Degradation and Ageing Mechanisms in Li-ion Cells

Introduction

A lithium-ion cell can deliver less capacity, less power or both as it ages. Those outcomes do not each point to one unique chemical cause. Lithium can become trapped in interphase products, host particles can lose electrical contact, metallic Li can plate during charging, and resistive films or damaged pores can increase polarisation. These mechanisms interact, so ageing diagnosis starts by distinguishing what is lost—cyclable lithium, active host, or accessible voltage at the requested rate.

Core explanation

Loss of lithium inventory , or LLI, means fewer lithium atoms remain available to shuttle reversibly between electrodes. SEI growth consumes Li into reaction products; plated lithium can become electronically isolated or react with electrolyte. Even if the cathode and anode crystal frameworks remain largely intact, LLI shifts the relative states of charge they can access in a fixed cell-voltage window. Capacity declines because the cell lacks enough mobile lithium to use both hosts fully.

Loss of active material , or LAM, means part of an electrode's storage capacity becomes unavailable. Particle fracture, disconnection from the conductive network, phase reconstruction, metal dissolution or pore blockage can contribute. LAM can occur on the positive or negative electrode and need not consume lithium directly. It may nevertheless accelerate LLI: remaining active area carries more local current, causing stronger interfacial reactions and possibly plating.

Lithium plating occurs when charging drives the negative-electrode surface potential sufficiently low relative to Li/Li⁺ that metallic lithium deposition competes with insertion. Cold temperature, high charge current, high state of charge or transport limitation can make this more likely. Plated Li may later strip, but some becomes isolated or reacts chemically; growth can also create safety concerns. A voltage or capacity change alone is not definitive proof of plating. Differential voltage analysis, careful relaxation measurements and direct chemical or imaging evidence can strengthen the assignment.

Impedance growth lowers power capability by increasing voltage drop at a given current. It may reflect thicker interphases, loss of electrolyte, surface reconstruction, contact damage or altered pore pathways. At low discharge rate a cell might still show near-normal capacity, while at high rate it reaches the voltage cutoff early. Consequently a capacity test should specify current, temperature and cutoffs. Calendar ageing at rest and cycle ageing under use can have different dominant processes.

Mechanisms are coupled. Silicon cracking exposes surface and drives new SEI; new SEI consumes lithium and may increase resistance; higher local resistance increases current nonuniformity; that can promote further cracking or plating. A complicated model can reproduce a fade curve with several different parameter combinations, so direct or orthogonal diagnostics are important.

Step-by-step reasoning

Record how capacity and power change separately at defined temperatures and rates. Compare low-rate reference tests with high-rate tests: a large rate-dependent deficit suggests growing polarisation. Use voltage-curve analysis to test relative LLI and electrode LAM, acknowledging model uncertainty. Examine EIS for resistance changes and material characterisation for cracks, film growth or plated Li. Relate each proposed mechanism to operating conditions and avoid claiming a unique cause from one capacity number.

Visual explanation

Draw a full cell with a fixed number of lithium tokens moving between two host reservoirs. In LLI, some tokens are locked in SEI; in LAM, part of one reservoir is crossed out. On the anode surface draw a metallic lithium layer for plating. Show two discharge curves: a high-current aged curve reaching cutoff early due to voltage drop and a low-current curve with smaller difference.

Real-world analogy

A shuttle system can fail because vehicles are lost, parking spaces close, or roads become slower. Fewer vehicles parallels LLI, fewer spaces parallels LAM, and slower roads parallel impedance growth. The analogy distinguishes modes, though a real battery's modes also change potentials and reaction chemistry.

Real-world example

A fast-charged cell shows growing high-rate capacity loss, while its slow-rate reference capacity declines more gradually. EIS shows increased resistance and an anode inspection finds evidence of surface products. Researchers cannot assign the entire loss to plating without measuring metallic Li or a compatible voltage signature; SEI growth and contact losses may contribute. Cycling a matched cell at lower charge rate helps identify rate-dependent stress.

Why?

Why can capacity fade with intact active particles? Lithium may be locked into side products. Why can power fade precede large low-rate capacity fade? Resistive films and transport gradients cause voltage cutoffs at high current. Why does plating often appear during charge? Insertion demand can exceed the rate at which Li enters the host, lowering the local anode potential until metallic deposition competes.

Common misconception

All “lost capacity” is not lost cathode material. It may be LLI or simply inaccessible at the test rate due to impedance. Likewise, observing an SEI does not prove it alone caused fade; all conventional graphite cells form an SEI during normal operation. A mechanism claim needs comparison with the cell's baseline and operating conditions.

Worked example

Question: A new cell delivers 2.00 Ah at a slow rate and 1.80 Ah at a fast rate. After ageing it delivers 1.90 Ah slowly but only 1.40 Ah rapidly. Compare slow-rate and fast-rate losses.

Reasoning: Slow-rate capacity loss is 0.10 Ah, or 5% of initial slow-rate value. Fast-rate loss is 0.40 Ah, about 22% of initial fast-rate value. The larger rate-dependent loss is consistent with increased polarisation or transport limitation, but LLI or LAM could still contribute to the slow-rate decline.

Answer: Slow-rate loss is 0.10 Ah; fast-rate loss is 0.40 Ah, suggesting a substantial rate-related penalty.

Quick check

1. Can a battery lose high-rate power while retaining much of its slow-rate capacity? Answer: Yes. Impedance or transport losses can cause early voltage cutoff at high current while slow operation still accesses material.

Exam focus

Define LLI, LAM and impedance growth before linking them to observations. State charge rate, state of charge and temperature when discussing plating. Use numerical capacity data with the same test conditions. Explain feedback among cracking, SEI growth and local current density rather than treating modes as independent.

Advanced insight

Voltage-curve fitting can estimate electrode slippage and active-capacity changes, but flat electrode voltage profiles can make parameters hard to identify uniquely. EIS likewise separates frequency ranges more readily than chemical identities. Combining reference performance tests with operando pressure, gas analysis or post-mortem imaging improves attribution. A model with more coupled degradation terms is not necessarily better if the data cannot constrain them.

Summary

Li-ion ageing combines loss of cyclable lithium, loss of usable host material and rising impedance. Lithium plating can both consume lithium and create safety concerns. These modes can reinforce one another and affect capacity and power differently. Diagnosis requires matched test conditions and multiple measurements before assigning a chemical cause.

Practice questions

1. Give one example of LLI without immediate destruction of the cathode host. Answer: Lithium fixed in newly grown anode SEI lowers cyclable inventory while the cathode lattice may remain intact.

2. Why may cold fast charging promote lithium plating? Answer: Slower transport and insertion increase anode polarisation, allowing metallic Li deposition to compete.

3. A cell retains slow-rate capacity but loses fast-rate capacity. Which broad degradation mode should be investigated first? Answer: Increased impedance or transport limitation should be investigated, without excluding other simultaneous modes.

4. How can particle cracking indirectly increase LLI? Answer: It exposes fresh surface for electrolyte reduction and new SEI, which consumes cyclable lithium.

Sources: Coupled degradation-mode analysis, Nature Communications; Inactive lithium quantification, Nature; Argonne, impedance rise and power fade study.