Publication date: 10th April 2024
Intercalation electrodes exhibit rich point defect chemistry with complex and dynamic local chemistry. Foundational to (electro)chemical and catalytic transformations in these electrodes are stable and reversible high-valent redox (HVR) couples. The "over" oxidation needed to create these cationic and anionic redox couples is strongly linked to structural disorder, resulting in vacancies, interstitials, Frenkel, and antisite defects. In this talk, using transition metal oxides (TM-O) as examples and combining computational and experimental methods, we report two major findings.
First, we developed a comprehensive mechanism and framework for understanding the source of the structure-redox coupling. [1,2] We discovered that during HVR, the highly oxidized species have a strong thermodynamic driving force to form short O-O covalent bonds to become stable. However, O-O dimerization induces significant and energetically unfavorable local bond strain, which is alleviated by point defect disorder, achieving overall energy savings. This disorder is mostly irreversible and leads to significant voltage hysteresis, preventing the harnessing of extra activity or capacity from HVR for (electro)chemical and catalytic applications in batteries, catalysis, and memory devices. Consequently, high-valent redox couples have been historically avoided.
Second, we demonstrate a mechanism by which structural disorder and voltage hysteresis can be completely avoided during HVR. [3,4] We discovered that the introduction of ordered TM point defects in intercalation materials during synthesis significantly changes the thermodynamic and kinetic energy landscape of defect formation during HVR. More specifically, the ordered cation vacancies provide an electrostatic templating platform that uniquely enhances the coulombic interaction between oxidized oxygen species and interlayer vacancies. This kinetically stabilizes localized O-hole polaron species and prevents O-O dimerization. In summary, our work provides an exhaustive understanding of the complex and dynamic defect chemistry in intercalation electrodes, underscoring the roles of covalent bonding, point defects, and electrostatics as critical governing factors and design principles.
1. Abate, I.I.*, Nazar, L. F., Chueh, W. C. et al., Angew. Chem. Int. Ed., 60 (2021), 10880-10887.
2. Gent, W. E., Abate, I. I.*, Chueh, W. C. et al., Joule, 4 (2020), 1369-1397
3. Abate, I. I.*, & Chueh, W. C. et al., Energy Environ. Sci., 14 (2021), 4858-4867
4. Smith, H.B., Abate, I.I. * et al., In Review.
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