Induced AlF3 segregation for the generation of reciprocal Al2O3 and LiF coating layer on self-generated LiMn2O4 surface of over-lithiated oxide based Li-ion battery

•The well-distributed surface modification with Al2O3 and LiF coating layers.•Self-generated structure of spinel-like LiMn2O4 phase enhancing electronic conductivity.•Al2O3 layer suppressing side-reactions between cathode and electrolyte.•Thin surface coating layers improving discharge capacity and cycling performance.

For Li-ion batteries, AlF3 coating has been known to modify the over-lithiated layered oxide (OLO) cathodes to produce stable cathodes, but during synthesis procedure, the environment of excess amount of Li metal and free-exposed oxygen may cause the formation of Al2O3 and LiF materials, separately. We investigated the possibility of separated coating formation of Al as Al2O3 and F as LiF from AlF3 using density functional theory calculation, which suggests a favorable binding affinity of both Al2O3 and LiF phases to the OLO surface to support the preferable formation of coating layer of Al2O3 and LiF. Meanwhile, we found the well-distributed surface modification with the coating layers and a small amount of AlF3 (<∼11 vol%) throughout various surface analyses using the well-known coating process of AlF3 formation. Importantly, during the coating process LiF led to the transformation of OLO surface from Li2MnO3 to a self-generated structure of spinel-like LiMn2O4 phase, which enhances electronic conductivity. In addition, Al2O3 plays a key role in suppressing reactions between cathode and electrolyte, leading to stable cyclability. Experimental findings suggest that the well-distributed coating layer of two materials leads to a synergic effect on the enhanced electrochemical performance for high voltage operating Li-ion batteries.

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