Mechanistic Insight into the Stability of HfO 2 ‐Coated MoS 2 Nanosheet Anodes for Sodium Ion Batteries

It is demonstrated for the first time that surface passivation of 2D nanosheets of MoS 2 by an ultrathin and uniform layer of HfO 2 can significantly improve the cyclic performance of sodium ion batteries. After 50 charge/discharge cycles, bare MoS 2 and HfO 2 coated MoS 2 electrodes deliver the specific capacity of 435 and 636 mAh g −1 , respectively, at current density of 100 mA g −1 . These results imply that batteries using HfO 2 coated MoS 2 anodes retain 91% of the initial capacity; in contrast, bare MoS 2 anodes retain only 63%. Also, HfO 2 coated MoS 2 anodes show one of the highest reported capacity values for MoS 2 . Cyclic voltammetry and X‐ray photoelectron spectroscopy results suggest that HfO 2 does not take part in electrochemical reaction. The mechanism of capacity retention with HfO 2 coating is explained by ex situ transmission electron microscope imaging and electrical impedance spectroscopy. It is illustrated that HfO 2 acts as a passivation layer at the anode/electrolyte interface and prevents structural degradation during charge/discharge process. Moreover, the amorphous nature of HfO 2 allows facile diffusion of Na ions. These results clearly show the potential of HfO 2 coated MoS 2 anodes, which performance is significantly higher than previous reports where bulk MoS 2 or composites of MoS 2 with carbonaceous materials are used.