Electrochemical Study On The Interfaces Of Cathode And Anode Of High-Energy-Density Lithium Metal Batteries In Ether Electrolytes

The replacement of the existing fossil energy with green renewable energy is an essential way to achieve the goal of "carbon neutrality".High-specific-energy secondary batteries provide a strong guarantee for the efficient usage of renewable energy.The energy densities of state-of-art lithium-ion batteries based on graphite anodes are approaching their theoretical limits,so the development of advanced highspecific-energy battery systems is imminent.Lithium metal is considered as an ideal anode material to replace the existing graphite anodes.However,almost all components of the electrolytes can irreversibly react with the lithium metal anode,especially the commercial organic carbonate solvents,accompanied by the severe parasitic reactions and dendrite growth conundrums.Ether electrolytes show lower reducing reactivity to Li metal and higher Coulombic efficiencies(>99%)compared to the conventional carbonate electrolytes.However,most ether electrolytes exhibit poor oxidation stability at high voltages above 4 V.Therefore,to simultaneously improve the energy density and cycling stability of lithium metal batteries,on the one hand,it is necessary to develop high-stability,high-energy cathode materials,such as transition metal sulfide cathodes with lower operating voltages but higher discharge capacities.On the other hand,novel electrolyte systems need to be developed to stabilize the electrochemical interfaces of both the strong reducing lithium metal anode and the aggressive oxidizing high-voltage cathodes.Therefore,it is essential to study the chemical/electrochemical processes as well as the composition and structure of the electrode interphases of lithium metal batteries.In view of the above problems and challenges,this thesis focuses on the structural design of the cathodes of high energy density lithium metal batteries in ether electrolytes,and the regulation mechanism studies of the electrolytes on the electrode interfaces.By combining various characterization methods,the interfacial chemical/electrochemical processes and interphasial structures of high-specific-energy cathodes and lithium metal anodes were systematically studied.The main research contents are as follows:1.The hollow structure of copper sulfide(CuS)nanoboxes is utilized to accommodate the volume change and facilitate the transport process of lithium ions.Even in the low-concentration ether electrolyte,the hollow CuS nanoboxes still exhibit a ultralong cycling life and excellent rate capability.The lithium storage mechanisms of CuS during charge-discharge processes are revealed by operando Raman spectroscopy,and the corresponding electrochemical reaction equations are supplemented.Our results demonstrate that the low-cost hollow CuS nanobox cathode has a broad application prospect in high-rate,high-energy Li metal batteries.2.The high-voltage cathode compatibility and the chemical process of aluminum corrosion reactions in ether electrolyte systems with a range of concentrations are systematically studied,revealing the key role of the free solvent ratios in the electrolyte solvation structure.As the salt concentration increases,the stability of the high-voltage cathode and the corrosion resistance of the aluminum current collector are enhanced accordingly.Combining high-resolution electron microscopy,Raman spectroscopy and theoretical calculations,we find that the network solvation structure can promote the formation of uniform LiF-rich interphasial layer,thereby improving the electrochemical stability of the battery.3.A single-salt single-solvent model electrolyte system is selected to systematically study the correlation between the electrolyte solvation structure,interphasial structure,and lithium metal deposition morphology.In the high-concentration electrolytes,the anion-dominated solvation structure initiates a rapid and thorough decomposition process of anions,thus resulting in the generation of an amorphous inorganic-rich interfacial layer.Such an amorphous interphase possesses a high specific interfacial energy,which induces dendrite-free bulk Li deposition.Our work helps to understand the origin of lithium dendrites and guides the rational design of high-performance electrolytes in lithium metal battery.4.By adding a branched chain structure to 1,2-Dimethoxyethane molecule,a new weakly solvating electrolyte is developed to stabilize both lithium metal anode and high-voltage cathodes.The high-voltage coin cells and pouch cells can both achieve stable cycling performance.Through cryo-electron microscopy,we have found that the interface layer formed on the surface of the high-voltage cathodes in the weakly solvating electrolyte contains a large number of uniform LiF nanoparticles,which can effectively passivate the high-voltage cathodes.Our work provides new insights on the molecular design of weakly solvating electrolytes and the understanding of the interfacial structure of high-voltage cathodes.