This study investigates the aging mechanisms of lithium-ion (Li-ion) cells with silicon/graphite (Si/Graphite) anodes under calendar aging conditions. A total of 121 single-layer pouch cells with either graphite or Si/Graphite anodes (3.0 wt–%, 5.8 wt–%, and 20.8 wt–% Si) and NMC622 cathodes were tested at different storage conditions (SoC: 30%, 60%, and 100%; temperature: 25°C, 45°C, and 60°C). The aging data was analyzed in terms of capacity fade, showing a square-root behavior with time. Differential voltage analysis (DVA) was used to gain further insight into electrode processes, and the rate dependence of capacity fade over time and temperature was best described by three-dimensional (3D) Arrhenius plots. Post-mortem analysis (SEM, EDX, GD-OES) was performed to investigate changes on electrode and material levels. The results show that Si-containing cells exhibit a combination of lithium inventory loss and a loss of accessible Si active material, both caused by SEI growth. The capacity fade of Si/Graphite cells is faster than that of pure graphite cells, with higher Si content leading to more significant capacity loss. The aging rate increases with temperature, following an Arrhenius-like behavior. The main aging mechanisms for calendar aging of Li-ion cells with Si/Graphite anodes are lithium inventory loss (LLI) and loss of active material (LAAM). The presence of Si in the anode leads to additional LAAM due to SEI growth and volume expansion. The SEI layer formed on the anode surface irreversibly binds lithium, leading to LLI. In Si/Graphite cells, the SEI also affects the Si active material, resulting in both LLI and LAAM. The study concludes that the main aging mechanism for the investigated cells is SEI growth, with the SEI irreversibly binding lithium in the anode. For graphite anodes, SEI growth only leads to LLI, while for Si/Graphite anodes, it leads to both LLI and LAAM. The findings highlight the importance of understanding the aging mechanisms of Si/Graphite anodes for the development of long-lasting Li-ion batteries.This study investigates the aging mechanisms of lithium-ion (Li-ion) cells with silicon/graphite (Si/Graphite) anodes under calendar aging conditions. A total of 121 single-layer pouch cells with either graphite or Si/Graphite anodes (3.0 wt–%, 5.8 wt–%, and 20.8 wt–% Si) and NMC622 cathodes were tested at different storage conditions (SoC: 30%, 60%, and 100%; temperature: 25°C, 45°C, and 60°C). The aging data was analyzed in terms of capacity fade, showing a square-root behavior with time. Differential voltage analysis (DVA) was used to gain further insight into electrode processes, and the rate dependence of capacity fade over time and temperature was best described by three-dimensional (3D) Arrhenius plots. Post-mortem analysis (SEM, EDX, GD-OES) was performed to investigate changes on electrode and material levels. The results show that Si-containing cells exhibit a combination of lithium inventory loss and a loss of accessible Si active material, both caused by SEI growth. The capacity fade of Si/Graphite cells is faster than that of pure graphite cells, with higher Si content leading to more significant capacity loss. The aging rate increases with temperature, following an Arrhenius-like behavior. The main aging mechanisms for calendar aging of Li-ion cells with Si/Graphite anodes are lithium inventory loss (LLI) and loss of active material (LAAM). The presence of Si in the anode leads to additional LAAM due to SEI growth and volume expansion. The SEI layer formed on the anode surface irreversibly binds lithium, leading to LLI. In Si/Graphite cells, the SEI also affects the Si active material, resulting in both LLI and LAAM. The study concludes that the main aging mechanism for the investigated cells is SEI growth, with the SEI irreversibly binding lithium in the anode. For graphite anodes, SEI growth only leads to LLI, while for Si/Graphite anodes, it leads to both LLI and LAAM. The findings highlight the importance of understanding the aging mechanisms of Si/Graphite anodes for the development of long-lasting Li-ion batteries.