Cell Degradation

Importance of degradation process

Degradation in lithium-ion batteries refers to the multifaceted processes that progressively diminish their performance over time. These processes manifest as capacity fade, the reduction in the amount of charge a battery can store, and power fade, the decrease in its ability to deliver energy quickly. Understanding degradation is crucial for several reasons.

• Extending Battery Lifespan: By understanding the mechanisms of degradation, researchers can develop strategies to mitigate their effects and prolong battery life. This involves optimising operating conditions, exploring new materials, and designing more robust battery architectures.
• Enhancing Safety: Degradation processes can lead to internal issues like lithium plating and dendrite growth, increasing the risk of internal short circuits and thermal runaway events, posing serious safety hazards. Knowledge of these mechanisms is critical for designing safer batteries and implementing appropriate safety measures.
• Enabling Effective Battery Management: Accurate models of battery degradation are essential for developing sophisticated Battery Management Systems (BMS). These systems rely on understanding how different degradation mechanisms influence battery behaviour to estimate crucial parameters like State of Health (SoH) and remaining useful life, enabling optimal charging and discharging strategies and ensuring safe and reliable battery operation.
  
• Improving Economic Viability: Batteries represent a significant cost in applications like electric vehicles and grid storage. Understanding and predicting degradation is vital for accurately assessing battery lifespan, enabling better warranty policies, optimising battery usage patterns, and maximising return on investment.
• Reducing Environmental Impact: The production of lithium-ion batteries has environmental implications due to raw material extraction and manufacturing processes. Extending battery lifespan through degradation management directly reduces the need for frequent replacements, thereby minimising the overall environmental footprint.

In conclusion, degradation is a complex challenge inherent to lithium-ion battery technology. A thorough understanding of its various mechanisms, modes of influence, and long-term effects is vital for advancing battery technology, optimising its performance, and unlocking its full potential across diverse applications.

Degradation Mechanisms

There are several degradation mechanisms affecting lithium-ion battery performance. These mechanisms, categorised by the affected component, are:

Electrolyte:

• Loss of Electrolyte: This involves the reduction in electrolyte volume and conductivity, primarily due to its consumption during SEI formation and growth. Other contributing factors include high voltages, elevated temperatures, lithium plating, and moisture contamination, all of which can lead to electrolyte decomposition.

Negative Electrode (NE):

• Solid Electrolyte Interphase (SEI) Layer Growth: The SEI is a passivation layer that forms on the NE surface upon contact with the electrolyte. While it initially consumes about 10% of the capacity during formation, it also protects the electrode from further electrolyte reactions. However, the SEI continues to grow over time, particularly on graphite anodes, due to factors like high temperatures, high currents, and the deposition of transition metal (TM) ions from the positive electrode.
• Lithium Plating: This side reaction involves the deposition of metallic lithium on the NE surface instead of intercalation. It occurs when the intercalation process is hindered, such as at low temperatures or high charge currents, or when the NE is fully lithiated. Factors like high cell voltage, insufficient NE mass or surface area, and defects in the separator or NE can exacerbate lithium plating.
• Particle Fracture: This mechanism, prevalent in both electrodes, arises from the volume changes experienced during lithium intercalation and deintercalation. Factors like extreme temperatures, high current loading, cycling across specific SoC windows, high silicon content, large particle size, and manufacturing processes like calendering can all contribute to particle fracture

Positive Electrode (PE):

• Structural Change and Decomposition: These mechanisms are highly dependent on the PE chemistry, which commonly involves lithium transition metal oxides like NMC.
• Phase Change: Delithiated layered structures of NMC materials can transform into disordered spinel and rock salt phases, particularly at high states of charge (SoC) and elevated temperatures. This process releases oxygen, leading to the formation of O2 and other gaseous products, and contributes to capacity fade and impedance rise.
• Particle Fracture: This mechanism, prevalent in both electrodes, arises from the volume changes experienced during lithium intercalation and deintercalation. Factors like extreme temperatures, high current loading, cycling across specific SoC windows, large particle size, and manufacturing processes like calendering can all contribute to particle fracture.
• Oxidation of Lattice Oxygen: At high voltages, oxide anions within the PE lattice can undergo electrochemical oxidation. This leads to TM dissolution and the formation of rock salt phases, further degrading the electrode.
• Electrolyte Decomposition and Loss: Highly oxidised nickel species, often found at high SoCs, can react with the electrolyte, causing its decomposition and the dissolution of Ni2+ ions.
• TM/Li+ Site Exchange: The similar ionic radii of Li+ and Ni2+ can result in their exchange within the PE crystal lattice, hindering Li+ diffusion and increasing impedance.
• Acid Attack: Moisture contamination in the electrolyte can lead to the formation of acidic species like HF, which attack the PE material, causing TM dissolution and electrolyte loss.
• Positive Solid Electrolyte Interface (pSEI) Formation: Similar to the SEI on the NE, a pSEI layer can form on the PE due to the reaction of dissolved TM ions with the electrolyte. pSEI growth contributes to electrolyte loss and increases cell impedance. (also called as cathode electrolyte interface or CEI)
  

 Interconnected Mechanisms

These degradation mechanisms are not isolated events but are interconnected. For example, the formation of dendrites on the anode can lead to electrolyte decomposition, which in turn can accelerate the degradation of both electrodes.

The Challenge of Battery Longevity

Addressing battery degradation is a complex challenge that requires a deep understanding of these interconnected mechanisms. Researchers and engineers are constantly working to develop materials and technologies that can improve battery life and performance. By unraveling the puzzle of battery degradation, we can pave the way for a future powered by more reliable and efficient energy storage solutions.

Three Main Stages of Battery Degradation

Battery degradation may be described mainly by three main stages: acceleration, stabilisation, and saturation.

• Acceleration Stage: This initial stage is characterised by a rapid decline in the battery's capacity. This early degradation is primarily attributed to the formation of the SEI layer on the negative electrode. While the SEI is crucial for protecting the electrode from further reactions with the electrolyte, its formation consumes lithium ions and reduces the battery's active material, leading to an initial capacity loss.

• Stabilisation (Linear Ageing) Stage: Following the initial acceleration phase, the degradation process tends to stabilise, resulting in a more gradual and linear decline in capacity. While the exact causes of this transition are still debated, the sources suggest that the established SEI layer, now acting as a barrier, slows down further electrolyte decomposition and SEI growth. This leads to a more balanced and predictable degradation pattern.

• Saturation (Nonlinear Ageing) Stage: In this final stage, the degradation process accelerates again, leading to a rapid and nonlinear drop in capacity. Several factors and their complex interplay have been proposed to explain this transition:

• SEI-Induced Pore Clogging and Lithium Plating: As the SEI layer grows, even at a slower rate, it can clog the pores within the electrode, hindering lithium-ion transport. This can lead to localised overpotentials, creating favourable conditions for lithium plating, even at room temperature. Lithium plating, being a highly irreversible process, consumes active lithium, reduces available capacity, and can further exacerbate degradation. 
• Electrolyte Loss and Cell Drying: Continuous SEI growth, even at a reduced rate, consumes the electrolyte solvent. This gradual electrolyte loss can lead to localised dry areas within the cell, hindering ionic conductivity and accelerating degradation.
• Positive Electrode Degradation: While early degradation stages often focus on the negative electrode, the positive electrode also undergoes degradation processes like structural changes, transition metal dissolution, and SEI growth. These processes, initially subtle, can accumulate over time and become more prominent in the later stages of battery life, contributing to the accelerated capacity fade.

A single universal explanation for the transition from linear to nonlinear ageing might not exist. The specific combination of degradation mechanisms driving this transition likely depends on factors like cell chemistry, operating conditions (temperature, charge/discharge rates, depth of discharge), and manufacturing variations. A more comprehensive understanding of these interacting factors is crucial for accurately predicting battery lifespan and developing effective mitigation strategies.

Reference

1.  Lithium ion battery degradation: what you need to know