Investigating the Charge and Discharge Mechanisms for Lithium-Sulfur Batteries Using Electrochemical Impedance Spectroscopy
Authors: Lateef Saheed Adewale , Mustain William Earl , Jalilvand Golareh 
Published: November 2022

The advent of electric vehicles, grid storage, and more powerful electronic devices has made it necessary to develop a new generation of high energy rechargeable batteries to overcome the inadequacies of the current lithium-ion batteries (LIB) technology. Replacing the conventional cathode material with Sulfur and pairing it with a Lithium (Li) metal anode to obtain the Lithium-Sulfur (Li-S) battery is a promising approach. Li-S has a theoretical specific energy density of 2600 Wh/kg, which is significantly higher than what the modern LIB can offer1. In addition, the low cost, natural abundance, and environmental friendliness of sulfur suit commercial consideration.
However, the path to commercialization and adoption of Li-S has several challenges. One of the major challenges in Li-S batteries relates to the undesired solubility of the sulfur products in the liquid electrolyte, resulting in so-called Li polysulfides (LiPSs) “shuttling”. During the discharge, the reaction of Li and sulfur results in the stepwise transformation of stable ring-shaped sulfur (S8) to a series of LiPSs (Li2Sx, x = 1, 2, 4, 6, 8). Unfortunately, the long-chain LiPSs have high solubility in the electrolyte that facilitates the back-and-forth transport of the LiPSs between the electrodes. The result is a “chemical short” in the cell, a loss of active material, and poor cyclability2. To overcome this problem, a clear understanding of the discharge and charge electrochemical reactions in Li-S battery is much needed.
Electrochemical Impedance Spectroscopy (EIS) is a powerful technique to study the electrochemical mechanisms at work in many technologies, including the Li-S battery. Because of this, multiple EIS-based measurements for the Li-S battery have been reported. However, those attempts carried out EIS measurements during the first cycle3–5 or the first few cycles6, which does not provide robust insight into the conditioned charge and discharge processes taking place in a real battery as it operates for many cycles. Hence, a clear understanding of the electrochemical processes and their mechanisms in Li-S batteries at extended cycle numbers is needed.
In this work, we expand our understanding of the Li-S charge and discharge mechanisms at a high number of cycles and various depths of discharge (DoD) and states of charge (SoC). An equivalent circuit model of the EIS spectra was proposed from the Nyquist plots. New insights about the individual contribution of charge transfer resistance and the impedance resulting from the deposition of non-conducting Li2S/Li2S2 at high number of cycles were achieved. These new insights are expected to provide a better understanding of the Li-S performance during extended cycle life.

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