Development of silicon polymer electrodes for all-solid-state lithium-ion batteries

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In this work, the development of silicon polymer electrodes is presented. The goal is to obtain silicon polymer electrodes that have a high capacity, capacity retention and coulombic efficiency (CE) as well as a sufficient rate capability (or current density acceptance). The silicon electrodes are produced in atmospheric conditions. Subsequently, they are impregnated with a liquid precursor for a hybrid inorganic-organic polymer electrolyte (HPE) that is then thermally cured. The HPE used in this work is based on a molecular hybrid polymer with polyether organic domains and an inorganic SiO2 network. It contains lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium bis(fluorosulfonyl)imide (LiFSI) as conducting salt and optionally an ionic liquid (IL) to increase the conductivity. At 60 °C, the conductivity of the HPE ranges from (0.24±0.11) mS/cm with LiFSI as conducting salt without further additives to (0.94±0.07) mS/cm with LiTFSI and an ionic liquid.
The electrochemical performance of the silicon polymer electrodes is examined in solid-state cells with a lithium counter electrode and a HPE membrane that serves as electrolyte and separator between the electrodes. Using LiTFSI as the conducting salt, electrodes with an areal capacity of 1.5 mAh/cm^2 achieve a high specific capacity of approx. 1100 mAh/gSi at a C-rate of C/10. However, the capacity decreases to approx. 700 mAh/gSi after 100 cycles. The use of LiFSI instead of LiTFSI significantly improves the capacity retention, however, at the expense of a reduced current density acceptance due to the lower conductivity of the LiFSI-containing HPE. In the TFSI–containing cells, the ionic liquid leads to a decrease of the coulombic efficiency while in the FSI–based cells, the IL does not significantly influence the CE. Nonetheless, the capacity retention is not as good as in the LiFSI-containing cells without IL. The simultaneous achievement of a high current density acceptance and a good capacity retention is one of the main remaining challenges. In addition, significant side reactions and a low CE in the early cycles need to be addressed in order to enable the application of the silicon electrodes in full cells where the lithium inventory is limited.

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