Cascading Degradations Artificially Improving the Lifetime of Li-ion Full Cells using DMC-based Highly Concentrated Electrolyte

The deployment of energy dense Ni-rich NMC (LiNi x Mn y Co z O 2 with x > 0.8) in Li-ion batteries is hampered by a poor interfacial stability above 4.2 V. Among the strategies to mitigate this instability, highly concentrated electrolytes (HCE) have shown a promising resilience at high potential. In this work, we demonstrate that although cells using HCE experience low capacity fading compared to conventional carbonate based-electrolyte, HCE does not prevent oxidation of dimethylcarbonate (DMC) at high potential. Even worse, this phenomenon cannot be fully offset by lithium intercalation at the negative electrode and eventually leads to lithium plating that precipitates the cell end of life. To circumvent lithium plating, cycling at high temperature is shown to build a more passivating solid electrolyte interphase (SEI); while promising at first, the lithium losses associated with the SEI formation trigger a jump of graphite staging. Only replacing DMC by ethyl carbonate (EC) solvent reduces efficiently the parasitic oxidation and prevents capacity rollover. This work, by the use of adapted testing protocols and analysis workflows, provides the necessary understanding to open new routes for tackling parasitic reaction at high voltage in Li-ion batteries, which including mastering of SEI formation conditions and the use of appropriate solvent.