In a groundbreaking study published in Advanced Science, a research team led by Dr. Duck Young Kim and Dr. Wenge Yang from HPSTAR has unveiled the stability and novel properties of proton superoxide (HO₂) under extreme pressure. This discovery not only expands our understanding of superoxide compounds but also sheds light on the superionic behaviors in energy storage devices, paving the way for the design of potential H-ion batteries.

HO₂, the lightest superoxide, was predicted and analyzed using density functional theory (DFT) calculations, combined with ab initio molecular dynamics (AIMD) simulations. The study reveals that HO₂ becomes thermodynamically stable above ~450 GPa and exhibits unique metallic behavior due to pressure-induced overlap of 𝜋* orbitals between adjacent O₂⁻ anions. HO₂ undergoes a metal-to-insulator transition, driven by changes in orbital overlaps—a phenomenon comparable to the Wilson transition which is observed in LiO₂.

Furthermore, AIMD simulations uncovered that HO₂ transitions into a superionic state at high pressures and high temperatures, characterized by rapid hydrogen diffusion within a fixed oxygen sublattice. This unique phase is associated with high ionic electrical conductivity, making HO₂ a promising candidate for future energy storage technologies. The researchers also proposed hydrogen-lithium mixed superoxide structures (Li_1-xO₂H_x) that could be stabilized at intermediated pressures, broadening the potential routes to stabilize HO₂ towards ambient pressure, which holds promise for practical applications.

These findings fill gaps in our knowledge of superoxide compounds and superionicity under extreme conditions, enriching the diversity of H-O binary system. They also suggest that proton superoxide HO₂ is a promising candidate for use as the lightest solid electrolyte in batteries, offering the potential to significantly advance the development of energy storage systems and fast-ion conductors.

Caption: The description of the crystal structure and the diffusive H⁺ in superionic HO₂ (left) and the pressure-temperature phase diagram of HO₂ (right).

近日，北京高压科学研究中心 (HPSTAR) 的Duck Young Kim研究员课题组，在理论上成功预测了稳定的质子超氧化物HO₂，并系统研究了其在极端条件下的一系列新奇特性，包括绝缘体-金属转变和超离子态转变。这项工作拓展了我们对超氧化物和超离子态的理解和认识，为能源材料和新型快离子导体的设计提供了新的思路和见解。这项工作以“Stability of Proton Superoxide and its Superionic Transition Under High Pressure”为题发表在近期的《Advanced Science》期刊上。