A Facile Synthesis of Al2O3-Coated LiNi1/3Co1/3Mn1/3O2 with Improved Cycle Life Prepared By a Wet-Chemical Method

Abstract

Aging of lithium-ion batteries (LIBs) and its prevention has been one of the focal points of LIB research in the recent years. The stability of the well-established anode material graphite far surpasses the stability of emerging high energy cathodes (NCM Ni83, Li- and Mn-rich NCM)1,2. Additionally, the degradation of the solid electrolyte interphase (SEI) on the anode side mainly originates from dissolved TM species of the cathode material3. Thus, to bring the high energy LIBs to market, the main focus has been on the mitigation of cathode degradation. Application of protective coatings is an excellent method for stability improvement and is often preferred over doping and structural modifications4. Nevertheless, a pressing need remains for a more sustainable, cost-effective, and easily scalable method to synthesize coatings on cathode materials.In this work, we study LiNi1/3Co1/3Mn1/3O2 (NCM 111) as a model material and coat it with Al2O3 via a wet-chemical process. Wet-chemical processes have in the past often used toxic precursors or solvents, rendering the sustainability of these methods questionable5,6. Thus, here a wet-chemical coating process is developed based on a green solvent (ethanol) and a non-toxic raw material (aluminium isopropoxide). The coating is based on hydrogen bond formation between a hydrolyzed precursor (Al(OH)3) and the oxygen groups on the surface of the commercially sourced active material (NCM111) to form a uniform coating of Al(OH)3 which is converted to Al2O3 after a sintering step in air at 500°C. In addition to characterizing the effects of coating on the cathode active material, we also investigated how the coating procedure alone, without adding the aluminium precursor, affects the active material. This is done to separate the effects of solution washing and sintering from those of the coating formation on the active material. The pristine, coated, and washed and sintered NCM powders were investigated by SEM, EDS, XPS, XRD, and TEM methods, and electrochemical tests in half cells.The capacity of the uncoated NCM decreases to 80% of initial capacity during the first 200 cycles at 1 C rate. A significant improvement in both capacity retention and rate capability is observed for both the washed and coated samples. Although coated samples display further cycle life improvements, most of the improvements can be attributed to the ethanol washing away residual lithium compounds (RLCs) such as LiCO3, LiOH and LiO, which are left on the particle surface after the synthesis or formed during the storage in ambient atmosphere. RLCs can either accelerate electrolyte and electrode decomposition via several reaction pathways7 or participate in cathode-electrolyte interphase formation, resulting in increased impedance and decreased cycle life. Further improvements in the Al2O3-coated samples due to the coating serving as a protective layer that helps to prevent unwanted reactions between the cathode and the electrolyte.This study demonstrates Al2O3-coated NCM 111 with improved electrochemical performance. Al2O3 is deposited via a facile and environmentally friendly wet-chemical coating process. The study also highlights the importance of washing the active material to remove residual lithium compounds, which can have a significant impact on cycle life and performance.Authors acknowledge the support of M-Era.net project "Inert Coatings for Prevention of Ageing of NMC Cathode for Lithium-Ion Batteries” (InCoatBat).