Abstract
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 al., Journal of Power Sources 2014, 248, 721-728.
[3] J. Lee, J.H. Moon, Korean Journal of Chemical Engineering 2017, 34 (12), 3195-3199.
[4] Y. Huang et al., Energy Fuels 2020, 34 (6), 7639-7647.
Figure 1