Key Takeaways & Executive Findings
- •• • The multi-stage sodium compensation strategy reduces initial active sodium loss by 36.53%, directly addressing the primary capacity degradation mechanism in sodium-ion batteries. • • A single activation event during cycling provides an additional 0.115 mAh cm−2 of active sodium, enabling sustained compensation for continuous SEI growth. • • The multi-replenished HC||NFPP-M cell maintains a specific capacity of 82.15 mAh g−1 after 350 cycles, a 55.56 mAh g−1 improvement over the untreated cell, demonstrating exceptional long-term cycling stability. • • The cell exhibits a capacity loss of only 0.059% per cycle over 350 cycles at 0.5 C, significantly outperforming conventional pre-sodiation approaches.
Abstract
Irreversible sodium loss, primarily caused by solid electrolyte interphase (SEI) formation during initial cycling, significantly degrades the capacity of sodium-ion batteries by depleting active sodium. While pre-sodiation mitigates initial sodium loss, it fails to address continuous loss throughout the battery lifecycle. To overcome this limitation, we propose a sustained sodium compensation strategy utilizing activation-releasing systems. Key to this approach are high-capacity sodium compensators, Na2C2O4 and Na2C4O4, supported on a B and N co-doped Mo2C-W2C (MoW-C) heterostructure catalyst. This configuration enables efficient sodium release at charging voltages of 3.53 and 3.78 V, respectively. By integrating the sodium supplement agent onto the separator, and precisely controlling voltage and charge, multiple sodium replenishment is achieved over the entire battery lifecycle. This strategy reduces initial active sodium loss by 36.53%. Furthermore, a single activation during subsequent usage provides an additional 0.115 mAh cm−2 of active sodium. As a result, the cell exhibits exceptional cycling stability, with a capacity loss of only 0.059% per cycle over 350 cycles at 0.5 C.
1. Introduction
Sodium-ion batteries (SIBs) are a promising alternative to lithium-ion batteries for grid-scale energy storage due to sodium's abundance and low cost. However, their commercial viability is hindered by irreversible active sodium loss (ASL), primarily from SEI formation during initial cycling and continuous SEI repair during operation. Since cathode materials are the sole sodium source, ASL depletes cyclable Na+ inventory, reducing energy density and cycle life. Hard carbon anodes, with their low initial Coulombic efficiency, exacerbate this issue. Conventional pre-sodiation methods, such as anode pre-sodiation with reactive agents or cathode pre-sodiation with sodium-rich additives, only address initial loss and fail to compensate for continuous loss, limiting long-term performance.
This work introduces a novel 'sodium reservoir' strategy that enables multi-stage sodium replenishment throughout the battery's lifetime. By integrating high-capacity Na2C2O4 and Na2C4O4 sodium sources onto a B, N-co-doped Mo2C-W2C heterocatalyst-modified separator, controlled sodium release is achieved at distinct voltages (3.53 V and 3.78 V). This design allows for precise activation via voltage/charge control, compensating for both initial and continuous ASL. The approach not only reduces initial loss by 36.53% but also provides additional sodium on demand, resulting in exceptional cycling stability with only 0.059% capacity loss per cycle over 350 cycles. This separator-based strategy avoids the polarization and cathode damage associated with conventional additives, offering a practical solution for long-life SIBs.
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Jingyu Xiang, Wei Zhong, Linfeng Peng, Shijie Cheng, Jia Xie (2026). A Heterocatalyst-Modified Separator Enables Multi-Stage Sodium Compensation for Long-Life Sodium-Ion Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3733-2
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Frequently Asked Questions
What is the specific mechanism by which the MoW-C heterocatalyst enables sodium release at distinct voltages, and how does this prevent adverse effects on the cathode?
The B, N co-doped Mo2C-W2C heterostructure acts as a catalyst that lowers the decomposition potential of Na2C2O4 and Na2C4O4 to 3.53 V and 3.78 V, respectively. By placing the sodium sources on the separator rather than the cathode, the decomposition occurs away from the cathode, preventing polarization-induced voltage shifts and avoiding direct damage to cathode materials from reactive decomposition products.
How does the multi-stage sodium compensation strategy address continuous sodium loss during cycling, and what is the quantitative impact on capacity retention?
The strategy allows for periodic activation of the sodium reservoir by adjusting the charge cut-off voltage and controlling charge capacity. This releases additional sodium to compensate for ongoing SEI growth. Quantitatively, a single activation provides 0.115 mAh cm−2 of active sodium, and after 350 cycles, the multi-replenished cell retains a specific capacity of 82.15 mAh g−1, which is 55.56 mAh g−1 higher than the untreated cell, demonstrating a significant improvement in long-term capacity retention.
What is the impact of the sodium compensation on the SEI layer composition and thickness, and how does this contribute to cycling stability?
After 200 cycles with multiple sodium replenishments, TEM analysis revealed a uniform SEI layer of 31.76 nm thickness on the hard carbon anode. The SEI is NaF-rich due to the FEC electrolyte additive, which provides high ionic conductivity, mechanical strength, and chemical stability. This stable SEI reduces side reactions and irreversible capacity loss, thereby enhancing cycling stability.
How does the separator-based sodium reservoir compare to conventional cathode additives in terms of electrochemical performance and practical implementation?
The separator-based approach avoids the polarization and cathode damage associated with cathode additives. It enables controlled, multi-stage sodium release without interfering with cathode reactions. In terms of performance, the multi-replenished cell achieves a capacity loss of only 0.059% per cycle over 350 cycles, which is superior to single-replenishment systems that show a capacity of 63.84 mAh g−1 after 200 cycles. This demonstrates better long-term stability and practical viability.
What are the potential scalability and cost implications of using MoW-C heterocatalyst and sodium oxalate salts in commercial sodium-ion batteries?
The materials used, including Mo2C, W2C, and sodium oxalates, are relatively abundant and cost-effective compared to lithium-based counterparts. The separator modification process is compatible with existing battery manufacturing lines, as it involves simple coating or impregnation. The improved cycle life (350 cycles with minimal degradation) reduces the frequency of battery replacement, potentially lowering the levelized cost of storage. However, the additional processing steps and catalyst synthesis may increase upfront costs, which need to be weighed against the extended lifespan.
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