Lithium battery internal resistance

The internal resistance is the resistance that current flows through the interior of the battery while the lithium battery is in operation. According to the test method, it can be divided into AC internal resistance and DC internal resistance. Battery internal resistance is an important parameter to identify the quality of lithium-ion battery. The internal resistance of the battery is large, which will generate a large amount of Joule heat, which will cause the battery temperature to rise, resulting in lower battery discharge working voltage, shorter discharge time, battery performance, life expectancy, etc. Has a serious impact. Internal resistance is also an important parameter to be examined in verifying the electrochemical performance of various factors for lithium batteries. Combined with the materials and processes of lithium batteries, we share with you the factors affecting the internal resistance of lithium batteries.

Generally, the internal resistance of the battery is divided into an ohmic internal resistance and a polarization internal resistance. The ohmic internal resistance is composed of the electrode material, the electrolyte, the diaphragm resistance, and the contact resistance of each part. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including electrochemical polarization internal resistance and concentration polarization internal resistance. The ohmic internal resistance of the battery is determined by the total conductivity of the battery, and the polarization internal resistance of the battery is determined by the solid phase diffusion coefficient of lithium ions in the electrode active material.

First, ohm internal resistance

The ohmic internal resistance is mainly divided into three parts, one is ion impedance, the other is electronic impedance, and the third is contact impedance. We hope that the smaller the internal resistance of the lithium battery, the smaller the need to take specific measures to reduce the ohmic internal resistance.

1.Ion impedance

Lithium battery ion impedance refers to the resistance that lithium ions are transmitted inside the battery. Lithium ion migration speed and electron conduction speed play an important role in lithium batteries. Ion impedance is mainly affected by positive and negative materials, separators and electrolytes. To reduce the ionic impedance, you need to do the following:

1 Ensure that the positive and negative materials and electrolyte have good wettability.

In the design of the pole piece, it is necessary to select a suitable compaction density. If the compaction density is too large, the electrolyte is not easily wetted, and the ion impedance is increased. For the negative pole piece, if it is formed on the surface of the active material during the first charge and dischargeSEIIf the film is too thick, it will also increase the ion impedance. At this time, it is necessary to adjust the formation process of the battery to solve it.

2 electrolyte effect

The electrolyte should have a suitable concentration, viscosity and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration with the positive and negative active materials. At the same time, the electrolyte also needs a lower concentration, and too high a concentration is also unfavorable for its flow infiltration. The conductivity of the electrolyte is the most important factor affecting the ion impedance, which determines the migration of ions.

3 diaphragm impact on ion impedance

The main factors affecting the ion impedance of the diaphragm are: electrolyte distribution in the diaphragm, diaphragm area, thickness, pore size, porosity and tortuosity coefficient. For ceramic diaphragms, it is also necessary to prevent ceramic particles from clogging the pores of the membrane to facilitate ion passage. While ensuring that the electrolyte fully wets the separator, there is no residual electrolyte remaining therein, and the use efficiency of the electrolyte is lowered.

2. Electronic impedance

There are many influencing factors of electronic impedance, which can be improved from materials and processes.

1 positive and negative plates

The factors affecting the electronic impedance of the positive and negative plates are: the contact between the living material and the current collector, the factors of the living material itself, and the parameters of the plate. The living material should be in full contact with the fluid collecting surface, and it can be considered from the current collector copper foil and the aluminum foil substrate, and the positive and negative electrode pastes are considered. The porosity of the living material itself, by-products on the surface of the particles, and uneven mixing with the conductive agent cause changes in the electronic impedance. The plate parameters such as the density of the living material are too small, and the particle gap is large, which is not conducive to electron conduction.

2 diaphragm

The main factors affecting the electronic impedance of the diaphragm are: diaphragm thickness, porosity and by-products during charge and discharge. The first two are easy to understand. After the battery is disassembled, it is often found that the diaphragm is covered with a thick layer of brown material, including the graphite anode and its reaction by-products, which will cause blockage of the diaphragm pores and reduce battery life.

3 current collector substrate

The material, thickness, width of the current collector and its degree of contact with the tabs affect the electronic impedance. The current collector needs to select a substrate that is not oxidized and passivated, otherwise it will affect the impedance. Poor soldering of copper foil and tabs can also affect the electronic impedance.

3. Contact impedance

The contact resistance is formed between the contact of the copper foil and the active material, and it is necessary to focus on the adhesion of the positive and negative electrodes.

Second, the polarization internal resistance

When a current passes through the electrode, the phenomenon that the electrode potential deviates from the equilibrium electrode potential is called the polarization of the electrode. Polarization includes ohmic polarization, electrochemical polarization, and concentration polarization, as shown in FIG. Polarization resistance refers to the internal resistance caused by polarization of the positive and negative electrodes of the battery during electrochemical reaction. It can reflect the internal consistency of the battery, but it is not suitable for production due to the influence of operation and method. The internal resistance of polarization is not constant and changes with time during charging and discharging. This is because the composition of the active material, the concentration and temperature of the electrolyte are constantly changing. The ohmic internal resistance follows Ohm's law, and the polarization internal resistance increases with increasing current density, but is not linear. It often increases linearly with increasing logarithm of current density.

Figure 1. OCV-operating voltage-polarization voltage diagram

In general, the DC internal resistance of the battery is equal to the sum of the polarization internal resistance and the ohmic internal resistance. The determination of DC internal resistance is of great significance. There are many factors affecting the internal resistance of polarization, such as charge and discharge rate, ambient temperature,SOCState, concentration of electrolyte, etc. Here is an example of the internal resistance of the lithium iron phosphate battery. The relevant literature is available in a private letter, as shown in the following figure:

Third, the current internal resistance measurement method used in the industry

In industrial applications, accurate measurement of battery internal resistance is performed by dedicated equipment. At present, there are two main methods for measuring internal resistance of batteries used in the industry:

1. DC discharge internal resistance measurement method

According to the physical formula R=U/I, test equipment to make the battery in a short time (usually2~3Second) forced through a large constant DC current (currently used40A~80AThe high current), measure the voltage across the battery at this time, and calculate the current internal resistance of the battery according to the formula.

The accuracy of this measurement method is high. If it is properly controlled, the measurement accuracy error can be controlled.0.1Less than %. But this method has obvious shortcomings:

(1) can only measure large capacity batteries or batteries, small capacity batteries can not be2~3Load in seconds40A~80AHigh current

(2When the battery passes a large current, the electrode inside the battery will be polarized, resulting in a polarization internal resistance. Therefore, the measurement time must be very short, otherwise the measured internal resistance value is very large;

(3A large current has a certain damage to the electrodes inside the battery through the battery.

2. AC pressure drop internal resistance measurement method

Because the battery is actually equivalent to an active resistor, we apply a fixed frequency and a fixed current to the battery (currently used1kHzfrequency,50mASmall current), and then the voltage is sampled, after a series of processing such as rectification, filtering, etc., the internal resistance value of the battery is calculated through the operational amplifier circuit. The battery measurement time of the AC voltage drop internal resistance measurement method is extremely short, generally100About milliseconds.

The accuracy of this measurement method is also good, and the measurement accuracy error is generally1%~2%between.

The advantages and disadvantages of this method:

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

(2The measurement accuracy of the AC voltage drop measurement method is likely to be affected by the ripple current and the possibility of harmonic current interference. This is a test of the anti-interference ability in the measuring instrument circuit.

(3) Measuring by this method does not cause much damage to the battery itself.

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