Electrochemical Properties And Energy Storage Mechanism Of Molybdenum-based Compounds As Novel Anode Materials

Rechargeable batteries,as the highly-efficiency energy storage and transformation devices,play a vital role in the clean energy development and application.However,the low theoretical capacity of commercial graphite anode of 372 m A h g^-1 cannot meet the great demand of high-energy-density lithium-ion batteries?LIBs?.Also,similar situation occurs in the studies of sodium-ion batteries?SIBs?.Especially,the low sodiation voltage is likely to form sodium dendrite,with the result of serious security issues.So,developing the novel anode materials with high specific capacity and high security is becoming more and more important.4d transition metal ion,Mo,has a diversity of valence changes ranging from 0 to +6,which shows a stronger ionic storage ability than 3d transition metal ions.More importantly,its higher potential avoids the formation of dendrite,which makes molybdenum-based compounds safer than graphite and hard carbon.Thus,Mo-based compounds are considered as one of the best choices in the anode materials with highly potential and performance.However,it should be noted that the unavoidable volume expansion and extraction of molybdenum-based anodes could result in the bad cycle stability and rate capability.We chose molybdenum-based compounds as the research target,focused on the ionic storage mechanism of these compounds and studied their structural and phase evolution during the electrochemical reaction.Also,by taking advantage of the synergic effect of different kinds of reaction mechanisms,we found several kinds of molybdenum-based electrode materials with excellent cycle and rate performances.Conclusions are given as followings:Firstly,we prepared polyanion-type material Li Fe?Mo O4?2 by solid-state reaction.As the anode materials for LIBs,Li Fe?Mo O4?2 gave a high specific capacity of 1034 m A h g^-1,corresponding to nearly 15 Li+ insertion.By using ex-situ XRD and TEM,we studied its phase evolutions at different discharged stages.As the increasing in the inserted lithium ions,an irreversible structural transition from triclinic to cubic symmetry was observed.When being discharged to 0.01 V,the anode material decomposed into Fe metal,Mo metal and Li2 O with the high discharge capacity of 1034 m A h g^-1.Secondly,we successfully synthesized brannerite-type Li VMo O₆,which exhibited high reversible capacity and good rate capability.Between 3.0 and 0.01 V,it showed a high specific capacity of around 910 m A h g^-1 and 584 m A h g^-1 under high applied current density of 2 A g^-1.When the inserted lithium was higher than 0.2 mol,Li VMo O₆ electrode was likely to decompose into Li2_Mo₂O₇ and V₂O₅,which acted as the mother compounds for the subsequent lithiated processes.When being discharged to 0.01 V,most of the V⁵⁺ were reduced to V³⁺ or V²⁺,dominating by insertion/extraction mechanism;while,Mo⁶⁺ would be reduced to Mo metal according to the conversion reaction.It was found that the synergic effect of these two mechanisms could induce the excellent cycle stability.When molybdenum oxides experienced the?de?lithiated processes,vanadium oxides played as the “matrix” to control the volume expansion.Li VMo O₆ is considered as the high-performance anode material for LIBs owing to the high reversible capacity and excellent rate performance.Following the synthesis strategy,we designed and prepared a new brannerite-type material Na VMo O₆,and studied its electrochemical properties both in LIBs and SIBs.In LIBs,Na VMo O₆ showed a high reversible capacity of 880 m A h g^-1,much higher than that value in SIBs(about 170 m A h g^-1).We also studied the phase evolution of Na VMo O₆ in both batteries.In LIBs,both the insertion/extraction processes of vanadium oxides and conversion reaction of Mo-based oxides worked together.In SIBs,Na VMo O₆ decomposed into Na₂ Mo O₄ and V₂O₅,which worked as the host compound for sodium storage with relatively low discharge capacity.At last,we studied the sodium storage properties of triclinic Ag₂Mo₂O₇ as the anode material for SIBs.At the current density of 20 m A g^-1,the anode delivered the discharge capacity of 190 m A h g^-1 after 70 cycles.Also,it showed a good long-term cycle stability with discharge capacity of around 100 m A h g^-1 for 1000 cycles at 500 m A g^-1.At the first discharge to 0.01 V,Ag₂Mo₂O₇ electrode decomposed into Na₂ Mo O₄ and Ag nanoparticles.The electrochemical inactive Ag was not involved into the subsequent charge-discharge processes and could increase the electronic conductivity of the working electrodes.And sodium ions were suggested to be inserted into Na——2 Mo O-4,with manifested good cycle stability.In this dissertation,we studied the electrochemical reaction of several molybdenum-based anodes in LIBs or SIBs.Besides acquiring several kinds of high-performance anode materials,we mainly focused on the evolution of the long-term and short-term structural evolutions,changes in the ionic valences,morphology and crystallization.We also discussed the influence of ionic storage mechanism on the cycle and rate performance.We think the insight obtained in this study is beneficial to the design of new anode materials in future.