Unveiling the Secrets of Lithium Battery Internal Resistance: Key Influencing Factors and Optimization Solutions

Internal resistance is the resistance encountered by current flowing through a lithium battery during operation. Based on the testing method, it can be divided into AC internal resistance and DC internal resistance. Battery internal resistance is an important parameter for evaluating the quality of lithium-ion batteries. High internal resistance generates a large amount of Joule heat, causing the battery temperature to rise, leading to a decrease in discharge voltage and a shortened discharge time, seriously affecting battery performance and lifespan. Internal resistance is also an important parameter to consider in electrochemical performance tests of lithium batteries. This article will share the factors affecting lithium battery internal resistance, considering the materials and manufacturing processes of lithium batteries.

Generally, battery internal resistance is divided into ohmic resistance and polarization resistance. Ohmic resistance consists of the resistance of the electrode materials, electrolyte, separator, and contact resistance of various components. Polarization resistance refers to the resistance caused by polarization during electrochemical reactions, including electrochemical polarization resistance and concentration polarization resistance. The ohmic resistance of the battery is determined by the total conductivity of the battery, while the polarization resistance is determined by the solid-phase diffusion coefficient of lithium ions in the electrode active material.

I. Ohmic Resistance

Ohmic resistance is mainly divided into three parts: ionic impedance, electronic impedance, and contact impedance. To minimize the internal resistance of lithium batteries, specific measures need to be taken to reduce these three components of ohmic resistance.

1. Ionic Impedance

Lithium battery ionic impedance refers to the resistance encountered by lithium ions during their transfer within the battery. In lithium batteries, the migration speed of lithium ions and the electron conduction speed play equally important roles. Ionic impedance is mainly affected by the positive and negative electrode materials, separator, and electrolyte. To reduce ionic impedance, the following points should be addressed:

① Ensure good wettability between the positive and negative electrode materials and the electrolyte.

When designing the electrode sheets, an appropriate compaction density should be selected. If the compaction density is too high, the electrolyte will not easily penetrate, increasing the ionic impedance. For the negative electrode sheet, if the SEI film formed on the surface of the active material during the first charge and discharge is too thick, it will also increase the ionic impedance. This requires adjusting the battery formation process to solve the problem.

 ② Influence of Electrolyte

The electrolyte should have appropriate concentration, viscosity, and conductivity. Excessive electrolyte viscosity hinders its wetting of the positive and negative electrode active materials. At the same time, the electrolyte needs to have a low concentration; too high a concentration is also detrimental to its flow and wetting. The electrolyte's conductivity is the most important factor affecting ionic impedance, as it determines ion migration.

③ Influence of Separator on Ionic Impedance

The main factors affecting ionic impedance in the separator include: electrolyte distribution within the separator, separator area, thickness, pore size, porosity, and tortuosity. For ceramic separators, it is also necessary to prevent ceramic particles from blocking the separator pores, which would hinder ion passage. While ensuring sufficient electrolyte wetting of the separator, there should be no excess electrolyte remaining within it, which would reduce the efficiency of electrolyte utilization.

2. Electronic Impedance

Electronic impedance is influenced by many factors, and improvements can be made from aspects such as materials and manufacturing processes.

① Positive and Negative Electrode Plates

The factors affecting the electronic impedance of the positive and negative electrode plates mainly include: the contact between the active material and the current collector, the properties of the active material itself, and the electrode plate parameters. The active material needs to be in full contact with the current collector surface, which can be considered from the copper and aluminum foil substrates of the current collector and the adhesion of the positive and negative electrode slurries. The porosity of the active material itself, by-products on the particle surface, and uneven mixing with the conductive agent can all cause changes in electronic impedance. Electrode plate parameters, such as low active material density, result in large inter-particle gaps, which are unfavorable for electron conduction.

② Separator

The factors affecting the electronic impedance of the separator mainly include: separator thickness, porosity, and by-products during charging and discharging. The first two are easy to understand. After disassembling the battery cell, a thick layer of brown substance is often found on the separator, which includes graphite negative electrode and its reaction by-products. This can cause separator pore blockage and reduce battery life.

③ Current Collector Substrate

The material, thickness, width of the current collector, and its contact with the electrode tabs all affect electronic impedance. The current collector needs to be made of an unoxidized and unpassivated substrate; otherwise, it will affect the impedance. Poor welding between the copper/aluminum foil and the electrode tabs will also affect electronic impedance.

3. Contact Impedance

Contact resistance is formed at the contact between the copper/aluminum foil and the active material, and the adhesion of the positive and negative electrode slurries needs to be given special attention.

II. Polarization Internal Resistance

When current flows through an electrode, the phenomenon of the electrode potential deviating from the equilibrium electrode potential is called electrode polarization. Polarization includes ohmic polarization, electrochemical polarization, and concentration polarization, as shown in the figure below. Polarization resistance refers to the internal resistance caused by polarization during the electrochemical reaction between the positive and negative electrodes of the battery. It can reflect the consistency of the battery's internal structure, but due to the influence of operation and methods, it is not suitable for use in production. Polarization internal resistance is not constant; it continuously changes over time during charging and discharging because the composition of the active material, the concentration of the electrolyte, and the temperature are constantly changing. Ohmic internal resistance obeys Ohm's law, and polarization internal resistance increases with increasing current density, but not linearly. It usually increases linearly with the logarithm of the current density.

Generally, the DC internal resistance of a battery is equal to the sum of the polarization internal resistance and the ohmic internal resistance. The measurement of DC internal resistance is of great significance. Many factors affect the polarization internal resistance, such as the charge and discharge rate, ambient temperature, SOC state, and electrolyte concentration.

III. Battery Internal Resistance Measurement Methods Currently Used in the Industry

In industrial applications, the accurate measurement of battery internal resistance is performed using specialized equipment. Currently, the main battery internal resistance measurement methods used in the industry are as follows:

1. DC Discharge Internal Resistance Measurement Method

According to the physical formula R = U/I, the test equipment forces a large constant DC current (currently generally 40A to 80A) through the battery for a short period (generally 2-3 seconds), measures the voltage across the battery at this time, and calculates the current battery internal resistance using the formula.

This measurement method has high accuracy; if properly controlled, the measurement accuracy error can be controlled within 0.1%.

However, this method has obvious drawbacks:

(1) It can only measure large-capacity batteries or accumulators; small-capacity batteries cannot withstand a large current of 40A to 80A within 2-3 seconds;

(2) When a large current passes through the battery, the electrodes inside the battery will undergo polarization, generating polarization internal resistance. Therefore, the measurement time must be very short; otherwise, the measured internal resistance value will have a large error;

(3) A large current passing through the battery can cause some damage to the electrodes inside the battery.

2. AC Voltage Drop Internal Resistance Measurement Method

Because a battery is essentially equivalent to an active resistor, we apply a fixed frequency and fixed current to the battery (currently, a frequency of 1 kHz and a small current of 50 mA are generally used), and then sample its voltage. After a series of processing steps such as rectification and filtering, the internal resistance value of the battery is calculated through an operational amplifier circuit. The battery measurement time using the AC voltage drop internal resistance measurement method is extremely short, generally around 100 milliseconds.

This measurement method also has good accuracy, with a measurement accuracy error generally between 1% and 2%.

Advantages and disadvantages of this method:

(1) The AC voltage drop internal resistance measurement method can measure almost all batteries, including small-capacity batteries. This method is generally used for measuring the internal resistance of laptop battery cells.

(2) The measurement accuracy of the AC voltage drop measurement method may be affected by ripple current, and there is also the possibility of harmonic current interference. This is a test of the anti-interference ability of the measuring instrument circuit.

(3) This method does not cause significant damage to the battery itself.

(4) The measurement accuracy of the AC voltage drop measurement method is not as good as the DC discharge internal resistance measurement method.