An overview of high-entropy materials for energy applications, including H₂ catalysis and storage, CO₂ conversion, O₂ catalysis and electrochemical energy storage, is given and the challenges and opportunities within this field are discussed.

Charge and discharge of lithium ion battery electrodes is accompanied by severe volume changes. In a confined space, the volume cannot expand, leading to significant pressures induced by local microstructural changes within the battery. While volume changes appear to be less critical in batteries with liquid electrolytes, they will be more critical in the case of lithium ion batteries with solid electrolytes and they will be even more critical and detrimental in the case of all-solid-state batteries with a lithium metal electrode. In this work we first summarize, compare, and analyze the volume changes occurring in state of the art electrode materials, based on crystallographic studies. A quantitative analysis follows that is based on the evaluation of the partial molar volume of lithium as a function of the degree of lithiation for different electrode materials. Second, the reaction volumes of operating full cells (“charge/discharge volumes”) are experimentally determined from pressure-dependent open-circuit voltage measurements. The resulting changes in the open-circuit voltage are in the order of 1 mV/100 MPa, are well measurable, and agree with changes observed in the crystallographic data. Third, the pressure changes within solid-state batteries are approximated under the assumption of incompressibility, i.e. for constant volume of the cell casing, and are compared to experimental data obtained from model-type full cells. In addition to the understanding of the occurring volume changes of electrode materials and resulting pressure changes in solid-state batteries, we propose “mechanical” blending of electrode materials to achieve better cycling performance when aiming at “zero-strain” electrodes.

In liquid electrolyte-type lithium-ion batteries, Nickel-rich NCM (Li_1+x (Ni_1−y−z Co _y Mn_z)_1−x O₂) as cathode active material allows for high discharge capacities and good material utilization, while solid-state batteries perform worse despite the past efforts in improving solid electrolyte conductivity and stability. In this work, we identify major reasons for this discrepancy by investigating the lithium transport kinetics in NCM-811 as typical Ni-rich material. During the first charge of battery half-cells, cracks form and are filled by the liquid electrolyte distributing inside the secondary particles of NCM. This drastically improves both the lithium chemical diffusion and charge transfer kinetics by increasing the electrochemically active surface area and reducing the effective particle size. Solid-state batteries are not affected by these cracks because of the mechanical rigidity of solid electrolytes. Hence, secondary particle cracking improves the initial charge and discharge kinetics of NCM in liquid electrolytes, while it degrades the corresponding kinetics in solid electrolytes. Accounting for these kinetic limitations by combining galvanostatic and potentiostatic discharge, we show that Coulombic efficiencies of about 89% at discharge capacities of about 173 mAh g_NCM ⁻¹ can be reached in solid-state battery half-cells with LiNi_0.8Co_0.1Mn_0.1O₂ as cathode active material and Li₆PS₅Cl as solid electrolyte.

High‐entropy sulfides (HESs) containing 5 equiatomic transition metals (M), with different M:S ratios, are prepared by a facile one‐step mechanochemical approach. Two new types of single‐phase HESs with pyrite (Pa‐3) and orthorhombic (Pnma) structures are obtained and demonstrate a homogeneously mixed solid solution. The straightforward synthesis method can easily tune the desired metal to sulfur ratio for HESs with different stoichiometries, by utilizing the respective metal sulfides, even pure metals, and sulfur as precursor chemicals. The structural details and solid solution nature of HESs are studied by X‐ray diffraction, transmission electron microscopy, energy‐dispersive X‐ray spectroscopy, electron energy loss spectroscopy, X‐ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, and Mössbauer spectroscopy. Since transition metal sulfides are a very versatile material class, here the application of HESs is presented as electrode materials for reversible electrochemical energy storage, in which the HESs show high specific capacities and excellent rate capabilities in secondary Li‐ion batteries.