Effects of Electrolytes, Additives, and Binders on Cycling Performance of Hard Carbon for Na-Ion Batteries

Lithium resources do not seem particularly limited but the access to this essential resource could be potentially uncertain. Additionally, the cost of lithium keeps increasing with the prospect of commercialization of Li-ion batteries. Thus, other abundant resources, preferably of low cost, must be investigated as the charge carrier to ensure energy sovereignty, therefore Na-ion batteries become the promising device in this regard. In our laboratory, by using carbon materials as negative electrodes for Na-ion batteries, we have demonstrated the practical satisfactory performance of the Na-ion batteries.^[1] The use of electrolyte additives is one of the most effective methodologies to improve the electrode performance. Recently we have reported that the FEC additive facilitates the formation of stable solid electrolyte interphase (SEI) on the surface of electrodes and reduces irreversible capacity.^[2] We have also reported, binders, which are used for the preparation of composite electrodes from powder materials, as essential components to improve the electrochemical performance and surface structures of graphite electrodes in Li cells.^[3]  In this study, the impact of polymer binders and electrolyte components on the hard carbon electrodes in Na cells is examined.

The hard carbon electrodes were prepared by mixing different binders, such as poly(vinilydene fluoride) (PVDF), carboxymethyl cellulose (CMC), and sodium polyacrylate (PAANa). As shown in Fig. 1, around 150 mAh g^-1 of capacity was lost after 100^th cycles in the case of PVDF binder. However, the capacity retention was dramatically improved by using electrodes with CMC and PAANa binders. To obtain information on the chemical components of the SEI layer, hard X-ray photoelectron spectroscopy (HAXPES) spectra of the SEI formed in electrolyte were collected in the synchrotron facility. Fig. 2 shows the HAXPES spectra of hard carbon electrodes before and after the first cycle with PVDF or CMC binder. The peak at 284.5 eV is assigned to C-C bonding originating from hard carbon. In the case of electrode with CMC, C-C peak intensity is relatively lower than that of PVDF. This result indicates that the thicker film is formed on the surface of hard carbon with CMC. The peak of alkyl carbonate exists in the case of PVdF after the first cycle whereas, in the case of CMC, the peaks of alkyl group and Na₂CO₃appear. The difference suggests that these components of surface film on hard carbon could be related to the improved cycleability. In this presentation, the effect of not only binders but also electrolytes (including solvents, salts, and additives) will be discussed from characterization with powerful surface analysis techniques such as SEM, TOF-SIMS, XPS and HAXPES.

References:

[1] S. Komaba et al., Adv. Funct. Mater., 21, 3859-3867 (2011).  

[2] S. Komaba et al., ACS Applied Mater. Interfaces, 3 (11), 4165-4168 (2011).

[3] S. Komaba et al., Journal of Power Sources, 189, 197-203 (2009).