PseudoOpenCircuitVoltageMethod

A technique used to measure the voltage of a cell under a low applied current as an estimate for the open-circuit voltage.

The pseudo-open-circuit voltage (pseudo-OCV) method is a practical approach for estimating the open-circuit potential (OCP) of intercalation electrodes, such as those in lithium-ion batteries. It enables the derivation of OCP versus stoichiometry functions from low-rate galvanostatic tests and is especially useful in the context of parameterizing physics-based battery models. This article outlines the method’s basis, implementation in half and full cells, and best practices for test protocols and data processing.

In lithium-ion batteries, the open-circuit voltage (OCV) at the cell level is the difference between the OCPs of the positive and negative electrodes. The half-cell OCP function is a key parameter for physics-based battery models, such as the Doyle–Fuller–Newman (DFN) model, and critical for accurate state estimation and degradation modelling.

Traditional methods to extract OCP include:

• GITT (Galvanostatic Intermittent Titration Technique): accurate but time-consuming
• Three-electrode-cell measurements: invasive and susceptible to artifacts
• Relaxation-based methods: slow and affected by hysteresis

The pseudo-OCV method approximates near-equilibrium voltages using slow constant-current cycling (typically C/30 or slower).

Half-Cell Approach

The pseudo-open-circuit voltage (pseudo-OCV) method is a practical and widely used approach for characterizing the equilibrium potential of intercalation electrodes as a function of their state of charge. When implemented in half-cell configurations, the method observes the working electrode voltage versus a reference electrode under near-equilibrium conditions. These measurements are critical for building accurate, physics-based models of electrochemical systems.

Purpose and Advantages of Half-Cell Measurements

Half-cell configurations allow the measurement of the open-circuit potential (OCP) of a single electrode without interference from the counter electrode. This is particularly important for model development, where accurate OCP–stoichiometry relationships are needed for each electrode independently. Full-cell measurements only provide access to the combined potential difference between two electrodes. This makes it mathematically impossible to resolve individual OCP curves without assumptions or external references—a limitation known as the "observability problem." Half-cell testing avoids this issue by providing direct access to the electrode under study.

This methodology supports model calibration, validation of new materials, and analysis of degradation mechanisms, and is applicable across a wide range of chemistries and formats.

Application to Freshly Manufactured Electrodes

The pseudo-OCV method is suitable for both harvested and freshly fabricated electrodes. When applied to newly manufactured electrodes, it enables early-stage evaluation of electrochemical behavior, thermodynamic reversibility, and suitability for integration into devices.

To ensure valid results, fresh electrodes should be adequately formed to stabilize their initial behaviour. It may be that the pseudo-OCV protocol itself is sufficient to do formation or conditioning on some electrodes; further research on this topic is needed.

Experimental Methodology

In a typical implementation, the electrode under study is assembled into a half-cell with a suitable reference/counter electrode. The assembly is then cycled using a slow constant current—commonly at a rate of C/30 or slower—to approximate equilibrium conditions. Voltage is recorded continuously, and at these low currents, it closely tracks the open-circuit potential.

The test is performed across the full accessible state-of-charge window of the electrode material. Both charge and discharge sequences are used to generate a continuous OCP–stoichiometry profile that reflects the electrode’s thermodynamic properties.

For intercalation electrodes used in systems such as lithium-ion batteries, this approach typically involves pairing with a lithium metal counter/reference. However, the method is general and can be applied to other chemistries with appropriate modifications to cell design and reference selection.

Stabilization at Stoichiometric Boundaries

To improve the accuracy of measurements near the limits of the stoichiometric range, additional steps are often introduced. Rest periods following current interruptions allow the system to relax toward its true equilibrium state. Constant-voltage holds can further support stabilization, particularly near the high and low ends of the potential window.

Some protocols incorporate dithering techniques, such as sinusoidal or chirped current pulses, to enhance relaxation and reduce the time required to reach near-equilibrium. These steps are especially valuable when accurate endpoint determination is needed for parameter fitting in models.

Assumptions and Limitations

The pseudo-OCV method relies on several key assumptions. First, it assumes that low-rate cycling sufficiently approximates equilibrium, with minimal overpotential or hysteresis. This assumption generally holds for common intercalation materials but may be less accurate for systems with pronounced kinetic limitations, strong hysteresis, or complex phase behavior.

The method also assumes that the reference electrode behaves in a stable and reversible manner across the entire test window. Any degradation, side reactions, or non-ideal behavior from the reference or counter electrode can introduce errors. Additionally, inaccessible capacity due to inactive material or side reactions can distort the apparent relationship between charge passed and stoichiometry.

These limitations must be understood and addressed through careful cell construction, proper formation procedures, and validation of repeatability.

Required Metadata for Interpretation

To analyze pseudo-OCV data and derive meaningful electrode models, a minimal set of metadata is required. This includes:

• mass loading of the active material / mg cm^-2
• active facial electrode area / cm²
• actual electrode coating thickness / µm
• active material density / g cm^-3

The test temperature must also be recorded, as electrode potentials are typically temperature dependent. If the electrode is newly fabricated, its formation and cycling history should be documented to confirm stability and representative behaviour.