With the rapid development of industrial technology, the reserves of non-renewable energy sources represented by fossil fuels have decreased sharply in recent years. There is an urgent need for a green and low-cost electrochemical energy storage technology to adapt to the rapid development of renewable energy and improve its utilization rate. Lithium-ion batteries (LIBs) have successfully been commercialized and are used for large-scale energy storage due to their excellent energy density and cycle life. However, the uneven distribution of lithium resources globally and their high prices have prompted people to gradually shift to other metal-ion batteries with significant cost advantages, such as sodium-ion batteries (SIBs), to reduce the dependence of energy storage technologies on lithium resources. Currently, a large number of researchers have focused their attention on the development of high-performance SIBs.

During the development work in the laboratory stage, the assembly and testing of button cells are indispensable. In the tests of symmetrical button cells and half cells, the negative electrode is a sodium metal disc. Sodium discs can generally be obtained in two ways:

1. They can be rolled and sliced from a small amount of metallic sodium blocks; 2. They can be directly purchased as commercial finished composite sodium discs.

I. Steps for Using Hand-rolled Sodium Metal Sheets

Required materials: Sodium blocks stored in kerosene, dust-free paper (or dust-free cloth), plastic sealing bag, plastic knife, central cylindrical mold, plastic chopping board, plastic roller.

Figure 1. Preparation steps of hand-rolled sodium sheets

1. Handling sodium blocks: In a glove box filled with argon gas and having a water and oxygen content of less than 0.1 ppm, remove the sodium blocks stored in kerosene. Wipe the surface of the sodium blocks clean with dust-free paper (or dust-free cloth), and use a plastic knife to scrape off the oxide layer on the surface of the sodium blocks, revealing the shiny sodium metal.

2. Packaging: Place the processed sodium blocks into a sealed plastic bag to prevent contamination of the sodium blocks due to direct contact with tools and other items.

3. Rolling: Use plastic rollers to roll the sodium blocks in different directions to form large sodium sheets (the thickness can be adjusted by changing the force).

4. Tabletting: Use the central cylindrical mold to punch out sodium tablets of different specifications in the form of circles.

5. Remove the sodium tablets: Open the plastic packaging bag, and the cut-out circular sodium tablets will automatically fall off. Place the excess sodium in the recycling bottle.

It should be noted that the dust-free paper or dust-free cloth needs to be vacuum-dried in advance to reduce moisture. During the preparation process, it should be carried out in a glove box filled with argon gas to prevent sodium metal from reacting with the air. At the same time, the operators need to wear additional protective gloves on top of the rubber gloves in the glove box to ensure safety.

II. Usage Steps of Commercial Compound Sodium Chips:

Figure 2. Operating steps for commercial composite sodium tablets before use

1.With the left hand, hold the flat and widened forceps to grip the round piece. With the right hand, use the pointed stainless steel forceps to remove the blue protective film. It is best not to use rubber forceps to remove the film.

2. Use the same method to tear off the transparent film on the other side

III. Comparison between commercial sodium tablets and hand-rolled sodium tablets

Purity contrast

1. For the preparation of hand-rolled sodium sheets, the usual procedure is to first remove the large sodium blocks from the kerosene and wipe the surface clean. However, this step is likely to result in incomplete cleaning of the kerosene, leading to low purity and high impurity content of the sodium sheets produced by hand; while commercial sodium sheets generally have a higher purity, usually above 99.7%, with less impurities, which reduces the occurrence of side reactions caused by impurities and extends the service life of the battery.

2. Comparison of Preservation Time

For hand-wound sodium sheets, they are generally made and used immediately. Due to the absence of a protective film, after slicing and during the battery assembly process, the surface will be exposed to the atmosphere of the glove box for a long time due to garbage cleaning, resulting in a significant degree of oxidation. While commercial composite sodium sheets are used as needed, the protective film can be removed within 10 seconds, increasing the battery assembly efficiency by more than twice. Moreover, the composite sodium electrode sheets are composed of high-purity metallic sodium combined with aluminum foil. Both sides are treated with inert coating and then undergo four layers of protective packaging, allowing for long-term storage without oxidation. (It can be stored in the glove box for more than three months.)

3. Uniformity Contrast

The production process of hand-wound sodium sheets involves significant uncertainties. As they are handmade, their thickness and shape cannot be made completely consistent, which may affect the repeatability of battery tests. The surface of hand-wound sodium sheets may be relatively rough, resulting in a smaller contact area and poorer contact quality with the electrode materials compared to commercial sodium sheets. This leads to fluctuations in battery performance. Compared to homemade circular sheets, it can avoid the problem of low battery assembly efficiency caused by non-standard production processes. At the same time, the smooth and shiny surface of the sodium sheets can also reduce the occurrence of micro-short circuits caused by burrs or scratches on the surface, ensuring the safe and stable operation of sodium batteries. These advantages make the composite sodium electrode sheets have broad application prospects in the field of sodium battery research, providing researchers with a better and more efficient option.

Overall, in battery tests, commercial sodium sheets, due to their high consistency, stability, flatness and purity, can provide researchers with more accurate and reliable test results. However, hand-milled sodium sheets, due to the limitations of the manufacturing process, perform relatively poorly in terms of performance. When researchers choose sodium sheets, they need to consider the specific experimental requirements and budget comprehensively.

IV. Comparison between Hand-rolled Sodium Tablets and Commercial Sodium Tablets

To visually observe the performance differences between the two sodium sheets, we used Cu-BTC as the precursor and carried out simple heat treatment and acid etching to prepare a simple nitrogen-doped porous carbon NC. Based on these two sodium sheets, we assembled half-batteries to evaluate the performance differences of the sodium sheets. The following are the performance differences of the two sodium sheet-based half-batteries:

Figure 3. Cycling performance of the half-cell based on hand-rolled sodium sheets using NC

Figure 4. Cycling performance of a half-cell based on NC composite sodium sheets

It can be seen from Figure 3 that the charging specific capacity ratio of Cu@NC and the Coulomb efficiency are shown to be related during 1000 cycles. Due to the rough surface of the hand-rolled sodium sheet, it is more prone to cause a soft short circuit. After the dendrites pass through the separator, the positive and negative electrodes come into contact with each other and cause a soft short circuit, and the Coulomb efficiency begins to drop sharply. Then, the battery shows a continuous charging phenomenon, but the voltage does not reach the upper limit of the window. In subsequent cycles, as the dendrites fall off from the electrode, the positive and negative electrodes no longer come into contact with each other, and the Coulomb efficiency returns to 100%. In contrast, using commercial composite sodium sheets can achieve stable cycling for 2000 cycles without any soft short circuit phenomenon, indicating that commercial composite sodium sheets have advantages in improving stability.

V. Conclusion

Overall, commercial composite sodium sheets have better uniformity compared to hand-rolled sodium sheets, and are more convenient to use. They can avoid the problem of low battery assembly efficiency caused by non-standard production processes, thereby improving performance consistency. Additionally, the smooth and shiny surface of the sodium sheets can reduce surface burrs or scratches, thereby minimizing abnormal situations such as micro-short circuits and enhancing the cycle stability of the battery. These advantages make commercial composite sodium sheets have broad application prospects in the field of sodium-ion battery research, providing researchers with a better and more efficient option.