Preparation Of Novel Unusual Transition Metal Oxide Energy Storage Materials Based On Molten Salt Electrochemistry

With the launch of various new energy storage devices and the construction of a new generation of power grid platforms,the sustainable development of our society requires urgently new structural and functional materials of high performance,long life and low environmental impact,and especially novel technologies in the production of these energy storage materials to reduce energy consumption,pollutant emission,and waste of resources.High temperature molten salts have unique physicochemical properties that can accommodate material production processes,especially the electrochemical ones,in very simple and environmentally friendly manners that promise very wide applications.In particular,molten salt electrochemical synthesis have attracted growing attention for the green production of transition metal oxide and other new energy storage materials.Recently,a facile and environment friendly method with low energy consumption and controlable reaction conditions to electrochemically extract metals and alloys directly from solid oxides in molten salts has received a great deal of attention from academic and industrial.Based on the feasibility in achieving multiple intermediate valences during the electrochemical reduction of solid metal oxides in molten salt,new functional materials for applications in the field of energy storage could be synthesized via novel electrochemical reduction in molten salts.In this dissertation,we have systematically studied the reduction processes of transition metal oxides in molten salts of various alkali metals chlorides using electrochemical technologies,and characterized the reduction products by spectroscopic analyses.Moreover,the effects of size,morphology,and material composition on the electrochemical performance of materials are investigated.The main researches and results are summarized below.1.A novel mothed of electrochemical synthesis of Li-Mo-O for lithium ion storage was developed for the first time.A series of Li-Mo-O（Li₂MoO₄@C and LiMoO₂@C）function materials were synthesized by electrolysis of solid-state MoO3 in LiCl-LiBr molten salt.Cyclic voltammetry using the MoO₃ powder filled Mo cavity electrode has revealed that the Li⁺insertion into MoO₃ takes place in two steps.Kinetic analysis has suggested that the diffusion of Li⁺in the solid oxide is the speed determining step for the formation of the Li-Mo-O compounds.Both Li₂MoO₄@C and LiMoO₂@C show high capacity and excellent cycling performance as cathode for lithium ion battery.After 200cycles,both Li₂MoO₄@C and LiMoO₂@C delivered stable reversible capacity 473 and565 mAh g^–11 for lithium storage,respectively.The LiMoO₂@C electrode delivers a high initial coulombic efficiency 72%,outstanding rate capability of 322 mAh g^-1at 1.6 A g^-1.The effects of various carbon sources on the performance of LiMoO₂ batteries were further explored.2.The insertion reactions of Na⁺into solid MoO₃ were systematically studied in respective NaCl-NaF-NaI molten salt.Na₂MoO₄@C product has high capacity and excellent cycle performance as a sodium ion anode material.Na₂MoO₄@C electrode delivered a reversible capacity 218 mAh g^-1at 0.2 A g^-1.Even at large current 0.5 A g^-1,a capacity 75 mAh g^-11 can still be achieved after 1400 cycles.Na₂MoO₄@C electrode exhibited excellent rate and cycling performance.3.In-situ carbon-coated TiO₂（TiO₂@C）was prepared by solvothermal reaction.The electrolysis product of solid TiO₂@C in molten KCl was hollandite Kx Ti8O16 thin rods placed in a star arrangement.The structure of this material with high stability could promote the de-intercalation of lithium ions and had a good cycle perfromance as an anode material for lithium ion battery.The K_1.35Ti₈O₁₆@C electrode had a reversible capacity 320mAh g^-1at 50 mA g^-1and 135 mAh g^-1after 2000 cycles at 1 A g^-1,this indicates excellent long cycle stability.The mechanism of lithium storage in K_1.35Ti₈O₁₆@C composites was further analyzed by ex-situ XRD.4.Li-V-O compound was obtained by electrochemically electrolyzing V₂O₅ in LiCl-LiBr molten salt at a specific voltage.The V₂O₅ powder filled Mo cavity electrode was investigated in molten LiCl-LiBr,confirming the mechanism for the electrochemical reduction of solid V₂O₅ to Li_0.37VO₂ in accordance with the process of V₂0₅?Li₃VO₄?Li_0.37VO₂.Li_0.37VO₂@C showed high capacity and excellent cycling performance as lithium ion battery cathode.After carbon coating,the Li_0.37VO₂@C delivered a high reversible capacity 625 mAh g^-11 at 100 mA g^-1.Moreover,the Li_0.37VO₂@C electrode had excellent rate performance.The discharge specific capacity can still be maintained at 190mAh g–1 after 300 cycles at a high current density 3.2 A g^-1.The conversion mechansim of Li_0.37VO₂@C materials are revealed by ex-situ XRD.And further optimized the composition of Li0.37VO₂ and C,10%of Super-P as a precursor can effectively improve the conductivity of Li_0.37VO₂.5.For the first time,constant voltage electrolytic synthesis of NaNbO₂@C compounds was conducted in molten salt NaCl-NaF-NaI with a graphite anode and an assembled cathode of Nb₂O₅ porous pellet.The NaNbO₂@C was firstly reported as a negative electrode material for sodium ion batteries with good sodium storage performance.NaNbO₂@C can provide a reversible sodium storage capacity 160 mAh g^-1at 100 mA g^-1.After 500 cycles,the NaNbO₂@C electrode still provided reversible sodium storage capacity 86.4 mAh g^-1at a current density 500 mA g^-1,possessing excellent cycling performance.And the Na storage mechanism of the NaNbO₂@C was futher elaborted.