Citrate-Assisted Solvothermal Synthesis of SnO2 Porous Microflowers as Efficient Cathode for Advanced Hybrid Supercapacitor

Herein, mesoporous tin dioxide materials with distinct structures (SnO₂ microflowers and SnO₂ microsheets) were, respectively, prepared via a citrate-mediated solvothermal route along with a post-annealing treatment in air. Electrochemical tests revealed a typical battery-type charge storage behavior of SnO₂ materials. Attributing to the 3D hierarchical flower-like structure and its good conductivity, the SnO₂ microflowers possessed a specific capacity of 177.2 C g⁻¹, slightly greater than 159.0 C g⁻¹ achieved by SnO₂ microsheets under 1 A g⁻¹. When assembled into hybrid supercapacitors (HSCs) utilizing activated carbon (AC) as an anode, the SnO₂ microflowers//AC HSC device delivered a high energy density (ED) of 29.5 W h kg⁻¹ at 883.9 W kg⁻¹, surpassing SnO₂ microsheets//AC HSC (26.9 W h kg⁻¹ at 881.7 W kg⁻¹). Furthermore, both SnO₂//AC HSCs exhibited long-term stability, showing 113.2% (SnO₂ microflowers//AC) and 106.4% (SnO₂ microsheets//AC) capacity retention over 5000 cycles. Notably, this synthesis strategy achieves facile morphological control and improved electrochemical properties of SnO₂ materials by adjusting the sodium citrate amount. These results indicate that SnO₂ microflowers and SnO₂ microsheets are attractive candidates for high-performance HSC assembly. Furthermore, this cost-effective approach can provide a reference for synthesizing other advanced metal oxide-based electrode materials.