The meaning of resistance in lithium-ion batteries
With the use of lithium batteries, the battery performance continues to decay, mainly manifested as capacity decay, internal resistance increase, power decline and so on, which will be considerable important impact on the battery longer lifetime and safety performance. So here let’s talk about the topic of what’s the internal resistance.
Resistance is the resistance that current flows through the interior of the battery when the lithium battery is working. Generally, the internal resistance of lithium batteries is divided into ohmic internal resistance and polarization internal resistance. Ohmic internal resistance consists of electrode material, electrolyte, diaphragm resistance and contact resistance of various parts. 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.
Ohmic resistance
Ohmic internal resistance is mainly divided into three parts, one is ionic 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 internal resistance, so we need to take specific measures to reduce the ohmic internal resistance for these three items.
1.Ion impedance
Lithium battery ion impedance refers to the resistance of lithium ions to transfer inside the battery. Lithium ion migration speed and electron conduction speed play an equally important role in lithium batteries, and ionic impedance is mainly affected by positive and negative electrode materials, separators, and electrolytes. To reduce the ionic impedance, the following points need to be done:
- ensure that the positive and negative materials and the electrolyte have good wettability—In the design of the pole piece, it is necessary to select an appropriate compaction density. If the compaction density is too large, the electrolyte will not easily infiltrate, which will increase the ionic impedance. For the negative pole piece, if the SEI film formed on the surface of the active material during the first charge and discharge is too thick, the ionic impedance will also be increased, and the formation process of the battery needs to be adjusted to solve this problem.
- The effect of electrolyte—The electrolyte should have suitable concentration, viscosity and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration between the electrolyte and the positive and negative active materials. At the same time, the electrolyte also needs a lower concentration, and if the concentration is too high, it is also not conducive to its flow and infiltration. The conductivity of the electrolyte is the most important factor affecting the ionic impedance, which determines the migration of ions.
- The effect of the diaphragm on ionic impedance—The main factors affecting the ionic 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 blocking the pores of the diaphragm, which is not conducive to the passage of ions. While ensuring that the electrolyte fully infiltrates the diaphragm, there must be no residual electrolyte remaining therein, which reduces the use efficiency of the electrolyte.
2.Electronic impedance
There are many influencing factors of electronic impedance, which can be improved from the aspects of materials and processes.
- Positive and negative plates—The main factors affecting the electronic impedance of the positive and negative plates are: the contact between the active material and the current collector, the factors of the active material itself, and the plate parameters. The active material should be fully in contact with the current collector surface, which can be considered from the current collector copper foil, aluminum foil substrate, and the adhesiveness of the positive and negative electrode paste. The porosity of the active material itself, the by-products on the surface of the particles, and the uneven mixing with the conductive agent will all cause changes in electronic impedance. Plate parameters such as the density of the active material are too small, the particle gap is large, which is not conducive to electron conduction.
- Diaphragm—The main factors affecting the electronic impedance of the diaphragm are: the thickness of the diaphragm, the porosity and the by-products during the charging and discharging process. The first two are easy to understand. After the dismantling of the battery cell, it is often found that a thick layer of brown material is attached to the diaphragm, including the graphite negative electrode and its reaction by-products, which will cause the diaphragm pores to block and reduce the battery life.
- Current collector substrate—The material, thickness, width and degree of contact of the current collector with the tabs all affect the electrical impedance. The current collector needs to choose an unoxidized and passivated substrate, otherwise it will affect the impedance. Poor welding between copper and aluminum foil and tabs will also affect electronic impedance.
3.Contact resistance
Contact resistance is formed between the contact between the copper and aluminum foil and the active material, and it is necessary to focus on the adhesiveness of the positive and negative electrode paste.
Polarization resistance
The phenomenon that the electrode potential deviates from the equilibrium electrode potential when a current passes through the electrode is called the polarization of the electrode. Polarization includes ohmic polarization, electrochemical polarization and concentration polarization. Polarization resistance refers to the internal resistance caused by the polarization of the positive and negative electrodes of the battery during the electrochemical reaction, which can reflect the internal consistency of the battery, but is not suitable for production due to the influence of operation and methods. The polarization internal resistance is not constant and changes with time during the charging and discharging process. This is because the composition of the active material, the concentration of the electrolyte and the temperature are constantly changing. Ohm’s internal resistance obeys Ohm’s law, and the polarization internal resistance increases with the current density, but it is not a linear relationship. Usually increases linearly with the logarithm of the current density.
Next time topic2 of The Factors affecting internal resistance of lithium ion batteries