The reversibility and cyclability of anionic redox in battery electrodes hold the key to its practical employments. Here, through mapping of resonant inelastic X-ray scattering (mRIXS), we have independently quantified the evolving redox states of both cations and anions in Na2/3Mg1/3Mn2/3O2. The bulk-Mn redox emerges from initial discharge and is quantified by inverse-partial fluorescence yield (iPFY) from Mn-L mRIXS. Bulk and surface Mn activities likely lead to the voltage fade. O-K superpartial fluorescence yield (sPFY) analysis of mRIXS shows 79% lattice oxygen-redox reversibility during initial cycle, with 87% capacity sustained after 100 cycles. In Li1.17Ni0.21Co0.08Mn0.54O2, lattice-oxygen redox is 76% initial-cycle reversible but with only 44% capacity retention after 500 cycles. These results unambiguously show the high reversibility of lattice-oxygen redox in both Li-ion and Na-ion systems. The contrast between Na2/3Mg1/3Mn2/3O2 and Li1.17Ni0.21Co0.08Mn0.54O2 systems suggests the importance of distinguishing lattice-oxygen redox from other oxygen activities for clarifying its intrinsic properties.

Though Li 2 MnO 3 was originally considered to be electrochemically inert, its observed activation has spawned a new class of Li-rich layered compounds that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compounds, the underlying origin of the initial charge capacity of Li 2 MnO 3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn 4+ , and surface degradation for Li 2 MnO 3 cathodes displaying capacities exceeding 350 mAh g −1 . Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn 4+ and bulk oxygen redox during activation of Li 2 MnO 3 . Quantitative gas-evolution and titration studies reveal that O 2 and CO 2 release accounted for a large fraction of the observed capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li 2 MnO 3 is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li 2 MnO 3 by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn 4+ valence.

Layered lithium transition metal oxides derived from LiMO 2 (M = Co, Ni, Mn, etc.) have been widely adopted as the cathodes of Li-ion batteries for portable electronics, electric vehicles, and energy storage. Oxygen loss in the layered oxides is one of the major factors leading to cycling-induced structural degradation and its associated fade in electrochemical performance. Herein, we review recent progress in understanding the phenomena of oxygen loss and the resulting structural degradation in layered oxide cathodes. We first present the major driving forces leading to the oxygen loss and then describe the associated structural degradation resulting from the oxygen loss. We follow this analysis with a discussion of the kinetic pathways that enable oxygen loss, and then we address the resulting electrochemical fade. Finally, we review the possible approaches toward mitigating oxygen loss and the associated electrochemical fade as well as detail novel analytical methods for probing the oxygen loss.

Nickel-rich layered metal oxide LiNi 1−y−z Mn y Co z O 2 (1 − y − z ≥ 0.8) materials are the most promising cathodes for next-generation lithium-ion batteries in electric vehicles. However, they lose more than 10% of their capacity on the first cycle, and interfacial/structural instability causes capacity fading. Coating and substitution are possible direct and effective solutions to solve these challenges. In this Letter, Nb coating and Nb substitution on LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) is easily produced through a scalable wet chemistry method followed by sintering from 400 to 800 °C. A Li-free Nb oxide treatment is found to remove surface impurities forming a LiNbO 3 /Li 3 NbO 4 surface coating, to reduce the first capacity loss and to improve the rate performance. Nb substitution stabilizes the structure, as evidenced by less heat evolution on heating, thus providing better long cycling stability with a 93.2% capacity retention after 250 cycles.

Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; however, its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity. Herein we report a sodium rich disordered birnessite (Na0.27MnO2) for aqueous sodium-ion electrochemical storage with a much-enhanced capacity and cycling life (83 mAh g−1 after 5000 cycles in full-cell). Neutron total scattering and in situ X-ray diffraction measurements show that both structural water and the Na-rich disordered structure contribute to the improved electrochemical performance of current cathode material. Particularly, the co-deintercalation of the hydrated water and sodium-ion during the high potential charging process results in the shrinkage of interlayer distance and thus stabilizes the layered structure. Our results provide a genuine insight into how structural disordering and structural water improve sodium-ion storage in a layered electrode and open up an exciting direction for improving aqueous batteries.

Tackling the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot heterostructures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photocatalytic proton reduction. Topochemical synthesis yields a metastable β-Sn 0.23 V 2 O 5 compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed, and β-Sn 0.23 V 2 O 5 /CdSe heterostructures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast subpicosecond hole transfer. The β-Sn 0.23 V 2 O 5 /CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semiconductor components to effect rapid charge separation necessary for photocatalysis.

Resonant inelastic x-ray scattering (RIXS) at the O K-edge is considered a prime technique to identify bulk oxidized oxygen formation but its fundamental interpretation is not straightforward. In this study, we intentionally induce RIXS signatures of oxidized oxygen upon beam exposure in LiAlO 2 polymorphs that are easily distinguished due to their wide band gaps. After careful consideration of beam exposure effects on LR-NMC (Li[Li 0.144 Ni 0.136 Mn 0.544 Co 0.136 ]O 2 ), we conclude that oxidized oxygen features are inherent at high states of charge and are lost upon aggressive beam exposure. The extracted oxidized oxygen lineshapes from our x-ray irradiation studies for both LiAlO 2 (induced) and LR-NMC (inherent) are found to have an additional oxidized oxygen RIXS feature not observed in O 2 gas studies. This study highlights the unique insight of O K-edge RIXS into determining the nature and stability of oxidized oxygen states.

Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes.