Why Does Low Internal Resistance Not Guarantee Long Lithium-Ion Battery Cycle Life?

During the cell-screening process, one often encounters the following scenario: two types of cells-tested under identical temperature, state-of-charge (SOC), and protocol conditions-show similar initial internal resistance, yet their subsequent capacity retention rates diverge significantly. In some cases, the cell with the lower initial internal resistance actually experiences faster capacity degradation. If one looks solely at the screening data, this result might appear contradictory.

In reality, initial internal resistance and cycle life address two distinct questions: How does the cell respond to an electrical current at a given moment? And, after repeated, long-term lithium insertion and extraction, how much usable capacity remains? A low initial internal resistance does not, in itself, cause a short lifespan, nor does it guarantee a long one. The key lies in what happens to the lithium inventory, the active materials, and the interfaces throughout the cycling process.

When current flows through a battery cell, the terminal voltage deviates from its equilibrium value. The rapid voltage response encompasses ohmic contributions from current collectors, as well as electron and ion transport pathways. This is followed by polarization effects related to interfacial charge transfer, concentration gradients, and solid-phase diffusion. In engineering terms, "internal resistance" is not a single, fixed constant; its value depends on the timescale of the measurement.

To evaluate lithium-ion battery performance more accurately, initial internal resistance should be considered alongside capacity and charge-discharge behavior. The ACEY-HK3560 high-precision battery internal resistance tester is designed for the new measurement requirements of the battery industry. It is suitable for low-resistance batteries, large-capacity lithium battery packs, and rapid product classification on production lines.

AC internal resistance (ACIR) is typically derived from the small-signal response at a specific AC frequency-for instance, the common 1 kHz measurement, which emphasizes faster processes. DC internal resistance (DCIR) is determined by the voltage change resulting from a defined current pulse; the duration of this pulse dictates which polarization processes are captured. These two metrics cannot be directly converted into one another, nor should they be compared without accounting for SOC, temperature, rest duration, and the direction of the current (charge vs. discharge).

Cycle life, by contrast, measures something different: how the dischargeable capacity evolves over repeated cycles under specified operating conditions and voltage cut-off limits. Factors such as loss of lithium inventory (LLI), loss of active material (LAM) in the electrodes, and an increase in impedance can all alter this outcome. A low initial internal resistance in a batch of cells likely indicates a favorable electrical response under the specific test conditions at that moment; however, it cannot predict the cumulative rate of side reactions that may occur over time.

1. Continuous loss of active lithium

The solid electrolyte interphase (SEI) layer on a graphite anode may continue to grow slowly even after the initial formation process. Electrolytes continue to undergo reduction at newly exposed or inadequately protected interfaces, leading to the sequestration of lithium within interfacial products. The causal chain is straightforward: interfacial side reactions consume lithium → the inventory of cyclable lithium decreases → the available lithium-intercalation/deintercalation window for both electrodes shifts → and when the lithium buffer is insufficient, reversible capacity declines. A low initial impedance does not, by itself, guarantee that these side-reaction pathways are sufficiently slow.

Under conditions such as fast charging, low temperatures, or significant localized polarization, lithium plating can occur on the graphite anode. While some of the deposited metallic lithium may be re-stripped during subsequent cycles, other portions become irreversible-either through reactions with the electrolyte or by losing electrical contact-ultimately contributing to the loss of active lithium. Although not every instance of lithium plating equates to an equivalent, permanent loss of capacity, repeated occurrences increase the risk of cell failure.

Why might capacity decline while internal resistance remains relatively stable? The loss of active lithium primarily alters the "stock" of available lithium, rather than necessarily causing a major disruption to the conductive pathways at a given state of charge (SOC). While the growth of the solid electrolyte interphase (SEI) may increase interfacial impedance, the measured internal resistance of a full cell is an aggregate value reflecting multiple processes, influenced by both the SOC and the timescale of the measurement. Consequently, at certain stages, capacity loss may be more pronounced than the increase in internal resistance; conversely, an rise in impedance might first manifest as a reduction in rate capability. While these two trends are generally correlated, they do not follow a fixed ratio.

2. Gradual Degradation of Active Materials

Cathode particles are subjected to strain during repeated lithium intercalation and deintercalation. In certain layered oxides, phenomena such as cracking, surface structural reconstruction, and interfacial side reactions can prevent some particles from fully participating in electrochemical reactions. If electrical contact is severed, material that otherwise retains the capacity to store lithium becomes electrochemically inaccessible-a form of "loss of active material" (LAM) that can further trigger localized current concentration and shifts in impedance. The severity of this process depends on the specific material system, the voltage window, and the temperature; therefore, these mechanisms cannot be generalized to all types of cathodes.

Anodes can also undergo deactivation. Damage to graphite particles or a deterioration in their contact with the conductive network reduces the number of available sites for lithium intercalation. Silicon-containing anodes exhibit more pronounced volumetric expansion and contraction, which can lead to particle fracture, loss of electrical contact, and the repeated reconstruction of the solid electrolyte interphase (SEI). This same process can simultaneously result in the loss of active material (LAM) and the loss of cyclable lithium (LLI). While a material may demonstrate rapid transport kinetics and low interfacial impedance in its fresh state, this only indicates favorable initial dynamics; it does not guarantee that the particles and interfaces can withstand long-term cycling.

3. Low overall internal resistance can mask localized non-uniformity

Cell-level AC or DC internal resistance (ACIR/DCIR) provides a composite response measured at the terminals. Within the electrode-both across its thickness and along its plane-porosity, electrolyte concentration, local state of charge (SOC), and active reaction area are not perfectly uniform. During charging, local current density and the overpotential at the anode can also vary. Even if the overall internal resistance is low, certain regions may reach a high degree of lithiation prematurely, or become more susceptible to lithium plating due to mass-transfer limitations.

Such localized lithium plating, along with the associated interfacial reactions, consumes cyclable lithium and alters pore structures and contact states over repeated cycles. This causes the electrochemical reactions to become increasingly concentrated in the remaining active regions. Consequently, a low macroscopic internal resistance does not guarantee safety throughout the entire electrode. Determining whether lithium plating has actually occurred requires an analysis of operating conditions, diagnostic signals, and-if necessary-physical evidence obtained through cell disassembly; one cannot draw a definitive conclusion based solely on an internal resistance curve.

First, standardize the test conditions-ensuring consistent SOC, temperature, rest duration, current direction, pulse duration, or AC frequency-before comparing the initial internal resistance value against its trajectory over the course of cycling. An increase in internal resistance suggests changes in power capability or polarization; however, a single ACIR or DCIR value is generally insufficient to distinguish which electrode-or which specific process-is the primary driver.

Next, examine periodic reference capacity measurements and degradation curves. A gradual decline versus the sudden appearance of an "inflection point" (a sharp change in the degradation trend) requires different investigative approaches. If capacity drops significantly at high C-rates while remaining relatively stable at low C-rates, the issue is likely related to polarization and cut-off voltage effects. Conversely, if the reference capacity also continues to decline at low C-rates, the investigation should focus on the loss of cyclable lithium (LLI) and the loss of active material (LAM). Coulombic efficiency and cycle-by-cycle irreversible capacity can provide clues regarding side reactions; however, because the differences involved are often minute, high-precision measurements are required. Furthermore, Coulombic efficiency cannot be directly converted into a specific quantity of lithium loss.

For battery manufacturers, regular charge-discharge testing is essential for tracking capacity retention and identifying performance changes during repeated cycling. A suitable battery charge discharge test equipment can evaluate discharge capacity and charge-discharge behavior under defined test conditions, helping engineers monitor battery performance over time.

By overlaying charge-discharge voltage curves obtained under identical conditions-and, where necessary, performing Incremental Capacity Analysis (ICA, or dQ/dV) or Differential Voltage Analysis (DVA, or dV/dQ)-one can observe shifts in peak positions, peak areas, and electrode capacity windows. These features help distinguish between Loss of Lithium Inventory (LLI), Loss of Active Material (LAM), and polarization; however, they cannot be interpreted in isolation from the electrode curves, test C-rates, and signal noise. If multiple lines of evidence remain inconsistent, Electrochemical Impedance Spectroscopy (EIS) can be used to differentiate processes based on their characteristic time scales, or post-mortem analysis (disassembly) can be conducted to inspect for cracks, lithium plating, and interfacial conditions. It should be noted that EIS fitting does not necessarily yield a unique solution.

Initial internal resistance represents an electrical response under specific, instantaneous conditions, whereas cycle life reflects the long-term stability of the materials, lithium inventory, and electrochemical interfaces. A low initial internal resistance is neither a cause of short cycle life nor proof of a low side-reaction rate or long-term structural stability.

To estimate the cycle life of a battery cell, one must evaluate capacity fade, impedance evolution, and voltage characteristics collectively under controlled test conditions, followed by targeted diagnostic analysis to verify the underlying degradation mechanisms. Relying solely on the factory-specified internal resistance to predict cycle life is insufficient.

ACEY New Energy specializes in the research, development, and manufacturing of advanced lithium battery manufacturing equipment. We provide customized, one-stop solutions for lithium-ion battery cell production, including cylindrical, coin, and pouch cells, as well as lithium battery pack assembly lines for various applications. Whether you are starting a new battery manufacturing business or upgrading your existing production capabilities, our experienced engineering team can provide professional technical support, equipment recommendations, and process guidance to help you build an efficient and reliable production line. Contact us to discuss your project requirements and find the right solution for your business.

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