Silicon is considered one of the most promising anode materials for next generation batteries due to its extremely high theoretical capacity of about 3579 mAh g⁻¹. However, its practical application in solid state batteries remains challenging because the large lithiation induced volume expansion exceeding 300% causes severe mechanical stress, cracking, and degradation of the electrode–electrolyte interface. Thin film deposition techniques provide a well controlled platform to systematically study these degradation mechanisms and identify structural design parameters that improve mechanical stability and electrochemical performance.
In this work, amorphous silicon thin films were deposited on copper current collectors by DC magnetron sputtering to investigate the influence of film thickness on the structural integrity and electrochemical behaviour of silicon anodes in soft solid state batteries. Silicon layers with thicknesses of 200 nm, 500 nm, 1000 nm and 2000 nm were prepared at a base pressure of approximately 4 × 10⁻⁶ mbar and an argon working pressure of about 1.5 × 10⁻² mbar without substrate heating. Structural characterization by scanning electron microscopy and X ray diffraction confirmed the formation of dense amorphous silicon films with thickness dependent surface morphology. Increasing film thickness resulted in rougher surfaces and the formation of crack networks.
Electrochemical performance was evaluated in CR2032 coin cells employing a polymer based soft solid electrolyte and galvanostatic cycling at 60 °C. Thin silicon layers of 200 nm exhibited stable cycling behaviour but limited reversible capacity due to the low amount of active material. In contrast, thick films of 1000 nm and 2000 nm delivered higher initial capacities but suffered from rapid capacity fading caused by cracking and delamination from the copper current collector. Films with an intermediate thickness of approximately 500 nm showed the best balance between reversible capacity and cycling stability.
Post mortem analysis revealed that thin films remained largely intact after cycling, while intermediate thickness layers developed controlled crack networks that partially maintained electrical connectivity. Thick films exhibited severe fragmentation and delamination, leading to electrical isolation of active material and pronounced performance degradation.
Overall, the results indicate an optimal thickness window around 500 nm for sputtered silicon thin film anodes that balances mechanical stability and electrochemical utilization. Maintaining stable interfacial contact between silicon, current collector and solid electrolyte is therefore essential for long term performance of silicon based anodes in solid state batteries.