With the use of lithium energy storage batteries, the performance of the battery is constantly attenuated, which is mainly manifested in battery capacity attenuation, increase in internal resistance, and decrease in power. Therefore, the factors affecting the internal resistance of the battery are expounded in combination with the battery structure design, the performance of raw materials, the process technology and the use conditions.
Resistance is the resistance that current flows through the interior of the battery when the lithium energy storage 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.
The 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.
01 Ionic 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 ionic impedance, you need to do the following:
Ensure that the positive and negative materials and 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.
02 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.
03 Contact resistance
The 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.
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