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This review aims to address the status of transition metal‐based cathode materials for Mg2+ and Ca2+‐based multivalent‐ion batteries on a critical standpoint, providing a comprehensive overview. Multivalent‐based ions battery (MIB) technologies are among the most promising post‐lithium electrochemical energy storage devices currently studied, but they still fall short in several aspects due to their early stage of research. In addition, difficult experimental conditions related to the electrolyte systems and the cathode materials require an additional quote of care when performing experiments. In this review, a global approach is undertaken, from an introduction to electrolytes to the studied insertion parameters that allow a fast (de)insertion of multivalent ions. Then, the currently studied structural classes of cathode materials and a critical comment on data reporting, which are among the focal points of the actual state‐of‐the‐art research, are thoroughly discussed.

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.

Next generation energy storage technologies need to be more sustainable and cheaper. Among Post-Li chemistries, Mg batteries are emerging as a possible alternative with desirable features like abundance of Mg on the Earth`s crust and a doubled volumetric capacity with respect to the current Li metal. However, research and development of Mg-batteries is still in its infancy stage and still many hurdles are to be understood and solved. For instance, cathode materials showing high capacities, operating at high potentials and with sufficient fast kinetics need to be designed and developed. Polyanionic materials are a class of sustainable and environmentally friendly materials that emerged as possible Mg2+ hosts. In this work the insertion of Mg cations inside the NASICON Na3V2(PO4)3 and, for the first time, in the mixed phosphate phase Na7V4(P2O7)4(PO4), is reported, structurally and electrochemically characterized.