Study On Porosity Distribution Of Lithium-ion Battery Electrode And Its Effect On Rate Performance

The increasingly widespread application of lithium-ion batteries in the field of electric vehicles places higher requirements on the energy density and rate performance of lithium-ion batteries.Therefore,how to improve the rate performance of lithium-ion batteries under the existing process is a hot topic at the moment.The existing electrode manufacturing process thickens the electrode and rolls it to reduce the porosity of the electrode and increase the volume ratio of the active material.Although the volumetric energy density of the battery can be increased,the discharge capacity of the electrode at a higher discharge rate still has Greater loss.In this paper,a systematic study of the electrode preparation process on the porosity distribution and the rate performance problems caused by the electrode is carried out.The main work is as follows:The cross-sectional porosity and planar porosity distribution of the lithium-ion battery electrode are studied through the SEM image of the electrode,which provides a basis for the subsequent modeling of lithium ion battery rate performance considering the porosity distribution.Analysis of the distribution of the thickness direction porosity of the lithium iron phosphate electrode,ternary material electrode and graphite electrode shows that the porosity of the lithium iron phosphate electrode has a larger surface layer and a smaller bottom layer,and the ternary electrode and graphite electrode have a smaller surface layer and a lower layer Larger.By analyzing the porosity of the unrolled and rolled graphite electrode surfaces,it is found that the frequency distribution of the unrolled electrode plane porosity is more dispersed,and the frequency distribution of the electrode plane porosity is more concentrated after rolling.A P2 D model considering the gradient distribution of electrode thickness to porosity and a P2 D model considering the uneven distribution of electrode plane porosity are established.According to the electrode thickness porosity distribution rule,the electrode thickness porosity gradient distribution model is proposed,coupled with the P2 D model,and the P2 D model considering the electrode thickness porosity gradient distribution is obtained;the electrode plane partition parallel circuit model is used to describe the electrode plane porosity The nonuniform distribution of the electrode,and the electrode plane partition parallel circuit model and the P2 D model are coupled through the sub-cells on the parallel branch,and the P2 D model considering the uneven distribution of the electrode plane porosity is obtained.The proposed model was verified and proved the rationality of the model.The P2 D model considering the thickness distribution of the electrode thickness was used to calculate the rate performance of the graphite electrode half-cells with different thicknesses.The results show that when the average electrode porosity is 0.2,the reverse gradient of the graphite electrode thickness porosity caused by the existing process Compared with the electrode thickness to no gradient distribution,the electrode rate performance is significantly reduced,and when the average porosity of the electrode is 0.4,there is no difference in the rate performance of the two electrodes.The results of the corresponding electrochemical mechanism analysis show that the electrolyte mass transfer difficulty caused by the reverse gradient distribution of the thickness of the electrode to the porosity leads to a decrease in the utilization rate of the active particles,and the increase in liquid resistance increases the capacity loss.According to the results of the mechanism analysis,it is inferred that the positive gradient distribution of electrode porosity can improve the rate performance and verified by calculation.The optimal values are 0.12 and 0.24,respectively.Based on the P2 D model considering the non-uniform distribution of electrode plane porosity,the concept of non-uniformity of porosity(standard deviation of electrode plane porosity)is proposed according to the rule that the frequency distribution of the electrode plane porosity is close to the normal distribution.To describe the degree of non-uniform distribution of porosity in the electrode plane.The rate performance of graphite electrode half-cells with the same average porosity and different electrode plane non-uniformities were simulated.The results show that the smaller the electrode porosity non-uniformity,the better the rate performance,but there is an optimal electrode plane porosity non-uniformity σp0 = 0.01,so that when the non-uniformity of the porosity of the electrode plane is lower than this value,the rate performance will no longer have a significant change.The corresponding electrochemical mechanism analysis results show that the lower the porosity heterogeneity of the electrode plane is,the closer the SOC of each area of the electrode plane is,and the closer the current density of each area is,and the greater the non-uniformity of the electrode plane porosity is the opposite.Therefore,try to ensure that the unevenness of the porosity of the electrode plane is reduced to around 0.01.