[Complete Battery Mastery]⑪ The Dream of '5-Minute Full Charge' for Electric Cars Achieved by Silicon Anode Materials

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Swedish electric vehicle brand Polestar announced at its 'Polestar Day' event on the 9th (local time) that it will pilot a battery capable of charging for a 100-mile (approximately 160 km) driving range within 5 minutes in the Polestar 5. This battery is the 'XFC (extreme fast charging)' developed by Israeli startup StoreDot. The XFC battery applies silicon to the anode material. The company aims to reduce the 100-mile charging time to 3 minutes by 2028 and 2 minutes by 2032. Korean battery material company Daewoo Electronics Materials announced on the 14th that it will supply silicon anode materials to SK On. This is the second time the company has supplied silicon anode materials, following LG Energy Solution.

Although electric vehicles have become more popular compared to the past, many people still hesitate to purchase them. The biggest reasons are inconvenient charging and the driving range per charge being unsatisfactory compared to traditional internal combustion engine vehicles. Silicon anode materials are gaining attention as a solution to these two challenges.

In batteries, the anode material plays the role of storing and releasing lithium ions and electrons during charging. It can be considered a kind of storage tank. The performance of this storage tank determines the battery's charging speed and lifespan.

Until now, lithium-ion batteries have focused on developing cathode materials to increase charging capacity. The development of high-nickel batteries, which increase the nickel content to boost capacity, is a representative example. Currently, NCM811 batteries (nickel:cobalt:manganese ratio of 8:1:1) with 80% nickel content have been commercialized, and technologies with 90% nickel content are also being developed. However, cathode materials have reached their limits in further expanding battery capacity.

Therefore, the battery industry is focusing on improving anode material performance. Graphite has mainly been used as the anode material. Natural and artificial graphite have the advantage of stably storing lithium ions. Next-generation anode materials that can replace graphite, such as silicon, tin oxide, and aluminum, are being researched. Among these, silicon is the most notable. Interest in silicon anode materials has increased further as China plans to control graphite exports starting December 1, 2023.

Storage Capacity 10 Times Greater... Overcoming the 'Swelling' Phenomenon is the Challenge

Silicon (Si) is easy to obtain anywhere. Silicon accounts for 28% of the Earth's crust, making it the second most abundant element after oxygen (46%). Graphite stores 1 lithium ion per 6 atoms, whereas silicon can store 15 lithium ions per 4 atoms. Theoretically, the energy capacity per unit weight of silicon anode material can reach up to 4200 milliampere-hours per gram (mAh/g), about 10 times the theoretical energy capacity of graphite (372 mAh/g).

Improving the storage capacity of the anode material can increase charging speed. Also, since fewer anode materials are needed to store lithium ions, the battery's weight (and volume) can be reduced. In other words, increasing the energy storage capacity per unit weight (or volume) enhances the energy density (Wh/kg or Wh/L).

However, silicon as an anode material has a fatal drawback: the swelling phenomenon that occurs during charging. Graphite expands by about 10% during charging, but silicon's expansion rate reaches 300-400%. Repeated expansion and contraction during charge-discharge cycles eventually cause cracks, rendering the battery unusable.

Silicon anode materials can store more lithium ions compared to graphite, but they exhibit swelling during charging. Repeated expansion and contraction cause cracks on the surface, rendering the battery unusable. (Photo by Nexon website)

Therefore, silicon anode materials are currently commercialized by adding a small amount of silicon to existing graphite. Due to technical limitations, only 5-10% silicon is added so far. It is known that adding 5% silicon improves energy density by about 10-15% compared to conventional graphite anode materials. The battery industry aims to increase this to 25-30% in the mid to long term.

Silicon anode materials also have the disadvantage of lower Initial Coulombic Efficiency (ICE) compared to graphite. ICE refers to the ratio of discharge capacity to charge capacity in the first cycle (discharge capacity/charge capacity × 100), also called initial charge-discharge efficiency. For example, if the first charge achieves 500 mAh/g and the discharge shows 450 mAh/g, the ICE is 90%. Graphite anode materials have an ICE of about 95%, while silicon anode materials remain in the 70-80% range.

The lower ICE in silicon anode materials is due to the irregular formation of the SEI (Solid Electrolyte Interphase) layer compared to graphite anode materials. SEI is a thin solid film formed on the anode surface during the first charge after battery manufacturing. If the SEI is uneven, it hinders lithium ion movement, reducing charge-discharge efficiency.

Silicon anode materials are also more expensive than graphite, which is another issue to overcome. To solve the technical problems of silicon, coatings or additives are applied, which increase the cost.

Currently commercialized silicon anode materials are mainly divided into two types: silicon oxide (SiOx) series and silicon-carbon composite (Si-C) series. Some companies are developing 'pure silicon' technology using only silicon, but it has not yet reached commercialization.

SiOx is made by coating nano-sized silicon particles with silicon oxide. Si-C is manufactured by mechanically bonding silicon and carbon. SiOx has less volume expansion than Si-C but suffers from lower ICE. Si-C has better ICE but is disadvantaged in volume expansion. Material companies are developing various processes to strengthen strengths and compensate for weaknesses.

Single-Walled Carbon Nanotube (SWCNT) Structure (Source: LG Energy Solution)

To reduce swelling, conductive materials such as carbon nanotubes (CNT) are sometimes used together. Conductive materials are used with cathode or anode materials to facilitate electron flow. Especially, adding CNT conductive materials to silicon anode materials can somewhat suppress expansion. CNT conductive materials have a structure where six carbon atoms are connected and rolled into a cylindrical shape.

Single-walled carbon nanotubes (SWCNT) are mainly used as CNT conductive materials for silicon anode materials. SWCNT was exclusively produced by Russia's OCSiAL, but Japanese company Zeon, Korean companies Nano New Materials, Corbon, and JO have newly entered the market, creating a competitive landscape.

Tesla Likely to Adopt in 4680 Batteries... GM and Volkswagen Also Interested

Despite technical challenges, the battery industry expects silicon anode materials to become widespread eventually. Tesla is pursuing the application of silicon anode materials in its 4680 cylindrical batteries. Panasonic, a battery supplier to Tesla, announced in July that it would purchase silicon anode materials from UK startup Nexeon. Panasonic plans to produce lithium-ion batteries using Nexeon's anode materials at its Kansas plant starting in 2025. Nexeon attracted attention when SKC invested $80 million (about 104.2 billion KRW) in 2021.

Besides Tesla, automakers such as General Motors (GM), Ford, and BMW plan to equip or are equipping silicon anode material batteries. The fact that Daewoo Electronics Materials, a leading Korean silicon anode material company, is supplying silicon anode materials to SK On also raises expectations. Daewoo Electronics Materials has been supplying silicon anode materials to LG Energy Solution since 2019. LG Energy Solution's battery was applied to the Porsche Taycan electric vehicle. Volkswagen announced at its 2021 'Power Day' that it would expand the application of silicon anode materials to improve energy density and reduce charging time.

BMW equipped its new electric vehicle i7 with Samsung SDI's P5 (formerly Gen5) battery. This battery applies high-nickel cathode materials and silicon anode materials. Samsung SDI applied its proprietary technology called SCN (Silicon Carbon Nanocomposite). GM announced in September 2022 that it would collaborate with silicon anode material startup OneD Battery Science located in Palo Alto, USA. GM Ventures also invested $25 million (about 3.25 billion KRW) in the company.

Major Corporations Like POSCO, SK, Lotte, and Hansol Also Declare Entry

Material companies are also actively moving. Globally, the leading silicon anode material companies are China's BTR (Beiterui) and Japan's Shin-Etsu Chemical. In Korea, Daewoo Electronics Materials was the first to commercialize silicon anode materials. Its production capacity is about 3,000 tons as of 2023, with plans to expand to 10,000 tons in 2024 and 20,000 tons in 2025.

Major corporations such as POSCO Group and SK Group are also rushing to develop silicon anode materials.

POSCO Holdings acquired Terra Technos, which holds SiOx technology, in July 2022, renamed it POSCO Silicon Solution, and incorporated it as a subsidiary. The company is constructing a first-phase plant with an annual capacity of 450 tons in the Yeongilman Industrial Complex in Pohang. POSCO Future M, which produces graphite anode materials, is developing SiOx and Si-C based anode materials and aims for mass production by 2026. POSCO Future M is also developing pure silicon anode materials made entirely of silicon.

SK Materials established a joint venture, SK Materials Group14, with US battery material company Group14 in 2021. The company completed a silicon anode material plant with an annual capacity of 2,000 tons in Sangju, Gyeongbuk, in 2023. SKC, which invested in and licensed technology from UK-based Nexeon, established a silicon anode material subsidiary, Ultimus. This company plans to pilot-produce silicon anode materials in January next year using Nexeon's technology.

Lotte Materials signed an equity investment agreement in July 2023 with French startup Enwires, which holds silicon anode material technology. The company plans to start mass production in 2027. Hansol Chemical is preparing for mass production of SiC-based silicon anode materials in Iksan, Jeonbuk, with an investment of 85 billion KRW. LG Chem is developing pure silicon.

Silicon anode battery under development by Enerbat (Photo by Enerbat)

Overseas companies are also active. Enevate, located in Irvine, California, USA, has received about $191 million in investments from Alliance Ventures jointly established by Renault-Nissan-Mitsubishi, LG Chem, and Samsung Ventures. The company is developing silicon anode batteries capable of charging for a 400 km driving range within 5 minutes. It uses 70% pure silicon in the anode active material, achieving an energy density of 3000 mAh/g and an initial coulombic efficiency above 90%. However, it is understood that commercialization has not yet been achieved. In September, the company announced a partnership with US battery company NantG to manufacture next-generation high-performance batteries.

Another US startup, Amprius, is also developing pure silicon. The company stated that it is testing silicon anode batteries with a specific energy of 450 Wh/kg and capable of charging up to 80% within 6 minutes with its partners. Hyundai Industries invested about $1.4 million in Amprius in 2021.

Digital Daily, 'Tesla to Apply Silicon Anode Materials in 4680 Batteries', 2023.5.4
CleanTechnica, 'Panasonic To Produce Higher-Energy-Density Batteries In USA With Silicon From Nexeon', 2023.7.25
TechCrunch, 'GM partners with and invests in OneD Battery Sciences in quest for cheap, energy-dense EV batteries', 2022.9.29
Shinhan Investment Corp., 'Next in Secondary Battery Materials: Si + CNT', 2023.3.15
Hana Securities, 'Silicon Anode and CNT Conductive Material Market Analysis', 2023.6.5
Enevate official website
Amprius official website