High-Temperature Performance of Lithium-Ion Batteries: A Focus on 18650 and 21700 Cells
High-Temperature Performance of Lithium-Ion Batteries: A Focus on 18650 and 21700 Cells
Introduction
Lithium-ion batteries (LIBs) have revolutionized the portable electronics industry and are increasingly being adopted for electric vehicles (EVs) and energy storage systems (ESS). Their high energy density, long cycle life, and relatively low self-discharge rates make them ideal for a wide range of applications. However, one of the critical challenges facing LIB technology is their performance at elevated temperatures. High temperatures can accelerate the degradation processes within the battery, leading to shorter cycle life, capacity fade, and potential safety hazards. This article focuses on the high-temperature performance of two popular LIB formats: 18650 and 21700 cells, with specific attention to their charge temperature limits and rechargeable capacity.
Overview of Lithium-Ion Battery Technology
Lithium-ion batteries operate on the principle of lithium ions moving from the negative electrode (anode) to the positive electrode (cathode) during discharge and vice versa during charging. The electrolyte, typically a lithium salt in an organic solvent, facilitates the movement of lithium ions between the electrodes. The electrodes are composed of active materials that can reversibly intercalate and de-intercalate lithium ions. The anode material is often graphite, while the cathode material can be a variety of compounds, such as lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (NCM), or lithium iron phosphate (LFP).
The 18650 and 21700 Formats
The 18650 and 21700 formats refer to the cylindrical shape and size of the battery cells. The 18650 cell has a diameter of 18 mm and a length of 65 mm, while the 21700 cell has a diameter of 21 mm and a length of 70 mm. The larger size of the 21700 cell allows for increased capacity and energy density compared to the 18650 cell, making it a popular choice for EVs and high-energy applications.
At elevated temperatures, several degradation processes within the LIB accelerate, leading to performance degradation. These include:
Thermal Runaway: High temperatures can cause a chain reaction of thermal events within the battery, leading to thermal runaway and potentially catastrophic failure.
SEI Degradation: The solid electrolyte interphase (SEI) that forms on the anode during the first charge cycles is critical for stable cycling. High temperatures can degrade the SEI, leading to increased internal resistance and capacity fade.
Electrolyte Decomposition: The organic solvents in the electrolyte can decompose at high temperatures, generating gases that can cause swelling and even rupture of the battery case.
Material Degradation: The active materials in the electrodes can undergo structural changes at high temperatures, reducing their ability to reversibly intercalate and de-intercalate lithium ions.
Increased Self-Discharge: High temperatures can increase the rate of self-discharge, reducing the shelf life of the battery.
Charge Temperature Limits
To ensure safe and reliable operation, LIBs have specified charge temperature limits. These limits are typically set to prevent the battery from exceeding critical temperatures where degradation processes accelerate significantly. For 18650 and 21700 cells, the charge temperature limit is often set at 60°C. Exceeding this limit can lead to the degradation processes mentioned earlier, reducing the cycle life and capacity of the battery.
Rechargeable Capacity of 18650 and 21700 Cells
The rechargeable capacity of a LIB refers to the amount of charge that can be stored and discharged from the battery. It is typically expressed in milliampere-hours (mAh) and is a critical parameter for determining the energy density and runtime of the battery.
18650 Cells
18650 cells are available in a range of capacities, with common values being 2000 mAh, 2200 mAh, 2500 mAh, and 3000 mAh. The actual capacity of a given cell can vary depending on the electrode materials, electrolyte composition, and manufacturing process. Higher-capacity cells generally have thicker electrodes and more electrolyte, which increases their energy density but can also lead to increased internal resistance and potential safety concerns.
At high temperatures, the rechargeable capacity of 18650 cells can be affected by several factors. As mentioned earlier, high temperatures can degrade the SEI, leading to increased internal resistance and capacity fade. Additionally, the active materials in the electrodes can undergo structural changes, reducing their ability to reversibly intercalate and de-intercalate lithium ions. These factors can result in a reduction in the rechargeable capacity of the cell over time.
However, some 18650 cells are designed to operate at elevated temperatures, with specialized electrolytes and electrode materials that can withstand higher temperatures without significant degradation. These high-temperature cells often have lower capacities compared to standard cells to compensate for the increased thermal stability.
21700 Cells
21700 cells offer increased capacity and energy density compared to 18650 cells due to their larger size. Common capacities for 21700 cells range from 3000 mAh to 5000 mAh, with some high-performance cells exceeding this range. As with 18650 cells, the actual capacity of a given 21700 cell can vary depending on the electrode materials, electrolyte composition, and manufacturing process.
The rechargeable capacity of 21700 cells can also be affected by high temperatures. The same degradation processes that occur in 18650 cells can also occur in 21700 cells, leading to reduced capacity over time. However, due to their larger size and increased electrolyte volume, 21700 cells may be able to tolerate higher temperatures for longer periods without significant degradation compared to 18650 cells.
Specialized high-temperature 21700 cells are also available, with electrolytes and electrode materials designed to withstand elevated temperatures. These cells may have lower capacities compared to standard 21700 cells but offer improved thermal stability and longer cycle life at high temperatures.
Managing High-Temperature Performance
To mitigate the effects of high temperatures on LIB performance, several strategies can be employed:
Thermal Management Systems: Incorporating thermal management systems (TMS) into battery packs can help maintain the battery within its optimal operating temperature range. TMS can include passive systems, such as heat sinks and thermal insulation, or active systems, such as liquid cooling and thermal fans.
Cell Design and Materials: Developing new cell designs and materials that can withstand higher temperatures without significant degradation can improve the high-temperature performance of LIBs. This includes optimizing the electrode materials, electrolyte composition, and cell structure.
Battery Management Systems (BMS): BMS can monitor the battery's state of charge, temperature, and other critical parameters in real-time. By adjusting the charging and discharging rates based on these parameters, BMS can help maintain the battery within its safe operating range and extend its cycle life.
Charging Protocols: Implementing charging protocols that limit the charge temperature and rate can help prevent the battery from exceeding critical temperatures where degradation processes accelerate. For example, using a lower charging current at higher temperatures can help reduce the heat generated during charging.
Conclusion
In conclusion, high temperatures present significant challenges for lithium-ion batteries, including accelerated degradation processes that can lead to shorter cycle life, capacity fade, and potential safety hazards. For 18650 and 21700 cells, the charge temperature limit is often set at 60°C to ensure safe and reliable operation. The rechargeable capacity of these cells can be affected by high temperatures, with degradation of the SEI, active materials, and electrolyte contributing to reduced capacity over time.
To mitigate the effects of high temperatures on LIB performance, several strategies can be employed, including the use of thermal management systems, optimized cell designs and materials, battery management systems, and charging protocols. By addressing these challenges, the high-temperature performance of LIBs can be improved, enabling their use in a wider range of applications and extending their cycle life.
Further Research and Development
As the demand for lithium-ion batteries continues to grow, further research and development will be essential to improve their high-temperature performance. Here are some areas of focus for future research:
Advanced Electrolytes: Developing new electrolytes with higher thermal stability and lower reactivity with the electrode materials can help improve the high-temperature performance of LIBs. Electrolytes with higher ionic conductivity and lower viscosity can also contribute to better battery performance at elevated temperatures.
New Electrode Materials: Research into new electrode materials that can withstand higher temperatures without significant degradation is ongoing. These materials may include new lithium-ion intercalation compounds, solid-state electrolytes, and composite electrodes that combine the benefits of multiple materials.
Cell and Pack Design: Optimizing the cell and pack design can help improve thermal management and reduce the risk of thermal runaway. This includes designing cells with better heat dissipation properties, incorporating thermal insulation to reduce heat transfer between cells, and using advanced cooling technologies, such as phase change materials and thermal pipes.
Battery Management Systems (BMS): Advancements in BMS technology can help improve the high-temperature performance of LIBs.
