Electrochemical Assessments of Lithium Plating Reversibility on Graphite Anode in Lithium-Ion Batteries

In recent years, lithium-ion batteries (LIB) have been widely adopted as the power source for transportation systems, especially electric vehicles, owing to their superior energy and power densities. However, the extended charging time of LIB remains a significant barrier to the widespread adoption of electric vehicles. To address this problem, numerous studies have been conducted from a variety of perspectives (e.g., material, structure, charging algorithm) and as a result, the charging time of LIB has been significantly reduced. Despite these technological improvements, however, the reduced durability and safety of the LIB during fast charging remains an issue that needs to be addressed. Specifically, lithium plating on the anode surface during fast charging is a critical side reaction to avoid. The impact of lithium plating on battery performance can be broadly classified into three categories, based on how the lithium reacts after plating: lithium that is spontaneously intercalated into the active material after plating and then deintercalated during discharge; lithium that remains in the plated form and is oxidized during subsequent discharge; lithium that remains in the plated form but is unable to react during subsequent discharge. The first two are termed reversible lithium plating as they contribute to the capacity while the last is referred to as irreversible lithium plating because it does not participate in the subsequent discharge reaction. Irreversible lithium plating typically results in loss of lithium inventory, increased internal resistance, and can even cause internal short circuits if lithium electrodeposits accumulate with repeated charge and discharge. Therefore, research on lithium plating during fast charging should place greater emphasis on detecting irreversible lithium plating and analyzing its effects. Among lithium plating detection methods, electrochemical techniques are particularly noteworthy, because they can analyze the lithium plating/stripping in real-time without dismantling the battery. However, existing studies using electrochemical methods were primarily focused on determining the presence or absence of plated lithium on the anode and rarely discussed irreversible lithium plating. In this presentation, we propose an electrochemical method for detecting and quantifying irreversible lithium plating on the anode during LIB charging. For this purpose, lithium was initially plated onto graphite anode under different charging conditions, and the lithium morphology was analyzed through stereological image analysis. The morphology analysis showed that the porosity and surface area of the plated lithium increased with higher charging rates, indicating that the mechanical strength of the lithium is weaker when plated at higher charging rates. It was also observed that the lithium deposit layer shrank in the open circuit state immediately after charging, resulting in a decrease in the overall volume of the plated lithium. From the electrochemical assessments, including interfacial capacitance, open circuit voltage, and irreversible capacity, it proved that as the charging rate increased, the capacitance increased and the degree of capacitance decrease over time also increased in the open-circuit state immediately after charging. Furthermore, it was confirmed that the time period of flat relaxation voltage closely coincided with the time period of capacitance reduction and plated lithium shrinkage. By determining the irreversible capacity through discharge capacity analysis, the amount of irreversible lithium plating was estimated and found to be linearly related to the degree of capacitance reduction. This strongly suggests that the amount of irreversible lithium plating can be quantified by the degree of capacitance reduction over time in the open circuit state immediately after lithium plating. In this presentation, we will describe an electrochemical technique for analyzing the evolution of the plated lithium over time and propose a method for detecting irreversible lithium plating. Furthermore, we will discuss in detail the correlation between the interfacial capacitance and irreversible capacity, and present the possibility of quantifying irreversible lithium plating by an interfacial capacitance analysis.