Lithium-ion batteries power the majority of portable energy storage applications in research and industry today. Understanding how they work at the electrochemical level is foundational knowledge for anyone conducting battery materials research, whether you are developing new electrode formulations, characterising electrolyte behaviour, or evaluating cell degradation mechanisms.
This article walks through the core principles of lithium-ion operation, from basic electrochemistry to the factors that limit cycle life, and connects those principles to the experimental methods used in laboratory settings.
What is a lithium-ion battery?
A lithium-ion battery is an electrochemical energy storage device that operates by shuttling lithium ions between two electrodes through an electrolyte, while electrons travel through an external circuit to do useful work. Unlike primary cells, lithium-ion cells are rechargeable, meaning the ion insertion and extraction reactions at both electrodes are reversible over many cycles.
The term “lithium-ion” distinguishes these cells from earlier lithium-metal designs, in which metallic lithium served as the anode. In lithium-ion systems, lithium is stored within host materials through a process called intercalation or alloying, depending on the electrode chemistry. This distinction is important: in a standard lithium-ion cell, lithium is not present as free metal during normal operation, which significantly improves safety compared with lithium-metal systems.
Lithium-ion cells are characterised by their relatively high specific energy, flat discharge profiles, and low self-discharge rates. These properties make them the dominant technology in portable electronics, stationary storage, and electric mobility applications, as well as a primary focus of electrochemical research worldwide.
How does a lithium-ion battery store and release energy?
A lithium-ion battery stores energy by driving lithium ions from the cathode into the anode during charging and releases energy by reversing that process during discharge. At the anode, lithium ions are inserted into the host material (intercalation); at the cathode, they are extracted. The corresponding electron flow through the external circuit constitutes the electrical current.
During charging
An external power source applies a potential that exceeds the cell’s open-circuit voltage, forcing lithium ions to deintercalate from the cathode and migrate through the electrolyte to the anode, where they intercalate into the host structure. Graphite, the most common anode material, forms lithium-graphite intercalation compounds (LixC6) during this process, with a theoretical specific capacity of 372 mAh/g.
During discharge
When the cell is connected to a load, lithium ions spontaneously deintercalate from the anode and migrate back through the electrolyte to the cathode. The driving force is the difference in electrochemical potential between the two electrodes, known as the cell voltage. The rate of discharge is expressed as the C-rate, where 1C corresponds to a full discharge in one hour relative to the cell’s rated capacity.
The overall energy stored is the integral of voltage over capacity, expressed in Wh/kg (gravimetric energy density) or Wh/L (volumetric energy density), depending on the application context.
What are the main components inside a lithium-ion cell?
A lithium-ion cell contains four primary components: the anode, the cathode, the electrolyte, and the separator. Each plays a distinct electrochemical role, and the performance of the cell depends on the properties and compatibility of all four.
- Anode: Typically graphite, though silicon-graphite composites and lithium titanate (LTO) are used in specific applications. The anode hosts lithium ions during charging. Silicon offers a much higher theoretical specific capacity than graphite but undergoes significant volume expansion during lithiation, which presents a key research challenge.
- Cathode: Common materials include lithium cobalt oxide (LCO), lithium iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), and lithium nickel cobalt aluminium oxide (NCA). The choice of cathode material determines the cell’s voltage, specific capacity, and thermal stability.
- Electrolyte: A lithium salt (commonly LiPF6) dissolved in an organic solvent mixture. The electrolyte must be ionically conductive but electronically insulating. Its electrochemical stability window defines the operating voltage limits of the cell.
- Separator: A porous polymer membrane that prevents direct contact between the anode and cathode while allowing ion transport. Separator integrity is critical for cell safety.
Current collectors, typically copper at the anode and aluminium at the cathode, are also essential structural components that conduct electrons to and from the external circuit. In research settings, electrode formulations also include binders and conductive carbon additives to maintain electrical contact and mechanical cohesion within the electrode coating.
What causes lithium-ion batteries to degrade over time?
Lithium-ion batteries degrade through a combination of irreversible electrochemical reactions, mechanical stress, and structural changes in the electrode materials. Degradation manifests as capacity fade, increased internal resistance, or both, and the dominant mechanisms depend on the cell chemistry, operating conditions, and C-rate.
Solid Electrolyte Interphase (SEI) formation and growth
During the first charge cycle, the electrolyte partially reduces at the anode surface, forming a passivating layer known as the Solid Electrolyte Interphase (SEI). The SEI is necessary for stable operation, as it prevents continuous electrolyte decomposition. However, the SEI consumes lithium irreversibly during formation, reducing the initial coulombic efficiency below 100%. Over subsequent cycles, the SEI can continue to grow, consuming additional lithium and increasing cell impedance.
Electrode structural changes
Repeated lithiation and delithiation cycles cause volume changes in electrode particles. In graphite, this is modest (around 10%), but in silicon-based anodes, volume expansion can exceed 300%, leading to particle cracking, loss of electrical contact, and accelerated SEI formation on newly exposed surfaces. At the cathode, structural transformations, transition metal dissolution, and cracking of secondary particles all contribute to capacity fade.
Lithium plating
At elevated C-rates or low temperatures, the rate of lithium-ion insertion into the anode can be exceeded by the rate of supply, causing metallic lithium to plate on the anode surface rather than intercalate. Lithium plating is a significant safety concern and a contributor to rapid capacity loss, as plated lithium can become electrically isolated (dead lithium) or react with the electrolyte.
Quantifying degradation mechanisms in the laboratory requires careful experimental design, including operando techniques and post-mortem analysis, to distinguish between competing failure modes.
How are lithium-ion batteries tested in the lab?
Laboratory testing of lithium-ion batteries involves assembling electrochemical test cells, subjecting them to controlled charge and discharge protocols, and measuring electrochemical responses to characterise performance and degradation. Standard techniques include galvanostatic cycling, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS).
Test cell formats
Research-grade test cells differ from commercial formats in that they are designed for scientific measurement rather than energy delivery. Coin cells, pouch cells, and cylindrical cells each have trade-offs in terms of pressure control, electrolyte volume, and accessibility for in situ measurements. Standardised research cell formats allow reproducible assembly and reliable comparison of electrode materials across different laboratories.
For example, the PAT-Cell is a research test cell designed for reproducible electrochemical measurements, with a modular architecture that supports a range of electrode configurations and electrolyte systems.
Key electrochemical measurements
- Galvanostatic cycling: Applies a constant current (defined by the C-rate) and measures the resulting voltage profile. Provides specific capacity, coulombic efficiency, and rate capability data.
- Electrochemical impedance spectroscopy (EIS): Applies a small AC perturbation over a range of frequencies and measures the impedance response. Used to separate contributions from ohmic resistance, the SEI layer, charge transfer, and diffusion processes.
- Cyclic voltammetry (CV): Sweeps the electrode potential at a defined scan rate and measures current, revealing redox potentials and kinetic information.
- Dilatometry: Measures electrode thickness changes during cycling to quantify volume expansion and contraction. The ECD-4-nano electrochemical dilatometer achieves a thickness resolution better than 5 nm, making it suitable for resolving subtle expansion behaviour in thin-film or composite electrodes.
Operando and in situ techniques, such as optical microscopy through transparent cell windows or gas analysis during cycling, provide additional mechanistic insight that post-mortem analysis alone cannot deliver.
What’s the difference between lithium-ion and next-generation battery technologies?
Next-generation battery technologies differ from conventional lithium-ion cells primarily in their electrode or electrolyte chemistry, aiming to overcome the energy density, safety, or cost limitations of current lithium-ion designs. The most actively researched alternatives include solid-state batteries, lithium-sulphur (Li-S) cells, lithium-air (Li-O2) cells, and sodium-ion batteries.
Solid-state batteries
Solid-state batteries replace the liquid electrolyte with a solid ionic conductor, which eliminates the flammability risk associated with organic solvents and enables the use of a lithium-metal anode. A lithium-metal anode offers a substantially higher specific capacity (3860 mAh/g) than graphite but requires a solid electrolyte to suppress dendrite formation. The primary research challenges include achieving sufficient ionic conductivity in the solid electrolyte, managing interfacial resistance between the electrolyte and electrodes, and accommodating volume changes during cycling. The PAT-Cell-Solid is designed specifically for testing solid-state electrolyte systems under controlled stack pressure.
Lithium-sulphur and lithium-air
Lithium-sulphur cells offer a theoretical specific energy that significantly exceeds that of lithium-ion cells because sulphur has a high theoretical specific capacity and is abundant. However, the polysulphide shuttle mechanism causes rapid capacity fade, and practical specific energy remains well below theoretical values. Lithium-air cells are even more energy-dense in principle but face severe challenges related to oxygen management, electrolyte stability, and cycle life.
Sodium-ion batteries
Sodium-ion batteries operate on the same intercalation principle as lithium-ion cells but use sodium ions as the charge carrier. Sodium is more abundant and less expensive than lithium, making sodium-ion an attractive option for stationary storage applications where cost matters more than specific energy. The larger ionic radius of sodium compared with lithium requires different host materials and presents its own set of structural challenges.
Each of these technologies requires adapted experimental methods and test cell designs to characterise accurately, which is why research-grade instrumentation capable of handling diverse chemistries is essential for laboratories working across multiple platforms.
How EL-Cell GmbH supports lithium-ion battery research
EL-Cell GmbH designs and manufactures electrochemical test equipment specifically for battery materials research, covering the full range of experimental needs described in this article. Our product ecosystem is built around the PAT Series, which integrates test cells, potentiostats, and software into a compatible, modular platform.
For researchers characterising how lithium-ion batteries work at the materials level, we offer:
- The PAT-Cell for reproducible galvanostatic cycling and EIS measurements across a wide range of electrode chemistries
- The ECD-4-nano for operando dilatometry with sub-5 nm thickness resolution, enabling direct measurement of electrode volume changes during cycling
- The PAT-Cell-Solid for solid-state electrolyte research under defined stack pressure
- The PAT-Tester-i-16, a 16-channel battery tester with an integrated potentiostat/galvanostat and EIS capability, housed in a temperature-controlled cell chamber
For laboratories that require outsourced testing, our Application Laboratory provides a full measurement service. Researchers send electrode materials or electrolytes to our Hamburg facility, and our team handles cell assembly, protocol design, high-throughput cycling across hundreds of channels, and delivery of a final evaluation report. If you would like to discuss how our instruments or testing services can support your research programme, please contact us directly.
Comments are closed.