Constructing The Compatible Interface Between Nasicon Solid Electrolyte And Electrode For All Solid State Battery

All solid-state batteries are considered to be the promising direction of the new generation lithium-ion batteries due to their excellent safety,high energy density,low self-discharge rate and stable long cycle life.In an all-solid-state battery,the organic electrolyte in the conventional secondary battery is replaced by solid electrolytes,and the wettability and compatibility of solid/solid interface formed with the positive and negative electrodes are inferior.The high interface impedance hinders the transmission of lithium ions between the solid/solid interfaces.Therefore,improving the solid/solid interface according to the transport properties of ions in the solid electrolyte material is the key to the development of all-solid-state batteries.In this context,the main work of this research paper is as follows:Lithium aluminum aluminum phosphate(LATP)solid electrolyte materials with different Al3+ doping amount were prepared by modified sol-gel method,and the relative density of LATP solid electrolyte materials with the increase of Al3+ doping amount was investigated.Studies have shown that the relative density of LATP solid electrolyte materials increases significantly with the introduction of Al3+,but it does not increase further with the increase of doping amount.A similar change rule was obtained for the morphology analysis of samples with different doping amounts.As a result of the alternating current impedance,the highest ion conductivity was 1.048×10-4 S/cm for the sample with a doping amount of 0.3,and activation energy of the lithium ion diffusion was 0.308 eV according to the Arrhenius equation.Based on the solid electrolyte LATP,a buffer material was prepared by compositing with two common cathode materials,LiCoO2 and LiFePO4.The structure and morphology of the two composites was characterized.Based on that the composites sintered at 700? were selected for further study.The specific components of the two composite materials,ionic conductivity,electronic conductivity,and electrochemical stability are discussed.The results show that the two composites have good ionic conductivities(ion diffusion coefficients are 1.24×10-10 cm2s-1,3.43×10-12 cm2s-1,respectively)and electron conductivity(like semiconductors).Cyclic voltammetry test and constant current charge and discharge test were employed to determine the electrochemical atability in the operating conditions of all solid-state battery(CV test voltage range:0-5V,constant current charge and discharge test voltage range:3.0-4.3V),at the beginning of charging process there is a weak activation process,that is,an oxidation reaction occurs,and a specific capacity of about 55 mAh/g is contributed during the first week of charging then maintained stable in the subsequent electrochemical cycle.This ensures the stability during the cycling of the all solid-state battery.For the all solid-state batteries employing buffer layer exhibit good electrochemical performance under RT and 0.1C test condition(LiCoO2 and LiFePO4 solid batteries have about 150 mAh/g and 155mAh/g stable charge and discharge capacity after 100 cycles)Focused Ion Beam(FIB)technique was employed to extract the sample interface between the electrode and the buffer layer before and after cycling,and the interface morphology and element distribution spectrum were performed.The results show that the employed buffer makes a sharp point-to-point contact converted into face-to-face contact with a transition region,which ensures the smooth transmission of ions at the interface.After the cycle,the interface undergoes obvious structural evolution,deducing the side reactions occurring in the electrochemical cycle and the reaction stress generated,but the all-solid-state battery still maintains good stability and electrochemical performance,and the bridge-connecting formed by the introduced buffer layer can be seen,which absorbs the product and stress caused by the side reactions.A carbon-rich oxygen-depleted intermediate is formed at the interface after the cycle,which is distinct from the interface element distribution before the cycle.These carbon-rich and oxygen-depleted intermediates are undoubtedly important to ensure the ionic conductivity of solid-state batteries over long periods of time,and the formation of such intermediates remains unclear.In addition,the capacity retention of the LiCoO2 battery is not as good as that of the LiFePO4 battery,which should be mainly attributed to the severe catalytic action of the surface Co ions at the LiCoO2 electrode,which corresponds to the interface region.Therefore,the stability of the interface region having Co ions will be weaker than the stability of the interface region having Fe ions.On the other hand,the binder PVDF used to prepare the electrode layer accelerates decomposition under the catalysis of Co ions,which not only contributes to the formation of carbon-rich intermediates,but also exacerbates battery instability and interface structure degradation,which in turn affects the electrochemical performance of solid-state batteries.