Lithium battery polarization

　　When the current passes through the electrode, the phenomenon that the electrode deviates from the equilibrium electrode potential is called battery polarization, and polarization generates an overpotential. According to the causes of polarization, polarization can be divided into ohmic polarization, concentration polarization and electrochemical polarization,

　　(1) Ohmic polarization: It is caused by the resistance of each part connected by the battery. Its voltage drop follows Ohmic law. When the current decreases, the polarization decreases immediately, and disappears immediately after the current stops.

　　(2) Electrochemical polarization: polarization is caused by the slow electrochemical reaction on the electrode surface. As the current decreases, it decreases significantly in microseconds.

　　(3) Concentration polarization: due to the slow diffusion process of ions in the solution, the concentration difference between the electrode surface and the solution body under a certain current causes polarization. This polarization decreases or disappears at the macro second level (from a few seconds to tens of seconds) as the current decreases.

　　The internal resistance of the battery increases with the increase of the battery discharge current, mainly because the large discharge current increases the polarization trend of the battery, and the larger the discharge current, the more obvious the polarization trend, as shown in Figure 2. According to Ohm's law: V=E0-I × RT, the increase of the internal overall resistance RT, the time required for the battery voltage to reach the discharge cut-off voltage is correspondingly reduced, so the discharged capacity is also reduced.

　　Lithium ion battery is essentially a kind of lithium ion concentration battery. The charging and discharging process of lithium ion battery is the process of lithium ion insertion and removal at the positive and negative electrodes. Factors affecting the polarization of lithium ion batteries include:

　　(1) Influence of electrolyte: low electrolyte conductivity is the main reason for lithium ion battery polarization. In the general temperature range, the conductivity of electrolyte for lithium ion battery is generally only 0.01~0.1S/cm, which is one percent of the aqueous solution. Therefore, when the lithium ion battery is discharged at high current, it is too late to supplement Li+from the electrolyte, and polarization will occur. Improving the conductivity of electrolyte is the key factor to improve the high current discharge capacity of lithium ion battery.

　　(2) Influence of positive and negative electrode materials: the channel of large lithium ion particles of positive and negative electrode materials diffusing to the surface is longer, which is not conducive to large rate discharge.

　　(3) Conductive agent: the content of conductive agent is an important factor affecting the high rate discharge performance. If the content of conductive agent in the positive electrode formula is insufficient, electrons can not be transferred in time during high current discharge, and the internal polarization resistance increases rapidly, so that the battery voltage quickly decreases to the discharge cut-off voltage.

　　(4) Influence of pole plate design:

　　Electrode plate thickness: in the case of large current discharge, the reaction speed of active substance is very fast, which requires that lithium ions can be embedded and removed quickly in the material. If the electrode plate is thick, the lithium ion diffusion path increases, and a large lithium ion concentration gradient will be generated in the electrode plate thickness direction.

　　Compaction density: if the compaction density of the pole piece is larger and the pores become smaller, the lithium ion movement path in the thickness direction of the pole piece is longer. In addition, if the compaction density is too high, the contact area between the material and the electrolyte will be reduced, the electrode reaction site will be reduced, and the internal resistance of the battery will also be increased.

　　(5) Influence of SEI film: The formation of SEI film increases the resistance of electrode/electrolyte interface, resulting in voltage hysteresis, i.e. polarization.

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