High-Energy Micro-grain Silicon Anodes for Lithium-Ion Technology
F.Farmakis, P.Selinis, N.Georgoulas, M.Hagen, P.Fanz, S.Schiestel, A.Kovacs
2015. 227th meeting of the Electrochemical Society (ECS), Chicago, May 2015
Abstract: It is well known that silicon presents one of the most important anode materials for the improvement of lithium-ion cells in terms of energy density. Indeed, silicon’s high theoretical specific capacity to lithium (more than 3800 mAh/g at room temperature), environmental friendliness, low potential compared to lithium and material abundance turns silicon to a strong candidate for the replacement of carbon-based anodes . However, one of the main drawbacks of silicon’s application to the lithium-ion technology is its poor electrochemical cycling stability over several galvanostatic cycles, mainly due to silicon’s huge volume change (around 300%) during lithiation and delithiation that lead to high internal mechanical stress . Many alternatives have been proposed that alleviate this mechanical expansion through the use of nanostructured silicon sometimes combined with carbon-based materials . However, most of proposed solutions are technically and financially demanding at least for production. In this presentation, we investigate the physical and the electrochemical properties of micro-grain structured silicon deposited on special copper with nodular grains. Silicon films with high loadings (0.8-1 mg/cm2) have been deposited with the aid of DC sputtering technique on top of copper foil with nodular grains. Half-cells with silicon as anode and Lithium metal as counter electrode were prepared. The gravimetric specific capacity was estimated by measuring the weight of silicon. Scanning Electron Analysis (SEM) has been performed in order to investigate the physical properties of the deposited material. Figure 1 demonstrates the SEM picture of the micro-grain structure of silicon when deposited on the copper foil with nodular grains. It is shown that silicon is organized in columnar form with grains of an average size of 1-2 μm. By KaptonTM tape tests, the adhesion of this material on the copper foil was found to be excellent. The equivalent silicon thickness that has been grown is estimated to be more than 4 μm (loading 0.8-1 mg/cm2). Figure 2 shows the electrochemical behavior of such half-cells when cycled with current of 0.6 mAh/cm2 (C/3 rate). Concerning the specific capacity, we observe a slight decrease from 2200 mAh/g to 1800 mAh/g and after the 5th cycle, an increase up to the initial value. It is also mentioned that the silicon anode achieves high specific areal capacity up to 2 mAh/cm2. According to McDowell at al. , amorphous silicon spheres with diameter of around 1 μm do not fracture upon lithiation and therefore anodes fabricated with such material demonstrate stable capacity over galvanostatic cycling. Thus, our experimental results further support that the DC sputtered amorphous spheres provide excellent capacity retention and good stability.