Matthew A. Wright

Layered Na$M$O$_2$ oxides are attractive cathode materials for Na-ion batteries due to their exceptional tunability of redox chemistry, voltage, and structural stability via transition-metal composition and layer ordering. Here, we demonstrate that microwave synthesis provides a rapid and efficient route to the preparation of Na$M$O$_2$ materials. Six oxide compositions in the Na--Mn--Fe--Ni--Cu--O phase space were synthesized on 7 min to 10 min timescales. Synchrotron X-ray diffraction, Raman spectroscopy, and electron microscopy confirm well-ordered layered frameworks composed of micrometer-scale particles. Compositional analysis using inductively coupled plasma optical emission spectroscopy and X-ray fluorescence shows that near-target stoichiometries are obtained without the excess sacrificial Na precursors typically required in conventional solid-state routes, consistent with suppressed Na volatilization during the short reaction. The rapid reaction thereby provides access to highly sodiated O3 phases that can be difficult to obtain under equilibrium conditions. The high mobility of Na$^+$, combined with the large size mismatch between Na and $M$, promotes the formation of well-ordered frameworks with no evidence of antisite disorder between Na and the transition metal ions. Electrochemical measurements reveal composition-dependent Na-ion (de)insertion behavior: Mn/Fe-rich compositions deliver high capacity but exhibit pronounced hysteresis, while Ni substitution increases operating potential and suppresses abrupt phase transitions. Dopant levels of Cu, previously reported to stabilize the structure, are readily incorporated and further smooth the potential profiles, albeit at the expense of capacity. These results demonstrate that microwave synthesis enables rapid and reliable access to O3--Na$M$O$_2$ phases, emphasizing its suitability as a platform for the rapid exploration of compositions.

Layered Na--Mn--Fe--O oxides are among the leading cathode candidates for Na-ion batteries owing to their low cost, earth-abundant constituents, and competitive electrochemical performance. Their rich structural chemistry, spanning O-type (octahedral Na) and P-type (prismatic Na) polymorphs with distinct stacking sequences, offers multiple avenues for property optimization. However, controllable access to specific polymorphs, particularly nonequilibrium ones, remains a synthetic challenge. Here we show that susceptor-assisted microwave heating can produce the P2, O3, and P3 phases of layered Na--Mn--Fe--O cathodes in minutes, with phase selectivity set by the nominal Na/$M$ ratio of the precursor alone. The rapid reaction times suppress Na volatilization, preserving the target stoichiometry without the excess sacrificial Na precursors typically required by conventional solid-state routes. The high mobility of Na$^+$, combined with the large size difference between Na and Mn/Fe, yields well-ordered frameworks with no evidence for antisite disorder between Na$^+$ and the transition metals. Accessing all three structures from one rapid route enables a controlled comparison of how stacking sequence governs electrochemistry. On cycling, we observe Fe$^{3+}$/Fe$^{4+}$ redox and behavior consistent with Fe$^{3+}$ migration into the Na layer at high voltage in all three polymorphs, with P2 showing the most stable high-voltage cycling. Operando diffraction shows that all three converge toward disordered O-type stacking on deep desodiation, and voltage-resolved distribution-of-relaxation-times analysis shows that Na$^+$ diffusion kinetics are governed by stacking transitions and Na--vacancy ordering, with pronounced kinetic barriers at glide-type structural transitions. These results establish microwave synthesis as a versatile route to both equilibrium and nonequilibrium layered Na cathodes and clarify how stacking sequence and local disorder jointly control redox behavior and ion transport.

A previously unreported low-temperature phase transition in bismuth halide double perovskite Cs$_2$AgBiCl$_6$ is reported, thereby establishing trends in the structural ground states across Cs$_2$NaBiCl$_6$, Cs$_2$AgBiCl$_6$, and Cs$_2$AgBiBr$_6$. Using the combined toolkit of variable-temperature synchrotron X-ray and neutron powder diffraction, Raman spectroscopy, and density-functional theory-based electronic structure modeling, we demonstrate a cubic $\rightarrow$ tetragonal $I4/m$ transition upon cooling with distinct onset temperatures. Neutron powder diffraction refinements and DFT calculations assign the low-temperature phase of Cs$_2$NaBiCl$_6$ to $I4/m$, rather than the previously reported $I4/mmm$ ground state. Cs$_2$AgBiCl$_6$ is also found to transform to a structure crystallizing in the $I4/m$ space group at low temperatures. Temperature-dependent Raman data and density-functional-theory-based modeling capture the softening and freezing of octahedral tilt modes and quantify relative instabilities. Solid-state nuclear magnetic resonance spectroscopy at room temperature completes the characterization and helps underpin the subtle differences in the covalency across the compounds. Trends in the phase transition temperature $T_{\rm s}$ and tilt magnitudes emerge from coupled effects of halide identity, $M$(I)--site bonding character, and a mismatch between interatomic distances. These results establish the structure--dynamics--bonding framework for tuning tilt-driven instabilities in halide double perovskites.

While redox activity in electrode materials is well studied in the context of energy storage, its connection to magnetic and electronic phenomena is perhaps not as widely appreciated. Electrochemistry can be leveraged to provide dynamic control over emergent electronic and magnetic behavior in solid materials. In this perspective we review how alkali-ion (de)insertion has been utilized to alter spin states, induce charge ordering, and drive transitions between insulating, metallic, and magnetically frustrated phases for a range of battery materials, including layered LiCoO$_2$ and LiNiO$_2$, olivine-LiFePO$_4$, and spinel LiMn$_2$O$_4$. We also describe less conventional systems like Wadsley--Roth shear phases and breathing-kagome networks to showcase the broader potential for quantum control via redox chemistry. The evolution of emergent properties like spin frustration, Jahn--Teller distortions, and electron localization/delocalization is related to ion content and crystallographic structure. We conclude that electrochemistry can provide a powerful platform for exploring and manipulating physical phenomena in correlated and quantum materials and discuss some experimental challenges.

Despite remaining enigmatic, strong hydrogen bonding provides an advanced design handle for tailoring the properties of functional materials. Here, 3$R$--(H/D)RhO$_2$ delafossites (prepared by ion exchange of Na$^+$ from NaRhO$_2$) contain H/D in linear coordination with O, linking Rh$^{\rm III}$O$_2$ layers. Bragg and real-space X-ray and neutron scattering analysis, vibrational and solid-state NMR spectroscopy, and density functional theory (DFT)--based electronic structure calculations have been employed to understand the nature of the hydrogen bonding. Despite short distances between H/D and the two O to which they are bonded, a clear double-minimum corresponding to a shorter and longer (H/D)--O distance is established. The triangular lattices formed by H/D appear to display ice-like disorder, corroborated by low-temperature heat capacity measurements.

Magnesium batteries offer a promising alternative to lithium-ion systems, but suitable electrodes remain limited. Covalent organic frameworks (COFs) are attractive candidates due to their structural tunability and open channels for ion transport. We report a pyrene-4,5,9,10-tetraone COF composite with carbon nanotubes as a Mg electrode, delivering 70 mAh g$^{-1}$ at 200 mA g$^{-1}$ and operating at 1.3 V. In situ Raman spectroscopy confirms carbonyl-centered redox on pyrene tetraone, supporting a Mg$^{2+}$-driven carbonyl reduction. Compared with Li$^+$, only partial carbonyl utilization occurs, attributed to steric and electrostatic constraints of divalent Mg$^{2+}$. This incomplete conversion to magnesium-enolate inspires future work toward structural optimization.

V$_2$Se$_9$ displays facile electrochemical insertion of up to 1.6 Mg$^{2+}$ per unit formula with fast diffusion (coefficients of 10$^{-10}$--10$^{-12}$ cm$^2$ s$^{-1}$) surpassing best-in-class materials like Mo$_6$S$_8$. Detailed structural characterization of synchrotron X-ray diffraction data with ab initio Maximum Entropy Method analysis reveals Mg$^{2+}$ insertion into octahedral sites within the large vdW space between [V$_4$Se$_{18}$]$_\infty$ chains. Fast rate performance is attributed to low structural perturbation and low diffusion barriers, calculated by bond valence pathway analysis, resulting from the low charge-per-size of anionic selenium. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy reveal that reversible insertion of Mg$^{2+}$ is facilitated by V$^{5+}$/V$^{3+}$ redox. V$_2$Se$_9$ demonstrates that selenides, despite their larger molecular weight, offer potential as fast-rate positive electrode materials for magnesium batteries over well-explored oxides and sulfides.

Magnesium batteries attract interest as alternative energy-storage devices because of elemental abundance and potential for high energy density. Development is limited by the absence of suitable cathodes, associated with poor diffusion kinetics resulting from strong interactions between Mg$^{2+}$ and the host structure. V$_2$PS$_{10}$ is reported as a positive electrode material for rechargeable magnesium batteries. Cyclable capacity of 100 mAh g$^{-1}$ is achieved with fast Mg$^{2+}$ diffusion of 7.2 $\times$ 10$^{-11}$--4 $\times$ 10$^{-14}$ cm$^2$ s$^{-1}$. The fast insertion mechanism results from combined cationic redox on the V site and anionic redox on the (S$_2$)$^{2-}$ site; enabled by reversible cleavage of S--S bonds, identified by X-ray photoelectron and X-ray absorption spectroscopy. Detailed structural characterisation with maximum entropy method analysis, supported by density functional theory and projected density of states analysis, reveals that the sulphur species involved in anion redox are not connected to the transition metal centres, spatially separating the two redox processes. This facilitates fast and reversible Mg insertion in which the nature of the redox process depends on the cation insertion site, creating a synergy between the occupancy of specific Mg sites and the location of the electrons transferred.