Si-based materials for lithium-ion batteries IV: Surface-modified Si/C/perfluorooctene powder

Commercial lithium-ion batteries (LIBs) typically contain graphite as the electrochemically active material in the negative electrode (anode). Silicon is being considered as the intercalation material to partially or fully replace graphite in next-generation, high energy density, LIBs because the theoretical lithium storage capacity of silicon is about ten times greater than that of graphite (Ref. 1). However, the commercialization of silicon-based electrodes has been hindered by the large volume changes (320% increase) that result from the insertion/extraction of lithium-ions into the silicon particles; graphite, in comparison, displays an ∼10% volume change. Additionally, the electrochemical alloying/dealloying with lithium fractures the silicon particles and increases their surface area, which, in turn, increases parasitic side-reactions that trap lithium-ions and degrades battery performance.

Various strategies are being pursued to improve the performance and longevity of silicon-containing lithium-ion batteries. Modifying the silicon particle surfaces to increase electronic conductivity and minimize reactivity with the electrolyte ranks high among these approaches. In this study, we report x-ray photoelectron spectroscopy data from surface-modified silicon powders obtained from a commercial vendor (Paraclete Energy, Inc.). All silicon particles have an oxide surface layer that forms during material synthesis. Coating these particles with carbon can prevent H₂ evolution reactions that occur during water-based electrode fabrication processes. Additional coatings can alter particle interactions with the electrolyte (such as wetting) and insertion/extraction of Li during charge/discharge cycling. Here, data are presented from Si particles coated with a thin carbonaceous layer (Si/C) and an additional coating based on perfluorooctene (PFO). The spectra indicate the expected silicon-carbon, carbon-fluorine, and silicon-fluorine species related to the surface modification process in addition to oxidized carbon and silicon due to atmospheric exposure.

XPS measurements of the powder and anode samples were made using a Kratos Axis Ultra x-ray photoelectron spectrometer (Kratos Analytical, Inc., Manchester, UK) using monochromatic Al K_α radiation (1486.6 eV). High-resolution spectra were collected from a single spot at an emission angle of 0° at a pass energy of 40 eV from a 0.7 × 0.3 mm² area using the hybrid (electrostatic and magnetic immersion) lens mode.

Host Material: Si/C/perfluorooctene powder

CAS Registry #: Unknown

Host Material Characteristics: Homogeneous; solid; polycrystalline; dielectric; inorganic compound; powder

Chemical Name: Silicon/carbon/perfluorooctene

Source: Paraclete Energy Inc.

Host Composition: Silicon + 2.5 wt. % carbon + 4 wt. % perfluorooctene

Form: Powder

Structure: Silicon has a diamond cubic structure

History and Significance: The surface-modified Si/C/PFO powder was obtained from Paraclete Energy, Inc. The particles were produced by milling and have a mean diameter of ∼150 nm. This material is used in the fabrication of anodes for high-energy silicon-based lithium-ion cells.

As Received Condition: Untreated material as-received from the supplier.

Analyzed Region: Same as the host material

Ex Situ Preparation/Mounting: Specimen was mounted onto a sample holder with an insulating double-sided office tape (3M 665) (Ref. 2). The sample holder was then loaded onto a grounded sample stage.

In Situ Preparation: None

Charge Control: Low energy flood gun/magnetic immersion lens combination, with filament current = 1.8 A, charge balance = 3.5 V, and filament bias = 1 V (Ref. 3).

Temp. During Analysis: 300 K

Pressure During Analysis: <3 × 10⁻⁷Pa

Preanalysis Beam Exposure: 0 s.

Manufacturer and Model: Kratos Axis Ultra

Analyzer Type: Spherical sector

Detector: Channeltron

Number of Detector Elements: 8

Analyzer Mode: Constant pass energy

Throughput (T = E^N): N = 0

Excitation Source Window: None

Excitation Source: Al K_α monochromatic

Source Energy: 1486.6 eV

Source Strength: 210 W

Source Beam Size: 2000 × 2000 μm²

Signal Mode: Multichannel direct

Incident Angle: 60°

Source-to-Analyzer Angle: 60°

Emission Angle: 0°

Specimen Azimuthal Angle: N/A

Acceptance Angle from Analyzer Axis: 0°

Analyzer Angular Acceptance Width: 40° × 40°

Manufacturer and Model: Kratos Minibeam I

Energy: 4000 eV

Current: 0.001 mA

Current Measurement Method: Biased stage

Sputtering Species: Ar⁺

Spot Size (unrastered): 1000 μm

Raster Size: 2000 × 2000 μm²

Incident Angle: 90°

Polar Angle: 45°

Azimuthal Angle: 90°

Comment: Sputtering was performed with a differentially pumped ion gun for calibration spectra only.

Energy Scale Correction: The binding energy scale of the instrument was calibrated using the Cu 2p (932.6 eV) and Au 4f (84.0 eV) photoelectron lines (Ref. 4). The data for the two survey spectra were referenced to Si 2p = 99 eV (Ref. 3). The data for the high-resolution spectra were referenced to Si 2p = 99.3 eV, as determined by peak fitting [GL(10)].

Recommended Energy Scale Shift: First survey 2 eV, high-resolution spectra 2.3 eV, and second survey 2 eV

Peak Shape and Background Method: Peak shape: See supplementary material (Ref. 2). Background: A noniterative Shirley background was used for all remaining regions (Ref. 5).

Quantitation Method: Quantification was done using region definitions for the survey and peak fitting component definitions for the high-resolution spectra (Refs. 6 and 7) with CasaXPS version 2.3.18. Sensitivity factors were supplied by Kratos Analytical.

This work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois. D.P.A. gratefully acknowledges support from the Office of Vehicle Technologies at the U.S. Department of Energy. This manuscript has been cocreated by UChicago Argonne, LLC, Operator of Argonne National Laboratory. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.