Preparation And Doping Of Li₇P₃S₁₁ Solid Electrolyte And Structural Design Of Lithium Metal Anodes

In recent years,the use of inorganic solid electrolytes instead of liquid electrolytes to solve the safety problems of lithium（Li）ion batteries has attracted more and more attention.Compared with oxide solid electrolytes,sulfide solid electrolytes have higher ionic conductivity,lower hardness and better interface contact,so they are considered to be one of the most promising solid electrolyte materials.However,sulfide solid electrolytes also have many shortcomings:（i）.The ionic conductivity of sulfide electrolyte synthesized by the liquid phase method is generally unsatisfactory,and the reaction mechanism is unclear;（ii）.The direct compatibility with high-voltage cathode materials and Li metal anodes is poor.The occurrence of side reactions leads to an increase in interface resistance;（iii）.The stability to water and oxygen in the air is poor.In recent years,anion and cation doping is often used to increase the ionic conductivity of the electrolyte,reduce the interface resistance and improve the chemical stability of the solid electrolyte,thereby effectively solving the electrolyte/electrode interface problem.However,the effects of doping on solid electrolytes lack systematic studies,and the effects of doping on the electrolyte/Li metal interface and Li deposition stability are unclear.Therefore,it is necessary to deeply study the influence of doping on sulfide solid electrolyte and the liquid phase synthesis mechanism of sulfide solid electrolyte.In addition,in order to increase the energy density and reach a level equivalent to that of traditional liquid batteries,solid-state batteries must use Li metal as the negative electrode.However,it is well known that Li metal has many problems such as unstable SEI,uncontrolled growth of Li dendrites,formation of dead Li,and unlimited volume expansion during deposition and stripping,which seriously hinder the application of Li metal anodes.Therefore,in order to obtain high-performance solid-state Li metal batteries,the research on the modification of Li metal negative electrodes becomes particularly important.Therefore,this thesis will conduct research from two aspects of sulfide solid electrolyte and Li metal anode.The main contents are as follows:（1）Solid-phase synthesis of Li₇P₃S₁₁ electrolyte is taken as the research object,and Zn²⁺,Mo⁴⁺,Fe²⁺,Sn⁴⁺,and Si⁴⁺is used as dopants to study the advantages and disadvantages of cation doping on sulfide electrolytes.First,taking Mo doping as an example,ab initio molecular dynamics（AIMD）calculations and experiments show that MoS₄^4-will preferentially replace the P₂S₇^4-group in Li₇P₃S₁₁,thereby broadening the Li⁺channel and generating Li vacancies to increase the ionic conductivity.However,excessive doping and an increase in the radius of doped cations will cause the body-centered cubic structure destroy of Li₇P₃S₁₁ and result in a decrease in ionic conductivity.Studies have shown that the doping of metal cations will cause the introduction of metal element or Li-metal alloy into the solid electrolyte/Li interface layer（SEI）.As an electronic conductor,the generated elemental metal and Li-metal alloy will cause the accumulation of electrons and lead to uneven deposition of Li metal,thereby promoting the growth of Li dendrites.In addition,the introduction of elemental metal will turn the originally single Li ion conductor into a mixed electronic and Li⁺ionic conductor,which will cause the SEI layer of the electrolyte/Li interface to thicken and increase the interface resistance,and it is not conducive to interface stability.Therefore,the resistance of Li/LPS-0.2Mo/Li keeps increasing with the increase of standing time.The critical current density of electrolytes doped with different cations（LPS-0.2Y）also shows that the critical current density is not positively related to the ions conductivity,whereas increases with the resistivity of the doped elements.It is interested to find that the doping of non-metallic silicon will not cause a decrease in the critical current density.Therefore,non-metal doping can effectively increase the ionic conductivity of Li₇P₃S₁₁ and improve the stability of the interface.This work provides important evidence and guidance for the selection of solid electrolyte dopants and the construction of electrolyte/Li interface SEI.（2）The reaction mechanism of liquid phase synthesis of Li₇P₃S₁₁ electrolyte is explored.In this work,the phase changes in the liquid phase reaction are traced,and the chemical compositions of the products at different ratios of Li₂S and P₂S₅ are studied.The results show that the liquid phase reaction is mainly a process in which the lone pair of Li₂S electrons attacking the bridge S on P₂S₅ to obtain compounds with different element ratios.It is found that the only precipitation phase is Li₃PS₄·ACN,which can react with soluble phases with higher P content(such as Li₂P₄S₁₁,Li₄P₄S₁₂,etc.)to form Li₄P₂S₇ to ensure that the final molar ratio of Li₃PS₄·ACN to Li₄P₂S₇·CAN is 1:1.Finally,during calcination,Li₇P₃S₁₁ is formed by the equimolar reaction of the above two compounds.Based on the above research,the liquid reaction equation of Li₇P₃S₁₁is 7Li₂S+3P₂S₅+4ACN=2Li₄P₂S₇·ACN+2Li₃PS₄·ACN.Furthermore,the unreacted gel-like Li₄P₂S₇·ACN can be decomposed into Li₄P₂S₆,which deduces low-ionic conductivity of the electrolyte compared to that synthesized by solid-state methods.（3）Co-electrodeposition method is conducted to uniformly deposit a thin layer of CuSn on the surface of the conductive carbon fiber paper（CP）.The CuSn/CP is used as the Li metal deposition framework to inhibit the growth of Li dendrites.Co-electrodeposited CuSn alloy has high structural stability and low nucleation barrier.The highly electronegative Cu in the alloy can reduce the electron cloud density of Sn,and the electron affinity between Sn and the surface of lithium atoms in a power-deficient state increases,thereby increasing the binding energy with lithium atoms and lowering the lithium nucleation barrier.The Cu in the alloy can also inhibit the huge volume expansion of Sn during the Li insertion process,and prevent the uneven Li deposition behavior caused by the uneven distribution of nucleation sites.In addition,the highly conductive Cu can equalize the charge distribution,make the electric field distribution of Li deposition more uniform,thus promote the uniform deposition of Li metal.The uniformly distributed lithophilic Sn can reduce the overpotential of Li nucleation to form large Li nuclei and avoid the formation of dendrites.Therefore,the prepared CuSn/CP electrode exhibits a cycle life exceeding 1000 h at a current density of 1mA/cm².The full battery assembled with CuSn/CP-Li composite anode and Li Fe PO₄positive electrode shows excellent rate and cycle capacity retention.A high discharge specific capacity of 132.5 mAh/g is obtained at 2C.（4）A novel Li deposition strategy is proposed to make full use of the three-dimensional framework to reduce the local current density.In this method,copper foam（CF）is used as the current collector,a layer of ZnO with weaker lithiophilicity is atomic layer deposited on the upper layer,and a layer of Au with stronger lithiophilicity is sputtered on the lower layer to obtain a ZnO/CP/Au electrode.It is well known that the Li⁺transmission resistance of the upper layer is lower than that of the lower layer.But the high lithiophilicity of Au reduces the electrochemical reaction barrier of Li metal.The Li Zn alloy formed by lithiation of ZnO has weaker lithophilicity than Au,so the electrochemical reaction barrier of the upper layer is higher than that of the lower layer.In addition,the Li₂O formed by the lithiation of ZnO also increases the ohmic resistance of the upper layer of the foamed copper due to the electronic insulation.Therefore,the total resistance of the deposition of Li in the upper layer and the lower layer can be made close,thereby guiding the simultaneous deposition of Li on the upper and lower layers of the foamed copper framework.This strategy changes the traditional"top-growth"and"bottom-up"deposition modes,thereby truly reducing the local current density of Li deposition and inhibiting the growth of dendrites.This omnidirectional deposition method could maximally reduce the local current density and prevent the rapid consumption of local Li ions to form a space charge layer,thereby ensuring uniform deposition of Li metal on the upper and lower layers of the copper foam,which is confirmed by SEM characterization.The as-prepared ZnO/CP/Au electrode presents lower deposition overpotential than the ZnO/CF and CF/Au electrodes,no Li dendrites is found at much high deposition areal capacity even at 10 mAh/cm²,exhibiting excellent cycle performance,high and stable coulombic efficiency.At the same time,the cycle stability is better than the CuSn/CP electrode in the previous chapter.