The opportunity begins with chemistry. Commercial lithium-ion batteries generally rely on transition-metal oxide cathodes such as lithium cobalt oxide and lithium iron phosphate. These materials typically release and accommodate approximately one lithium ion per formula unit, limiting their practical capacities to below about 250 milliampere-hours per gram. Extracting more lithium can destabilize the crystal lattice, trigger irreversible phase transitions, and generate mechanical damage as the electrode repeatedly expands, contracts, and changes composition during cycling.

Halide cathodes, which contain fluorine, chlorine, or other halogen elements, can follow more complex electrochemical pathways. Instead of relying solely on lithium-ion intercalation, they may combine intercalation with conversion reactions. During conversion, the original cathode structure is partially reorganized as metal-halide bonds break and new phases form. This process can transfer several electrons per formula unit, creating a route to capacities substantially higher than those of conventional oxide cathodes.

Iron trifluoride, or FeF₃, illustrates the scale of the promise. It has a theoretical capacity of approximately 712 milliampere-hours per gram. At an average operating voltage near 2.7 volts, that corresponds to a theoretical specific energy of around 1,950 watt-hours per kilogram at the active-material level—several times the energy associated with many commercial cathode materials. Halide chemistry may also offer economic benefits. Recent work involving iron chloride has reported retention of 83 percent of its capacity after 1,000 cycles, while the estimated material cost was described as roughly 2 percent of that of lithium iron phosphate.

The shift to all-solid-state lithium batteries is central to making these materials viable. In liquid-electrolyte cells, many halide compounds can dissolve or react with the electrolyte, causing active material loss and rapid performance deterioration. Solid electrolytes remove the liquid solvent that drives this dissolution, allowing researchers to reconsider halides as practical cathode candidates. The solid environment may also improve safety by eliminating flammable liquid components, although the resulting batteries still face major manufacturing and interface challenges.

Another advantage is that some halide compounds can contribute to both ionic and electronic transport within a composite cathode. Conventional solid-state electrodes usually require substantial quantities of solid electrolyte and conductive carbon. These inactive components reduce the fraction of energy-storing material, while the boundaries between cathode particles, electrolyte particles, and carbon create solid-solid interfaces that can restrict charge movement. Poor physical contact can become especially damaging as particles change volume during repeated conversion reactions.

The review highlights Li₁.₃Fe₁.₂Cl₄ as an example of a halide material with unusually high transport properties. Reported ionic conductivity reaches approximately 10⁻⁴ siemens per centimeter, while electronic conductivity can approach 10⁻⁵ siemens per centimeter. Such a combination could allow the cathode itself to participate in the movement of lithium ions and electrons, reducing the need for large amounts of separate conductive additives. In an all-solid-state electrode, this “all-in-one” behavior could increase the proportion of active material and has been associated with an energy density of 529.3 watt-hours per kilogram under the reported conditions.

The same reactions that create high capacity, however, make halide cathodes difficult to control. At high voltage, excessive delithiation can weaken the structure, promote irreversible phase changes, and potentially release reactive halogen-containing gases. At low voltage, metallic products and highly lithiated halides may form passivating layers. These layers can block lithium-ion transport, isolate active particles electronically, and make subsequent charge and discharge reactions less reversible. The result is a narrow operating window in which a material must deliver high energy without undergoing destructive chemical transformation.

Researchers are pursuing several strategies to widen that window. Protective coatings can limit unwanted reactions at cathode–electrolyte interfaces, while stronger metal–halogen bonding may improve structural stability. Nanostructuring can shorten lithium-ion diffusion distances and accommodate mechanical strain, although it may increase surface reactivity and complicate large-scale manufacturing. Controlling the reaction pathway is another approach: rather than allowing uncontrolled conversion, scientists aim to guide the formation of intermediate phases that preserve electrical contact and remain accessible to lithium ions.

According to the review, the next stage of halide-cathode development will require more than discovering a material with a high theoretical capacity. Machine-learning models and high-throughput calculations could screen the vast chemical space of halides for combinations of capacity, voltage, conductivity, and stability. Advanced characterization will be needed to track phase evolution and identify the precise mechanisms governing intercalation and conversion. At the device level, cathode composition, solid electrolyte, current collector, pressure, and manufacturing method must be designed together. The researchers argue that progress in these areas could move halide cathodes from promising laboratory compounds toward safer, more affordable, and substantially higher-energy all-solid-state batteries for electric vehicles and grid storage.

Subject of Research: Halide cathode materials for all-solid-state lithium batteries

Web References: https://doi.org/10.1093/nsr/nwag438

References: National Science Review, DOI: 10.1093/nsr/nwag438

Image Credits: © Science China Press

Keywords

Halide cathodes, all-solid-state lithium batteries, ASSLBs, lithium-ion batteries, FeF₃, iron chloride, conversion chemistry, energy density, solid electrolytes, battery materials, machine learning, electric vehicles, grid storage

Sodium-ion batteries are attracting intense interest because sodium is far more abundant than lithium and is widely distributed across Earth’s crust. That abundance could help reduce raw-material costs and ease pressure on lithium supplies. Sodium-based cells also offer safety and low-temperature advantages, making them attractive for applications ranging from stationary energy storage to electric vehicles. However, their commercial progress depends on solving a major problem: many sodium-ion cathodes lose capacity rapidly after repeated charging and discharging.

The Tokyo University of Science team focused on O3-type sodium nickel manganese oxide, written chemically as O3-Na[Ni1/2Mn1/2]O2, or NNMO. This layered material begins with a favorable, stoichiometric sodium arrangement and can deliver relatively high reversible capacity. Yet sodium ions move in and out of its crystal structure during battery operation, causing large changes in the spacing and volume of the layered lattice. Over time, these repeated structural shifts can trigger cracking, phase transformations, loss of crystallinity and severe capacity fading.

To investigate how scandium works, the researchers introduced Sc3+ ions into NNMO in two ways. In the first approach, scandium was incorporated directly into the bulk crystal structure through a doping process. The resulting materials were labeled NNMSOx, with the number representing the scandium content. The researchers paid particular attention to NNMSO8, which demonstrated the strongest cycling performance among the doped compositions. In the second approach, they treated NNMO particles with a scandium isopropoxide solution and then annealed them at 800 degrees Celsius, producing a surface-modified material known as NNMO-SC800.

The difference between the two approaches became strikingly clear when the materials were tested in coin-type sodium cells. After 100 charge-discharge cycles, undoped NNMO retained only 18.6 percent of its original capacity. By comparison, NNMSO8 retained 67.8 percent, while NNMO-SC800 retained 75.4 percent. These results show that both bulk doping and surface coating can dramatically improve durability, although they do so through different chemical and structural mechanisms.

Inside the doped material, electrochemically inactive Sc3+ ions occupy positions normally associated with transition metals. Their ionic size is comparable to that of the nickel and manganese ions in the host lattice, allowing them to become part of the layered framework without simply forming a separate phase. Because scandium does not participate in the same redox reactions as the active transition metals, it helps immobilize nearby sodium ions. These relatively fixed sodium ions function like structural pillars, supporting the layers as sodium is extracted and reinserted during operation.

This internal stabilization also changes the battery’s electrochemical signature. NNMSO8 displayed a smoother charging and discharging profile than the undoped cathode. The researchers attributed this behavior to suppression of sodium-ion and vacancy ordering, a process in which sodium ions and empty sites arrange themselves into ordered patterns during cycling. Such ordering can promote abrupt structural changes and voltage plateaus. By disrupting it, scandium doping allows sodium ions to move through the cathode more smoothly while reducing the size of harmful volume fluctuations.

The coated material followed a different path. In NNMO-SC800, scandium was found mainly at the particle surface, where it formed a phase resembling O3-NaScO2. This protective layer did not substantially alter the crystal structure inside the cathode. Instead, it acted as a barrier between the active electrode and the electrolyte, suppressing parasitic reactions that gradually consume active sodium, damage the surface and accelerate interfacial degradation. The coating therefore improved cycling stability without producing the smoother voltage profile observed in the bulk-doped material.

Tests in full sodium-ion cells further demonstrated the practical significance of the findings. The researchers paired the modified cathodes with hard-carbon anodes and operated the cells for 300 cycles. The full cell using NNMSO8 retained 71.4 percent of its initial capacity, while the cell using NNMO-SC800 retained an impressive 91.2 percent. The results suggest that surface protection is especially powerful for preserving capacity over extended operation, while bulk doping provides important resistance to structural collapse. Neither strategy alone solved every degradation pathway: coating could not fully prevent long-term loss of crystallinity, and doping did not completely eliminate capacity fading.

The researchers say the most promising future direction may be to combine both approaches, creating cathodes that are reinforced internally and protected externally. Scandium provides an exceptionally clear model for understanding how these mechanisms operate, but its cost and limited availability make it unlikely to be the final commercial solution. The next challenge will be to identify more abundant elements that can reproduce scandium’s ability to stabilize sodium-ion battery structures and protect their surfaces. If successful, this design principle could help transform sodium-ion batteries into longer-lasting, lower-cost alternatives for the rapidly expanding energy-storage market.

Subject of Research: Not applicable

Article Title: Scandium doping and coating for improving O3-NaNi1/2Mn1/2O2 electrode in sodium battery

News Publication Date: 8 August 2026

Web References: https://www.tus.ac.jp/en/mediarelations/

References: Small, DOI: 10.1002/smll.75049

Image Credits: Professor Shinichi Komaba and Associate Professor Shinichi Kumakura, Tokyo University of Science, Japan

Keywords

Sodium-ion batteries, scandium doping, surface coating, cathode materials, energy storage, battery technology, electrochemistry, electric vehicles, sustainable energy, materials science