In the above, we introduce the main cause of accelerated cycling of decay in parallel LFP batteries - negative electrode copper foil dissolution, then we will continue to introduce how to use CT imaging tools from the positive and negative active material particle level analysis led to battery capacity decline the reason.
From the previous analysis we can see that the LFP positive electrode did not change significantly after cycling, indicating the good stability of the LFP material. In order to further analyze the cathode material microstructure changes, Rachel Carter using CT on the positive electrode structure (As shown in the figure below). As can be seen from the figure, the positive electrode of LFP is very uniform. The change of cathode active material and current collector thickness after cycling is very small, which can basically eliminate the volume expansion of positive electrode and the loss of active material The analysis at the particle level also showed that the LFP material is well-stabilized inside the cell and therefore the LFP material is not a major contributor to the decay of the LFP cell.
LFP material in the exclusion of the factors that led to the decline of LFP battery falls on the negative electrode in the previous battery anatomy analysis we can see that the cycle of the negative electrode not only in the electrode on both sides of the edge of the region The phenomenon that the active material and the copper foil peeled off occurred and a part of lithium intercalation (stage 1 lithium intercalation) graphite material was deactivated in the middle of the electrode, resulting in a light orange phenomenon in this part of the electrode, We found that copper deposition occurred on the surface of the negative electrode and the separator, and the deposited Cu element on the surface of XPS analysis appeared in the metal state.All the indications are that the main factor leading to the decay of the LFP cell is negative-polarity Change, so Rachel Carter focused on the negative analysis.
The figure above shows the results of CT imaging of the negative electrode after 1200 cycles of a single battery and 750 cycles of the parallel battery. It can be seen from the above figures a and b that both negative electrodes basically maintain the same structure, but in the parallel cycle After the battery, due to the dissolution of the copper foil and the redeposition of the Cu element on the negative electrode and the separator surface, we can see the structure of the separator in the CT image.
Through the analysis of CT images, we can see that there is no visible damage on the LFP battery negative electrode copper foil, but we found a lot of pitting damage on the copper foil of the LCP battery in parallel circulation.Further analysis of copper foil The thickness of the copper foil was found to be 0.5nm thinner than that of the LCP battery Cu foil which was cycled alone after the parallel cycle was found, which proves the dissolution of the copper foil in the cell after parallel cycle we found before. CT imaging analysis The results show that there is a spatial relationship between the position of Cu deposition on the negative electrode and the location of copper foil dissolution damage, indicating that the copper foil is deposited on the negative electrode surface and the separator during the migration to the positive electrode after dissolution.
Analysis of the particle level of the negative electrode (as shown in the following figure) showed that the size of the partially damaged area on the copper foil reached about 10 μm and had penetrated the copper foil. Further analysis showed that as the copper foil oxidized and dissolved, the surface characteristics of the copper foil Has changed in the surface of the copper foil dissolved in the vicinity of the porous negative electrode active material and copper foil between the force will be significantly weakened, resulting in the active material delamination and glass stripping, causing the negative electrode electrochemical Reduced activity leads to a decrease in capacity during cycling.
Rachel Carter's work shows that while lithium-ion batteries are paralleled in parallel, they undergo a rigorous internal resistance matching<0.1毫欧) 和容量匹配 (差别<0.2%) , 但是在经过大电流脉冲放电循环后仍然出现了明显的电流分布不均匀的现象, 在脉冲放电后的静置过程中, 并联电池之间的再均衡电流达到了1A, 这表明在脉冲放电的过程中并联电池之间也出现了明显的放电容量差异, 这就非常容易导致并联的电池出现部分电池过充或者过放. 循环测试结果也验证了上述推断, 单独循环的电池在大电流脉冲放电循环1200次后, 容量衰降了28%, 但是并联的电池在循环750次后, 容量就衰降了35%, 远远高于单独循环的电池.
The research on the mechanism of battery decay in parallel cycle shows that the main factor of the capacity decay of LFP battery in impulse discharge cycle is not the positive electrode of LFP but the negative electrode.The partial discharge of the battery occurred in the cycle due to the uneven current distribution Release, caused the dissolution of copper foil, and in the negative electrode and the deposition occurred on the diaphragm, resulting in the active material and the copper foil peeling and delamination, causing the negative active material electrochemical activity decreased, resulting in parallel circulating LFP battery capacity decline accelerate.
