Also known for their high theoretical energy densities and low costs dating back to the 1960s, halide cathode materials are now emerging as a transformative platform for next-generation energy storage. A comprehensive review published in National Science Review by Xiaofei Yang and Xianfeng Li from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, together with Xueliang Sun from the Eastern Institute of Technology, Ningbo, charts a transformative path for all-solid-state lithium batteries (ASSLBs), spotlighting halide cathode materials as the key to unlocking dramatically higher energy densities through a fundamental shift in battery chemistry.
For decades, the cathode active materials (CAMs) inside lithium-ion batteries (LIBs) have been dominated by transition metal oxides, like LiCoO 2 in your smartphone or LiFePO 4 powering electric vehicles. These workhorses of portable energy operate on a simple principle: they store and release one lithium ion (Li + ) per formula unit, typically delivering a capacity less than 250 mAh g -1 . Yet, under the demand for releasing greater energy, they crack under the strain—lattice collapse, irreversible phase changes, and chemo-mechanical degradation that compromise both performance and safety.
Halide CAMs: from LIBs to ASSLBs
“Halide cathode materials have long been overlooked due to their dissolution in liquid electrolytes. The advent of ASSLBs fundamentally changes this picture, enabling us to revisit these high-capacity materials and unlock their multi-electron reaction potential,” says Xiaofei Yang.
Halide materials, first explored as battery materials as early as the 1960s, have long attracted researchers with their high capacity and low cost. Typically, FeF 3 delivers a capacity of 712 mAh g -1 , nearly triple that of conventional oxide cathodes. At an average voltage of roughly 2.7 V, it translates to a theoretical energy density of 1,950 Wh kg -1 . For context, that is roughly 3 to 4 times higher than today’s commercial cathode materials. The economic case is equally compelling: recent work on FeCl 3 demonstrated not only impressive stability, retaining 83% of its capacity after 1,000 cycles, but also a material cost 2% of LiFePO 4 .
The review identifies two major paradigm shifts that define the evolution of halide cathodes from liquid-electrolyte systems to solid-state devices.
Halide CAMs: from multi-phase to single-phase interface
Conventional solid-state cathodes suffer from two inherent drawbacks: the large fraction of inactive electrolyte and carbon additives dilutes energy density, while the solid-solid triple-phase interfaces hinder both ion and electron transport. “It's a double penalty—dead weight eats into energy density, and poor interfacial contact chokes charge transport,” says Xueliang Sun.
Halide cathodes rewrite these rules. The latest halide, Li 1.3 Fe 1.2 Cl 4 , exhibits remarkable ionic conductivities reaching 10 -4 S cm -1 and electronic conductivities up to 10 -5 S cm -1 , enabling these materials to effectively function as their own charge-transport network. This unique all-in-one electrode dramatically reduces or even eliminates inactive additives, thereby achieving a significantly enhanced energy density (529.3 Wh kg -1 ).
Halide CAMs: from single-electron to multi-electron
Traditional oxide cathodes shuffle one electron per metal center through intercalation chemistry. Halides, by contrast, can marshal multiple electrons per formula unit through more complex reaction pathways that combine intercalation with conversion chemistry. This multi-electron capability is the fundamental lever that enables energy densities far beyond what intercalation chemistry alone can achieve.
“The challenge is that multi-electron reactions come with a narrow electrochemical stability window,” the authors note. At high voltages, excessive delithiation triggers structural degradation, irreversible phase transitions, and hazardous halogen gas evolution. At low voltages, metallic species and lithiated halides form passivating layers that impede ion transport and reduce reversibility. Protective coatings, strengthened metal-halogen bonding, nanostructuring, and reaction pathway modulation are emerging as key strategies to widen this window, thereby addressing the central dilemma between high energy density and stable cycling.
“By bridging fundamental mechanistic understanding with practical engineering strategies, we hope this review can serve as a roadmap for researchers working toward the next generation of high-energy, cost-effective all-solid-state lithium batteries,” Xianfeng Li says.
Looking forward, the review outlines a detailed roadmap for next-generation halide cathode development. Artificial intelligence and high-throughput computational screening emerge as powerful tools for navigating the vast compositional space of halide materials, searching for formulations that optimize the delicate balance among capacity, voltage, and stability. Advanced characterization techniques are highlighted as essential for unraveling the complex intercalation-conversion mechanisms that remain poorly understood. At the device level, the review advocates for integrated all-in-one architectures that co-design the cathode, electrolyte, and current collector interfaces from the ground up. These efforts could one day deliver cheaper, safer, and dramatically more energetic power sources for everything from electric vehicles to grid-scale storage.
National Science Review
Systematic review