The Study Of Hcp-type Solid Electrolyte Based On First-principle Calculation

The factors affecting the lithium-ion diffusion in solid electrolyte include lithium-ion concentration,non-lithium cationic doping,non-lithium cationic arrangement and so on.How these factors affect the lithium-ion diffusion is one very significant scientific problem,which is lack of systematic research at present.In view of this,based on first-principles calculation,this work established different models to study the thermal stability,intrinsic electrochemical window,solid-solid interface stability and electronic conductivity of hcp anionic sublattice materials（including LISICON type materials,diamond-like structural materials and Li₇Nb O₆ and its doping phases）.Furthermore,AIMD and NEB methods were used to study the diffusion kinetics of lithium ions.The main research contents are as follows:Firstly,based on LISICON solid electrolyte Li_2+2xZn_1-xSi O₄,the influence of lithium-ion concentration on the lithium-ion diffusion kinetics was studied.The results prove that in Li_2+2xZn_1-xSi O₄ with different lithium-ion concentrations,lithium ions are primarily transmitted by concerted migration through interstitial sites（octahedral sites）and lattice sites（tetrahedral sites）,rather than through the bottleneck of ab plane.When x=0.375,the lithium-ion conductivity of the material is the highest,reaching 2.006 m S/cm（300 K）and the activation energy E_a decreases to0.255 e V.This result explains the experimental phenomenon studied by HONG et al.Secondly,based on Li_16-2xM_xGe₄O₁₆（M=Zn,Mg）,the impact of non-lithium cationic doping on lithium-ion diffusion kinetics was studied and the hypothesis that non-lithium cationic doping with larger radius could improve the lithium-ion conductivity was proposed.To increase the universality of the research results,several Li_16-2xM_xGe₄O₁₆（M=Zn,Mg）models with different lithium-ion concentrations were established,Cd was doped in the Mg and Zn sites respectively.The results show that Cd doping can improve the lithium-ion conductivity when the lithium-ion concentration is low.When the lithium-ion concentration exceeds a certain value（the optimal lithium-ion concentration）,Cd doping decreases the lithium-ion conductivity.The non-lithium cations doping strategies were proposed,doping non-lithium cations with larger radius at lower lithium-ion concentration and doping non-lithium cations with smaller radius at higher lithium-ion concentration.These two doping strategies are also applicable to thio-LISICON materials and are consistent with the results obtained from relevant experimental studies.Thirdly,the effect of different stacking modes of non-lithium cations in hcp anionic sublattice on electrochemical properties of diamond-like structural materials Li_16-2xZn_xM₄N₁₆（M=Ge,Sn;N=S,Se）was studied.By the calculation results,four materials(Li₁₅Zn_0.5Ge₄S₁₆,Li₁₄Zn Sn₄S₁₆,Li₁₅Zn_0.5Ge₄Se₁₆ and Li₁₅Zn_0.5Sn₄Se₁₆)with high lithium-ion conductivity were found.Among them,sulfides have the lithium-ion conductivity and electrochemical window comparable to Li₁₀Ge P₂S₁₂（LGPS）,selenides could be used as solid electrolyte of Li-Se all solid-state battery.In addition,based on the lithium-ion diffusion characteristics in the fixed Se-hcp anionic sublattice,three design strategies for fast sulfur/selenide lithium-ion conductors are proposed:（1）possessing high lithium-ion concentration;（2）having large cavity structure,proper transmission distance and penetration radius and other specific spatial configuration;（3）The unit volume of S/Se anion more 42.3（?）³/49.3（?）³.Finally,Li₇Nb O₆ with all non-lithium cations in octahedral sites was explored.The lithium-ion diffusion energy barrier in this material is 0.446e V and the lithium-ion conductivity at room temperature is 0.008 m S/cm.Mg/Zn doped in Li sites and Y/W doped in Nb sites were applied to adjust the lithium-ions concentration of models,then the lithium-ion conductivity of models with different lithium-ion concentrations were studied.Based on the analysis of thermodynamic stability,the thermal stability of the material with Mg/Zn doped at Li sites decreases,which is not conducive to the synthesis of the material.Y doping does not improve the lithium-ion conductivity;the W-doped Li₅₅Nb₇WO₄₈ possess high lithium-ion conductivity（0.28 m S/cm）at room temperature,good electronic insulation and the stable electrochemical window（1.21-2.90 V）.This thesis contains 70 figures,22 tables and 222 references...