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Material characterization
This protocol is extracted from research article:
Ultrathin conformal polycyclosiloxane films to improve silicon cycling stability
Sci Adv, Jul 19, 2019; DOI: 10.1126/sciadv.aaw4856

The thickness and optical constants of the polymer coatings deposited on the Si monitor wafers were determined by measuring spectral reflectivity and fitting to an appropriate optical model (F20-UV, Filmetrics). The composition of the polymer coatings on the Si electrodes was characterized using FTIR spectroscopy (Tensor 27, Bruker) with an ATR accessory having a monolithic diamond crystal with a 45° incident angle (Specac Golden Gate). Measurements were collected from 600 to 4000 cm−1 with a resolution of 4 cm−1. A total of 128 scans were integrated for an improved signal-to-noise ratio. The spectra were baseline-corrected in the Bruker OPUS software.

XPS was performed with a monochromatized Al Kα source operated at 15 kV and 10 mA (AXIS Ultra spectrometer, Kratos Analytical). Samples were loaded onto a sample holder using a double-sided copper tape. Survey scans were performed from 1200 to −5 eV with a 1-eV step. Survey scan pass energy was set at 160 eV. Spectra binding energies were calibrated to the aliphatic C 1s peak position at 284.8 eV. Ten scans were integrated for high-resolution spectra for C 1s, O 1s, Li 1s, and Si 2p with a pass energy of 20 eV. CasaXPS software was used to analyze and deconvolute peaks using a Gaussian-Lorentzian mix with a baseline modeled using the Shirley method. Uncoated and pV4D4-coated silicon thin film electrodes were imaged using a Zeiss Auriga scanning electron microscope with variable pressure operated between 10 and 15 kV.

Silicon electrodes were cycled against lithium metal in a coin cell format, assembled in an Ar-filled glovebox. Before battery assembly, the silicon electrodes were dried at 100°C in nitrogen gas. Aromatic polyamide (aramid) fiber–based separators (Gold 40, Dreamweaver) were used. These separators were infiltrated with 100 μl of 1.0 M LiPF6 in 1:1:1 EC:EMC:DMC electrolyte (Gotion). The cells were cycled galvanostatically from 0.05 to 1.5 V using a battery cycler (BT-2043, Arbin Instruments). The first two cycles were performed at a C/10 rate to establish the SEI, and the rate was then increased to 1C for long-term cycling. Cell temperature was maintained at 25°C.

Potentiostatic EIS was performed using a three-electrode split cell configuration (EQ-3ESTC15, MTI Corporation) to isolate behaviors on the silicon electrode. The silicon electrode (diameter, 7/16″) was the WE. Lithium metal (diameter, 9/16″) was the CE. A small piece of foil (~1.5 mm by 1.5 mm) at the end of a copper foil strip (~1 mm by 12 mm) served as the RE. The three electrodes were separated by two separators (diameter, 3/4″; Gold 40, Dreamweaver). Electrolyte used was 200 μl of 1.0 M LiPF6 in 1:1:1 EC:EMC:DMC (Gotion). For impedance as a function of cycling, cells were cycled from 0.05 to 1.5 V at 1C (SP-200, Bio-Logic Science Instruments). Before each impedance measurement every five cycles, the silicon electrode was lithiated to 0.05 V and potentiostatically held for 2 hours to achieve equilibrium. Impedance spectroscopy was performed with a frequency sweep from 3 to 25 mHz with an amplitude of 10 mV.

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