Ternary materials + silicon/carbon composite system is the mainstream high-energy-density battery research and development direction of major battery manufacturers. The theoretical specific capacity of silicon materials can reach 4200mAh/g, which is more than ten times that of graphite materials. The lithium insertion voltage platform is close to that of graphite. It is an ideal negative electrode material for lithium-ion batteries. However, the volume expansion of Si materials can reach more than 300% during the lithium insertion process, which will not only cause the pulverization and fragmentation of Si materials themselves, but also cause the destruction of the structure of the silicon-containing negative electrode itself, resulting in the loss of active substances.
The application of silicon-carbon materials puts forward higher requirements on the binder and conductive agent system of the negative electrode. The traditional PVDF binder is too rigid and is not suitable as a binder for the Si negative electrode system. Polyacrylic Acid (PAA) binders have a high number of carboxyl groups. These groups can form hydrogen bonds with certain groups on silicon material surfaces. This helps create a negative electrode SEI film, which improves the cycling performance of silicon negative electrodes. Therefore, PAA binders are a very excellent binder for Si negative electrode. Studies have shown that the performance of Li-substituted acrylic acid (LiPAA) is better than that of PAA binder itself, but the reason is not clear.
Kevin A. Hays used 15% nano-Si and 73% artificial graphite, 2% carbon black, and 10% binder (PAA or LiPAA) system to study the two binders. After the above electrodes were initially dried, Kevin A. Hays also performed secondary drying at a temperature between 100-200°C to completely remove the moisture in the electrodes. The test of button batteries showed that the capacity of the negative electrode using LiPAA binder was about 790mAh/g, while the capacity of the negative electrode using PAA binder was about 610mAh/g.
The figure above shows the cycle performance curve of the full battery using NMC532 as the positive electrode. From Figure A, we can see that the cycle performance of the full battery using LiPAA binder has no obvious relationship with the temperature of the secondary drying. The initial capacity of the NMC532 positive electrode at C/3 rate is 127mAh/g, and the capacity decays to about 91mAh/g after 90 cycles. From Figure B, we can find that the cycle performance of the battery using PAA binder has a significant relationship with the temperature of the secondary drying of the negative electrode (120℃ red, 140℃ gold line, 160℃ green line, 180℃ blue line). Based on initial capacity, the PAA battery with the NMC532 positive electrode shows the greatest capacity when dried at 160℃, and the lowest when dried at 120℃. Judging by cycle performance, the battery dried at 160℃ degrades most quickly, losing 62mAh/g after 90 cycles. The battery dried at 140℃ degrades more slowly, with a loss of about 71mAh/g after 90 cycles.
From the perspective of the first efficiency, the first efficiency of the battery with LiPAA binder is about 84%, and the first efficiency of the LiPAA battery dried at 200℃ is slightly lower at about 82%, and the coulomb efficiency quickly rises to about 99.6% in the first 5 cycles. The initial battery efficiency with the PAA binder is around 80%. When the PAA battery is dried at 180℃, the initial efficiency decreases slightly to about 75%. After 40 cycles, the PAA battery’s efficiency only reaches 99.6%, a slower rate than the LiPAA battery.
In order to study the reactions occurring in the battery during the cycle, Kevin A. Hays conducted a pulse discharge test on the battery at a discharge depth of 50% DOD and found that the internal resistance of the LiPAA battery is significantly lower than that of the PAA battery (as shown in the figure below). At the same time, we also noticed that there is no obvious relationship between the internal resistance of the LiPAA battery and the secondary drying temperature of the negative electrode, while the internal resistance of the PAA battery has increased significantly with the increase of the secondary drying temperature of the negative electrode.
In view of the close relationship between the performance of PAA binder and the secondary drying temperature, Kevin A. Hays conducted thermogravimetric analysis on the negative electrode using LiPAA and PAA binders (the results are shown in the figure below). It can be seen that the dehydration of the two binders is mainly divided into two steps: 1) removal of free water (40°C); 2) removal of adsorbed water (LiPAA binder 75°C, PAA binder 125°C). In addition to removing these water, we also found other weight loss peaks, PAA binder 140-208°C, LiPAA binder 85-190°C, these weight losses mainly come from the polymerization of some carboxyl groups in PAA and the release of water, which causes weight loss. For LiPAA, the impact of this polymerization reaction is relatively small because 80% of the H in the carboxyl group is replaced by Li.
The reduced interaction between the PAA and Si material, potentially due to polymerization of PAA binder’s carboxyl groups at high temperatures, could explain the poor cycling after high-temperature secondary drying in PAA negative electrodes. But, peel strength tests show PAA adhesion, while decreasing with higher secondary drying temperatures, remains better than LiPAA adhesion overall, suggesting other factors are at play in LiPAA binder’s improved cycling.
