78 79 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Figure 3: Illutrated charge reaction pathways of (a) Fe x O y and (b) Mn x O y . Copyright 2021, Nature Publishing Group. High-energy X-rays reveal non-equilibriumbattery chemistry Related publication: Hua, X., Allan, P. K., Gong, C., Chater, P. A., Schmidt, E. M., Geddes, H. S., Robertson, A.W., Bruce, P. G., & Goodwin, A. L. Non-equilibriummetal oxides via reconversion chemistry in lithium-ion batteries. Nature Communications 12 , 561. (2021) DOI: 10.1038/s41467-020- 20736-6 Publication keywords: Lithium-ion batteries; Metal oxides; Non-equilibriumphases; Pair distribution function C oncerns such as global warming and the depletion of fossil fuel reserves are driving up demand for lithium-ion batteries in the automotive industry. Developing next-generation batteries with improved performance requires new, affordable materials. A class of transition metal oxides (M x O y ) provides cost-effective electrode candidates. However, we have a limited fundamental understanding of how these materials change during battery cycling, a complex process involving multiple highly nanostructured (or amorphous) phases. Researchers from the University of Oxford used X-Ray Pair Distribution Function (XPDF) beamline (I15-1) to characterise the nanoscopic phases present in these battery materials. The beamline’s high flux and high energy X-rays, together with its optimised in operando electrochemistry setup, allowed the researchers to collect high-quality pair distribution function (PDF) data. This was crucial for investigating any nanostructured components and their phase behaviours in real time. Their results showed that the mechanisms of these metal oxides have a metal dependence and their reactions follow a topotactic pathway, contradicting the commonly-presumedmechanismvia a complete structure de- and reconstruction. This newmechanistic understanding of how thesematerials react rationalised the origin of their slow electrochemical performances, providing insights into effective strategies for further development. In addition, this study also reported the experimental observation of the non-equilibrium metal monoxide polymorphs for the first time. This opens up exciting new avenues for electrochemically assisted synthesis to explore non-native metal oxides with new functionalities. The conventional electrode materials for lithium-ion batteries operate via insertion chemistry 1 , whose reversibility is restricted by the homogeneity range of materials’ crystal structures upon charge and discharge, limiting capacity. In the search for the next-generation electrode materials, conversion -type binary metal oxides (M x O y ) have attracted considerable interest. These materials were believed to undergo a complete structure de- and re-construction upon battery cycling and involve a full reduction of the metal centre (2 y Li + M x O y ↔ x M + y Li 2 O), thus resulting in a large capacity. Despite significant synthetic efforts giving rise to a library of M x O y systems with diverse nano-morphologies 2 , a mechanistic understanding of these materials’ reversible battery chemistry remains poor. Consequently, critical issues such as low power and energy efficiency persist, hindering their practical use. To conceive viable strategies for further development requires fundamental knowledge of these materials’ phase behaviours underpinning their unique reactivities within a battery. In this work, a series of Fe and Mn oxides (Fe x O y andMn x O y ) were selected as model compounds because they are the most attractive candidates of M x O y due to their low cost and facile syntheses. The metal oxides’ battery chemistry is heterogeneous and nanoscopic in nature making it challenging to study via traditional crystallographic methods, so instead the pair distribution function (PDF) technique was employed in- operando on the I15-1 beamline. I15-1 allowed the local structure of these materials to be studied and tracked in real time during battery cycling. Based on the acquired PDFs of the reversible cycles, the patterns between the Fe x O y and Mn x O y series are notably different, indicating their metal dependent phase behaviours. However, the peak evolution within the same Fe or Mn series shows a single common trend among different species, implying that the reversible reactionmechanismof the same M x O y family is independent of the composition and the initial crystal structure of the oxide species. For the Fe oxides (represented by α -Fe 2 O 3 ), the PDFs (Fig. 1a) surprisingly show subtle peak shifts with slight peak broadening and intensity reduction upon charge, suggesting that the α-Fe lattice with a body-centred cubic ( bcc ) ordering is retained without transformation to the rocksalt (rs) FeO as commonly believed. Remarkably, while most of the PDF peak intensities progressively decrease upon charge, a peak at 1.9 Å that corresponds to the Fe-O atom pair continues to grow (Fig. 1a), implying oxygen insertion into the α-Fe lattices to form bcc -FeO phases. To determine their crystal structures, a series of bcc -FeO x models (0 ≤ x ≤1) were constructed. These models include octahedrally-coordinated oxygen concerning its lower energy than the tetrahedral counterpart (Fig. 1b). For the selected x values, local structure relaxation was introduced by using a Metropolis Monte Carlo algorithm to incorporate O-O interactions. The results showed that upon increase of the oxygen concentration, the bcc -FeO x system experiences a disorder-to-order transition regarding the O distribution and eventually transforms to a structure with locally ordered domains (Fig. 1c) that mirror the tetragonally-distorted rs -FeO, hinting at an underlying link between the bcc - and rs -FeO (Fig. 1d). Using these derived structures, their PDF patterns were calculated (Fig. 1e) and showed an excellent agreement with the experimental data, confirming the reliability of this modelling analysis. In contrast to the Fe series, the PDFs of the Mn oxides (represented by Mn 3 O 4 , Fig. 2a) show a drastic change in peak profiles reflecting a significant Mn atomic rearrangement during the reversible reactions. The short structure coherence lengths (< 20 Å) of the PDF patterns persist, indicating that the average grain sizes of Mn-containing species remain small (or near amorphous) upon cycling. Given the practical challenges to study highly nanoscopic structures, as well as the analytic uncertainties concerning the intermediate, a recently-developed method 3 based on Metropolis non-negative matrix factorisation (NMF) 4 was employed, allowing for robust deconvolution of complex mixtures without a priori knowledge of the number and nature of the constituent components. The analysis incorporated three members representing the starting, intermediate and end phases, and gave rise to distinct PDF patterns that could be respectively modelled by using α-Mn, distorted rs -MnO, and zincblende ( zb ) MnO structures (Fig. 2b). Their corresponding phase ratios (Fig. 2c) also supported the presence of an intermediate. Although a one-step conversion reaction fromMto rs -MOhas been widely accepted as the charge reaction pathway, our study showed that both Fe and Mn oxides exhibit a two-step mechanism forming bcc- FeO and zb- MnO phases. Based on the structure coherence among the constituent phases, the charge reaction in Mn x O y manifests as insertion of Mn 2+ into the face centred cubic ( fcc ) O sublattice, whereas a different pathway based on O 2− insertion into the bcc -Fe sublattice was observed in the Fe system (Fig. 3). These topotactic reactions highlight a displacement -like mechanism, which is subjected to a path hysteresis due to the mobility differences amongst displaced species. This new insight rationalised the distorted voltage polarisations observed within the reversible cycles of M x O y , and on the one hand, pointed to future strategies to improve their kinetic performances by employing displaced species with fast mobilities. It is important to note that this study also reported the first experimental observation of the non-equilibrium bcc -FeO and zb -MnO polymorphs. Their formation under well-defined electrochemical conditions is likely stabilised by substantial surface energies of very finite particle sizes, suggesting electrochemical devices may offer an alternative synthesis strategy to explore non-native metal monoxides with new functionalities. References: 1. Cabana, J. et al . Beyond intercalation-based Li-Ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions. Advanced Materials 22 , E170–E192 (2010). DOI: 10.1002/ adma.201000717 2. Ren, Y. et al . Ordered mesoporous metal oxides: synthesis and applications. Chemical Society Reviews 41 , 4909 (2012). DOI:10.1039/c2cs35086f 3. Geddes, H. et al. Structural characterisation of amorphous solid dispersions via metropolis matrix factorisation of pair distribution function data. Chemical Communications 55 , 13346–13349 (2019). DOI: 10.1039/C9CC06753A 4. Lee, D. D. et al . Learning the parts of objects by non-negative matrix factorization. Nature 401 , 788–791 (1999). DOI: 10.1038/44565 Funding acknowledgement: This project was supported by the European Commission via M.S.C.A. (Grant 798169), the E.R.C. (Grant 788144), the Henry Royce Institute (Grant ref EP/R010145/1) and a Birmingham Fellowship. Corresponding author: Dr Xiao Hua, Lancaster University,
[email protected] Crystallography Group Beamline I15-1 Figure 1: In operando PDF data of (a) the Fe oxides upon charge and (b) modelling analysis. The results gave rise to a (c) bcc-FeO structure with well-ordered domains resembling (d) distorted rs-FeO. (e) Simulations using the derived structure models show an excellent agreement with the experiment. Copyright 2021, Nature Publishing Group. Figure 2: In operando PDF patterns for (a) the Mn oxides during the first charge. The NMF analysis gave rise to (b) three distinct components and (c) the evolution of the associated phase ratios. Copyright 2021, Nature Publishing Group.