What is the nominal voltage of a lithium-ion battery?

The nominal voltage of a lithium-ion battery is typically 3.6 V or 3.7 V per cell, depending on the active materials used. This value represents the average voltage a cell delivers over a full discharge cycle under standard conditions, and it serves as the reference point for cell characterisation, pack design, and electrochemical modelling. Understanding what nominal voltage means—and how it differs from the actual voltage measured during cycling—is fundamental to interpreting battery test data correctly.

For researchers working with electrode materials and test cells, voltage is not simply a fixed property. It reflects the thermodynamics of the electrode couple, the state of charge, and the kinetics of the electrochemical reactions taking place. The sections below address the most common questions about lithium-ion battery voltage in the context of materials research.

What is the nominal voltage of a lithium-ion battery?

The nominal voltage of a lithium-ion battery cell is the average, or representative, voltage across its discharge curve, typically cited as 3.6 V or 3.7 V. It is not a fixed thermodynamic quantity but a practical convention used to characterise a cell’s voltage output with a single, comparable figure. The exact value depends on the cathode and anode chemistries used in the cell.

In electrochemical terms, the cell voltage at any given moment is the difference between the cathode and anode potentials, each measured against a reference electrode. During discharge, the cathode acts as the positive electrode and the anode as the negative electrode. The nominal voltage reflects the midpoint of the voltage plateau observed during galvanostatic discharge, which is closely related to the thermodynamic open-circuit voltage of the electrode couple at mid-state-of-charge.

In research settings, nominal voltage is used to calculate theoretical energy density (Wh/kg), to set voltage windows in cycling protocols, and to compare the performance of different electrode material combinations. It is a convenient shorthand, but researchers should always refer to the full voltage profile when characterising a new material or cell configuration.

Why does lithium-ion battery voltage change during cycling?

Lithium-ion cell voltage changes during cycling because the electrode potentials shift continuously as lithium ions intercalate into and de-intercalate from the active materials. The cell voltage at any point in a charge or discharge cycle reflects the instantaneous difference between the cathode and anode potentials, both of which are state-of-charge dependent.

Thermodynamic and kinetic contributions

The equilibrium, or open-circuit, voltage of a cell is governed by the Nernst equation and reflects the free energy change of the lithium insertion reaction. However, under applied current, the measured voltage deviates from this equilibrium value due to overpotential. Overpotential arises from three main sources:

• Ohmic resistance — voltage drop across the electrolyte, separator, and contact resistances
• Charge-transfer resistance — kinetic barrier at the electrode/electrolyte interface
• Diffusion limitations — concentration gradients within the electrode particles and electrolyte

During charge, these contributions cause the measured cell voltage to be higher than the equilibrium value. During discharge, they cause it to be lower. The magnitude of the deviation increases with C-rate (the charge or discharge current relative to the cell’s capacity).

Degradation effects over repeated cycles

Over many cycles, the voltage profile also changes due to degradation mechanisms. Growth of the solid electrolyte interphase (SEI) layer on the anode surface increases internal resistance, widening the gap between charge and discharge voltage curves. Loss of active lithium through side reactions reduces capacity and shifts the relative utilisation of the cathode and anode, which can alter the shape and position of voltage plateaus. Tracking these changes in the dV/dQ or dQ/dV profile is a standard method for diagnosing degradation in research cells.

What is the difference between nominal, minimum, and maximum voltage?

The nominal voltage is the average representative voltage of a cell during normal operation. The minimum voltage (also called the lower cut-off voltage) is the lowest voltage to which a cell should be discharged before irreversible damage occurs. The maximum voltage (upper cut-off voltage) is the highest voltage permitted during charging, beyond which electrolyte oxidation, structural degradation, or safety hazards arise.

For a standard lithium-ion cell with a graphite anode and a lithium cobalt oxide (LCO) or nickel manganese cobalt oxide (NMC) cathode, typical voltage limits are approximately 2.5 V to 2.8 V at the lower cut-off and 4.1 V to 4.2 V at the upper cut-off. These values are material-dependent and must be established empirically or from the literature for each new electrode combination.

In half-cell testing, where a single electrode is tested against a lithium metal counter and reference electrode, the voltage window is defined relative to the Li/Li⁺ reference potential. Researchers must be careful not to conflate half-cell voltage windows with full-cell voltage windows, as the relationship between the two depends on the capacity balance and potential alignment of the paired electrodes.

How does nominal voltage vary across lithium-ion chemistries?

Nominal voltage varies significantly across lithium-ion chemistries because it is determined by the redox potentials of the cathode and anode active materials. Different cathode materials operate at different average potentials versus Li/Li⁺, and the choice of anode material also shifts the cell voltage.

Common cathode materials and their approximate average discharge potentials versus Li/Li⁺ include:

• Lithium cobalt oxide (LCO) — approximately 3.9 V, giving a full-cell nominal voltage near 3.7 V with a graphite anode
• NMC (lithium nickel manganese cobalt oxide) — approximately 3.7 V to 3.8 V versus Li/Li⁺, depending on Ni content
• Lithium iron phosphate (LFP) — approximately 3.4 V versus Li/Li⁺, resulting in a nominal cell voltage near 3.2 V to 3.3 V
• Lithium manganese oxide (LMO) — approximately 4.0 V versus Li/Li⁺, yielding a nominal cell voltage near 3.7 V to 3.8 V
• Lithium nickel oxide (LNO) and high-Ni NMC variants — operate at higher potentials and are of particular interest in next-generation research

On the anode side, graphite operates at a low and relatively flat potential of approximately 0.1 V to 0.2 V versus Li/Li⁺ during lithiation, which is why it contributes little to reducing the cell voltage. Silicon-based anodes operate at a slightly higher average potential, which marginally reduces the full-cell nominal voltage compared to graphite. Lithium titanate (LTO) anodes operate near 1.55 V versus Li/Li⁺, which substantially reduces the full-cell nominal voltage to approximately 2.3 V when paired with NMC—a trade-off accepted in exchange for improved rate capability and cycle life.

Why does nominal voltage matter for battery research and testing?

Nominal voltage matters for battery research because it directly affects the calculation of energy density, the design of cycling protocols, and the interpretation of electrochemical data. Setting incorrect voltage windows during galvanostatic cycling can lead to irreversible electrode damage, artificially inflated capacity values, or premature capacity fade—all of which compromise the reproducibility and validity of experimental results.

Researchers use nominal voltage in several practical ways:

• Calculating gravimetric energy density (Wh/kg) from specific capacity (mAh/g) using the relation: energy density = nominal voltage × specific capacity
• Setting appropriate upper and lower cut-off voltages in cycling programmes to avoid electrolyte decomposition or deep discharge of active materials
• Comparing electrode materials on a consistent basis across different studies and publications
• Interpreting differential capacity (dQ/dV) plots, where phase transitions in the active material appear as peaks at characteristic voltages

For researchers developing new electrode materials, understanding how the nominal voltage of a candidate material compares to existing benchmarks is essential for assessing its practical relevance. A high-capacity cathode material that operates at a significantly lower potential than existing cathodes may not deliver a net improvement in energy density at the cell level, even if its specific capacity in mAh/g is superior.

How EL-Cell GmbH supports lithium-ion battery voltage research

Accurate voltage measurement and reproducible cycling protocols are central to any study of lithium-ion cell electrochemistry. EL-Cell GmbH designs and manufactures electrochemical test cells and instrumentation specifically for this type of research, with a focus on minimising experimental artefacts that can distort voltage data.

Our products address the specific needs of researchers studying nominal voltage, voltage profiles, and electrochemical degradation:

• The PAT-Cell and ECC series test cells provide standardised, reproducible cell geometries for half-cell and full-cell cycling, ensuring that voltage measurements reflect material properties rather than cell assembly variability
• The PAT-Tester-i-16 offers up to 16 independent test channels with galvanostatic and potentiostatic control, electrochemical impedance spectroscopy (EIS) capability, and precise voltage resolution—enabling detailed characterisation of voltage profiles and overpotential contributions across multiple samples simultaneously
• The ECD-4-nano electrochemical dilatometer allows simultaneous measurement of voltage and electrode thickness change, providing insight into the volumetric behaviour of electrode materials at different states of charge
• Specialised cells such as the PAT-Cell-Solid and PAT-Cell-Press extend this capability to solid-state and gas-evolving systems, where voltage behaviour under pressure or in the presence of evolved gases requires dedicated cell designs

If you are designing a cycling protocol, selecting voltage windows for a new electrode material, or building a reproducible testing workflow, contact EL-Cell GmbH to discuss which test cell and instrumentation configuration is most appropriate for your experimental requirements.