Effect Of Spark Plasma Sintering On The Microstructure And Electrochemical Properties Of All-solid-state Lithium Battery

All-solid-state battery,as a new generation of lithium-ion battery with higher safety performance and higher energy density,has been the ideal choice of the power battery for the electric vehicles.The main feature of the all-solid-state lithium battery is using solid-state electrolyte to replace conventional organic liquid electrolyte,but the introduction of solid-state electrolyte makes the contact interface inside the battery become "solid to solid" interface.This interface has the inherent characteristics of small contact area and poor contact state,which affect the conduction of lithium ions significantly and hinders the development of all-solidstate batteries seriously.Therefore,it is the key to improve the interface contact for improving the electrochemical performance of all-solid-state lithium batteries and promoting their commercial development.Among the numerous methods to improve the interface contact,the co-sintering is one of the easiest and most effective methods,especially for the spark plasma sintering(SPS).Due to the low sintering temperature and short sintering time,SPS has shown unique advantages in the preparation of solid electrolyte and the assembly process of all-solid-state lithium batteries.However,at present,the influence of spark plasma sintering conditions on the internal microstructure and electrochemical performance of allsolid-state lithium batteries is still lack of systematic research.Based on the above research background,the solid electrolyte,the electrodeelectrolyte half battery and the composite electrode were prepared by spark plasma sintering technology in this paper.The influence of sintering conditions on the microstructure and electrochemical properties of each part was investigated.Then,combined with the best sintering process parameters of each part,all-solid-state lithium battery was assembled and electrochemically cycled.Finally,the all-solidstate lithium battery after electrochemical cycles was disassembled,the influence of the electrochemical cycle on the microstructure evolution of the battery was also investigated.Firstly,the Li1.5Al0.5Ge1.5(PO4)3(LAGP)solid-state electrolyte was prepared by SPS,and the effects of loading method,sintering temperature and carbon contamination on the microstructure and electrochemical performance of the samples were investigated.The results showed that the loading method of heating first and then loading was more beneficial to improve the relative density of the sample than the method of heating and loading simultaneously.With the sintering temperature increasing,both the relative density and the ionic conductivity of the sample increased first and then decreased,and achieved the maximum at 700℃.The maximum relative density of the sample was 97%,and the maximum ionic conductivity was 2.12 × 10-4 S·cm-1.During the SPS process,the samples would be affected by carbon contamination inevitably.The carbon contamination inside the sample not only changed the sample color,but also increased the grain boundary impedance.However,this carbon contamination could be removed by annealing treatment at 500℃ in air atmosphere.Secondly,the electrode-electrolyte half battery was prepared using LiFePO4(LFP)as electrode by SPS,and the effects of the sintering temperature and sintering current direction on the microstructure and electrochemical performance of the interface were investigated.The results showed that at the relative high sintering temperature(650℃),the electrode-electrolyte interface presented an initial contact state with the interface layer formed;at the relative low sintering temperature(500℃),the interface presented a loose contact state without any interface layer.The formation of the interface layer not only increased the interface impedance,but also affected the normal electrochemical cycle.In addition,it was also found that the sintering current direction could affect the formation of the interface layer and the contact state of the interface significantly.At the 650-LFP/LAGP interface,a Fe-rich layer composed of Fe2P2O7 was found,while at the 650-LAGP/LFP interface,an Al-rich layer composed of AlPO4 was found.The interface contact was relative loose at the 500-LFP/LAGP interface,while it was relative compact at the 500-LAGP/LFP interface.Based on these observed results,the formation mechanism of electrode-electrolyte interface during spark plasma sintering was proposed.Thirdly,the 30%LFP-55%LAGP-15%C composite cathode was prepared by SPS,the effects of the sintering temperature on microstructure and electrochemical properties of the composite cathode were investigated,and the spatial distribution of each component in the composite cathode was also determined by FIB threedimensional reconstruction technology.The results showed that at the sintering temperature of 550℃,each component of the composite cathode maintained its original phase structure,but with the sintering temperature increasing,the LAGP were decomposed and transformed into LAGP nanocrystals and AlPO4 and GeO2 impurities.Meanwhile,with the sintering temperature increasing,the volume fraction and connectivity of the pores inside the composite cathode decreased significantly.Both of the formation of the impurities and the decrease of the pore connectivity could deteriorate the electrochemical performance of the composite cathode.In addition,although the composite cathode sintered at 550℃ showed the largest initial discharge specific capacity about 138 mAh·g-1,it had a poor cycling stability with capacity fading and voltage fluctuation.On the contrary,the composite cathodes sintered at 600℃ and 650℃ showed stable electrochemical cycles.By the postmortem analyses,it was found that the capacity fading of the composite cathode was related to the cracking during the electrochemical cycle.Finally,combined with the optimized sintering conditions of the above three parts,the all-solid-state lithium battery was assembled by SPS and its electrochemical performance was tested.The effects of the cycle time on the microstructure evolution and electrochemical performance of the battery were investigated.The results showed that at the beginning of the electrochemical cycle,the battery showed a stable cycle performance,and its discharge specific capacity exceeded 150 mAh·g-1.However,with the cycle number increasing,the battery had a serious capacity fading.By the postmortem analyses,it was found that a large number of cracks were formed inside the battery,the composite cathode was partially separated from the solid-state electrolyte,and the interface between the solid-state electrolyte and the anode had a serious interface reaction.The LAGP grains expanded and became seriously amorphous in the reaction region.These changes of microstructure finally led to the capacity fading.Furthermore,it was also found that a SEI layer composed of large numbers of Li2CO3 grains was formed,and it extended along the cracks to the cathode side in the form of dendrites.Based on the above observations,the failure mechanism of the all-solid-state lithium battery was proposed.