Sodium-ion batteries promise efficient, affordable and sustainable electrical energy storage that avoids critical raw materials such as lithium, cobalt and copper. In this work, a manganese-based, cobalt-free, layered NaxMn3/4Ni1/4O2 cathode active material for sodium-ion batteries is developed. A synthesis phase diagram was developed by varying the sodium content x and the calcination temperature. The calcination process towards a phase pure P2-Na2/3Mn3/4Ni1/4O2 material was investigated in detail using in-situ XRD and TGA-DSC-MS. The resulting material was characterized with ICP-OES, XRD and SEM. A stacking fault model to account for anisotropic broadening of (10l) reflexes in XRD is presented and discussed with respect to the synthesis process. In electrochemical half-cells, P2-Na2/3Mn3/4Ni1/4O2 delivers an attractive initial specific discharge capacity beyond 200 mAh g−1, when cycled between 4.3 and 1.5 V. The structural transformation during cycling was studied using operando XRD to gain deeper insights into the reaction mechanism. The influence of storage under humid conditions on the crystal structure, particle surface and electrochemistry was investigated using model experiments. Due to the broad scope of this work, raw material questions, fundamental investigations and industrially relevant production processes are addressed.

Rechargeable sodium-ion batteries are viable candidates as nextgeneration energy storage devices. Nonetheless, the development of high-potential and stable cathode materials is still one among the open tasks. Here, we propose a combined experimental/theoretical approach to shed light on the effect of magnesium doping on the layered P2-Na 0.67 Mn 0.75 Ni 0.25 O 2 cathode material. The P2-Na 0.67 Mn 0.75 Ni 0.25 O 2 baseline material and doped P2-Na 0.67 Mn 0.75 Ni 0.20 Mg 0.05 O 2 , synthesized via coprecipitation route followed by thermal treatment, have been physically and chemically characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), as well as electrochemically via galvanostatic cycling and galvanostatic intermittent titration technique (GITT). The Mg-doped material showed stabilization of the high potential plateau and improved cycle life. The analysis of the phase transition with synchrotron operando XRD (SXRD) shows multiple possible intermediate phases ("Z-phase") rather than a pure OP4-like structure. Based on our experimental data and periodic density functional theory (DFT) calculations, the stability of the O2, P2, and OP4 phases for the pristine and Mg-doped systems was investigated to elucidate the origin of the "Z"phase formation in the Mg-doped material.

Structural evidence of a Li+ induced phase segregation on particle level in Co-free layered Li–Mn–Ni-oxide cathode materials for Li-ion batteries is presented, illustrating the importance of correlative SEM–Raman microscopy in battery research.

Exceeding the electrochemical stability window of battery cells causes side reactions that are often accompanied by the release of gas molecules. A powerful operando method to analyze such side reactions and their onset potentials is differential electrochemical mass spectrometry (DEMS). While the method provides valuable information, the correct assignment of the DEMS signals and deriving quantitative information on the amounts and types of gases released can be challenging. A frequent limitation is that gas concentrations are often calculated from single m/z ratios only. This has the drawback of overlooking unexpected gases which can moreover cause misinterpretation of the signal intensities, or even the attribution to gases which are not actually formed. Here, we describe a multiple concentration determination (MCD) algorithm, that uses the full MS spectra as a basis. The approach allows a more reliable determination of the gas release and is, to our knowledge, for the first time applied to DEMS for batteries. As study case, Na-ion half cells with P2-Na0.67Mn3/4Ni1/4O2 (NaMNO) as cathode active material (CAM) are chosen. The gassing behavior for two electrolyte formulations (1M NaPF6 in propylene carbonate (PC) and 1M NaPF6 in diglyme (2G)) and for two different upper cut-off potentials (4.25 and 3.80 V) is determined. Against the general belief that glymes lead to more gassing at high potentials, we find that gas evolution for PC electrolytes is generally larger compared to 2G electrolytes. In case of 2G, dimethyl ether is found as decomposition product. Pressure change measurements in a closed cell are used as a second, independent method to validate the gas quantification of the MCD algorithm. The study also highlights the relevance for implementing a reference electrode into DEMS cell setups.

Expanding energy generation from renewables is inevitable to reduce the impact of man-made climate change. With that, the need for intermediate energy storage is gaining in significance. Today, lithium-ion batteries (LIBs) are dominating mobile drive-trains and also play a key role in stationary energy storage. LIBs are incorporating critical raw materials in view of availability and economic importance, such as cobalt, lithium, copper and graphite. Sodium-based batteries, in which sodium replaces lithium as ionic charge-carrier, utilize the same working principles but substitute critical raw materials for abundant and cost-effective alternatives. Hard carbon replaces graphite as anode active material, copper foils are substituted by aluminum current collectors and manganese-based cobalt-free layered host lattices offer promising performance as cathode active material 1,2. By applying established production processes, investment costs are reduced and a rapid scale-up is enabled (Drop-In technology) 3, making SIBs a sustainable, efficient and cost-effective complementary technology to LIBs 4. Among various known cathode active materials for SIBs, the family of layered sodium transition metal oxides (NaxMO2, 1>x>0) offers promising electrochemical performance 2,5–7. These compounds show a wide structural variety (O3, P3, P2) due to the ionic radius of sodium and the tendency for Na+/vacancy ordering 8. The scope of our presentation will be low-cost manganese-based, cobalt-free layered NaxMnyNi1-yO2 cathode active materials for SIBs. We will discuss the influence of the transition metal stoichiometry y on the structure based on Neutron and X-ray diffraction experiments. Using advanced electrochemical methods and diffraction experiments, these structural models are then correlated with physical and electrochemical properties such as Na+/vacancy orderings, solid diffusion coefficients and potential profiles. For y = 3/4, a synthesis phase diagram will be presented covering a broad range of sodium content x and calcination temperature. For phase-pure P2-NaxMn3/4Ni1/4O2, we will present the influence of the calcination process on the structure and discuss the electrochemical properties in half-cells in-depth. For optimized materials, attractive initial specific discharge capacities beyond 220 mAh g-1 are obtained in sodium half-cells between 1.5 – 4.3 V. A capacity decay occurs during electrochemical cycling within this full voltage window. The origin of the capacity decay will be discussed based on electrochemical studies and ex-situ investigations of the morphology with SEM and local structure with HRTEM. Finally, we will present the influence of storage in ambient air to gain insights on the large-scale processability of the materials. The chosen synthesis route adapts industrially established processes for NCM production for SIB cathode materials, enables to tune powder properties to technical specifications and is highly scalable. The broad scope of this work addresses raw material questions, fundamental investigations and industrially relevant production processes. ACKNOWLEDMENTS: The German Federal Ministry of Education and Research (BMBF) supported this work within the project TRANSITION (03XP0186C) and ExcellBattMat (03XP0257A and 03XP0257C). REFERENCES Larcher, D. & Tarascon, J.-M. Towards greener and more sustainable batteries for electrical energy storage. Nature Chem 7, 19–29; 10.1038/nchem.2085 (2015). Hasa, I. et al. Challenges of today for Na-based batteries of the future: From materials to cell metrics. Journal of Power Sources 482, 228872; 10.1016/j.jpowsour.2020.228872 (2021). Tarascon, J.-M. Na-ion versus Li-ion Batteries: Complementarity Rather than Competitiveness. Joule 4, 1616–1620; 10.1016/j.joule.2020.06.003 (2020). Vaalma, C., Buchholz, D., Weil, M. & Passerini, S. A cost and resource analysis of sodium-ion batteries. Nat Rev Mater 3, 1–11; 10.1038/natrevmats.2018.13 (2018). Nagore Ortiz-Vitoriano, Nicholas E. Drewett, Elena Gonzalo & Teófilo Rojo. High performance manganese-based layered oxide cathodes: overcoming the challenges of sodium ion batteries. Energy Environ. Sci. 10, 1051–1074; 10.1039/C7EE00566K (2017). Nuria Tapia-Ruiz et al. 2021 roadmap for sodium-ion batteries. J. Phys. Energy 3, 31503; 10.1088/2515-7655/ac01ef (2021). Gonzalo, E., Zarrabeitia, M., Drewett, N. E., López del Amo, Juan Miguel & Rojo, T. Sodium manganese-rich layered oxides: Potential candidates as positive electrode for Sodium-ion batteries. Energy Storage Materials 34, 682–707; 10.1016/j.ensm.2020.10.010 (2021). Kubota, K., Kumakura, S., Yoda, Y., Kuroki, K. & Komaba, S. Electrochemistry and Solid‐State Chemistry of NaMeO 2 (Me = 3d Transition Metals). Adv. Energy Mater. 8, 1703415; 10.1002/aenm.201703415 (2018).