Migration Performance Regulation Of Lithium Ion In Li₇La₃Zr₂O₁₂ And Modification Of Interface Between Li₇La₃Zr₂O₁₂ And Lithium Anode

Compared with lithium-ion batteries,all-solid-state batteries are considered to be the next generation of lithium batteries due to its better safety and higher energy density.As an important component of all-solid-state batteries,solid electrolytes play a important role in it.Among various solid electrolytes,Li₇La₃Zr₂O₁₂(LLZO)with garnet structure is a promising one because of good performances.However,LLZO also have some problems needed to be solved:the Li⁺conductivity is relatively low compared to liquid electrolyte;in addition,the contact between LLZO and electrodes are insufficient,resulting in large interfacial impedance.In this thesis,the Li⁺conductivity was improved by doping on Li site to change the type of crystal structure of LLZO and doping on Zr site to regulate the Li⁺migration channel.The regulation mechanism have also been clarified.At the same time,the surface of LLZO was modified to increase its effective contact area with lithium anode and improve the migration of Li⁺at the anode interface.The type of crystal structure of Li₇La₃Zr₂O₁₂ was changed by doping on Li site and the influence of it on Li⁺migration was studied.Firstly,the Li⁺concentration was adjusted by amount of lithium source,and the optimal lithium ion concentration has been determined,the results shown that when the Li⁺concentration is 6.50 pfu,activation energy reaches the minimum of 0.21 eV and the Li⁺conductivity reaches the maximum of 1.31×10^-3 S·cm^-1.Then,the effect of the content of Ga on the phase and crystal structure were studayed.The results of neutron diffraction and ⁷Li MAS NMR test shown that with the increase of Ga content,the occupancy of lithium ions in octahedral voids increase first and then decrease,when the doping amount is 0.20 pfu,the occupancy reaches the highest,which means the Li⁺conductivity is the highest.Finnally,different elements have been tried to dope on Li site and the influence of these elemnts on the crystal structure type and properties have been studied.By testing XRD and SEM,only Al,Fe,Ga can stabilize LLZO as a cubic phase at room temperature.On the other hand,the space group of Fe or Ga-doped LLZO is transform Ia-3d to I-43d which is better for Li⁺transfer,as the doped ions does not block Li⁺migration channels in this structure.Furthermore,the distance of different Li⁺of Ga-doped LLZO is shorter than that of Fe-doped LLZO while the bottleneck size is larger.Beyond that,the grain size of Ga-doped LLZO is larger.Hence,the Ga-doped LLZO shows the highest Li⁺conductivity.The Li⁺migration channel was regulated by doping on Zr site of LLZO and the influence mechanism has also been studied.When doping tetravalent ions with different radius,the M-O bond length,cell parameter and bottleneck size increase with the increase of the radius of doped ions,and the Li⁺conductivity increase while the Ea decrease gradually.When doping pentavalent ions,by testing neutron diffraction,as the radius of doped pentavalent ions increase,the M-O bond length rise while the La-O bond length decrease because of larger interionic force,the cell parameter and bottleneck size decrease gradually.With the increase of radius of doping ions,the Ea firstly decrease and then increase due to the bottleneck size and M-O bond length are regulated synergistically.Based on previous research,doping Ga on Li site,and Sc on Zr site,Li_6.4+xGa_0.2La₃Zr_2-xSc_xO₁₂(0?x?0.15)have been prepared and the Li⁺concentration and Li⁺migration channel have been regulated synergistically to further improve the conducivity.With the increase of Sc content,the bottleneck size and the M-O bond length increase gradually,the Li⁺conductivity increase firstly and then decrease.The rise of conductivity is result from the rise of bottleneck size and M-O bond length,and the drop of conductivity is due to the Li⁺concentration larger than 6.6pfu.When doping 0.05 pfu of Sc,the Li⁺conductivity reaches the highest of 1.5×10^-3S·cm^-1.Finally,in order to improve of Li⁺migration at the interface between LLZO and lithium anode,the surface of LLZO was modified by sputtering a layer of metal film to enhance the interfacial contact.After adopting Cu to modified the surface of LLZO,the contact angle between LLZO and lithium metal reduce from 122°to 88°,and the modified LLZO connect closely with lithium anode at the interface,while there are many gaps between unmodified LLZO and lithium negative electrode.After modification,the interfacial impedance decrease from 677?·cm² to 29?·cm²,and the lithium symmetrical cell can operated at an electric current density of 0.4 mA/cm²rather than 0.1mA/cm².Then,a layer of Ag,which is eiser to form alloy with Li,was coated on the surface of LLZO.After modification,the contact angle is further reduced to 67.2°and the interfacial impedance decreased to 26?·cm²,but the modifited lithium symmetrical cell also can operated at 0.4 mA/cm².Based on this,using Ag to modify the surface of LLZO,and adopting LLZO with different Li⁺conductivity to assemble the modified lithium symmetric batteries.The results shown that higher Li⁺conductivity is benefit for the batteries to operate at higher current density and the modified lithium symmetric battery can operated even at 0.5 mA/cm² when doping LLZO with the highest Li⁺conductivity.