How a cellulose-based bacterial separator can help lithium-sulfur batteries

A team of researchers recently published a paper in the journal Advanced material interfaces that demonstrated the advantages of bacterial cellulose (BC)-based separators in lithium-sulfur (Li-S) battery applications.

Study: Understanding the advantages of cellulose-based bacterial separator in Li–S battery. Image credit: RESTOCK images/Shutterstock.com

Record

Li-S batteries have gained much attention as next-generation energy storage systems due to their environmental friendliness and high energy density. The separator is one of the critical components in Li-S batteries that affect battery performance, as it separates the anode and cathode and prevents short-circuiting in the battery.

The separator affects the homogeneity between the electrolyte and the electrode and the ion transport between the electrodes. Polypropylene (PP) and polyethylene (PE) separators are widely used as battery separators. However, these separators exhibit low selectivity for ion transport and poor compatibility with the Li anode, making them less suitable for Li–S battery applications.

Cellulose, an abundant, natural and renewable resource, is considered a suitable alternative to conventional polyolefin separators used in various secondary batteries due to its greater mechanical strength and high thermal stability, which ensures improved safety performance.

The hydroxyl groups in cellulose facilitate the regulation of the ion transport process in the battery. Cells using cellulose-based masks as spacers exhibited uniform lithium anode morphology and excellent charge-discharge performance for 200 h. In addition, nanoporous cellulose spacers exhibited dendrite-free performance for 500 hours.

Although several studies have shown the excellent performance of batteries using different cellulose-based separators, the lack of detailed understanding of the different advantageous characteristics of cellulose-based separators, especially BC-based separators, in Li-S batteries hinders the use big scale. from these spacers.

SEM images of the surface and cross-section of BC-1 a, b) airgel and CD) PP separator. mi) N₂ adsorption-desorption isotherm of different BC aerogels. eat) PP and BC separator electrolyte storage capabilities. G) The thermal stability of different separators at different temperatures. Image credit: Li, J et al., Advanced Materials Interfaces

The study

In this study, the researchers prepared models of BC separators with different thicknesses and compared them with PP separators in terms of electrochemical performance. The aim of the study was to confirm the ability of cellulose-based separators to improve Li-S battery performance by modulating lithium ions (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 spacer is controlled in a microbial fermentation process. In this process, a common aqueous medium was used to grow the bacteria, and then BC was produced as an exopolysaccharide at the medium/air interface.

A thick gel, referred to as a pellicle, consisting of 99% water and an interconnected three-dimensional (3D) porous network of BC nanofibers was obtained. Then, the protein in the BC hydrogel was eluted by immersion in 5% sodium hydroxide solution, heating it to 80°C ^TheC for several hours, and finally washing with deionized water.

The resulting purified BC hydrogel was stored in aqueous solution at 4°C ^TheC and then lyophilized for 24 hours at 10°C^-6 bar pressure and −52 ^TheC temperature to obtain BC airgel. Finally, the BC airgel was cut into 19 mm disks as five, three, and one mm thick spacers and designated as 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 as-synthesized samples. Dimensional changes of spacers were recorded at different temperatures to measure their thermal stability.

The researchers evaluated the rate of electrolyte absorption, electrolyte storage capabilities, and porosity of the separator. The Vienna ab initio simulation package was used to perform the theoretical simulation of the binding energies. The researchers also performed electrochemical measurements on the prepared BC spacer samples and polysulfide shuttle optical tests.

one) Voltage profiles of symmetrical lithium cells with PP and BC-1. SEM images of circular lithium anode surface (after 680 h) in symmetrical cell with b) PP or do) BC-1. High-resolution XPS O 1s surface spectra of recycled lithium anode in a symmetric cell with d) PP or e) BC-1. Image credit: Li, J et al., Advanced Materials Interfaces

Observations

BC separators with various thicknesses were successfully synthesized. BC spacers showed better electrolyte uptake and wettability compared to commercial PP spacers due to the abundance of hydroxyl groups and higher porosity, which improved Li⁺ porting and improved interface compatibility.

The spacers also inhibited the formation of Li dendrites, leading to uniform Li deposition on the anode. In addition, the spacers exhibited high thermal stability, which improved the safety performance of the battery. In addition, BC oxygen functional groups suppressed the displacement of soluble polysulfides by effectively adsorbing polysulfides through electrostatic interactions.

one) Transfer currents of different spacers. si) The transfer current decay rates are calculated after the steady state current is reached for a lithium-sulfur cell. Algebraic equations express the decay rate. Image credit: Li, J et al., Advanced Materials Interfaces

The symmetric cell with a BC spacer showed safe electrochemical performance for 680 hours due to the formation of lithium oxide (Li₂O) of cellulose and Li metal and 3D fiber structure. The quantitative relationship between capacitance loss and separator physical properties was successfully described by optimizing a mathematical model. Cellulose played a unique role in the mass transfer process of LiPS.

In summary, the findings of this study demonstrated the feasibility of using BC separators as a suitable battery material for high energy density Li-S and other Li-metal batteries and validated the hypothesis regarding the advantages of cellulose-based separators for Li-S battery applications.

In the future, the knowledge gained in this study on the advantageous features of the BC separator may provide a theoretical basis for the design of functional BC separators for batteries.

bibliographical references

Li, J., Li, Y., Li, Z., et al. (2022) Understanding the advantages of cellulose-based bacterial separator in Li–S battery. Advanced material interfaces. https://onlinelibrary.wiley.com/doi/10.1002/admi.202201730

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