Ultrathick Self-Standing Foam for Use in Sodium-Ion Batteries

By Bismay Prakash RoutReviewed by Susha Cheriyedath, M.Sc.Feb 1 2022

In a recent study published in the journal ACS Materials Letters, researchers developed a new physical structure with high areal capacity for the electrodes of high-energy-density sodium-ion batteries (SIBs). The purposed electrode is a vertically aligned self-standing foam with an ultrathick (1500 μm) wall made up of carbon framework and embedded 1T-MoS₂ nanosheets.

Study: Confined Iterative Self-Assembly of Ultrathick Freestanding Electrodes with Vertically Aligned Channels for High Areal Capacity Sodium-Ion Batteries. Image Credit: gcarnero/Shutterstock.com

The foam electrode consisting of a mesh of vertical channels provided a high areal mass of 20.74 mg cm⁻² without the need of any binder, current collector, or carbon additive. Further, a confined iterative self-assembly strategy was used to control the morphology and areal mass of the foam.

SIBs with High Areal Capacity

SIBs are a cheap and readily available alternative to lithium-ion batteries (LIBs); however, their low energy density limits their demand. The most common approach to increase the energy density of any metal-ion battery is finding new electrode materials with a high specific capacity, such as silicon, sulfide, carbon-based materials, and alloys.

However, the energy density of the electrode can also be increased by increasing the mass loading of active materials, which requires a better design of the physical structure of the electrode specific to its field of application. In the case of a conventional solid electrode, higher mass loading means higher impedance and increased volume change during cycling. Thus, most solid electrodes have a thickness of fewer than 100 μm.

Meanwhile, self-standing electrodes are themselves the composites of active materials, thus do not require separate binders, current collectors, or carbon additives to hold together up to very high thicknesses. Additionally, carbon frameworks can make the whole mass of the electrode active material for enhanced capacity. High areal mass means more mass of active materials per unit area, thus more energy density.

About the Study

In this study, researchers developed an electrode for SIBs with vertically aligned self-standing foam with an ultrathick (1500 μm) wall made up of carbon framework and embedded 1T-MoS₂ nanosheets. The vertical compartments of the foam were used as the sodium storage, whereas the high conducting 2D metallic 1T-MoS₂ nanosheets facilitated a path for sodium-ion (Na⁺) diffusion. The carbon framework was the core structure of the electrode walls that provided high conductivity, improved mechanical stability, and adequate internal space to accommodate large volumetric changes during sodiation/desodiation.

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Firstly, a vertically aligned microchannel-shaped mold was cooled on one side by using liquid nitrogen. After that, 1T-MoS₂ and chitosan (CS) water solution was poured onto that mold which gradually started to solidify unidirectionally from the cold side.  Moreover, a confined iterative self-assembly strategy was used to increase the height of vertical channels and reduce the corresponding width, which resulted in increased areal mass density in successive iterations.  

Observations

SEM images showed that 1T-MoS₂ had a fluffy ball-like shape with individual 1T-MoS₂ nanosheets extending from the center to the periphery of the ball of 200 nm diameter.  The FT-IR I_D/I_G ratio of MoS₂@CSC (the last C corresponds to the carbon framework) was 0.93, indicating the formation of a high crystalline structure due to the reaction between MoS₂ and CS. XPS spectra revealed the binding energies of Mo 3d3/2, Mo 3d5/2, S 2s, C 1s located at 231.7, 228.6, 226.0, and 284.8 eV, respectively. The Mo 3d3/2 and Mo 3d5/2 binding energies were attributed to 1T-MoS₂ polymorphs.

The CV curves of the MoS₂@CSC electrode showed two oxidation peaks at 1.70 and 0.40 V and three reductive peaks at 0.05, 0.25, and 0.75 V.  Each reductive peak corresponds to the formation of solid electrolyte interphase, conversion of Na_xMoS₂ to Na₂S and Mo, and Na⁺ insertion into electrodes. The confined iterative self-assembly strategy led to an areal mass of 12.61, 19.45, and 22.00 mg cm⁻², and a corresponding areal capacity of 2.03, 4.45, and 4.72 mAh cm⁻² for MoS₂@CSC-I, MoS₂@CSC-II, and MoS₂@CSC-III, respectively.

Conclusions

In summary, the researchers of the study developed a vertically aligned self-standing foam electrode made up of a carbon framework embedded with 1T-MoS₂ and CS on the wall. The vertical microchannels provided efficient electrolyte and ion diffusion paths, whereas the hollow compartments accommodated large volume change during cycling. More importantly, the high areal capacity contributed to high energy density. Hence, self-standing micro-channeled MoS₂@CSC foam is a promising areal capacity tunable electrode for SIBs.

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