Unraveling Effects of Current Density and Silicon on Silicon-Graphite Composite Anodes By in-Situ Synchrotron X-Ray Diffraction
Authors: Hua Weicheng , Schweigart Philipp , Nylund  , Lian Camilla , Cova Federico Hector , Svensson Ann Mari , Blanco Maria Valeria 
Published: February 2024

The development of lithium-ion batteries (LIBs) with high capacity and rate capabilities is imperative to meet energy demands for both transport and grid storage in the future. Due to its relative abundance and excellent cycling stability, state of the art LIBs contains graphite as the main anode component. However, graphite suffers from a low theoretical specific capacity of 372 mAhg-1 [
1
]. Alternatively, silicon (Si) has emerged as a promising energy-dense anode material with a theoretical specific capacity 10 times higher than graphite, but their widespread application is impeded by low electrical conductivity and extremely poor cycling stability due to large volume variations when alloying with lithium during electrochemical cycling [
2
]. Interestingly, silicon-graphite (SiG) composites have been pinpointed as promising options for producing improved anodes, which aims to combine graphite’s high electronic conductivity and structural stability with silicon’s high specific capacity. As expected, increasing the silicon content of SiG anodes reduces the cycling stability [3]. Optimization of SiG with respect to silicon content and utilization of silicon is not straightforward, and a comprehensive analysis on the effect of variations of silicon content and current density on SiG andoes is key. To investigate these interactions, in-situ synchrotron X-ray diffraction (XRD) analysis of composite anodes with varying silicon content at increasing c-rates was performed.

In situ experiments were performed at BM01 of the European Synchrotron Radiation Facility (ESRF). An in-situ electrochemical cell consisting of two current collectors, in which the lower current collector is made of brass and the upper current collector is made of stainless steel, was utilized [
4
]. A half-cell was then assembled between the two current collectors in a coin cell configuration using lithium metal as the counter electrode and a glass fiber separator. 1M LIFSI in 1:1 vol EC: DMC was used as the liquid electrolyte. The current collectors each have a thin single crystal sapphire window that is mostly transparent to X-ray but have distinct signals that can be masked out in post-processing. To avoid diffraction signal from the underlying copper current collector of the electrode, the composite anode consists of electrodes cast on copper foil as well as a free-standing section. The assembled cell was galvanostatically cycled between 1 V and 50 mV while XRD data was acquired. In this study 4 different electrodes containing 0, 20 %wt., 30 %wt. and 70 %wt. of Si in the active material of the electrode were cycled at C/5, C/2, 1C and 2C rates.

Analysis of the XRD results showed that tracking of the evolution of graphite main reflections during electrochemical cycling while completely avoiding the copper signal has been achieved. The degree of graphite lithiation was tracked by following the distinct 002, 100 and 110 peaks of graphite where phase transitions to LiC12 and LiC6 can be identified. Consistent with the results of other studies, graphite has been found to be both lithiated and delithiated at lower potentials than silicon [
3
]. Increasing silicon content was found to decrease the degree of graphite lithiation as silicon will be preferentially lithiated. Increasing the current density was found to reduce the overall degree of lithiation of graphite as the specific capacity decreases and the material is not sufficiently lithiated to undergo the expected phase transitions. This study has therefore unraveled the effect of silicon content and current density on composite anode behavior which could allow for the design of better next-generation composite anodes.

Tarascon, J.-M. & Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature
414, 359–367 (2001).

Ashuri, M., He, Q. & Shaw, L. L. Silicon as a potential anode material for Li-ion batteries: where size, geometry and structure matter. Nanoscale
8, 74–103 (2015).

Moyassari, E. et al. The Role of Silicon in Silicon-Graphite Composite Electrodes Regarding Specific Capacity, Cycle Stability, and Expansion. J. Electrochem. Soc.
169, 010504 (2022).

Drozhzhin, O. A. et al. An electrochemical cell with sapphire windows for operando synchrotron X-ray powder diffraction and spectroscopy studies of high-power and high-voltage electrodes for metal-ion batteries. J. Synchrotron Radiat.
25, 468–472 (2018).

Figure 1