Evaluating the Influence of SiOx on the Calendar Lifetime of Li Ion Batteries across Varied Temperatures and State of Charge Conditions

Lithium-ion batteries (LIBs) have a wide range of application in various fields, such as automotive and stationary applications. The incorporation of silicon (Si) in the anode is an attractive strategy to increase energy density and enable fast charging. This is due to the excellent theoretical specific capacity of Si, which exceeds 3000 mAh/g. However, due to the alloying behavior of Si, a significant volume change can be observed by Si based anodes when cycled. In literature the rapid cyclic degradation is related to these volume changes. A practical approach is to use silicon oxide (SiO x ) as anode material, which undergoes lower volume changes during cycling but still exhibits a specific capacity >2000 mAh/g. Studies on the cycle life of SiO x -containing cells are frequently published in literature, but calendar life studies are scarce. Here, the calendar life of two different 1 Ah pouch cell configurations, possessing different anode chemistries, were investigated. One cell contained artificial graphite and 10% SiO x (SiO-cell) while the other cell only used artificial graphite as anode material (G-cell). Both cell configurations were built with an NMC811 cathode. 1 M LiPF 6 in EC: EMC (3:7 (wt.)) + 2% wt. VC was used as electrolyte. Temperatures of 20 °C, 40 °C and 60 °C and state of charge (SoC) levels of 20%, 40%, 60%, 80% and 100% were investigated in this study. It has been observed that for 20 °C and 40 °C, the degradation rate of both cell types intensifies when stored at 100% SoC. At 60 °C, however, the capacity fade already starts intensifying drastically at around 60% SoC. The capacity fading during storage experiments of the SiO- cells, depending on the parameters tested, is higher by a factor of around 1.1 to 4.1 than the capacity fading of the G- cells. Interestingly, the capacity fade ratio of the SiO-cell to G-cell is at maximum when cells were stored below 40% SoC at 20 °C. When stored at low SoC, SiO x is the anode active material being lithiated majorly. These findings indicate that SiO x indeed causes stronger calendaric degradation compared to graphite. Hence, in future studies on SiOx based anodes calendaric degradation is an interesting topic to address additionally to cyclic degradation.