Rechargeable batteries with high energy density, long cycle life, and low cost are considered key enablers for sustainable consumer electronics, electric vehicles (EVs), and smart grid energy storage. Lithium-ion batteries (LIBs) have been emerged

Spectroscopic and X-ray diffraction operando techniques were used to investigate polycrystalline antiperovskite (Li 2 Fe)SO as cathode materials in a Li-battery setup. During Li removal, several intermediate, relatively stable phases exist. At low charging, Fe is oxidized from +2 to +3, but at higher charging, S 2− is also partly oxidized to elemental sulfur, suggesting a cathode bifunctionality, and both redox processes seem reversible. On cycling (Li 2 Fe)SO in a battery, spectroscopy data suggest that a part of the Fe atoms irreversibly vacate the high-symmetry positions in the crystal lattice, in line with the broadening of X-ray diffraction peaks. Instead, new, relatively broad reflections appear in the X-ray patterns that might be explained by a crystallographic superstructure, corresponding to a doubling of the cubic unit cell axis, but the peak broadness indicates a lowering in crystallographic symmetry. Using a standard electrolyte and a moderate charging rate of C/10 results in typical capacity loss per cycle, but by using an electrolyte with low sulfur solubility, the (Li 2 Fe)SO cathode is stabilized, and charge densities of more than 200 mAh g −1 at a 1C charging rate are obtained. Additionally, a Li-deficient precursor (Li 0.8 Fe)SO served as a cathode material in a Na battery, providing presumably reversible Na intercalation and removal.

A series of solid solutions (Li 2 Fe 1−y Mn y )SO with a cubic antiperovskite structure was successfully synthesized. The composition (Li 2 Fe 0.5 Mn 0.5 )SO was intensively studied as a cathode in Li-ion batteries showing a reversible specific capacity of 120 mA h g −1 and almost a 100% Coulombic efficiency after 50 cycles at 0.1C meaning extraction/insertion of 1 Li per formula unit during 10 h. Operando X-ray absorption spectroscopy confirmed the redox activity of both Fe 2+ and Mn 2+ cations during battery charge and discharge, while operando synchrotron X-ray diffraction studies revealed a reversible formation of a second isostructural phase upon Li-removal and insertion at least for the first several cycles. In comparison to (Li 2 Fe)SO, the presence of Mn stabilizes the crystal structure of (Li 2 Fe 0.5 Mn 0.5 )SO during battery operation, although post mortem TEM studies confirmed a gradual amorphization after 50 cycles. A lower specific capacity of (Li 2 Fe 0.5 Mn 0.5 )SO in comparison to (Li 2 Fe)SO is probably caused by slower kinetics, especially in the two-phase region, as confirmed by Li-diffusion coefficient measurements.

A known strategy for improving the properties of layered oxide electrodes in sodium-ion batteries is the partial substitution of transition metals by Li. Herein, the role of Li as a defect and its impact on sodium storage in P2-Na 0.67 Mn 0.6 Ni 0.2 Li 0.2 O 2 is discussed. In tandem with electrochemical studies, the electronic and atomic structure are studied using solid-state NMR, operando XRD, and density functional theory (DFT). For the as-synthesized material, Li is located in comparable amounts within the sodium and the transition metal oxide (TMO) layers. Desodiation leads to a redistribution of Li ions within the crystal lattice. During charging, Li ions from the Na layer first migrate to the TMO layer before reversing their course at low Na contents. There is little change in the lattice parameters during charging/ discharging, indicating stabilization of the P2 structure. This leads to a solidsolution type storage mechanism (sloping voltage profile) and hence excellent cycle life with a capacity of 110 mAh g -1 after 100 cycles. In contrast, the Li-free compositions Na 0.67 Mn 0.6 Ni 0.4 O 2 and Na 0.67 Mn 0.8 Ni 0.2 O 2 show phase transitions and a stair-case voltage profile. The capacity is found to originate from mainly Ni 3+ /Ni 4+ and O 2-/O 2-δ redox processes by DFT, although a small contribution from Mn 4+ /Mn 5+ to the capacity cannot be excluded.

in further markets, such as electromobility and large-scale grid storage. Although LIBs offer high energy density and have reached a high level of maturity, there are still many technological challenges to meet established user habits and increasing demands. [1] Among the remaining challenges of LIBs, one of the most crucial issues is the electrochemical instability of anodes toward electrolytes. When using anode materials with favorably low electrode potentials close to Li/Li + , common liquid electrolytes are decomposed. Ideally, this leads to the formation of a stable so-called solid-electrolyte interphase (SEI), preventing further electrolyte decomposition and leading to stable cycling performance. Furthermore, the SEI has decisive influence on the charge/discharge kinetics of the battery, as this is where the Li-ions overcome the interface between the electrolyte and the electrode. Accordingly, the SEI is a key component that determines the performance of LIBs. The "natural SEI" that forms at the anode/electrolyte interface during initial cycling represents a thin layer made of salts, oxides, polymers. [2] The conceptual model of SEI was introduced by Peled in 1979 [3] and further developed by other groups. [4][5][6][7][8][9][10][11][12] According to this model, natural SEIs possess only ionic conductivity, while serving as a barrier to electron transfer. In this regard, the thickness and conformity of SEI An intrinsic challenge of Li-ion batteries is the instability of electrolytes against anode materials. For anodes with a favorably low operating potential, a solid-electrolyte interphase (SEI) formed during initial cycles provides stability, traded off for capacity consumption. The SEI is mainly determined by the anode material, electrolyte composition, and formation conditions. Its properties are typically adjusted by changing the liquid electrolyte's composition. Artificial SEIs (Art-SEIs) offer much more freedom to address and tune specific properties, such as chemical composition, impedance, thickness, and elasticity. Art-SEIs for intercalation, alloying, conversion and Li metal anodes have to fulfil varying requirements. In all cases, sufficient transport properties for Li-ions and (electro-)chemical stability must be guaranteed. Several approaches for Art-SEIs preparation have been reported: from simple casting and coating techniques to elaborated Phys-Chem modifications and deposition processes. This review critically reports on the promising approaches for Art-SEIs formation on different type of anode materials, focusing on methodological aspects. The specific requirements for each approach and material class, as well as the most effective strategies for Art-SEI coating, are discussed and a roadmap for further developments towards next-generation stable anodes are provided.

Two new structural forms of Na x Co0.5Ti0.5O2, the layered O3- and P3-forms, were synthesized and comprehensively characterized. Both materials show electrochemical activity as electrodes in Na-ion batteries. During cell charging (desodiation of the Na x Co0.5Ti0.5O2 cathode), we observed a structural phase transformation of O3-Na0.95Co0.5Ti0.5O2 into P3-Na x Co0.5Ti0.5O2, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-Na0.65Co0.5Ti0.5O2. During Na insertion (cell discharging), the reconversion of the P3-form into O3-Na x Co0.5Ti0.5O2 was impeded for both materials and occurs well below 1 V versus Na+/Na only. The reconversion is hindered by the charge and spin transfers of Co (LS-Co3+ → HS-Co2+) and by a significant unit cell volume expansion at the P3 → O3 transformation, as revealed from the magnetization, crystallographic, and spectroscopic studies. As the kinetics of such transformations depend on numerous parameters such as time, temperature, and particle size, a large cell overpotential ensues. An extended cutoff voltage at 0.2 V versus Na+/Na during discharging allows to complete the P3 → O3 transformation and increases the specific discharging capacity to 200 mA h g–1. Moreover, a quasi-symmetrical full cell, based on the O3- and P3-forms, was designed, eliminating safety concerns associated with sodium anodes and delivering a discharge capacity of 130 mA h g–1.

Blended electrode materials containing high‐capacity silicon (Si) and robust graphite (Gr) materials are considered advanced alternatives to pure graphite electrodes used in Li‐ion batteries. Understanding the component‐specific lithiation and delithiation behavior and electrochemical interactions between the blended materials is of crucial importance for targeted optimization of composition and microstructural design, yet hardly addressed to date. Herein, a model‐like Si/Gr blended electrode and special electrochemical cell are introduced to directly capture the component specific behaviors for the first time. This includes studies of the formation cycles, the reaction distribution between Si and Gr, the component‐specific contributions to the capacity at different charge and discharge rates, and the internal dynamics during pulse loads and subsequent relaxation. The deconvolution of the components’ behavior during operation provides fundamental insights that contribute to a profound understanding and targeted optimization of Si/Gr blended electrodes. Furthermore, the application of the presented experimental approach can serve scientists to identify and study other advanced materials combinations as blended electrodes for rechargeable batteries.

O3-type layered NaNi0.5Ti0.5O2, which has been reported previously as a promising cathode material for Naion batteries, has been characterized using comprehensive operando techniques combined with electrochemical and magnetization measurements. Synchrotron diffraction revealed a reversible O3-P3 transformation during charge and discharge without any intermediate phases, which stands in contrast to NaNiO2 and NaNi0.5Mn0.5O2. X-ray absorption studies showed that the electrochemical process in the potential window of 1.5-4.2 V vs. Na + /Na is sustained exclusively by Ni oxidation and reduction while Ti remains inactive. These findings are further supported by ex situ magnetization measurements, yielding a lower paramagnetic moment in the charged state. On the basis of these insights, we elaborate on the beneficial stabilizing effect of Ti. However, a strong Crate dependence for NaNi0.5Ti0.5O2 and NaNi0.5Mn0.5O2 during cycling known from the literature points at a rather high influence of the original structure stacking and the associated Na migration paths. ASSOCIATED CONTENT Supporting Information. Structural parameters of O3 and P3 phases, OCV evolution, XAS edge shifts, Forward Fourier Transformed XAS spectra, EXAFS fit parameters, inverse susceptibility plots, XRD in cycled state and cycling performance. This material is available free of charge via the Internet at http://pubs.acs.org.