Performance Investigation On Garnet-type Solid Electrolyte And Anode/Electrolyte Interface

Solid state lithium metal battery with high energy density and high safety is one of the most significant development directions of next generation lithium batteries.As a typical oxide solid electrolyte,Li7La3Zr2O12(LLZO)presents high lithium-ion conductivity,wide electrochemical window,high mechincal strength and good thermal stability.Thus,LLZO in recent years has attracted extensive attention in academic and industrial fields.Nevertheless,the depolyment of LLZO solid-state lithium metal battery is still hindered by the densification issue and large electrolyte-electrode interface resistance.In this work,we take Li6.4La3Zr1.4Ta0.6O12(LLZTO)as research target,and the behind reason of its poor sinterability was investigated.Based on thermodynamic calculation,additives are selected and introduced to promote the densification of LLZTO material and meanwhile lithium-ion conductors are formed on grain boundary to enhance conductivity of solid electrolyte.As to the lithium anode interface,lithium reactive elementary substance is added into molten lithium to improve wettability of lithium towards LLZTO solid electrolyte and so reduce the interface resistance.The mechanisms of additive in solid electrolyte and second component in lithium anode are both investigated.Impacts of additives on electrolyte and interfaces microstructures,material conductivities and interface resistances,and electrochemical performances of symmetric and full cells are studied.Garnet-type solid-state lithium metal battery with improved specific capacity,cycling stability and rate performance are achieved.The main contents of this thesis are as follows:(1)The effects of Li2CO3 amount on sintered microstructure and relative density of LLZTO were investigated.The results indicate that the spontaneously formed Li2CO3 on particle surface of LLZTO,which is difficult to decompose and has highly low ionic conductivity,is one of main causes for the poor densification.Al2O3 is chosen as additive,based on thermodynamic calculation,to eliminate Li2CO3 and generate ionic conductor LiAlO2,which is helpful to densify LLZTO solid electrolyte and increase ionic conductivity.AC impedance test and analysis suggest that the introduction of Al2O3 significantly improves the grain boundary conductivity of LLZTO.The 3%Al2O3(nAl:nLi=3%)coated LLZTO demonstrates a relative density of 92.07%and an ionic conductivity of 4.16×10-4 S cm-1.Its Li-based symmetric cell can cycle steady for 800 h at room temperature and 0.1 mA cm-2 current density.(2)Based on thermodynamic calculation results,selected as addtitive,SiO2 is introduced to LLZTO solid electrolyte to efficiently scavenge Li2CO3 on the surface of LLZTO particles during sintering and form Li4SIO4 along grain boundary,which facilitates the densification of electrolyte and improves the total ionic conductivity.An addition of 1%SiO2(nSi:nLi=1%)remarkably boosts the relative density and ionic conductivity of LLZTO pellet up to 94.71%and 3.84×10-4 S cm-1,respectively.Compared with pristine LLZTO,a symmmtric lithium cell assembled with 1%SiO2-LLZTO has highly stable polarization voltage and preferable ability to surppress lithium dendrite.(3)Low-cost sulfur was used to prepare sulfur-modified lithium anode.Li2S is formed through the reaction of sulfur and lithium anode,so as to improve the wettability of lithium anode on LLZTO electrolyte and reduce interface resistance.The influences of sulfur amount on anode wettability,interface contact and symmetric cell interface resistance are investigated.The experimental results show that the interface resistance can be effectively reduced to 12.4 ? cm2 by introducing 10%S(ms:ms+Li=10%),and 10%S modified lithium anode exhibits excellent cycle stability in both symmetric and full cells.The 10%S-Li symmetric cell assembled with LLZTO mantains a stable cycling for 970 h at current density of 0.2 mA cm-2 and at room temperature.The 10%S-Li|LLZTO|LiFePO4 full cell still has a discharge specific capacity of 149.7 mAh g-1 and a coulomb efficiency of 100%after 100 cycles at 0.1C.(4)Phosphorus modified lithium anode was prepared by adding phosphorus elementary substance into molten lithium metal by simple fusion method.Such anode demonstrates outstanding cycling stability and rate capability,which can be attributed to:1)Li3P as reaction product of phosphorus lithiation is lithiophilic.Phosphorus modified lithium anode has good wettability with LLZTO solid electrolyte,which can effectively reduce the interface resistance;2)Li3P has relatively high lithium-ion conduction,which can enlarge reaction area and so accelerate electrode reaction kinetics.Electrochemical test results indicate that 5%P can remarkably decrease the interface resistance between Li and LLZTO to 12.1? cm2,and its symmetric cell exhibits stable cycling for 430 h at current density of 0.1 mA cm-2 and at room temperature.The 5%P-Li|LLZTO|LiFePO4 full cell maintains a specific capacity of 156 mAh g-1 at 0.05C after 85 cycles and 125 mAh g-1 at 1C,demonstrating the excellent rate capability.(5)Silicon modified lithium anode was synthesized by introducing silicon into molten lithium.Second phase Li22Si5 is formed by lithiation of silicon,which can effectively improve the wettability of lithium anode to electrolyte and improve the interface contact between anode and electrolyte.First-principles calculation suggests a weak reaction happens between modified anode and electrolyte,which is helpful to form a robust anode interface and further decrease interface resistance.The 15%Si-Li anode shows a dramatically reduced interface resistance of 2.7 ?cm2,and the corresponding symmetric cell demonstrates steady cycling over 1200 h at 0.1 and 0.2 mA cm-1 current density.The 15%Si-Li|LLZTO|LiFePO4 full cell dilivers a greatly high reversible capacity of 169 mAh g-1 at 0.05C with a coulombic efficiency of 100%after 100 cycles.