Quantifying Reaction Products of Silicon Monoxide Electrodes during Initial Cycle in Lithium-Ion Batteries

Abstract

Silicon monoxide (SiO) is a potential high-capacity anode material for lithium-ion batteries. However, its low initial Coulombic efficiency hinders its adoption in commercial batteries. The irreversible capacity of SiO is typically attributed to the formation of lithium silicate (such as Li4SiO4), lithium oxide (Li2O) and solid electrolyte interphase (SEI), but the amount of each product is typically not quantified. Here, we use an electrochemical approach, depth of discharge test, which we developed earlier [1] to estimate the amount of reaction products. Based on the first cycle discharge-charge capacities as well as the proportion of reversible capacity at different state of discharge (see Figure 1a and 1e), the lithiation mechanism of pristine SiO1.12 is estimated to be: SiO1.12 + 3.10 Li+ + 3.10 e- → 0.63 Li3.75Si + 0.15 Li2O + 0.11 Li4SiO4 + 0.26 SiO2 where inactive SiO2 in the material accounts for the lower-than-theoretical capacity, Li4SiO4 takes in lithium but cannot release them back, while Li3.75Si and Li2O give partial reversibility upon delithiation. We found that the amount of reaction products depends strongly on how the material is treated. Table I summarizes the corresponding reaction mechanisms [2]. In particular, while carbon-coating (SiO@C) increases the reversible capacity (y) as it increases its electrical conductivity which allows easier access to the lithium from Li2O, larger irreversible capacity is also observed with more Li4SiO4 formation. On the other hand, disproportionation (d-SiO@C-900°C and 1000°C) decreases the irreversible capacity in the initial cycle with the formation of large cluster of SiO2 which is electrochemically inactive. Though, the reversible capacity is reduced. Our results suggest that there are opportunities to further improve the first cycle efficiency of SiO by finding ways to control the reaction products during the initial lithiation process. More results and discussions will be presented at the meeting.