Batteries hold the key ingredient for powering almost everything, from laptops and mobile devices to electric vehicles and space missions. However, despite being hailed as a solution to fossil fuel dependence, today’s lithium-ion batteries are not quite ready to fuel tomorrow’s technologies solely for one reason: they are too heavy.
Take the electric vehicle Tesla Model S, for example, which uses a lithium-ion battery that measures a whopping 2,200 kg, about a quarter of the car’s total weight.
To lessen the load, a team led by Zhaolin Liu, a Senior Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE), is developing a new wave of high energy density batteries. These lithium-sulfur (Li-S) cells are much lighter than their predecessors, have superior specific energy and are less prone to overheating during use, making them ideal for transportation and aviation applications.
Li-S batteries are still far from perfect. Although sulfur has a much higher theoretical specific capacity than those of current cathode for lithium-ion batteries (1672 vs 287 mAh/g), Li-S batteries suffer from fast capacity fading, low rate capability (the maximum charge/discharge rate of a battery) and low volumetric energy density (the amount of energy a system contains in comparison to its volume)—all of which hinder their commercial adoption.
To overcome these limitations, Liu and colleagues took a closer look at the electrochemical reactions that cause Li-S batteries to drain their charge so quickly. Using electrochemical impedance spectroscopy (EIS) for measuring battery resistance change during discharging of a Li-S battery, the team looked for specific chemical factors that influence Li-S cell deterioration.
“The reduction from sulfur to lithium sulfide forms various intermediates, including Li2S8, Li2S6, Li2S4, Li2S3 and Li2S2,” explained Liu. “The discharge capacity and output current are mainly determined by the conversion of these intermediates during discharge.”
Using EIS, the team observed two distinct voltage dips over time, coinciding with the transition from Li2S4 and Li2S2. According to Liu, these sudden voltage drops correspond to energy barriers, proving to be the culprit behind sulfur’s poor conductivity.
Electrochemical impedance spectroscopy confirmed voltage drops correspond with two energy barriers in sulfur reduction during discharge.
© A*STAR Research
With this leap forward in our understanding of chemical dynamics in batteries, the researchers have their sights set on making a better Li-S battery.
“Our fundamental research could facilitate the development of Li-S batteries, especially fast-discharging sulfur batteries,” noted Liu. “We will work in line with our findings to explore effective catalysts for sulfur reduction.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
