Development And Mechanism Of Modification Strategies For Lithium Cobalt Oxide Cathode Materials

Lithium cobalt oxide(LCO)cathode plays a dominating role in the lithium ion battery(LIB)cathode material market due to their stable electrochemical performance and high specific capacity.In order to further improve the energy density of the battery,the development of the next generation of batteries is attracting more and more attentions.The typical examples including high-voltage liquid-electrolyte LIBs and allsolid-state LIBs both require cathode materials with high bulk and surface structural stability.Aiming at the development of stable high-voltage LCO cathode materials for liquid-electrolyte and all-solid-state batteries,this thesis has developed a series of novel strategies and functional coating materials for efficient surface engineering and bulkstructure modification of the LCO materials,the relationship between the technological parameters for modification,the composition and structure of the coating material and its interaction with the LCO core were clarified,and their role in stabilizing the cathodeelectrolyte interface as well as in maintaining the bulk structural integrity of the bulk was also studied.Firstly,a high-speed solid-phase coating method was employed for encapsulation of Al-doped LCO in a continuous Li4Ti5O12(LTO)layer(LTO@LCO).The structure and morphology of the core-shell LTO@LCO were investigated by EPMA,XPS,TEM and other characterization techniques,and the role of the coating layer in enhancing the electrochemical performance of the relevant liquid-electrolyte LIBs under high-voltage was studied.The cycling performance of the composite exhibits a significant improvement compared with the bare LCO.Typically,the 0.15%LTO@LCO could yield a capacity of 160.4 mAh g-1,which transforms into a retention of 89.9%after 100 cycles.Moreover,a stronger signal intensity of Al was observed on the LCO surface than in the interior after cycling,revealing outward dissolution of Al dopants and the formation of surface AIF3 in the early cycles;thus,on-site generated coating layer and the pre-coated LTO layer work together to stabilize the electrolyte/cathode interface and suppress the dissolution of transition metal.To further study the interaction between titanium oxides and LCO,the hightemperature reaction between TiO2 and LCO was manipulated,through which a continuous ultrathin pure-phase Li2CoTi3O8(LCTO)layer was formed on-site on the LCO surface.When the resulting LCTO@LCO was applied as cathode material in Li10GeP2S12(LGPS)-based all-solid-state lithium-ion batteries ASSLIBs,such a LCTO layer functions as an interlayer between the LCO and the LGPS solid electrolyte(LGPS),which contains a stable 3D network of spinel structures,relatively low electronic conductivity(2.5×10-8 S cm-1),high lithium diffusion coefficient(DLi+=8.22×10-7 cm2 s-1),strong mechanical strength.Furthermore,the LCTO is not only thermodynamically and electrochemically compatible,but has high interfacial affinity with both LCO and LGPS,which provides a favorable bridge that connects tightly with both LCO and LGPS during cycling,thus preventing generation of gaps and inhibiting increase of impedance.With enhanced solid-solid interfacial contact between the cathode and the solid electrolyte,the cycling stability of the LCTO@LCO is notably improved,which yields an initial capacity as high as 140 mAh g-1 with a retention of 82.9%after 200 cycles.Through the compositional and structural analysis of the cathode-electrolyte interface after electrochemical cycling,it was found that LCTO,as the interface layer,could greatly suppress the oxidative decomposition of LGPS.Moreover,the cross-sectional SEM and EPMA images of the cycled cathode material reveal that both the LCTO coating layer and the LCO active materials maintain high structural integrity after cycling,while the bare LCO material suffers from obvious cracks,which would lead to peneratration of adverse side reactions deep into the interior of the particles.Last but not least,barium lithium borate(LiBa(B3O5)3,LBBO),was selected as coating material of LCO in all-solid-state battery based on the previous theoretical calclation researches.The electrochemical stability and lithium ion diffusion properties of LBBO materials were explored thereotically.Various cross-sectional characterization techniques such as FIB-SEM,FIB-EPMA,FIB-TEM reveal that the element Ba could diffuse into the bulk phase of LCO,which serve as a "pillar" in the layered structure and enlarge the interlamellar spacing.On the other hand,the composition of coating layer consists of crystalline LBBO,Li3BO3 and some amorphous regions.With addition of 0.3 wt%LBBO,the resulting coating layer was observed to be 10 nm in thickness,and the cycling stability of the LBBO@LCO is improved notably,enabling a high initial capacity of 155 mAh g-1 with a retention of 80%after 150 cycles.