Lithium-ion battery cathode materials are generally poor in conductivity, so in the use of conductive agents are generally required to increase the conductivity, common conductive agents include carbon black conductive agents, carbon nanotubes, carbon fibers, and the current fiery graphite materials , if divided from the structure, these types of conductive agents can be divided into three categories: 1) zero-dimensional conductive agents, such as carbon black; 2) one-dimensional conductive agents, such as carbon nanotubes and carbon fibers; 3) two-dimensional conductive Agents, such as graphene materials, each have their own unique properties, such as carbon black materials have advantages in short-range conduction, and carbon nanotubes conductive agent has advantages in long-range conduction.
When lithium-ion batteries work, we generally think that there may be two links that limit the battery rate performance: 1) electronic conduction; 2) ion transmission, many studies have surface electronic conduction links are the key links affecting the rate performance and capacity of lithium-ion batteries. More conductive agents are beneficial to improve the electrical properties of lithium-ion batteries. Samantha L. Morelly and others from Drexel University studied ion conduction, long-range conduction and short-range conduction through different homogenization processes. The effect of lithium-ion battery's 'short-range conductivity' on the rate performance is more significant for the lithium-ion battery's rate performance.
In the experiment, Samantha L. Morelly chose NCM111 material as the research object. Carbon black was used as the conductive agent and PVDF as the binder. The formulation of the slurry was divided into two types, one was 95% of NCM, and the other was 2.5% of CB. 2.5 % of PVDF, the second is 94.5% of NCM, 3% of CB, and 2.5% of PVDF. Use the following two processes for homogenization. From the flow chart below we can see that Samantha L. Morelly The carbon black conductive agent CB is added in two ways. One is all the carbon black CB is added into the PVDF glue together with the NCM material; the other is that first the ball mill is used to dry mix part of the CB and the NCM, and then the remaining The CBs are added together into the PVDF solution. The ratio of dry-blended CBs is 1-f (f=0, 0.25, 0.5, 0.75, and 1). Generally, we believe that the CB can be adsorbed on the NCM particles through the ball milling process. The surface, the formation of 'fixed carbon black', while the wet blended carbon black CB will be present between the NCM particles, Samantha L. Morelly call it 'free carbon black'.
The following figure a is the SEM photograph of NCM material. It can be seen from the figure that the particle size of NCM is about 10um, the lower image b is 2.5% of CB content, and f=0 (that is, all the CB and NCM are ball milled and dry mixed together). The picture of the electrode, we can see that there are many CBs that are not adsorbed on the surface of the NCM particles, but that there is obvious agglomeration. The following figure c is the SEM picture of the electrode with 3% CB content and f=0. It can be seen that most of the CBs are adsorbed on the surface of the NCM particles with less agglomerated particles. This result shows that not all carbon black CBs added during the dry blending process become fixed carbons adsorbed on the surface of the NCM particles. We say that the proportion of carbon black added during the wet mixing process is not all of the 'free carbon black', and that a portion of the carbon black that was not adsorbed on the surface of the NCM particles during the dry mixing process also becomes 'free carbon black'. Characterizing the true proportion of 'free carbon black' in the slurry, Samantha L. Morelly studied the rheological properties of different slurries and characterized the amount of 'free carbon black' in the slurry.
Carbon black CB is a nano-particle, the density is relatively small, while NCM particles are relatively large, and the density is relatively high. Therefore, the amount of 'free carbon black' in the slurry will significantly affect the rheological properties of the slurry system, and we can also pass the slurry. The rheological properties of the slurry reversely deduce the amount of 'free carbon black' in the slurry. From the following figure a (carbon black content is 2.5%) we can see that when f = 1 (that is, all the carbon black is in the wet The slurry added during the mixing process has the highest elastic modulus and viscous modulus, and is almost independent of the shear rate. The elastic modulus G' is always greater than G'', indicating that the slurry exhibits a The state of the gelled gel. As f decreases from 1 to 0.75 and 0.5, the modulus of the slurry drops significantly. At f = 0.25, the modulus of the slurry drops further and we follow the curve. It can be seen that there is no significant relationship between the elastic modulus G' and the frequency, but the viscous modulus G'' increases as the frequency increases, and at the frequency of 10 to 100 G' and G'' are superimposed This phenomenon indicates that the slurry exhibits a weak gel state. At f=0, the rheological properties of the slurry almost match that at f=0.25. In the same way, it shows that ball milling and dry mixing do not make all CB adsorb on the surface of NCM particles.
From Figure b below we can observe that the slurry with 3.0 wt% CB has similar characteristics, but we also found that at f=0, the 3% CB slurry has a lower modulus, which is similar to the above SEM. The observations are consistent, indicating that the amount of 'free carbon black' in the slurry after dry milling with 3% CB is less.
Based on the above research results, Samantha L. Morelly plots the modulus of the slurry with different f values at a frequency of 1 rad/s, as shown in the figure below. According to the modulus of the slurry, Samantha L. Morelly divided the slurry. For two zones: one is a strong gel zone and one is a weak gel zone. It can be seen from the figure that the amount of 'free carbon black' has a significant effect on the modulus of the slurry, 3% CB's slurry model The amount is significantly higher than the 2.5% CB paste, but when f = 0.5, the sizing of the two pastes is the same, indicating that the amount of 'free carbon black' in the paste is the same, when f is further After decreasing to 0.25, the modulus of the 3% CB slurry is even lower than 2.5% of the slurry, indicating that by ball milling, the amount of 'free carbon black' in the 3% CB slurry is less than 2.5% Carbon black.
The figure below shows the rate performance of slurries prepared with different homogenization processes. When the amount of carbon black added is 2.5%, we can see that the performance with f=0 is the worst and the performance with f = 0.25 is the best. When the addition amount of black is 3%, the electrodes with f=0 and f=0.25 exhibit the best performance, while the capacity loss of NCM materials with 3% CB content at a high magnification is also significantly less than 2.5. % of slurry.
In order to analyze the factors influencing the rate characteristics of NCM electrodes, Samantha L. Morelly conducted conductivity tests on electrodes with 2.5% CB content. The results are shown in the figure below. It can be seen from the figure that the electrode with f = 1 has the highest conductivity, f = 0 And 0.25 electrode conductivity is low, this is mainly because f = 1, when all the carbon black CB in the wet mixing process to form a 'long-range conduction' structure, increase the conductivity of the electrode, this point also got Confirmation of the SEM results, but f=0 and 0.25, because most of the carbon black is adsorbed on the surface of the NCM particles during ball milling and dry mixing, resulting in too little 'free carbon black' and therefore 'long-range conductivity' Poor performance, resulting in lower conductivity.
From the above analysis results, it is not difficult to see that the effect of lithium ion battery on the rate characteristics is not what we normally think of as the 'ion diffusion' process, but is more influenced by the electronic conductivity. For example, Samantha L. Morelly’s study found that The rate characteristics of the slurry with 3% CB content are significantly better than that of the electrode with 2.5% CB content. If the theory of 'ion transport' is used as the limiting part, more conductive agent means more tortuous Li+ diffusion channels in the electrode. Instead, it will reduce the rate performance of the electrode. Second, Samantha L. Morelly's work also reveals the impact of 'short-range conduction' in the rate performance of lithium-ion batteries. Studies have shown that the CB adsorbs on the surface of NCM particles to form a better 'short range' by ball milling. Conductive' networks can significantly increase the rate capability of the electrodes, and 'short-range conductivity' is even more important in the electrode rate characteristics than 'long-range conductivity'.
