Study On Negative Interface Modification Of Garnet Based Solid-State Lithium Metal Batterries

In recent years,the solid state lithium metal batteries?SSLBs?have been focused due to they offer significantly improved energy density and safety far beyond the conventional lithium-ion batteries.These advantages arise from the solid-state electrolytes?SSEs?,which are important part of SSLBs.The garnet-type SSEs have advantages of chemical and electrochemical stability against lithium and relatively high ionic conductivity(as high as above 1 mS cm^-1 at room temperature)in the various solid electrolytes.In addition,the garnets can be used as active fillers for polymer electrolytes to further enhance their overall performances.However,the huge interfacial resistances resulting from the poor physical contact between the lithium anode and the rigid electrolytes,which hinder the practical application of garnet based SSLBs.In this work,we have worked on the interface problems based on the garnets and the composite electrolytes.The detailed results are as follows:?1?As for the garnets,the lithiophobicity of the garnets not only cause the poor interface contact,but also trigger the lithium dendrite growth.Even worse,lithium dendrite growth can easily occur at the garnets,causing short circuit of the SSBs.Herein,we present a strategy to solve such problem by coating 20 nm SnO₂ films on the surfaces of the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ pellets.Through conversion reaction of SnO₂ with Li at200°C,the nanocomposite layers consisting of crosslinked Li_xSn and Li₂O are formed between the Li and the LLZTO electrolytes.This leads to transition from lithiophobicity to lithiophilicity,thus greatly reducing interfacial resistance from 1100?cm² to 25?cm².Furthermore,taking advantage of suppressing volume change of Li_xSn alloy,the intermediate layers maintain integrity under the current densities of 0.2 mA cm^-2 for650 h cycles.In addition,the critical current density of lithium symmetrical cells can be as high as 1.15 mA cm^-2.?2?Actually,the poor wettability of Li on the garnet is resulted from a thin Li₂CO₃layer on the garnet surface.Therefore,removing the Li₂CO₃ layer can achieve an intrinsically lithiophilic surface to maintain intimate contact and thus suppress the lithium infiltration.Herein,a novel and simple method of rapid acid treatment is proposed to perfectly remove the Li₂CO₃ layer on the LLZTO,thus achieving an intrinsic Li/LLZTO interface.Dipping the LLZTO pellet into 1 M hydrochloric acid?HCl?solution for 30 s can efficiently reduce interfacial resistance from 940?cm² to26?cm².Moreover,the favorable interface enables stable Li plating/stripping for over700 h under a current density of 0.2 mA cm^-22 at 30 ^oC.In addition,the interfacial resistance with Li decreases to 28?cm² and 31?cm² through treatment of H₂SO₄ and H₃PO₄.The Li symmetric cells based on acid treated LLZTO exhibit stable cycling over 600 h with no obvious overpotential.These results indicate that rapid acid treatment is a simple and high-effective strategy to well address the interfacial issue of garnet-type solid-state electrolytes.?3?To overcome the brittleness of LLZTO ceramics,flexible organic-inorganic composite solid electrolyte membranes composed of PEO polymers and LLZTO powders were prepared.However,the huge interfacial resistances and low lithium ion conductivity at room temperature limit their application toward lithium metal solid-state batteries?SSBs?.Herein,we prepared the PEO-LLZTO-SN composite electrolyte by tape casting,and then systematically investigated the effect of SN and LLZTO on the performance of PEO-based electrolytes.The electrolyte presents a maximum ion conductivity of 2×10^-4 S cm^-1 at 30?.Moreover,the Li/PEO-60%LLZTO-40%SN?PLS?/Li cells exhibit low interfacial resistance and cycle over 200 h at 0.2 mA cm^-2and 30?.This excellent performance of PEO-LLZTO-SN is ascribed to the synergistic effect of LLZTO and SN.All-solid-state Li/PLS/LiFePO₄ cells were assembled at 30?,which show reversible cycles with a capacity of 125 mAh g^-1 at the first discharge,and retention of 80%capacity after 100 cycles.