Study On The Preparation And Performance Of Electrolytes For All-Solid-State Li-CO₂ Batteries

Li-CO₂ battery uses the greenhouse gas CO₂ as the working gas and has a high theoretical energy density as 1876 Wh/kg,which makes it one promising option of the next generation of high-energy-density secondary batteries.Many researches have been made on the application of the solid-state catalysts and redox medium（RM）and received good results.However,the organic liquid electrolytes have the nature of flammability and volatility and some of them will decompose under the high charge voltage of Li-CO₂ batteries.These drawbacks of liquid electrolyte bring the semi-open batteries safety hazards and performance degradation.Therefore,non-flammable and non-volatile solid-state electrolytes have been applied in Li-CO₂ batteries.However,the Li-CO₂ batteries using solid-state electrolytes are still facing many challenges.Firstly,the discharge product of Li-CO₂ battery,Li₂CO₃,has poor conductivity and is difficult to decompose,which results in a large charge overpotential.Secondly,the solid-solid interface between solid-state electrolyte and electrodes doesn’t contact as well as the solid-liquid interface in liquid electrolyte.This will lead to a larger interface impedance,which affects the battery performance badly.Thirdly,some solid-state electrolytes are unstable with the lithium metal negative electrode and require interface protection.Therefore,we designed a few all-solid-state Li-CO₂ batteries using inorganic solid-state electrolyte Li_1+xAl_yGe_2-y（PO₄）₃（LAGP）and polymer electrolytes in this essay,hoping to propose some solutions to the problems mentioned above.Firstly,when LAGP was chosen as the electrolyte of Li-CO₂ battery,RuO₂nanoparticle was added in the cathode material which uses single-walled carbon nanotubes as electronic conductors and LAGP as ion conductors.In this way,the decomposition kinetics of Li₂CO₃ can be promoted.Then,the composited cathode material was directly coated on the surface of LAGP film to improve the interface contact.Finally,a germanium protective layer is sputtered on the other surface of LAGP to avoid lithium reacting with the solid-state electrolyte.This Li-CO₂ battery could work between 25℃and 60℃,reach a discharge capacity of 2499 m Ah/g at 60℃and sustain 30 cycles under a capacity limitation of 500 m Ah/g at a current density of 50m A/g.Secondly,less expensive catalyst NiO has been applied in the Li-CO₂ battery to replace relatively costly RuO₂ catalyst.At the same time,PEO-Li TFSI-SN polymer electrolyte has been used instead of Ge layer to skip the complicated procedure of melting lithium.By comparing the conductivity and the width of electrochemical window,PEO-Li TFSI electrolytes with 20%SN inside was chosen in this work.The double-layered Li-CO₂ battery could achieve a specific capacity of 2024 m Ah/g at 60℃,and also could be charged and discharged reversibly at room temperature.30 cycles has been achieved at a constant capacity of 500 m Ah/g at 60℃.Thirdly,in order to explore the application of Li-CO₂ battery in flexible devices,we tried to use poly（propylene）carbonate（PPC）based electrolyte in the Li-CO₂ battery.Experiments showed that the impedance of the interface between PPC-Li TFSI polymer electrolyte and lithium metal anode decreased after activating under 80℃.At this time,the flexible Li-CO₂ battery can be charged and discharged for 20 cycles at a constant capacity of 1000 m Ah/g at room temperature.But we also found that after recycling,the lithium metal anode corroded seriously,which limited the cycle life of the battery.Hence,The protection for lithium metal anodes under this system remains to be studied.In summary,we have designed and prepared some all-solid-state Li-CO₂ batteries that can be charged and discharged reversibly,studied the impact of different anode protection methods and different cathode catalysts on battery performance.Preliminary exploration of flexible all-solid-state Li-CO₂ batteries has also been carried out,providing some ideas for the study of next-generation high-energy-density secondary batteries.