Wettability and plasticizing effect of CO₂ on Si/C electrode in lithium-ion batteries

Silicon (Si) is widely used material in Li-ion batteries because of its high specific capacity. However, Si anodes have some drawbacks because of poor life cycles and low coulombic efficiency due to significant volume changes during processing. Supercritical CO2 (scCO(2)) annealing as a low-temperature process (36 degrees C) has been applied to anode electrode for battery efficiency optimization concerning Si particles' microcracking after cycles. Styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were used as binders in that electrode structure. Within the scope of the study, Si/C/CMC/SBR and LiCoO2 were preferred as the anode and the cathode, respectively. As the results of the tests, CO2 annealing has been proven to increase capacity preservation from 20.7% to 79.7% after 103 cycles. At the same time, coulombic efficiency has been enhanced from 79.9% to 82.4% after scCO(2). CO2 in supercritical state is believed to be a suitable wetting agent, especially for Si particles. Battery performance tests suggest that CO2 wetting enhances the system's tolerance to Si particles' expansion on cyclic use. From the first charge/discharge capacities it is understood that CO2 wetted Si particles perform 25-30% less in capacity which is believed to be a result of particle/CO2 interface acting as a barrier; however, CO2 annealed samples' stability is higher according to the tests. Considering all these results, the novel scCO(2) annealing can be an attractive post-processing method for the new generation silicon-based lithium-ion batteries.