Design of carbon-bimetallic oxide nanofiber cathodes with hybrid charge storage for advanced aluminum-ion batteries

Aluminum-ion storage is a compelling Li-ion alternative due to high theoretical volumetric capacity (∼8040 mAh cm^-3) and trivalent (Al³⁺) multi-electron redox reactions, enabling higher charge density and energy storage per unit volume. Additionally, aluminum is the third most abundant element in the Earth's crust, non-toxic, and costs ∼0.002 USD per Wh, making it a low-cost, sustainable alternative for large-scale grid and stationary energy storage. In this work, we designed a novel carbon-bimetallic oxide (V₂O₅/SnO₂) nanofiber composite (C_x-V₂O₅-SnO₂) as a cathode and evaluated its electrochemical performance for efficient aluminum-ion (Al-ion) storage. Among the prepared samples, the optimized C_1.2-V₂O₅-SnO₂ nanofiber cathode delivers a high reversible capacity of 155.4 mAh g^-1 at 0.2 A g^-1 and retains 86.5 mAh g^-1 even at a high current density of 5.0 A g^-1 with over 99% coulombic efficiency across 400 cycles. This is due to the optimal surface area (240 m² g^-1) and structural stability of the C_x-V₂O₅-SnO₂ electrode and interfacial stability. Compared to state-of-the-art V₂O₅ cathodes, which deliver 130-140 mAh g^-1 at 0.5 A g^-1, the C_x-V₂O₅-SnO₂ nanofiber composite exhibits ∼11% higher specific capacity (155 mAh g^-1) at twice the current density (1 A g^-1), highlighting its superior rate capability. Furthermore, C_x-V₂O₅-SnO₂ exhibits a symmetric charge-discharge voltage profile, characteristic of a faradaic-dominated charge-storage mechanism involving highly reversible redox reactions with minimal voltage hysteresis. The observed linear voltage-capacity relationship further suggests a pseudocapacitive contribution, highlighting a hybrid charge-storage mechanism that combines surface-controlled redox processes with diffusion-limited intercalation. As the current density increases from 1 to 5 A g^-1, the specific capacity (155 mAh g^-1) decreases to 86 mAh g^-1 due to ion diffusion limitations, leading to under-utilization of active material and increased polarization. Cyclic voltammetry (CV) results exhibit broad redox peaks indicative of reversible Al-ion intercalation/deintercalation and a mixed kinetic behavior. Rate performance studies show capacity fade at high currents due to diffusion constraints, yet full capacity recovery at lower currents highlights excellent structural resilience and electronic conductivity. Collectively, the hybrid charge-storage mechanism, rate capability, and long-term cycling stability position the C_1.2-V₂O₅-SnO₂ composite as a promising cathode material for Al-ion storage.