Gabriel E. Pérez

Neutron powder diffraction (NPD) is a non-destructive technique uniquely suited to investigating industrially relevant sodium-ion cathode materials. Unlike X–ray diffraction, it offers excellent contrast between neighbouring transition metal ions and is sensitive to lighter elements such as O/Na, which are key to revealing superstructural orderings characteristic of sodium cathodes. However, in situ/operando NPD battery studies are hindered by sample quantity requirements, and lack of suitable sample environments. Using a cell developed at ISIS Neutron and Muon Source, we observed no detrimental effects from prolonged vacuum exposure required for Time-of-Flight NPD. We collect/analyse data in two modes: operando , in which the electrochemical structural evolution is tracked, and in situ, wherein higher signal-to-noise data is collected during voltage holds, for phase-pure samples at set voltage cutoffs. Using Na _x NiO ₂ as a model system, we present a methodology to account for the complex background profile characteristic of electrochemical environments enabling, for the first time, quantitative Rietveld refinements against in situ electrochemical NPD, and providing a systematic framework for future studies. Structural models are refined against in situ data and benchmarked against combined refinement of ex situ SXRD/NPD measurements. This work unlocks potential for previously inaccessible insights into structure-property relationships and degradation, helping to facilitate the development of next-generation cathodes with enhanced useful lifetime, capacity, and performance.

Neutron powder diffraction (NPD) is a non-destructive technique uniquely suited to investigating industrially relevant sodium-ion cathode materials. Unlike X-ray diffraction, it offers excellent contrast between neighbouring transition metal ions and is sensitive to lighter elements such as O/Na, which are key to revealing superstructural orderings characteristic of sodium cathodes. However, in situ/operando NPD battery studies are hindered by sample quantity requirements, and lack of suitable sample environments. Using a cell developed at ISIS Neutron and Muon Source, we observed no detrimental effects from prolonged vacuum exposure required for Time-of-Flight NPD. We collect/analyse data in two modes: operando, in which the electrochemical structural evolution is tracked, and in situ, wherein higher signalto-noise data is collected during voltage holds, for phase-pure samples at set voltage cutoffs. Using Na _x NiO ₂ as a model system, we present a methodology to account for the complex background profile characteristic of electrochemical environments enabling, for the first time, quantitative Rietveld refinements against in situ electrochemical NPD, and providing a systematic framework for future studies. Structural models are refined against in situ data and benchmarked against combined refinement of ex situ SXRD/NPD measurements. This work unlocks potential for previously inaccessible insights into structure-property relationships and degradation, helping to facilitate the development of next-generation cathodes with enhanced useful lifetime, capacity, and performance.

Operando diffraction experiments enable real-time, nondestructive correlation of the electrochemical behavior of battery cells with the physicochemical changes occurring in their internal components. These experiments can now be directly performed on standard cell formats, identical to those used for electrochemical testing, at unprecedented scattering, temporal, and spatial resolutions. This can generate significant volumes of new multimodal datasets, opening exciting possibilities for probing and parameterizing complex battery performance and degradation mechanisms. To fully leverage the wealth of data produced by such experiments via data-driven machine learning methods, the open sharing of data via standardized, machine-readable formats, in line with FAIR principles, is essential. Currently, this is hindered by differences in the cell design, experimental protocols, and data-processing workflows. In this perspective, we discuss the need for standardizing operando battery diffraction experiments, the associated challenges, and propose a NeXus-based application definition for the standardized exchange of experimental metadata and results as a pathway toward reproducible research practices and transparent analytical workflows.

Water-based cathode slurries for lithium-ion batteries can replace polyfluoroalkyl binders (PFAS) with biodegradable alternatives. Dispersing carbon black in water is the main formulation challenge. For the first time, an amphiphilic block copolymer, Pluronic F68, is blended with a water-soluble homopolymer, polyethylene oxide, to simultaneously disperse carbon black in water while controlling the viscosity of the slurry. Two series of binder blends are prepared: one systematically varies the viscosity of the binder solution, and the other systematically varies the solid loading of the slurry. These orthogonal series demonstrate that the polymer surfactant has a strong effect on half-cell performance independent of the role of viscosity. At the optimum formulation, water-processed half-cells using LiNi _0.6 Mn _0.2 Co _0.2 O ₂ achieve a first specific discharge capacity up to 166 mAh g ^–1 and capacity retention of 89% after 100 cycles at 1C, comparable to cells made using a PFAS binder. Water-processed cells outperform the conventional cell at a fast charging rate of 5C. The results suggest that the affinity of the polymer binder(s) to carbon black, measured via adsorption, can be used as a selection criterion to identify binders that optimize the electrochemical performance of water-processed battery cells.

The bond valence sum (BVS) method has been extensively used to probe the ionic diffusion pathways within solids possessing high ionic conductivities, both anionic and cationic. For the former, the presence of lone-pair cations such as Tl ⁺ , Sn ²⁺ , Pb ²⁺ , Sb ³⁺ and Bi ³⁺ is known to enhance the anionic conductivity of solids by promoting disorder within the anion substructure and lowering the energy barriers between lattice sites. However, the BVS formalism assumes spherical ions and does not include the possibility of the more irregular coordination environments associated with electron lone pairs. We present here a simple modification of the BVS approach, which has been implemented in the form of a Python program that permits the preferred F ⁻ diffusion pathways to be determined in fluoride ion conductors containing lone-pair cations. The method has been benchmarked by calculating energy barriers that broadly correlate with the F ⁻ ionic conductivities of known compounds, and successfully reproduces the F ⁻ distribution within the highly conducting compound β-PbSnF ₄ determined by neutron powder diffraction and molecular dynamics (MD) methods. Using this new approach, fluoride compounds within the Inorganic Crystal Structure Database that contain at least one lone-pair cation have been screened and several new potential F ⁻ ion conductors have been identified.

The advent of diverse battery chemistries necessitates the development of novel diagnostic techniques to investigate their degradation. Due to its sensitivity to light elements and ability to distinguish between 3d transition metals, operando neutron diffraction is well-suited to study the crystallographic transformations in battery materials and their associated degradation pathways. However, it is currently not used academically due to limited data quality and time resolution, and the lack of a cost-effective electrochemical cell that can be reproducibly fabricated at the laboratory scale with benchmarked performance. Here, using the high-resolution cold-neutron diffractometer, WISH, we demonstrate that neutron diffraction can be performed in operando on laboratory-scale 70-mAh single-layer pouch cells without any modification, deuteration or isotope enrichment, cycled under practically relevant conditions. This advances the battery diagnostics toolkit beyond X-ray techniques and permits continuous academic investigations of established, and next-generation technologies like lithium-metal, anode-free and lithium-sulphur batteries for further development.

Lithium nickel oxide, LiNiO₂ (LNO), and its doped derivatives are promising battery cathode materials with high gravimetric capacity and operating voltages. They are also of interest to the field of quantum magnetism due to the presumed S = 1/2 triangular lattice and associated geometric frustration. However, the tendency for Li/Ni substitutional defects and off-stoichiometry makes fundamental studies challenging. In particular, there is still a discrepancy between the rhombohedral (R3̅m) bulk structure and the Jahn-Teller (JT) distortions of the NiO₆ octahedra inferred on the basis of local structural probes. Karger et al. (Chem. Mater. 2023, 35, 648-657) recently used Na/Li ion exchange to synthesize "defect-free" LNO by exploiting the absence of antisite disorder in NaNiO₂ (NNO). Here we characterize the short- and long-range structure of this ion-exchanged material and observe splittings of key Bragg reflections at 100 K in X-ray and neutron diffraction (XRD and NPD), indicative of a monoclinic distortion induced by a cooperative collinear JT distortion, similar to that seen in NNO. Variable temperature XRD reveals a second-order phase transition from the monoclinic (C2/m) low-temperature structure to a rhombohedral (R3̅m) structure above ∼400 K. We propose that this collinear JT ordering is also present in solid-state synthesized LNO with the domain size and extent of monoclinic distortion controlled by defect concentration. This new structural description of LNO will help advance our understanding of its electronic and magnetic properties and the series of phase transformations that this material undergoes upon electrochemical cycling in Li-ion batteries.

Dual-phase electrocatalysts help to enhance Li–O ₂ battery performances with a sheet-like Li ₂ O ₂ morphology.