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The formation and growth of the Li2O2 discharge product impacts the reversibility of the oxygen evolution and reduction reactions in Li-O2 batteries which may lead to a shorter cycle life. A clear understanding of the surface reactions and the growth mechanism of Li2O2 requires probing dynamic changes on the surface of the positive electrodes in situ during the discharge of a Li-O2 battery. To investigate this, we establish an experimental system by adopting a multi-beam optical sensor (MOS) and developing a custom-made battery cell. First, the accuracy and reliability of the system was demonstrated by analyzing the stress accumulation on the Au negative electrode during Li plating/stripping, and the results were consistent with an earlier single-beam scanning deflectometry report. Then, the Li-O2 battery was discharged in LiNO3 in diglyme electrolyte by applying either linear sweep voltammetry or by applying constant current under an O2 environment. Control experiments in Argon-saturated electrolytes indicate surface stress generation due to charge-induced stress. The stress generation on Au positive electrode is attributed to the formation of Li2O2 reaction products on the Au surface and charge-induced stress.

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The rapid increase in the human population starting from the 20th century resulted in the excessive utilization of non-renewable energy sources. Environmental concerns caused by greenhouse gas emissions forced society to find renewable ways to produce energy. Their irregular nature, however, requires the development of large-scale energy storage technologies. While Li-ion batteries (LIBs) predominates the market, growing demand and increasing prices limit their large-scale application. As a cost-effective alternative, Na-ion batteries (NIBs) have gained attention from the research community. With similar chemical characteristics and no alloying with aluminum, existing LIB facilities can produce NIBs at a much lower cost while requiring minimal upgrades.1 Higher reactivity and the larger ionic radius of sodium creates mechanical and interfacial instabilities on the electrode materials due to sodium chemistry. Large and sudden changes in the lattice parameters of active materials originating from Na concentration may cause particle fracturing and eventually, capacity degradation with repeated sodium insertion/removal.2 Understanding the fundamentals of the mechanical deformation of the Na-ion active materials is crucial for the pursuit of better-performing NIBs. In our study, our goal was to investigate the effect of cycling rate on the mechanical response of sodium iron phosphate (NaFePO4, NFP) cathode material during sodium intercalation via galvanostatic cycling at different rates. We implemented digital image correlation (DIC) and galvanostatic intermittent titration technique (GITT), and a mathematical model to understand this phenomenon. Similar to our previous study on NFP electrodes, in situ DIC measurements showed that there is a linear relationship between Na concentration and strain regardless of cycling rate.3 It was also observed that the slower the scan rate, the rate of strain generation was also lower. A mathematical model was developed; using the in-situ XRD results, to estimate the strain evolution and concentration gradient in the composite electrode.4 Simulations suggested the nonuniform distribution of Na ions in the electrode particle, which can lead to the larger strain generation in the composite electrode. Effective utilization of in situ strain measurement system in conjunction with an analytical model helps us to understand the volumetric changes observed in NFP cathode during electrochemical reactions. Acknowledgment: The work was supported by the Department of Energy and we are thankful to Vijay Murugesan, Damien Saurel, and Monserrat Casas for fruitful discussions. References Kubota K, Dahbi M, Hosaka T, Kumakura S, Komaba S. Towards K-Ion and Na-Ion Batteries as “Beyond Li-Ion.” Chem Rec. 2018;18(4):459-479. doi:10.1002/tcr.201700057 Moreau P, Guyomard D, Gaubicher J, Boucher F. Structure and stability of sodium intercalated phases in olivine FePO 4. Chem Mater. 2010;22(14):4126-4128. doi:10.1021/cm101377h Özdogru B, Dykes H, Padwal S, Harimkar S, Çapraz Ö. Electrochemical strain evolution in iron phosphate composite cathodes during lithium and sodium ion intercalation. Electrochim Acta. 2020;353. doi:10.1016/j.electacta.2020.136594 Galceran M, Saurel D, Acebedo B, et al. The mechanism of NaFePO4 (de)sodiation determined by in situ X-ray diffraction. Phys Chem Chem Phys. 2014;16(19):8837-8842. doi:10.1039/c4cp01089b