Tomoya Kawaguchi

Hollandite-type α-MnO2 is a prototypical tunnel-structured host for rechargeable batteries, yet the atomic-scale mechanisms governing multivalent-ion insertion remain poorly understood. Here, we reveal how guest-ion species dictate electrochemical reaction pathways through distinct occupation modes within the one-dimensional tunnels of α-MnO2. By combining electrochemical analyses, atomic-resolution scanning transmission electron microscopy, and soft X-ray absorption spectroscopy, we directly correlate reaction kinetics with guest-ion configurations and the accompanying structural and electronic responses of the host framework. Electrochemical measurements reveal substantial diffusion-controlled contributions for both Na+ and Mg2+, whereas Ca2+ exhibits predominantly surface-controlled behavior. Atomic-resolution imaging demonstrates that, despite their comparable macroscopic kinetic contributions, Na+ and Mg2+ exhibit distinctly different microscopic insertion modes: Na+ homogeneously occupies the tunnel-center sites throughout the particles, whereas Mg2+ occupies both tunnel-center and off-center sites, with pronounced near-surface enrichment accompanied by local tunnel distortion. In contrast, the Ca2+ reaction is largely confined to the near-surface region, with no clearly detectable occupation of the internal tunnel sites. Soft X-ray absorption spectroscopy further reveals spatially heterogeneous Mn reduction, with more pronounced reduction near the structurally less constrained particle surface. These results establish a direct relationship between guest-ion occupation modes, electrochemical reaction pathways, and the resulting structural evolution, providing an atomic-scale design principle for engineering tunnel-type host materials for multivalent-ion insertion.

Rechargeable magnesium batteries (RMBs) have faced challenges in utilizing oxide cathodes due to the inherently sluggish Mg diffusion and poor compatibility with electrolytes, despite the high redox potential. Herein, we present a prototype RMB that is operational at room temperature, consisting of a nanoparticulate amorphous oxide cathode, fluorinated alkoxyborate (Mg[B(HFIP)₄]₂) as the electrolyte, and a Mg metal anode. The amorphous Mg _x Ti_1/9Mo_2/9O cathode contains a considerable free volume formed by ion exchange between monovalent and divalent cations, facilitating Mg diffusion and eventually realizing reversible Mg insertion/extraction at room temperature. The reasonable compatibility of the present cathode with the electrolyte enables full cell operation with a Mg metal anode, and various analyses have demonstrated that Mg intercalation is responsible for the battery performance. The discharging capacity is ~150 mAh g⁻¹, and 70 mAh g⁻¹ is maintained after 200 cycles. These findings demonstrate the feasibility of RMBs with oxide cathodes that are operational at room temperature.

Oxide cathode materials are promising candidates for high-energy-density rechargeable magnesium batteries (RMBs) due to their high electrode potential. However, it is still challenging to reversibly insert/extract Mg into/from oxide cathodes since Mg insertion generally induces a partially irreversible phase transformation to a rocksalt structure (rocksalt transformation). This rocksalt transformation generally degrades the cyclability because the densely packed cations in the rocksalt structure impede the Mg migration. We reported that a defect spinel oxide of ZnMnO₃ successfully suppressed the rocksalt transformation within a limited capacity of ~70 mAh g^-1 [1]. However, the excessive Mg insertion resulted in the rocksalt transformation, limiting the available capacity. Elucidating the activation mechanism of the rocksalt oxide cathodes for RMBs is of great importance in circumventing this issue and ultimately utilizing rocksalt oxides themselves as promising cathode materials for RMBs. To tackle this problem, we designed a disordered-rocksalt oxide of Mg_0.35Li_0.3Cr_0.1Mn_0.05Fe_0.05Zn_0.05Mo_0.1O (M7O), in which initially contained Li is extracted in the first charging to yield substantial cation vacancies that facilitate the Mg migration in the structure, eventually activating the reversible Mg insertion/extraction in the subsequent cycles [2]. High configurational entropy derived from many constituent cations also plays a pivotal role. For example, high valence cations of Cr(III) and Mo(IV) do not solely form the rocksalt oxides. However, these elements were successfully incorporated into the structure. Furthermore, the formed vacancies were expected to be stabilized by high configurational entropy during the battery operation. The single-phase oxide of the disordered rocksalt structure was successfully synthesized using the Pechini method. The reversible discharge capacity of ~80 mAh g^-1 was constantly observed at a current density of 10.4 mA g^-1 at 90 °C. Inductively coupled plasma optical emission spectroscopy confirmed the Li extraction in the 1st charging and subsequent reversible Mg insertion/extraction via the cation vacancies formed by the Li extraction, which was further corroborated by X-ray diffraction. X-ray absorption spectroscopy indicated that Mn, Fe, and Mo compensate for the charge upon charging and discharging by changing their valence states, while those of Cr and Zn hardly changed. The ab initio calculations demonstrated that adjacent two cation vacancies significantly decrease the activation energy of the Mg migration in the rocksalt structure from ~2000 to 800 meV. The composition range to ensure that such a fast diffusion path is thoroughly connected in the particle was evaluated using the percolation theory. The present study opens a new frontier of the oxide cathodes for RMBs. [1] K. Shimokawa et al., Advanced Materials 33, 2007539 (2021). https://onlinelibrary.wiley.com/doi/10.1002/adma.202007539 [2] T. Kawaguchi et al., J. Mater. Chem. A (2024). https://pubs.rsc.org/en/content/articlelanding/2024/ta/d3ta07942b Image caption: (Left) Schematic of the reversible Mg insertion/extraction mechanism in the present rocksalt oxide cathode. (Top right) Energy dispersive spectroscopy images of the constituent elements. (Bottom right) Charge/discharge curves of the present cathode material. Figure 1

Lithium-rich layered oxides (LLOs) have attracted attention as high-capacity cathode materials for lithium-ion secondary batteries. Its unique charge-compensation mechanism and structure changes during delithiation and lithiation yield a large capacity. LLOs reversibly extract and insert ~1 mol of Li (> 250 mAh/g) per Li_1+x Me _{1 − x} O₂ (Me=Ni, Co, Mn) notation. In contrast, conventional layered oxides such as LiCoO₂ reversibly extract and insert ~0.6 mol (~160 mAh/g), which is limited by the irreversible change of the host structure by the excessive Li extraction. However, such a large amount of delithiation in LLOs still induces various structural changes. Although various structure analyses, including conventional XRD, have revealed the structural changes of LLOs accompanying charging and discharging, the inherently disordered structure of LLOs requires a more elaborate experimental approach to elucidate the relationship between the structural changes and electrode properties. Because conventional XRD only provides structure information of the well-ordered crystalline phase, the structure evolution of LLOs and their charging/discharging mechanisms should be elucidated to reveal such disordered structures. Pair distribution function (PDF) analysis, which is an advanced XRD technique using X-ray total scattering, enables quantitative observations of the short-, medium-, and long-range orders with high spatial resolutions in real space even from highly disordered structures simultaneously. In addition, spectroscopic analyses have revealed the unique charge compensation mechanisms associated with the high capacity of LLOs. In LLOs, the contribution of O anions is a key factor for the reversible high capacity. Another consideration is the valence bands of both O and Me from simultaneous observations of constituent elements, since the O 2p and Me 3d orbitals are hybridized. Soft X-ray emission spectroscopy (SXES) is a useful technique to observe the electronic structure of multiple elements simultaneously. Among them, electron-induced X-ray emission spectroscopy (EXES) is an accessible technique using laboratory X-ray or electron beam sources for excitation. In this study, we performed EXES and PDF analyses along with X-ray photoelectron spectroscopy (XPS) on the same states of samples for the LLO cathode, 0.4Li₂MnO₃−0.6LiNi_0.5Mn_0.5O₂, to reveal the relation between the charge compensation mechanism and changes in the crystal structure. The PDF analysis suggested that charging caused Me cations to migrate to the octahedral and tetrahedral sites in the Li layer. EXES independently observes the energy distribution of each orbital, revealing their specific roles in the charge compensation accompanying electrochemical charging and discharging. EXES with the aid of XPS revealed that the charge was compensated by Ni, Mn, and O. The contribution from each element depended on the charging/discharging state. The changes in the electronic/crystalline structures that yield the unique charge-compensation mechanism and the large capacity of the LLO electrode will be discussed.

Compositional distribution, strain, and defects in alloy nanoparticles significantly affect the catalytic properties for a wide range of applications such as gas reforming, fuel cells, electrolysis, etc. while various alloy compositions of nanoparticles have been investigated to improve performance and reduce the amount of precious metals used. It was reported that the composition distribution of the alloy particles, expectedly homogeneous in the whole particle, changed dynamically depending on the operating environment, resulting in altering the catalytic activity [1]. Furthermore, defects such as dislocations, grain boundaries, and elastic strain of the crystalline lattice also significantly affect catalytic properties. Since these factors dynamically change and evolve under operating environments, in situ measurements are essential to reveal these factors and ultimately to exploit them to enhance the catalytic properties. Bragg coherent diffraction imaging (BCDI) is a lens-less imaging method that measures speckle patterns near Bragg diffraction from a crystalline sample and reconstructs the 3D particle images. Since Bragg diffraction is inherently sensitive to displacements of the crystal lattice, BCDI yields the displacement field within the sample as well as the electron density, i.e., the “shape” of the particle. This displacement field information further allows us to evaluate strain distribution, dislocations, and composition distribution in the particle. BCDI is also advantageous for in situ measurements because the 3D image can be obtained by rotating the sample by only a few degrees, which eases the geometrical constraint of an electrochemical cell for the measurement. This is in contrast to conventional 3D imaging techniques such as X-ray CT, which usually requires 180 degrees of sample rotation. Furthermore, it is expected that the measurement capabilities of BCDI will be rapidly advanced by upgrading synchrotron radiation facilities such as APS-U, which delivers high-flux coherent X-rays. This should expand the applications of BCDI to wider electrochemical materials. In this presentation, we will briefly overview the principle of BCDI followed by specific examples of in situ measurements of catalytic alloy particles [2-4] conducted at 34-ID-C at Advanced Photon Source. [1] F. Tao et al., Science 322, 932 (2008). [2] T. Kawaguchi et al., Phys. Rev. Lett. 123, 246001 (2019). [3] T. Kawaguchi et al., Nano Lett. 21, 5945 (2021). [4] D. Sheyfer, R. G. Mariano, T. Kawaguchi, et al., Nano Lett. 23, 1 (2023).

Oxide cathode materials have promising applications in rechargeable magnesium batteries (RMBs) due to their high redox potential, which allows the exploitation of the low potential of Mg metal anodes and the eventual realization of RMBs with high energy densities. However, the capacity and cyclability of oxide cathodes, such as spinel oxides, are frequently limited by the irreversible formation of rocksalt oxides during discharging because the rocksalt structure with densely packed cations hinders subsequent facile Mg extraction by charging, eventually leading to electrode deterioration. It is, therefore, of great importance to reveal a mechanism that realizes reversible Mg extraction/intercalation in rocksalt oxides to enhance the conventional oxide cathodes and exploit the rocksalt oxides as novel cathode materials for RMBs. Herein, we show an activation mechanism of rocksalt oxides as a cathode material for RMBs by exemplifying Li-extracted defect disordered rocksalt oxide obtained from Mg_0.35Li_0.3Cr_0.1Mn_0.05Fe_0.05Zn_0.05Mo_0.1O, which is capable of reversible Mg insertion/extraction. Extracting the Li cations in the first charging leads to a substantial amount of cation vacancies in the rocksalt structure, facilitating Mg diffusion in subsequent cycles. This vacancy is secured even after fully discharging the material by inserting Mg into the structure because of the lowest possible valence states of the constituent transition metal cations, contributing to the reversible charging/discharging of this material.

Stable photocharging has been demonstrated by applying an appropriate cation for spinel LiMn _1.5 M _0.5 O ₄ . This work paves the way for designing spinel-oxide cathode materials to develop high-performance photo-rechargeable batteries.