In-situ Deformation Characterization And Mechano-chemical Coupling Properties Of Fluorinated Carbon Electrodes

Lithium/fluorinated carbon（Li/CF_x）batteries have the advantages of high theoretical specific energy and stable voltage platform,and are widely used in medical and military equipment.However,the gradual aggregation of the product lithium fluoride（Li F）during the discharge process can cause the volume deformation of the CF_x material,destroying the electrode structure and affecting the release of battery capacity,which is particularly obvious at large discharge rates.Therefore,studying the stress/strain evolution law of CF_x electrodes during the discharge process has guiding significance for the design of electrode materials and the improvement of capacity.Therefore,in this paper,based on digital image correlation（DIC）,in-situ mechano-electrochemical testing system of electrodes suitable for Li/CF_x batteries was built,and the effects of material structure and discharge rate on electrode strain/stress evolution were studied.The relationship between strain/stress and the evolution of electrochemical properties are analyzed.The main research contents are as follows:Electrode design and performance testing and construction of in situ mechanical-electrochemical testing system.First,the flake-structured graphite fluoride and the commercial bulk-structured graphite fluoride were used as positive electrode active materials to prepare electrode.Then,different kinds of speckle particles were sprayed on the electrode and assembled into coin cells and in-situ simulated cells.The electrochemical performance test results show that the electrode performance of sprayed acrylic self-spraying paint particles is the best.Finally,in-situ mechanical-electrochemical test system for Li/CF_x battery electrodes was built by combining the DIC test system,the in-situ simulated battery with sprayed speckle,and the battery performance test system.Deformation in situ characterization and strain analysis of electrodes and pouch cells.The non-uniform distribution of the plane strain field of the CF_x electrode during the discharge process was characterized in real time by DIC technology,and the effects of material structure and discharge rate on the electrode strain evolution were studied.The results show that the（?）₁ of the flake graphite fluoride at 100%DOD is 0.501%,which is 61.5%lower than that of the commercial bulk graphite fluoride,indicating that the larger specific surface area and multi-layer structure can provide the growth of Li F crystals.space to relieve electrode volume deformation.Meanwhile,when the discharge rate increased from 0.5 C to 1 C,（?）₁ increased by28.6%,and showed worse electrochemical performance,the same phenomenon was also found in the in-situ observation of pouch cells.This may be due to the fact that under the large discharge rate,the Li F crystals did not have time to grow and aggregated,resulting in more serious damage to the electrodes and affecting the performance of the battery capacity.Lithium-ion concentration and electrode stress analysis.First,the change of the lithium-ion concentration of the CF_x electrode along the thickness direction of the active layer during the discharge process was studied,and the effects of the material structure and current density on the lithium-ion concentration of the active layer were analyzed.Then,the evolution law of plane stress of CF_x electrode is calculated and analyzed in combination with constitutive equation.At a discharge rate of 0.5 C,the（?）_xx and（?）_yy of the flake graphite electrode are-0.62 MPa and-2.53 MPa,which are significantly smaller than the plane stress of the bulk graphite fluoride electrode.When the discharge rate was increased to 1 C,the（?）_xx and（?）_yy of the electrode increased to-1.56 MPa and-5.60 MPa,respectively.Finally,the plane stress is disassembled into mechanically induced stress and electrochemically induced stress,and the results show that the electrochemically induced stress has a greater influence on the plane stress.