Recent reports post conflicting results on the atmospheric stability of Cs2TiBr6, a nontoxic, Earth-abundant solar energy conversion material. Here, a high-temperature melt of CsBr and TiBr4 yielded large-grain samples with >1 mm2 facets as verified by optical microscopy and scanning electron microscopy (SEM). With pristine-material properties of particular interest, we investigated a series of physicochemical surface treatments including rinsing, abrasion, and cleaving in ultrahigh vacuum (UHV). For each surface treatment, X-ray photoelectron spectroscopy (XPS) quantified surface chemical species, while ultraviolet photoelectron spectroscopy (UPS) established valence-band structure as a function of surface treatment. Amorphous titanium oxide with crystalline cesium bromide dominates the surfaces of nascent Cs2TiBr6 material. UHV cleaving yielded oxide-free surfaces with excellent alignment between valence-band structure and a density functional theory (DFT)-calculated density of states, a 3.92 eV work function, and 1.42 eV Fermi energy vs the valence band maximum. Band energetics are commensurate with moderate n-type doping for this melt-synthesized large-grain Cs2TiBr6. Titanium oxide once again dominates UHV-cleaved samples following a 10 min exposure to an air ambient. We discuss the implications of these surface chemical and electronic results for photovoltaics.
We quantified the chemical species present at and reactivity of the (100) face of tetrahedral single-crystal methylammonium lead iodide, MAPbI3(100), and polycrystalline cesium tin bromide, CsSnBr3. For these ABX3 perovskites, experiments utilized the orthogonal reactivity of the A+-site cation, the B2+-site cation, and the X–-site halide anion. Ambient pressure exposure to BF3 solutions probed the reactivity of interfacial halides. Reactions with p-trifluoromethylanilinium chloride probed the exchange reactivity of the A+-site cation. A complex-forming ligand, 4,4′-bis(trifluoromethyl)-2,2′-bipyridine, probed for interfacial B2+-site cations. Fluorine features in X-ray photoelectron spectroscopy (XPS) quantified reaction outcomes for each solution-phase species. XPS revealed adsorption of BF3, indicating surface-available halide anions on both MAPbI3(100) and on CsSnBr3. Temperature-programmed desorption quantified a ∼200 kJ mol–1 desorption activation energy from MAPbI3(100) and a ∼215 kJ mol–1 desorption energy from CsSnBr3. Adsorption of the fluorinated anilinium cation included no concomitant adsorption of chlorine as revealed by the absence of Cl 2p features within the limits of XPS detection. We interpret the observation of the anilinium species as exchanging for interfacial methylammonium species on MAPbI3(100) surfaces and interfacial cesium on the polycrystalline CsSnBr3 surface. Within detection limits, the bipyridine ligand demonstrated no adsorption to MAPbI3(100), suggestive of a Pb2+ deficient surface, but adsorption to the polycrystalline CsSnBr3 that suggests surface-accessible Sn2+. The combination of results implies that methylammonium cations and iodide anions dominate tetragonal MAPbI3(100) surface that, respectively, enables cation exchange and Lewis adduct formation for surface derivatization. We discuss the present results in the context of interfacial stability, passivation, and reactivity for perovskite-based energy conversion.
Performance and cost requirements for emerging storage applications challenge existing battery technologies and call for substantial improvements in cell energy and rate capability. Convection batteries can reduce mass transport limitations commonly observed during high current operation or with thick electrodes. In prior proof-of-concept work, while convection was shown to improve cell performance, its effectiveness was limited in the select cases studied. To understand the feasibility of the convection battery more comprehensively, we develop a mathematical model to describe convection in a Li-ion cell and evaluate performance as a function of a broad range of cell dimensions, component properties, as well as electrochemical and flow operating conditions. Qualitatively, we find that electrolyte flow enhances accessible capacity for cells with large electrolyte diffusive transport resistance and low initial amounts of electrolyte salt by reducing spatial concentration gradients and, thus, allowing for efficient high current operation. Quantitatively, by leveraging dimensional analysis that lumps >10 physical and cell parameters into representative dimensionless groups, we describe the efficacy, trade-offs, and upper performance bounds of convection in an electrochemical cell. Our analyses suggest that this format has the potential to enable high-power energy-dense storage which, in turn, may offer new application spaces for existing and emerging intercalation chemistries.
Ubiquitous in consumer electronics and emergent in transportation and stationary applications, lithium-ion batteries (LIB) are the state-of-the-art energy storage technology due to their energy density, roundtrip efficiency, and cycle life.1,2 While the past decade has seen a steady decline in battery price and concomitant increase in energy density due to a combination of materials development, manufacturing advances, and market scale,3,4 current LIBs are still unable to meet the often incongruous power and energy requirements of newer applications (e.g., fast charging of energy-dense batteries).5,6 In addition, these more extreme operating environments challenge battery longevity and safety, necessitating responsive balance-of-plant systems which include thermal management systems that control cell temperatures using heat transfer media. At elevated temperatures, accelerated solid-electrolyte interphase growth and component decomposition may lead to capacity/power fade,7,8 and, in the worst cases, thermal runaway and hazardous releases. Within the battery cell, temperature gradients lead to non-uniform electrode reaction distribution, and subsequently reduced cell performance and cycle life.8 Thus, typical LIB operating temperature ranges are constrained between 20 ℃ and 40 ℃, with minimal temperature differences across the cell. Most current thermal management systems rely on heat exchange through the surface or tab of the cell with a cooling media (air, liquid, phase-change materials, etc.).9,10 While generally sufficient under many of today’s applications (low C-rates), this approach can be challenged by newer applications, such as those that require high power input/output (EV fast charging, electric aviation), or need large battery formats (stationary storage systems). In this presentation, we will describe a novel concept of thermal management through forced convection of the electrolyte through the porous electrodes and separator. By leveraging battery simulation and dimensional analysis, we demonstrate that: (1) electrolyte convection provides efficient heat removal capability by carrying the generated heat out of the cell through the flowing medium; (2) the elimination of electrolyte concentration gradient by flow, and the resulting smaller ohmic resistance, concentration and activation overpotentials, help prevent cell temperature rise through reduced heat generation rate. Compared to current thermal management systems, this approach offers several important potential advantages, including (1) reduced internal temperature gradient, (2) rapid response time to temperature regulation, (3) simplifications to manufacturing, and ultimately, and (4) reduced system costs and improved battery safety. References: Zubi, G. et al. Renew. Sustain. Energy Rev. 89, 292–308 (2018). Blomgren, G. E. J. Electrochem. Soc. 164, A5019–A5025 (2017). Nykvist, B. & Nilsson, M. Nat. Clim. Change 5, 329–332 (2015). Schmuch, R.et al. Nat. Energy 3, 267–278 (2018). Ahmed, S. et al. J. Power Sources 367, 250–262 (2017). Bills, A.et al. ACS Energy Lett. 5, 663–668 (2020). Tomaszewska, A. et al. eTransportation 1, 100011 (2019). Wu, W. et al. Energy Convers. Manag. 182, 262–281 (2019). Zichen, W. & Changqing, D. Renew. Sustain. Energy Rev. 139, 110685 (2021). Tete, P. R. et al. J. Energy Storage 35, 102255 (2021).
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