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Best Practices for Operando Battery Experiments: Influences of X‑ray Experiment Design on Observed Electrochemical Reactivity
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cell, although the volume of the sample being probed may represent less than 0.2% of the total electrode volume. Here, we directly assess how electrochemical reactivity can be impacted by experiment design, including by the X-ray measurements themselves or by common features or adaptations of electrochemical cells that enable X-ray measurements. By mapping the state of charge in whole electrodes recovered following cycling under simulated operando experiment conditions, we decouple the X-ray beam and electrochemical cell parameters that are critical to achieving reliable electrochemical performance and reactivity. Specifically, we compare cycling with or without exposure to hard (17 keV) and high energy (58 keV) X-ray beams and with or without modifying a standard electrochemical cell to add an X-raytransparent, thin-film window. For this study, the wellcharacterized LiFePO4 to FePO4 (de)intercalation reaction was selected as a model system. In LiFePO4, two-phase reaction behavior has been well established under equilibrium cycling conditions or for recovered electrodes.18,19 Accordingly, the state of the reaction can be directly inferred from the relative abundance of the two end-members, LiFePO4 and FePO4, evaluated based on the relative intensity of (ideally nonoverlapping) diffraction peaks from each phase. Although the coin cells are a standard for electrochemical testing, providing exemplary electrochemical performance when used as intended, modifications implemented to allow X-rays to penetrate the cell and the electrode within can degrade the cell integrity and performance. In spite of this, modified coin cells remain widely used for operando experiments.20 Modifications to the coin cell to facilitate X-ray measurements, typically involves perforation of the X-ray attenuating, conductive stainless steel casing, which is then resealed with an X-ray transparent, thin-film polymer window (see Figure 1). This disrupts several important cell characteristics: flexibility of the window can reduce stack pressure on the electrode; the nonconductive character of the window can reduce the electrical conductivity to the electrode. Further, there is potential for leaking of the fluid electrolyte from the cell or contamination by atmospheric gases. To mitigate against the effect of the reduced conductivity provided by thin-film polymer windows, some experiments cover the window with a thin metal foil. To separate the impact of cell stack-pressure (i.e., flexible windows) and conductivity on the electrode reactivity, modified coin-cell configurations were simulated within a customdesigned electrochemical cell, the AMPIX cell.21 Although the AMPIX cell itself mimics the physical geometry and electrochemical performance of a standard coin cell, with rigid, conductive windows providing uniform stack pressure and conductivity, the ease with which this cell can be disassembled to recover cycled electrodes and its versatile architecture that
he dynamic processes and multiscale complexities that govern electrochemical energy storage in batteries are most ideally interrogated under simulated operating conditions within an electrochemical cell. Although more experimentally demanding, probing reactions within an operating electrochemical cell offers many advantages over ex-situ or postmortem analysis of components recovered from dismantled cells. Operando studies increase measurement efficiency, precision, and reliability by allowing a single, individual system to be probed at fine intervals without risking contamination or relaxation of metastable species. Moreover, operando studies can provide insight into dynamic effects such as ratedependence of the reaction mechanism1−3 or time-dependent processes (e.g., relaxation of the electrode during gravimetric intermittent titration technique studies). Applications of such operando studies are growing rapidly.4−11 For any operando experiment, there is an implicit expectation that the data provides an accurate representation of the reaction behavior found under normal operating conditions. That is, the reactivity is not altered by the measurement itself or by changes to the operating conditions required to enable the measurement. For example, for electron microscopy measurements, the strong interaction of the electron beam with matter means that true battery operating conditions can be difficult to simulate without degrading the resolution of the measurements12 and it is possible for the material to restructure within the electron beam.13−15 By contrast, having only weak interactions with matter neutron and very high energy X-ray beams are able to penetrate the battery casing (typically stainless steel) to probe electrode reactions without perturbing the reaction itself.16,17 For operando X-ray scattering and spectroscopy experiments, the X-ray methodology often dictates a need for specialized electrochemical cells that are optimized for X-ray transmission, scattering geometry, and background contribution. This includes, for example, typical powder X-ray diffraction, X-ray absorption spectroscopy, pair distribution function, X-ray tomography, and coherent X-ray diffraction imaging measurements. Not only does any modification to the electrochemical cell have potential to perturb the electrochemical reactivity, but depending on the X-ray beam energy (and X-ray absorption), the beam itself has potential to influence the electrochemical reactivity. Without recognizing the impact that experiment and electrochemical cell design can have on operando data, inconsistencies and potentially conflicting results may be reported, leading to incorrect conclusions on reaction mechanisms. However, the influence of operando electrochemical cell design and X-ray beam exposure during measurements on the electrochemical reactivity has not been directly assessed. Instead, many operando studies presume reliability of the experiment based on the electrochemistry of the entire system being within an arbitrary tolerance (perhaps within a few percent) of expected behavior for a standard coin © 2015 American Chemical Society
Published: June 4, 2015 2081
DOI: 10.1021/acs.jpclett.5b00891 J. Phys. Chem. Lett. 2015, 6, 2081−2085
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The Journal of Physical Chemistry Letters
Figure 2. Electrochemical profiles for the (de)intercalation of LiFePO4 under different simulated experimental configurations.
Figure 1. Cross sections of (a) a standard coin cell, (b) a coin cell modified to facilitate X-ray transmission (with a photo), and (c) the AMPIX cell, an X-ray compatible coin-cell alternative.
The reaction state of the cycled electrodes was evaluated at each point on a 0.5 × 0.5 mm grid based on the relative intensities of the 211 and 020 diffraction peaks from LiFePO4 and FePO4, respectively (Figure 3). Uniform electrode reactivity was found in the coin-cell-like AMPIX cell cycled both (A) without X-ray exposure and (C) with high-energy Xray exposure. With exposure to lower energy X-rays, nonuniformities in the reaction state were evident (B). The region in which the reaction was retarded overlapped with the cell window, on which the X-ray beam was centered, however, the affected region was much larger than either the area illuminated by the direct X-ray beam (0.5 × 0.5 mm, i.e. 1−4 pixels in the reaction state map) or the window itself. For the simulated modified coin cells (D, E, F), the reaction state for the region under the window was less advanced than the rest of the electrode wherein a uniform reaction state was evident. The degree of retardation of the reaction was most pronounced at the center of the window where, presumably, the nonuniformities were also most pronounced. The configuration combining both nonuniform conductivity and pressure (D) showed the most variable reactivity. The cells with either nonuniform pressure or conductivity (E, F) showed similar levels of reduced reactivity. The mapping data show that X-ray beam interactions can influence the electrochemical reactivity. Mechanisms for beam interactions are often complex and can involve radical formation. This may account for the diffuse shape of the area in which electrode reactivity was impacted. Beam interactions are directly proportional to the X-ray absorption, which is minimized for higher X-ray energies. This is consistent with the
can be adapted to simulate different modified coin-cell geometries makes it a convenient device for the present study. To reproduce the effects of beam exposure during an operando measurement, electrodes were cycled in the coin-cell equivalent standard AMPIX cell without beam and with continuous exposure to 17 and 58 keV X-rays. To reproduce the effects of cycling within an intact coin cell, with nonuniform stack pressure, with nonuniform conductivity, and with nonuniformity in both stack pressure and conductivity, these cell configurations were simulated within the AMPIX cell (see Table 1) and electrodes cycled without exposure to the X-ray beam. The electrodes were delithiated to 4.6 V then lithiated to 3.35 V at which point the electrochemistry was terminated. For the different cell assemblies, electrodes were cycled at a rate of C/10. To cycle electrodes under X-ray exposure, within the limited beamtime available, electrodes were cycled at a faster rate, 1C. The cycled electrodes were recovered and sealed between Lexan plates to map the reaction state using spatially resolved powder X-ray diffraction. Bulging evident in electrodes cycled with nonuniform stack pressure made it difficult to recover these electrodes without their cracking. The electrochemical voltage profiles (Figure 2) for all experimental configurations were relatively consistent, the initial discharge capacities differing by
