From new materials to air batteries, large-scale battery research is on the right track (Part 2): Negative electrode materials and separators

Anode Materials

Silicon-based new materials, popular LTO

In the field of negative electrode materials, the research and development directions for large batteries are divided into two. One is to increase capacity, and the other is to improve safety and life. At present, the specific capacity of the mainstream material graphite is about 370mAh/g. In order to achieve large capacity, research on silicon materials exceeding 1000mAh/g is very prosperous.

Since the potential of graphite to lithium is only about 0.1V, the precipitation of lithium at the negative electrode will cause safety problems, and compounds with the electrolyte are easily formed at the graphite interface, making it difficult to ensure the cycle characteristics. Therefore, lithium titanate (Li4Ti5O12, LTO), which has a potential of up to 1.5V to lithium and excellent safety and cycle characteristics, has attracted attention (Figure 5) (Note 7).

(Note 7) Toshiba has already started selling lithium-ion rechargeable batteries using LTO under the trade name "SCiB."

At this battery discussion meeting, Nissan Motor, NEC, Dow Corning, Toyota Central Research Institute, etc. successively presented their views on silicon-based materials. Research on silicon-based materials, including oxides such as SiO, and composite electrode bodies with carbon are very active. Toyota Central Research Institute presented its new silicon-based material, layered polysilane (S6H6) (Note 8).

(Note 8) He gave a presentation titled “Characteristics of Layered Polysilane (Si6H6) as Negative Electrode for Lithium Rechargeable Batteries” [Speech No. 1D04].

Toyota Central Research Institute said that the thickness of layered polysilane is at the nanometer level, and it has the same structure as graphite, which is formed by stacking flakes (Si6H62)n with a micrometer-level in-plane size and strong anisotropy.

The results of the electrode test using this material showed that the first charge capacity was 1170mAh/g, which is nearly twice the capacity of the silicon particle prototype. The volume expansion rate after 10 repeated charge and discharge cycles was 150%, which is less than the 156% of silicon particles. Toyota Central Research Institute believes that maintaining a layered structure helps to reduce expansion and said that this material will become one of the hot candidates for silicon materials.

Murata Manufacturing and Toyota Motors have successively announced LTO, which can improve safety and life. Since the specific capacity of LTO is only 175mAh/g, it is expected to achieve a balance between capacity and safety by mixing it with silicon materials in the future.

Although there is no problem when LTO is used alone, when it is combined with materials containing conductive additives such as acetylene black to form a composite electrode, the rate performance during charging will decrease. As a solution to this problem, Murata Manufacturing introduced a method of adding other elements to LTO to improve its properties (Note 9).

(Note 9) He gave a presentation titled “Improving the Charge-Discharge Rate Performance of Lithium Titanate by Adding Other Elements” [Speech No.: 2D16].

Murata Manufacturing has found that adding zirconium (Zr) and strontium (Sr) when synthesizing LTO can improve the rate performance during charging. The principles of Zr and Sr to improve charging characteristics are different. Zr is achieved by reducing the particle size of LTO and increasing the reaction area. Strontium improves the characteristics by generating Li2SrTi6O14, where lithium can be separated and inserted.

On the other hand, Toyota reported that replacing oxygen in the LTO crystal structure with nitrogen can improve conductivity (Note 10). Specifically, oxygen defects were introduced and nitrogen was replaced simultaneously in a N2/NH3 environment, and the conductivity increased by 5 orders of magnitude from less than 10-7S/cm to 2×10-2S/cm.

(Note 10) He gave a presentation titled “Studying the improvement of electronic conductivity of Li4Ti5O12 by introducing defects and impurities” [Speech number: 2D11].

Diaphragm

Using high heat-resistant nonwoven fabric

As for the separator, safety research is being conducted for its application in large batteries. The substrates of the old separators are microporous films such as polyethylene (PE) and polypropylene (PP), which have poor heat resistance. Therefore, recently, measures to increase heat resistance by setting a ceramic layer on the surface of the old separator have become the mainstream.

Figure 6: Nonwoven separator with excellent heat resistance and rate performance
In the 180°C holding test, the nonwoven separator did not change much (a). The test results using the stacked unit showed that the nonwoven separator was more excellent in both rate performance and cycle characteristics (b, c). The image was created by this site based on data from Mitsubishi Paper.

[page]

At this year's battery symposium, Mitsubishi Paper and Tokyo University of Science presented a nonwoven separator that uses high-heat-resistant cellulose and polyethylene terephthalate (PET) instead of a low-heat-resistant substrate (Figure 6)(Note 11).

(Note 11) He gave a presentation titled “Characteristics of lithium-ion batteries using nonwoven separators” [Speech number: 1B25].

The nonwoven separator and the PP separator with a heat-resistant ceramic layer of alumina (Al2O3) were compared by placing them at 180°C for 3 hours. The results showed that the nonwoven separator did not shrink after 3 hours, while the PP separator with a heat-resistant ceramic layer shrank after 5 minutes.

Intended for use in stacked units

Next, the two also made a stacked unit with a capacity of about 30mAh using LiMn2O4 as the positive electrode, and compared its rate performance and charge-discharge cycle characteristics. The non-woven fabric separator not only has good heat resistance, but also has excellent electrolyte permeability due to its large porosity. Therefore, in terms of rate performance, compared with the PP-made separator, the higher the discharge rate, the higher the capacity retention rate of the non-woven fabric separator.

In terms of charge and discharge cycle characteristics, the capacity retention rate after 100 cycles is as high as 85% or more for non-woven fabric separators at 25°C and about 70% at 50°C. In contrast, the capacity retention rate for PP separators is 77% at 25°C and only 66.5% at 50°C.

In addition, when the cells that were repeatedly charged and discharged were disassembled and the separators were observed, the results showed that the non-woven separator showed little change, while the surface of the PP separator changed color and deteriorated due to oxidation (Note 12).

(Note 12) Charge and discharge tests were performed for 5 cycles at rates of 0.2/0.5/1/3/5/10/20C.

Figure 7: Improving the performance of all-solid batteries by covering with solid electrolytes
Osaka Prefecture University published a report on the unit characteristics of all-solid batteries with solid electrolytes covering the positive electrode active material (a). Cross-sectional observation confirmed that the surface of the active material of the positive electrode material was covered with a high density of solid electrolytes (b). The prototype unit showed good charge and discharge cycle characteristics (c). The image was created by this site based on data from the Tatsumi Suna Laboratory at Osaka Prefecture University.

Nonwoven separators do not have a shutdown function that stops conducting lithium ions after reaching a certain temperature*, and their mechanical properties, such as ductility, are also very different from those of conventional separators. As a result, Mitsubishi Paper is looking to expand sales of nonwoven separators for large-scale laminated units that require a different manufacturing method than before.

*Shutdown function = When an internal short circuit occurs, the microporous membrane diaphragm dissolves, plugging the holes in the short circuit portion and preventing ion conduction.