Key Takeaways & Executive Findings
- •• • Initial discharge capacity of 5400 mAh g−1 (carbon mass basis) and reversible capacity of 3500 mAh g−1 at room temperature, with a discharge plateau near 3.9 V, exceed prior Na/Cl2 systems using amorphous carbon nanospheres or graphite cathodes; this directly addresses the energy-density bottleneck for stationary storage and electric vehicles. • • Stable cycling for over 140 cycles at rates up to 2 C with near 100% Coulombic efficiency, enabled by the dual-function KFSI additive that forms a NaF/KF-rich SEI and suppresses dendrites via electrostatic shielding from the lower K+/K redox potential; this mitigates sodium-metal anode failure, a persistent barrier to commercialization. • • In situ Raman spectroscopy identifies reversible SCl2 and S2Cl2 species at the end of charge, contributing an additional ~3.9 V plateau, while the main Cl−/Cl2 redox plateau remains at ~3.55 V; this clarifies the reaction mechanism and demonstrates that MWCNTs trap active species, preventing shuttle-induced electrolyte depletion. • • Cryogenic TEM reveals NaCl nanocrystals deposited within the hollow cores of MWCNTs, and EELS confirms uniform chlorine distribution on nanotube surfaces in the charged state; the high defect density (Raman D/G ratio = 1.01) and large pore volume (2.48 cm3 g−1) are critical for accessing internal space and achieving superior performance.
Abstract
Rechargeable sodium-chlorine (Na/Cl2) batteries derived from thionyl chloride (SOCl2) primary systems offer high theoretical energy density and wide-temperature operation, but their reversibility is constrained by chlorine shuttle and unstable sodium-metal interfaces. Dai et al. demonstrate a Na/Cl2 battery using multi-walled carbon nanotubes (MWCNTs) as the cathode host, a sodium-metal anode, and a SOCl2-based electrolyte containing AlCl3, potassium bis(fluorosulfonyl)imide (KFSI), and NaCl additives. The cell delivers an initial discharge capacity of 5400 mAh g−1 (carbon mass basis), a reversible capacity of 3500 mAh g−1, and a discharge plateau near 3.9 V at room temperature, sustaining over 140 cycles at rates up to 2 C with near 100% Coulombic efficiency. The dual-function KFSI additive suppresses sodium dendrites via electrostatic shielding from the lower K+/K redox potential and forms a NaF/KF-rich solid-electrolyte interphase. In situ Raman spectroscopy reveals reversible SCl2 and S2Cl2 formation at the end of charge, contributing an additional ~3.9 V plateau, while the main Cl−/Cl2 redox plateau remains at ~3.55 V. Cryogenic transmission electron microscopy shows NaCl nanocrystals deposited within the hollow cores of MWCNTs, and electron energy loss spectroscopy confirms uniform chlorine distribution on nanotube surfaces in the charged state. A high defect density (Raman D/G ratio = 1.01) and large pore volume (2.48 cm3 g−1) are identified as critical enablers of superior battery performance.
1. Introduction
Rechargeable Na/Cl2 batteries inherit the high theoretical energy density and wide-temperature operability of SOCl2 primary cells, but their practical deployment has been stalled by chlorine shuttle and unstable sodium-metal interfaces. Prior cathode designs—amorphous carbon nanospheres or graphite—fail to simultaneously trap electrochemical oxidation products and sustain reversible Cl−/Cl2 redox, leading to rapid capacity fade and electrolyte depletion. The strong retention of Cl2 and related species is essential to suppress shuttling and stabilize NaCl discharge products, yet structural engineering of carbon hosts to achieve this remains an unresolved technical challenge.
Dai et al. address this bottleneck by employing multi-walled carbon nanotubes (MWCNTs) as the cathode host in a Na/Cl2 cell with a sodium-metal anode and a SOCl2-based electrolyte containing AlCl3, KFSI, and NaCl additives. The MWCNT architecture provides a high defect density (Raman D/G ratio = 1.01) and large pore volume (2.48 cm3 g−1), enabling chlorine redox chemistry within the nanotube cores. The dual-function KFSI additive stabilizes the sodium anode via electrostatic shielding and a NaF/KF-rich SEI, while in situ Raman and cryogenic TEM reveal reversible SCl2/S2Cl2 formation and NaCl nanocrystal deposition inside the nanotubes. This integrated approach delivers 5400 mAh g−1 initial capacity, 3500 mAh g−1 reversible capacity, and over 140 cycles at 2 C with near 100% Coulombic efficiency.
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Debin Kong, Linjie Zhi (2025). Nanoconfined Chlorine Redox Chemistry in Multi-Walled Carbon Nanotubes: A Breakthrough for Rechargeable Na/Cl2 Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3542-8
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Frequently Asked Questions
What is the dominant failure mechanism of the Na/Cl2 battery under extended cycling, and how does the KFSI additive mitigate it?
The primary failure modes are sodium dendrite growth and chlorine shuttle. The KFSI additive mitigates both: the lower redox potential of K+/K creates electrostatic shielding that regulates Na+ deposition and suppresses dendrites, while its decomposition forms a stable NaF/KF-rich SEI layer. This enables over 140 cycles at up to 2 C with near 100% Coulombic efficiency.
How does the MWCNT cathode compare to conventional amorphous carbon nanospheres or graphite in terms of capacity and rate capability?
MWCNTs deliver an initial discharge capacity of 5400 mAh g−1 and a reversible capacity of 3500 mAh g−1, with a discharge plateau near 3.9 V. This represents a significant improvement over previous Na/Cl2 systems using amorphous carbon nanospheres or graphite cathodes, which exhibit lower capacities and poorer cycling stability.
What is the role of the additional voltage plateau at ~3.9 V, and how does it affect the overall energy density?
In situ Raman spectroscopy reveals that SCl2 and S2Cl2 species form at the end of charging and are reduced during early discharge, contributing an additional plateau at ~3.9 V. This supplements the main Cl−/Cl2 redox plateau at ~3.55 V, thereby increasing the overall capacity and energy density of the cell.
What are the scalability bottlenecks for producing MWCNT-based cathodes with the required defect density and pore volume?
The critical parameters are a Raman D/G ratio of 1.01 and a pore volume of 2.48 cm3 g−1. Achieving these consistently at scale requires precise control of MWCNT synthesis and purification. Current chemical vapor deposition methods can produce MWCNTs with tunable defects, but uniform pore volume and defect density across batches remain challenging, impacting cost and yield.
What is the cost parity outlook for this Na/Cl2 system against lithium-ion batteries, considering the SOCl2-based electrolyte and KFSI additive?
Sodium and chlorine are earth-abundant and low-cost, but the SOCl2-based electrolyte with AlCl3, KFSI, and NaCl additives introduces material costs. KFSI is relatively expensive, though used in small amounts. At scale, the raw material cost could be competitive with lithium-ion, but processing and safety measures for SOCl2 handling add overhead. The 140-cycle life at 2 C is currently below commercial requirements, necessitating further cycle-life improvements for cost parity.
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