Sodium Metal Battery Electrolyte Keeps 90.4% Capacity After 1,000 Cycles in Lab Cells

Interest is growing in technologies that use sodium, an abundant and relatively inexpensive resource, as a next-generation alternative to lithium-ion batteries. Among them, the "sodium metal battery" is seen as a path to higher energy density. Instead of the hard carbon typically used for the anode in conventional sodium-ion batteries, it uses metallic sodium itself, which has a high theoretical capacity. However, metallic sodium is highly reactive, and side reactions readily occur when it comes into contact with conventional organic liquid electrolytes. Repeated charging and discharging also causes needle-like metal crystals known as dendrites to grow, which can lead to internal short circuits and fires, so major obstacles to commercialization remain.

A research team led by Professor Chan-Jin Park of Chonnam National University in South Korea developed a new electrolyte based on a polymerizable deep eutectic solvent (PDES) to address these problems, and confirmed stable operation at laboratory scale. The results were published in 2026 in Energy Storage Materials, Volume 90 (DOI: 10.1016/j.ensm.2026.105430, article number 105430; authors: Kale, Dam, Sangabathula, Kim, Park). The team used an "in-situ polymerization" approach, in which a liquid deep eutectic solvent is thermally polymerized inside the battery and turned into a shape-retaining polymer matrix. This combines structural stability that suppresses leakage with sodium-ion conductivity, and the team reports high capacity retention after 1,000 charge-discharge cycles in small experimental cells.

From liquid to solid: molecular design that cures a deep eutectic solvent inside the cell

A major factor making sodium metal batteries hard to commercialize is the tendency of the electrode-electrolyte interface to become unstable. Metallic sodium repeatedly dissolves and deposits during charging and discharging, so the electrode's surface condition and volume change, and the protective film formed at the interface is easily broken. When the film is damaged and fresh metal surface is exposed, side reactions with the electrolyte proceed, consuming electrolyte and active sodium and accelerating battery degradation. Inorganic solid electrolytes can avoid leakage, but if tiny gaps form between a hard electrolyte and the electrode, poor contact increases interfacial resistance, which is a different problem.

Professor Park's team focused on combining the solvation properties unique to deep eutectic solvents (DES) with the structural stabilization offered by polymerization. The "PDES-1" electrolyte they developed consists of three components: the sodium salt sodium perchlorate (\(\mathrm{NaClO}_4\)), the polymerizable monomer N-isopropylacrylamide (NIPAM), and succinonitrile (SN), which acts as a plasticizer. Combining these allows the electrolyte to be polymerized inside the battery while maintaining a state in which ions move easily.

When the three compounds are mixed in specific ratios, hydrogen bonding and ion-dipole interactions act between the molecules, lowering the melting point of the mixture to below room temperature. The resulting deep eutectic mixture is liquid when injected into the cell, so it easily penetrates fine irregularities and pores on the electrode surface. After injection, heating causes the vinyl groups of NIPAM to polymerize, transforming the liquid into an electrolyte with a flexible polymer matrix. Compared with pressing a pre-solidified electrolyte against an electrode, this method is thought to reduce the problem of gaps forming between electrode and electrolyte.

Each component of PDES-1 plays a distinct role. Sodium perchlorate supplies the sodium ions that carry charge. NIPAM coordinates with sodium ions through its carbonyl group (\(\mathrm{C=O}\)), promoting salt dissociation while also forming the backbone of the polymer matrix. Succinonitrile maintains polymer chain mobility and helps ions move within the matrix. The aim of the design is to reduce the risk of liquid electrolyte leakage while also mitigating the poor electrode contact that tends to be a problem with rigid inorganic solid electrolytes.

Molecular interactions secure high ionic conductivity and a 5.0 V potential window

Polymerizing an electrolyte generally tends to lower its ionic conductivity. When the mobility of polymer chains is restricted, the movement of ions that depends on them also slows. In particular, if sufficient ionic conductivity cannot be secured near room temperature, charging and discharging at practical current densities becomes difficult.

The optimized PDES-1 electrolyte recorded an ionic conductivity of 1.39 mS/cm (millisiemens per centimeter) at 30°C. Its sodium-ion transference number, which represents the fraction of current in the electrolyte carried by sodium-ion movement, reached 0.58. The transference number of alkali metal ions in typical liquid electrolytes is said to be around 0.2 to 0.4, so in PDES-1 sodium ions carry the majority of charge transport. A high sodium-ion transference number makes it easier to suppress concentration polarization near the electrode interface during charging and discharging, which can be expected to reduce uneven deposition of metallic sodium.

The team investigated the mechanism behind these conduction properties using spectroscopic analysis combined with molecular dynamics (MD) simulations. The results suggested that non-covalent interactions between sodium ions and NIPAM or succinonitrile may alter the structure of the solvation shell formed around the ions. In the computational model, the contact ion pairs formed by sodium ions and perchlorate anions weakened, making the sodium ions more mobile. However, this is a molecular-level explanation derived from combining experimental results with a computational model, and it has not been demonstrated that the same behavior occurs in all deep eutectic electrolytes.

Electrochemical stability tests also reported a wide potential window of 5.0 V. This suggests an advantage when pairing the electrolyte with cathode materials that operate at high voltage. However, the measured window alone does not guarantee long-term stability at the cathode interface in an actual battery, or safety during overcharge. Verification in real cells combined with high-voltage cathodes will be needed separately.

90.4% capacity retained after 1,000 cycles in small cells

When evaluating battery performance, it is necessary to distinguish between the properties of the electrolyte alone, symmetric cells that examine metal electrode deposition and dissolution, and full cells that combine a cathode and anode. In this study, the team evaluated these in stages to examine both the ion-conduction properties of PDES-1 and its operational stability as a battery. Two results stand out: a symmetric cell test that repeated sodium metal deposition and dissolution over a long period, and a full-cell durability test spanning 1,000 cycles.

First, in "sodium symmetric cells" using metallic sodium for both electrodes, stable deposition and dissolution (plating and stripping) was confirmed for more than 1,000 hours. During the test there was no sudden voltage rise or voltage drop indicating a short circuit, and the overpotential remained stable. The team believes this result stems from a stable protective layer forming at the electrode-electrolyte interface, which suppressed uneven deposition of metallic sodium. However, this test evaluates metal electrode deposition and dissolution on an hours basis, which is a different metric from the cycle life of full cells that include a cathode.

Next, in full cells using sodium vanadium phosphate (\(\mathrm{Na}_3\mathrm{V}_2(\mathrm{PO}_4)_3\), NVP) as the cathode and metallic sodium as the anode, discharge capacity and rate performance were examined. At a low rate of 0.1C, the initial discharge capacity reached 110.2 mAh/g, with Coulombic efficiency approaching 99%. Even at 5C, a much faster rate, the cell maintained a discharge capacity of 90 mAh/g. This indicates that, despite being a polymerized electrolyte, it secures ion conductivity sufficient for relatively fast charging and discharging.

In the long-term durability test, 90.4% of the initial capacity was retained after 1,000 cycles at a 2C rate. The team attributes this high capacity retention to a stable interfacial layer formed between the electrolyte and electrodes, which suppressed side reactions and prevented uneven deposition of metallic sodium. This suggests that the interfacial degradation that has plagued conventional sodium metal batteries may be curbed through the molecular design of a polymer electrolyte.

However, the 90.4% capacity retention was measured using an NVP cathode and repeated 2C cycling in a specific laboratory environment. The same lifetime is not necessarily obtained at different temperatures or rates, or with other cathode materials. In addition, the abstract-level information on the paper does not allow confirmation of the number of test repetitions, statistical uncertainty, or detailed results compared with existing commercial electrolytes under identical conditions. Although the figure is a promising result, further verification is needed before concluding that an advantage over commercial batteries has been established.

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  "type": "bar",
  "title": "Discharge capacity by charge-discharge rate (NVP full cell)",
  "unit": "mAh/g",
  "categories": ["0.1C (initial)", "5C (high rate)"],
  "series": [
    { "name": "Discharge capacity", "values": [110.2, 90] }
  ],
  "caption": "Reported values at each rate in a laboratory-scale small full cell",
  "source": "Energy Storage Materials (DOI: 10.1016/j.ensm.2026.105430)"
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Even when the charge-discharge rate was raised 50-fold from 0.1C to 5C, discharge capacity fell only about 18%, from 110.2 mAh/g to 90 mAh/g. This indicates that pathways for sodium-ion movement are secured within the polymer matrix, allowing charging and discharging to continue even at relatively high current densities. However, this was obtained in small experimental cells with thin electrodes, and it is not yet clear whether similar rate performance can be achieved in larger cells with thick electrodes.

The table below summarizes the main measurements reported and the extent of what can be concluded from each.

Pouch cells that withstand flame and cutting, and the challenges that remain

The team also examined the electrolyte's combustion behavior and its resistance to mechanical damage, not just electrochemical performance. In tests exposing the synthesized PDES-1 electrolyte directly to flame, it was hard to ignite, showing high flame retardancy. The gel-like polymer matrix formed by thermal polymerization is also less likely to flow out than a liquid electrolyte, which can be expected to reduce the risk of leakage if the casing is damaged. These properties are important for mitigating the safety problems associated with flammable liquid electrolytes.

The team also built a small pouch cell using PDES-1 and demonstrated its resistance to physical damage. With an LED connected to the pouch cell and lit, the LED stayed on without ignition even when the cell was bent or cut through the casing with scissors. The experiment visually shows the flexibility of the electrolyte and the possibility of maintaining some power supply even when a cell is physically damaged.

However, keeping an LED lit is not the same as confirming the safety of practical large batteries. A test that lights a low-power LED with a small cell cannot adequately evaluate the thermal runaway and ignition risks that matter in high-energy-density cells for electric vehicles and power grids. For this demonstration, quantitative safety indicators such as temperature rise during an internal short circuit, the amount of decomposition gas generated, changes in internal resistance, and cell energy density have not been presented. It has also not been confirmed whether the cells meet evaluations based on international industrial safety standards, such as nail penetration and external heating tests.

What this study demonstrates is the material properties, the molecular-level ion-conduction mechanism, and operation in laboratory-scale coin cells and pouch cells. It does not show that equivalent lifetime or safety would be obtained in the large cells used in stationary storage or electric vehicles. Judging practicality will also require examining manufacturing constraints and performance losses that accompany scaling up cells.

Particularly important is reproducibility when electrode thickness and area are increased. In small laboratory cells, performance is often evaluated with thin electrodes using only a few milligrams of active material. Commercial cells, by contrast, use thick, densely compressed electrodes with higher active material loading to raise energy density. Whether the pre-polymerization electrolyte can penetrate uniformly into such thick electrodes, and whether sufficient ionic conductivity and interface-protecting effects are maintained after polymerization, remains to be confirmed.

The amount of electrolyte used and the thickness of the metallic sodium anode are also key evaluation items for practical use. In laboratory tests, good lifetimes can sometimes be obtained more easily by using plenty of electrolyte or thick sodium metal foil as the anode. These conditions are disadvantageous for overall cell energy density and material cost, however, so they cannot necessarily be applied directly to commercial cells. The question is whether this durability can be reproduced with minimal electrolyte volume or ultra-thin sodium foil.

The team points to possible applications of the technology in low-cost grid energy storage systems (ESS) and emergency backup power. If the instability of the metal anode can be improved through molecular design of the electrolyte while using abundant sodium, it could become a new option for stationary batteries. Still, the results remain at the demonstration stage in small experimental cells, and manufacturing reproducibility at larger scale, reliability across a wide range of temperatures, and management of heat generation and safety all remain to be verified. A key factor in judging practicality will be whether the high capacity retention of 90.4% after 1,000 cycles can be reproduced in larger cells with practical electrode areas and electrolyte volumes.