Capacity and energy are two of the most frequently used metrics in battery research, yet they are not interchangeable. Understanding the distinction is essential for interpreting electrochemical data correctly, comparing electrode materials, and designing rigorous experiments. This article addresses each concept in turn, then examines how they interact across charge–discharge curves and long-term cycling.
What is capacity in a lithium-ion battery?
Capacity is the total amount of electric charge a battery or electrode can store and deliver, expressed in ampere-hours (Ah) or milliampere-hours (mAh). It quantifies how many charge carriers—lithium ions—can be reversibly inserted into and extracted from an electrode material under defined conditions. Capacity is measured by integrating current over time during a charge or discharge process.
In a research context, capacity is most commonly reported as specific capacity in mAh/g, normalised to the mass of active electrode material. This normalisation allows direct comparison between different materials regardless of electrode loading. Areal capacity, expressed in mAh/cm², is used when electrode geometry or coating uniformity is the focus of the study.
It is important to distinguish between theoretical capacity, which is derived from the stoichiometry of the host material, and practical capacity, which reflects what is actually measured under real experimental conditions. Practical capacity is always lower than theoretical capacity due to kinetic limitations, incomplete lithiation, and irreversible side reactions.
What is energy in a lithium-ion battery?
Energy is the product of capacity and voltage, expressed in watt-hours (Wh) or milliwatt-hours (mWh). Where capacity tells you how much charge a battery can deliver, energy tells you how much work that charge can perform. A cell with a high capacity but a low average discharge voltage will store less energy than one with a moderate capacity at a higher voltage.
In electrochemical testing, energy is calculated by integrating the product of instantaneous voltage and current over the full discharge duration. This makes energy inherently dependent on the shape of the voltage profile, not just the total charge passed. Two electrode materials with identical specific capacity values can exhibit significantly different energy outputs if their average lithiation potentials differ.
For research purposes, energy is often normalised to mass or volume to give energy density in Wh/kg or Wh/L, enabling comparison across material classes and cell formats.
What’s the difference between capacity and energy in a battery?
The key distinction is that capacity measures charge storage (Ah or mAh), while energy measures the ability to do work (Wh). Capacity depends solely on the amount of lithium that can be reversibly cycled. Energy depends on both that amount and the voltage at which the cycling occurs. A battery with high capacity at low voltage may deliver less energy than one with lower capacity at higher voltage.
Consider two hypothetical cathode materials: one with a specific capacity of 200 mAh/g operating at an average potential of 3.0 V versus lithium, and another with 150 mAh/g at 4.0 V. The first delivers 600 mWh/g; the second delivers 600 mWh/g as well—identical energy despite different capacities. This illustrates why reporting capacity alone is insufficient for a complete materials assessment.
In practice, the distinction matters for how researchers evaluate new electrode candidates. A material that appears promising based on its gravimetric capacity may rank differently when energy density is calculated, particularly when the operating voltage window is narrow or the discharge profile is steeply sloping.
What are specific capacity and energy density, and why do they matter?
Specific capacity (mAh/g) and energy density (Wh/kg or Wh/L) are normalised metrics that allow researchers to compare electrode materials and cell designs on a common basis. Specific capacity normalises charge storage to electrode mass; energy density normalises total energy output to mass or volume. Both are indispensable for materials screening and cell optimisation.
Why normalisation is critical in research
Without normalisation, raw capacity or energy values are artefacts of electrode geometry, loading mass, and cell format. A thicker electrode will always show higher absolute capacity than a thin one made from the same material. Normalising to mass or area removes this dependence and makes results transferable between laboratories and comparable across publications.
Gravimetric metrics (per gram) are standard for early-stage materials research, where the primary goal is ranking candidate materials. Volumetric metrics (per litre) become more relevant as research moves toward practical cell design, where physical space is a constraint. Researchers working on solid-state or thick-electrode configurations often report both.
The role of C-rate in reported values
Both specific capacity and energy density are C-rate dependent. The C-rate expresses the charge or discharge current relative to the nominal capacity of the cell. At high C-rates, kinetic limitations reduce the amount of lithium that can be reversibly accessed within the voltage window, lowering both reported capacity and energy. Comparing values across studies requires confirming that the same C-rate was applied.
How does the charge–discharge curve affect capacity and energy measurements?
The shape of the voltage versus capacity curve directly determines how energy is calculated. Capacity is read from the x-axis of a charge–discharge curve as the total charge passed. Energy is the area under that curve—the integral of voltage over capacity. A flat, high-voltage plateau contributes far more energy per unit of capacity than a steeply sloping profile at a lower average voltage.
Materials with well-defined two-phase reaction mechanisms, such as lithium iron phosphate (LFP), produce flat voltage plateaux. This means their energy output is predictable and closely tied to their capacity. Materials with solid-solution intercalation mechanisms produce sloping profiles, where the average voltage, and therefore the energy output, depends on the state of charge at any given moment.
Overpotential also affects the measured energy. During charge, the applied voltage must exceed the thermodynamic equilibrium potential to drive the reaction; during discharge, the terminal voltage falls below it. The gap between charge and discharge curves, visible as hysteresis on a voltage–capacity plot, represents energy lost to internal resistance and kinetic barriers. This hysteresis directly reduces the round-trip energy efficiency of the cell.
For accurate energy measurements in a half-cell configuration, a stable and well-defined reference electrode is essential. Errors in the reference potential propagate directly into voltage readings and therefore into all derived energy calculations.
Why do capacity and energy fade differently over battery cycles?
Capacity fade and energy fade are related but distinct degradation phenomena. Capacity fade occurs when fewer lithium ions can be reversibly cycled—due to loss of active lithium, structural degradation of electrode materials, or growth of resistive surface layers. Energy fade occurs when either capacity decreases, the average discharge voltage drops, or both. Because voltage can decline independently of capacity, energy often fades faster than capacity over long-term cycling.
Mechanisms behind capacity loss
The primary sources of capacity fade include:
- Irreversible lithium consumption by the solid electrolyte interphase (SEI) layer, which forms on the anode surface during the first cycles and continues to grow with cycling
- Structural degradation of the cathode, including particle cracking, phase transitions, and transition metal dissolution
- Lithium plating on the anode at high C-rates or low temperatures, which removes active lithium from the reversible cycle
- Electrolyte decomposition that reduces ionic conductivity over time
Why energy fades faster than capacity
As cells age, increasing internal resistance raises the overpotential during discharge, pulling the terminal voltage below its initial value. Even if the same total charge is delivered, the lower average voltage means less energy is extracted. This is why coulombic efficiency—the ratio of discharge capacity to charge capacity—can remain high while energy efficiency declines. Tracking both metrics separately across cycles gives a more complete picture of cell degradation than monitoring capacity alone.
Researchers studying degradation mechanisms benefit from measuring differential capacity (dQ/dV) and incremental capacity analysis alongside standard cycling data. These techniques reveal subtle shifts in reaction potentials that are invisible in raw capacity versus cycle number plots.
How EL-Cell GmbH supports capacity and energy measurements in battery research
Accurate measurement of capacity and energy requires test hardware that introduces minimal artefacts, maintains stable temperature conditions, and records voltage and current with sufficient resolution. EL-Cell GmbH designs its electrochemical test cells and instrumentation specifically to meet these requirements for laboratory-scale battery research.
Key tools from the EL-Cell product range that are directly relevant to capacity and energy measurements include:
- PAT-Tester-i-16: A fully integrated battery tester with up to 16 independent channels, potentiostat and galvanostat (PStat/GStat) functionality, electrochemical impedance spectroscopy (EIS) capability, and a temperature-controlled cell chamber. Precise current control and voltage measurement across all channels ensure that capacity and energy data are consistent and comparable between experiments.
- PAT-Cell: A standardised research test cell designed for reproducible half-cell and full-cell measurements. Consistent electrode geometry and controlled compression reduce experimental variability, which is critical when comparing specific capacity values across different electrode formulations.
- ECD-4-nano: A high-resolution electrochemical dilatometer that measures electrode thickness changes during cycling with sub-5 nm resolution. Correlating dimensional changes with capacity and energy data provides mechanistic insight into volume expansion, SEI growth, and structural degradation.
If you are designing experiments to characterise new electrode materials or study degradation mechanisms, contact us to discuss which test cell configuration and measurement protocol best fits your research requirements.
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