How Lithium-Ion Batteries Work and Why They Degrade Over Time

The Battery That Changed Everything

The lithium-ion battery is one of the most transformative technologies of the past 40 years. It powers smartphones, laptops, electric vehicles, power tools, and grid-scale energy storage. Its development — for which John Goodenough, M. Stanley Whittingham, and Akira Yoshino received the 2019 Nobel Prize in Chemistry — made portable electronics practical and is now enabling the transition from fossil fuels to renewable energy. Understanding how it works requires a basic grasp of electrochemistry, but the core principles are elegant and accessible.

A battery stores electrical energy as chemical energy and converts it back on demand. All batteries work by separating charged particles in a way that creates a potential difference (voltage) between two electrodes — the anode (negative) and cathode (positive). When the battery discharges and powers a device, electrons flow through the external circuit from anode to cathode, doing useful electrical work. Inside the battery, ions flow through the electrolyte (a conductive liquid or gel) to maintain charge balance. Charging reverses all of this, using external electrical energy to push the chemistry backward.

Lithium-Ion Electrochemistry

What distinguishes lithium-ion batteries is the specific chemistry they use. During discharge, lithium ions (Li+) move from the anode to the cathode through the electrolyte, while electrons simultaneously flow through the external circuit. During charging, lithium ions move in the opposite direction — back into the anode — while electrons are pushed back by the charger.

The anode in most lithium-ion batteries is graphite, a layered form of carbon whose structure has gaps between layers that can accommodate lithium ions (a process called intercalation). The cathode is typically a metal oxide compound — lithium cobalt oxide (LiCoO2) in early batteries, now more commonly lithium iron phosphate (LiFePO4), lithium nickel manganese cobalt oxide (NMC), or lithium nickel cobalt aluminum oxide (NCA), depending on the application. The electrolyte is a lithium salt dissolved in an organic solvent, which allows lithium ions to move between electrodes but does not conduct electrons (forcing them through the external circuit where they do work).

Voltage, Energy Density, and Why Lithium

Lithium was chosen as the active ion for good chemical reasons. Lithium is the lightest metal and has the most negative standard electrode potential of any element — meaning it gives up electrons very readily, producing a high cell voltage. Higher voltage means more energy delivered per unit of charge, which translates directly into energy density. A lithium-ion cell produces about 3.6 to 4.2 volts, compared to 1.5 volts for an alkaline AA battery or 2 volts per cell for lead-acid batteries. This high voltage, combined with light weight, gives lithium-ion batteries exceptional energy density — the amount of energy stored per kilogram of battery mass.

Modern lithium-ion cells achieve 150-300 Wh/kg (watt-hours per kilogram), compared to 30-50 Wh/kg for lead-acid batteries. This is why your phone can run for a day on a battery that weighs tens of grams, and why electric vehicle batteries can store 50-100 kWh in a pack that weighs 400-700 kg — achievable range without unreasonable weight penalty. No other battery chemistry currently available approaches this combination of energy density, power, long cycle life, and cost.

The Solid Electrolyte Interphase: Why Batteries Are Complex

The practical performance of lithium-ion batteries depends critically on a thin film that forms spontaneously on the anode surface during the first charge cycle: the Solid Electrolyte Interphase (SEI). The organic electrolyte is thermodynamically unstable at the low voltages of the graphite anode and reacts to form this solid film. A well-formed SEI is essential — it blocks further electrolyte decomposition while remaining permeable to lithium ions. Without it, the electrolyte would continue reacting with the anode, consuming lithium and electrolyte irreversibly.

The SEI is also a major source of battery aging. Over many charge-discharge cycles, the SEI gradually thickens, increasing the resistance to lithium-ion transport and consuming lithium ions that are no longer available to store energy. Battery engineers spend enormous effort optimizing electrolyte formulations and additives to produce stable, thin SEI layers that minimize degradation while providing adequate protection. Understanding and controlling the SEI is one of the central research frontiers in battery science.

Why Lithium-Ion Batteries Degrade Over Time

Battery degradation — capacity fade and increased internal resistance — results from several simultaneous processes that operate over charge cycles and calendar time. The most important are:

• Lithium plating: At low temperatures or during fast charging, lithium ions arrive at the graphite anode faster than they can intercalate, depositing as metallic lithium on the surface. This metallic lithium can form dendrites (spiky metallic filaments) that may pierce the separator between anode and cathode, causing a short circuit — a significant safety risk. Even without dendrites, lithium plating permanently removes active lithium from the cycle.
• Cathode degradation: Metal oxide cathodes experience structural changes over repeated cycling — the crystal lattice cracks, metal ions dissolve into the electrolyte, and the cathode's capacity declines. High temperatures and high states of charge accelerate this.
• Electrolyte decomposition: The organic electrolyte gradually breaks down, especially at high temperatures and voltages, producing gases and consuming electrolyte.
• SEI growth: As described above, ongoing SEI thickening consumes lithium and increases resistance.

This is why keeping a phone battery at 20-80% charge, avoiding extreme temperatures, and not fast-charging constantly extends battery life — these practices minimize the conditions that accelerate each degradation mechanism.

Battery Management Systems

The sophisticated electronics that accompany lithium-ion batteries in phones, laptops, and vehicles — collectively called the Battery Management System (BMS) — are essential for safe and efficient operation. The BMS monitors cell voltage, current, and temperature in real time. It prevents overcharging (which can cause thermal runaway and fire), overdischarging (which can permanently damage cells), and current flows that could cause lithium plating.

In electric vehicle battery packs, which contain hundreds or thousands of individual cells, the BMS also handles cell balancing — redistributing charge among cells to ensure all cells reach the same state of charge, preventing any single cell from being overcharged or over-discharged. The BMS estimates the battery's state of charge (how full it is) and state of health (how much capacity remains relative to when new), providing the information that drives the battery indicator on your dashboard or phone screen. Modern BMS systems use machine learning models trained on degradation data to provide accurate remaining-life predictions.

The Next Generation: Solid-State Batteries

The most anticipated development in battery technology is solid-state batteries, which replace the liquid or gel electrolyte with a solid ionic conductor. Solid electrolytes promise several advantages: they are less flammable than organic liquid electrolytes, they may suppress lithium dendrite formation, and they could enable the use of a lithium metal anode (pure lithium metal, rather than graphite) which has nearly ten times the energy density of graphite. A solid-state battery with a lithium metal anode could potentially achieve 400-500 Wh/kg — roughly double current lithium-ion performance.

The challenges are formidable: solid electrolytes have lower ionic conductivity than liquid electrolytes at room temperature (slowing charging and discharging), they crack under the volume changes that occur during cycling, and manufacturing them at scale is far more difficult than manufacturing liquid-electrolyte cells. Toyota, Samsung, QuantumScape, and Solid Power are among the companies pursuing solid-state batteries, with commercial products for electric vehicles expected within the decade. Whether they will deliver on their promise — or whether liquid electrolyte chemistry will advance fast enough to maintain its dominance — remains one of technology's most consequential open questions.