In Situ Measurement And Experimental Mechanical Research Of Lithium Diffusion In Graphite Electrodes

Solid-phase diffusion of lithium in electrode is a fundamental issue in research of lithium-ion battery.The phase transition of structure,electrode deformation and stress caused by Li diffusion in electrode are important to understand degeneration mechanisms of electrode strength and battery cycle,which are hot issues in fields of mechanics,chemistry and material science.The experimental research about evolution of potential,phase transition,battery capacity,Li concentration,and electrode deformation is helpful to adjustion of battery performance and optimization of lithium storage quality.In this paper,an experiment research on Li solid-phase diffusion is performed.It is concerned about cooperative in-situ measuring techniques about Li concentration field,phase field,deformation field during Li intercalation/deintercalation,quantitative characterizations of concentration field,and phase field,and experimental analysis of influence factors of Li diffusion and phase transition.There are three parts in this paper.First,a well-sealed optical electrochemical half-cell with electrodes installed side-by-side is designed,coordinating with an image acquisition system,to present the variations of graphite electrode color during lithiation.According to the RGB numerical model of color,a calibration method for the proportion of original red(R),green(G),blue(B)composed typical graphite electrode color is proposed based on a correspondence between electrode color,phase and Li concentration,which realize a visible and quantitative characterization of Li solid-phase concentration field,phase field.The Digital Image Correlation(DIC)with active optics technology is implemented to relieve a decorrelation of electrode digital images caused by color changing,and the corresponding optical path is designed,which couples with ordinary image acquisition optical path to compose a cooperative in-situ measuring system with dual-optical paths.Li concentration field,phase transition field and corresponding electrode deformation field are measured in-situ synchronously by cooperative measuring system.The results indicate that the measuring system alleviates decorrelation of electrode images,improves precision of the DIC measureing and provides an experimental platform to further study of mechanics-electrochemical coupling in lithium ion battery.Second,the experiment in-situ presents variation of electrode color and three phase transitions of graphite electrode,namely C-Li C₁₈,Li C₁₈-Li C₁₂,Li C₁₂-Li C₆,during lithiation and delithiation with mechanics-electrochemical coupling.The Li concentration distribution curve along direction of Li transfer and its time evolution have been quantificationally shown by the RGB calibration method.The apparent Li diffusion rates are calculated based on concentration curves.Influence factors of Li diffusion,such as potential,concentration(gradient),phase transition and deformation(gradient),are discussed based on time evolution of apparent Li diffusion rate.The asymmetry of lithiation and delithiation is discussed from two aspects of phase transitions and distribution of Li concentration respectively.Furthermore,results of cooperative in-situ measurements indicate that there is an obvious kinetic causal relationship between Li concentration and electrode deformation,which confirms a feasibility of the dual-optical path measurement method proposed in this paper.Third,The Li intercalation/de-intercalation in electrode at C-rates of C/7(high),C/10(medium),C/15(low)and cycles of 1?4 are further researched experimentally and evolution of electrode color are presented.The phase volume fraction,migration rate and width of phase interfaces,distribution of Li concentration and evolutions of them over time and potential at different C-rate are quantitatively analyzed during phase transition of C-Li C₁₈,Li C₁₈-Li C₁₂,Li C₁₂-Li C₆.The influence of C-rate and cycles are discussed quantitatively from two aspects of Li diffusion and phase transition respectively.The experimental results display a correspondence between potential,phase transition and capacity and the effects of C-rate.The influence mechanisms of C-rate are analyzed from the perspective of energy.There are two effects of“accelerate”and“restrict”of high C-rate have been summarized.Based on the experimental analysis,an optimization suggestion with balancing of battery capacity and lithiation rate is further proposed.