The continuous development of high specific energy lithium-ion battery has made the application of silicon anode become the trend of the times. Although the high-capacity Si anode material provides a qualitative leap for the specific energy of lithium-ion batteries, it also brings about 300% Swelling, which will not only cause the negative pulverization of the material, but also affect the stability of the negative interface, resulting in sustained capacity decline.Mechanical studies have shown that the silicon anode in the cycle of decline mainly in two forms: 'local Failure 'and' global failure '. The' local effectiveness' mode is mainly due to the Si particles losing their connection to the conductive network, resulting in a failure to participate in the charge-discharge reaction. The 'global failure' is mainly due to the fact that the Si- Expansion, resulting in the powder out of material, resulting in the loss of active material.
The methods to solve the volume expansion of silicon negative electrode can be divided into two categories: 1) to overcome the volume expansion of Si negative electrode and reduce the powdering out phenomenon of silicon negative electrode in the circulation process by adopting better performance binders; and 2) Materials composite and nanostructured, to reduce the volume expansion in the cycle of Si anode, such as the new dendritic Si-C composite materials, new Si materials and so on.
Binder selection has always been an important way to improve the cycle performance of silicon negative electrodes. A good binder can significantly reduce problems such as pulverization of the silicon negative electrode during the cycle, and improve the interfacial stability of the electrodes. However, Often neglected the electrode preparation process on the silicon anode - the homogenization process, rolling process, according to the University of Nantes, France, Z. Karkar et al. 'Step-by-step homogenization' process can significantly improve the electrode uniformity , And enhance the stability of the electrode, and the rolling process will destroy the binder between the negative particles of the bridge, reducing the mechanical strength of the electrode.However, the laminated electrode under a certain humidity 'ripening' a few days can be significant Improve the distribution of the negative electrode binder, enhance the mechanical strength of the electrode, thereby enhancing the cycling performance of the battery.
Z. Karkar employed a silicon negative formulation of 80% Si, 12% conductive agent and 8% CMC and was homogenized in a buffer of pH 3 (capable of promoting the -OH functionality on the surface of the Si particles and the -COOH functional groups to enhance cohesion.) The graph below shows the two homogenization modes (1) standard homogenization (SM), where all substances are added together and homogenized; 2) fractional homogenization (RAM) That is, firstly, the Si material and the CMC glue are homogenized, and then the conductive agent is added for homogenization) and the SEM images of the different electrodes of the roller pressure. From the figure, we can see that in the stepwise homogenized electrode, the conductive agent GM15 tablets also showed a parallel state between each other, the electrode structure is more orderly, porosity is also higher.
The graph below shows the curves of porosity, electrode density and volumetric capacity of two hybrid electrodes under different pressures. Comparing the data of two electrodes, we can notice that the electrode prepared by the "stepwise homogenization" method The porosity was significantly higher than the electrodes prepared by the standard homogenization method (72% and 60%, respectively) .The electrodes prepared by the standard homogenization method were more easily rolled, and in order to achieve an electrode porosity of 35% The electrode prepared by the "homogenization" method requires a pressure of 15 tons / cm2, but the electrode prepared by the standard homogenization method requires only 5 tons / cm2 of pressure (at the maximum pressure, the "stepwise homogenization" and the "standard homogenization 'The maximum volumetric capacity of the electrode prepared by the method was 1300 and 1400 mAh / cm3, respectively, which is about 2.5 times that of the graphite negative electrode).
The graph below shows the cycling performance curves of the electrodes with different coating volumes prepared by the two homogenization methods. It can be seen from the figure that the difference between the two methods is not great at a lower coating amount of 1.8 mg / cm2, but Electrodes prepared by the 'stepwise homogenization' (RAM) method were significantly better than those prepared by the 'standard homogenization' (SM) method at a coating amount of 2.6 mg / cm2 and above.Z. Karkar considered that for both The electrodes prepared by the method all have a 'limit coating amount'. After the coating amount is exceeded, the electrode capacity decay will obviously be accelerated, but the limit coating amount of the electrode prepared by the RAM homogenization method is obviously high Electrode prepared by SM homogenization method.
In this process we also found an interesting phenomenon (c and d in the following figure). After being pressed for a period of time under a certain humidity condition, the rolled electrode can significantly improve the cycling performance of the electrode. The root cause of this phenomenon is that the binder in the electrode migrates from the surface of the Si particles to the junctions between the particles during 'curing', thereby significantly increasing the overall strength of the electrode while rolling During the process of copper foil cracks will be oxidized, so that the formation of Cu (OC (= O) -R) 2 bond between the chemical bonds, which significantly enhance the negative electrode and copper foil between the intensity of Under the dual action described above, the mechanical strength of the negative electrode is improved and the cycle performance is improved.
As we know, the Si negative electrode will cause the degradation of cycle performance as the compaction density increases. Combined with the SEM analysis above, Z. Karkar considers the reason that the cycling performance of Si negative electrode decreases with increasing rolling pressure. Rather than Si particles crushing during rolling (no visible Si particles were observed in the SEM image), the binder bridge between the particles was destroyed during the deformation of the electrode, resulting in a decrease in the overall mechanical strength of the negative electrode Lower, and the laminated maturation process can well restore the damaged binder structure during rolling, thereby improving the cycling performance of the electrode.
The graph below shows the volumetric expansion of the laminated film after being pressed by a 'ripening' electrode (1.96 mg / cm2, 19 μm thick, porosity: 33%) during cycling. From Figure a, The maximum volume expansion can reach 258%, which is even higher than the theoretical 193%, and the irreversible volume expansion at the first discharge is about 40%, mainly due to the formation of the SEI film on the surface of the abdominal muscles. The maximum expansion of the electrode proceeded to progressively decrease, reaching 220% for the fourth time, but the irreversible volume expansion of the electrode continued to increase.
Z. Karkar's research shows that the homogenization process has a significant effect on the structure and the stability of the Si negative electrode. Electrodes made using the 'staged homogenization' RAM have better uniformity of structure, higher porosity and therefore better . Although the structure of waterborne binder (CMC) in the electrode is destroyed during the rolling process, the curing of the binder in the Si negative electrode can be well restored by 'curing' at a certain humidity Structure, so as to improve the structural stability of the rolled electrode and improve the cycling performance of the battery.
