The stakes are considerable. Lithium-ion batteries dominate the market, but the high cost of lithium salts and the limited availability of terrestrial lithium resources have pushed scientists toward alternatives. Sodium, abundant in the Earth’s crust and inexpensive to extract, is the obvious candidate. Yet sodium ions are bulkier than lithium ions, with an ionic radius of 1.02 angstroms compared to 0.76 angstroms, and they carry a higher reduction potential and slower reaction kinetics. That makes finding suitable electrode materials harder. MoS2, a layered two-dimensional transition metal dichalcogenide with a theoretical capacity of roughly 670 mAh/g, has emerged as a leading contender thanks to its natural abundance, straightforward synthesis, and generous interlayer spacing that eases ion intercalation.

The research team, led by Luis A. Selis of Universidad Nacional de Ingeniería in Peru, Utkarsh Vijay, and Diego E. Galvez-Aranda of Texas A&M University, focused on the stable 2H phase of MoS2, the polymorph most widely studied for battery applications. Using the Quantum ESPRESSO software package with the Perdew-Burke-Ernzerhof exchange-correlation functional, they built periodic simulation cells containing a 48-atom MoS2 slab and a single electrolyte anion. The five anions examined were hexafluorophosphate (PF6-), tetrafluoroborate (BF4-), perchlorate (ClO4-), bis(fluorosulfonyl)imide (FSI-), and bis(trifluoromethanesulfonyl)imide (TFSI-), all staples of real battery electrolytes.

Crucially, the team did not model the surface at a single static state. Instead, they simulated three degrees of sodiation, corresponding to 0 percent, 50 percent, and 90 percent sodium content in the electrode, capturing how the interface evolves as the battery fills with sodium. For each anion and each sodium concentration, multiple molecular orientations were tested, and the researchers evaluated interaction energies, charge redistribution, the electron localization function, work function changes, and electronic structure modifications. The models deliberately exclude explicit solvent molecules and reactive decomposition pathways, but the authors note that similar simplified interface models have been widely used in prior DFT studies to probe early interfacial processes relevant to SEI evolution.

The results paint strikingly different portraits for each anion. Tetrafluoroborate, BF4-, behaves like a committed guest that changes its mind. At 0 percent sodium, it binds strongly to the MoS2 surface, with charge density difference maps showing pronounced electron accumulation at a fluorine atom contacting the surface, suggesting partial bond formation and even a possible weakening of the B-F bond. But as sodiation increases, the interaction collapses: at 50 percent sodium the interaction energy turns slightly positive, and at 90 percent it reaches +0.33 eV, an outright unfavorable configuration. The work function difference progressively increases, indicating reduced electronic perturbation of the anode as sodium accumulates.

Perchlorate, ClO4-, tells a subtler story. Its interaction energy starts strongly negative at -2.55 eV on the pristine surface but weakens to -1.34 eV and -1.09 eV at 50 and 90 percent sodium. What makes this case intriguing is that the total charge transfer barely changes across the entire range, remaining between 0.10 and 0.16 electrons. The weakening, the researchers found, is instead driven by a spatial redistribution of electrons: the electron localization function reveals growing localization in regions associated with intercalated sodium atoms rather than the MoS2 surface itself. In other words, the anion keeps transferring charge, but the sodium steals the electronic spotlight.

Hexafluorophosphate, PF6-, is the outright failure of the group. It never forms a stable interface configuration on MoS2 at any sodium concentration. At 0 percent sodium it sits tilted against the surface with no preferential interaction, and as sodium content rises it reorients toward a vertical configuration while sodium atoms migrate toward its fluorine atoms. The interaction energies turn positive, reaching +0.44 eV at 90 percent sodium, and the anion effectively abandons the surface in favor of bonding with intercalated sodium. This finding matters because PF6- is among the most common salts in commercial electrolytes, and its inability to anchor to MoS2 could shape how the SEI nucleates.

The large, flexible anions TFSI- and FSI- display the most complex behavior, governed by molecular orientation. For TFSI-, only oxygen-coordinated configurations proved favorable at low sodium, with the O-attack orientation yielding -0.49 eV while fluorine-dominated orientations remained energetically unfavorable at positive energies. Molecular electrostatic potential calculations explain why: the regions around oxygen atoms are consistently more negative than those near fluorine atoms, making oxygen the preferred contact point with the positively charged molybdenum and sodium sites. FSI- showed something even more dramatic. In two of its three tested orientations, at 90 percent sodium the S-F bond within the anion spontaneously dissociated, with the freed fluorine atom forming a stable bond with a surface sodium atom. The interface energies plunged to -8.64 eV and -9.06 eV, and charge transfer surged to -1.29 and -1.54 electrons, marking a transition from a molecular interface to a dissociative one.

That spontaneous FSI- decomposition is arguably the paper’s most consequential discovery. The authors conclude that highly sodiated surface regions dramatically lower the energetic barrier for FSI- decomposition, identifying the Na-F interaction as the primary driving force for initial electrolyte degradation. Since electrolyte decomposition is precisely what builds the SEI, the thin passivating layer that protects an anode from further parasitic reactions, this suggests that the state of charge itself dictates when and how the interphase begins to form. A general trend also emerged across all five anions: increasing sodium concentration weakens the direct anion-surface interaction while promoting sodium-mediated interactions, a shift the authors link to the changing electrostatic potential of the electrode, which concentrates around both molybdenum and intercalated sodium ions as sodiation deepens.

The work is candid about its limits. Static DFT calculations cannot capture the dynamic behavior of a complete electrolyte environment with salt and solvent molecules, and the authors position their approach as an initial screening framework ahead of more computationally demanding ab initio molecular dynamics simulations. Still, the practical payoff is real: the calculated interface energies, preferred orientations, charge redistribution patterns, and work-function shifts provide atomistic descriptors that could guide the selection of electrolyte salts for MoS2-based sodium-ion batteries before any experimental validation. Smaller, symmetric anions behave predictably, while larger, flexible ones produce complex, orientation-dependent interfaces, and electronegative oxygen atoms emerge as key stabilizers. As sodium-ion batteries push toward commercialization for renewable energy storage, understanding these first atomic moments at the electrode surface may prove just as important as the electrodes themselves.

Subject of Research: First-principles simulation of electrolyte anion stability and orientation on MoS2 surfaces for solid electrolyte interphase formation in sodium-ion batteries

Article Title: First-principles study of electrolyte components stability and orientation on MoS2 surfaces: implications for SEI formation in sodium-ion batteries

Article References: Selis, L. A., Vijay, U., & Galvez-Aranda, D. E. (2026). First-principles study of electrolyte components stability and orientation on MoS2 surfaces: implications for SEI formation in sodium-ion batteries. Frontiers in Batteries and Electrochemistry, 5, Article 1845442. https://doi.org/10.3389/fbael.2026.1845442

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1845442

Keywords: sodium-ion batteries, MoS2, solid electrolyte interphase, density functional theory, electrolyte anions, SEI formation, anode materials, charge transfer, sodiation, work function, electrode-electrolyte interface, FSI decomposition

The material in question is LiNi0.90Co0.05Mn0.05O2, often abbreviated as NCM90 or simply a high-nickel NCM cathode. With 90 percent of its transition-metal sites occupied by nickel, it delivers a high specific capacity and a strong voltage platform, the two properties that translate directly into longer driving ranges and better energy density for electric vehicles. The trade-off is well documented in the literature. As the nickel fraction climbs, the layered crystal structure that stores lithium becomes increasingly unstable, particularly at high states of charge where the lattice contracts, cracks form, and reactive nickel species are exposed to the liquid electrolyte.

That exposure triggers a cascade of degradation. Electrolyte decomposition products accumulate on the particle surface as a resistive layer, impeding the flow of lithium ions into and out of the cathode. Microcracks admit more electrolyte deeper into secondary particles, accelerating the damage. At elevated temperatures, the reactions run faster and the structural collapse accelerates. Previous studies have attributed the poor cycling of nickel-rich materials to this combination of interfacial parasitic reactions and internal mechanical stress, which is why surface coatings have become one of the most intensively explored remedies in battery research.

Coatings, however, come with a catch. A thick, dense protective layer shields the cathode from the electrolyte effectively, but it also adds resistance in series with the lithium-ion pathway, degrading rate capability. The ideal coating must be thin, chemically robust, electrochemically inert, and somehow favorable to ion transport all at once. The Chinese team addressed this challenge with cerium fluoride, CeF3, applied at just 1.0 weight percent through what the authors describe as a facile in-situ liquid-phase method. The process forms a coating layer roughly 10 nanometers thick, thin enough to barely register on the scale of the cathode particles yet continuous enough to act as a functional barrier.

The electrochemical results are striking. At a moderate cycling rate of 0.3 C and room temperature of 25 degrees Celsius, the CeF3-coated cathode delivered a high initial discharge capacity of 201.71 milliampere-hours per gram. After 200 cycles, the material retained 82.83 percent of its capacity, a figure that stands out for a cathode with such a high nickel content. Capacity fade in uncoated high-nickel materials over comparable cycling windows is typically much more severe, driven by the interfacial reactions the coating is designed to suppress.

High-temperature performance is where the coating truly earns its keep. Batteries in real vehicles rarely enjoy laboratory temperatures; summer heat inside a pack can push cells well above 40 degrees Celsius, and degradation chemistry accelerates sharply with temperature. When the researchers cycled their coated cathode at 0.3 C and 55 degrees Celsius, it retained 83.32 percent of its capacity after 100 cycles. Maintaining that level of retention at elevated temperature suggests the cerium fluoride layer remains a stable, protective interface under conditions that would otherwise rapidly consume an unprotected nickel-rich surface.

Equally important is what the coating did not cost in speed. At a demanding rate of 5 C, meaning a full discharge in roughly twelve minutes, the coated material still delivered 155.76 milliampere-hours per gram. This rate capability underscores the central argument of the paper: the CeF3 layer does more than passivate the surface. According to the authors, the tailored coating establishes a functional barrier that mitigates adverse reactions at the electrode-electrolyte interface while significantly improving lithium-ion diffusion kinetics across the interface. In other words, the coating is not a toll booth on the ion highway but a smoother on-ramp.

The choice of cerium fluoride is chemically deliberate. Fluoride-based coatings are inherently resistant to the acidic species, principally hydrofluoric acid, that form when electrolyte decomposes, making them durable candidates for interfacial protection. Rare-earth fluorides add another dimension, as prior work has explored cerium doping in spinel cathodes and shown that rare-earth elements can confer structural benefits. The in-situ liquid-phase route is also notable for its simplicity: rather than requiring energy-intensive gas-phase deposition, the coating forms directly from solution during synthesis, a process more compatible with the large-scale manufacturing that high-volume battery production demands.

The study positions itself within a crowded field of surface engineering approaches. Aluminum oxide, zirconium oxide, titanium dioxide, lithium phosphates, lithium niobate, and lithium-conducting garnets have all been tested as cathode coatings, each with strengths and limitations. What distinguishes the CeF3 result is the demonstrated balance between interfacial protection and diffusion kinetics, a combination the authors argue is essential for practical adoption. Coatings that protect but slow the cell down simply trade one problem for another, and the 5 C performance reported here indicates the nanometer-scale fluoride layer threads that needle.

The broader significance lies in what it means for the commercial trajectory of high-nickel cathodes. Automotive manufacturers have been pushing nickel content ever higher to squeeze more range out of every kilogram of cell material, but cycle life and thermal robustness remain gating factors for warranty guarantees and fast-charging protocols. A coating that simultaneously improves room-temperature cycling, high-temperature retention, and fast-discharge capability addresses the full spread of real-world stresses. As the authors conclude, this work introduces a novel strategy to mitigate structural instability in high-nickel cathodes, effectively balancing interfacial protection with lithium-ion diffusion kinetics, and thereby accelerating their application in high-performance lithium-ion batteries. If the approach survives the leap from coin cells to full-size pouch cells, a ten-nanometer whisper of cerium fluoride could prove to be one of the quieter breakthroughs in the race for better batteries.

Subject of Research: Cerium fluoride surface coating to enhance the interfacial stability and lithium-ion diffusion kinetics of high-nickel lithium-ion battery cathodes

Article Title: CeF₃-coated LiNi0.90Co0.05Mn0.05O2cathode with enhanced interfacial stability and lithium-ion diffusion kinetics for high-performance lithium-ion batteries

Article References: CeF₃-coated LiNi0.90Co0.05Mn0.05O2cathode with enhanced interfacial stability and lithium-ion diffusion kinetics for high-performance lithium-ion batteries. (n.d.). https://doi.org/10.1007/s10853-026-13545-7

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13545-7

Keywords: lithium-ion batteries, high-nickel cathode, NCM90, cerium fluoride coating, interfacial stability, lithium-ion diffusion, capacity retention, electrode-electrolyte interface, energy materials, surface modification, electric vehicles, rate capability

A new study published in the journal Ionics by Zhenyun Zhao, Naihui Hou, and colleagues at Zhejiang Sci-Tech University, working with Yuwen Cheng of Anhui University of Technology, tackles that question head-on. The team’s central insight is that the counter anions in the electrolyte, long treated as passive spectators because they do not carry charge between electrodes, actually play a decisive role at the electrode-electrolyte interface. By combining electrochemical measurements with density functional theory simulations and spectroscopic evidence, the researchers show that ammonium ions and electrolyte anions form interfacial ion pairs that reshape the local electric field, alter charge-transfer barriers, and ultimately govern both capacitance and rate performance. The work provides one of the most systematic pictures to date of how anion identity controls interfacial ammonium-ion storage.

To isolate interfacial effects from the confounding variables of complex electrode architectures, the team needed a structurally well-defined model electrode. They chose TatDha-COF, a fully conjugated covalent organic framework synthesized through the condensation of 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline with 2,3-dihydroxyterephthalaldehyde in a sealed glass tube heated at 120 degrees Celsius for 72 hours, yielding red powders at 60% efficiency. The framework features a 3.2-nanometer pore size, a hollow microtubular morphology visible in electron microscopy, and a crystalline hexagonal lattice confirmed by powder X-ray diffraction peaks at 2.8 and 26.38 degrees. X-ray photoelectron spectroscopy verified the expected carbon, nitrogen, and oxygen chemistry, including imine C=N groups, C-N linkages, and hydroxyl C-OH functionalities. Critically, the one-dimensional channel architecture and highly exposed surface groups promote pseudocapacitive behavior, making the material an ideal platform for studying what happens at the interface rather than optimizing a commercial device.

The electrochemical comparison across three representative ammonium electrolytes produced a result that initially looks like a paradox. In 1 M ammonium acetate, TatDha-COF delivered the highest specific capacitance of the three salts tested, yet it suffered the poorest rate capability, with capacitance falling off more steeply as scan rates increased from 1 to 5 millivolts per second than in either 0.5 M ammonium sulfate or 1 M ammonium chloride. In ammonium sulfate, the opposite held true: excellent rate performance came at the expense of specific capacitance. Cyclic voltammetry revealed three pairs of redox peaks in the sulfate and chloride electrolytes but only two pairs in ammonium acetate across a stable window of -0.9 to 0.9 volts versus Ag/AgCl. A two-electrode coin cell in ammonium acetate retained 61% of its initial capacitance after 100 cycles at 1 ampere per gram while maintaining a Coulombic efficiency near 100%, indicating highly reversible charge-discharge processes despite gradual decay.

Kinetic analysis using the well-known b-value method, in which the slope of log peak current versus log scan rate distinguishes diffusion-controlled from adsorption-controlled behavior, added an important layer of nuance. Values approaching 1 indicate adsorption-controlled, capacitive processes, while values near 0.5 indicate diffusion control. For the dominant redox pair between -0.3 and 0 volts, the b values were 1.03 and 0.85 in ammonium sulfate, 0.82 and 0.70 in ammonium chloride, and 0.84 and 0.74 in ammonium acetate. This shows that the redox reaction was predominantly adsorption-controlled in all three electrolytes, but with a stronger adsorption contribution in ammonium sulfate. Taken together with the capacitance and rate data, the results suggest that ammonium acetate and ammonium chloride favor pronounced pseudocapacitive behavior, whereas ammonium sulfate leans toward electric-double-layer characteristics.

The theoretical heart of the paper lies in the density functional theory calculations, performed with the Vienna ab initio simulation package using the PBE functional, projected augmented wave method, and Grimme’s DFT-D3 dispersion correction. The team first identified the preferential adsorption site for bare NH4+ on the framework by comparing two distinct imine nitrogen environments, labeled N1 and N2. Adsorption at N1 proved more thermodynamically favorable because the ammonium ion can simultaneously form two hydrogen bonds, an N-H…N bond with an adjacent C=N group and an N-H…O bond with a neighboring hydroxyl, anchoring it firmly to the surface. All subsequent simulations therefore centered on the N1 site, providing a consistent atomic-scale stage on which to test how different anions modify the adsorption picture.

What emerged from the ion-pair calculations is the study’s most consequential finding. Although anions do not adsorb directly onto the negatively biased electrode, they can coordinate with the already-adsorbed ammonium ion, sitting on its distal side and forming stable interfacial complexes. The calculated adsorption energy for a [NH4+(SO4^2-)] pair was -4.09 electron volts, while a [NH4+(AC^-)3] complex with three acetate ions reached -3.05 electron volts, both substantially more favorable than bare NH4+ adsorption alone and both far lower than a single [NH4+(AC^-)] pair. This confirms that interfacial ion-pair formation is thermodynamically spontaneous. The stronger association of three acetate ions with one ammonium ion, compared with one sulfate paired with one ammonium, means the acetate complex is harder to rearrange and desorb during charging, which raises the interfacial charge-transfer barrier and slows ion diffusion, exactly the behavior observed in impedance measurements.

Experimental evidence backed the simulations. Ex situ X-ray photoelectron spectroscopy of the discharged electrode revealed a weak but discernible high-binding-energy component between 402 and 405 electron volts in the N 1s spectrum, consistent with chemically adsorbed ammonium, alongside a weakening of C=N peaks and strengthening of C-N and N…H-N features. More tellingly, the O 1s spectrum showed a new component at approximately 531.3 electron volts after discharge, assignable to the carboxylate group of acetate anions retained at the surface through strong association with ammonium. Electrochemical impedance spectroscopy at different voltage states completed the picture: the ammonium acetate electrolyte exhibited a notably larger charge-transfer resistance and a steeper Warburg slope than ammonium sulfate, indicating higher interfacial barriers and slower ion diffusion, in direct agreement with the computed ion-pair energetics.

The final piece of the puzzle came from Bader charge analysis, which quantifies how electrons redistribute across the interface. The total charge transferred to the [NH4+(AC^-)3] pair was 0.38 electrons, nearly eight times the 0.05 electrons transferred to [NH4+(SO4^2-)]. The reason lies in polarizability: acetate is far more polarizable than sulfate, and it strongly perturbs the ammonium ion’s symmetric tetrahedral electronic environment. The charge dispersion among the four hydrogen atoms of adsorbed ammonium grew from roughly 0.04 electrons in the sulfate complex to about 0.11 electrons in the acetate complex, a signature of strong anisotropic polarization. This distortion enhances the local positive charge density on the electrode-proximal side of the ammonium ion and increases the number of electrons transferred at the interface. Since more transferred electrons mean more stored charge, this mechanism neatly explains why ammonium acetate yields higher capacitance than ammonium sulfate.

The study thus resolves the capacitance-rate trade-off into a single coherent picture rooted in interfacial coordination chemistry. Acetate anions, by pairing tightly with adsorbed ammonium, boost the electron transfer number and pseudocapacitive charge storage but simultaneously immobilize the interface, degrading rate performance. Sulfate forms a less stabilizing, more mobile ion pair that sacrifices capacitance for speed. The work, which builds on earlier interface-focused studies of acetate electrolytes with MXene electrodes, establishes that electrolyte anions are not merely charge-balancing bystanders but active architects of the ammonium storage process. For a technology whose appeal rests on safety and cost, that realization could prove pivotal: rational electrolyte design, tuned at the level of the anion, now stands out as a powerful and largely unexplored lever for engineering the next generation of ammonium-ion supercapacitors.

Subject of Research: Role of electrolyte anions in interfacial ammonium-ion storage in supercapacitors

Article Title: Role of electrolyte anions in governing ammonium-ion storage performance

Article References: Zhao, Z., Hou, N., Ming, S., Su, Q., Cheng, Y., Wang, T., Chen, H., & Chen, W. (2026). Role of electrolyte anions in governing ammonium-ion storage performance. Ionics. https://doi.org/10.1007/s11581-026-07539-3

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07539-3

Keywords: ammonium-ion supercapacitors, electrolyte anions, electrode-electrolyte interface, covalent organic framework, interfacial ion pairs, pseudocapacitance, density functional theory, Bader charge analysis, electrochemical impedance spectroscopy, aqueous energy storage, ammonium acetate, charge transfer kinetics