Atomic Layer Deposition of a Nanometer-Thick LiPO Protective Layer on LiNiMnO Films: Dream or Reality for Long-Term Cycling?

LiNiMnO (LNMO) is a promising 5V-class electrode for Li-ion batteries but suffers from manganese dissolution and electrolyte decomposition owing to the high working potential. An attractive solution to stabilize the surface chemistry consists in mastering the interface between the LNMO electrode and the liquid electrolyte with a surface protective layer made from the powerful surface deposition method. Here, we show that a 7400 nm thick sputtered LNMO film coated with a nanometer-thick lithium-ion-conductive LiPO layer was deposited by the atomic layer deposition method. We demonstrate that this "material model system" can deliver a remarkable surface capacity (∼0.4 mAh cm at 1C) and exhibits improved cycling lifetime (×650%) compared to the nonprotected electrode. Nevertheless, we observe that mechanical failure occurs within the LNMO and LiPO films when long-term cycling is performed. This in-depth study gives new insights regarding the mechanical degradation of LNMO electrodes upon charge/discharge cycling and reveals for the first time that the surface protective layer made from the ALD method is not sufficient for long-term stability applications.