Degradation Analysis of Commercial Lithium-Ion Batteries in Long-Term Storage. Lithium-ion batteries have become indispensable across various industries due to their high energy density and efficiency. However, their performance deteriorates over time, particularly during extended storage periods. Understanding the mechanisms and factors influencing this degradation is crucial for optimizing battery lifespan and maximizing their effectiveness. This article delves into the degradation analysis of commercial lithium-ion batteries in long-term storage, offering actionable strategies to mitigate performance decline and extend battery life.
Key Degradation Mechanisms:
Self-discharge
Internal chemical reactions within lithium-ion batteries cause a gradual loss of capacity even when the battery is idle. This self-discharge process, though typically slow, can be accelerated by elevated storage temperatures. The primary cause of self-discharge is side reactions triggered by impurities in the electrolyte and minor defects in the electrode materials. While these reactions proceed slowly at room temperature, their rate doubles with every 10°C increase in temperature. Therefore, storing batteries at temperatures higher than recommended can significantly increase the self-discharge rate, leading to a substantial reduction in capacity before use.
Electrode reactions
Side reactions between the electrolyte and electrodes result in the formation of a solid electrolyte interface (SEI) layer and degradation of electrode materials. The SEI layer is essential for the normal operation of the battery, but at high temperatures, it continues to thicken, consuming lithium ions from the electrolyte and increasing the internal resistance of the battery, thus reducing capacity. Moreover, high temperatures can destabilize the electrode material structure, causing cracks and decomposition, further decreasing battery efficiency and lifespan.
Lithium loss
During charge-discharge cycles, some lithium ions become permanently trapped in the electrode material’s lattice structure, making them unavailable for future reactions. This lithium loss is exacerbated at high storage temperatures because high temperatures promote more lithium ions to become irreversibly embedded in lattice defects. As a result, the number of available lithium ions decreases, leading to capacity fade and shorter cycle life.
Factors Affecting Degradation Rate
Storage temperature
Temperature is a primary determinant of battery degradation. Batteries should be stored in a cool, dry environment, ideally within the range of 15°C to 25°C, to slow down the degradation process. High temperatures accelerate chemical reaction rates, increasing self-discharge and the formation of the SEI layer, thus speeding up battery aging.
State of charge (SOC)
Maintaining a partial SOC (around 30-50%) during storage minimizes electrode stress and reduces the self-discharge rate, thereby extending battery life. Both high and low SOC levels increase electrode material stress, leading to structural changes and more side reactions. A partial SOC balances stress and reaction activity, slowing down the degradation rate.
Depth of discharge (DOD)
Batteries subjected to deep discharges (high DOD) degrade faster compared to those undergoing shallow discharges. Deep discharges cause more significant structural changes in electrode materials, creating more cracks and side reaction products, thus increasing the degradation rate. Avoiding fully discharging batteries during storage helps mitigate this effect, prolonging battery life.
Calendar age
Batteries naturally degrade over time due to inherent chemical and physical processes. Even under optimal storage conditions, the chemical components of the battery will gradually decompose and fail. Proper storage practices can slow down this aging process but cannot entirely prevent it.
Degradation Analysis Techniques:
Capacity fade measurement
Periodically measuring the battery’s discharge capacity provides a straightforward method to track its degradation over time. Comparing the battery’s capacity at different times allows for assessing its degradation rate and extent, enabling timely maintenance actions.
Electrochemical impedance spectroscopy (EIS)
This technique analyzes the battery’s internal resistance, providing detailed insights into changes in electrode and electrolyte properties. EIS can detect changes in the battery’s internal impedance, helping identify specific causes of degradation, such as SEI layer thickening or electrolyte deterioration.
Post-mortem analysis
Disassembling a degraded battery and analyzing the electrodes and electrolyte using methods like X-ray diffraction (XRD) and scanning electron microscopy (SEM) can reveal the physical and chemical changes occurring during storage. Post-mortem analysis provides detailed information on structural and compositional changes within the battery, aiding in understanding degradation mechanisms and improving battery design and maintenance strategies.
Mitigation Strategies
Cool storage
Store batteries in a cool, controlled environment to minimize self-discharge and other temperature-dependent degradation mechanisms. Ideally, maintain a temperature range of 15°C to 25°C. Using dedicated cooling equipment and environmental control systems can significantly slow the battery aging process.
Partial charge storage
Maintain a partial SOC (around 30-50%) during storage to reduce electrode stress and slow down degradation. This requires setting appropriate charging strategies in the battery management system to ensure the battery remains within the optimal SOC range.
Regular monitoring
Periodically monitor battery capacity and voltage to detect degradation trends. Implement corrective actions as needed based on these observations. Regular monitoring can also provide early warnings of potential issues, preventing sudden battery failures during use.
Battery management systems (BMS)
Utilize BMS to monitor battery health, control charge-discharge cycles, and implement features such as cell balancing and temperature regulation during storage. BMS can detect battery status in real-time and automatically adjust operational parameters to extend battery life and enhance safety.
Conclusion
By comprehensively understanding degradation mechanisms, influencing factors, and implementing effective mitigation strategies, you can significantly enhance the long-term storage management of commercial lithium-ion batteries. This approach enables optimal battery utilization and extends their overall lifespan, ensuring better performance and cost efficiency in industrial applications. For more advanced energy storage solutions, consider the 215 kWh Commercial and Industrial Energy Storage System by Kamada Power.
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Post time: May-29-2024