研究目的
To study the kinetics of phase transformation and phase boundary propagation during initial lithiation of silicon electrodes in lithium ion batteries, and to develop a non-invasive, in situ method for measuring phase boundary progression under controlled conditions.
研究成果
The developed non-destructive, in situ method combining picosecond ultrasonics with an electrochemical cell successfully measures phase boundary propagation in crystalline silicon during lithiation with nanometer resolution. Key findings include a phase boundary velocity of approximately 12 pm/s at 40 μA/cm2, an average x value of 3.73 in LixSi, a volume expansion ratio of 3.85, and a sound velocity in the lithiated layer of about 7.15 nm/ps. The reaction rate coefficient is estimated at 4.5e-5 mm?·s?1·mol?1 using a Deal-Grove model. The technique provides precise control over electrochemical parameters and real-time monitoring, facilitating better understanding of kinetics in battery materials.
研究不足
The technique has limited sensitivity to fine structural details such as gradients in lithium concentration in LixSi. The sound velocity in the lithiated layer is not independently measured, relying on literature values for calculations. Charge losses due to solid electrolyte interphase (SEI) formation are approximated and not fully accounted for, potentially affecting accuracy. The method requires planar sample geometries and may not be directly applicable to complex shapes.
1：Experimental Design and Method Selection:
The study employs picosecond ultrasonics (PU) integrated with an electrochemical cell for real-time, non-invasive measurement of phase boundary propagation in crystalline silicon during lithiation. The PU technique uses ultra-fast laser pulses to generate and detect ultrasound pulses, allowing precise thickness measurements.
2：Sample Selection and Data Sources:
Silicon wafers with (100) orientation, approximately 40-55 μm thick, are used as samples. An aluminum thin film is sputter-deposited on one side as a transducer, and a current collector is microfabricated on the other side.
3：List of Experimental Equipment and Materials:
Equipment includes a mode-locked Ti-Sapphire pulsed laser (wavelength 800 nm, pulse duration 100 fs, repetition rate 80 MHz), half-waveplate, polarized beamsplitter, electro-optic modulator, retroreflectors, lenses, non-polarized beamsplitter, photodiode, lock-in amplifier, translation stage, data acquisition card, and an Autolab PGSTAT128N potentiostat. Materials include silicon wafers, aluminum film, nickel grid current collector, lithium metal electrode, Woven Celgard C2500 separator, electrolyte (1.2 M lithium hexafluorophosphate in ethylene carbonate/diethyl carbonate), transparent epoxy, and a custom polyether ether ketone (PEEK) cell.
4：2 M lithium hexafluorophosphate in ethylene carbonate/diethyl carbonate), transparent epoxy, and a custom polyether ether ketone (PEEK) cell.
Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: The sample is assembled in the electrochemical cell with controlled lithium flux. Lithiation is performed in steps: constant current to 0.1 V, hold at 0.1 V for 1.5 h, then constant current lithiation at 40 μA/cm2. PU measurements are taken before and after each lithiation step, involving laser pulse generation, modulation, delay variation, and signal detection to measure echo arrival times.
5：1 V, hold at 1 V for 5 h, then constant current lithiation at 40 μA/cm2. PU measurements are taken before and after each lithiation step, involving laser pulse generation, modulation, delay variation, and signal detection to measure echo arrival times.
Data Analysis Methods:
5. Data Analysis Methods: Data processing includes subtracting a polynomial background to remove thermal effects, calculating second derivatives to eliminate oscillatory components, and analyzing echo arrival times to determine phase boundary position and sound velocities. Kinetic parameters are extracted using a Deal-Grove type model.
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