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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.

With the use of more green energy, more batteries are needed to store the energy for when it is needed. Most batteries today consist of expensive and scarce metals such as lithium and cobalt. In order to meet the increased demands, other chemistries need to be investigated. One of these is sodium-ion batteries (NIBs) that potentially consist of more abundant and affordable materials. An interesting cathode material for NIBs is Prussian White (PW), that is both environmentally friendly and cheap. During use in a battery PW undergoes a phase transition that induces a large volume change. It has been proposed that small ions or neutral guest molecules could mitigate this volume change. This thesis investigates the use of lithium to stabilise the framework of PW. To investigate this, a method to intercalate lithium into PW with different Na-content was developed. Four Na/Li ratios were achieved using electrochemical synthesis. These samples were analysed using galvanostatic cycling to determine the electrochemical properties, X-ray diffraction (XRD) to investigate the structure and Operando XRD to study the changes of the material during cycling. Depending on the Li/Na ratio, three phases were identified: Cubic, rhombohedral with an 18% smaller volume and a new phase with a smaller, but not determined, volume. With addition of lithium, the cubic phase transition was shifted to a higher state of discharge and a lower state of charge. This means that more Li/Na would be intercalated before a phase transition occurred, possibly leading to lower strain in the material. This indicates that Li intercalate into the PW framework, and that Li could potentially be used to stabilise the PW framework.