Chinese Researchers Use a Precision Carbon Shell to Curb Thermal Battery Energy Loss

A tiny filter delivers a large gain at 500 degrees Celsius

A cathode developed by scientists at the Institute of Process Engineering of the Chinese Academy of Sciences has reached a specific energy of 882 watt-hours per kilogram while operating at 500 degrees Celsius. Its protective carbon shell contains channels just 0.54 nanometers across, designed to keep harmful cobalt complexes inside while letting lithium ions move through.

The research, led by Professors Wang Song and Zhu Yongping and published in Advanced Science on January 4, 2026, addresses the shuttle effect, a chemical process that allows active material to dissolve and migrate through a battery. That movement wastes material, causes unwanted reactions and reduces the energy a battery can deliver.

The team used cobalt difluoride, known by its chemical formula CoF2, a transition metal fluoride cathode material. A cathode is one of a battery's two electrodes, where reactions help produce electrical energy during discharge. Around the material, the researchers built a carbon shell derived from covalent organic frameworks, or COFs, porous structures whose openings can be controlled with exceptional precision.

At a current density of 100 milliamps per square centimeter, the resulting cathode delivered a discharge plateau above 2.5 volts and a specific capacity of 365 milliampere-hours per gram. Its voltage and specific energy were described as the highest reported values among high-voltage thermal battery cathodes to date.

As of February 2, 2026, the advance remains a laboratory demonstration. Its immediate promise is for specialized power systems used in military equipment, aerospace and deep drilling, where reliability in extreme conditions matters more than the convenience of recharging.

Why these batteries need extreme heat

Thermal batteries use molten salt electrolytes and typically operate between 350 and 550 degrees Celsius. An electrolyte carries ions between the electrodes. In these batteries, heating turns the salt into a liquid that can carry those ions and allow the battery to supply power.

Ordinary lithium-ion batteries work near room temperature. The heat needed by thermal batteries makes them unsuitable for smartphones, consumer electronics and typical electric vehicles. Their value lies in situations where conventional batteries would struggle or fail.

Military hardware, including missiles and guided munitions, already uses thermal batteries. Other applications include spacecraft, aircraft emergency power units, equipment used in deep wells and certain industrial processes. Such systems may need to work in environments ranging from the vacuum of space to the high pressures encountered in boreholes.

A thermal battery can remain inactive for years, then activate instantly when heated to its operating temperature. These devices generally function as power sources used once, delivering bursts of high power for tasks where failure could have serious consequences. Their operational life means the period during which they can provide useful power after activation, alongside their ability to remain ready during storage.

How cobalt escapes into the electrolyte

The molten salt that makes a thermal battery work also creates a difficult chemical environment. Traditional cathode materials can dissolve into it and move away from the places where they are needed. The loss reduces usable capacity, while unwanted reactions consume energy and damage the structure over time.

In the cobalt difluoride system, CoF2 undergoes anion exchange with lithium chloride, or LiCl, in the electrolyte. Anions are negatively charged ions. This exchange produces cobalt tetrachloride complexes, written as CoCl4 2-, which can migrate through the molten salt.

Those dissolved complexes are central to the shuttle effect. Like bricks drifting away from a building site, they carry useful material away from the cathode. The resulting losses can be irreversible, reducing capacity and charge efficiency and limiting both energy density and operating duration.

Earlier attempts included different types of sulfur electrodes and barriers within the battery. Some reduced dissolution, but the challenge was to stop the harmful material without also stopping the ion movement needed to generate electricity.

A shell that separates ions by size

Wang and Zhu's team focused on the size difference between lithium ions and cobalt tetrachloride complexes. Lithium ions, written as Li+, need to move through the battery. The larger cobalt complexes need to stay confined. A useful barrier therefore has to act as a selective filter.

Conventional solid barriers could block too much movement and interrupt battery operation. Barriers with larger or less consistent pores could allow the cobalt complexes to escape. Producing openings sufficiently uniform to separate the two was the central manufacturing challenge at the scale of nanometers.

Covalent organic frameworks offered a way to control that structure. COFs are crystalline, porous materials with highly ordered arrangements and uniform pore sizes. Their structures can be engineered with precision below one nanometer, allowing researchers to start with a carefully defined framework.

The team converted a COF precursor into a carbon coating through controlled thermal processing. The resulting shell had channels measuring 0.54 nanometers across, small enough to block the larger cobalt complexes while allowing lithium ions to pass rapidly.

The structure is described as a plum pudding@shell architecture, with active CoF2 particles enclosed within the porous carbon shell. The researchers call its filtering action a size-sieving confinement mechanism. In plain terms, the openings sort chemical species by size, retaining the troublesome complexes while keeping the battery's useful ion traffic moving.

Thermodynamic analysis and experimental validation supported the mechanism. Together, they indicated that the tailored channels could block diffusion of cobalt complexes and suppress the shuttle effect without compromising ionic conductivity, the electrolyte system's ability to carry ions.

What the performance figures mean

The tested cathode is named CoF2@CSC700-24. CSC denotes the COF-derived shell carbon, processed at 700 degrees Celsius for 24 hours. Those preparation conditions are separate from the 500 degrees Celsius used for the reported battery performance test.

The current density of 100 milliamps per square centimeter describes how much electrical current passed through a given electrode area. Stating that value alongside temperature helps explain the conditions under which the voltage, capacity and specific energy were measured.

A discharge plateau above 2.5 volts means the cathode maintained a relatively steady voltage during a portion of discharge. The capacity of 365 milliampere-hours per gram describes how much electrical charge it delivered for each gram of material. Specific energy, measured here at 882 watt-hours per kilogram, describes the electrical energy delivered relative to mass.

Professor Wang Song, a corresponding author of the study, described how the findings could guide future battery design:

Our findings provide a mechanistical foundation for designing next-generation high-energy-density thermal batteries through precise interfacial engineering.

Interfacial engineering means controlling the surfaces where materials meet. Here, that boundary is the shell around the cathode material and its contact with the electrolyte. The advance combines a physical structure with a chemical purpose: keeping active material in place while preserving the movement required for discharge.

The reported results set new benchmarks for high-voltage thermal battery cathodes. They also show why the shell matters beyond a single energy figure. A barrier that retains active material could help a battery keep supplying useful power for longer.

Where longer operation could matter first

Military and aerospace systems are the most immediate settings for the technology. Thermal batteries already power guided missiles, torpedoes and emergency systems in combat aircraft, making improvements to their duration and energy per unit of mass relevant to established applications.

Greater energy density could let a battery provide power for longer without increasing its weight. Alternatively, a smaller battery might provide the same energy as an existing design. In aerospace engineering, reducing mass can affect range and maneuverability, while extending power delivery can support a longer mission.

Deep drilling is another potential beneficiary. Equipment can encounter high temperatures and high pressures miles beneath the Earth's surface. A cathode that better retains its active material could help supply sustained power in those demanding conditions.

Suppressing the shuttle effect also suggests a possible route toward reusable thermal battery systems or other applications requiring longer operation. Current thermal batteries are essentially disposable devices activated for specific missions, so moving toward reuse would require development beyond this cathode result.

More distant possibilities include energy storage for electricity grids in extreme climates, emergency power for industrial facilities and specialized electric vehicles operating in Arctic or desert environments. These remain theoretical applications. The high operating temperature continues to restrict where thermal batteries are practical, even when their electrode chemistry improves.

A wider push across battery technologies

The thermal battery work sits alongside several other research efforts in China aimed at improving energy storage through different chemistries. Each approach targets a different balance of cost, safety, charging speed, energy density and operating conditions.

Researchers at the Chinese Academy of Sciences have also developed a self-healing interface for solid-state batteries. It uses migrating iodide ions to form adaptive interphases, the thin regions where battery materials meet, removing the need for high external pressure to maintain contact between layers. The technology could enable energy densities exceeding 500 watt-hours per kilogram.

Tianjin University has developed lithium-metal batteries with energy densities described as twice those of current Tesla packs. Shanghai Jiao Tong University has unveiled an anode-free sodium-sulfur battery achieving 1,198 watt-hours per kilogram, with estimated costs of $5.03 per kilowatt-hour.

Other tracks include graphene-based batteries promising charging in five minutes and lifespans four times those of lithium-ion batteries. Sodium-ion alternatives using hard carbon anodes have also been described as charging faster than conventional lithium systems.

Government support includes a special project worth 6 billion yuan for solid-state battery development launched in 2024. The Ministry of Industry and Information Technology has conducted reviews at the project's midpoint. The funding and parallel research reflect a strategy of pursuing several technologies rather than relying on one chemistry for every application.

Thermal batteries serve extreme environments. Solid-state systems promise safer electric vehicles, sodium-ion technology offers a route toward less expensive grid storage, and lithium-metal research seeks higher energy density. Advancing all of them could position Chinese institutions to supply a range of future battery markets.

The dates and conditions behind the advance

The key dates place the cathode result within that broader research programme. The government funding initiative began before the thermal battery paper, while the January publication brought the new shell design into the scientific record.

• 2024: China launched a special project worth 6 billion yuan for solid-state battery development.
• January 4, 2026: The thermal battery research led by Wang Song and Zhu Yongping was published in Advanced Science.
• February 2, 2026: The cathode advance remained at the laboratory demonstration stage, with manufacturing and complete battery validation still ahead.

The thermal battery test operated at 500 degrees Celsius, within the stated operating range of 350 to 550 degrees Celsius. Its shell was produced under different conditions, at 700 degrees Celsius for 24 hours. Keeping those temperatures separate helps explain both how the material was made and how it performed.

Manufacturing and reliability still need work

Producing the carbon shell at larger scale is a central challenge. Manufacturers would need to maintain the uniform 0.54-nanometer channels across more material, because the filtering mechanism depends on a narrow size range. Changes in pore structure could affect either confinement or lithium ion movement.

Thermal batteries also require specialized handling because of their molten salt electrolytes and high temperatures. Those demands limit their use to applications where the operational benefits justify the complexity of the complete power system.

Further validation must examine stability during extended operation and thermal cycling. Aerospace and defense applications call for reliability over years of shelf storage, and the development requirements described for this technology include thousands of potential activation cycles. Testing must establish whether the shell retains its structure and selective permeability under repeated heating and prolonged exposure to high temperatures.

Molten salts can also corrode materials, creating compatibility problems for housings and seals. The new cathode addresses a major degradation pathway, while a complete battery still needs a suitable anode, electrolyte management and durable components around the cell.

Optimizing the full cell around the cathode is therefore a likely next focus. The route from this laboratory result to deployed equipment requires production control, testing and integration. No deployment date or schedule for those next steps has been announced.

The Bottom Line

• Researchers at the Chinese Academy of Sciences developed a carbon shell with channels measuring 0.54 nanometers to suppress the shuttle effect in cobalt difluoride thermal battery cathodes.
• The shell blocks larger cobalt tetrachloride complexes while permitting lithium ions to move through, helping retain active material without stopping useful ion transport.
• At 500 degrees Celsius and 100 milliamps per square centimeter, the cathode delivered a plateau above 2.5 volts, capacity of 365 milliampere-hours per gram and specific energy of 882 watt-hours per kilogram.
• The study, led by Professors Wang Song and Zhu Yongping, was published in Advanced Science on January 4, 2026.
• Military, aerospace and deep drilling applications are the immediate focus, while reuse and wider energy storage applications remain possibilities requiring further development.
• Scaling production, testing durability and integrating the cathode into a complete battery remain necessary before deployment.