Key Takeaways & Executive Findings
- •• • Bi@NC anode delivers a specific capacity of 300 mAh g−1 at 0.5 A g−1, with rate capability sustained above 75 A g−1, enabling high-power operation for SICs. • • Exceptional cycling stability of over 12,000 cycles in half-cell configuration, indicating robust structural integrity and long-term durability for practical applications. • • Full SIC device achieves a maximum energy density of 115 Wh kg−1 at 434 W kg−1, and retains 72 Wh kg−1 at an elevated power density of 45,535 W kg−1, demonstrating superior kinetic matching. • • Three-electrode Swagelok cell tests confirm minimal voltage variation across current densities, validating well-matched kinetics between Bi@NC anode and activated carbon cathode, crucial for device reliability.
Abstract
Sodium-ion capacitors (SICs) typically feature a hybrid design, incorporating a battery-type anode that operates by faradaic redox reactions and an activated carbon cathode that functions through electrical double-layer (EDL) adsorption/desorption. However, the kinetics of faradaic processes are inherently slower than those of EDL processes, leading to a fundamental problem known as kinetic imbalance between the electrodes, which hinders the development of high-performance SICs. To address this, we synthesized composites of bismuth nanoparticles in N-doped carbon (Bi@NC) by a high-temperature sintering method. The resulting Bi@NC anode has a specific capacity of 300 mAh g−1 at 0.5 A g−1, an exceptional rate capability (maintaining performance at currents exceeding 75 A g−1), and outstanding cycling stability over 12,000 cycles. Three-electrode Swagelok cell tests revealed that this high-rate Bi@NC composite effectively decreases the kinetic gap with the activated carbon cathode, as shown by an analysis of their respective potential swing windows (vs. Na/Na+). This enables the fabricated SIC to achieve a maximum energy density of 115 Wh kg−1, a peak power density of 45,535 W kg−1, and a long cycle life exceeding 8,000 cycles.
1. Introduction
Sodium-ion capacitors (SICs) have emerged as a promising alternative to lithium-ion systems due to the abundance and low cost of sodium. However, the inherent kinetic mismatch between the faradaic battery-type anode and the capacitive cathode limits their performance. Conventional carbon anodes suffer from sluggish Na+ diffusion, while alloying anodes like bismuth offer high capacity but suffer from volume expansion and poor rate capability. This study addresses this bottleneck by synthesizing a bismuth-carbon composite (Bi@NC) via a one-step thermal decomposition method, which enhances conductivity and buffers volume changes, thereby achieving fast kinetics.
The Bi@NC anode demonstrates superior rate capability and cycling stability compared to the activated carbon cathode, effectively reducing the kinetic gap. Full SIC devices fabricated with this anode achieve high energy and power densities, with three-electrode tests confirming minimal voltage polarization. This work provides a novel strategy for designing anode materials that enable kinetic compatibility, advancing the practical implementation of high-performance SICs.
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Man Xiaoge, Huang Xinli, Min Xinyue, Yan Yijie, Shi Yuanchang, Li Tao, Wang Chengxiang, Zhang Zhiwei, Yin Longwei, Wang Rutao (2026). A fast bismuth-carbon composite anode for achieving kinetic matching between the anode and cathode of sodium-ion capacitors. New Carbon Materials. https://doi.org/10.1016/S1872-5805(26)61072-4
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Frequently Asked Questions
What is the specific capacity and rate capability of the Bi@NC anode, and how does it compare to the activated carbon cathode?
The Bi@NC anode delivers a specific capacity of 300 mAh g−1 at 0.5 A g−1 and maintains performance at current densities exceeding 75 A g−1, demonstrating superior rate capability compared to the activated carbon cathode, which typically exhibits lower capacity and poorer rate performance.
How does the Bi@NC composite mitigate volume expansion during cycling, and what is the resulting cycle life?
The carbon framework in Bi@NC not only improves conductivity but also effectively alleviates stress caused by volume expansion of bismuth during electrochemical cycling. This structural design enables outstanding cycling stability, with the half-cell retaining performance over 12,000 cycles.
What are the energy and power densities of the full sodium-ion capacitor device, and how do they reflect kinetic matching?
The fabricated SIC achieves a maximum energy density of 115 Wh kg−1 at a power density of 434 W kg−1, and retains 72 Wh kg−1 at an elevated power density of 45,535 W kg−1. Three-electrode tests show minimal voltage variation across current densities, confirming well-matched kinetics between the anode and cathode.
What is the synthesis method for Bi@NC, and how does it contribute to the material's performance?
Bi@NC composites are prepared by a one-step thermal decomposition method using metal Bi salts with organic ligands as precursors. This method yields a uniform dispersion of bismuth nanoparticles within an N-doped carbon matrix, which enhances electronic conductivity and buffers volume changes, leading to improved rate capability and cycle stability.
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