Electrochemical Evaluation and Phase-related Impedance Studies on Silicon–Few Layer Graphene (FLG) Composite Electrode Systems

Silicon-Few Layer Graphene (Si-FLG) composite electrodes are investigated using a scalable electrode manufacturing method. A comprehensive study on the electrochemical performance and the impedance response is measured using electrochemical impedance spectroscopy. The study demonstrates that the incorporation of few-layer graphene (FLG) results in significant improvement in terms of cyclability, electrode resistance and diffusion properties. Additionally, the diffusion impedance responses that occur during the phase changes in silicon is elucidated through Staircase Potentio Electrochemical Impedance Spectroscopy (SPEIS): a more comprehensive and straightforward approach than previous state-of-charge based diffusion studies.

13g PAA powder was dissolved in 87g of deionised water using a Turbula (T10B Glen Mill) for 24 hours, followed by an overnight degassing period.
For the partial neutralisation, 5g sodium hydroxide pellets (NaOH, Sigma-Aldrich) were dissolved in deionised water added to the above PAA solution and blended using a spatula for 10 minutes. The resulting partially neutralised PAA solution was allowed to de-gas for 24 hours before it could be used.

Electrode Comparison Study
For further comparison with the forth formulation, bare Si and bare FLG electrodes were made according to the formulation in Table 2. Cross-sections were obtained through HITACHI Ion Milling System (IM4000 plus) at an acceleration voltage of 6 kV for 10 minutes.
The cycled cells are dissemble in the dry room and washed with dimethyl carbonate (DMC), and then follow the same procedure as stated above to obtain the cross-section images.

Electrode Density
The electrode density was obtained through measuring the mass and the thickness of the electrode.
The area of electrode is known as 1.72cm 2 , so the electrode density ρ can be calculated by:

Nano Indentation Test
The nano-indention test is conducted on nano-indenter (NanoTest Extreme, Micro Materials Ltd, UK) with a Berkovich indenter which diameter is 25um.
The test process is: 1. Applied the load to the electrode until it reached 15mN.
2. Hold it for 300s to ensure the creep exponent has been removed during unloading.
3. Remove the load and thermal drift correction for 60s.
The tensile curves are shown as Figure 1. Hardness and modulus are calculated using data taken from the slope of the tangent to the unloading curve, summarized as Table 3. It can be observed from Table 3 that the Young's modulus has been significantly reduced with incorporating FLG into Si electrode. Phase Change Process for dQ/dV quasi-plateaus

PEIS Result Fitting
Typically, the Nyquist plot of electrodes in a Li-ion battery contains two semi-circle and a linear drift of 45˚ to follow. It has been widely agreed that the intersection point of the X-axis refers to the series resistance, which is also called electrolyte resistance 4,5 . The first semi-circle represents the resistance when Li-ions are diffusing through the SEI layer, and the second refers to the charge transfer at the interface between the electrolyte and the active material. However, previous impedance studies on Si based electrodes 6,7 have suggested that interphase electronic contact resistance between the active material and the current collector (which normally reacted at medium-high frequency range) should be also taken into consideration. The straight-line tail followed is referred to as Warburg impedance, which includes the diffusion impedance of Li ions within active material. Based on the EIS spectra of the electrodes in this study and the previous literatures, the fitting equivalent circuit is shown in Figure 2. It consists of a resistor representing series resistance, and is followed by a series of three resistors in parallel with a constant phase elements and a Warburg diffusion element at the end. They respectively account for the SEI resistance, interphase electronic contact resistance and charge transfer resistance.

Figure 2
The equivalent circuit for fitting the impedance spectra