The internal resistance of lithium battery and its influencing factors

Lithium battery internal resistance, static resistance and working resistance are often different in different environment and different temperature. What factors influence the lithium battery internal resistance?

1 the working process of the lithium battery

As shown in the above, lithium ion batteries charge and discharge process of physical model. Blue arrows indicate the charging, the red arrows indicate discharge. The lattice structure of blue and green as the anode material, black layer as the cathode material. The current mainstream of lithium ion battery, generally according to the positive electrode material type, named the lithium iron phosphate, lithium manganese acid which is a type of cathode material; Negative extremely graphite material; The positive set fluid aluminum foil, negative set fluid for copper foil.

The following discharge, for example, describe the physical process of lithium battery is discharging.

Externally load after connecting the formation of a current path outside the battery ontology. Because is negative, and electric potential difference between the cathode nearby electronic first set of fluid and the external conductor is moved to the anode; Around the cathode of lithium ion concentration increased. Electrons from the cathode through an external circuit to the anode, and near the anode of lithium ion, embedding the anode material, near the anode of lithium ion concentration decreased. Is the cathode lithium ion concentration difference between formation. So, completes the battery discharge process, the first push.

As lithium ion under the impetus of the ion concentration away from the cathode, a vacancy arises near the cathode, anode materials in lithium ion, from the cathode, embedded into the electrolyte; A lot from lithium ion electrolyte through the diaphragm, move to the positive ego extremely. At the same time, the original lithium ions in the form of the combination of electronic via an external circuit to the anode. Battery started according to the load demand of the discharge process.

Charge is a reverse process of discharge, the same embedded and mobile, embedded in several stages, just power from the charger to promote the development of process, and the movement direction of the ion is from the positive to the negative movement. Here is no longer here.

2 lithium battery internal resistance

Understand the working process of the lithium battery, so in the process of obstacles, then formed the lithium battery internal resistance.

Battery internal resistance includes the ohmic resistance and polarization resistance. Under the condition of constant temperature, basic stable, ohmic resistance and polarization resistance will change as the factors influencing the level of polarization.

Ohmic resistance is mainly composed of electrode material, electrolyte and membrane resistance and collection of fluid, such as ear connection of various parts of the contact resistance, and the size of the battery, the structure, connecting method and so on.

Polarization resistance, load current moment of resistance, is all hinder the charged ions inside the battery arrives the trend of the combined. Polarization resistance can be divided into two parts of electrochemical polarization and concentration polarization. Electrochemical polarization is electrolyte in the electrochemical reaction speed cannot be achieved due to electron movement speed; Concentration polarization, is lithium ion embedded out the speed of moving anode materials and in the material is less than the speed of the lithium ions to the electrode assembly.

3 lithium battery internal resistance factors

From the above process can deduce the influence factors of battery internal resistance.

3.1 plus factor

Temperature, ambient temperature is one of the important influence factors of various resistance, specific to lithium batteries, is due to the temperature influence the activity of electrochemical material, determine the electrochemical reaction speed and the speed of the ion movement.

Current or load demand, on the one hand, the size of the current is directly associated with the polarization resistance. This trend is the greater the current, the bigger the polarization resistance. On the other hand, the heating effect of the current, impact on the electrochemical activity of material.

3.2 the battery itself

Cathode materials, anode materials, lithium ion embedding and the ease of embedded, decided to the size of the internal resistance of the material, is part of the concentration polarization resistance.

Electrolyte, lithium ion electrolyte in the moving speed, the influence of electrolyte conductivity, is the main part of electrochemical polarization resistance.

Diaphragm, diaphragm itself resistance, direct constitute part of the ohm internal resistance, at the same time for the lithium ion mobility barriers, and formed a part of electrochemical polarization resistance.

Collection of fluid resistance, parts connected resistance, is the main component of battery ohm internal resistance.

Process level, plate making process, coating is uniform, the compaction density, the level of technology in the manufacturing process of these batteries, will also directly effects on the polarization resistance.

4 lithium battery internal resistance measurement

Lithium battery internal resistance measurement, generally can be divided into dc measurement method and two ac measuring method.

4.1 dc resistance measurement

Use a current source, to exert short pulse batteries, measuring the terminal voltage and the open circuit voltage of the poor. With this difference divided by the test current is considered to be the dc resistance of the battery.

Lithium battery polarization resistance will be affected by the size of the load current, in order to try to stay away from this factor, dc resistance measurement method of the polarization time is shorter, and the load current is larger.

In theory, the measuring current is smaller, the more won't cause polarization reaction, reduce the interference of polarization resistance. But as a result of battery internal resistance itself is tiny, is milliohm level, too little electricity, voltage testing instrument is limited by accuracy of measurement, measurement error of the results cannot be ruled out. As a result, people weighing instrument accuracy and the influence of polarization resistance, to find a balance the relationship between the two measuring current value.

For ordinary battery monomer, measure the current general in 5 c - around 10 c, very big. With the increment of capacity of batteries, or multiple batteries in parallel, its internal resistance is reduced, therefore, if there is no instrument accuracy improvement, measuring current is difficult to come down.

4.2 ac internal resistance measuring methods

For smaller values of the battery to load a picture ac input as an incentive, monitoring the response of terminal voltage. Using a specific program to analyze data, draw ac internal resistance of the battery. Analysis of resistance, only related to the battery itself characteristic, has nothing to do with the size of the excitation signal.

As a result of the existence of battery capacity characteristics, the frequency of the excitation signal is different, the measured resistance is different also. Software analysis results can be expressed with a set of complex number, the horizontal axis as the real component, the vertical axis of the imaginary part. Thus form a map, the so-called ac impedance spectrum, as shown in the above.

Through further analysis of data, one can get it only from the ac impedance spectroscopy battery ohm resistor, SEI film diffusion resistance, capacitance value of the SEI film, the equivalent capacitance of charge transfer in the electrolyte and charge in the electrolyte diffusion resistance, equivalent model, then map the battery for the further study of the battery performance. A model of equivalent battery, as shown in the figure below.

5 resistance application in engineering practice

Internal resistance, as one of the key characteristic of lithium battery, for its research achievements, can be applied in engineering, and other fields.

Resistance have close relationship with the battery charge capacity, therefore be applied to battery SOC estimation of management system;

Resistance directly reflect the battery aging degree, someone put batteries resistance as the SOH battery state of health of the assessment basis;

Monomer high internal consistency directly affects the mode of group after group of capacity and life, and therefore was used as a widely used batteries sorting with set of static index;

Resistance is an important indication of the battery failure, in power battery package in the fault diagnosis system, used by research;

Internal impedance matching capacity indicators, such as the loss analysis can determine whether the battery existence phenomenon of lithium, use retired batteries used in pilot.

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