Abstract
Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/extraction of charge carriers (for example, lithium ions). Common synthesis methods can introduce large cations such as potassium, barium and ammonium ions into the tunnels, but how these cations affect charge storage performance is not fully understood. Here, we report the role of tunnel cations in governing the electrochemical properties of electrode materials by focusing on potassium ions in α-MnO2. We show that the presence of cations inside 2 × 2 tunnels of manganese dioxide increases the electronic conductivity, and improves lithium ion diffusivity. In addition, transmission electron microscopy analysis indicates that the tunnels remain intact whether cations are present in the tunnels or not. Our systematic study shows that cation addition to α-MnO2 has a strong beneficial effect on the electrochemical performance of this material.
Highlights
Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/ extraction of charge carriers
In the case of Li þ being the charge carrier, many researchers believe that the presence of large cations (K þ, Ba2 þ ) inside the 2 Â 2 tunnels impedes the diffusion of Li þ by physical blocking and repulsive electrostatic forces[18,21,22,23]
In this study, the effect of tunnel cations (K þ ) on the electrochemical performance of a-MnO2 cathodes was examined using a powerful combination of analytical aberration-corrected scanning transmission electron microscopy (ACSTEM), in situ
Summary
Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/ extraction of charge carriers (for example, lithium ions). Recent research shows that large cations such as K þ , Ba2 þ and NH4þ can be introduced during synthesis of a-MnO2 (refs 17,18) These cations, which partially occupy the tunnel cavities at certain stabilized lattice sites, are expected to interact with the charge carrier (Li þ , Na þ and Mg2 þ ) and affect the charge storage performance of a-MnO2 where tunnel-driven (de)intercalation contributes to the overall capacity[16,18,19,20]. The effect of varying K þ concentration on the rate performance of a
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