Synthesis And Performance Study Of Nb₂O₅ Based Magnesium-lithium Hybrid Ion Battery Cathode

Magnesium-lithium hybrid ion batteries（MLIB）combine the advantage of rapid diffusion kinetics in lithium ion batteries and high safety of metal Mg anode,which can significantly broaden the selection range of traditional magnesium ion batteries cathode materials,and thus is regarded as a new type of secondary battery with promising development prospects.As a typical intercalation-type electrode material,Nb₂O₅ has high stability and strong compatibility of charge/discharge potential window with the electrolyte of MLIB.Moreover,its unique crystal structure can provide a channel for rapid ion diffusion and has a great potential in terms of high rate performance.However,the intrinsic low electronic conductivity of Nb₂O₅leads to undesirable electron transfer rate and excessively high ohmic resistance when used as an electrode material,which seriously affects the electrochemical performances.In allusion to the issues of Nb₂O₅,the strategies such as micro-nano structure construction and heteroatom doping are employed to modify Nb₂O₅,which improve the electronic conductivity and further facilitate ion diffusion,resulting in improving electrochemical performances of Nb₂O₅ in MLIB.Nb₂O₅ holey nanosheets with different crystal structures（TT-Nb₂O₅,T-Nb₂O₅ and H-Nb₂O₅）are synthesized by a graphene oxide sacrificial template method and corresponding electrochemical performances in MLIB are investigated,and the effect of holey nanosheet structure on the performances of T-Nb₂O₅ is also explored.The results show that the unique quasi-two-dimensional ion diffusion channel bestows T-Nb₂O₅the strongest pseudocapacitance effect,and thus exhibiting the best electrochemical performances among three different crystal structures,its mechanism is single-phase solid solution reaction.Holey nanosheet structure provides fast mass transfer channels for the material,which is conducive to improve the rate performance of the material.Moreover,holey structure can also alleviate the volume expansion caused by ion intercalation and enhances structural stability.Based on the above advantages,the capacity of T-Nb₂O₅holey nanosheets maintains 73.3 m Ah g^-1 after 300 cycles at 5C and corresponding capacity retention is 73.2%.Although the crystal structure of T-Nb₂O₅ provides quasi-two-dimensional channel for the rapid diffusion of ion,it is limited by poor electronic conductivity,resulting in undesirable rate performance and specific capacity.In allusion to this issue,the surface of T-Nb₂O₅ array electrode is coated with high-conductivity Mo O₂through hydrothermal reaction,dipping and heat treatment to construct T-Nb₂O₅@Mo O₂heterostructure.The heterostructure exhibits better electronic conductivity,which is conducive to the electron transfer in electrochemical process.In addition,theoretical calculation results show that the heterostructure prompts the redistribution of electrons at the heterointerface of T-Nb₂O₅ and Mo O₂,and the formed built-in electric field significantly reduces ion diffusion energy barrier,resulting in facilitated ion diffusion.Based on the above advantages,the capacity of T-Nb₂O₅@Mo O₂heterostructure maintains 76.3 m Ah g^-1 after 1500 cycles at5C and corresponding capacity retention is 76.0%.Heteroatom doping strategy is adopted to adjust the energy band structure of the material in order to effectively enhance the bulk conductivity of T-Nb₂O₅,and is not restricted to improve surface conductivity in the previous work.The V-doped T-Nb₂O₅ is synthesized through solvothermal reaction and heat treatment to explore the influence of V-doping on the electrochemical performance of T-Nb₂O₅and the optimal V-doping amount.The results show that V-doping not only reduces the band gap of T-Nb₂O₅,improving the bulk conductivity of the material,which is conducive to the electron transfer in the electrochemical process,but also expands the interlayer spacing of（001）plane in T-Nb₂O₅,promoting ion diffusion,and thus improving the rate performance of the material.Moreover,V doping can enhance the stability of the T-Nb₂O₅ structural framework,resulting in improving the cycling performance of electrode material.Based on the above advantages,the capacity of optimized V-doped T-Nb₂O₅（when V doping amount is 3%）maintains 119.3 m Ah g^-1 up to 5000 cycles at 5C and corresponding capacity retention is 82.2%.