Research On The Designing,Preparation And Properties Of In-Situ Gel Electrolyte For Low Temperature Li-air Battery

Lithium-air battery（LABs）is the secondary battery system with the highest energy density at present,which is expected to meet the high energy density requirements for the commercialization of energy storage devices.However,electrolyte instability and insufficient ionic conductivity are considered to be the main problems affecting the application of Li-air batteries in sub-zero temperatures.The operation of electrolyte in low-temperature lithium-air battery involves not only the improvement of electrolyte material itself,but also the exploration of ion transport mechanism inside electrolyte.In addition,some CO₂ will inevitably participate in the reaction in the air battery,resulting in by-products at the positive electrode,which will hinder the long-term stable circulation of the air battery.Therefore,we have carried out the following researches:（1）A polymer-confined gel（PCG）electrolyte suitable for low-temperature LABs is developed by confining high donor number dimethyl sulfoxide（DMSO）within an in situ polymerized 4,4’-bis（stearoylamino）diphenyl ether（BSDE）matrix.The strong hydrogen bonds between the BSDE framework and DMSO molecules has been destroyed and thus the ionic conductivity at lower temperature has thus been improved.Furthermore,a local high-concentration electrolyte is constructed using DMSO as a solvent to dissolve the Li salt.The ionic conductivity of PCG in the fabricated electrolyte at-20℃is maintained at 0.15 m S cm^-1,which is 5.5 times higher than the corresponding liquid DMSO electrolyte.As a result,the LABs equipped with PCG display a long cycle life of 450 h（-20℃）and 760 h（25℃）,whereas the LABs with a liquid DMSO electrolyte fail at-20℃and can only operate for 40 h at 25℃.This study provides a feasible but facile strategy for the design of gel electrolytes suitable for low-temperature LABs,and this strategy can be extended to other energy-storage systems.（2）Based on the above work,in order to further study the ion transport mechanism inside the electrolyte,PVDF-HFP was added into the PCG to form a D-N electrolyte with dual network structure.The mechanism of action of each component in D-N electrolyte was deeply studied by theoretical calculation and various characterization techniques,and it was revealed that BSDE triggered the structural modification of PVDF-HFP polymer,which promoted the transport efficiency of lithium ions by fixing anions in polymer chains and constructing multiple heavy ion transport channels.Significantly improved ionic conductivity and Li⁺migration（0.67）.On the other hand,through the hydrogen bonding between PVDF-HFP and BSDE,a more solid polymer network structure is constructed,which enhances the limiting effect on liquid DMSO and improves the electrochemical performance of electrolyte in a wider temperature range（-20-50℃）.The lithium-air battery assembled in situ with D-N electrolyte was improved from 450 hours of stable cycling to 600 hours at-20℃,from 760 hours to more than 1200 hours at 25℃,and from less than 100 hours to more than 200 hours at50℃.This work establishes a multi-component gel polymer electrolyte system,dissects and reveals a multi-heavy ion transport model,and promotes a deeper understanding of advanced electrolyte design and ion transport with new structures.（3）The existence of Li₂CO₃ was found in the discharge product detection of the above two works,which was mainly due to the participation of CO₂ in the air.Li₂CO₃,as a substance with poor conductivity,high oxidation potential and difficult decomposition,is covered on the surface of the cathode material,which will inevitably lead to low round-trip efficiency and short cycle life of the air battery,and seriously hinder the long-term stable cycle of the lithium-air battery.Thus,the in-situ gel polymer electrolyte is introduced into Li-CO₂ batteries.The presence of the electrolyte additive ethylenediamine（EDA）can not only induce TEGDME crosslinking polymerization to form in-situ gel electrolyte.Protect the lithium metal anode while efficiently capturing CO₂ to reduce its accumulation.The total discharge capacity of Li-CO₂ battery assembled with this gel electrolyte is up to 32 A h g^-1,and the cycle life is more than2000 hours.This work takes the recycling of polluted gas as the innovation point,which provides a strategy of killing two birds with one stone for the improvement of environment and the development of energy storage devices.In addition,the influence of CO₂ concentration on the open-circuit current voltage is revealed by measuring GITT,and the relationship between concentration and voltage in Nernst equation is also proved.