Preparation Of Multiple Transition Metal Oxides/Sulfides As Conversion-Type Anode And Investigation Of The Battery Performance

Lithium ion batteries（LIBs）have been widely applied in the fields of portable electronic devices,electric vehicles and power stations since its commercialization,and have become one of the greatest inventions of the 21^st century as well.Recently,with the rapid development of electric vehicles industry and power stations,new-generation LIBs should be developed with higher energy density,higher power density and lower cost as to meet the ever increasing demands.The graphite based materials are the widely used anodes in LIBs so far,which only deliver a theoretical capacity of 372 m Ah g^-1 and hindering the development of high energy-density batteries.Therefore,it is urgently need to develop new anode materials with higher capacity and stable cycle performance for new-generation LIBs.The transition metal oxides/sulfides based on the conversion mechanism has attracted wide attentions due to their higher capacity and lower potential platform.However,transition metal oxides/sulfides experience electrode pulverization,successive electrolyte decomposition at the interface of electrode/electrolyte which deteriorates the cycle life of batteries and prohibits their practical applications.Therefore,the development of new transition metal oxides/sulfides materials with the improved stability is the only way to promote the practical application of such materials.Based on this,the works mainly includes:（1）Sn S2 nanosheets was synthesized by hydrothermal method and characterized as pure 1T phase by TEM.Compared with commercialized Sn S2 mixed with 1T and1H phase,it displays a higher capacity,lower voltage plateau and lower charge-transfer resistance.However,the Sn S2 nanosheets delivered only 443.5/438.2m Ah g^-1 of discharge and charge capacity at the 30^th cycle,only about 30%of the initial capacity.The morphology of cycled Sn S2 electrode indicates that the micro-morphology of Sn S2 is destroyed and the decomposition of electrolyte takes place on the surface of electrode.There is a thicker solid electrolyte interface（SEI）layer on the surface of electrode,then tremendously damaging the cycle life of1T-Sn S2.（2）In order to improve stability of metal oxide electrodes,four-component entropy-stabilized metal oxides（MEOs）nanoparticles were synthesized by hydrothermal method as anodes for LIBs at first.The MEO prepared at 200°C for 6 h deliver the best cycle performance(discharge capacity of 730.9 m Ah g^-1@2^nd cycle;755.8 m Ah g^-1@90^th cycle)and rate performance(the capacity retention is 92.04%returned to the current intensity of 100 m A g^-1after 50 cycles).Then high entropy metal oxide（HEO）composed of five elements was further prepared by co-precipitation method,whose morphology,composition,and electrochemical properties were investigated carefully by XRD,SEM and electrochemical tests.The average particle size of HEO is 0.5-3μm.Compared with the single/mixed/medium entropy transition metal oxides,HEO exhibits excellent cycle stability(capacity retention of 99.6%/814.9 m Ah g^-1@115^th cycle).The active particles on the electrode surface after cycling are uniform,which maintain the integrity and uniformity of the electrode,without obvious crack and thick SEI layer.The results show that the HEO based materials can withstand the stress changes during the discharging and charging process,and display a weak effect on catalyzing decomposition of electrolyte,so as to maintain an excellent cycle stability.（3）Na Ge3/2Mn1/2O4（NGMO）was synthesized by solid state method,whose main phase was indexed into Na4Ge9O20 with a tunnel structure.NGMO shows an average particle size of 1-3μm with good crystallinity.The tunnel structure is in favor of the insertion and de-insertion of Na⁺,which effectively relieves volume expansion and shrinking.Compared with pure Na4Ge9O20,NGMO displays a higher energy density(200.32 m Ah g^-1@2^nd cycle;173.8 m Ah g^-1@50^th cycle,while NGO delivered 27 m Ah g^-1in the 50^th cycle),which also delivers a safe voltage plateau of～0.36 V.In conclusion,the design of multi-components transition metal oxides/sulfides would promote the cycle stability as anode of LIBs.In HEO,the formation of solid solution due to the intensive mixing and the entropy-stabilized effects together promote the Li-storage stability.Moreover,the fast ionic diffusion capability in HEO endow its high rate performance.Therefore,the HEO based mateirals are promising anodes for LIBs.For SIBs,the low-cost,tunnel-structure anodes reduce the volume changes resulting from the insertion/desertion of Na⁺with large radius,which undoubtedly provides a new reference for the development of anodes for SIBs.