Effect of Water-Soluble CMC/SBR Binder Ratios on Si-rGO Composites Using µm- and nm-Sized Silicon as Anode Materials for Lithium-Ion Batteries

Abstract

Silicon-containing materials are still the most promising alternatives to graphite as the negative electrodes of lithium-ion batteries. However, the different Li+ storage mechanism combined with the high capacity result in new requirements for the passive electrode components, such as the binder. To ensure sufficient cycling stability, silicon must be embedded in a suitable carbonaceous matrix. For this purpose, we used a simple ball milling process with reduced graphene oxide (rGO) to produce Si-rGO composites with µm- and nm-sized silicon particles. The rGO was synthesized previously from a two-step thermal synthesis method developed in-house. Subsequently, electrodes with varying CMC/SBR ratios (3:1, 1:1, and 1:3) were prepared from the composites containing the different Si particle sizes. It was found that the optimal binder ratio depends on the size of the Si particles. For the nm‑Si‑rGO composite, a CMC/SBR ratio of 3:1 results in a total capacity over 51 cycles of 20.6 Ah g−1, which means an improvement of 20% compared to CMC/SBR = 1:3 (17.1 Ah g−1). In contrast, we demonstrate that for µm-Si-rGO composites with an optimal CMC/SBR ratio of 1:1 (13.0 Ah g−1), compared to nm-Si-rGO, a higher SBR content is beneficial for the cycling behavior. Moreover, a comparison with graphite from the literature indicates that a rGO-matrix reduces the need for SBR.

Impact of Functional Groups in Reduced Graphene Oxide Matrices for High Energy Anodes in Lithium-Ion Batteries

“…This exfoliation increases the fraction of freely accessible oxygen groups on the surface, which can be thermally reduced without a drastic volume expansion compared to the exfoliation of encapsulated oxygen groups. 41 During the reactive spray drying process, the GO gets thermally exfoliated but…”

“…Due to the prior exfoliation of the material in the RSD process despite a heating rate of 10 K min −1 , the surface enlargement is comparatively small to a thermal one-step reduction. 41 The subsequent reduction of the specific surface area is probably due to a smaller number of defects distributed within the graphitic lattice, which allows the individual graphene layers to stack closer together. Consequently, an increase in temperature and the associated removal of lattice defects leads to a further decrease in the BET surface area for rGO 700Ar/H2 and rGO 1000Ar/H2 .…”

“…However, all measured specific surfaces areas for (r)GO materials are significantly higher compared to the standard anode material graphite but lower compared to rGO materials which are prepared by classic one-step thermal exfoliation, resulting in specific surface areas of 100 to 500 m 2 g −1 . 21,41 In Fig. 2d the results of the Raman spectroscopy for each (reduced) graphene oxide material are presented.…”

Investigation of The Failure Mechanisms of Li-Ion Pouch Cells with Si/Graphite Composite Negative Electrodes and Single Wall Carbon Nanotube Conducting Additive

“…SBR (styrene butadiene rubber) is often used in conjunction with CMC (carboxymethyl cellulose) to enhance mechanical flexibility and overall electrode stability. [15][16][17] With the addition of single walled carbon nanotubes (SWCNT) to the electrode, the particles within a Si-Gr composite electrode should remain electrically connected during the cell's lifetime without the need for complicated binder networks. 18 If this is the case, there should be no active mass loss and the major degradation mechanism will be loss of lithium inventory in the ever growing negative electrode SEI leading to what is called "shift loss."…”