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Ultrahigh Molecular Weight polyethylene oxide (UHMW PEO) and NaPF₆ diglyme produce solid electrolytes with chemical and electrochemical stability in Prussian White (PW)//Hard Carbon (HC) and Na//Al cells. Both the NaPF₆ concentration and the polymer content rule the rheology, and the mechanical and electrochemical properties of the electrolytes. The use of UHMW PEO allows to incorporate minimum concentrations with maximum rheological impact. The solid character is achieved by the formation of rigid scaffold consisting of PEO/Na⁺ crystalline complexes, which stops flow and provides a mechanical barrier between electrodes. To maximize the benefits of polymer addition, it is worthwhile to test formulations in which the concentration of Na⁺ and PEO are varied. This is because the PEO/Na⁺ crystalline complexes reduce the amount of mobile Na⁺ cations. Na⁺ complexation and crystalline phase distribution is studied by FTIR, XRD and DSC in NaPF₆ in diglyme solutions of concentration 0.9 to 1.5 m, and their solid electrolytes where PEO varies from 5 to 10 wt%. Ion mobility (conductivity and diffusivity) is studied in relation to their chemical composition and phase distribution, and their electrochemical behaviour is investigated in PW//HC and Na//Al cells. Cells with solid polymer electrolytes show remarkable performance even at room temperature and high current rate (1C).

The incorporation of small amounts of polymers into liquid electrolytes to produce gel electrolytes has many benefits. Decreasing, or even avoiding flow reduces accidents caused by leaks, allows for simpler and more flexible geometries and configurations, stabilizes electrochemical cycling, and may even permit the separate recycling of each cell component. Direct mixing of polymers with liquid electrolytes is very frequently possible, and it is a sustainable and scalable procedure that avoids evaporation stages and produces thermoreversible materials. In this work, some of the polymers most commonly used for gel electrolyte formation-polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyvinyl chloride (PVC), poly(methyl methacrylate) (PMMA), and poly(ethylene oxide) (PEO)-are employed to produce gels with the electrolyte NaPF6 1.1 m in diglyme, and their behavior and performance are described. All these polymers are soluble in the electrolyte, and the ultrahigh molecular weight (UHMW) PEO and the halogenated polymers are able to form self-standing gels at concentrations ranging from 5 to 10 wt %. The fluorinated polymer gels left standing in an Ar glovebox progressively become colored, turning almost black after one month, showing a lack of long-term chemical stability. PVC 10 wt % and UHMW PEO 5 wt % gels, however, become hard gels with no macroscopic phase separation and remain stable for periods of months. Their electrochemical stability against Na electrodes was tested, showing that the PVC gel is not stable, and only the UHMW PEO 5 wt % gels were further tested with Prussian white (PW) cathodes. The PEO gel electrolyte, without any physical separator, showed equivalent performance in PW||Na half-cells as the baseline electrolyte with a glass fiber separator, while also being self-standing and solid-like in consistency. The PEO gel electrolytes also showed applicability to PW||HC full cells. Moreover, after electrochemical testing, the coin cells with PEO gels were easily disassembled, and their components were recovered.

Prussian blue analogues for sodium-ion battery cathodes are growing in popularity as next-generation energy storage devices. Prussian White (PW) with formula Na_xFe[Fe(CN)₆]_y•nH₂O is leading the trend, having already been commercialized. However, capacity fade (PW/electrolyte degradation) and safety concerns (cyanide/cyanogen release) still raise concerns. Online electrochemical mass spectrometry, supported by both operando Fourier transform infrared spectroscopy and Mössbauer spectroscopy, is herein used to analyze degradation processes in PW-based Na-ion full cells. Apart from the typical cell formation reactions, hydrogen is observed to evolve during cell discharge and evidenced to stem from oxidation of NaH, accumulated upon charge. Over-oxidation of PW after full desodiation releases CN, which not only forms (CN)₂ but also degrades the electrolyte. Loss of CN likely results in a nanometric (≈4 nm) surface-reconstructed passivation layer on PW, thus inhibiting further degradation. Fundamental understanding of degradation reactions in PW full-cells, as gathered herein, shows that the aforementioned capacity fade and safety concerns are wholly addressable and hence guides the further development of Na-ion batteries for wider ranges of applications.

This study aims to demonstrate the feasibility of the atmospheric plasma spraying (APS) technique to fabricate individual constituents of solid-state batteries (SSBs) such as anode, solid electrolyte (SE) and cathode as well as further produce their half-cell (anode|SE) and full-cell (anode|SE|cathode) configurations. The materials targeted in this work were Li4Ti5O12 (LTO) as an anode, Li7La3Zr2O12 (LLZO) as a SE and LiNi1/3Mn1/3Co1/3O2 (NMC111) as a cathode, with aluminium substrates being used as current collectors. The microstructure of the LTO and LLZO layers exhibited a characteristic lamellar structure along with the presence of a secondary phase attributed to delithiation at high temperatures, whereas the NMC111 layer was found to undergo substantial structural change. X-ray diffraction (XRD) analysis suggested that both LTO and LLZO layers retain most of the characteristic peaks along with the presence of secondary phases while NMC111 layers undergone significant change in the crystal structure. The XPS analysis confirms the presence of expected elements and oxidation states for the LTO layer. In the case of the LLZO layer, a metal carbonate surface reaction layer was observed, while the NMC111 layer reveals the presence of Li, Ni, Mn, Co, and O along with feeble metal carbonate. Fabrication of half-cell and full-cell configurations shows encouraging results by revealing a well-intact interface demonstrating the feasibility of the APS technique to accomplish such layered structures. This proof-of-concept effort provides valuable insights into the efficacy of APS for fabricating SSB components for further development, benefiting both the battery and thermal spray communities.

While polymer-based solid-state sodium batteries promise both safe operation and utilization of sustainable materials, they are held back by the insufficient ionic conductivity of the involved solid polymer electrolytes (SPEs). In this study, the conductivity and cation transference number are significantly improved through the construction of a composite polymer electrolyte (CPE) system based on poly(trimethylene carbonate) (PTMC) with sodium bis(trifluorosulfonylimide) (NaTFSI), combined with NaAlO₂ (NAO) ceramic filler at loadings ranging from 10 to 40 wt%. The NAO-based CPEs show the highest conductivity at 20 wt% NAO, with a Na⁺ transference number of ∼0.9 at 60 °C also being obtained for the same material, which is notably higher than that for the NAO-free SPE. Solid-state batteries composed of a Prussian white cathode and a Na metal anode and employing these CPEs reach a cycling performance of ∼100–150 mA h g⁻¹ at C/10 and 55 °C for more than 200 cycles without additives or plasticizers, thus opening the door to the potential exploration of CPEs for Na-based battery chemistries.

Anodic dissolution of aluminum (commonly called aluminum corrosion) is a potential issue in sodium-ion batteries. Herein, it is demonstrated how different sodium-ion battery electrolyte solutions affect this phenomenon. The type of electrolyte was critical for the presence of anodic dissolution, while the solvent appeared to alter the dissolution process.

In selecting electrolytes for Na-ion batteries, simply importing the analogue of common lithium-ion battery electrolytes to sodium-ion batteries does not address safety concerns like toxicity and flammability. Electrolytes based on sodium bis(oxalato)borate (NaBOB) in organophosphates like triethyl phosphate (TEP) largely alleviate these specific safety concerns. However, it may be beneficial to obtain solutions with higher ionic conductivities than NaBOB in TEP, and compare the performance in Na-ion batteries with high mass loading electrodes. Here, we have shown that N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc), and N-methyl pyrrolidone (NMP) cosolvents almost double the ionic conductivity (from similar to 3.5 mS/cm to similar to 7 mS/cm) without sacrificing the flame-retarding properties of the base NaBOB in TEP electrolyte. The physical properties of these cosolvent electrolyte mixtures are investigated, along with the electrochemical performance of these electrolytes full-cells based on hard carbon anodes and Prussian white cathodes with near-commercial areal capacity (similar to 2 mAh/cm(2)).

The development of liquid electrolytes receives significant attention within the field of battery research. New concepts are emerging, and one of these groundbreaking ideas is localized high concentration electrolytes (LHCEs). The fundamental characteristic of this type of electrolyte is related to its solvation structure. However, despite the progress made, the solvation process and its relationship towards physicochemical and electrochemical properties are not yet fully understood. A comprehensive understanding of LHCEs and their solvation structure requires further dedicated research and analysis. This concept review offers a thorough examination of the design principles governing LHCEs, elucidates methodologies for investigating solvation structures, connects these insights to interphase chemistry, and explores potential applications in future battery technology.

Prussian white (PW), Na_2−xFe[Fe(CN)₆], is an attractive cathode material for sodium-ion batteries due to its porous framework enabling fast sodium-ion extraction and insertion, environmentally safe elements, scalable synthesis, and performance comparable to current lithium-ion technologies. However, PW suffers from large volume changes between rhombohedral and cubic phases during cycling which is suggested to be detrimental over time because of structural degradation and increased ion insertion resistance. In particular, studies on PW hard carbon full cells revealed that most of the capacity is lost from the lower potential plateau, where this phase transition occurs. It is proposed that cycling in a restricted potential range, where the phase transition is avoided, could benefit the cycle lifetime and capacity retention. Here, we show an operando X-ray diffraction study aiming at determining how the structure evolves after prolonged cycling in different restricted potential ranges and how this impacts the cycling stability and capacity fade in PW. No signs of structural degradation were observed independently of the pre-cycling conditions used. In addition, more of the rhombohedral phase and capacity were recovered in the discharged state when a more restricted potential range had been applied. Thus, it was shown that the phase transition and corresponding volume changes have little impact on the capacity fade. Instead, the main source for capacity fade was proved to be sodium inventory loss, especially during the initial cycles, in combination with, to a lesser extent, polarization. This study gives a new perspective on PW-based batteries in that neither volume changes nor phase transitions are detrimental to battery performance. These results aid the development of improved cycling protocols and battery systems comprised of PW where the lifetime of the material is prolonged.