State-of-Charge Dependent Change in the Microporous Structure of Graphite Electrodes

Abstract

Regarding the negative electrode, the most used electrode material in commercially available lithium-ion batteries (LIBs) up to the present day is graphite.1 Upon battery operation, an irreversible volume change in the microporous structure of the graphite electrode is detected. It is associated with the formation of the solid-electrolyte interphase (SEI) within the first cycles and an irreversible thickness increase of the electrode. Consequently, one observes a change in the electrode’s porosity after the initial formation cycles.2,3 Next to irreversible microstructure changes, one observes reversible changes in the microstructure associated with the intercalation of Li-ions into the host structure of the graphite particles upon charging the battery. The electrode particles expand during this process, increasing particle volume by ~13% when fully lithiated.4 In commercial LIBs, electrode materials, separators, and current collectors are tightly packed for a high volumetric energy density. This restricts the thickness expansion of the electrodes.5 Consequently, the graphite particles must expand into the internal pores of the electrodes, resulting in a decrease in the void volume of the porous network. Since the porous electrodes and the separator are filled with the electrolyte and the particle expansion of graphite is more significant than that of the active material in the positive electrode, one observes electrolyte motion in commercial LIBs during charge and discharge. It can be tracked by techniques requiring neutron beam facilities or expensive devices such as X-ray CT instruments.2,6 Recently, Aiken et al. developed a relatively simple apparatus to track the electrolyte motion by monitoring the changing moment of inertia of cylindrical cells during operation and simplifying the approach to study the discussed effect.In this study, we developed a method to track the reversible change of the microporous structure of an artificial graphite (MAGE-5, Hitachi, Japan) with an LP572 electrolyte (1 M LiPF6 in EC:EMC 7:3 w:w with 2 wt% VC, Gotion) during a charge and discharge cycle after formation employing electrochemical impedance spectroscopy (EIS) and operando dilatometry with both cell setups having a 1.5 bar cell compression Since the electrode’s microstructure is connected to the macroscopic properties using the MacMullin number of:NM=τ/ε=RIon·A·κ/dit correlates the porosity , electrodes thickness , ionic pore resistance , area of the electrode , conductivity of the electrolyte , and the tortuosity .8 Thus, we analyzed the change of the ionic pore resistance of graphite electrodes employing EIS in a MAGE//Li half-cell setup with a micro-reference electrode and a free-standing graphite electrode.Due to the particle expansion, we monitor an increase of the ionic pore resistance of up to 40% during charging, i.e., lithiation, of the graphite electrode.Next, we determined the SOC-dependent increase in thickness of the graphite electrode revealing a reversible thickness increase of 3%. Knowing the relation of porosity and tortuosity in a generalized Bruggeman form and using Equation 1 as described above with a constant electrode area and assuming a constant electrolyte conductivity , allowed us to extract the relative SOC-dependent decrease in porosity and void volume of the electrodes. At the end of the charge, both values decreased by approx. 10% and 9%, respectively, confirming the electrolyte repression from the graphite electrode.