Interesting Electrochemical Mechanism of Hybrid Sodium-ion Batteries

Assis. Prof. Yang Xu, University College London, UK

09:30-10:30,  July 20, 2023

Yiucheng Lecture Hall (500), Xu Zuyao Building

Biography

Dr Yang Xu is an Assistant Professor in Electrochemical Energy Storage in the Department of Chemistry at University College London (UCL), UK. He received his bachelor’s and PhD degrees at the University of Science and Technology of China, and he carried out his postdoctoral work at Boston College (US), University of Alberta (Canada), and Technical University of Ilmenau (Germany). He joined UCL Chemistry and started his independent research group in 2019. His group consists of 3 postdocs, 7 PhD students and several master’s students. His research focuses on emerging battery technologies including Na, K, and Ca-based systems, with particular interest in Na/K/Ca-ion intercalation chemistry, electrodeposition of Na/K metal anodes for metal anode-less (anode-free) batteries, and electrochemical mechanism of Na/K-S chemistry. His group recently developed an interest in hybrid ion battery systems and has expanded to Li battery chemistry after receiving new fundings this year. He has received over ₤1.7M research funding as the Principal Investigator in the last three years from various funders including the EPSRC, Royal Society, Faraday Institution, Leverhulme Trust and UCL. He has published >70 peer-review papers (h-index 42, >7k citations) in leading international journals and is the recipient of the MINE Outstanding Young Scientist Award (2019), the EPSRC New Investigator Award (2020), and the STFC Early Career Award (2023). In addition, he is a member of the editorial board of JPhys Mater. (IOP), the advisory boards of J. Mater. Chem. A and Mater. Adv. (RSC), and the youth editorial boards of Sci. China. Mater. (Springer) and eScience (Elsevier). He is also the director of the master’s program Materials for Energy and Environment at UCL Chemistry.

Abstract

Conventional sodium-ion battery (SIB) cathodes are pre-sodiated but interestingly, pre-potassiated materials are demonstrated to be appealing cathodes for SIBs. Potassium Prussian blue analogue (K-PBA) KxFeFe(CN)6 is an excellent example. I will show in my talk that in a hybrid SIB cell, where Na+ is in the electrolyte and K+ is in the K-PBA cathode, not only is the electrochemical mechanism of cation intercalation in K-PBA dominated by K-ion rather than Na-ion, but the mechanism can be significantly affected by the [Fe(CN)6]4- anion vacancies present in K-PBA. K-PBA with 25% anion vacancies exhibits two cation intercalation steps, and both are dominated by K-ion, displaying a ~0.2 V increase in the intercalation voltage compared with the voltage of Na-PBA in a SIB cell. In contrast, K-PBA with 7% anion vacancies exhibited a split intercalation at the lower voltage range (<3.2 V vs. Na+/Na), showing a K-ion dominating intercalation above 2.8 V and a Na-ion dominating intercalation below 2.8 V. The difference is caused by K-ion kinetics regulated by [Fe(CN)6]4- anion vacancies. A higher vacancy level enhances K-ion diffusion in the PBA framework, which facilitates K-ion intercalation and suppresses Na-ion intercalation. A lower vacancy level deteriorates K-ion diffusion and thus enhances Na-ion intercalation. As a result, in the SIB half-cells, the K-PBA cathode with a higher vacancy level outperformed the one with a lower vacancy level. In the SIB full-cells consisting of the K-PBA cathode and hard carbon anode, the electrochemical mechanism is shown a similar manner as the half-cells, where K-ion dominates the intercalation in the cathode while Na-ion dominates the intercalation in the anode, forming hybrid SIBs.