Electrode-Resolved Impedance and Potential Measurements to Investigate the Rate Capability of High Loading Micron-Silicon Anodes in All-Solid-State Batteries
Authors: Conforto Gioele , Schuster Robin , Bohn Moritz , Kutsch Tobias , Gasteiger Hubert Andreas 
Published: February 2024

In order to enable the market introduction of all-solid-state batteries (ASSBs), several anode concepts are under investigation to increase the energy density compared to graphite anodes, while maintaining long cycle-life. At the same time, the production costs must be the same or even reduced in comparison with classical lithium-ion batteries. Besides lithium metal as anode material, which still poses significant technical challenges, more and more interest is being directed toward materials that form lithium alloys. One of the most attractive candidates is silicon, which has a low delithiation potential of around 0.45 V vs. Li+/Li and an attractive theoretical gravimetric capacity of 3579 mAh/gSi, which in the fully lithiated state corresponds to a volumetric capacity of ≈2190 mAh/cm3, even higher than the one reachable by lithium metal.

Silicon-containing anodes are already being introduced in classical lithium-ion batteries. For that system, the lithium-ion conduction across the anode electrode is mainly provided by the liquid electrolyte that fills the pores of the electrode, while the electrical conduction is enhanced by the addition of graphite and/or conductive carbon additives, because crystalline silicon is only a semiconductor. As is well known, the major issue with silicon anodes is the volume expansion of 300% upon lithiation of silicon, inducing severe cracking of µm-sized silicon particles as well as continuous solid electrolyte interphase (SEI) growth, both of which lead to a fast capacity fading.

The intrinsic solid nature of all-solid-state batteries opens the possibility of a new electrode concept that has the potential to mitigate the above-mentioned drawbacks of silicon. Indeed, the silicon particles themselves could be used for the lithium-ion conduction across the electrode, so that anode electrodes can be prepared that are free of electrolyte, thereby largely restricting SEI formation to the anode/separator interface .

In this work, anodes with >90% weight of silicon are prepared by coating a water-based slurry that contains microscale silicon particles (ca. 2-4 µm diameter) as active material, the rest being made up of LiPAA and NaCMC as binders as well as C65 carbon as conductive additive. The combination of the water-based process and the microscale dimension of the silicon particles make this electrode concept easily scalable and cost-effective.

To reach a competitive areal silicon anode capacity of 3 mAh/cm2, with the full utilization of the silicon, only a thin layer is needed (pristine thickness of 8 µm when the anode porosity is 50%). The thickness variation and the anode morphology are being investigated by examining cross-sectional scanning electron microscopy (SEM) images at different states-of-charge (SOC). After the first complete lithiation, the anode consists of a fully dense and compact Li3.75Si alloy block of 15 µm thickness. Upon its complete delithiation, a peculiar porous columnar morphology is formed that on account of the void space between the columns and within the columns retains a thickness of 11 µm. This peculiar morphology was recently reported in the literature (Tan et al., Science 373 (2021), 1494–1499); and seems to be reversible during subsequent lithiation and delithiation processes. For example, our half-cells utilizing an InLi counter electrode and operating at 70 MPa have more than 80% capacity retention after 350 cycles at a rate of C/5, with an average coulombic efficiency higher than 99.9%. The reason for this promising stability could be attributed to the minimal loss of lithium into the SEI, which is limited to the geometrical contact area between the silicon and the solid electrolyte separator.

To better understand the electrochemical and mechanical characteristics of these silicon anodes, they are investigated in full-cells, utilizing an NCM composite cathode, while the anode potential is monitored by using a gold wire micro reference electrode (µ-RE) which was recently developed in our group (Sedlmeier et al., J. Electrochem. Soc. 170 (2023) 030536). This specific µ-RE, which has a defined and stable potential, allows to precisely deconvolute the potential curves of cathode and anode, and also to measure and separate the impedance contributions of the two electrodes. Therefore, the impedance of the silicon anode is measured at different SOC to understand how the contact resistances, the charge-transfer resistance, and the solid-state diffusion affect its overall impedance. In particular, the low impedance at high SOC enables to reach a good fast charging rate capability up to 1C without lithium metal plating and dendrite formation, as reported in Figure 1. However, in order to explore whether the fast charging goal of 4C (80% of the capacity in 15 minutes) can be reached, the influence of the carbon additive, operating temperature and applied pressure are being investigated.

Figure 1