Design And Optimization Of Anode-free Lithium Metal Batteries Based On Lithium-rich Manganese Oxides Cathode

As an electrochemical energy storage device,lithium-ion batteries have been widely used in mobile power devices,cell phones and new energy vehicles since the21st century.The cathode active materials of conventional lithium-ion batteries are lithium iron phosphate,lithium cobaltate and ternary laminate materials,and the negative active materials are graphite and silicon.In order to further meet consumers’pursuit of battery energy density,lithium metal batteries have returned to the researchers’vision.Lithium metal is the"holy grail"among various cathode materials due to its extremely high specific capacity(3860 m Ah g^-1),the lowest reduction potential（-3.04 V vs SHE）and low density(0.534 g cm^-3).By using battery structures of commercial cathode materials and without negative active materials,the problem of lithium production and energy storage can be bypassed.However,conventional commercial materials have limited lithium storage capacity.And anode-free lithium metal batteries are particularly sensitive to active lithium consumption because all lithium sources only come from the positive electrode.Unfortunately,during the first charge of anode-free lithium metal batteries,SEI film is generated,which consumes a large amount of lithium ions,reducing its actual specific capacity.Both SEI and dead lithium consume the limited amount of active lithium in the battery to varying degrees,reducing the cycle life of the battery.The production of both SEI and"Dead Li"consume the limited active lithium in the battery to varying degrees,reducing the cycle life of the battery.In order to solve the above problems,this paper starts from lithium-rich manganese-based materials and uses MXene current collector and PVDF-HFP porous film to improve the cycle performance of anode-free lithium metal batteries,respectively.The main research results are as follows:（1）Two-dimensional flexible MXene films were prepared for use as anode collectors in Li-rich Mn-based anode-free lithium metal batteries using a vacuum filtration method to enhance the cycle stability of the full battery.The excess lithium ions from the Li–rich Mn-based material can be used to compensate for the consumption of SEI in the first cycle and mitigate the loss of active lithium in the next cycles.The reaction between the F element on the surface of MXene film and lithium metal produces Li F-rich SEI.Li F lowers the diffusion barrier of lithium ions and provides nucleation sites,which enables uniform nucleation and horizontal deposition of lithium metal,inhibits lithium dendrite growth,and improves the cycle reversibility of lithium metal.The specific capacity can reach 200 m Ah g^-1 for 15 cycles,and the capacity retention rate is 50%for 40 cycles.（2）Porous PVDF-HFP films were prepared by solvent evaporation method,The resulting porous PVDF-HFP films were used as an interlayer for Li-rich Mn-based anode-free lithium metal batteries to improve the cycling performance of the batteries.The porous PVDF-HFP film is an organic porous film that changes the path of lithium uniform deposition,resulting in a slower capacity decay during cycling compared to a full cell without interlayer protection.Moreover,the polarβ-phase structure of the porous PVDF-HFP film induces the generation of high Li F content at the negative interface,which increases the reversibility of lithium deposition/exfoliation at the negative electrode.In addition,the lithium deposition behavior was observed in Li||Cu half-cells at different current densities,and the lithium nucleation was more uniform and the electrode surface was smoother in the half-cells protected by the porous PVDF-HFP film.Li-rich Mn-based anode-free lithium metal cells protected with PVDF-HFP porous films still had 50%content retention at 40 cycles at a current density of 0.2 C.In summary,this thesis investigates an anode-free lithium metal battery based on Li-rich Mn-based material as the cathode active material.Firstly,we quantitatively analyzed the lithium replenishment effect of Li-rich Mn-based cathode material,and on this basis,we prepared two-dimensional flexible MXene collector and PVDF-HFP porous film from collector and interlayer respectively,and successfully improved the cycle life of Li-rich Mn-based anode-free lithium metal battery,and analyzed the influence of collector and interlayer on lithium ion deposition process and electrochemical performance from the perspective of interface The results show that the anode-free lithium-metal battery can be used as a solution for the development of anode-free lithium metal.