Electropolymerization Triggered in Situ Surface Modification of Electrode Interphases: Alleviating First-Cycle Lithium Loss in Silicon Anode Lithium-Ion Batteries

We report on interphase modification by in situ generated protons (H+) via electropolymerization. The protons are released from oxidative electropolymerization of indole-3-carboxylic acid (InAc) used as an additive in a LiNi0.8Co0.15Al0.05O2 (NCA) based cathode during cycling against lithium metal and silicon-graphite composite (Si-Gr) electrodes. Electrochemical data supported by ex situ NMR spectroscopy and X-ray photoelectron spectroscopy (XPS) prove that the H+ produced in the lithium metal cell are used for cathode interphase formation and thereby improve the Coulombic efficiency during cycling. With the help of Li3PO4, which scavenges H+ by the release of Li+ it can be established that H+ is reduced at the cathode at a potential of similar to 3 V vs Li/Li+ to form H-2 without large-capacity fade. The H+ have a more pronounced effect on the anode side when replacing lithium metal by Si-Gr. This is due to the facile Fr reduction at the anode during charge which modulates the solid electrolyte interphase (SEI) as well as the Si surface which is proven by ex situ XPS. The reduction of H+ at the anode is found to have a positive effect in mitigating the irreversible Li+ loss (10% capacity gain with 10% InAc) at the Si-Gr electrode, which was maintained over 100 cycles.