All-solid-state Thin Film Lithium Batteries Based On Garnet-type Solid Electrolytes

Compared to commercial lithium ion batteries based on liquid electrolyte,all-solid-state lithium batteries has seen a resurgence of research interests in recent years for their potential to offer high energy density and excellent safety.This is mainly because of the advantages of solid electrolyte composed of inorganic materials with nonflammability and wide electrochemical window.Although the concept of all-solid-state lithium battery has a long history,it still faces many scientific and technical difficulties in its practical application.Recently the garnet-type solid electrolytes Li₇La₃Zr₂O₁₂?LLZO?provides the basis for the development of all-solid-state lithium batteries due to excellent stability with lithium,high ionic conductivity and wide electrochemical window.However,there are some problems in building all-solid-state lithium batteries based on LLZO solid electrolytes.Firstly,it's difficult to get stable structure of cubic LLZO,and the grain boundary resistance of ceramic is large.Secondly,the interface problems between electrode material and LLZO solid electrolyte limit the development of high performance lithium batteries.Therefore,based on the preparation of LLZO solid electrolytes with high ionic conductivity,this thesis carried out the research to investigate the compatibility and interface characteristic between anode material and LLZO solid electrolytes and the major results are as follows:?1?LLZO solid electrolytes with high ionic conductivity were prepared by element doping.The doping modification of LLZO solid electrolytes was carried by adjusting the content of W elements.And hot-pressing sintering was used to prepare the Li_7-2xLa₃Zr_2-xW_xO₁₂ ceramics.It was found that the introduction of W element can stabilize the cubic garnet structure of LLZO solid electrolyte.The cross-section SEM images shows hot-pressing sintering was beneficial to improve contact between grains,decrease the pores and increase relative density.With a W doping content of X=0.4,the relative density of Li_7-2x-2x La₃Zr_2-xW_xO₁₂ ceramics was 98%with the highest ionic conductivity of 8.73×10^-4 S·cm^-1.The electronic conductivities of LLZWO ceramics tested by direct current polarization method were three orders of magnitude lower than ionic conductivity.CV method indicated the electrochemical windows of LLZWO ceramics was more than 5 V.Besides,Li_6.4La₃Zr_1.4Ta_0.6O₁₂ ceramics with relative density of 99%and ionic conductivity of 1.01×10^-3 S·cm^-11 were prepared by Ta element doping.?2?In order to investigate the interface characteristic between Si anode and LLZO solid electrolytes,amorphous Si layers were deposited on LLZO solid electrolytes via direct current magnetron sputtering and the Li/LLZTO/Si all-solid state thin film lithium cells were fabricated.The thickness of Si layers has great influence on the cycling performance of the Li/LLZTO/Si cells.It is found that Si layer anodes thinner than 180 nm can maintain good contact with the LLZTO solid electrolytes,leading the Li/LLZTO/Si cells to exhibit excellent cycling performance with a capacity retention over 85%after 100 cycles.As the Si layer thickness is increased to larger than 300 nm,the capacity retention of Li/LLZTO/Si cells becomes 77%after 100 cycles.When the thickness is close to 900 nm,the cells can cycle only for a limited number of times because of the destructive volume change at the interfaces.The Si/LLZTO/LFP full cells with the Si anode can work stably at room temperature.They show first discharge capacities of120 mAh g^-1,which can cycle 100 times with a capacity retention of 72%.?3?To study the interface characteristic between Ge anode and LLZO solid electrolytes,Ge layers were deposited on LLZO solid electrolytes to build the Li/LLZTO/Ge all-solid state thin film lithium cells by direct current magnetron sputtering.The capacity of the cells decayed seriously during the charge and discharge process because of the large volume change at the interface between Ge and LLZTO solid electrolytes.The introduction of Au intermediate modification layers between Ge and LLZTO can improve the performance of the Li/LLZTO/Ge all-solid state thin film lithium cells.The effect on 50-nm-thick Ge layers was obvious,and the capacity retention of cells improved from 28%to 90%.The reason was because Au intermediate modification layers provide better contact.When the thickness of Ge layers became larger,the effect of Au intermediate modification layers was limit due to large volume change.?4?The principle of space charge layer based on defect chemistry near the two-phase boundary from the thermodynamical point of view is firstly presented.The space charge layer effects in several typical conducting systems as well as the influence on properties are reviewed.On this basis,the space charge layer effects in rechargeable solid state lithium batteries reported so far are reviewed.Characterization techniques of the space charge layer effects are introduced.