li₂MnSi O₄ is rather difficult,and there are many internal defects,such as low electron conductivity and poor ion diffusivity.In addition,the structure of Li₂MnSiO₄ is prone to irreversible changes in the cycle,leading to a rapid drop in cycle capacity,which limits its application in next-generation lithium-ion batteries.In view of the above problems,carbon coating and particle size reduction were used to modify Li₂MnSiO₄ materials in this paper.Furthermore,this paper introduced a method for efficient utilization of biomass rice husks.The lithium manganese silicate cathode material coated with porous carbon was prepared by using high temperature solid-state method.In the synthesis process,silicon dioxide in rice husk and cellulose,lignin and hemicellulose were selected as silicon source and carbon source,respectively.Finally,the electronic conductivity of Li₂MnSiO₄/C composites was further improved by using excellent conductivity of graphene and carbon nanotubes.The main research contents and conclusions are as follows:?1?Li₂MnSiO₄/C cathode materials were synthesized by high temperature solid-state method,using LiOH·H₂O,MnCO₃,SiO₂ as raw materials and citric acid as carbon source.The results show that the prepared samples have high crystallinity indexing to the orthorhombic of the Pmn2₁ space group.The sample particles are nanocubes with regular morphology and relatively uniform distribution.The size is 50 nm,and the thickness of the carbon layer coated on the surface is about 5 nm.The electrochemical test results indicate that the sample has a high initial discharge capacity of 245 mAh g^-1at 0.2 C and a reversible capacity of 188.8 mAh g^-1after 50 cycles,the capacity retention rate is above 77%,exhibiting excellent electrochemical performance.?2?Biomass rice husks were used as raw materials to prepare the porous Li₂MnSiO₄/C cathode materials under the protection of argon,using the method in Chapter III.ZnCl₂ was used as an activator to treat rice husks,and their effects on the morphology and properties of the rice husks were also studied.The results show that the cubic Li₂MnSiO₄ nanoparticles are uniformly distributed in the porous carbon layer of rice husks after ZnCl₂ activation,exhibiting excellent electrochemical performance.The initial discharge capacity is as high as321 mAh g^-1at 0.2 C,which is close to the theoretical capacity of Li₂MnSiO₄.The discharge capacity can still reach 242.2 mAh g^-1after 50 cycles,which is significantly better than LMS/C obtained directly carbonized rice husks.?3?Under the protection of argon,the effects of calcination temperature and time on the morphology and crystallinity of the composite were studied by adjusting the reaction parameters in the synthesis process.Results show that the the appropriate reaction time and temperature can promote the growth of sample lattice to obtain more complete and more homogeneous grains,which affects the electrochemical properties of composite materials.And the optimum reaction conditions are calcined 700°C for 8 h.Compared with chemical reagent SiO₂,Li₂MnSiO₄ prepared from rice husks has no obvious difference in structure and morphology.However,it shows excellent electrochemical performance because of its unique porous structure,which has a great potential for development.?4?In order to further improve the conductivity of composites,graphene and carbon nanotubes were respectively used to compound them.Firstly,the modified Hummers method was used to prepare graphene oxide,and the carbon nanotubes?MWCNTs?were treated with HNO₃.Then,5 wt%of the graphene oxide and acidified MWCNTs were taken respectively,combined with porous LMS/C-700/8 obtained under the optimal condition to synthesis porous LMS/C-rGO and porous LMS/C-MWCNTs composites.The experimental results show that the addition of graphene and MWCNTs did not change the crystal structure of Li₂MnSiO₄,and the Li₂MnSi O₄ nanoparticles are uniformly dispersed in the three-dimensional network structure composed of graphene with rice husks carbon and porous carbon layer with MWCNTs,respectively,displaying excellent electrochemical properties.After 50 cycles at 0.2C,the discharge capacity of porous LMS/C-rGO is maintained at 254.7 mAh g^-1,and the reversible specific capacity of porous LMS/C-MWCNTs is also 251.9 mAh g^-1.These composites have excellent cycle stability even at high rate.After 100 cycles at 0.5 C,the reversible capacities of porous LMS/C-rGO and porous LMS/C-MWCNTs are 179.1 mAh g^-1and 185.3 mAh g^-1,respectively.