Lithium-ion batteries with Si anodes are still attracting increasing attention, particularly due to the high specific energy density. The main disadvantage of silicon as anode material is its reduced cell performance in terms of cycling stability. One promising approach to improve this is embedding silicon nanoparticles in a graphene-like matrix via spray drying. All processes described so far need a time- and energy-intensive two-step-synthesis to obtain the graphene-like rGO structure. Here, we present a reactive spray drying process for synthesis of Si/rGO composites. For proper reactor design, the reaction kinetics are investigated by simultaneous thermal analysis in various atmospheres. We can describe thermal decomposition of GO to rGO as a second-order reaction. STA data also show that additional presence of water in the atmosphere due to the one-step synthesis is negligible at temperatures below 600 °C for both the reaction of GO and the additional oxidation of Si. To evaluate the electrochemical performance, the composites are cycled in a half cell setup. Delithiation capacity after cell formation could be raised from 252 mAh g-1 for GO to 327 mAh g-1 for rGO. In addition, we are able to synthesize Si-containing composites suitable for the anode of LiB using our process.

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.

In recent years, lithium-ion batteries (LiB) with Si anodes attracted increasing attention, particularly due to their high specific energy density. An optimization of this characteristic is extremely important for their extended use as power source not only in portable devices but also in larger applications, such as electric vehicles.The main disadvantage of using silicon as anode material in LIBs is its high volume expansion during lithiation, which leads to various negative effects, such as pulverization, delamination, and an unstable solid electrolyte interface (SEI) [1]. This eventually results in a reduced cell performance, especially in terms of cycling stability. One promising approach to mitigate these issues is to wrap silicon nanoparticles in a graphene-like matrix via spray drying. A suitable starting material for this approach is graphite oxide (GO) [2, 3, 4].In all processes described so far, an additional time- and energy-intensive calcination step is necessary to obtain the graphene-like “reduced graphene oxide” (rGO) structure. Here, we present a new reactive spray drying process for a simplified one-step synthesis of Si/rGO composites using water as solvent. To design the reactor properly, it is necessary to understand the reaction kinetics. Therefore, the reaction is investigated by simultaneous thermal analysis (STA) in various atmospheres. We can describe the thermal decomposition of GO to rGO as a second-order reaction (Arrhenius behaviour). The STA data also show that the additional presence of water in the atmosphere due to the one-step synthesis is negligible at temperatures below 600 °C for both the reaction of GO and the unwanted additional oxidation of Si. We use crystalline silicon nanoparticles (dp ≈ 100 nm) and micrometer-sized graphite oxide prepared via modified hummers method as starting materials. Si and GO are dispersed in water and ultrasonically treated to obtain a stable dispersion. The prepared dispersion is sprayed continuously into a hot reactor (> 500 °C), where the droplets dry in the first reaction zone, forming a Si/GO composite, followed by a thermal decomposition reaction to Si/rGO in the second one. The Si/rGO particles are separated from the hot gas stream via a cyclone. To prevent oxidation, the entire system is operated in an inert atmosphere. To evaluate the electrochemical performance, the as-prepared composites are coated on a copper foil using a water-based binder mixture of styrene-butadiene rubber and caboxymethyl cellulose (SBR:CMC = 1:1). The electrodes are cycled against Li/Li+ in a half cell setup.The specific capacity after cell formation is raised from 275 mAh g-1 for GO to 335 mAh g-1 for rGO due to the reactive spray drying process. A Si/rGO composite with a Si content of 11.5 % shows a capacity of 505 mAh g-1 after the formation. Future studies will focus on further reduction of the oxygen content of rGO and an improved integration of silicon in the carbon matrix.[1] J.W. Choi, D. Aurbach, Nature Reviews Materials 2016, 1 (4), 16013.[2] M. Li et...