What Does a Battery Technician Do?

When a battery passes a current, the electrode deviates from the equilibrium electrode potential, which is called electrode polarization. The phenomenon of battery polarization exists in common batteries such as lead-acid batteries, lithium batteries, and nickel-metal hydride batteries.

The electrode polarization produced by anode current is called anodic polarization; the electrode polarization produced by cathode current is called cathodic polarization. The larger the current passing through the unit area of the electrode, the more serious the potential deviates from the equilibrium electrode. For example, iron rusts because there are impurities in the iron (usually C). In the electrolyte solution, iron will become the negative electrode, and carbon as the positive electrode, which accelerates the corrosion of iron. ^[1]

According to the causes of polarization, polarization can be divided into three types: electrochemical polarization, concentration polarization, and ohmic polarization.

1. Electrochemical polarization is polarization caused by various types of electrochemistry itself;

2. Concentration polarization is caused by the consumption of reactants, which causes the electrode surface not to be replenished in time (or a certain product accumulates on the electrode surface and cannot be evacuated in time). For example, the accumulation of hydrogen in the positive electrode of the battery causes the electrode potential to deviate from the current as a whole. Average value of concentration calculation;

3. Ohmic polarization is the polarization caused by the contact resistance between the electrolyte, electrode materials, and conductive materials.

The above three polarizations are resistance to electrochemical reactions. Therefore, the internal resistance of the battery is the sum of ohmic internal resistance, electrochemical polarization internal resistance, and concentration polarization internal resistance. ^[1]

1. Influence of electrolyte: The low conductivity of electrolyte is the main reason for the polarization of lithium ion batteries. The conductivity of the electrolyte for lithium ion batteries is generally only 0.01 to 0.1 S / cm, which is one hundredth of the aqueous solution. Therefore, when the lithium ion battery is discharged at a large current, it is too late to replenish Li ⁺ from the electrolyte, and polarization occurs. Improving the conductivity of the electrolyte is a key factor in improving the high-current discharge capacity of lithium-ion batteries.

2. The effect of positive and negative electrode materials: The channel for large lithium ion diffusion of the positive electrode material particles to the surface is lengthened, which is not conducive to large-rate discharge.

3. Conductive agent: The content of the positive electrode conductive agent is a key factor affecting the high-rate discharge performance. The main reason that normal batteries cannot perform high-rate discharge is that the conductive agent content in the normal positive electrode formula is insufficient, and electrons cannot be transferred in time during high current discharge, and the polarization internal resistance rapidly increases, causing the battery voltage to quickly decrease to the end of discharge Voltage.

4. The effect of the thickness of the pole piece: under the condition of large current discharge, the active material reacts quickly, and ions can be quickly inserted and extracted in the material. If the pole piece is thick, it will undoubtedly lengthen the path of ion movement and increase. Resistance to ion movement. However, if the compacted density of the pole piece is large, the small path and small pores of the ion movement inside the active material become smaller. As the internal resistance of the battery increases and the porosity of the material decreases, the compaction density is too large, the contact surface between the material and the electrolyte decreases, and the internal resistance of the battery increases. The pulping process is too short, the slurry is uneven, and the lithium ion may diffuse slowly during charging and discharging. When charging and discharging at high current, the polarization and resistance are large, and the battery safety is poor. If the time is too long, the slurry is too fine, and the internal resistance of the battery is too large. Secondly, the coating thickness and its uniformity also affect the insertion and extraction of lithium ions in the active material. For example, the negative electrode film is thick and non-uniform. Due to the different polarizations in the charging process, local deposition of metallic lithium on the surface of the negative electrode may occur. The compacted density of the positive electrode sheet has an impact on the rate discharge performance. If the compacted density is too large, the discharge capacity is low, and the discharge capacity is small during large-rate discharge. However, the compacted density may not be high.

5. The influence of the SEI film: The formation of the SEI film increases the resistance at the electrode / electrolyte interface, causing voltage hysteresis or polarization. The SEI film is generated when the formation voltage is above 2.5V, and the SEI film is generated by the voltage between 3.0 and 3.5V. The main stages. The internal resistance of the battery increases with the increase of the discharge current of the battery, which is mainly due to the large discharge current that increases the polarization tendency of the battery, and the larger the discharge current, the more obvious the polarization tendency, according to Ohm's law : V = E0-I × RT, the increase of the overall internal resistance will reduce the time required for the battery voltage to reach the discharge cut-off voltage, so the discharged capacity will also be reduced. ^[2]