Recently, atomic layer deposition (ALD) has emerged as a promising and powerful technique to fabricate nanostructured electrodes and improve electrode/electrolyte interface in energy storage devices such as Li and Na ion batteries. In this project, we have deposited an electrochemically inert oxide material, hafnium oxide (HfO₂), on different anode materials. We demonstrate that a nanoscale layer of HfO₂ on zero dimensional (0D) SnO₂ nanospheres, one dimensional (1D) MoO₃ nanorods and two dimensional (2D) MoS₂ nanosheets significantly improves the cyclic stability, when these materials were applied as anode in Li ion batteries. Moreover, HfO₂ coated MoS₂ nanosheets were tested as anode material in Na ion batteries and, after 50 charge/discharge cycles, HfO₂ coated MoS₂ electrodes retained 91% of the initial capacity and bare MoS₂ electrodes retained only 63%.

Most importantly, the mechanism of capacity retention was explained by ex situ high resolution transmission electron microscopy (HRTEM), electrical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and X-ray diffraction. We showed that capacity retention is directly related to the degree of crystallinity retention after charge/discharge process. For instance, in case of MoO₃, ex-situ HRTEM analysis showed that after 50 charge/discharge cycles, the crystal structure of bare MoO₃ electrodes degrades severely while HfO₂ preserves the crystal structure during charge/discharge process. Furthermore, it was found that thickness and amorphous nature of HfO₂ layer plays critical role because Li⁺ ions has to diffuse through this layer. In summary, a mechanistic insight into capacity retention after HfO2 coating has been provided by post – electrochemical ex-situ analysis.