State of charge and electrode potential are directly linked: as a lithium-ion battery charges or discharges, the electrochemical potential at each electrode shifts in a characteristic and measurable way. Understanding this relationship is fundamental to battery materials research because it governs how researchers interpret capacity, detect phase transitions, and assess electrode behaviour under realistic cycling conditions.
Whether you are running half-cell tests on a new cathode material or characterising anode kinetics, the SOC–potential relationship is one of the most informative signals available. The sections below address the key questions surrounding this relationship, from basic definitions through to practical considerations in the lab.
What is state of charge in a battery?
State of charge (SOC) is a measure of the remaining electrochemical capacity in a battery or electrode, expressed as a percentage of its fully charged capacity. An SOC of 100% indicates a fully charged state; 0% indicates full discharge. In research contexts, SOC is most commonly tracked by integrating current over time, a method known as coulomb counting.
In battery materials research, SOC is often expressed relative to the specific capacity of a single electrode material, reported in mAh/g or mAh/cm². This distinction matters: full-cell SOC reflects the balance between both electrodes, whereas half-cell testing isolates one electrode against a reference, giving a clearer view of that material’s intrinsic behaviour. Researchers working on new active materials typically start at the half-cell level, where SOC can be mapped directly onto the lithiation or delithiation state of the material under study.
What is electrode potential and how is it measured?
Electrode potential is the electrical potential of an electrode relative to a reference electrode, measured in volts. It reflects the thermodynamic driving force for electrochemical reactions at the electrode surface. In lithium-ion systems, electrode potential is typically reported versus a lithium metal reference (Li/Li⁺), though other reference electrodes may be used depending on the electrolyte system and experimental design.
How reference electrodes affect measurement accuracy
Accurate electrode potential measurement requires a stable, well-defined reference electrode. In a two-electrode full cell, the measured voltage is the difference between the potentials of both electrodes, which makes it impossible to deconvolute the individual electrode contributions. A three-electrode configuration, by contrast, introduces a dedicated reference electrode that allows the potential of the working electrode to be measured independently and precisely.
This is why three-electrode test cells are strongly preferred in fundamental research. Any drift, contamination, or impedance mismatch at the reference electrode will introduce artefacts into the potential measurement, which in turn distorts the apparent SOC–potential relationship. Choosing an appropriate reference electrode for the electrolyte system in use is therefore a critical experimental decision.
How does state of charge affect electrode potential?
As SOC changes, electrode potential shifts in a characteristic pattern determined by the thermodynamics of lithium insertion and extraction within the active material. At equilibrium, the electrode potential at a given SOC corresponds to the open-circuit voltage (OCV) of that electrode, which reflects the chemical potential of lithium in the host material at that state of lithiation.
For many intercalation materials, this relationship is not linear. Some materials exhibit flat voltage plateaux over wide SOC ranges, indicating a two-phase coexistence region in which lithium inserts into a distinct crystallographic phase. Others show sloping profiles, indicative of single-phase solid-solution behaviour. Conversion and alloying materials show more complex profiles still. The shape of the potential–SOC curve is therefore a direct fingerprint of the electrochemical mechanism operating within the material, making it one of the most diagnostic measurements in electrode characterisation.
What is a dQ/dV plot and why does it matter?
A dQ/dV plot is a derivative analysis technique in which the differential capacity (dQ/dV, in mAh/V) is plotted against electrode potential. Peaks in the dQ/dV plot correspond to voltage plateaux in the charge–discharge curve, making phase transitions and electrochemical processes far easier to resolve and compare between cycles or samples.
The technique is particularly valuable because small changes in electrode behaviour, such as the gradual shift or broadening of a peak over repeated cycles, can indicate structural degradation, loss of active material, or evolving kinetic limitations that would be difficult to detect in the raw voltage profile alone. Researchers use dQ/dV analysis to:
- Identify phase transitions and their SOC positions
- Track capacity fade mechanisms over cycling
- Compare electrochemical behaviour between different electrode formulations
- Assess the reversibility of lithiation and delithiation processes
Because dQ/dV analysis depends on the quality and resolution of the underlying voltage and capacity data, it requires precise, low-noise current and potential measurements throughout the full SOC window.
How do you accurately track electrode potential vs. SOC in the lab?
Accurate tracking of electrode potential versus SOC requires a combination of controlled cycling conditions, a stable reference electrode, and a well-designed electrochemical test cell that minimises parasitic resistances and artefacts. The measurement must capture both the potential and the cumulative charge passed at sufficient resolution to resolve the features of interest.
Key practical steps
- Use a three-electrode cell: This isolates the working electrode potential from counter-electrode contributions, which is essential for meaningful SOC–potential data.
- Apply low C-rates for near-equilibrium data: High charge/discharge rates introduce overpotential, which shifts the apparent potential away from the thermodynamic value. Slow cycling (C/10 or lower) yields data closer to the true open-circuit voltage profile.
- Allow adequate rest periods: Measuring OCV after interrupting current flow reveals the relaxed electrode potential at a fixed SOC, which is useful for building equilibrium potential curves.
- Control temperature: Electrode potential is temperature-dependent. Isothermal conditions are necessary for reproducible SOC–potential curves, particularly when comparing results across experiments or laboratories.
- Ensure good electrical contact and uniform current distribution: Poor contact or non-uniform current flow across the electrode introduces local SOC gradients that distort the measured potential.
Reproducibility across repeat measurements and across different cell assemblies is the practical benchmark. If the SOC–potential curve shifts significantly between nominally identical cells, the source of variability must be identified and eliminated before the data can support publishable conclusions.
What factors can distort the SOC–potential relationship?
Several experimental and material-related factors can cause the measured SOC–potential relationship to deviate from the true thermodynamic behaviour of the electrode material. Identifying and controlling these factors is a central challenge in rigorous battery research.
Overpotential and kinetic effects
When current flows, overpotential shifts the measured electrode potential away from its equilibrium value. The magnitude of this shift depends on the applied current density (mA/cm²), the electrode’s intrinsic kinetics, and the ionic conductivity of the electrolyte. Comparing data collected at different C-rates without accounting for overpotential can lead to incorrect conclusions about the material’s true SOC–potential profile.
Side reactions and coulombic efficiency losses
During the first cycles, the formation of the solid electrolyte interphase (SEI) layer on the anode consumes charge without contributing to reversible capacity. This irreversible capacity loss means that the SOC calculated by coulomb counting will overestimate the true lithiation state of the electrode if first-cycle losses are not properly accounted for. Coulombic efficiency, the ratio of charge extracted to charge inserted in a given cycle, is the key metric for quantifying these losses.
Cell design and reference electrode stability
Poorly designed test cells can introduce resistive artefacts, electrolyte depletion, or non-uniform pressure on the electrode, all of which distort the SOC–potential curve. Reference electrode instability, in particular, is a common source of systematic error that is difficult to detect without independent validation. Using a well-characterised, purpose-built electrochemical test cell with a defined geometry and controlled assembly conditions significantly reduces these sources of distortion.
How EL-Cell GmbH supports accurate SOC and electrode potential measurements
EL-Cell GmbH designs and manufactures electrochemical test cells and instrumentation specifically for the kind of rigorous electrode characterisation described in this article. Our product ecosystem addresses the practical requirements of accurate SOC–potential measurement directly:
- Three-electrode capability: The PAT-Cell supports true three-electrode configurations, enabling independent measurement of working-electrode potential against a stable reference, which is essential for clean SOC–potential data.
- Temperature-controlled testing: The PAT-Tester-i-16 integrates a temperature-controlled cell chamber with up to 16 test channels, potentiostat/galvanostat (PStat/GStat) functionality, and electrochemical impedance spectroscopy (EIS) capability, providing the isothermal conditions needed for reproducible potential measurements.
- High-resolution dilatometry: The ECD-4-nano electrochemical dilatometer tracks electrode thickness changes with a resolution better than 5 nanometres, allowing volume changes to be correlated directly with SOC and electrode potential during cycling.
- Standardised cell geometry: All PAT-Series cells use a defined, reproducible geometry that minimises assembly variability, a key requirement when comparing SOC–potential curves across experiments or research groups.
If you are setting up or optimising a battery materials characterisation workflow, contact EL-Cell GmbH to discuss which test cell configuration best fits your electrode system and research objectives. Our team can advise on cell selection, reference electrode options, and measurement protocols suited to your specific materials and cycling conditions.
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