Safety Analysis of 21700 Batteries

Safety Analysis of 21700 Batteries

---A&S Power

The safety of 21700 batteries is closely tied to their structural design, material properties, and application scenarios. Compared to the earlier 18650 batteries, 21700 cells incorporate safety optimizations, though risks still exist and should be evaluated based on specific use cases. Below is a detailed analysis.

I. Safety Design and Advantages of 21700 Batteries

1. Structural Optimization to Reduce Thermal Runaway Risks
The diameter of 21700 batteries increases from 18mm (18650) to 21mm, reducing the number 

of electrode winding layers and improving electrolyte saturation. This shortens heat dissipation paths, 

evenly distributing heat during high-rate charging/discharging and minimizing local overheating. 

Enhanced casing materials (stainless steel or aluminum) and explosion-proof pressure relief valves in 

some models (e.g., Tesla’s customized cells) quickly release internal pressure to prevent explosions 

under extreme conditions.

  2.Material System Improvements for Stability 21700 batteries often use high-nickel cathode materials 

(e.g., NCA/NCM811). Although their thermal stability is slightly lower than lithium iron phosphate, 

surface coating (e.g., LiAlO₂) and single-crystal processing reduce crystal cracks, minimizing side reactions 

from oxygen release during charging/discharging. Flame-retardant additives (e.g., fluorinated carbonates) 

in the electrolyte raise the flash point from ~60°C to over 80°C, slowing combustion during thermal runaway.

3.Enhanced Safety Management via BMS Integration
With lower internal resistance (10–15mΩ vs. 15–25mΩ for 18650), 21700 batteries allow BMS to precisely 

monitor voltage and temperature changes, triggering warnings for overcharging, overdischarging, or overheating 

(e.g., activating cooling when temperature exceeds 60°C). Fewer series/parallel connections in battery packs 

(due to higher single-cell capacity) reduce solder points and contact resistance, minimizing heat generation 

from poor connections.

II. Safety Risks and Challenges of 21700 Batteries

1.Thermal Management Pressure from High Energy Density
With an energy density of 240–300Wh/kg (20% higher than 18650), thermal runaway in 21700 cells releases 

more energy per unit volume. For example, a 5000mAh 21700 cell can release energy equivalent to ~20g of 

TNT, 1.2–1.5 times that of an 18650 cell. Charging at temperatures below -10°C promotes lithium dendrite 

growth on the anode, potentially piercing the separator and causing internal short circuits. 21700 cells may 

experience faster dendrite growth than 18650 due to reduced electrolyte conductivity, requiring heating 

films or limited charging currents.

2. Safety Hazards Under Abusive Conditions
Overcharging a high-nickel 21700 cell beyond 4.3V lowers the onset temperature of oxygen release and 

electrolyte reactions from 200°C to 180°C. Overdischarging below 2.5V dissolves the copper current collector, 

increasing short-circuit risks. Mechanical damage (e.g., crushing or puncturing) can generate short-circuit 

currents exceeding 100A (vs. ~80A for 18650), leading to rapid heat generation and potential ignition.

3. Safety Hazards Under Abusive Conditions
Overcharging a high-nickel 21700 cell beyond 4.3V lowers the onset temperature of oxygen release and 

electrolyte reactions from 200°C to 180°C. Overdischarging below 2.5V dissolves the copper current collector, 

increasing short-circuit risks. Mechanical damage (e.g., crushing or puncturing) can generate short-circuit 

currents exceeding 100A (vs. ~80A for 18650), leading to rapid heat generation and potential ignition.

4. Quality Variations Among Manufacturers
Inferior 21700 cells from small producers may have uneven separator thickness or electrode coating defects, 

increasing the probability of micro-shorts. For instance, early failure rates of non-branded 21700 cells can be 

30% higher than those from major manufacturers. Some vendors overstate discharge rates (e.g., claiming 15C 

support when actual capacity is 10C), risking overheating during high-current use.

III. Safety Usage Recommendations and Scenario Adaptation

1. Industrial and Consumer Electronics Applications
Power tools/drones: 21700 batteries suit high-power scenarios but require active cooling (e.g., fans or liquid cooling) 

to prevent overheating during prolonged high-rate discharge (e.g., maintaining temperatures below 50°C for 15C discharge in drills). 

Portable chargers/laptops: Though less common in consumer electronics due to size, 21700 cells must have BMS with overcharge 

protection (cutoff voltage 4.2V±0.05V) and over-temperature protection (70°C shutdown)

2. Automotive and Energy Storage Scenarios
EV battery packs: Tesla uses serpentine liquid cooling systems to keep 21700 cell temperature differences within ±2°C, with fireproof 

aerogel between cells to contain thermal runaway. Energy storage stations: 21700 batteries should integrate with fire suppression systems, 

using smoke sensors and heptafluoropropane extinguishers to activate at 80°C.

3. Daily Usage Precautions
Avoid extreme temperatures (above 45°C or below -20°C) and ensure ventilation during charging. Purchase cells from reputable manufacturers 

(e.g., Panasonic, Samsung SDI, CATL) to avoid low-quality products (e.g., off-brand 21700 cells may run 15°C hotter than branded ones at 10C discharge). 

Regularly check cell voltage (differences >0.1V or internal resistance >20mΩ indicate replacement needs) and inspect for swelling/leakage, immediately 

if issues are found.

IV. Safety Comparison with Other Battery Types

Conclusion: Balancing Technological Advancement and Risk Control

21700 batteries offer improved safety over early 18650 cells through structural and material innovations, but their high energy density still poses thermal 

runaway risks under abusive conditions. Safety primarily depends on manufacturing quality, BMS efficiency, and user compliance. For consumers, choosing 

branded products and following usage guidelines mitigates risks, while industrial applications require robust thermal management and safety systems to 

leverage their performance advantages.

A&S Power，a lithium battery manufacturer in China over 15 years with customer of Philips, Skullcandy, Mitel, Keytronic, Jabil etc.