Robust AlF₃ Atomic Layer Deposition Protective Coating on LiMn_1.5Ni_0.5O₄ Particles: An Advanced Li-Ion Battery Cathode Material Powder

The most promising LiMn1.5Ni0.5O4 (LMNO) ultrahigh voltage cathode material is not yet commercialized because it is suffering from capacity fading during cycling, especially at elevated temperatures. Manganese ions dissolution from the cathode and their precipitation on the graphite anode are the main cause of failure of Li-ion batteries (LIBs) utilizing LMNO cathode material. In order to mitigate this issue, an AlF3 layer was coated directly on LMNO powder particles via atomic layer deposition (ALD). A few nanometer thick coating was individually formed on each particle. The coating protected the particles from the corrosion-like phenomenon, when immersed in LIB electrolyte at room temperature (RT) and at 45 °C. Half-cell electrochemical measurements showed superior performance for the ALD coated AlF3 material over the uncoated material. In the full-cell configuration enhanced capacity retention was observed for cells comprised from cathode materials coated by different AlF3 ALD coatings. Complete Li-ion cells utilizing ALD coated cathode powder in the cathode and a graphite anode exhibited lower initial capacity, which was recovered continuously during cycling at RT and dramatically at 45 °C during the first ∼30 cycles. A different and modified formation process and cycling method significantly improved the lower initial capacity of the Li-ion cells on the expense of a rather shorter cycle life. Even with the new formation cycling, Li-ion cells utilizing ALD coated materials exhibited better cycling performance than cells utilizing pristine material. Fluorination of oxygen impurities in the coating layer or its lithiation are suggested as mechanisms for the recovered capacity. Li-ion cells utilizing ALD AlF3 coated cathode powder were cycled up to 180 cycles, when 150 of them were at 45 °C.