Materials can be transformed from one crystalline phase to another by using an electric field to control ion transfer, in a process that can be harnessed in applications such as batteries, smart windows and fuel cells. Increasing the number of transferrable ion species and of accessible crystalline phases could in principle greatly enrich material functionality. However, studies have so far focused mainly on the evolution and control of single ionic species (for example, oxygen, hydrogen or lithium ions). Here we describe the reversible and non-volatile electric-field control of dual-ion (oxygen and hydrogen) phase transformations, with associated electrochromic and magnetoelectric effects. We show that controlling the insertion and extraction of oxygen and hydrogen ions independently of each other can direct reversible phase transformations among three different material phases: the perovskite SrCoO (ref. 12), the brownmillerite SrCoO (ref. 13), and a hitherto-unexplored phase, HSrCoO. By analysing the distinct optical absorption properties of these phases, we demonstrate selective manipulation of spectral transparency in the visible-light and infrared regions, revealing a dual-band electrochromic effect that could see application in smart windows. Moreover, the starkly different magnetic and electric properties of the three phases-HSrCoO is a weakly ferromagnetic insulator, SrCoO is a ferromagnetic metal, and SrCoO is an antiferromagnetic insulator-enable an unusual form of magnetoelectric coupling, allowing electric-field control of three different magnetic ground states. These findings open up opportunities for the electric-field control of multistate phase transformations with rich functionalities.

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17–xNi0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.

This work experimentally identifies the charge-transfer energy as a key factor governing the catalytic oxygen evolution reaction (OER) activity and mechanism across a wide range of perovskite chemistries.

LiCoO 2 is a dominant cathode material for Li-ion batteries due to its high volumetric energy density, which could potentially be further improved by charging to high voltage. Practical adoption of the high-voltage charging is, however, hindered by LiCoO 2 's structural instability at the deeply delithiated state and the associated safety concerns. Here, we achieve stable cycling of LiCoO 2 at 4.6 V (vs. Li/Li +) through trace Ti-Mg-Al co-doping. By using state-of-the-art synchrotron X-ray imaging and spectroscopic techniques, we confirm the incorporation of Mg and Al into the LiCoO 2 lattice, which inhibits the undesired phase transition at voltages above 4.5 V. On the other hand, even in trace amount, Ti segregates significantly at grain boundaries and on the surface, modifying the microstructure of the particles while stabilizing the surface oxygen at high voltage. These dopants contribute though different mechanisms and synergistically promote the cycle stability of LiCoO 2 at 4.6 V.

The interaction between the transition metal 3d and the oxygen 2p states via hybridization underpins many of the phenomena in transition metal oxide materials. We report the empirical trend of this interaction using the pre-edge feature of the O Kedge X-ray absorption spectrum. Our assessment method is built on the dipole approximation and the configuration interaction between the transition metal 3d and the oxygen 2p states. We found that hybridization increases with the number of 3d electrons, consistent with the expected electronegativity trend. We support this analysis with density functional calculations, which reveal a systematic increase in the transition metal 3d and the oxygen 2p state mixing with increasing 3d-electron number. Oxidation of the transition metal was also found to increase hybridization, which we believe reflects the reduced transition metal 3d and oxygen 2p energy difference, causing increased covalency. We compare the analysis from the surface-sensitive electron-yield and the bulk-sensitive fluorescence-yield spectra, revealing that either method can be used to study the hybridization trend. We finally compare and discuss the influence of the lanthanide ions and the influence of the covalency on oxygen electrocatalysis. Our study describes an efficient and simple approach to understand the hybridization trend in transition metal oxides, which has considerable implications for electrochemical energy conversion processes.