Preparation And Electrochemical Properties Of Cathode Materials For Molybdenum-based Lithium-ion Batteries

The cathode material of lithium-ion batteries is the most important component of portable electronic devices.In recent years,the lithium-rich layered oxide Li₂MoO₃avoids the problem of oxygen release due to the electron transfer from Mo⁴⁺to Mo⁶⁺,which provides a guarantee of safety.At the same time,Li₂MoO₃ also has a high theoretical capacity of 339 mAh g^-1.These features provide a new idea for constructing novel electrode materials with Li₂MoO₃ as the structural unit.However,Li₂MoO₃ also has some disadvantages.For example,Mo⁴⁺is easily oxidized to Mo⁶⁺in the air and the conductivity of the material is poor.At high potential,there is the problem of Moion dissolution.These disadvantages greatly limit the application of Li₂MoO₃.To improve the above shortcomings,this paper explores three aspects:synthesis method,bulk doping and material composite.Through the combination of experimental characterization and theoretical calculation,the effect of structural change on the redox reaction and electrochemical performance of Li₂MoO₃ was discussed.To explore the effect of microstructure on the materials,Li₂MoO₃ cathode materials were prepared by ball milling method,molten salt method and two-step solvothermal method.SEM and TEM tests show that the Li₂MoO₃ material prepared by the molten salt method has a nanosheet-like surface structure,and the XPS test proves that the molten salt method can form a relatively closed environment during the preparation process,which helps to inhibit the formation of Mo⁶⁺.Electrochemical performance tests show that the initial discharge capacity of LMO-2 is 286.5 mAh g^-1 at a current density of 34mA g^-1,and it still has a reversible specific capacity of 102.3 mAh g^-1 after 50 cycles.To improve the structural stability and electronic conductivity of Li₂MoO₃,the Ru element was introduced into Li₂MoO₃ to construct Li₂Mo_1-xRu_xO₃ material after the molten salt method was determined.The results calculated by density functional theory(DFT)show that the Ru doping strategy does not change the crystal structure,but enlarges the interplanar spacing of the TM plane and forms stronger interplanarity between Mo_4d/Ru_4dand O_2p orbitals.Covalent interactions also lead to a significant reduction in the band gap.The theoretical calculation results are consistent with the experimental results of XRD,TEM and XPS.Moreover,the results of electrochemical performance tests show that the LMRO-2 sample can achieve a first discharge capacity of 299.1 mAh g^-1 at a current density of 1 C and a reversible specific capacity of 125.2 mAh g^-1 after 100 cycles.Meanwhile,the EIS test showed that the LMRO-2 sample had the lowest electrochemical impedance and the highest lithium-ion diffusion coefficient.To further improve the interfacial stability and Li-ion diffusion kinetics of Li₂MoO₃.Li₂MoO₃ cathode material was synthesized by molten salt method,g-C₃N₄ was synthesized by simple temperature programming,and then Li₂MoO₃/g-C₃N₄ composite was constructed by ball milling method.The g-C₃N₄ with abundant nitrogen-containing carbon skeleton can provide active sites and surface defects for the material during the reaction,and g-C₃N₄ can also reduce the corrosion of HF to the electrode material.At the same time,according to the calculation results of density functional theory(DFT),the(001)crystal planes of the two materials are well matched to form a relatively stable interface.The stable metal interface is beneficial to reduce the interface impedance and enhance the charge transfer and Li-ion diffusivity.The experimental results and theoretical calculations are also matched,and the GLMO-5 sample with a compound stoichiometric ratio of 5%has the best electrochemical performance,with a discharge capacity of 64.6 mAh g^-1 at a current density of 1700 mA g^-1,which is much larger than that of the LMO sample(29.9 mAh g^-1)under the same conditions.The GLMO-5 sample is still stable at 170.9 mAh g^-1 after 50 cycles at a current density of 34 mA g^-1.EIS further shows that GLMO-5 has the highest lithium-ion diffusion coefficient(1.94×10^-14cm~2 s^-1).