Fundamental Study On Solid-state Electrolyte For All-solid-state Lithium Battery

Lithium-ion batteries have rapidly become the dominant power sources for portable electronic devices,electric vehicles and energy storage systems,due to their high energy density and long cycle life.However,safety issues still prevent full utilization of these batteries owing to the use of flammable liquid electrolytes,and thus safety problems have become a significant concern especially in large capacity applications such as electric vehicles and energy storage systems.In this respect,the development of all-solid-state lithium batteries with non-flammable solid electrolytes may provide a fundamental solution to the safety issue of lithium batteries.Inorganic solid electrolytes present potential advantages,such as absence of electrolyte leakage,high electrochemical stability,non-flammability,high thermal stability and absence of problems relating to vaporization of organic solvents.In the past decades,many researches have been conducted on solid-state electrolytes such as NASICON-type Li+ion conductors with a general formula of LiM2(PO4)3(M = Sr,Ti,Ge,Zr,Sn,and so on.).Among the discovered solid-state electrolytes,Li1.5Al0.5Ge1.5(PO4)3(LAGP)conductive microcrystalline glass is so far regarded as a hopeful candidate for inorganic solid electrolyte materials,which shows high Li+ ion conductivity,excellent chemical stability,wide applicable temperature range and low cost.Li+ ion conductive microcrystalline glass is fabricated by crystallizing its basic glass at high temperature,so that the deformation easily occurs during annealing treatment.Unfortunately,many obvious flaws must be improved to realize the practical application of LAGP in all-solid-state Li batteries.First,a dense and uniform electrolyte membrane is difficult to produce directly using LAGP raw material.Second,the solid LAGP material shows poor interface contact performance with electrodes.Finally,LAGP is unstable during contact with the Li metal.Aiming at the above-mentioned problems,we undertake the research work from two sides of both electrolyte and electrodes/electrolyte interfaces performances.The main results were summarized as follows.1.LAGP solid electrolyte was prepared by high temperature solid-state fusion method.In order to realize the practical application of LAGP in all-solid-state Li batteries,we prepared LAGP-PEO(LiTFSI)composite solid electrolyte using LAGP,polyethylene oxide(PEO)and lithium bis(trifluoromethanesulfonyl)imide(LiTFSI).The effects of PEO content in the composite solid electrolyte were studied.The results showed that the ratio of PEO in the composite solid electrolyte can be reduced to a low level of 1 wt.%and the PEO remains stable even at a high potential of 5.12 V(vs.Li/Li+).2.In order to prevent the direct contact of LAGP from Li metal,PEO(LiTFSI)was coated on the Li anode.The effects of the molecular weight of PEO used to modify the Li anode were studied.The results showed that PEO-500000(LiTFSI)film on a Li anode possesses good mechanical properties and satisfactory interface contact features.The PEO-500000(LiTFSI)film can also prevent Li from reacting with LAGP.Furthermore,the formation of lithium dendrites can be effectively inhibited as the composite solid electrolyte is combined with the PEO film on the Li anode.3.In order to prevent the direct contact of LAGP from Li metal,LiPON was deposited on the Li anode by radio-frequency(RF)magnetron sputtering method with Li3PO4 as the target material.The effects of the thickness of LiPON on Li anode performance were studied.The results showed that LiPON film with a thickness of 500 nm exhibits satisfactory interface property in both Li metal anode and the LAGP-PEO(LiTFSI)solid electrolyte.The LiPON film can provide a uniform Li+flux across the interface and effectively inhibit the formation of Li dendrites in all solid-state Li batteries.The assembled all-solid-state lithium battery Li(LiPON)/LAGP-PEO(LiTFSI)/LiFePO4 can deliver an initial discharge capacity of 154.4 mAh g-1 and exhibit good cycling stability and rate performance at 50 ℃.4.The interfacial properties between different cathode materials and composite electrolytes were studied.The results showed that the interfacial properties between cathode materials(LiFePO4,LiFeo.2Mno.8PO4)and composite electrolytes were good.In addition,it was found that the composite electrolyte can effectively prevent the occurrence of Jahn-Teller effect.Finally,the self discharge test of all-solid-state lithium batteries shows that there was not self-discharge phenomenon in all-solid-state lithium batteries.