The series of LiMO2 (M: transition metal) materials are highly relevant as cathode materials of Li-ion batteries. The stability of such systems remains an important factor for their usability in batteries, and depends strongly on the electronic configuration of the transition-metal ions. In particular, the promising class of multi-transition-metal systems exhibits complicated valence states due to intermetallic charge transfer and charge disproportionation. Here we perform a systematic study on the valence of the transition-metal ions using x-ray absorption spectroscopy on the M-L2,3 edges and O-K edges. In Li(Ni0.5Mn0.5)1-xCoxO2 we established that the valence is Co3+ and Ni0.52+Mn0.54+ throughout the whole series. Meanwhile, in LiNi1-xCoxO2 we found that the Ni displays a behavior consistent with a charge disproportionated negative charge transfer system, and that with increased concentration of Co3+, the disproportionation signal decreases. Since the number of O 2p holes also gets reduced, we infer that the material will also become more unstable.

Redox-flow batteries (RFBs) are promising electrochemical energy storage devices to load-level intermittent power from renewable energy. In particular, aqueous RFBs using aqueous electrolytes possess several advantages over nonaqueous ones, such as low fabrication cost, nontoxicity, safety, and environmental benignity. Therefore, developing high-performance, abundant, less-expensive iron-based catholytes for aqueous RFBs is essential toward their wide deployment in a power grid. In this Perspective, we summarize the recent progress of iron-based catholytes for aqueous RFBs. We emphasize that iron-based catholytes possess widely ranged redox potentials (−1.0 to 1.5 V vs standard hydrogen electrodes) and solubility in water (0.2–4.0 mol L−1), thereby providing a wide range of cell performance. The molecular design, such as ligand functionalization, counter ion mixing, and asymmetrization, allows for rationally improving solubility, redox potential, and energy density. Furthermore, we demonstrate a simple evaluation method of the redox potential of iron-based catholytes using the calculated energy levels of the lowest unoccupied molecular orbital of ligand molecules. Finally, we rationalize the design strategy of iron-based catholytes for advanced aqueous RFBs.

Two-dimensional transition-metal carbides/nitrides (MXenes) have been intensively investigated as electrode materials for electrochemical energy storage devices, such as batteries and supercapacitors, due to their high capacitance, high-rate capability, and good cycle stability. Although MXenes possess various surface termination groups (e.g., -O, -OH, -F, -Cl, and -Br) that directly interact with adsorbed cations to exhibit charge transfer, the influence of each surface termination group on the electrochemical properties of MXene remains elusive because of difficulty in achieving exclusively modified termination. Herein, we report the electrochemical properties of MXenes with surface termination groups controlled by using fluorine-based aqueous solutions and molten salts as etchants. In aqueous electrolytes, Ti3C2Cl x and Ti3C2Br x synthesized using molten salts show no electrochemical activity in contrast to Ti3C2T x (T = O, OH, F, and Cl). Meanwhile, in a nonaqueous electrolyte, Ti3C2Cl x and Ti3C2Br x undergo amorphization upon the initial lithiation. These results suggest that the -O, -OH, and -F terminations play an important role in the electrochemical properties of MXene electrodes.

Increasing the energy density of lithium-ion batteries is an important step towards flexible electricity supply, which can be achieved by developing large-capacity positive electrodes. Lithium-rich oxides have been a longstanding research target because of their large capacity involving extra oxygen-redox reactions. In this work, we report the synthesis, electrochemical properties, electronic structure, and structural evolution of O2-type lithium-rich layered oxide Li_1.22‒xRu_0.78O₂. A robust Ru‒O layered framework without Ru migration allows for unveiling the solid-state electrochemistry of O2-type lithium-rich layered oxides with possibility of a large yet stable extra capacity for oxygen-redox reaction. Using a combination of X-ray photoelectron spectroscopy, X-ray absorption/emission spectroscopy, and in situ/ex situ X-ray diffraction, we clarified that O2-Li_1.22‒xRu_0.78O₂ delivers a large capacity of 200 mAh g^‒1 in association with Ru⁵⁺/Ru⁴⁺ and Ru⁴⁺/Ru³⁺ two-electron redox reactions under a solid-solution process, but with no contribution from the extra oxygen-redox reaction.

Increasing the energy density of sodium-ion battery systems requires highvoltage cathode materials to compensate for the inherently higher redox potential of the Na/Na+ couple compared to the Li/Li+ couple (difference of 0.3 V). Distinct from the high-voltage (>4.2 V) operation using late transition metals (Co-3+Co-/(2+) or Ni3+/Ni2+) in previously reported polyanionic compounds, here we identify the record-high Cr4+/Cr3+ (3d(2)/3d(3)) redox potential in Na3-xCr2(PO4)(2)F-3 (0 < x < 1) at 4.7 V vs Na/Na+. Despite higher d-band position at early 3d transition metal with smaller effective nuclear charge compared to late transition metals, Cr 3d(t(2g)) in less than half-filled state possesses no energy level increments arising from crystal field splitting and intra-orbital Coulombic penalty, leading to extremely high voltages of the Cr4+/Cr3+ (3d(2)/3d(3)) redox couple.

<title>Abstract</title>Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis &gt;0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O₂²⁻ dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na_{2-<italic>x</italic>}Mn₃O₇, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O^−• was identified in Na_{2-<italic>x</italic>}Mn₃O₇. Computations reveal that O^−• is thermodynamically favorable over the peroxide-like O₂²⁻ dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.

<p>NASICON-type Na₂CrTi(PO₄)₃ offers a stable redox reaction of Cr⁴⁺/Cr³⁺ at 4.5 V <italic>vs.</italic> Na/Na⁺.</p>