Study On Structure And Lithium Storage Properties Of Nickel-Titanium Alloy Regulated Molybdenum Trioxide For Fast-Charging Anode Materials

Lithium-ion batteries have been widely used in mobile intelligent devices due to their outstanding advantages such as high energy density,long cycle life and low pollution.However,due to low theoretical specific capacity and poor rate performance of commercial graphite anode,lithium-ion batteries cannot meet their urgent needs for electric vehicle power batteries with long endurance and fast charging capacity.Because of limited capacity improvement of new cathode materials,developing new anode materials with high specific capacity,excellent rate performance and long life,which is great significance for promoting the development of fast-charging lithium-ion power batteries.Among all the alternative materials,molybdenum trioxide material has the advantages of high theoretical specific capacity(1117 m Ah g^-1),environmental friendliness,diverse structure and low cost.However,there are also some key issues such as large volume changes（300%）during charging and discharging,severe capacity attenuation during cycling,low intrinsic electron conductivity,low ion diffusion rate and large voltage hysteresis.In this paper,focusing on the prominent shortcomings of slow Li ion diffusion and poor invertibility of conversion reaction in molybdenum trioxide anode materials,in order to improve the fast charging performance of molybdenum trioxide anode materials,a simple one-step hydrothermal method was used to introduce transition metals Ti and oxygen vacancy into molybdenum trioxide materials using nickel-titanium alloy powder as the additive for the first time,so that to improve its intrinsic structural properties and lithium storage characteristics.A series of excellent performance molybdenum trioxide anode materials have been synthesized by adjusting the adding amount of nickel-titanium alloy powder（Ni Ti）,the amount of nitric acid in the reaction system and the reaction time.The synthesized materials include flower-like titanium-doped orthogonal Mo O₃（Ti-α-Mo O₃）,flower-like titanium-doped anoxic orthogonal Mo O₃(Ti-α-Mo O_3-x),and two-dimensional nano-lamellar titanium-doped anoxic hexagonal Mo O₃ coated by a layer of Ti O₂(Ti-h-Mo O_3-x@Ti O₂).The main contents of the paper are as follows:Firstly,the structural properties and lithium storage properties of hexagonal M-h-Mo O_3-xmaterials doped with transition group metal element M（M=Sc,Ti,Zr,V,Cr,Mn,Fe,Co,Ni）are calculated by First Principles.It is found that the doping of Ti element can make the hexagonal h-Mo O_3-x material show the best electronic structure and it is favor to be synthesized.Furthermore,it is found that the hexagonal Ti-h-Mo O_3-x material has better lithium storage capacity and lithium diffusion capacity than orthogonalα-Mo O₃ material,and its theoretical specific capacity is more than 50%higher than that of orthogonalα-Mo O₃material.Secondly,a flower-like hierarchical structure Ti-α-Mo O₃ was designed and constructed by one-step hydrothermal method by adjusting the adding amount of Ni Ti and reaction time.It was found that the reasons cause that from the porous Ti core by preferentially corroding Ni atoms in the dealloying process of Ni Ti,and the preferred growth direction of Mo O₃nuclei changed by the continuous releasing of Ti atoms.Ti⁴⁺can be doped intoα-Mo O₃ structure to increase the（020）layer spacing,thus increasing the intrinsic conductivity and Li⁺transport performance of orthogonalα-Mo O₃ material,which is consistent with the theoretical calculation results.The Ti-α-Mo O₃ material shows excellent electrochemical performance used as the anode of Li-ion battery.At the current density of 100 m A g^-1,it exhibits as high as initial Coulombic efficiency（ICE）of 92.6%.At the current density of 1 A g^-1,the Ti-α-Mo O₃still shows a reversible specific capacity of 425 m Ah g^-1 after 1000 cycles.Moreover,through in-depth analysis of the lithium storage mechanism,it is found that the Li⁺intercalate into Mo O₃ for the first time cannot completely escape during deintercalation period,but it eventually forms the Li_0.33Mo O₃ phase.Since the second cycle,the Li_0.33Mo O₃ phase and Li₂Mo O₃ phase of the Li⁺intercalation and deintercalation process,and the subsequent conversion reaction mechanism has not changed,which is the same with the previous result.Thirdly,the flower-like Ti-α-Mo O_3-x material assembled by nanosheets were prepared by one-step hydrothermal method through precise regulation of the amount of nitric acid added in the reaction system.It was found that the material shows superior rate performance,and the reversible specific capacity of 707.1,579.1 and 498.5 m Ah g^-1,after 500 cycles at the current density of 1 A g^-1,2 A g^-1 and 3 A g^-1,respectively.Moreover,by analyzing the d Q/d V curve and combining with the microstructure characteristics of the Ti-α-Mo O_3-x material,it is concluded that the coexistence of oxygen vacancy and Ti⁴⁺doping can effectively reduce the displacement length of Li⁺and Mo⁶⁺during the cycling of the electrode material,and narrow the difference of reaction kinetics during lithiation/delithiation process.Therefore,the voltage hysteresis phenomenon of the molybdenum trioxide anode material can be improved to a certain extent.Finally,the Ti O₂ coated two-dimensional nanosheet Ti-h-Mo O_3-x@Ti O₂ material was successfully prepared by one-step hydrothermal method through the regulation of reaction time.It is found that the material exhibits high reversible specific capacity and extraordinary rate performance.At the current density of 1 A g^-1,the reversible specific capacity of charging/discharging after 800 cycles is 1321.3/1326.8 m Ah g^-1,which exceeds the theoretical specific capacity of orthogonalα-Mo O₃ material,and is consistent with our theoretical predictions.Even at the higher current density of 5 A g^-1,the reversible charge/discharge capacity can still reach as high as 606.6/611.2 m Ah g^-1 after 2000 cycles.It is mainly attributed to the facts that the preferred orientation of Ti-h-Mo O_3-x@Ti O₂ along the（100）crystal plane can significantly increase the electrochemical reactive surface area,and the unique one-dimensional channel of hexagonal h-Mo O₃ can shorten the transport path of electrons and Li⁺,and the existence of Ti O₂ coating layer can effectively buffer the large volume changes generated during lithiation/delithiation.