Optimized Modification Of Yttrium-based Rare Earth Halide Solid-state Electrolytes

All-solid-state batteries have been widely studied and discussed by researchers all over the world.Typical inorganic solid-state electrolytes include oxides,sulfides and halides.Each type of inorganic solid-state electrolyte has its own advantages and shortcomings:oxide electrolytes have high ionic conductivity and wide electrochemical window,but the inherent mechanical rigidity of the material is not conducive to preparation and mechanical processing;sulfide electrolytes have excellent ionic conductivity,but they cannot be directly matched with cathode oxides and have poor air stability;halide electrolytes also have high ionic The halide electrolyte also has high ionic conductivity and oxidation stability,but is unstable when matched with lithium metal cathode.Based on the advantages of halide solid-state electrolytes and the demand to enhance the added value of rare-earth materials in China’s high-end automobile manufacturing,yttrium(Y)-based rare-earth halide solid-state electrolytes were selected as the research object in this paper.A new Y-based halide solid-state electrolyte with higher ionic conductivity and wider electrochemical window is investigated and realized for application in all-solid-state batteries by adopting a modulation strategy of adjusting the chemical composition of the material and combining it with doping modification.The main research of this thesis is as follows:Pure-phase LiaYClb solid-state electrolytes were prepared by the solid-phase sintering method,and the optimal sintering process regime was determined.This was followed by the design of a series of lithium-deficient solid-state electrolytes.The modulation of the space group structure of LiaYClb solid electrolyte from P-3ml to space group Pnma structure was achieved,and the computational simulation confirmed that the composition of the underlithium state was favorable to generate materials with Pnma structure.the ionic conductivity of Li2.375YCl5.375 with Pnma structure could reach 3.89×810-4 S/cm,which was improved nearly three times compared with that before the modification.The constructed full cell has a first-cycle discharge specific capacity of 123.8 mAh/g,a first-cycle coulomb efficiency of 93.57%,and a Coulomb efficiency close to 100%in 40 cycles.The mechanism of the effect of the high-valent cation Nb5+on the ion conduction of LiaYClb solid electrolyte was explored by computational simulations,and it was found that Nb5+ doping introduced additional lithium vacancy sites,while enhancing the vibration of ions in the structure and increasing the chance of lithium ion synergistic diffusion.the disordered lithium ion sites near the Y(Nb)cation sites led to the reduction of the ab-plane diffusion barrier,which led to the lithium ion migration to be significantly improved.A series of Nb5+-doped modified underlithium state LiaYClb solid-state electrolytes were synthesized to obtain structurally stable Li2.31Y0.98Nb0.02Cl5.31 with high ionic conductivity.The first discharge specific capacity of the full cell was 138.1 mAh/g with a first coulombic efficiency of 89.1%.Compared with the undoped Li2.375YCl5.375 solid-state electrolyte,the cycling stability was significantly improved,and the 100 cycling capacity retention rate reached 65%.The F-anion doping modification study was carried out for the Li2.375YCl5.375 and Li3YBr6 systems.Li2.375YCl5.075F0.3 and Li3YBr5.7F0.3 solid electrolytes with high stability to lithium metal reduction were prepared.The lithium symmetric cells using Li2.375YCl5.075F0.3 and Li3YBr5.7F0.3 can be stably cycled at room temperature for more than 1000 h.It is found that the improved stability is due to the dense and reticulated F-rich interfacial layer formed in situ at the electrolyte/Li-metal interface after F-doping,which stabilizes the interfacial structure and improves the long-term electrochemical cycling stability.In addition,the full cell also exhibits good electrochemical performance.The first discharge specific capacity is 121.6 mAh/g,the first coulombic efficiency can reach more than 90%,and the capacity retention rate can reach 65%in 70 cycles.The co-doping modification of Li2.375YCl5.375 solid-state electrolytes with space group Pnma by Nb5+and F-was carried out.The ionic conductivity of the prepared Li2.33Y0.975Nbo.025Cl5.03F0.3 solid-state electrolyte reached 6.8×10-4 S/cm,and the electrochemical stability window was about 0.56-4.21 V.The experimental results showed that the F-,Nb5+co-doped solid-state electrolyte also had good electrochemical stability.The Li-symmetric cell with Li2.33Y0.975Nb0.025Cl5.03F0.3 solid-state electrolyte matched with lithium metal cathode can provide stable cycling at a current density of 0.9 mA/cm2 for more than 1000 h.The in situ generation of a stable interfacial passivation layer between the lithium cathode and Li2.33Y0.975Nb0.025Cl5.03F0.3 solid-state electrolyte is the most important factor for the electrochemical stability.interfacial passivation layer,which is an important factor for the improved electrochemical cycling stability.In addition,the full cell also possesses the same high cycling stability.the 70-cycles cycling capacity retention is 55%,and the coulombic efficiency is maintained at about 99%over 70 cycles.