| The demand for energy is increasing with the rapid development ofsocial economy and technology.We are facing the severe problem of energy exhaustion because the traditional fossil energy in the earth is limited and non-renewable.At the same tiome,the excessive use of fossil energy has caused serious environmental problems such as river pollution and air pollution.In the era of sustainable development,it is extremely urgent to study and apply renewable clean energy.However,new energy such as wind,tidal and solar energy cannot directly provide usable energy for human beings restricted by the environment.Energy storage equipments are needed to convert and store them.In addition,the development of wearable devices and electric vehicles puts forward higher demands for energy storage devices and materials including large capacity,strong stability,non-pollution,low cost and safety.Lithium-ion battery can meet the need of social development with high energy density,long cycle life and low self-discharge rate and other advantages.The application of commercialized liquid lithium battery is limited due to packaging difficulties,shape limitations and poor safety.Solid-state electrolyte with wide electrochemical stable window and strong safety has solved many problems of traditional liquid battery.In this work,PDVF-based solid polymer electrolyte was prepared by solution casting method,and the influence of LLZTO doping on the structure and performance of the electrolyte was studied.The results showed that LLZTO provided an alkaline environment for PVDF,which led to the defluorination of PVDF,reduced the crystallinity of the polymer and improved the lithium ion conductivity.The electrochemical stability window of composite polymer electrolyte(CPE)was 2 V?5 V,and it was stable to lithium during the cycle,which can realize the application of high-potential cathode materials and lithium metal anode.The CPE with 20 wt.%-LLZTO applied to the following research has the best electrochemical performance,with ?=1.15×10-3 cm-1,Ea=0.23 eV at 50?.LiNi0.8Co0.15Al0.05O2 is regarded as one of the most promising cathode materials for its high energy density and low cost.However,the formation of rock salt phase on the surface of secondary particles will lead to structure failure and capacity attenuation during cycles.Therefore,the effect of surface phase transition on the interface and cycle performance was studied based on CPE.Intergranular fracture,even transgranular fracture occurred on the surface of cathode after cycles,which resulted the formantion of surface crack and the increase of inpedance.XRJD and TEM were used to analyze the structural changes of NCA.The phase change on the surface of cathode was severer under the influence of kinetic.Therefore,the diffusion path ofions increased caused by the cracks.Also,the rock salt structure reduced the surface activity and led to the increasing of impedance and decreasing of capacity.In this work,a gradient cathode(GC)was proposed to regulate the phase changes on the surface with gradient kinetics conditions based on the above issues.The degree of structural changes of NCAmaintained small during cycles.As a result,particles on the surface of GC kept spinel structure so that GC had a good contact with solid electrolyte and the capacity retention rate maintained 89%after cycles.The specific structural changes were analysised with TEM and EELs and concluded that obvious localized cation mixing(LCM)appeared in the particle on the surface of GC after 50 cycles.The rapid migration rate of lithium ions on the surface of GC led to the insufficient removal of lithium ions.The residual lithium ions inhibited Ni2+from entering the lithium layer,resulting in the formation of LCM.As a result,GC improved the structural stability of NCA particles,maintained a good contact with electrolyte during cycles.This study established the relationship between the kinetic regulation and interface optimization,and provided a new way for improving the performance of solid-state lithium batteries. |
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