The Influence Of Electric Field On The Measured Diffusion Coefficient Of Lithium Ion

At present,the influence of the electric field is not considered when measuring the diffusion coefficient(D_Li+)of lithium ions in lithium intercalation materials.In fact,it is affected by the direction of the electric field in the system.Based on this,in this paper,different electric field directions are constructed by selecting different counter electrodes,and then the current pulse relaxation（CPR）technique is used to measure D_Li+ by means of positive disturbance and negative disturbance respectively.The apparent D_Li+was characterized by the recovery time of the electrode potential after the perturbation.For the recovery time of the electrode potential,three criteria were used to analyze the obtained potential-time data to determine the law of the influence of the electric field on the measurement results,and the sensitivity and reliability of the three criteria were compared.The work of this paper is as follows:（1）For the graphite electrode system,when the counter electrode is lithium metal,after lithium intercalation,the potential recovery time after positive disturbance（hereinafter referred to as T_P）is shorter than the potential recovery time after negative disturbance（hereinafter referred to as T_N）;after delithiation,the first In positive and negative disturbances,T_P>T_N,in the second positive and negative disturbances,T_P<T_N.When the coumter electrode is Li（NiCoMn）O₂,in the first positive and negative disturbance after delithiation,T_P<T_N,in the second positive and negative disturbance,T_P>T_N;after lithium intercalation,in the two positive and negative disturbances,T_P>T_N.There are two possible reasons why the two positive and negative disturbances lead to opposite results:one is due to the sequence of positive and negative disturbances,and the other is that the potential recovery speed is too fast,the selected point is too close to the initial stage of recovery,and there are too few selectable points to choose from,resulting in Errors form and ultimately affect the results.（2）For the Li₄Ti₅O₁₂ electrode system,when the counter electrode is metal lithium,T_P<T_N,and the second positive and negative perturbation results are more obvious.When the counter electrode is Li₄Ti₅O₁₂,there is almost no difference between T_P and T_N,but there is a tendency that T_P is slightly longer than T_N.When the counter electrode is Li（NiCoMn）O₂,T_P>T_N during delithiation,but there is no obvious regularity in the results after lithium intercalation.There are two possible reasons:one is that the selected value of the recovery ratio is unreasonable,the selected value is too close to the initial stage of recovery,and the lithium ion diffuses too fast,resulting in inconsistent results;the other is that the lithium ion is in the process of extraction or intercalation.（3）For the Li（NiCoMn）O₂ electrode system,when the counter electrode is metal lithium,after lithium intercalation,T_P<T_N;after delithiation,in the first positive and negative disturbance,T_P<T_N,in the second positive and negative disturbance,T_P>T_N.Li（NiCoMn）O₂Li（NiCoMn）O₂ batteries are the same as Li₄Ti₅O₁₂-Li₄Ti₅O₁₂ batteries,positive and negative disturbances have no effect on the potential recovery time.（4）The system was analyzed based on electrochemical theory,and the sensitivity and reliability of the three criteria were compared.The results show that the recovery ratio and recovery value criteria are more sensitive.There are three problems in the recovery rate criterion:①When there are too few points to be selected,the potential recovery time after the two disturbances will have no point with the same slope,resulting in the situation that the recovery rate cannot be selected;②When the data points are too dense,a slope It can be corresponded by multiple data points,resulting in the situation that the recovery rate cannot be selected;③poor sensitivity.