Energy storage technology has received significant attention for portable electronic devices, electric vehicle propulsion, bulk electricity storage at power stations, and load leveling of renewable sources, such as solar energy and wind power. Lithium ion batteries have dominated most of the first two applications. For the last two cases, however, moving beyond lithium batteries to the element that lies below-sodium-is a sensible step that offers sustainability and cost-effectiveness. This requires an evaluation of the science underpinning these devices, including the discovery of new materials, their electrochemistry, and an increased understanding of ion mobility based on computational methods. The Review considers some of the current scientific issues underpinning sodium ion batteries.

Using in-depth structural and spectroscopic analysis, we unravel the nature of phenomena specific to the Fe3+/Fe4+ redox couple in P2-Na0.67−z[NiyMn0.5+yFe0.5−2y]O2.

Batteries based on sodium layered transition metal oxides are a promising alternative to current state-of-the-art lithium-ion systems for large-scale energy storage, resulting in recent intensive efforts to develop high-energy density, low cost, stable cathode materials. Some of the most promising degrade on exposure to ambient atmosphere; however, the process is not understood. Here, using neutron/X-ray diffraction coupled with mass spectroscopy and thermal analysis, we reveal the nature of the reactivity. We demonstrate the unprecedented insertion of carbonate ions in the vacancy-rich layered structure of P2-Na0.67[Mn0.5Fe0.5]­O2 on exposure to CO2 and moisture, concomitant with oxidation of Mn­(III) to Mn­(IV). The material exhibits much higher charge/discharge polarization and lower capacity than rigorously air-protected P2-Na0.67[Mn0.5Fe0.5]­O2; a detailed study by online electrochemistry mass spectroscopy reveals that the inserted carbonate ions decompose during electrochemical charging, accounting for the differences observed between the first and second cycles. Furthermore, we show that Ni-substituted materials P2-Na0.67[Ni x Mn0.5+x Fe0.5−2x ]­O2 are less prone to such reactivity and thus are more promising candidates for scalable processing. Understanding these mechanisms provides a vital guide for future sodium metal oxide battery research.

We report excellent cycling performance for P2− Na 0.6 Li 0.2 Mn 0.8 O 2 , an auspicious cathode material for sodium-ion batteries. This material, which contains mainly Mn 4+ , exhibits a long voltage plateau on the first charge, similar to that of high-capacity lithium and manganese-rich metal oxides. Electrochemical measurements, X-ray diffraction, and elemental analysis of the cycled electrodes suggest an activation process that includes the extraction of lithium from the material. The "activated" material delivers a stable, high specific capacity up to ∼190 mAh/g after 100 cycles in the voltage window between 4.6−2.0 V versus Na/Na + . DFT calculations locate the energy states of oxygen atoms near the Fermi level, suggesting the possible contribution of oxide ions to the redox process. The addition of Li to the lattice improves structural stability compared to many previously reported sodiated transition-metal oxide electrode materials, by inhibiting the detrimental structural transformation ubiquitously observed with sodium manganese oxides during cycling. This research demonstrates the prospect of intercalation materials for Na-ion battery technology that are active based on both cationic and anionic redox moieties.

We present an overview of the procedures and methods to prepare and evaluate materials for electrochemical cells in battery research in our laboratory, including cell fabrication, two- and three-electrode cell studies, and methodology for evaluating diffusion coefficients and impedance measurements. Informative characterization techniques employed to assess new materials for batteries are also described, including operando XRD, pair-distribution function analysis, X-ray photoelectron spectroscopy, and operando X-ray absorption spectroscopy. Examples of insightful information that each technique has provided in the research areas of Li-S, Na-ion, and Mg batteries are presented along with excellent references for detailed descriptions of the theory, experimental procedures, and various designs, as well as methods for data processing and analysis.

We report the synthesis and structural evolution of P2−Na 0.67 [Mn 0.66 Fe 0.20 Cu 0.14 ]O 2 during charge and discharge as a positive electrode for Na-ion batteries. Operando X-ray diffraction analysis revealed the existence of two phase transitions in the voltage window between 1.5−4.3 V. Ex situ pair distribution function analysis was used to characterize the local structure of the high-voltage phase which is disordered along the c-axis and is derived by the migration of a fraction of iron ions into the interlayer space. Operando Xray absorption spectroscopy was employed to monitor the local structural evolution of the transition metals and shows the existence of Mn 3+/4+ redox below ∼3.4 V, followed by the redox activity of Cu and Fe ions at higher voltages. However, no change was observed in the K-edge X-ray absorption near-edge spectrum of any of the transition metals at voltages above 4.1 V, where the growth of the high-voltage phase is initiated. The combination of these results implies the reversible contribution of oxide ion redox to the capacity, which is coincident with metal migration. The above processes result in voltage fading over cycling, similar to that exhibited by Li 2 MnO 3 -based positive electrode materials in Li-ion batteries.

Fortschrittliche Energiespeichertechnologien sind von immenser Bedeutung für tragbare elektronische Geräte, Elektrofahrzeuge, die Stromspeicherung in Kraftwerken und den Lastausgleich von Stromnetzen auf der Basis erneuerbarer Energien wie Solarenergie und Windkraft. Für die ersten zwei dieser Anwendungen haben sich Lithiumionenbatterien erfolgreich bewährt. Für die zwei letzteren Fälle scheint es jedoch sinnvoll, einen Schritt weiterzugehen – hin zum nächsten Alkalimetall im Periodensystem, dem Natrium. Eine flächendeckende Einführung von Natriumionenbatterien erfordert deren intensive Erforschung, einschließlich der Entdeckung neuer Materialien und ihrer Elektrochemie. Dieser Aufsatz fasst den aktuellen Stand der Forschung auf dem Gebiet der Natriumionenbatterien zusammen.

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