Abstract In the study of complex systems, the intricate interdependence among individual components leads to emergent properties that cannot be solely attributed to the properties of the components themselves. This principle is central to compositionally complex materials (CCMs), where interactions between different elements introduced into the structure result in unprecedented material properties. The emergence of high entropy materials (HEMs) in 2004 further increase complexity by achieving high configurational entropy (Sconfig) leading to unique properties and stabilizing phases that would otherwise undergo demixing due to positive reaction enthalpy. HEMs and CCMs represent an emerging family of materials where multiple principal elements occupy equivalent crystallographic sites. This atomic architecture gives rise to extraordinary properties such as tailorable electronic structures, lattice distortion effects, and synergistic interactions, resulting from their vast, near-infinite combinatorial space. Although the field is still in its infancy, early discoveries highlight their disruptive potential, particularly in energy technologies where robustness and durability are critical. Their exceptional thermal stability, corrosion resistance, and electro-chemo-mechanical durability position HEMs and CCMs as game-changers for applications demanding resilience under harsh operating conditions, such as batteries, fuel cells, and hydrogen storage systems. Beyond performance advantages, CCMs challenge traditional materials discovery frameworks. Their extensive design space makes conventional trial-and-error approaches impractical, creating an ideal platform for deploying AI-driven high-throughput computational screening, multiscale modeling, and autonomous experimental workflows. This convergence of complexity and innovation offers unprecedented opportunities to accelerate the identification of next-generation energy materials. This roadmap compiles insights from leading experts in the field of CCMs across key energy domains, including electrochemical storage, catalysis, thermoelectrics, and turbomachinery. Their contributions critically assess the current state of this material family highlighting unresolved scientific challenges, technological barriers, and the key advancements needed to move beyond the current state-of-the-art. Special emphasis is placed on combinatorial synthesis and high-throughput approaches and their potential to trigger exponential development of this emerging family of materials. Focused on energy applications, this roadmap provides a comprehensive overview of a time-critical topic, emphasizing the need for material innovation and joint efforts from academia and industry.

Abstract Capacity fade in Si-based porous anodes of Li-ion batteries has been mainly attributed to fracture of the Si particles. However, it was recently shown that even when the Si itself does not fracture, it is still not possible to retain a stable capacity, since significant cracking occurs in the binder. It is, therefore, vital to shift focus on the binder selection. In this work, it is shown that tannic acid is an effective binder for enhancing the mechanical stability of the Si porous electrodes, and hence increasing the electrochemical stability, since for nano Si-tannic acid anodes, at 0.2 C, the initial discharge capacity was 2284 mAh g-1 and after 200 cycles it had dropped to 1279 mAh g-1. Extended cycling was performed for 0.5 C, giving an initial capacity of 1414 mAh g-1, which dropped to 440 mAh g-1 after 1000 cycles. The aforementioned capacity fade for nano-Si/tannic-acid based porous electrodes is significantly lower than that recorded when employing other binders such as alginate (850 mAh g-1 after 200 cycles), carboxymethyl cellulose (900 mAh g-1 after 100 cycles) and polyvinylidene fluoride (~10 mAh g-1 after 50 cycles). Micron-sized Si-tannic acid electrodes also showcased an improved capacity retention of ~1150 mAh g-1 after 200 cycles when compared with other binders such as block co-polymers (~500 mAh g-1 after 100 cycles). Scanning electron microscopy revealed that the tannic acid binders, in Si-based anodes, resulted in only a 50% increase in the electrode thickness over cycling, whereas polyvinylidene fluoride gave a 183% overall electrode increase. The ability of tannic acid to significantly suppress the electrode thickness expansion during cycling reduced crack formation and hence improved capacity retention.

We explored the in-situ doping of cadmium telluride thin films with indium to produce n-type absorbers as an alternative to the near-universal choice of p-type for photovoltaic devices. The films were grown by close space sublimation from melt-synthesised feedstock. Transfer of the indium during film growth was limited to 0.0014%–0.014%—unless reducing conditions were used which yielded 14%–28% efficient transport. While chunks of bulk feedstock were verified as n-type by the hot probe method, carrier type of thin film material was only able to be verified by using hard x-ray photoelectron spectroscopy to determine the Fermi level position within the band gap. The assignment of n-type conductivity was consistent with the rectification behaviour of a p-InP/CdTe:In junction. However, chloride treatment had the effect of compensating n-CdTe:In to near-intrinsic levels. Without chloride, the highest dopant activation was 20% of the chemical concentration of indium, this being for a film having a carrier concentration of n = 2 × 1015 cm−3. However, the activation was often much lower, and compensation due to over-doping with indium and native defects (stoichiometry) are discussed. Results from preliminary bifacial devices comprising Au/P3HT/ZnTe/CdTe:In/CdS/FTO/glass are presented.