What Are Cylindrical Lithium Batteries?

1. Definition of Cylindrical Batteries

Cylindrical lithium batteries are divided into different systems based on their cathode materials: lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium manganese oxide (LMO), cobalt-manganese blends, and ternary (NMC) materials. The casing comes in two types: steel and polymer. Batteries with different material systems have different advantages. Currently, cylindrical batteries are primarily steel-cased LFP batteries. These batteries exhibit characteristics such as high capacity, high output voltage, good charge/discharge cycle performance, stable output voltage, high current discharge capability, stable electrochemical performance, safety in use, wide operating temperature range, and environmental friendliness. They are widely used in solar lights, lawn lights, backup power supplies, power tools, and toy models.

2. Structure of Cylindrical Batteries

The structure of a typical cylindrical battery includes: casing, cap, positive electrode, negative electrode, separator, electrolyte, PTC (Positive Temperature Coefficient) element, gasket, safety vent, etc. Typically, the battery casing serves as the negative terminal, and the cap as the positive terminal. The battery casing is usually made of nickel-plated steel plate.

3. Advantages of Cylindrical Lithium Batteries

Compared to pouch and prismatic lithium batteries, cylindrical lithium batteries have the longest development history, higher standardization, more mature manufacturing processes, higher yield rates, and lower costs.

• Mature production processes, lower PACK (battery assembly) costs, higher battery product yield rates, and good heat dissipation performance.

• Cylindrical batteries have formed a series of internationally unified standard specifications and models. The process is mature, suitable for mass continuous production.

• The cylindrical shape provides a large surface area, resulting in effective heat dissipation.

• Cylindrical batteries are generally sealed cells, requiring no maintenance during use.

• The battery casing has high pressure resistance; issues like swelling seen in prismatic or pouch batteries do not occur during use.

4. Cathode Materials for Cylindrical Batteries

Currently, the mainstream commercial cathode materials for cylindrical batteries mainly include lithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄), ternary (NMC), and lithium iron phosphate (LiFePO₄). Batteries with different material systems have distinct characteristics, as compared below:

5. Anode Materials for Cylindrical Batteries

Anode materials for cylindrical batteries can be roughly divided into six categories: carbon-based anode materials, alloy-based anode materials, tin-based anode materials, lithium-containing transition metal nitride anode materials, nanoscale materials, and nano-anode materials.

• Carbon Nanoscale Anode Materials: Currently, the anode materials practically used in lithium-ion batteries are basically carbon materials, such as artificial graphite, natural graphite, mesocarbon microbeads (MCMB), petroleum coke, carbon fibers, pyrolytic resin carbon, etc.

• Alloy-based Anode Materials: Include tin-based alloys, silicon-based alloys, germanium-based alloys, aluminum-based alloys, antimony-based alloys, magnesium-based alloys, and others. Currently, there are no commercial products.

• Tin-based Anode Materials: Can be divided into tin oxides and tin-based composite oxides. Oxides refer to oxides of metallic tin in various valence states. Currently, there are no commercial products.

• Lithium-containing Transition Metal Nitride Anode Materials: Currently, there are no commercial products.

• Nanoscale Materials: Carbon nanotubes, nano-alloy materials.

• Nano-anode Materials: Nano-oxide materials.

II. Cylindrical Lithium Battery Cells

1. Brands of Cylindrical Lithium-ion Cells

Cylindrical lithium batteries are more popular among Japanese and South Korean lithium battery companies, and there are also sizable enterprises producing cylindrical lithium batteries in China. The earliest cylindrical lithium battery was invented by SONY of Japan in 1992.

Well-known cylindrical lithium-ion cell brands include: SONY, Panasonic, Sanyo, Samsung, LG, Wanxiang A123, BAK, Lishen, etc.

2. Types of Cylindrical Lithium-ion Cells

Cylindrical lithium-ion cells are typically represented by a five-digit number. Reading from left to right, the first and second digits indicate the battery diameter, the third and fourth digits indicate the battery height, and the fifth digit indicates the round shape. Cylindrical lithium batteries have many models, with more common ones being 10400, 14500, 16340, 18650, 21700, 26650, 32650, etc.

① 10440 Battery
A 10440 battery is a lithium battery with a diameter of 10mm and height of 44mm, the same size as what we commonly call a "AAA battery." This type of battery usually has a very small capacity, only a few hundred mAh, and is mainly used in mini electronic products such as flashlights, mini speakers, and amplifiers.

② 14500 Battery
A 14500 battery is a lithium battery with a diameter of 14mm and height of 50mm. It typically has a voltage of 3.7V or 3.2V, with a relatively small nominal capacity, slightly larger than the 10440 battery, usually around 1600mAh. It has excellent discharge performance and is mainly used in consumer electronics, such as wireless speakers, electric toys, digital cameras, etc.

③ 16340 Battery
A 16340 battery is a lithium battery with a diameter of 16mm and height of 34mm. Due to its slightly shorter height and not-too-small capacity, it frequently appears in high-intensity flashlights, LED flashlights, headlamps, laser pointers, lighting fixtures, etc.

④ 18650 Battery
An 18650 battery is a lithium battery with a diameter of 18mm and height of 65mm. Its biggest feature is its very high energy density, reaching almost 170 Wh/kg. Therefore, this battery offers good cost-performance. It is the most commonly seen type due to its maturity and relatively good overall system stability. It is widely used in applications requiring around 10 kWh of battery capacity, such as mobile phones, laptops, and other small appliances.

⑤ 21700 Battery
A 21700 battery is a lithium battery with a diameter of 21mm and height of 70mm. Its increased volume allows for greater space utilization, potentially improving both cell and system energy density. Its volumetric energy density is much higher than that of 18650 batteries. It is widely used in digital products, electric vehicles, self-balancing scooters, solar lithium battery street lights, LED lights, power tools, etc.

⑥ 26650 Battery
A 26650 battery is a lithium battery with a diameter of 26mm and height of 65mm. It has a nominal voltage of 3.2V and nominal capacity of 3200mAh. This battery type boasts excellent capacity and high consistency, gradually becoming a trend to replace 18650 batteries. Many products in the power battery field are also starting to favor it.

⑦ 32650 Battery
A 32650 battery is a lithium battery with a diameter of 32mm and height of 65mm. This battery type has strong continuous discharge capability, making it suitable for electric toys, backup power supplies, UPS batteries, wind power generation systems, and wind-solar hybrid power generation systems.

III. Market Development of Cylindrical Lithium Batteries

Technological progress in cylindrical lithium-ion batteries primarily comes from innovative research and application advancements in key battery materials. Developing new materials further improves battery performance, quality, cost, and safety. To meet the downstream demand for higher specific energy, one approach is to adopt higher specific capacity materials, and another is to increase the charging voltage by using high-voltage materials.

From the 14500 to Tesla's 21700 battery, the near-to-mid-term development of cylindrical lithium-ion batteries involves optimizing existing lithium-ion power battery technologies to meet the needs of large-scale new energy vehicle development. Simultaneously, the focus is on developing new types of lithium-ion power batteries, improving key technologies such as safety, consistency, and lifespan, while conducting forward-looking research on next-generation battery systems.

For the medium-to-long-term development of cylindrical lithium-ion batteries, while continuously optimizing new lithium-ion power batteries, the focus will be on researching and developing next-generation battery systems, significantly improving specific energy, greatly reducing costs, and achieving practical and large-scale application of these new systems.

IV. Comparison Between Cylindrical and Prismatic Lithium Batteries

1. Battery Shape: Prismatic batteries can be designed in almost any size, whereas cylindrical batteries have fixed dimensions.

2. Rate Capability: Due to process limitations in welding multiple tabs, the rate capability of cylindrical batteries is slightly inferior to that of prismatic batteries with multiple tabs.

3. Discharge Platform: Theoretically, lithium batteries using the same cathode/anode materials and electrolyte should have the same discharge platform. However, prismatic lithium batteries often have a slightly higher internal discharge platform.

4. Product Quality: Cylindrical battery manufacturing processes are more mature. The probability of defects from secondary slitting of electrode sheets is lower, and the maturity and automation level of the winding process are relatively high. The stacking (laminating) process still mostly uses semi-manual methods, which can adversely affect battery quality.

5. Tab Welding: Welding tabs on cylindrical batteries is easier than on prismatic lithium batteries; prismatic batteries are more prone to cold soldering, affecting quality.

6. PACK Assembly: Cylindrical batteries are easier to use, making PACK technology simpler with better heat dissipation. For prismatic lithium batteries, solving heat dissipation issues during PACK is crucial.

7. Structural Characteristics: The chemical activity at the corners of prismatic lithium batteries is relatively poor. Long-term use can easily lead to energy density degradation and shorter runtimes.

V. Comparison Between Cylindrical and Pouch Lithium Batteries

1. Safety Performance: Pouch batteries offer better safety. Structurally, they use aluminum-plastic film packaging. In case of a safety issue, pouch batteries typically swell and split open, unlike steel or aluminum-cased cells which may explode. Their safety performance is superior to cylindrical lithium batteries.

2. Weight: Pouch batteries are relatively lighter. They are about 40% lighter than steel-cased lithium batteries and about 20% lighter than cylindrical aluminum-cased lithium batteries of the same capacity. They have low internal resistance, which greatly reduces self-discharge.

3. Cycle Performance: Pouch batteries have better cycle performance and longer cycle life. After 100 cycles, their capacity fade is 4% to 7% less than that of cylindrical aluminum-cased batteries.

4. Design Flexibility: Pouch battery design is more flexible. The shape can be varied, made thinner, and customized according to customer needs for new cell models. Cylindrical lithium batteries do not have this capability.

5. Disadvantages of Pouch vs. Cylindrical Batteries: Pouch batteries have poorer consistency, higher costs, and are more prone to leakage. Higher costs can be addressed through mass production, and leakage issues can be mitigated by improving the quality of the aluminum-plastic film.