Investigating the Change in the Microporous Structure of the Graphite Electrode during Formation

Abstract

Till today, the graphite-based electrode is the most widely used negative electrode in commercially available lithium-ion batteries (LIBs).[1] While extended research has been conducted in the investigation of the graphite electrode’s properties, especially the formation of the solid-electrolyte interphase (SEI) within the first cycles of LIBs remains a field of ongoing research.[2] The SEI forms during the first cycle(s) on the anode surface at potentials below the stability window of the electrolyte components, due to the reductive decomposition of the respective solvents, additives, and conductive salts.[2,3] The initially formed SEI has an influence both on the long-term cycling stability of the cell, depending on how good the passivation, i.e., electronic insulation, of the graphite surface is,[4] and on the rate performance of the anode, depending on the thickness of the SEI, its chemical composition, and its lithium-ion conductivity, referred to as SEI resistance.[5] Additionally, with a thickness of 10-20 nm when employing additives such as vinylene carbonate (VC), the SEI takes up parts of the void volume of the pristine electrode and may change the pores and the electrolyte conduction pathways within the electrode (see Figure 1).[6] In this study, we investigated the change of the microporous structure of an artificial graphite (MAGE-5, Hitachi, Japan) with an LP572 electrolyte (1 M LiPF6 in EC:EMC 3:7 w:w with 2 wt% VC, Gotion, USA) during formation at 45 °C. The electrode’s microstructure is often connected to the macroscopic properties using the McMullin number N M in the form of:[7] N M = τ / ε = R Ion ·A · κ / d, (1)correlating the porosity ε, electrodes thickness d, ionic pore resistance R Ion, area of the electrode A, conductivity of the electrolyte κ, and the tortuosity τ (see Figure 1).Here, we analyzed graphite electrodes with varying initial porosities from 20 to 50 %, employing electrochemical impedance spectroscopy (EIS) in a MAGE//Li half-cell setup with a micro-reference electrode and a free-standing graphite electrode.[8,9] Measuring EIS at temperatures of -5 °C allowed the reliable determination of the ionic resistance before and after formation. Due to the formed SEI covering the graphite particles and partially filling the void of the porous electrode, we discovered an increase of the ionic pore resistance throughout the whole range of initial porosities.Next, we determined a porosity dependent increase in thickness of the electrodes from electrodes cycled at the same conditions in coin cells that were subsequently harvested from the cells and analyzed for thickness changes. Additionally, mercury intrusion porosimetry (MIP) was used to determine the porosity of formed graphite electrodes, revealing a decrease in porosity after formation for all samples. The combination of these findings - using Equation 1 as described above with a constant electrode area A and assuming a constant electrolyte conductivity κ - allowed us to evaluate the change in the tortuosity of the graphite electrodes, giving insights into the overall changes in the graphite anode properties due to SEI formation.