Suppressing Irreversible Capacity Loss and Improving Stability during High Voltage Cycling of Li-Ion Battery Cathodes via a Carefully Designed “Formation Cycle” Protocol

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High irreversible capacity loss, low first cycle Coulombic efficiency, and inferior cyclic stability are the major challenges associated with high Ni-containing “layered” Li-transition metal (TM)-oxide cathodes. These problems get exacerbated with high upper cut-off potential and Ni-content, and are caused due to excessive Li-loss for the formation of unstable/thick/resistive cathode electrolyte interface layer (CEI) and kinetic hindrance towards Li-insertion during discharge owing to structural changes (including, “cation mixing”). To address these, a “formation cycle” protocol has been designed here, which involves two times charging to 3.8 V (vs Li/Li+), followed by holding at 3.8 V until an optimized current drop and then discharging to 2.6 V, prior to actual electrochemical cycling. The “formation cycle” protocol helps decompose surface residual species slowly/uniformly, facilitating the formation of thinner, less resistive, CEI. Furthermore, the 3.8 V hold facilitates preferential removal of Li-ions from TM-layer and concomitantly drives Ni-ions present in the Li-layer to the TM-layer vacancies created; thus, reducing “cation-mixing” and hindrance towards Li-insertion. Hence, the formation cycles reduce, and then suppress increments in impedance and “cation-mixing” during charge/discharge cycles; leading to improved first cycle Coulombic efficiency of ~85% (from ~60%, sans “formation cycle”) and ~88% higher reversible capacity after 100 cycles for Li-NMC811 based cathode, despite being subjected to deep-delithiation.