A phenomenon called "stratification" is one of the many challenges that impact the efficiency, life cycle, and safety of Lithium-ion batteries.
What is Stratification?
Stratification refers to the uneven distribution of lithium ions in the electrolyte within the battery cell. This uneven distribution typically results from prolonged periods of inactivity, incomplete charge or discharge cycles, or operation under extreme temperatures.
The electrolyte in the battery can be conceptualized similarly to a solution of salt in water. When the solution is agitated or mixed, the salt (analogous to lithium ions) is evenly distributed. But, if the solution is allowed to sit idle, the salt may settle or concentrate in particular areas, leading to stratification.
Impact of Stratification on Battery Performance
Reduced capacity: Stratification can lead to reduced battery capacity because certain areas in the battery cell contain higher concentrations of lithium ions than others. This non-uniformity means that the battery may not fully charge or discharge, reducing its overall capacity.
Degraded life cycle: Due to the uneven distribution of ions, some areas of the battery may experience more intense chemical reactions during charge/discharge cycles than others. This uneven wear and tear can lead to a shortened life cycle for the battery.
Safety concerns: Lithium plating is one of the most severe outcomes of stratification. When lithium ions concentrate near the anode during charging, it can cause lithium metal to plate onto the anode surface. Over time, this can lead to the formation of dendrites, needle-like structures that can potentially puncture the separator, causing a short circuit and, in the worst-case scenario, thermal runaway.
Counteracting Stratification
Battery management systems (BMS): A well-designed BMS can implement charging protocols that help to redistribute lithium ions evenly throughout the electrolyte. These might include slow or "trickle" charging, or periodic "equalizing" charges.
Improved battery design: Innovations in battery cell design, such as advanced electrode structures or new electrolyte materials, may help to promote more uniform ion distribution. For example, solid-state batteries replace the liquid electrolyte with a solid one, eliminating the risk of stratification.
Regular use and maintenance: For the end-user, maintaining a regular charge/discharge cycle and avoiding extreme temperatures can help to reduce the likelihood of severe stratification.
Stratification is among the many challenges in the quest for longer-lasting, safer, and more efficient lithium-ion batteries.