Research On Preparation And Electrical Performance Of LLZTO-LGO Composite Solid Electrolytes

Cubic phase Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）has been recommended as one of the best solid electrolytes（SEs）in all solid state lithium batteries（ASSLBs）owing to the high ionic conductivity,wide eletrochemical window,good thermal and chemical stability and excellent mechanical properties and so on.However,there are still urgent issues in grain boundary regions of LLZTO,i.e.large grain boundary resistance and low elastic modulus,which prohibit the development and commercial application of ASSLBs seriously.In view of the problems mentioned above,low-melt lithium germanates（LGO）equipped with Li⁺conduction capability are proposed to be introduced into LLZTO to prepare LLZTO-LGO composite solid electrolytes（CSEs）.By this,the promotion of Li⁺conduction and charge-discharge cycling performance can be realized at the reduced sintering temperature and shortened process period.In this work,the LGO1.5 sintering additive with n（Li₂O:Ge O₂）=1.5 is firstly prepared and applied in liquid sintering of Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）electrolytes.The impacts of sintering temperature and additive content of LGO on the densities and ionic conductivity of the electrolytes are studied.The effects of LGO in sintering process of the electrolytes and Li⁺conduction are analyzed in detailed.Experimental results show that the relative density and the ionic conductivity can be promoted by increasing sintering temperature and raising LGO content.The LLZTO@Li₄Ge O₄/Li₂O composite electrolytes with the largest relative density of96.0%and the highest ionic conductivity of 6.27×10^?4 S·cm^?1 can be produced at1160 ^oC for 3 h with 2wt.%LGO1.5 added.However,a higher sintering parameter（>1160 ^oC）and increased content of LGO（>2wt.%）cause excessive volatilization of lithium,which resulted in the decreasing of electrolytes ionic conductivity.During the sintering process,LGO1.5 acts as a solder to facilitate the growth and bonding of LLZTO grains.Therefore,the sintering temperature can be reduced and sintering period can be shortened.The generated Li₄Ge O₄/Li₂O eutectic phase establishes consecutive Li⁺conduction pathways in the electrolytes.Consequently,microstructure of LLZTO and Li⁺transportation of LLZTO are optimized.Aiming at the degradation of electrolyte performance caused by excessive lithium volatilization,three kinds of compensate powders（CPs）are designed by adding lithium to the sintering additive of CPs and adding lithium to LLZTO,respectively.A Li⁺concentration gradient from CPs to the electrolyte green body can be built to compensate Li to electrolytes by supplementing lithium to the sintering additive of CPs.However,the appearance of liquid phase of the CPs during the sintering procedure leads to a much higher surface energy than that of the electrolytes.The reduction of the sintering system energy would mainly rely on the shrinkage of CPs,which seriously hinders the densification process of the electrolytes.Hence,the relative density and ionic conductivity of the electrolytes would be decrease.The LLZTO powders with 25wt.%Li₂CO₃ excess are formed by additing lithium to LLZTO.There is not any liquid phase generated in it during the heating process.This kind of CPs compensate lithium to to electrolytes by constructing Li₂O（g）concentration gradient from the CPs to the electrolyte green bodies.Meanwhile,a large amount of Li₂O（g）volatilizes into the electrolytes,cooperating with the liquid phase for sintering.By this,the densification process of the electrolytes can be accelerated.The relative density of the electrolytes is promoted to 97.2%and the ionic conductivity is raised to 8.56×10^?4S·cm^?1 with the application of this kind of CPs.In addition,the CPs can be employed either in solid sintering of LLZO doped with various elements or liquid sintering of LLZO by the sintering additives with different contents and compositions.Not only the excessive lithium volatilization can be avoided,but can even increase the ionic conductivity of the electrolytes to over 2times higher than that produced by the traditional liquid sintering process.For optimizing the Li⁺condcution in the grain boundary regions furtherly and promoting the charge-discharge cycling performance with Li anode,the network structure of the electrolyte grain boundary regions are regulated by adjusting the composition of the sintering additive under the Li compensation condition constructed by the LLZTO powders with 25wt.%Li₂CO₃ excess.The experimental results show that the migrated Li⁺concentration in the grain boundary domains decreases with the application of LGO0 additive with n（Li₂O:Ge O₂）=0.When sintered by LGO2.0 with n（Li₂O:Ge O₂）=2.0,the amount of Li-O bonds increases.Both of the two factors mentioned above lead to the elevation of grain boundary resistance and reduction of ionic conductivity.The increase of Li-O bonds also results to a significant decreasing in the cycling performance of Li symmetraical cells of the electrolytes,or even inability in cycling.As LGO1.0 used as the sintering additive,Li-O bonds disappear and the network structure of the grain boundary domains is only consisted of[Ge O₄]groups.This shortens the Li⁺conduction pathways,promotes the mobiled Li⁺concentration and enhances the ability to inhibit the growth of lithium dendrites.Thereby,the grain boundary resistance reduces greatly.The ionic conductivity of the electrolytes is promoted to 8.74×10^?4 S·cm^?1 successfully and the critical current density is improved to 0.70 m A·cm^?2.The Li symmetrical cell based on the LLZTO-LGO1.0 composite electrolytes can be stably cycle for more than 2000 h at the current density of 0.15 m A·cm^?2.The Li Fe PO₄（LFP）all solid state battery based on the LLZTO-LGO composite electrolyte performs a discharge specific capacity up to 149.75m Ah·g^?1 at the discharge rate of 0.05 C.After 100cycles,the Coulombic efficiency is still as high as 99.7%and the capacity retention rate is 93.41%.