Cycling and Calendar Degradation

The concepts of cycling and calendaring degradation in lithium-ion batteries is defined on the basis of usage state, whether battery is used or not, it is described below

Cycling degradation

Cycling degradation refers to the irreversible capacity and power fade that occurs due to repeated charge and discharge cycles. This degradation is influenced by several operational parameters:

• State of Charge (SOC): Higher SOC levels generally lead to faster degradation.
• Depth of Discharge (DOD): High DOD rates contribute to capacity and energy reduction.
• C-rate: High charge and discharge currents accelerate degradation processes.
• Temperature: Extreme temperatures, both high and low, negatively impact battery lifespan.
• Voltage Limits: Operating outside the safe voltage window of the battery accelerates degradation.

The cycling process causes various chemical and mechanical stresses within the battery, leading to:

• Formation and growth of the Solid-Electrolyte Interphase (SEI) layer: This layer, while protective, consumes lithium ions and can become unstable over time, leading to capacity fade and impedance growth.
• Mechanical stress in electrode materials: Repeated expansion and contraction of electrodes during cycling can cause particle cracking and loss of active material.
• Lithium plating: Especially at low temperatures and high charge rates, lithium can deposit on the anode surface instead of intercalating, reducing capacity and potentially leading to safety hazards.

Calendar degradation

Calendar degradation, also referred to as calendar ageing, pertains to the capacity loss that transpires over time, regardless of whether the battery is being actively cycled or not. This degradation mode is primarily influenced by:

• Storage Time: Longer storage periods generally lead to more pronounced capacity fade.
• Temperature: High temperatures significantly accelerate calendar degradation.
• State of Charge (SOC): Higher SOC during storage exacerbates capacity loss.

Calendar degradation arises from various chemical processes within the battery, even in the absence of cycling:

• Electrolyte Decomposition: Electrolyte solvents and salts can decompose over time, leading to the formation of unwanted products that can hinder battery performance.
• Growth of Surface Films: Passivation layers, similar to the SEI layer, can form on both electrodes, consuming active lithium and increasing impedance.
• Self-Discharge Reactions: Even without external current flow, internal self-discharge reactions can occur, leading to capacity loss.

It is important to note that cycling and calendar degradation mechanisms are interrelated. The extent of each type of degradation depends on the specific operating conditions and battery chemistry. For instance, high-temperature storage can exacerbate both cycling and calendar degradation. Therefore, understanding both modes of degradation is crucial for predicting battery lifespan, optimising performance, and ensuring safe operation.