Research On Interfacial Behavior Of Li Metal Anode In Concentrated Electrolyte And Electrolyte With Inorganic Additive

Lithium-ion batteries(LIBs)have been successfully employed in many fields,such as portable electronic products,electric vehicles and large-scale energy storage systems.These products efficiently improve people's quality of life and further promote the development of society.However,LIBs employing graphite anodes(with a theoretical specific capacity of 372 m Ah g^-1)have limited room for specific energy improvement and can no longer meet the requirements for high-power electronic products and long-life electric vehicles.Therefore,the booming demand for higher energy density energy storage systems has led researchers focus back to the lithium metal anode(with a theoretical capacity of 3860 m Ah g^-1).With the support of Li metal anode,the specific energy of the battery can be evidently improved.However,the introduction of Li metal anode will also contribute to some new challenges.First of all,the electrochemical potential of lithium metal is very low and can react with almost all electrolytes.Thus,it is particularly important to select electrolytes with stable chemical properties.Secondly,when Li metal batteries cycled at high rate,the uneven distribution of charge on the surface of current collector will cause the uncontrollable growth of dendritic lithium and aggravated side reactions.Moreover,the excessive utilization of Li metal will significantly decrease the volumetric energy density of Li metal battery,so the development of anode-free Li metal batteries is the ultimate goal of research.During Li metal batteries'cycle,the generation of“dead Li”cannot be avoided(“dead Li”refers to the unreacted Li that is wrapped by the thick solid electrolyte interface(SEI)layers).Finding the approach of reactivating“dead Li”is of great significance for further improving the cycling performance of Li metal batteries.In this work,to deal with these problems,we design new strategies to improve the cycling performance of Li metal batteries by optimizing the electrolyte components and the electrochemical behavior of the Li/electrolyte interface is also studied.The main research results are summarized as follows:(1)A high-concentration double-salt electrolyte(4MLiFSI-LiNO₃/DOL)is developed to adjust the composition of SEI layers to obtain a uniform Li deposition morphology.As a result,the cyclic Coulombic efficiency of the Li metal anode is improved.We have found that,Li FSI in the electrolyte is easily hydrolyzed to generate Lewis acid,which accelerates the ring-opening reaction of DOL.When Li NO₃ is introduced into this system,the formation of Lewis acid is suppressed,and the continuous polymerization of DOL can be successfully inhibited.In-situ Raman test and electrochemical impedance test show that,the prepared electrolyte remains stable during long-term storage.In this electrolyte,sufficient lithium salt can quickly passivate the fresh Li deposition and generate a passivation layer rich in Li F;DOL can form a flexible organic layer on Li metal's surface,significantly improving the SEI layer's mechanical stability during plating and stripping of Li.Assisted with this Li F-Li₂O-oligomer composite film,Li depositions are densely packed and uniform in size.Under1.0 m A cm^-2 and 1.0 m Ah cm^-2,the Li metal anode can cycle stably for 450 times,and the Coulombic efficiency reaches 99.40%.(2)In 4MLiFSI-LiNO₃/DOL,a stable and dense organic-inorganic composite SEI layer can be quickly formed on the surface of fresh Li deposition,which effectively improves the mechanical stability of the SEI layer during the rapid dissolution and deposition of Li metal.The battery's rate performance is thus significantly improved.When the current density increases from 1.0 m A cm^-2 to 8.0 m A cm^-2,the distribution of Li deposition becomes loose slightly.However,the overall dense and uniform morphology remains unchanged.When the test currents are 2.0 m A cm^-2 and 4.0 m A cm^-2,the Li metal anode can cycle stably for 300 times and the average Coulombic efficiency reaches 99.35%and 99.20%respectively.Even at a current density as high as 8.0 m A cm^-2,the Li metal anode remains stable for over 240 cycles,and the average Coulombic efficiency reaches 99.14%.Based on this electrolyte,the Li-LFP full battery is designed and assembled with only 0.58×excess Li.It is found that the battery remains stable for 120 cycles with the capacity retention of 78.7%,and the average Coulombic efficiency is 99.70%.(3)In order to improve the utilization ratio of lithium in anode-free lithium metal battery,we introduce Li I,commonly used as a redox mediator,into the carbonate electrolyte:1 M Li PF₆/EC-DMC-DEC.We plan to activate the“dead Li”via the constant voltage operation triggered reaction between I^-/I₃^-.After adding Li I,the battery's electrochemical performance changes.I₃^-can pass through the thick SEI layer to react with“dead Li”to form Li⁺and I^-.The X-ray diffraction(XRD)results show that the residual amount of the unreacted Li metal on the anode is significantly reduced,and the“dead Li”can be successfully recovered and embedded into the Li Fe PO₄cathode in this way.Besides,scanning electron microscopy(SEM)images exhibit a denser Li deposition and the Li F concentration of SEI layer increases after adding Li I.The cycling performance of the LFP-SS battery is tested,and it can be seen that the addition of Li I can extend the cycle life of anode-free lithium metal batteries.This thesis puts forward several new strategies to improve the Coulombic efficiency and utilization ratio of Li metal anode,thereby improving the cycling performance of Li metal batteries.Our work also provides a new direction for the design of diverse of Li metal batteries with better performance.