Exploration Of Molybdate And Tungstate Anode Materials For Lithium Ion Batteries

The rapid development of application field puts forward higher requirements for the performance of lithium-ion batteries（LIBs）,especially energy density and temperature adaptability.The performance of battery mainly depends on the performance of cathode and anode materials.Compared with cathode materials,commercial anode materials have fewer candidates and worse performance.Therefore,the design and synthesis of new high-performance anode materials is the key to improve the performance of LIBs.In this thesis,a series of polyanionic molybdate and tungstate anode electrode materials were designed and synthesized,and their physicochemical properties and electrochemical properties were studied.Both Li Cr（Mo O₄）₂/C and Li₃Cr（Mo O₄）₃/C have a multi-dimensional ion transport channel,which can provide fast ion transport.Carbon-coating provides sufficient electronic conductivity and good electrolyte wetting property.Therefore,Li Cr（Mo O₄）₂/C and Li₃Cr（Mo O₄）₃/C display good electrochemical properties at room temperature and low temperature.In the voltage range of 0.01～3.00 V vs.Li⁺/Li,Li Cr（Mo O₄）₂/C and Li₃Cr（Mo O₄）₃/C show high specific capacities of 1158 and1077 m Ah·g^-1at 50 m A·g^-1,respectively.Moreover,they present excellent rate performance and cycle performance.Li Cr（Mo O₄）₂/C delivers a specific capacity of 352m Ah·g^-1at 5000 m A·g^-1.At 250 m A·g^-1,both Li Cr（Mo O₄）₂/C and Li₃Cr（Mo O₄）₃/C maintain a reversible capacity of more than 600 m Ah·g^-1after 500 cycles.Even at low temperatures of 0 ^oC,-10 ^oC and-20 ^oC,Li₃Cr（Mo O₄）₃/C still provides high capacities of1061 m Ah·g^-1,891 m Ah·g^-1and 742 m Ah·g^-1,respectively.The introduction of K⁺with larger radius can expand the ion transmission channel and reduce the cost.Therefore,this paper synthesized and studied the electrochemical properties of K₂M₂（Mo O₄）₃/C（M=Ni,Co,Mn）.These compounds show good lithium storage performance and temperature adaptability.K₂M₂（Mo O₄）₃/C（M=Ni,Co,Mn）delivers a high discharge specific capacity over 1000 m Ah·g^-1at 50 m A·g^-1.K₂Mn₂（Mo O₄）₃/C retains a reversible capacity of 586 m Ah·g^-1after 100 cycles at room temperature.The specific capacities of K₂Co₂（Mo O₄）₃/C are 935,896 and 651 m Ah·g^-1at50 m A·g^-1at 0 ^oC,-10 ^oC and-20 ^oC,respectively.Low-temperature CV and EIS tests show that the decrease of electrochemical activity and charge transfer ability are the main reason for the decline of low temperature electrochemical performance.Tungstate has similar functional and structural groups to molybdate,and also shows good lithium-ion storage capacity.The unique three-dimensional skeleton structure of Li₂Ni（WO₄）₂provides a large channel for the migration of lithium ions and good stability.In addition,the conductive carbon layer coated by ball milling further enhances the electronic conductivity of the material,reduces the particle size and improves the electrochemical reaction area.Based on the above advantages,Li₂Ni（WO₄）₂/C anode material shows a good lithium storage performance and electrochemical reversibility.To further improve the stability,the working voltage is set to 0.50～3.00 V.At 20 m A·g^-1,the specific capacity of Li₂Ni（WO₄）₂/C reaches 325 m Ah·g^-1.Under a high current density of2000 m A·g^-1,the reversible capacity of 86 m Ah·g^-1is still maintained after 500 cycles.This thesis enriches the material system of anode materials,deepens the understanding of the electrochemical and low-temperature properties of polyanion anode materials,and provides some experience and ideas for the exploration of new high-performance anode materials.