Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO_3-x

The short charging times and high power capabilities associated with capacitive energy storage make this approach an attractive alternative to batteries. One limitation of electrochemical capacitors is their low energy density and for this reason, there is widespread interest in pseudocapacitive materials that use Faradaic reactions to store charge. One candidate pseudocapacitive material is orthorhombic MoO₃ (α-MoO₃), a layered compound with a high theoretical capacity for lithium (279 mA h g^-1 or 1,005 C g^-1). Here, we report on the properties of reduced α-MoO_3-x(R-MoO_3-x) and compare it with fully oxidized α-MoO₃ (F-MoO₃). The introduction of oxygen vacancies leads to a larger interlayer spacing that promotes faster charge storage kinetics and enables the α-MoO₃ structure to be retained during the insertion and removal of Li ions. The higher specific capacity of the R-MoO_3-x is attributed to the reversible formation of a significant amount of Mo⁴⁺ following lithiation. This study underscores the potential importance of incorporating oxygen vacancies into transition metal oxides as a strategy for increasing the charge storage kinetics of redox-active materials.