Engineering of Conformal Electrode Coatings by Atomic Layer Deposition for Aqueous Na-ion Battery Electrodes

The application of atomic layer deposition on active material particles or as conformal layers directly on electrodes is an effective and viable approach for protecting the battery materials from degradation. Al2O3, TiO2, and HfO2 coatings are applied on NaTi2(PO4)(3), which is among the most studied negative electrode materials for aqueous Na-ion batteries. The coated electrodes are characterized in terms of electrochemical kinetics, charge capacity retention, and electrochemical impedance spectra. Al2O3, a widely used protective coating in non-aqueous batteries, is shown to be insufficient to suppress parasitic processes and is eventually dissolved by reaction with hydroxide during extended cycling in aqueous Na2SO4. However, this process provides a local buffering effect making the protective action of this coating mainly of chemical nature. TiO2 is found to be very resistant to increase in pH and remains almost intact during electrochemical cycling. However, we provide strong evidence that TiO2 itself is electrochemically active in aqueous electrolytes at negative potentials. The protonation of TiO2 leads to an additional increase in local pH which is detrimental to NaTi2(PO4)(3) and results in even faster capacity loss than in uncoated electrodes. Only HfO2 is found to be sufficiently stable and electrochemically inert ALD coating for negative NaTi2(PO4)(3) electrodes operating in aqueous electrolytes.