For recent lithium ion batteries(LiB) application, the demands for higher energy, higher power and longer life have been increasing. Therefore all chemical components in the LiB have been improved to meet the demands corresponding to various application. As for acetylene black(AB) it is one of the major conductive agents in the positive electrode. The smaller particle size ABs can make effective conductive paths for higher energy requiring a few amount of conductive agents^(1,2. However, these smaller size ABs are difficult to be dispersed due to its high surface area and low interaction with NMP in the slurry.  Even though oil absorption number(OAN) which is an evaluation method of aggregate in carbon blacks tends to be used as an index of the interaction with the solvent, the actual interaction with the solvent must be different from this classical evaluated results. Also, effects of dispersants which are often added in order to disperse AB homogeneously in the slurry cannot be evaluated with this index.  In this research, we tried to acquire the exact information about actual interaction between the conductive agents and solvents like NMP with pulse NMR method. Then we defined the new index and researched the relationship between the index and practical properties for LiB application.

(Experimental & Results)

 In the pulse NMR method, the relaxation time of NMP molecule which interacts with AB in the slurry is known to be shorter than of the NMP molecule not interacting⁽³.

Therefore, we defined solvent interaction index(W_AB) as the formula below.

W_AB=(Rav-Rb)/(Rb-S_AB)

Rav: The reciprocal of the relaxation time in the sample                                          

Rb: The reciprocal of the relaxation time in the blank(supernatant)

S_AB: Surface area of AB in the solvent

Larger W_ABmeans higher solvent interaction between AB and NMP

At first, four different specific surface areas of AB have been prepared and evaluated each solvent interaction index(W_AB) with pulse NMR and the surface characteristics with desorption gas measurement (Fig.1). These results showed smaller particle size AB had lower solvent interaction index and there were associations between W_AB and the amount of functional groups on the surface. That is to say, not only larger surface area but also low interaction with NMP(few functional groups) make smaller size AB difficult to be dispersed⁽⁴.

AB4 was selected as a conductive agent and three different molecular weights of polyvinylpyrrolidone (PVP) , a major effective dispersant was prepared in this experiment.

Then, four slurry samples were prepared by using a planetary centrifugal mixer and each solvent interaction index was evaluated with pulse NMR method (Fig.2).

According to our results, PVP can improve solvent interaction between AB and NMP also larger molecular weight PVP showed better effect on the solvent interaction than the smaller one.

This is because larger molecular weight PVP is thought to absorb the AB surface easier. In addition, slurry which have higher solvent interaction index shows low viscosity.

 Finally, in order to evaluate the effect of improved W_ABfor battery performance, we measured rate capability with AB4 and AB4+PVP samples (Fig.3). The mass was set to LCO:AB:PVdF=97.5:1:1.5 in the electrode. 3wt%(vs AB) of PVP was externally added into the electrode. This result showed battery containing AB4+ PVP 3wt% sample have good rate capability. Therefore, this pulse NMR method which can evaluate solvent interaction related to dispersibility is a useful tool to acquire the information for battery performance.

In this presentation, further discussions about more detail results and other solvent interactions(Electrolyte etc.) will be presented.

(Reference)

(1)T. Saito, et al., 81th Electrochemical society of Japan, 2Q04, (2014).

(2)Y. Nako, et al., 56th battery discussion, 3D13, (2015).

(3) G.P. van der Beek, et al., Langmuir,Vol. 7, No.2 (1991).

(4) T. Sonoda, et al., 42st Carbon material society, 1A09, (2015).