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The sodium iron hexacyanoferrate compound with chemical formula Na_2.04(2)Fe[Fe(CN)₆]·2.24(2)H₂O, also known as Prussian white (PW), contains disordered and dynamic water molecules that have a dualistic effect on its battery performance. Furthermore, the material exhibits severe strain when dehydrated, which over time diminishes the performance. To understand the complex role of water on the sodium ion conduction and the structural changes happening upon dehydration, local structural characterization is needed. Here, we report the first neutron total scattering study of PW. Reverse Monte Carlo (RMC) fitting reveals that local octahedral distortion of the nitrogen-bound iron octahedra contributes to the disorder of the framework. The strain observed in the dehydrated material comes from a combination of the Fe–N bond elongation and a disordered distribution of sodium throughout the larger structure. In the hydrated material, the sodium exhibits more order due to the presence of water, which constrains the sodium movement. However, the sodium ordering affects the orientation of the water molecules. In the low temperature P2₁/n phase, sodium orders into planes with the oxygen atoms in the water molecules being in the plane, while the hydrogen atoms are pointing away from the sodium plane. In the room temperature R phase, the sodium and water are less ordered despite similar frameworks. Sodium can take a wide range of positions, especially if no water molecule blocks its way, to obtain optimal bonding conditions. These results show that the relationship between sodium and water is co-dependent, and demonstrate that the local structure of framework materials has a crucial link to their properties.

Prussian blue analogues (PBAs) are interesting cathode materials for sodium-ion batteries, especially the iron-based, [Fe(CN)₆]ⁿ⁻ vacancy-free PBA Na_2–xFe[Fe(CN)₆]·zH₂O. However, the presence of water has an opposing role in the application of PBAs as electrode materials: the water provides structural stability ensuring minimum volume changes during sodium extraction and insertion, however, water can react with the electrolyte leading to unwanted side reactions. Therefore, water must be replaced with another compatible small molecule to ensure optimal performance. To achieve this, insights into the dynamics of water are crucial. Two samples with compositions of Na_1.90(9)Fe_0.90(7)²⁺Fe_0.10(3)³⁺[Fe²⁺(CN)₆]·2.12(2)H₂O and Na_0.34(5)Fe³⁺[Fe^2.66(5)+(CN)₆]·0.360(4)H₂O were investigated using quasi-elastic neutron scattering (QENS). The results show that the water dynamics strongly depend on the sodium content. The water was found to diffuse within a spherical cavity in the porous framework with a radius of 2.6 Å for the high sodium-containing sample and 1.8 Å for the low sodium-containing sample consistent with the pore sizes in the crystal structures. In addition to the water diffusing within the pores, it was found that a small fraction of the water exhibits a rattling or rotational motion suggesting that this water strongly interacts and binds to the sodium ions. For the high sodium-containing sample, this rattling or rotational motion transforms into quantum rotational tunneling of the water below 75 K. These results give new fundamental insight into the role of water in PBAs, laying the groundwork for substituting water with another small molecule compatible with nonaqueous battery systems while also ensuring structural stability.

Layered Na₃Fe₃(PO₄)₄ can function as a positive electrode for both Li- and Na-ion batteries and may hold advantages from both classical layered and phosphate-based electrode materials. Using a combination of ex-situ and operando synchrotron radiation powder X-ray diffraction, void space analysis, and Mössbauer spectroscopy, we herein investigate the structural evolution of the Na₃Fe₃(PO₄)₄ framework during Li- and Na-ion intercalation. We show that during discharge, Li- and Na-intercalation into Na₃Fe₃(PO₄)₄ occurs via a solid solution reaction wherein Na-ions appear to be preferentially intercalated into the intralayer sites. The intercalation causes an expansion of the unit cell volume, however at open circuit conditions after ion-intercalation (i.e., after battery discharge), Na_3+xFe₃(PO₄)₄ and Li_xNa₃Fe₃(PO₄)₄ undergo a structural relaxation, wherein the unit volume contracts below that of the pristine material. Rietveld refinement suggests that the ions intercalated into the intra-layer sites diffuse to the sites in the inter-layer space during the relaxation. This behavior brings new perspectives to understanding structural relaxation and deviations between structural evolution observed under dynamic and static conditions.

Lithium-rich layered oxides (LRLOs) are one of the most attractive families among future positive electrode materials for the so-called fourth generation of lithium-ion batteries (LIBs). Their electrochemical performance is enabled by the unique ambiguous crystal structure that is still not well understood despite decades of research. In the literature, a clear structural model able to describe their crystallographic features is missing thereby hindering a clear rationalization of the interplay between synthesis, structure, and functional properties. Here, the structure of a specific LRLO, Li_1.28Mn_0.54Ni_0.13Co_0.02Al_0.03O₂, using synchrotron X-ray diffraction (XRD), neutron diffraction (ND), and High-Resolution Transmission Electron Microscopy (HR-TEM), is analyzed. A systematic approach is applied to model diffraction patterns of Li_1.28Mn_0.54Ni_0.13Co_0.02Al_0.03O₂ by using the Rietveld refinement method considering the Rm and C2/m unit cells as the prototype structures. Here, the relative ability of a variety of structural models is compared to match the experimental diffraction pattern evaluating the impact of defects and supercells derived from the Rm structure. To summarize, two possible models able to reconcile the description of experimental data are proposed here for the structure of Li_1.28Mn_0.54Ni_0.13Co_0.02Al_0.03O₂: namely a monoclinic C2/m defective lattice (prototype Li₂MnO₃) and a monoclinic defective supercell derived from the rhombohedral Rm unit cell (prototype LiCoO₂).

Layered TiS₂ has been proposed as a versatile host material for various battery chemistries. Nevertheless, its compatibility with aqueous electrolytes has not been thoroughly understood. Herein, we report on a reversible hydration process to account for the electrochemical activity and structural evolution of TiS₂ in a relatively dilute electrolyte for sustainable aqueous Li-ion batteries. Solvated water molecules intercalate in TiS₂ layers together with Li⁺ cations, forming a hydrated phase with a nominal formula unit of Li_0.38(H₂O)_2−δTiS₂ as the end-product. We unambiguously confirm the presence of two layers of intercalated water by complementary electrochemical cycling, operando structural characterization, and computational simulation. Such a process is fast and reversible, delivering 60 mAh g^–1 discharge capacity at a current density of 1250 mA g^–1. Our work provides further design principles for high-rate aqueous Li-ion batteries based on reversible water cointercalation.

Traditionally, Ni-rich-layered oxide cathodes for lithium-ion batteries are produced utilizing N-methyl-2-pyrrolidone (NMP)-processed casting. However, to avoid using the reprotoxic solvent NMP, aqueous processing becomes one of the options. In this study, H2O-processed LiNi0.8Mn0.1Co0.1O2 (NMC811) electrodes have been prepared to compare with the NMP-processed counterparts to investigate the degradation mechanism. The thick cathode-electrolyte interphase (CEI), NiO-like phase formation, and the growth of electrochemically inactive NMC particles after long-term cycling lead to capacity decay. In addition, phosphoric acid (H3PO4) was utilized to lower the pH value during the water-processed electrode preparation, to avoid corrosion of the aluminium current collector. The use of H3PO4 enhanced the capacity retention of NMC811 electrodes, likely owing to the formation of a LiF-rich CEI layer in the initial cycle(s) and the alleviated formation of electrochemically inactive NMC particles. Additionally, reaction inhomogeneity is present in H3PO4-modified electrodes, which is attributed to various Li-ion reinsertion resistances throughout the porous electrode during long-term cycling. Although the performance of the water-processed NMC811 electrode is not reaching the level of NMP-processed electrodes, this study provides key insights into the involved degradation mechanisms and demonstrates a viable pathway for the development of sustainable battery manufacturing processes. A slightly thinner CEI layer and lower charge transfer resistance were achieved by H3OP4 modification during the water processing of Ni-rich cathodes, compared to non-treated counterparts.

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.

In this study, Synchrotron X-ray diffraction (XRD) radiography was utilized to investigate the ageing heterogeneity in 48 Ah prismatic lithium-ion cells with Ni-rich LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) as the positive electrode active material and graphite as the negative electrode active material after ∼2800 cycles. The study revealed that the area closest to the positive electrode tab is most vulnerable to degradation, particularly impacting the NMC material. Application of principal component analysis allowed to differentiate and visualize part of positive electrode material that has a different degradation due to the lithium plating. A comparison of non-destructive X-ray diffraction-based methods and electrochemical characterization method which was performed on the opened cell has shown an importance of a complementary approach. Our results highlight the feasibility of employing non-destructive techniques to study large prismatic cells, thereby presenting extensive opportunities for advancements in battery research and industry.

To enhance battery safety, it is of utmost importance to develop non-flammable electrolytes. An emerging concept within this research field is the development of localized highly concentrated electrolytes (LHCEs). This type of liquid electrolyte relies on the concept of highly concentrated electrolytes (HCEs), but possesses lower viscosity, improved conductivity and reduced costs due to the addition of diluent solvents. In this work, two different hydrofluoroethers, i.e., bis(2,2,2-trifluoroethyl) ether (BTFE) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), are studied as diluents in a phosphate-based non-flammable liquid electrolyte. These two solvents were added to a highly concentrated electrolyte of 3.0 M lithium bis(fluorosulfonyl)imide (LiFSI) in triethyl phosphate (TEP) whereby the salt concentration was diluted to 1.5 M. The solvation structures of the HCE and LHCE were studied by means of Raman spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy, where the latter was shown to be essential to provide more detailed insights. By using molecular dynamics simulations, it was shown that a highly concentrated Li+-TEP solvation sheath is formed, which can be protected by the diluents TTE and BTFE. These simulations have also clarified the energetic interaction between the components in the LHCE, which supports the experimental results from the viscosity and the NMR measurements. By performing non-covalent interaction analysis (NCI) it was possible to show the main contributions of the observed chemical shifts, which indicated that TTE has a stronger effect on the solvation structure than BTFE. Moreover, the electrochemical performances of the electrolytes were evaluated in half-cells (Li|NMC622, Li|graphite), full-cells (NMC622|graphite) and Li metal cells (Li|Cu). Galvanostatic cycling has shown that the TTE based electrolyte performs better in full-cells and Li-metal cells, compared to the BTFE based electrolyte. Operando pressure measurements have indicated that no significant amount of gases is evolved in NMC622|graphite cells using the here presented LHCEs, while a cell with 1.0 M LiFSI in TEP displayed clear formation of gaseous products in the first cycles. The formation of gaseous products is accompanied by solvent co-intercalation, as shown by operando XRD, and quick cell failure. This work provides insights on understanding the solvation structure of LHCEs and highlights the relationship between electrochemical performance and pressure evolution.

Prussian blue analogues (PBAs), A_xM[M’(CN)₆]_1–y·zH₂O, are a highly functional class of materials with use in a broad range of applications, such as energy storage, due to their porous structure and tunable composition. The porosity is particularly important for the properties and is deeply coupled to the cation, water, and [M’(CN)6]n– vacancy content. Determining internal porosity is especially challenging because the three compositional parameters are dependent on each other. In this work, we apply a new method, positron annihilation lifetime spectroscopy (PALS), which can be employed for the characterization of defects and structural changes in crystalline materials. Four samples were prepared to evaluate the method’s ability to detect changes in internal porosity as a function of the cation, water, and [M’(CN)₆]^n– vacancy content. Three of the samples have identical [M’(CN)₆]^n– vacancy content and gradually decreasing sodium and water content, while one sample has no sodium and 25% [M’(CN)₆]^n– vacancies. The samples were thoroughly characterized using inductively coupled plasma-optical emission spectroscopy (ICP-OES), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Mössbauer spectroscopy as well as applying the PALS method. Mössbauer spectroscopy, XRD, and TGA analysis revealed the sample compositions Na_1.8(2)Fe²⁺_0.64(6)Fe^2.6+_0.36(10)[Fe²⁺(CN)₆]·2.09(2)H₂O, Na_1.1(2)Fe²⁺_0.24(6)Fe^2.8+_0.76(6)[Fe^2.3+(CN)₆]·1.57(1)H₂O, Fe[Fe(CN)₆]·0.807(9)H₂O, and Fe[Fe(CN)₆]0.75·1.5H₂O, confirming the absence of vacancies in the three main samples. It was shown that the final composition of PBAs could only be unambiguously confirmed through the combination of ICP, XRD, TGA, and Mössbauer spectroscopy. Two positron lifetimes of 205 and 405 ps were observed with the 205 ps lifetime being independent of the sodium, water, and/or [Fe(CN)₆]^n– vacancy content, while the lifetime around 405 ps changes with varying sodium and water content. However, the origin and nature of the 405 ps lifetime yet remains unclear. The method shows promise for characterizing changes in the internal porosity in PBAs as a function of the composition and further development work needs to be carried out to ensure the applicability to PBAs generally.