Further improvements to lithium-ion and emerging battery technologies can be enabled by an improved understanding of the chemistry and working mechanisms of interphases that form at electrochemically active battery interfaces. However, it is difficult to collect and interpret spectra of interphases for several reasons, including the presence of a variety of compounds. To address this challenge, we herein present a vibrational spectroscopy and X-ray diffraction data library of ten compounds that have been identified as interphase constituents in lithium-ion or emerging battery chemistries. The data library includes attenuated total reflectance Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction data, collected in inert atmospheres provided by custom sample chambers. The data library presented in this work (and online repository) simplifies access to reference data that is otherwise either diffusely spread throughout the literature or non-existent, and provides energy storage researchers streamlined access to vital interphase-relevant data that can accelerate battery research efforts.

Data Collection

Prior to any characterization, all pristine compounds were stored in an argon glovebox with base oxygen and water concentrations of ~0.1 ppm and ~0.5 ppm, respectively. The sources and purities of the studied chemicals are provided in Table 1 of the associated paper.

ATR-FTIR Spectroscopy

ATR-FTIR spectra were collected from 370 to 4000 cm^-1 at a spectral resolution of 2 cm^-1 using a Shimadzu IRTracer-100 instrument with an IRIS single reflection diamond accessory. Herein, we generally report data in the mid-IR range, with a low-energy cutoff of about 500 cm­^-1. This approach was taken because the mid-IR range is accessible to experimentalists and because we use some data below 500 cm­^-1 to aid in the generation of a baseline for subtraction (see Data Processing for further details). We encourage those particularly interested in data around and below ca. 450 cm­^-1 to consult the raw data available in the online data library.

The ATR-FTIR instrument was housed in a nitrogen-filled glovebox with an oxygen concentration below 20 ppm. We note that to the best of our knowledge, none of the compounds in this study react with nitrogen at room temperature, the main offenders are oxygen and water, whose concentrations were analogous to levels in an Ar glovebox.  Compounds were transferred into the ATR-FTIR enclosure in sealed vials and then immediately placed on a clean diamond crystal for the ATR-FTIR measurement. This transfer approach was effective at minimizing unwanted reactions as described in detail in the Technical Validation section below. Most data presented here is an average of 512 individual spectra (CH₃COOLi, Li₂CO₃, ⁷LiF, ⁶LiF, Li₂O, PEO) which was used to maximize the signal to noise ratio, while only 50 spectra were accumulated for some of the more reactive compounds (LiH, LiPF₆, MnF₂, NiF₂) to minimize acquisition time and thereby reduce the likelihood of undesired reactions with trace amounts of oxygen or water.

Raman Spectroscopy

Raman spectra were collected using a 2 cm-square and 5 mm thick custom-made polyether ketone (PEEK) sample chamber with an optical window (2.5 cm-square and 1 mm thick glass microscope slide). The chamber, which kept samples in an inert argon environment during measurement, is illustrated in Fig. 1a. Prior to cell assembly, PEEK pieces and glass slides were sonicated with acetone and then isopropyl alcohol and baked at 40^oC for at least 4 hours before being transferred into an argon glovebox for assembly. After each sample chamber was assembled, it was isolated in a heat-sealed bag before being transferred to a Renishaw Qontor microscope where Raman spectroscopy was conducted. A 488 nm excitation laser was used at a power ranging from 1 to 10 mW to collect data over 25 acquisitions from 100 to 3200 cm^-1. An additional measurement of the lithium oxide sample was performed on the same instrument using a 633 nm laser (see Table 6). Unwanted contributions to the Raman spectra from the glass optical window were avoided by focusing the laser on the surface of the sample inside the chamber.

X-Ray Diffraction

The sample chambers used for XRD measurements were similarly assembled in an argon glovebox. Small quantities of each compound were placed on clean 2.5 cm-square and 1 mm thick glass microscope slides (cleaned and dried using the method described above) and covered with several sealing overlayers of polyimide tape (Kapton, Ted Pella, silicone adhesive, 70 µm thick) before being heat-sealed in individual plastic bags. The sample chambers were then transferred to a Bruker Phaser D2 instrument (wavelength, λ = 1.54 Å) where X-ray diffraction patterns were collected over a 2θ range of 10 to 90 degrees using an acquisition time of 0.2 seconds per step and a step size of 0.02 degrees per step. All samples remained in their sealed bags until right before the measurement was started. Kapton tape is not perfectly air-tight but was a sufficient barrier to enable the acquisition of data, which was completed within the first 15 minutes after the sample chambers were brought into ambient air. The XRD patterns were collected through the tape, rather than through the glass slide, to prevent significant XRD contributions from the glass. The relatively smooth XRD background from the amorphous tape was removed via processing as described in the Data Processing subsection. The Technical Validation section provides evidence that this approach successfully minimized unwanted reactions.

Data Processing

The collected raw data was processed to isolate features of the spectra and diffraction patterns that can be used to identify the presence of these compounds in complex data collected from interphases. Unwanted instrumental and background contributions were also removed through this processing. All FTIR measurements of inorganic compounds – and some organic ones – contained strong and broad absorption features below 600 cm^-1 which increased in  intensity as the wavenumber decreased. These features were so broad that the decreasing intensity side of the feature (e.g. see Palik and Hunter’s data on LiF in the Handbook of Optical Constants of Solids) was not observed above the instrument’s low-energy detection limit of 370 cm^-1. Because most researchers have detectors with similar limits (or even higher in energy), the FTIR data was processed to focus on the mid-IR regions (above ca. 500 cm^-1) that are most commonly accessible experimentally . As a result, low-wavenumber features (below our reporting window) were fit as part of the baseline when defining baseline profiles to be subtracted from the raw data across the entire measured range, even though they are technically not part of the background. Raw, unsubtracted data spanning the entire measured range is available in the data library. Panels 2a and 2b provide a representative baseline fitting of the raw data (which resulted in the removal of the downward-sloping part of the spectral feature that is observed below 600 cm^-1) and the resulting subtracted data, respectively. Spikes in the Raman spectra attributable to cosmic ray excitation were removed immediately after collection and are not included in the raw data. Raw Raman spectra were processed through the subtraction of Gaussian and/or polynomial fits to eliminate background contributions that could have been caused by several phenomena including fluorescence, glass effects, and surface roughness. Because our instrument generated raw Raman data with unevenly spaced wavenumber values, we performed an interpolation was required to use a fast Fourier transform filter for data smoothing. Gaussian fits to determine peak positions from the data before and after interpolation confirmed that this transformation did not affect the location or shape of spectral features.

Gaussian fits were used to subtract the amorphous background that the Kapton overlayer generated in XRD measurements. This background between 10 and 30 degrees appeared in all measurements through Kapton tape (but not in control measurements of bare metal foils) and was at lower 2θ than the first diffraction peak of most compounds, allowing subtraction of a consistent background in the few patterns (like that of lithium acetate) where the first diffraction peak was found below 30 degrees. This approach was deemed suitable because the there were no consistent and unidentifiable peaks in the processed XRD patterns which would have indicated a residual contribution from the Kapton overlayer.  A fast Fourier filter was applied to reduce high frequency noise (with care taken to avoid distorting spectral features) and small vertical offsets were used in some cases to align the high-2θ baseline near zero. All data was normalized to take on values from 0 to 1. To facilitate comparison with data collected on other instruments, d-spacing values (calculated using Braggs Law with λ = 1.54 Å) are included in the online repository in addition to a 2θ x-axis.

The data files are in a .xlsx format which can be opened using Excel (including the free Excel Viewer) and Google Sheets among other applications. These files can then be exported as a .csv or in other format as desired for use in other software (including data processing and plotting programs).