This study investigates the calendar aging of Li-ion batteries with Si/graphite anodes, focusing on the mechanisms and factors influencing capacity fade. A total of 121 pouch cells with different Si content (0%, 3.0 wt%, 5.8 wt%, and 20.8 wt%) and NCM622 cathodes were tested at various State of Charge (SoC) levels (30%, 60%, and 100%) and temperatures (25 °C, 45 °C, and 60 °C). The aging behavior was analyzed through capacity fade, differential voltage analysis (DVA), and post-mortem characterization. Key findings include: - Capacity fade follows a square-root function with time, indicating a time-dependent Arrhenius-like behavior. - The aging rate increases with temperature, with the highest rates observed at the beginning of aging and decreasing over time. - Cells aged at 100% SoC showed the fastest capacity loss, followed by 60% and 30% SoC. - Si/graphite cells generally exhibited faster capacity loss compared to pure graphite cells, with higher Si content leading to faster aging. - Post-mortem analysis revealed that SEI growth on graphite leads to lithium inventory loss (LLI) and capacity fade, while SEI growth on Si also causes loss of accessible Si active material (LAAM), contributing to both LLI and LAAM. The study concludes that the main aging mechanism for these cells is SEI growth, which irreversibly binds lithium in the anode. For graphite anodes, SEI growth occurs only on the surface, leading to LLI. For Si/graphite anodes, SEI growth affects both the Si active material and the graphite particles, resulting in both LLI and LAAM. The presence of Si enhances irreversible Li accumulation in the anode, leading to higher capacity losses.This study investigates the calendar aging of Li-ion batteries with Si/graphite anodes, focusing on the mechanisms and factors influencing capacity fade. A total of 121 pouch cells with different Si content (0%, 3.0 wt%, 5.8 wt%, and 20.8 wt%) and NCM622 cathodes were tested at various State of Charge (SoC) levels (30%, 60%, and 100%) and temperatures (25 °C, 45 °C, and 60 °C). The aging behavior was analyzed through capacity fade, differential voltage analysis (DVA), and post-mortem characterization. Key findings include: - Capacity fade follows a square-root function with time, indicating a time-dependent Arrhenius-like behavior. - The aging rate increases with temperature, with the highest rates observed at the beginning of aging and decreasing over time. - Cells aged at 100% SoC showed the fastest capacity loss, followed by 60% and 30% SoC. - Si/graphite cells generally exhibited faster capacity loss compared to pure graphite cells, with higher Si content leading to faster aging. - Post-mortem analysis revealed that SEI growth on graphite leads to lithium inventory loss (LLI) and capacity fade, while SEI growth on Si also causes loss of accessible Si active material (LAAM), contributing to both LLI and LAAM. The study concludes that the main aging mechanism for these cells is SEI growth, which irreversibly binds lithium in the anode. For graphite anodes, SEI growth occurs only on the surface, leading to LLI. For Si/graphite anodes, SEI growth affects both the Si active material and the graphite particles, resulting in both LLI and LAAM. The presence of Si enhances irreversible Li accumulation in the anode, leading to higher capacity losses.