A team of researchers recently published an article in the journal Advanced Material Interfaces who demonstrated the advantageous characteristics of bacterial cellulose (BC)-based separators in lithium-sulfur (Li-S) battery applications.
Study: Understand the advantageous features of bacterial cellulose separator in Li-S battery. Image Credit: RESTOCK images/Shutterstock.com
Background
Li-S batteries have garnered considerable attention as next generation energy storage systems due to their environmental friendliness and high energy density. The separator is one of the critical components of Li-S batteries that influence battery performance, as it separates the anode and cathode and prevents short circuits in the battery.
The separator influences the homogeneity between the electrolyte and the electrode and the transport of ions between the electrodes. Polypropylene (PP) and Polyethylene (PE) separators are most widely used as battery separators. However, these separators exhibit low selectivity for ion transport and poor compatibility with the Li anode, which makes them less suitable for Li-S battery applications.
Cellulose, an abundant, natural and renewable resource, is considered as a suitable alternative to conventional polyolefin separators used in various secondary batteries due to its greater mechanical strength and high thermal stability, which ensures increased safety performance. .
The hydroxyl groups in cellulose help regulate the ion transport process in the battery. Cells using cellulose-based masks as separators demonstrated uniform Li anode morphology and outstanding charge-discharge performance for 200 h. Additionally, cellulose separators with nanopores showed dendrite-free performance for 500 h.
Although several studies have demonstrated the outstanding performance of batteries using different cellulose-based separators, a lack of detailed understanding of the various advantageous characteristics of cellulose-based separators, especially BC-based separators, in Li-S batteries hinders large-scale use. of these separators.
BC-1 surface and cross-section SEM images a B) airgel and CD) PP separator. e) NOT2 adsorption-desorption isotherm of different BC aerogels. F) Electrolyte storage capacities of PP and BC separators. g) The thermal stabilities of different separators under different temperatures. Image Credit: Li, J et al., Advanced Materials Interfaces
The study
In this study, researchers prepared models of BC separators with different thicknesses and compared them with PP separators in terms of electrochemical performance. The objective of the study was to confirm the ability of cellulose-based separators to improve the performance of Li-S batteries by regulating lithium-ion (Li+) deposition on the anode and regulation of the movement of ionic species, such as Li+ and lithium polysulfides (LiPS).
BC hydrogel, sodium hydroxide and lithium sulfide were used as raw materials. The thickness of the BC separator has been controlled in a microbial fermentation process. In this process, a common aqueous nutrient medium was used to grow the bacteria, and then BC was produced as an exopolysaccharide at the medium/air interface.
A thick gel, called a pellicle, composed of 99% water and an interconnected three-dimensional (3D) porous BC nanofiber network was obtained. Subsequently, the protein in the BC hydrogel was removed by soaking it in 5% sodium hydroxide solution, heating it to 80 ohC for several hours, and finally wash it with deionized water.
The resulting purified BC hydrogel was stored in a 4 ohC then freeze-dried for 24 h at 10-6 pressure bar and −52 ohTemperature C to obtain the airgel BC. Eventually, the BC airgel was cut into 19 mm discs as spacers with thicknesses of five, three, and one mm and designated BC-5, BC-3, and BC-1, respectively.
X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were performed to characterize the synthesized samples. The dimensional changes of the separators were recorded at different temperatures to measure their thermal stability.
The researchers evaluated the electrolyte absorption rate, the electrolyte storage capacities and the porosity of the separator. The Vienna ab initio simulation package was used to perform the theoretical simulation of binding energies. The researchers also performed electrochemical measurements on the prepared BC separator samples and visual polysulfide shuttle tests.
a) Voltage profiles of symmetrical lithium cells with PP and BC-1. SEM images of the surface of the cycled lithium anode (after 680 h) in a symmetric cell with b) PP or vs) BC-1. High resolution O 1s XPS spectra of lithium anode surface cycled in a symmetric cell with d) PP or e) BC-1. Image Credit: Li, J et al., Advanced Materials Interfaces
Comments
BC separators of varying thicknesses have been successfully synthesized. BC separators exhibited better electrolyte absorption and wettability compared to commercial PP separators due to the abundance of hydroxyl groups and higher porosity, which improved Li+ transport and improved interface compatibility.
The separators also inhibited Li dendrite formation, leading to uniform Li deposition on the anode. Additionally, the separators demonstrated high thermal stability, which improved the safety performance of the battery. In addition, BC oxygen functional groups suppressed the shuttling of soluble polysulfides by effectively adsorbing polysulfides through electrostatic interactions.
a) Shuttle currents of different separators; b) Shuttle current decay rates are calculated after steady-state current is reached for a lithium-sulfur cell. Algebraic equations express the rate of decay. Image Credit: Li, J et al., Advanced Materials Interfaces
The symmetric cell with BC separator demonstrated safe electrochemical performance for 680 h due to the formation of lithium oxide (Li2O) from cellulose and Li metal and 3D fibrous structure. The quantitative relationship between the capacity loss and the physical properties of the separator has been successfully described by optimizing a mathematical model. Cellulose played a unique role in the LiPS mass transfer process.
In summary, the results of this study demonstrated the feasibility of using BC separators as a suitable battery material for high energy density Li-S batteries and other Li-metal batteries and validated the hypothesis on the advantageous characteristics of cellulose-based separators for Li-S. battery apps.
In the future, the knowledge gained in this study on the advantageous characteristics of BC separators can provide a theoretical basis for the design of functional BC separators for batteries.
References
Li, J., Li, Y., Li, Z., et al. (2022) Understanding the Advantageous Features of Bacterial Cellulose Separator in Li-S Battery. Advanced Material Interfaces. https://onlinelibrary.wiley.com/doi/10.1002/admi.202201730
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