Binder-Free V2O5 Cathode for High Energy Density Rechargeable Aluminum-Ion Batteries

Abstract

Nowadays, research on electrochemical storage systems moves into the direction of post-lithium-ion batteries, such as aluminum-ion batteries, and the exploration of suitable materials for such batteries. Vanadium pentoxide (V₂O₅) is one of the most promising host materials for the intercalation of multivalent ions. Here, we report on the fabrication of a binder-free and self-supporting V₂O₅ micrometer-thick paper-like electrode material and its use as the cathode for rechargeable aluminum-ion batteries. The electrical conductivity of the cathode was significantly improved by a novel in-situ and self-limiting copper migration approach into the V₂O₅ structure. This process takes advantage of the dissolution of Cu by the ionic liquid-based electrolyte, as well as the presence of two different accommodation sites in the nanostructured V₂O₅ available for aluminum-ions and the migrated Cu. Furthermore, the advanced nanostructured cathode delivered a specific discharge capacity of up to ~170 mAh g^-1 and the reversible intercalation of Al³⁺ for more than 500 cycles with a high Coulomb efficiency reaching nearly 100%. The binder-free concept results in an energy density of 74 Wh kg^-1, which shows improved energy density in comparison to the so far published V₂O₅-based cathodes. Our results provide valuable insights for the future design and development of novel binder-free and self-supporting electrodes for rechargeable multivalent metal-ion batteries associating a high energy density, cycling stability, safety and low cost.

Keywords: V2O5 cathode; aluminum-ion battery; binder-free electrode; paper-like thin films; post-lithium-ion batteries.

Figure 1

(a) Schematic illustration of vanadium pentoxide (V₂O₅) thin film preparation. A Si wafer is placed in a beaker that is filled up with the V₂O₅ dispersion. Drying at ambient conditions and removing the Si wafer from the V₂O₅ thin film results in the self-supporting V₂O₅ paper-like thin film. (b) AFM image of the paper-like surface showing the nanofiber alignment. (c) SEM image of the V₂O₅ paper revealing a layered structure. (d) Shaped V₂O₅ paper showing the flexibility. (e) XRD pattern of the V₂O₅ paper.

Figure 2

(a) Cyclic voltammetry curve of the second cycle at a scan rate of 0.1 mV s⁻¹ revealing two de-/intercalation potentials. (b) Schematic illustration of the two different intercalation sites near the planar oxygen atom (site-a) and close to the apical oxygen atom (site-b) of the VO₅ units.

Figure 3

Simplified schematic representation of the occurring reactions during the discharge and charge process in the Cu doped V₂O₅ cathode. (a) In the discharge process, Al³⁺ are electrochemically stripped from the Al anode and are intercalated into the V₂O₅ cathode via an intermediate Al₂Cl₇⁻ complex. (b) During the charge process, Al³⁺ is de-intercalated from the V₂O₅ cathode forming the intermediate Al₂Cl₇⁻ complex and metallic Al is deposited on the anode.

Figure 4

Images obtained during ex-situ TEM investigation of the cross-section of a cathode cycled eight times during CV investigations. An overview of the investigated cross-section is shown in image (a). The black regions correspond to the Cu-rich precipitations, which are homogeneously distributed over the complete sample. The images (b–d) are the higher magnification spots from the image (a).

Figure 5

Cyclic voltammetry curves at a scan rate of 0.1 mV s⁻¹ of (a) second cycles for p-V₂O₅ and V₂O₅/Cu revealing the importance of the migrated Cu, as well as for (b) cycles 2, 4 and 8 of V₂O₅/Cu.

Figure 6

Galvanostatic charge and discharge measurements with various current densities, as displayed in the diagrams. Specific storage capacity as a function of the cycle number. (a) Specific discharge capacity of p-V₂O₅. (b) Potential vs. specific capacity plot for p-V₂O₅. (c) Specific discharge capacity of p-V₂O₅ and V₂O₅/Cu. (d) Potential vs. specific capacity plot for the V₂O₅/Cu. (e) Charging/discharging capacity and Coulomb efficiency of the V₂O₅/Cu cathode.