Tommi Hendrik Aalto

We introduce NaI as a mild reagent for low-temperature topochemical defluorination of Ruddlesden-Popper oxyfluorides. In a topochemical fluorination and subsequent defluorination process La₂CoO₃F₃ is obtained from La₂CoO₄, using poly(vinylidene fluoride) (PVDF) or polytetrafluoroethylene (PTFE) as fluorine sources, and converted to La₂CoO₃F₂ by the addition of NaI. Both processes were studied by laboratory in situ X-ray diffraction, which reveals a stepwise fluorine uptake through four crystalline intermediates. Subsequent NaI treatment enables controlled F^- removal to form La₂CoO₃F₂. This Co(II) phase is not accessible by direct topochemical fluorination of La₂CoO₄ and is not observed along the La₂CoO₃F₃ formation pathway. X-ray and neutron powder diffraction establish La₂CoO₃F₃ as monoclinic (P2₁/c) with full occupation of the interstitial anion layer, whereas La₂CoO₃F₂ is isotypic to La₂NiO₃F₂ (Cccm) and exhibits a channel-like interstitial anion arrangement. Thermal analysis by in situ XRD is used to scan the temperature ranges over which both oxyfluorides retain their structure, and magnetization measurements indicate the change in cobalt oxidation and spin state upon fluorination/defluorination. This study uses the La₂CoO₄ → La₂CoO₃F₃ → La₂CoO₃F₂ reaction sequence as model system to demonstrate that, sequential topochemical fluorination and NaI-mediated defluorination provides access to metastable, anion-ordered RP oxyfluorides under milder conditions than conventional hydride-based reductions, avoiding both reduction of the metal cations to their metallic state and anion substitution reactions due to size effects of the iodide ion.

We present a comparative study of n = 1 and n = 2 Ruddlesden–Popper (RP) titanates Sr₂TiO₄ and Sr₃Ti₂O₇, as well as their oxyfluorides Sr₂TiO₃F₂ and Sr₃Ti₂O₅F₄, synthesized via low-temperature topochemical fluorination to study structure–property relationships in photocatalytic hydrogen (H₂) evolution. In Sr₂TiO₄, fluorination lowers the symmetry from I4/mmm to P4/nmm, placing Ti in a locally non-centrosymmetric site and producing ordered, asymmetric TiO₅F units. By contrast, Sr₃Ti₂O₇ retains its tetragonal I4/mmm symmetry upon fluorination with Ti remaining in a locally non-centrosymmetric environment. Photocatalytic H₂ evolution shows a ∼6.3-fold enhancement for Sr₂TiO₃F₂ compared to Sr₂TiO₄, which might be attributed to inversion symmetry breaking and the resulting internal local dipole fields that promote charge separation. In contrast, Sr₃Ti₂O₇ exhibits much higher intrinsic activity than Sr₂TiO₄ (∼30-fold), likely governed by dimensionality, while its fluorinated phase Sr₃Ti₂O₅F₄ shows suppressed performance due to the absence of local polarity, which limits beneficial fluorination effects and diminishes charge-separation efficiency, as supported by transient absorption spectroscopy (TAS) results. TAS reveals distinct charge-carrier dynamics, with Sr₃Ti₂O₇ showing the highest photogenerated electron population and Sr₂TiO₃F₂ exhibiting enhanced long-lived electrons, consistent with improved charge separation. Density functional theory (DFT) calculations show that fluorination deepens the O 2p valence band, slightly raises the Ti 3d conduction edge, and reduces c-axis dispersion, consistent with the UV-Vis and Butler–Ginley analyses. The resulting increase in electron effective mass suppresses charge mobility along the c-axis, lowering conduction dimensionality. These findings establish anion sublattice engineering as a selective route to tune local symmetry, charge-carrier dynamics, and photocatalytic performance in RP-type titanates.

In this work, we report the gas evolution behavior of two common intercalation-based cathode materials for fluoride-ion-batteries (La₂NiO₄ and La₂CoO₄) by chemical fluorination and electrochemical fluorination, as well as the thermal stability of the electrochemically fluorinated active materials. Variable-temperature X-ray diffraction with coupled mass spectrometry shows that the fluorinated phases of both active materials release oxygen after oxidative fluorination using AgF. Differential electrochemical mass spectrometry revealed that significant CO₂, CO and NO evolution takes place during electrochemical fluorination, but O₂ evolution could not be detected. Consequently, the gas evolution observed does not compromise the long-term stability of the cell performance. Variable-temperature X-ray diffraction of the charged cathode composites reveals that the electrochemically fluorinated products decompose once heated beyond 300 °C, then undergoing similar phase evolution behavior as observed in chemical fluorination. Thus, we conclude that oxygen evolution in Ruddlesden Popper-type materials on oxidative fluorination requires thermal activation beyond the operational temperature of 163 °C, and that it can be kinetically suppressed regardless of the reduced Fermi Level induced on fluorination. Our findings indicate that concepts used for the oxidative topochemical fluorination of materials need to be reconsidered: the choice of fluorination conditions must be carefully adopted to the changes of electronic structure induced and the temperature dependence of the oxygen evolution of the highly oxidized state.

We present a comparative study of n = 1 and n = 2 Ruddlesden−Popper (RP) titanates Sr2TiO4 and Sr3Ti2O7, as well as their oxyfluorides Sr2TiO3F2 and Sr3Ti2O5F4, synthesized via low-temperature topochemical fluorination to study structure−property relationship in photocatalytic hydrogen (H2) evolution. In Sr2TiO4, the fluorination lowers the symmetry from I4/mmm to P4/nmm, placing Ti in a locally non-centrosymmetric site and producing ordered, asymmetric TiO5F units. By contrast, Sr3Ti2O7 retains its tetragonal I4/mmm symmetry upon fluorination with Ti remaining in a locally non-centrosymmetric environment. Photocatalytic H2 evolution shows a ~6.3-fold enhancement for Sr2TiO3F2 compared to Sr2TiO4, might be attributed to inversion symmetry breaking and the resulting internal local dipole fields that promote charge separation. In contrast, Sr3Ti2O7 exhibits much higher intrinsic activity than Sr2TiO4 (~30-fold), likely governed by dimensionality, while its fluorinated phase Sr3Ti2O5F4 shows suppressed performance due to the absence of local polarity limits beneficial fluorination effects. Density Functional theory (DFT) calculations show that fluorination deepens the O 2p valence band, slightly raises the Ti 3d conduction edge, and reduces c-axis dispersion, consistent with the UV−Vis and Butler−Ginley analyses. The resulting increase in electron effective mass suppresses charge mobility along the c-axis, lowering conduction dimensionality. These findings establish anion sublattice engineering as a selective route to tune symmetry and photocatalytic performance in RP-type titanates.

In this work, we report the gas evolution behavior of two common intercalation-based cathode materials for fluoride-ion-batteries (La2NiO4 and La2CoO4) by chemical fluorination and electrochemical fluorination, as well as the thermal stability of the electrochemically fluorinated active materials. Variable-temperature X-ray diffraction with coupled mass spectrometry shows that the fluorinated phases of both active materials release oxygen after oxidative fluorination using AgF. Differential electrochemical mass spectrometry revealed that significant CO2, CO and NO evolution takes place during electrochemical fluorination, but O2 evolution could not be detected. Consequently, the gas evolution observed does not compromise the long-term stability of the cell performance. Variable-temperature X-ray diffraction of the charged cathode composites reveals that the electrochemically fluorinated products decompose once heated beyond 300 °C, then undergoing similar phase evolution behavior as observed in chemical fluorination. Thus, we conclude that oxygen evolution in RP-type materials on oxidative fluorination requires thermal activation beyond the operational temperature of 170 °C, and that it can be kinetically suppressed regardless of the reduced Fermi Level induced on fluorination. Our findings indicate that concepts used for the oxidative topochemical fluorination of materials need to be reconsidered: the choice of fluorination conditions must be carefully adopted to the changes of electronic structure induced and the temperature dependence of the oxygen evolution of the highly oxidized state.

Fluoride-Ion-Batteries are a promising battery technology to achieve high energy densities exceeding those of traditional lithium-ion batteries. Reports on intercalation-based electrode materials for Fluoride-Ion-Batteries have mostly focused on cathode materials, while only a few intercalation-based anode materials have been reported. Their performance was heavily affected by carbon-based reductive side reactions, limiting reversibility and introducing high overpotentials. In this study, we present the successful substitution of carbon by metallic copper in solid-state FIBs, enabling the use of La2NiO3F2, Pr2NiO3F2, Sr2TiO3F2 and Sr3Ti2O5F4 as intercalation-based anode materials by avoiding parasitic side reactions associated with the conductive carbon additive.

In this study, conductive, fluorine and antimony codoped tin oxide nanoparticles (FATO‐NPs) are highlighted as a possible alternative for conductive carbon additives in fluoride ion batteries, successfully circumventing oxidative side reactions. Since good cyclability with high and stable discharge capacities is achieved with both types of conductive additive at a high stack pressure of 180 MPa, it is concluded that conductive carbon is well‐suited for high‐voltage fluoride ion batteries, contrary to prior assumptions. However, FATO‐NP‐based cathodes outperform those based on conductive carbon at lower stack pressures of 50 MPa, emphasizing the importance of avoiding carbon fluorination at low stack pressures.

In this study, conductive, fluorine and antimony co-doped tin oxide nanoparticles (FATO-NPs) are highlighted as a possible alternative for conductive carbon additives in fluoride ion batteries, successfully circumventing oxidative side reactions. Since good cyclability with high and stable discharge capacities was achieved with both types of conductive additive, it was concluded that conductive carbon is well suitable for high-voltage fluoride ion batteries, contrary to prior assumptions. The utilization of almost completely side-reaction free FATO-NPs as the conductive additive enables the deconvolution of other side reactions in initial cycles.

If all‐solid‐state fluoride‐ion batteries want to compete with existing battery technologies, significant improvements in terms of cyclic stability are necessary to fully access the high specific capacities, which this battery concept can provide in theory. Herein, the development of a high‐pressure, high‐temperature battery operation stand for battery cycling under inert conditions inside a glovebox is reported. This stand is then used to investigate the effect of stack pressure on the cell performance of conversion‐based as well as intercalation‐based electrode materials for fluoride‐ion batteries. It is found that cyclic stability as well as energy efficiency is strongly increased compared to nonpressure conditions, which is assigned to sustained interparticle contact. Thus, the cell design must be considered carefully to be able to distinguish intrinsic material properties from percolation‐ and interphase‐related impacts on the cell behavior. Further, the effect of pressure on the ionic conductivity of common solid fluoride‐ion conductors is investigated.