Logo: to the web site of Uppsala University

The development of high-performance, sustainable sodium-ion batteries requires a mechanistic understanding of cathode redox processes and structural stability. Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material due to its non-toxicity, high average working voltage, and excellent structural and thermal stability. However, its practical application is hindered by impurity phase formation and low intrinsic electronic conductivity. To address these challenges, Na₄Fe_3-xM_x(PO₄)₂(P₂O₇) (M: Mn, Ni) composites are synthesized via a sol-gel method. A comprehensive characterization approach combining X-ray diffraction (XRD), X-ray absorption spectroscopy (XANES/EXAFS, soft XAS), and resonant inelastic X-ray scattering (RIXS) revealed that low-level substitution of Mn²⁺ and Ni²⁺ into Fe sites suppresses the formation of electrochemically inactive maricite NaFePO₄ and modifies the Fe-O coordination environment. These effects may result in lower ion migration energy barriers and better electrochemical reversibility. Among the doped samples, Mn-NFPP exhibited the best electrochemical performance, delivering a discharge capacity of ∼92 mAh g⁻¹ at 0.1 C and ∼80 mAh g⁻¹ at 2 C, with 99.5 % capacity retention after 100 cycles at 0.1 C. This work provides fundamental insights into the redox mechanism and atomic-scale structure–property relationship of NFPP, guiding the design of high-performance polyanionic cathodes for sodium-ion batteries.

Binders are essential components in battery systems that maintain the electrode structure and integrity throughout cycling. The choice of binder affects processing, electrochemical behavior, and end-of-life recovery. The widely used poly(vinylidene difluoride) (PVDF) binder is increasingly scrutinized since it is a polyfluoroalkyl compound and requires processing in organic solvents like N-methyl-2-pyrrolidone. Consequently, efforts are underway to identify more sustainable options. In this work, we present chitin nanofibers (ChNFs), derived from fisheries waste, as a biobased, fluorine-free alternative that enables fully aqueous electrode fabrication. ChNFs disperse more than 90% of graphite in water without the need for auxiliary agents such as surfactants. Colloidal probe microscopy shows that adhesion between protonated ChNFs and graphite depends on pH, likely being governed by cation-π interactions. This strong affinity facilitates the remarkable dispersing capability of ChNFs through a multifaceted mechanism. Adsorbed nanofibers confer electro-steric stabilization by extension into the aqueous phase. The strong ChNF network can physically entrap larger particles, greatly enhancing the long-term stability of ChNF-graphite suspensions. The ChNF-graphite dispersion exhibits rheological behavior suited for forming uniform electrode coatings. Electrodes prepared with 4% ChNFs deliver specific capacities of 370 mA h g^-1 and enhanced capacity retention over 100 cycles compared to PVDF-based counterparts. Electron microscopy, X-ray photoelectron spectroscopy, and online electrochemical mass spectrometry analyses reveal that the nature of the binder dictates the solid electrolyte interphase (SEI) characteristics. The ChNF produces a more robust and stable SEI that suppresses electrolyte reduction, directly contributing to the enhanced electrochemical performance.

The design of iron complexes with long-lived charge transfer states suitable for applications as photosensitizers remains a formidable challenge. Here, we investigated the effect of an extended ligand pi-system on the ground- and excited-state properties of iron(II) complexes with N-heterocyclic carbene (NHC) ligands. For this purpose, three iron complexes based on the established [Fe(II)(pbmi)2]2+ motif (pbmi = (1,1 '-(pyridine-2,6-diyl)bis(3-methylimidazole-2-ylidene))) have been modified with phenylethynyl moieties attached to the pyridine part of the ligand. In general, the introduction of the phenylethynyl units served to red shift the main absorption band, as well as to increase the extinction coefficient of the same, compared to the parent complex. The lowered MLCT energies are in line with the electrochemical data that revealed substantially easier reduction of the phenylethynyl-modified ligands, while the potentials of the Fe(III/II) couple are only moderately increased. Only minor modifications of the electronic effect intrinsic to the phenylethynyl moieties could be implemented with bromide and dimethylamino substituents on the phenylene units. As a result, all three complexes experience similar stabilization of their 3MLCT states, about 0.3 eV compared to the parent complex, and feature transient absorption data in line with ES dynamics that are dominated by a moderately long-lived (similar to 17 ps) 3MLCT state. These values exceed the 3MLCT lifetimes reported for the parent complex (up to 9 ps) and resemble the results for carboxylic acid and imidazolinium derivatives with comparable 3MLCT energies and lifetimes.

The real-time interface chemistry between the lithium cobalt oxide (LCO) working electrode and the LiClO4/propylene carbonate (PC) electrolyte is investigated during lithiation/delithiation using dip-and-pull ambient pressure photoelectron spectroscopy (APXPS). The APXPS results appear to exhibit the seldom discussed Co2+ state in the LCO structure, where the operando measurements indicate electron transfer among Co2+, Co3+, and Co4+ states. Specifically, the lithiation of LCO reduces the Co4+ state to both Co3+ and Co2+ states, where, as a function of voltage, reduction to the Co2+ state is initially more pronounced followed by Co3+ formation. In addition, a delay in surface delithiation is observed during the reverse potential steps. This is discussed in terms of overpotential at the interface measurement position as a consequence of the dip-and-pull setup for this experiment. Finally, the shifts in the apparent binding energies of the spectral features corresponding to the electrolyte and LCO at their interface show that the electrochemical potentials at delithiation voltage steps are different from the lithiation steps at the same applied voltages. This further explains the non-responsive delithiation. The BE shift observed from the LCO surface is argued to be dominantly due to the semi-conductive nature of the sample. Overall, this article shows the importance of operando APXPS for probing non-equilibrium states in battery electrodes for understanding electron transfer in the reactions.

The magnetic properties of Fe_2−2xMn_2xP_1−xSi_x (0 ≤ x ≤ 0.5) compounds are studied by neutron diffraction, Mössbauer spectroscopy, and magnetometry. DC magnetization measurements indicate that compounds with 0.2 ≤ x ≤ 0.5 undergo a paramagnetic to ferromagnetic transition, with the Curie temperature increasing as x increases. In contrast, compounds with 0 < x ≤ 0.15 show unclear magnetic ordering in DC magnetization measurements, while AC magnetization measurements display frequency-dependent peaks, indicating glassy spin dynamics. For the x = 0.125 sample, AC magnetization measurements under applied DC fields suggest that the transition at 150 K corresponds to a complex antiferromagnetic (AFM) structure. Mössbauer spectroscopy reveals four distinct regions of hyperfine interactions for different x values, suggesting extreme sensitivity in the magnetic behaviour with Mn and Si substitutions. For 0 < x < 0.15, a drop in the magnetic hyperfine field supports the existence of a complex AFM structure. Neutron diffraction on the x = 0.1 sample confirms an incommensurate AFM structure with a propagation vector q_x = 0.2204(4), consistent with the Mössbauer and magnetization results.

Porous silicon (Si) has gained significant interest in various applications due to its high surface area, tunable pore structure, excellent chemical reactivity, biocompatibility, and surface functionalization potential. Traditional methods for synthesizing porous Si often rely on hydrofluoric acid, a hazardous chemical that poses significant environmental and safety risks, limiting its scalability and sustainability. In this study, a green and scalable approach for synthesizing porous Si microparticles through urea-assisted etching is presented and evaluated as a function of temperature and container conditions (crucible vs. autoclave). The urea etching transformed pristine silicon microparticles, with a non-porous structure and a BET surface area of 2.3 m²/g, into porous silicon with surface areas as high as 26.7 m²/g. The highest porosity was achieved at 400 °C, while higher temperatures (600 °C and 800 °C) led to diminished porosity and surface restructuring. Quantitative analysis revealed a maximum etching yield of 17.5 %, etching rate of 14.6 mg/h, and a pore formation efficiency of ∼43 %. The crystalline structure of silicon remained intact across all treatments, with minor surface disorder observed at higher temperatures. The urea-assisted etching produced a temperature-and environment-dependent surface oxidation and nitrogen incorporation. At 220 °C and 400 °C, a thick oxide layer formed, particularly under high-‍pressure conditions, while oxidation was less pronounced at 600 °C and 800 °C, likely due to rapid thermal decomposition limiting sustained gas-solid interactions. Nitrogen incorporation was most significant in Si-220-HP, where multiple nitrogen environments were detected, including Si–N, NH₂/NH₃⁺, and NO_x species. At higher temperatures, only stable Si–N bonds persisted, while other nitrogen species diminished.

Fe^III complexes based on the [Fe^III(ImP)₂]⁺ motif (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)), where the ligand contains both carbene and cyclometalated moieties, are a promising class of photoactive materials made from this abundant metal. In this work, it is shown that bromo or furanyl substituents attached to the cyclometalating moiety of the ImP ligands stabilize the ²LMCT excited state to very different extent resulting in opposing effects on the ²LMCT lifetime. For [Fe^III(ImPBr)₂]⁺, the lifetime (255 ps) of its moderately stabilized ²LMCT state (1.85 eV) is slightly increased compared to the parent complex (1.90 eV, 240 ps) pointing to an increased barrier for deactivation via the ⁴MC state and enabling applications as photoredox catalyst. In contrast, the ²LMCT energy of [Fe^III(ImPFur)₂]⁺ is lowered substantially to a value of 1.63 eV due to the extended π-system of the ligands and the reduced energy gap favors internal conversion directly to the ground state resulting in a considerably reduced ²LMCT lifetime of 59 ps. These findings have general implications for design of ligand modifications aiming at extended LMCT lifetimes and/or modified ground and excited state potentials.

Two different types of electrolytes (co-solvent and multi-salt) are tested for use in high voltage LiNi_0.5Mn_1.5O₄||Si/graphite full cells and compared against a carbonate-based standard LiPF₆ containing electrolyte (baseline). Ex situ postmortem XPS analysis on both anodes and cathodes over the life span of the cells reveals a continuously growing SEI and CEI for the baseline electrolyte. The cells cycled in the co-solvent electrolyte exhibited a relatively thick and long-term stable CEI (on LNMO), while a slowly growing SEI was determined to form on the Si/graphite. The multi-salt electrolyte offers more inorganic-rich SEI/CEI while also forming the thinnest SEI/CEI observed in this study. Cross-talk is identified in the baseline electrolyte cell, where Si is detected on the cathode, and Mn is detected on the anode. Both the multi-salt and co-solvent electrolytes are observed to substantially reduce this cross-talk, where the co-solvent is found to be the most effective. In addition, Al corrosion is detected for the multi-salt electrolyte mainly at its end-of-life stage, where Al can be found on both the anode and cathode. Although the co-solvent electrolyte offers superior interface properties in terms of the limitation of cross-talk, the multi-salt electrolyte offers the best overall performance, suggesting that interface thickness plays a superior role compared to cross-talk. Together with their electrochemical cycling performance, the results suggest that multi-salt electrolyte provides a better long-term passivation of the electrodes for high-voltage cells.

Recently, polyanionic compounds have received great interest as alternative cathode materials to conventional oxides due to their different advantages in cost, safety, structural stability, as well as being environmentally friendly. However, the vast majority of polyanionic materials reported so far rely exclusively upon the redox reaction of the transition metal for lithium/sodium transfer.

The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is a promising strategy to enhance the energy density of polyanionic cathode materials for Li/Na-ion batteries. In addition to transition metal redox activity, the oxalate group also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution.

Herein, we report NaLiFe(C₂O₄)₂ as a new positive electrode and use different characterization techniques such as Raman spectroscopy and Mössbauer analyses to characterise this dual-ion redox process experimentally. First-principles calculations also help to understand the interactions between the transition metal and the oxalate group as the main factor that modulates the cationic and polyanionic redox couples in these materials.

Na4Fe3(PO4)(2)(P2O7) (NFPP) as a promising cathode material for sodium-ion batteries possesses excellent structural stability, minimal volume change, low cost, and non-toxicity. However, its practical application is hindered by the formation of impurity phases and its intrinsically low electronic conductivity. Herein, crystalline high purity carbon-coated NFPP (NFPP/CC) is synthesized by performing a green and scalable combustion method to enhance its overall electrochemical performance. The effects of pre-treatment and the calcination atmosphere on the structure and purity of NFPP are systematically investigated for a variety of synthesis parameters. The electrochemical performance of NFPP cathodes is evaluated in both half-cells with the sodium metal anode and full-cells with the hard-carbon anode via galvanostatic charge-discharge cycling measurements. The "combustion" synthesized NFPP/CC cathode delivers a reversible discharge capacity of similar to 102 mA h g(-1) at 0.1C in an operating voltage window of 1.8-3.8 V (vs. Na/Na+) retaining 99.7% of its initial capacity over 100 cycles. Furthermore, it demonstrates enhanced rate capability in comparison to the NFPP/CC cathode synthesized via the conventional calcination route. This study sheds light on using the combustion method as a facile and effective strategy to simultaneously mitigate the formation of impurity phases, reduce the carbon content, enhance the quality of carbon coating, improve the homogeneity of nanoparticles and pores within the structure, and enhance the electronic conductivity and physical stability of NFPP cathodes, paving the way for their practical application in high-performance sodium-ion batteries.