New research led by Dr. Hengzhong Zhang from the Center for High Pressure Science and Technology Advanced Research (HPSTAR) gave a systematical study on the electrical conductance of several undoped and Li-doped HEO compounds. Their work, published this week in JACS Au, provided a deep understanding of the electrical conduction mechanism of HEOs in a wide range of temperature and pressure conditions.

Li-doped high-entropy oxides (HEO) are promising electrode materials for Li-ion batteries. However, their electrical conduction and the pertinent mechanism in a wide range of temperatures and/or at high pressure are unknown, hindering their applications in extreme conditions.

In this work, the team measured the electrical conduction of the semiconductive HEO compounds at temperatures in 79 – 773 K and/or at pressures up to 50 GPa, determined the major electronic carrier type (holes). The electrical conductivity measurements unveiled that the in different temperature regions, different electronic conduction mechanisms operate in HEO samples: in a lower-temperature region, 79 – 170 K, ionization conduction dominates; in a middle temperature region 170 – 300 K, the extrinsic conduction dominates and above 300 K, the intrinsic conduction dominates.

Their combined electrical and optical absorption measurements depicted the electronic band structure, showing the intrinsic conduction proceeds by the thermal excitations of the electrons in the defect energy levels to the conduction band. The electrochemical impedance spectroscopy (EIS) unveiled the coexistence of the ionic conduction with the electronic conduction with a comparable magnitude at ambient pressure, and the intrinsic conduction mechanism also operates at high pressure. This work provides us with fundamental knowledge for developing new generations of HEO-based Li-ion electrode materials that would function in both ambient and extreme conditions.

Caption: (a) Electrical resistivities of Li_x-HEO samples as a function of temperature. Lines are the fitting curves. (b) ln(conductivity) as a function of 1000/T for the Li_0.200-HEO sample and the fitting curves. (c) UV-vis-NIR absorption spectra of Li_x-HEO samples: the plot of (A·E)² as a function of photon energy E. (d) Schematic diagram of the EIS setup. (e) EIS data (points) and the corresponding fitting for Li_0.200-HEO at 23 ℃. Inset in (e) is the equivalent circuit. (f) Thermoelectric open circuit voltages (OCV) of the Li_x-HEO samples. (g) Electron paramagnetic resonance (EPR) spectra of the Li_x-HEO samples. (h) Schematic illustration of ionization, extrinsic, and intrinsic thermal excitations of electrons in Li_x-HEO.

锂掺杂高熵氧化物是一种富有广阔前景的锂离子电池电极材料， 然而，其在宽温域和高压条件下的导电性能尚不清楚，这将限制它们在极端条件下的进一步应用。近日，北京高压科学研究中心的张衡中研究员课题组深入研究了几种锂掺杂高熵氧化物在79 – 773 K温区和高达50 GPa压力下的导电行为，发现了其以空穴载流子和离子迁移为输运的混合导电行为，并揭示了通过缺陷中间能带电子向导带跃迁和空穴向价带回落的本征导电机理。相关研究发表在近期《JACS Au》上（DOI:10.1021/jacsau.3c00693）。文章第一作者为北京高压科学研究中心博士生宋猛。