Lithium-Ion Cell Chemistry

Introduction

Lithium-ion cells power phones, laptops and electric vehicles because they combine a high voltage, around 3.6–3.8 V per cell, with a light, rechargeable chemistry. Unlike a lead–acid or alkaline cell, no electrode dissolves or is rebuilt. Instead, lithium ions slide in and out of solid host materials, like books moving between two bookshelves. Understanding this insertion chemistry explains both why these cells perform so well and why they must be kept within strict operating limits.

Core explanation

The two hosts. The usual negative electrode is graphite. Lithium ions can slip between its carbon layers to form LiC₆ when fully charged. The positive electrode is a transition-metal compound with open channels or layers, such as lithium cobalt oxide (LiCoO₂), a nickel–manganese–cobalt oxide (NMC) or lithium iron phosphate (LiFePO₄). Between them is a porous polymer separator soaked in an electrolyte: a lithium salt such as LiPF₆ dissolved in a mixture of organic carbonate solvents. Water cannot be used, because it would be electrolysed at these voltages.

Discharge. During discharge, at the negative electrode lithium leaves graphite:

LiC₆ → C₆ + Li⁺ + e⁻

Electrons travel through the external circuit, doing work, while Li⁺ ions travel through the electrolyte to the positive electrode, where both are taken up:

Li₁₋ₓCoO₂ + xLi⁺ + xe⁻ → LiCoO₂

The cobalt is reduced from a higher oxidation state towards Co(III). Charge neutrality is kept at every moment: every lithium ion that crosses the electrolyte is matched by an electron in the wire. On charging, an external supply drives both processes in reverse. Because Li⁺ simply shuttles back and forth, this is called a rocking-chair cell.

Why the voltage is high. The cell voltage reflects the difference in the chemical potential of lithium in the two hosts. Lithium in graphite is almost as reactive as lithium metal (about 0.1 V from it), while lithium in a cobalt or nickel oxide is strongly stabilised. The difference, roughly 3.7 V, sets the cell voltage.

The SEI. Graphite at low potential lies outside the stability window of the electrolyte, so on the first charge some solvent and salt are reduced, forming a thin solid-electrolyte interphase. This film consumes some lithium irreversibly (the first-cycle loss) but then passivates the surface: it lets Li⁺ through while blocking electrons, preventing continuous electrolyte decomposition. A stable SEI is essential for long cycle life.

Operating limits. Each chemistry has an upper cut-off voltage (commonly about 4.2 V) and a lower cut-off (about 2.5–3.0 V). Overcharging removes too much lithium from the positive host, destabilising its structure and oxidising the electrolyte. Over-discharging can dissolve the copper current collector. Charging too fast or in the cold makes lithium insertion into graphite slower than the arrival of ions, so the graphite potential falls below 0 V versus Li and lithium metal plates on the surface. Plated lithium can grow as needles that pierce the separator. Excess heat can trigger exothermic decomposition reactions that accelerate one another, known as thermal runaway. For these reasons every pack contains a battery management system that monitors voltage, current and temperature.

Step-by-step reasoning

To follow a lithium-ion discharge:

1. At the negative electrode, lithium is oxidised and leaves graphite as Li⁺. 2. Electrons flow through the external circuit to the positive electrode. 3. Li⁺ ions migrate and diffuse through the electrolyte and separator. 4. At the positive electrode, Li⁺ and e⁻ enter the oxide host and the metal is reduced. 5. The cell voltage falls gradually as the lithium content of each host changes.

Visual explanation

Picture two layered stacks facing each other across a separator. When charged, the graphite stack on the left is full of lithium dots. During discharge the dots stream rightwards through the liquid while an arrow for electrons loops through a lamp above; at the end, the oxide stack on the right is full.

Real-world analogy

A lithium-ion cell is like a car park shuttle. Passengers (lithium ions) ride a bus through the tunnel (electrolyte) while their luggage (electrons) goes round by a separate road (wire). Both must arrive together, and the gates at each car park (the host lattices) can only accept so many at a time.

Real-world example

Lithium iron phosphate cells run at about 3.2–3.3 V, lower than NMC cells, so they store less energy per kilogram. However, the phosphate framework holds its oxygen very firmly, making the cells much more resistant to thermal runaway. They are therefore popular in buses and home storage where safety and long life matter more than mass.

Why?

Why do lithium-ion cells lose capacity over years of use? The SEI slowly thickens, trapping lithium; electrode particles crack from repeated expansion and contraction; and transition metals may dissolve from the positive electrode. Each process removes cyclable lithium or active host material.

Common misconception

"A lithium-ion battery contains lithium metal." In normal operation lithium exists only as ions in the electrolyte and as inserted lithium in the hosts. Lithium metal appears only when something has gone wrong, such as plating during fast or cold charging.

Worked example

Question: LiCoO₂ (M ≈ 97.9 g/mol) can reversibly release only about half its lithium (x ≈ 0.5). Estimate its practical specific capacity.

Reasoning: The full theoretical capacity is 1 × 96 485 ÷ (3.6 × 97.9) ≈ 274 mA h/g. Using only half the lithium gives about 0.5 × 274 ≈ 137 mA h/g.

Answer: About 137 mA h/g, close to the roughly 140 mA h/g achieved in practice.

Quick check

1. Which electrode material is oxidised during the discharge of a graphite/LiCoO₂ cell, and what leaves it? Answer: Lithiated graphite (LiC₆) is oxidised, releasing lithium ions into the electrolyte and electrons into the circuit.

Exam focus

Be ready to write the half-equations for both electrodes during charge and discharge, to explain why a non-aqueous electrolyte is required, and to describe the role of the SEI. Link lithium plating to fast or cold charging and to the risk of internal short circuits.

Advanced insight

Many positive electrodes release oxygen from the lattice when strongly delithiated and heated. That oxygen can react with the flammable organic electrolyte inside the sealed cell, which is why high-nickel chemistries need careful voltage limits and why solid-state electrolytes, which are non-flammable and could permit lithium-metal negative electrodes, are an active research goal.

Summary

Lithium-ion cells shuttle Li⁺ between a graphite negative electrode and a metal-oxide or phosphate positive electrode, with electrons flowing through the external circuit. The voltage reflects the difference in lithium's chemical potential in the two hosts. A passivating SEI enables long life, while strict voltage, current and temperature limits avoid plating, structural damage and thermal runaway.

Practice questions

1. Write the half-equation for the negative electrode during charging. Answer: C₆ + Li⁺ + e⁻ → LiC₆. 2. Why is an aqueous electrolyte unsuitable for a 3.7 V lithium-ion cell? Answer: Water decomposes at about 1.23 V under standard conditions, so it would be electrolysed to hydrogen and oxygen. 3. What is the function of the separator? Answer: It keeps the electrodes apart to prevent an internal short circuit while allowing lithium ions to pass through the electrolyte in its pores. 4. Explain why charging a cell rapidly at −10 °C is dangerous. Answer: Slow insertion kinetics and diffusion at low temperature cause lithium to plate as metal on the graphite, which can form dendrites that short the cell.