Design And Optimization Of Garnet-type Li~+ Ion Solid Electrolytes For Dendrite-suppression

The commercial lithium ion batteries（LIBs）usually use organic liquid electrolytes with potential safety problems such as easy conbustion and even explosion,so the research of all solid-state batteries（ASSBs）is in an urgent situation.ASSBs have the advantages of low self-discharge,strong plasticity and high specific energy,and the key to ASSBs is the excellent electrolyte materials.Among plenty of solid electrolytes,Li-Garnet solid electrolyte Li₇La₃Zr₂O₁₂（LLZO）has become the hot spot recently for its chemical stability,high ionic conductivity and wide electrochemical window.LLZO and its doping systems have already been researched by scientists since reporting this material in 2007.The electrolyte system discussed in this article is the LLZTO with tantalum doped at Zr site named Ta-LLZO.Ta-doping is capable of reducing the phase formation temperature,stabilizing the cubic structure at room temperature and improving the ionic conductivity.The ASSBs system with Li-LLZTO could easily encounter the event of lithium dendrites which lead to lithium penetration by destroying the electrolyte structure severely,finally causing the failure with short circuit.The dendrite problem would result in a low critical current density and seriously restricting high-rate performance of the batteries.Therefore,it’s very meaningful to research how to improve the dendrite-suppression ability of LLZTO electrolyte.There are three respects for the reasons of dendrite production:1)Bad uniformity of the anode/electrolyte interface with poor contact,which could lead to partial high current density and induce the nucleation of lithium to form dendritic morphology along one-dimensional direction;2)The defects inside the ceramic electrolyte such as pores or flaws are easy to weaken its mechanical properties,as the breach for dendrites disruption;3)A high electronic conductivity of electrolyte is capable of consuming inner Li ions by the rection:Li⁺+e^-→Li,causing the nucleation of lithium within the electrolyte.The result is that the dendrites originated from interface and electrolyte have a cooperation in damaging the electrolyte.Based on the above mechanisms,there are some measures to solve the dendrite problem of LLZTO:（Ⅰ）Interfacial modification——Improving the uniformity and wettability;（Ⅱ）Grain boundary modification——Blocking off the growth route of dendrites along the grain boundary;（Ⅲ）Sintering optimization——Rising the quality and reducing the inner defects of ceramic.On the premise of Au layer and congruent Li-melting method as interfacial modification,this paper explores the improvement of dendrite-suppression by adding the nano-alkaline-earth metal oxide of Mg O/Ca O as grain boundary modification.The detailed research content are as follows:（1）Preparation and research on the composite electrolyte Mg O-LLZTO and related physical&chemical properties:Comprehensive mechanical property,density,electrical conductivity,CCD（Critical current density,as the symbol reflecting the ability for dendrite-suppression）and related performance of batteries.We ensured the effectiveness of Mg O modification at grain boundary for dendrite problem and 4wt%was confirmed as the optimal compound content,which could elevate the CCD from0.38 to 1.25 m A·cm^-2at room temperature.Thereafter,the lifespan of the quasi-solid state battery of Li-NCM₆₂₂ based on 4Mg O-LLZTO was improved from 10 cycles to over 50 cycles at high rates of 0.5/1.0C.（2）Preparation of the composite electrolyte Ca O-LLZTO,characterization of related properties and comparison of the performance with Mg O as the second-phase.The optimal compound content of Mg O was also 4wt%and CCD could be improved to more than 1 m A·cm^-2,suggesting a similar dendrite-suppression effect.（3）Discussion of the mechanism between dendrite-suppression and grain boundary modification with Mg O/Ca O from three points:sintering-resisting,toughening and reducing electronic conductivity.In addition,the effect of the second-phase at triple junctions of the grains instead of doping effect by the FESEM,and we found the direct connection between CCD and mechanical properties by designing gradient sintering experiment.In conclusion,the grain boundary modification with alkaline-earth metal oxide Mg O/Ca O is effective to improve the CCD of LLZTO,and it can be applied for dendrite-suppression of the LLZTO electrolyte,giving rise to positive effect in enhancing the high-rate performance of solid batteries based on LLZO electrolyte.