Commercial Sodium Battery Cathode Doubles as Lithium Ion Host in Water-Based Electrolyte

A cathode material already commercialized as a third-generation sodium-ion battery component has been shown, for the first time, to store lithium ions efficiently in an entirely water-based electrolyte. The compound, known by its chemical shorthand NFPP, belongs to the mixed polyanionic family of iron phosphates and pyrophosphates, and researchers have now demonstrated that it can shuttle lithium ions in and out of its crystal framework with a specific capacity that approaches the theoretical limit of the material. The finding, reported in the journal Ionics, opens a pathway toward rechargeable aqueous batteries that could sidestep many of the safety, cost and sustainability concerns that shadow conventional lithium-ion cells built with flammable organic solvents.

The team synthesized a carbon-coated version of the compound, NFPP/C, using a gel-combustion method, a synthesis route prized for its simplicity and scalability. The in-situ carbon coating serves a dual purpose: it improves the electronic conductivity of what is otherwise a relatively insulating phosphate framework, and it helps stabilize the electrode surface during repeated cycling. This combination is critical for polyanionic cathodes, whose inherently poor electronic transport has historically limited how fast they can be charged and discharged without sacrificing storage capacity.

What makes the new study scientifically compelling is the mechanism the researchers uncovered. When the NFPP/C electrode is first cycled in an aqueous lithium nitrate electrolyte, it does not initially behave as a lithium-ion host. Instead, the initial cycles trigger a gradual ion-exchange process in which lithium ions from the electrolyte replace sodium ions within the polyanionic structure. The team confirmed this structural transformation using ex-situ X-ray diffraction, which revealed the evolving crystalline phases as the exchange progressed. Only after this conditioning period does the electrode settle into a stable redox regime corresponding to genuine lithium-ion insertion and de-insertion from the framework.

Once that stabilization was achieved, the performance was striking. The electrode delivered a highly stable specific capacity of approximately 117 milliampere-hours per gram at a scan rate of 20 millivolts per second, a figure close to the theoretical capacity predicted for the compound. For context, capacities measured at such high scan rates reflect the material’s ability to operate under fast-charging conditions, so reaching near-theoretical values at speed is a meaningful benchmark rather than a laboratory curiosity achievable only at impractically slow rates.

The comparison with sodium storage revealed an intriguing split personality in the material. When tested for sodium-ion storage in an analogous aqueous sodium nitrate electrolyte, the same composite delivered a comparable capacity of 122 milliampere-hours per gram at the 20 millivolts per second scan rate. But when the researchers varied the scan rate, the two ions behaved differently in ways that illuminate the underlying physics. At low scan rates of 1 millivolt per second, lithium storage outperformed sodium storage, delivering 128 versus 118 milliampere-hours per gram. At high rates of 50 millivolts per second, the ranking flipped, with sodium pulling ahead at 97 versus 79 milliampere-hours per gram.

The explanation lies in the balance between two competing factors: the intrinsic thermodynamics of ion insertion and the kinetics of ion transport through the electrolyte and the electrode. Lithium ions are smaller and carry a higher charge density, which favors their insertion into the framework when time is not a constraint. Sodium ions, by contrast, are larger but benefit from the openness of NFPP’s three-dimensional polyanionic framework, and at high scan rates the ionic conductivity of the electrolyte becomes the determining factor, allowing the more diffusively mobile sodium ions to win. Chronopotentiometry measurements, which track the voltage response of the electrode under constant current, showed the same trend, reinforcing the reliability of the observation across two independent electrochemical techniques.

Long-term cycling tests over more than 100 cycles in both lithium nitrate and sodium nitrate solutions added a further layer of nuance. Capacity retention proved better in the lithium nitrate electrolyte, suggesting that the lithium-conditioned structure is somewhat more robust under repeated insertion and extraction. However, the cycling data also indicated that both lithium and sodium ions are co-inserted during operation rather than acting in strict isolation. This co-insertion behavior means the electrode does not behave as a pure single-ion intercalation host in practice, a detail that will matter for engineers designing cells around this chemistry but which also underscores the flexibility that makes the material attractive.

The broader significance of the work lies in what it says about the dual Na-Li functionality of NFPP/C in aqueous systems. A cathode that can operate with either ion, or with a mixture of both, gives battery designers an unusual degree of freedom. Water-based electrolytes are inherently non-flammable, inexpensive and tolerant of manufacturing imperfections that would be dangerous in organic-solvent cells, and pairing them with an earth-abundant iron phosphate cathode pushes the chemistry toward the low-cost, safe, sustainable end of the battery spectrum that grid storage and stationary applications demand.

The result also connects to a vigorous research frontier. Polyanionic cathodes have emerged as leading candidates for next-generation sodium batteries precisely because their rigid frameworks can be tuned through phosphate and pyrophosphate building blocks to stabilize multiple redox couples, and iron-based compositions avoid the cobalt and nickel supply chains that plague conventional cathodes. The new study extends that logic by showing that the same framework, already proven in commercial sodium cells, can be repurposed for lithium chemistry in water, effectively letting one material serve two distinct battery ecosystems.

Challenges remain before such aqueous dual-ion chemistry reaches commercial hardware. The ion-exchange conditioning period must be understood and controlled during cell manufacturing, the co-insertion of mixed ions will need to be managed to guarantee predictable long-term performance, and the modest capacities of polyanionic materials still trail the best organic-electrolyte cathodes in energy density. Yet the study provides a clear proof of concept: a commercially established sodium cathode, synthesized by a simple combustion route, can approach its theoretical capacity while storing lithium ions in water. For a field racing to make batteries safer, cheaper and more sustainable, that is a result worth watching closely.

Subject of Research: Lithium and sodium ion storage in Na4Fe3(PO4)2(P2O7)/C cathodes in aqueous electrolytes

Article Title: Electrochemical behaviour of Na4Fe3(PO4)2(P2O7)/C in an aqueous Li+ electrolyte

Article References: Gezović-Miljanić, A., Mišurović, J., Grudić, V., Dominko, R., & Vujković, M. (2026). Electrochemical behaviour of Na4Fe3(PO4)2(P2O7)/C in an aqueous Li+ electrolyte. Ionics. https://doi.org/10.1007/s11581-026-07475-2

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07475-2

Keywords: aqueous batteries, NFPP, cathode material, sodium-ion battery, lithium-ion storage, polyanionic cathode, gel-combustion synthesis, ion exchange, iron phosphate, capacity retention, mixed ion insertion, energy storage