Battery calendar aging refers to the gradual degradation of electrochemical performance over time, even when the battery remains inactive. Unlike cycle aging, which results from repeated charge-discharge processes, calendar aging occurs due to passive chemical and electrochemical reactions that proceed regardless of use. The primary manifestations of calendar aging are capacity fade and impedance growth, both of which reduce the usable energy and power delivery of lithium-ion batteries.
The fundamental mechanisms behind calendar aging involve complex interactions between electrode materials, electrolytes, and external conditions such as temperature and state of charge. These factors accelerate or decelerate degradation pathways, ultimately determining the lifespan of a battery in storage or standby applications.
**Chemical Degradation Processes**
Calendar aging is driven by irreversible side reactions that consume active lithium and degrade electrode materials. One of the most significant contributors is the continuous growth of the solid-electrolyte interphase (SEI) layer on the anode. The SEI forms during initial cycles as a passivation layer, preventing further electrolyte decomposition. However, over time, even at rest, the SEI can thicken due to residual electrolyte reduction reactions. This consumes lithium ions, reducing the available capacity. Additionally, the SEI growth increases interfacial resistance, leading to impedance rise.
Cathode materials also undergo chemical degradation, particularly at high states of charge. Transition metal dissolution, especially in layered oxides like NMC or LCO, occurs when the cathode is held at elevated voltages. The dissolved metal ions can migrate through the electrolyte and deposit on the anode, further destabilizing the SEI and accelerating capacity loss.
**Electrolyte Decomposition**
The electrolyte in lithium-ion batteries is thermodynamically unstable at both high and low potentials, leading to continuous decomposition even without cycling. At the anode, solvents such as ethylene carbonate (EC) and additives undergo reduction, contributing to SEI growth. At the cathode, oxidation reactions break down electrolyte components, generating gaseous byproducts like CO₂ and increasing internal pressure.
Elevated temperatures exacerbate electrolyte decomposition by increasing reaction kinetics. For example, linear carbonates like dimethyl carbonate (DMC) are more prone to oxidation at higher temperatures, leading to faster degradation. Additionally, trace impurities such as water or hydrofluoric acid (HF) can catalyze parasitic reactions, further accelerating aging.
**Passive Electrode Reactions**
Even without cycling, electrode materials experience structural changes over time. Graphite anodes can suffer from particle cracking due to mechanical stress induced by lithium intercalation, especially at high states of charge. This exposes fresh surfaces to the electrolyte, triggering additional SEI formation.
Cathode materials, particularly nickel-rich NMC, are susceptible to phase transitions when stored at high voltages. These structural rearrangements reduce the material’s ability to intercalate lithium, decreasing capacity. Similarly, lithium iron phosphate (LFP) cathodes, while more stable, can still experience slow degradation due to iron dissolution in the presence of moisture or acidic species.
**Time-Dependent Factors Influencing Calendar Aging**
Three primary factors govern the rate of calendar aging: state of charge (SOC), temperature, and material stability.
1. **State of Charge (SOC):** Higher SOC accelerates degradation due to increased electrode potentials. At elevated SOC, the anode is lithiated to a greater extent, increasing the driving force for SEI growth. The cathode is also held at a higher voltage, promoting transition metal dissolution and electrolyte oxidation. Storing batteries at intermediate SOC (e.g., 40-60%) minimizes these effects.
2. **Temperature:** Elevated temperatures exponentially increase reaction rates, as described by the Arrhenius equation. SEI growth, electrolyte decomposition, and transition metal dissolution all proceed faster at higher temperatures. For every 10°C increase, the rate of capacity fade can approximately double.
3. **Material Stability:** The intrinsic stability of electrode materials determines their susceptibility to calendar aging. For example, silicon-containing anodes degrade faster than graphite due to larger volume changes that disrupt the SEI. Similarly, high-nickel cathodes degrade more rapidly than LFP due to their structural instability at high voltages.
**Differences Between Calendar Aging and Cycle Aging**
While both calendar and cycle aging lead to capacity fade and impedance growth, their underlying mechanisms differ. Cycle aging is dominated by mechanical stresses from repeated volume changes, particle cracking, and active material loss. In contrast, calendar aging is primarily driven by chemical and electrochemical side reactions that proceed even without cycling.
However, the two aging modes are not entirely independent. Calendar aging can influence cycle aging by altering electrode surfaces and increasing impedance before cycling begins. Conversely, cycling can exacerbate calendar aging by continuously refreshing electrode-electrolyte interfaces, exposing new surfaces to degradation.
**Common Degradation Pathways**
Several well-documented degradation pathways contribute to calendar aging in lithium-ion batteries:
- **SEI Growth:** Continuous electrolyte reduction at the anode consumes lithium ions and increases resistance.
- **Transition Metal Dissolution:** Cathode degradation leads to metal ion migration and anode contamination.
- **Electrolyte Oxidation:** High cathode potentials break down solvents, generating gas and resistive byproducts.
- **Binder Degradation:** Polymeric binders can decompose over time, reducing electrode adhesion and increasing impedance.
Each of these pathways is influenced by storage conditions, with higher temperatures and SOC levels accelerating degradation. Understanding these mechanisms is critical for predicting battery lifespan and optimizing storage protocols to minimize performance loss over time.
In summary, calendar aging in lithium-ion batteries is an inevitable consequence of thermodynamic instability in electrochemical systems. The interplay between chemical degradation, electrolyte decomposition, and passive electrode reactions dictates the rate of capacity fade and impedance growth. By controlling external factors such as SOC and temperature, it is possible to mitigate some of these effects, though material-level instabilities remain a fundamental challenge for long-term battery storage.