The essential demand for functional materials enabling the realization of new energy technologies has triggered tremendous efforts in scientific and industrial research in recent years. Recently, high-entropy materials, with their...

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202103090.

Multicomponent materials may exhibit favorable Li‐storage properties because of entropy stabilization. While the first examples of high‐entropy oxides and oxyfluorides show good cycling performance, they suffer from various problems. Here, we report on side reactions leading to gas evolution in Li‐ion cells using rock‐salt (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O (HEO) or Li(Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)OF (Li(HEO)F). Differential electrochemical mass spectrometry indicates that a robust solid‐electrolyte interphase layer is formed on the HEO anode, even when using an additive‐free electrolyte. For the Li(HEO)F cathode, the cumulative amount of gases is found by pressure measurements to depend strongly on the upper cutoff potential used during cycling. Cells charged to 5.0 V versus Li+/Li show the evolution of O2, H2, CO2, CO and POF3, with the latter species being indirectly due to lattice O2 release as confirmed by electron energy loss spectroscopy. This result attests to the negative effect that lattice instability at high potentials has on the gassing.

P2-type layered oxides with the general Na-deficient composition NaxTMO2 (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na+ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V, combined with metal dissolution at low potentials upon discharge, results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na0.67(Mn0.55Ni0.21Co0.24)O2, Na0.67(Mn0.45Ni0.18Co0.24Ti0.1Mg0.03)O2, and Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)O2 with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications.

saying "technology is always limited by the materials available" still holds true today. [1] Therefore, developing and optimizing new materials will remain of tremendous importance in the coming years. This particularly applies in the light of ever-increasing performance requirements and a transition toward more effective and more sustainable technologies. Based on the functionality of these materials, vital tasks could be executed more efficiently, and tools could be manufactured, which by themselves supported the further search for even more sophisticated materials. The strive for materials to execute more complicated tasks demands an increased complexity of the materials; therefore, people started mixing different components, preparing the first complex alloys and composite materials already at an early age of history. Nowadays, very complex materials with various incorporated elements are known and used for a wide variety of different applications. Well-known examples for such materials with many different incorporated elements are found in the field of electrochemical energy storage (batteries) with electrodes composed of layered delafossite structures, such as NCM (Li(NiCoMn)O 2 ), Li(NiCoAl)O 2 , or the spinel LiNi 0.5 Mn 1.5 O 4 . [2] A similar trend toward a more complex composition to enable better and tailored performance is seen for the rapidly growing material family of MXene, [3] which are 2D metal carbides/ nitrides/carbonitrides with the unique ability to form solid solutions while maintaining their nanolamellar structure. [4] MXenes are obtained from removing A-site atoms from the MAX phase crystal lattice; we find for MAX phases M n+1 AX n (n = 1-4), where M represents an early transition metal element (e.g., V, Nb, Ti, Cr), A is an element typically from group 13 or 14 (e.g., Si, Al, Ga, Ge), and X is C and/or N. [4,5] MXenes have already demonstrated their tailored properties. For example, Han et al. studied the TiVNb MXene system [5] and effectively modified electronic and optical properties. The properties of MXenes are also greatly influenced by the surface groups, often referred to as T x or T z ; they strongly impact the electronic, electrochemical, and electrocatalytic properties. [6] Tailored surface functionality can also be seen as one more "element" to modify in addition to the chemical modification of the M-and X-site atoms.A temporary peak of materials complexity was developed independently by Cantor and Yeh, who both described the formation of an equimolar multi-element single-phase alloy, High-entropy materials (HEMs) with promising energy storage and conversion properties have recently attracted worldwide increasing research interest. Nevertheless, most research on the synthesis of HEMs focuses on a "trial and error" method without any guidance, which is very laborious and time-consuming. This review aims to provide an instructive approach to searching and developing new high-entropy energy materials in a much more efficient way. Toward materials design for future technologie...

High entropy oxides (HEOs) with chemically disordered multi-cation structure attract intensive interest as negative electrode materials for battery applications. The outstanding electrochemical performance has been attributed to the high-entropy stabilization and the so-called ‘cocktail effect’. However, the configurational entropy of the HEO, which is thermodynamically only metastable at room-temperature, is insufficient to drive the structural reversibility during conversion-type battery reaction, and the ‘cocktail effect’ has not been explained thus far. This work unveils the multi-cations synergy of the HEO Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O at atomic and nanoscale during electrochemical reaction and explains the ‘cocktail effect’. The more electronegative elements form an electrochemically inert 3-dimensional metallic nano-network enabling electron transport. The electrochemical inactive cation stabilizes an oxide nanophase, which is semi-coherent with the metallic phase and accommodates Li+ ions. This self-assembled nanostructure enables stable cycling of micron-sized particles, which bypasses the need for nanoscale pre-modification required for conventional metal oxides in battery applications. This demonstrates elemental diversity is the key for optimizing multi-cation electrode materials.

We developed a simple, scalable and high-throughput method for fabrication of large-area three-dimensional rose-like microflowers with controlled size, shape and density on graphene films by femtosecond laser micromachining. The novel biomimetic microflower that composed of numerous turnup graphene nanoflakes can be fabricated by only a single femtosecond laser pulse, which is efficient enough for large-area patterning. The graphene films were composed of layer-by-layer graphene nanosheets separated by nanogaps (~10–50 nm), and graphene monolayers with an interlayer spacing of ~0.37 nm constituted each of the graphene nanosheets. This unique hierarchical layering structure of graphene films provides great possibilities for generation of tensile stress during femtosecond laser ablation to roll up the nanoflakes, which contributes to the formation of microflowers. By a simple scanning technique, patterned surfaces with controllable densities of flower patterns were obtained, which can exhibit adhesive superhydrophobicity. More importantly, this technique enables fabrication of the large-area patterned surfaces at centimeter scales in a simple and efficient way. This study not only presents new insights of ultrafast laser processing of novel graphene-based materials but also shows great promise of designing new materials combined with ultrafast laser surface patterning for future applications in functional coatings, sensors, actuators and microfluidics.

No abstract