Alginic Acid As a New Aqueous Slurry-Based Binder for Cathode Materials of Lib

Lithium-ion batteries (LIBs) are one of the most promising energy sources for use in portable electric devices or automotive applications because of their high energy density. In commercial LIBs, poly(vinylidene fluoride) (PVdF) is widely used as binder for positive electrode (cathode) due to its enough mechanical properties, relatively high oxidation durability and good electrochemical stability.

However, a large amount of PVdF binder (over 5 wt.%) is needed to obtain an electrode with enough adhesion strength, which also has disadvantage of high cost. Moreover, N-methyl-2-pyrrolidone (NMP), which is commonly used as dispersant for PVdF, suffers from the safety problem as they are toxic as well as flammable. Thus, the switching from a using PVdF/ NMP to an aqueous-based system is desired.

Recently, it has been reported that CMC can be promising aqueous-based binder for a cathode.[1-2] However, few articles discuss its long-term cycle performance depending on the pH value of aqueous slurry, which is important because Al current collector is not stable in a strong alkaline solution.

In this work, we report a new aqueous-based cathode by using alginate as a binder. Alginate is natural polymer, which is categorized as polysaccharide derived from brown seaweeds. In our previous study, we reported on the LIB performance of carbon anode with Alg-salt binder both in organic and ionic liquid electrolyte systems, and showed its excellent cycle performance.

              Here, we propose another idea: using alginic acid (AlgH) as an aqueous slurry-based binder for a cathode. Because the AlgH is soluble only in alkaline water (It is insoluble in pure water), we successfully obtained the pH-controlled aqueous slurry by an acid-base reaction between AlgH and alkali cathode-material-surface (exchange between H⁺ and Li⁺). The present LIB cell with the cathode with AlgH binder exhibited excellent cycle performance that is comparable to that with conventional PVdF binder. Thus we believe that AlgH is a promising candidate as an aqueous slurry-based binder for a cathode of LIB.

The NMC(LiNi_1/3Co_1/3Mn_1/3) cathode was prepared by mixing 90 wt.% carbon material, 8 wt.% conductive agent (carbon black), and 2 wt.% AlgH binder, which were dispersed in pure water. The resulting slurry was cast onto Al foil and dried at 120 ̊C for 12 h under a vacuum condition. The obtained electrode film was punched to be a disk shape with a diameter of 12 mm for the LiB cell test. For comparison, the NMC cathode with PVdF binder (dispersed in NMP) was also prepared. A carbon (graphite) anode was also prepared for full test cell assembly. The electrolyte used was 1mol dm^-3 LiPF₆ in EC:DMC (1:1 by volume). The electrochemical measurements were carried out on coin-type cells CR2032.

 Figure 1 shows the performance of charged and discharged full cell composed of the NMC cathode with the AlgH binder at 1C rate (1C = 150 mAg^-1) and maximum operation voltage of 4.5 V. It exhibits excellent charge discharge behavior with a reasonable discharge capacity. The observed high coulombic efficiency proves that AlgH is stable without its any oxidative decomposition even at a high operation voltage of 4.5V. In addition, this result also suggests that corrosion of Al foil that significantly affects the cycle performance should be negligible. Thus, the present results suggest that AlgH acts as not only aqueous-based binder but also pH regulator. Actually, estimated pH of slurry with AlgH was 8.4, which is an acceptable value for Al since the corrosion of Al occurs in stronger alkaline solution (pH > 10).[3]

Another merit of using AlgH is that we can reduce the amount of binder component. We successfully prepared the cathode with adequate mechanical property only by using 2 wt.% of AlgH. As for a conventional cathode with PVdF, however, a large amount of binder, more than twice as much as AlgH, is need to achieve adequate mechanical properties (especially adhesion strength). The present results demonstrate that AlgH is a promising candidate for binder material useful for high-voltage LIB cathodes.

Figure.1  Cycle performance of full cell with NMC-based cathode based on AlgH binder at maximum operation voltage of 4.5 V. Rate: 1C = 150 mA g^-1.

References

[1] Q. Wu, S. Ha, J. Prakash, D. W. Dees, W. Lu, Electrochim. Acta, 114, 1 (2013).

[2] Z. Wang, N. Dupré, A. C. Gaillot, B. Lestriez, J. F. Martin, L. Daniel, S. Patoux, D. Guyomard, Electrochim. Acta, 6277 (2012).

[3] R. D. Armstrong, V. J. Braham, Corros. Sci, 38, 1463 (1996).