Why does internal resistance increase as a battery ages?

Internal resistance is one of the most informative indicators of battery health. As a battery ages through repeated charge and discharge cycles, its internal resistance rises—reducing power delivery, increasing heat generation, and accelerating capacity loss. Understanding the mechanisms behind this increase is essential for researchers developing more durable electrode materials and electrolyte formulations.

This article addresses the key questions surrounding internal resistance in lithium-ion and next-generation battery systems, from the electrochemical origins of resistance growth to the measurement techniques used in laboratory research.

What is internal resistance in a battery?

Internal resistance in a battery is the opposition to current flow within the cell itself. It arises from multiple sources: the electronic resistance of electrode materials and current collectors, the ionic resistance of the electrolyte, and the interfacial resistance at electrode–electrolyte boundaries. Together, these contributions determine how much voltage is lost during operation under load.

In electrochemical terms, internal resistance is not a single fixed value. It is frequency-dependent and can be decomposed into distinct components using techniques such as electrochemical impedance spectroscopy (EIS). The ohmic resistance (often denoted R₀) captures purely resistive contributions, while charge-transfer resistance and diffusion-related impedance reflect kinetic and transport limitations at the electrode surfaces and within the bulk electrolyte.

For battery materials researchers, distinguishing between these components is critical. A rise in ohmic resistance points to degradation of contacts or reduced electrolyte conductivity, whereas an increase in charge-transfer resistance often indicates surface film growth or particle cracking at the electrode level.

Why does internal resistance increase as a battery ages?

Battery internal resistance increases with age primarily because of irreversible chemical and structural changes that accumulate at the electrode–electrolyte interface and within the electrode particles themselves. These changes impede both ion transport and electron transfer, raising the overall impedance of the cell.

The dominant mechanisms include:

• Solid electrolyte interphase (SEI) layer growth: The SEI layer forms on the anode surface during the first cycles as the electrolyte decomposes. Over time, the SEI continues to grow, consuming active lithium and increasing ionic resistance at the anode interface.
• Cathode surface film formation: Analogous passivation layers can form on cathode materials, particularly under elevated voltage or temperature conditions, further raising interfacial resistance.
• Electrode particle cracking: Volume changes during lithiation and delithiation induce mechanical stress. Repeated cycling leads to particle fracture, which disrupts electronic contact within the electrode and increases resistance.
• Loss of active material contact: As the binder degrades and particles lose contact with the conductive network, effective electrode conductivity decreases.
• Electrolyte decomposition: Gradual electrolyte oxidation or reduction reduces ionic conductivity and generates resistive by-products.

The cumulative effect of these processes is a steady increase in battery impedance over cycle life. This is directly linked to battery capacity loss, since higher internal resistance means greater overpotential under current, which limits the usable voltage window of the cell.

What are the signs that a battery’s internal resistance is too high?

A battery with excessively high internal resistance shows a pronounced voltage drop under load, reduced deliverable capacity, and increased heat generation during cycling. In research cells, this manifests as wider charge–discharge voltage hysteresis and a shift in the midpoint voltage of galvanostatic profiles.

Researchers typically identify elevated resistance through the following observations:

• Increased IR drop (instantaneous voltage step) at the start of a discharge pulse
• Reduced coulombic efficiency as side reactions consume more charge
• Capacity fade that is disproportionate to cycle number, suggesting resistance-limited rather than thermodynamic capacity loss
• Asymmetric impedance growth observed in EIS spectra, particularly in the mid-frequency semicircle associated with charge-transfer resistance

These diagnostic signatures allow researchers to distinguish resistance-driven degradation from other failure modes such as lithium plating or active material dissolution.

How does temperature affect battery internal resistance?

Temperature has a strong and direct effect on battery internal resistance. At lower temperatures, ionic conductivity in the electrolyte decreases significantly, and charge-transfer kinetics slow down, both of which raise impedance. At elevated temperatures, resistance decreases in the short term, but accelerated degradation mechanisms lead to faster long-term resistance growth.

Low temperature effects

At sub-ambient temperatures, lithium-ion diffusion in both the electrolyte and electrode materials becomes sluggish. The charge-transfer resistance at the electrode–electrolyte interface rises sharply, and the risk of lithium plating on graphite anodes increases substantially during charging. This is a critical consideration for materials researchers designing electrolytes or electrode structures for low-temperature applications.

Elevated temperature effects

Higher temperatures accelerate SEI layer growth, electrolyte decomposition, and transition-metal dissolution from cathode materials. While resistance may appear lower in the short term due to improved kinetics, the long-term consequence is faster battery degradation and a steeper resistance increase over cycle life. Controlled temperature environments in testing are therefore essential for reproducible ageing studies.

How is internal resistance measured in battery research?

Internal resistance in battery research is most comprehensively measured using electrochemical impedance spectroscopy (EIS). EIS applies a small sinusoidal perturbation across a range of frequencies and records the resulting impedance response, allowing researchers to separate ohmic resistance, charge-transfer resistance, and diffusion contributions in a single measurement.

Beyond EIS, researchers use two additional approaches:

• DC pulse methods (DCIR): A short current pulse is applied, and the instantaneous voltage response is used to calculate resistance via Ohm’s law. This captures predominantly ohmic resistance and is simpler to implement but provides less mechanistic detail than EIS.
• Galvanostatic intermittent titration technique (GITT): Current pulses are applied with rest periods, allowing separation of kinetic and thermodynamic contributions. GITT is particularly useful for characterising diffusion limitations in electrode materials.

For EIS measurements to be meaningful, the test cell hardware must introduce minimal parasitic impedance. Three-electrode cell configurations are strongly preferred in research settings because they allow the impedance of the working electrode to be measured independently of the counter electrode, avoiding convolution of the two electrodes’ responses.

Can internal resistance be reduced or reversed in aged batteries?

In most cases, the increase in internal resistance due to battery ageing is not fully reversible. The structural and chemical changes responsible—SEI growth, particle cracking, and contact loss—are largely irreversible under normal cycling conditions. However, certain strategies can slow resistance growth or partially recover performance under specific circumstances.

From a research perspective, the following approaches are under active investigation:

• Electrolyte additives: Functional additives can stabilise the SEI layer and reduce its ongoing growth, limiting the rate of resistance increase over cycle life.
• Electrode architecture optimisation: Structuring electrodes to accommodate volume changes (for example, through hierarchical porosity or buffering coatings) reduces particle cracking and preserves electronic contact.
• Reconditioning protocols: In some systems, low-rate charge cycles or rest periods at specific states of charge can partially redistribute lithium and reduce localised resistance, though this does not address underlying structural degradation.
• Temperature management: Maintaining cells within an optimal temperature window during cycling slows the kinetics of degradation reactions, reducing the rate of resistance increase.

Understanding which mechanism dominates resistance growth in a given material system is a prerequisite for designing effective mitigation strategies. This requires precise, reproducible impedance measurements across cycle life—a task that demands well-designed test hardware and consistent experimental protocols.

How EL-Cell GmbH supports internal resistance and battery ageing research

Accurate measurement of battery impedance and resistance growth requires test cells that introduce minimal artefacts and support reliable three-electrode configurations. EL-Cell GmbH designs and manufactures electrochemical test equipment specifically for this type of research.

Our product range addresses the key requirements of internal resistance and battery degradation studies:

• Three-electrode test cells: The PAT-Cell and PAT-Cell-Force support independent working- and counter-electrode measurements, enabling clean EIS data that separates anode and cathode contributions to impedance growth.
• Integrated EIS capability: The PAT-Tester-i-16 combines a fully featured potentiostat/galvanostat with EIS functionality across up to 16 independent channels, allowing parallel ageing studies with concurrent impedance monitoring.
• Temperature control: The PAT-Tester-i-16 integrates a temperature-controlled cell chamber, ensuring consistent thermal conditions across measurements—critical for separating temperature effects from true ageing-related resistance changes.
• High-resolution dilatometry: The ECD-4-nano quantifies electrode thickness changes with a resolution better than 5 nm, providing complementary mechanical data alongside impedance measurements to correlate particle expansion with resistance growth.

If you are designing a battery ageing study or need guidance on selecting the right test cell configuration for EIS measurements, contact our team to discuss your experimental requirements. You can also learn more about who we are and our background in electrochemical research to understand the expertise behind our instrumentation.