Degradation Overview

The main battery degradation mechanisms (SEI growth, lithium plating, loss of active material) and modes of capacity and resistance loss.

Battery aging is not caused by a single factor but rather by a combination of degradation mechanisms—the physical and chemical processes that occur inside the cell—each affecting different components. These mechanisms manifest as degradation modes that can be measured and tracked. Understanding this distinction helps in diagnosing battery health and predicting remaining life.

Degradation mechanisms ultimately lead to:

• Loss of capacity
• Increase in resistance

Key Degradation Mechanisms

Understanding these processes is crucial for improving battery technology and extending battery lifespan. The three mechanisms below are among the most widely studied, though many other processes contribute to aging depending on the chemistry and operating conditions.

SEI Growth

A protective layer forms on the anode surface. While essential for stable performance, it continues to grow over time, consuming lithium ions and increasing internal resistance.

Lithium Plating

Under certain conditions, metallic lithium deposits on the anode surface instead of intercalating properly. This reduces available lithium and can form dangerous dendrites.

Mechanical Degradation

Repeated expansion and contraction during cycling causes cracking of electrode particles and loss of electrical contact.

Degradation Modes

Regardless of the specific mechanism, degradation manifests as one of the following modes:

Loss of Lithium Inventory (LLI)

When lithium is consumed by unwanted side reactions—such as SEI formation or lithium plating—it becomes unavailable for energy storage. The electrodes may remain structurally intact, but with less cyclable lithium, the battery’s capacity decreases. LLI is the dominant mode for calendar aging and is accelerated by high temperatures and high states of charge.

Loss of Active Material in the Negative Electrode (LAM_NE)

When anode material becomes electrically isolated (due to particle cracking, binder degradation, or delamination), it can no longer participate in the electrochemical reactions. LAM_NE is often caused by SEI-induced particle isolation or mechanical cracking from volume changes during cycling. It is more prominent at high C-rates or with materials that experience large volume changes (like silicon).

Loss of Active Material in the Positive Electrode (LAM_PE)

When cathode material becomes electrically isolated or structurally degraded, it can no longer store lithium. LAM_PE can result from structural changes, transition metal dissolution, oxygen release, or mechanical stress. It is accelerated by high voltages and high temperatures.

Resistance Increase

In addition to capacity loss, degradation mechanisms also cause the cell’s internal resistance to increase over time. This manifests as higher overpotentials during charge and discharge, reduced power capability, and increased heat generation. Resistance increase is caused by SEI thickening, loss of electrical contact, and electrolyte degradation.

When quantifying degradation effects on theoretical capacity, we typically focus on LLI, LAM_NE, and LAM_PE—these three modes directly determine how much charge the battery can store. Resistance increase affects power and efficiency but not the theoretical capacity itself.

Interaction of Degradation Modes

In practice, these modes occur simultaneously and often reinforce each other. For example:

• SEI growth (LLI) can increase local stresses that promote particle cracking (LAM_NE)
• Particle cracking (LAM_NE) exposes fresh surface area that forms new SEI (LLI)
• Lithium plating (LLI) can cause mechanical stresses that damage the anode (LAM_NE)
• Cathode structural changes (LAM_PE) can release oxygen that accelerates electrolyte decomposition

The relative contribution of each mode depends on battery chemistry, usage patterns, and environmental conditions. High-temperature storage primarily causes LLI through accelerated SEI growth, while aggressive cycling at low temperatures may cause both LLI (plating) and LAM_NE (mechanical stress).

Linking Mechanisms to Modes

The table below shows some common examples of how mechanisms contribute to degradation modes. This is not comprehensive—many other mechanisms exist and the relationships depend on specific battery chemistry and operating conditions. See the pages on SEI growth, lithium plating, and mechanical degradation for more details on specific mechanisms.

Mechanism	Primary Mode(s)	Accelerating Factors
SEI growth	LLI, Resistance increase	Time, high temperature, high SoC
Lithium plating	LLI	Low temperature, fast charging, high SoC
Mechanical degradation	LAMNE, LAMPE	High C-rates, deep cycling, volume changes
Cathode degradation	LAMPE, Resistance increase	High voltage, high temperature

Understanding how specific mechanisms contribute to each mode helps in designing batteries and usage strategies that minimize degradation.