EV Topics: Determining Factors of Internal Resistance for Lithium Battery

Original Author: 电动知家

Translate & Edit: Andy, Kewell

Summary

Battery performance continues to decay as we use it, primarily in respects of capacity fading, power level dropping, and rising internal resistance etc. Internal resistance, in particular, is prone to the influence of working conditions including temperature and depth of discharge. The article aims to discuss the determining factors that set the internal resistance for lithium batteries on several fronts, structure design, raw material performance, manufacturing process, and working conditions.

Impact of Structure Design 

The structure design of a battery impacts the internal resistance level. In addition to riveting and welding process of a battery, the battery tab number, size, and position can make a difference to it as well. As we increase battery tabs, the internal resistance level may be significantly reduced.

Influence of Raw Material Performance

1. The Active Materials of Electrodes

The material of positive electrode determines the performance of a lithium battery. Its conductivity can be improved through coating and doping. For example, the doping of Ni can enhance the strength of P-O bond, stabilizing the structure of LiFePO4/C and optimizing the cell volume, which can effectively reduce the charge transfer impedance of the positive electrode material.

Through the simulation analysis of the electrochemical thermal coupling model, it is found that under the condition of high-rate discharge, the significant increase of activated polarization, especially of the negative electrode, is the main reason for the serious polarization. To reduce the activated polarization of the negative electrode, particle size must also be reduced. The activated polarization can be reduced by 45% if the particle size of the negative solid phase is reduced by half. Therefore, it is essential to work on the improvement of electrode material in terms of battery design.

2. Conductive Additive

Graphite and black carbon are widely used in lithium battery application for their superior qualities. Black carbon, comparing to graphite, performs better as a conductive additive to the positive electrode. A graphite based conductive additive, is prone to the increase of porosity and Gurley rate under high-rate discharge thanks to its flaky form, therefore resulting in limited discharge capacity during the diffusion process of Li liquid phase. Batteries that utilize CNTs is lower in internal resistance for there is wider contact surface between the fibrous CNTs and active material, comparing to the point-contact between graphite/black carbon based additive and active material.

3. Current Collector

Reducing the interfacial resistance between the collector and the active substance and improving the bond strength between them are important means to improve the performance of lithium batteries. The interfacial impedance of the battery can be effectively reduced by coating the aluminum foil with conductive carbon coating and corona treatment. Compared with the ordinary aluminum foil, the internal resistance of the battery can be reduced by about 65% by using the carbon-coated aluminum foil, and the increase of the internal resistance of the battery can be reduced in the process of use.

The AC internal resistance of aluminum foil, after corona treatment, can be reduced by about 20%. In the state of charge level between 20-90%, the DC internal resistance is low on the whole. With the increase of discharge depth, the rate of increase for internal resistance is gradually smaller.

4. Membrane

The ion conduction in the battery depends on the diffusion of Li ions in the electrolyte through the porous membrane. The liquid absorption and wetting ability of the membrane is the key to form a good ion flow channel. When the membrane has a higher liquid absorption rate and porous structure, it can improve the conductivity and reduce the impedance of the battery, improving the overall performance of a battery. Compared with an ordinary membrane, a ceramic membrane and gelled membrane can not only greatly improve the high temperature shrinkage resistance of the membrane, but also enhance the liquid absorption and wetting ability of the membrane. Adding SiO2 ceramic coating on the PP membrane can increase the liquid absorption capacity of the membrane by 17%. When PVDF-HFP of 1μm was coated on the PP/PE composite membrane, the liquid absorption rate of the membrane increased from 70% to 82%, and the internal resistance of the cell decreased by more than 20%.

Manufacturing

1. Mixing

The uniformity of the slurry dispersion affects whether the conductive agent can be evenly dispersed in the active substance and in close contact with it, which is related to the internal resistance of the battery. By increasing the high-speed dispersion, the uniformity of the paste dispersion can be improved, and the internal resistance of the battery is smaller. By adding surfactant, the distribution uniformity of the conductive agent in the electrode can be improved and the electrochemical polarization can be reduced to increase the median discharge voltage.

2. Coating

Surface density is one of the key parameters of battery design. When the battery capacity is certain, increasing the extremely one-sided density will inevitably reduce the total length of collector and diaphragm, and the internal ohmic resistance of the battery will decrease accordingly. Therefore, within a certain range, the internal resistance of the battery will decrease with the increase of surface density. The migration and detachment of solvent molecules during coating and drying are closely related to the temperature of the oven, which directly affects the distribution of binder and conductive agent in the electrode sheet, and then affects the formation of conductive grid inside the electrode sheet. Therefore, the temperature of coating and drying is also an important technological process to optimize the performance of the battery.

3. Roll-in

To a certain extent, the internal resistance of the battery decreases with the increase of the compaction density, because the distance between the raw material particles decreases with the increase of the compaction density. The more the contact between the particles, the more the conductive Bridges and channels, and the battery impedance decreases. The compaction density is controlled mainly by rolling thickness. Different roll thickness has a great influence on the internal resistance of the battery. When the roll thickness is larger, the contact resistance between the active substance and the collector increases due to the fact that the active substance is not rolled tightly, and the internal resistance of the battery increases. Moreover, after the battery cycle, cracks appear on the surface of the cathode of the battery with larger thickness by rolling, which will further increase the contact resistance between the active substance on the electrode surface and the fluid collector.

4. Pole Plate Turnaround Time

The internal resistance of the battery increases slowly due to the effect of the carbon coating on the surface of LiFePO4 and the effect of the application time on the internal resistance of the battery. When the battery is used for a long time (more than 23h), the internal resistance of the battery increases obviously due to the reaction of lithium iron phosphate with water and the bonding effect of the adhesive. Therefore, it is necessary to strictly control the electrode plate turnover time in actual production.

5. Liquid Injection

The ionic conductivity of the electrolyte determines the internal resistance and ratio characteristics of the battery. The conductivity of the electrolyte is inversely proportional to the viscosity of the solvent, and is also affected by the concentration of lithium salt and the size of the anion. In addition to the research on the optimization of conductivity, the amount of liquid injection and the infiltration time after the injection also directly affect the internal resistance of the battery. A small amount of liquid injection or insufficient infiltration time will cause the internal resistance of the battery to be larger, thus affecting the capacity of the battery.

Working Conditions

1. Temperature

The effect of temperature on the size of internal resistance is obvious, the lower the temperature, the slower the ion transfer inside the battery, the greater the internal resistance of the battery. The battery impedance can be divided into body phase impedance, SEI film impedance and charge transfer impedance. The body phase impedance and SEI film impedance are mainly affected by the ionic conductivity of the electrolyte, and their variation trend at low temperature is consistent with that of the electrolyte conductivity. Compared with the increase of the volume phase impedance and SEI film resistance at low temperature, the charge reactive impedance increases more significantly with the decrease of temperature. The charge reactive impedance accounts for almost 100% of the total internal resistance of the battery below -20℃.

2. SOC

When the battery is in different SOC, the size of its internal resistance is also different, especially the DC internal resistance directly affects the power performance of the battery, which further reflects the battery performance in the actual state: The DC internal resistance of lithium battery increases with the increase of the battery discharge depth DOD, and the value of the internal resistance is basically unchanged at the discharge range of 10%~80%. Generally, the internal resistance increases significantly at the deeper discharge depth.

3. Storage

With the increase of the storage time of lithium-ion battery, the battery is aging and its internal resistance is increasing. The internal resistance of different types of lithium batteries varies to different degrees. After 9 to 10 months of storage, LFP cells have a higher rate of increase in internal resistance than NCA and NCM cells. The increase rate of internal resistance is related to storage time, storage temperature and storage SOC.

4. Cycle

The effect of temperature on the internal resistance of the battery is the same regardless of the storage or cycle. The higher the cycle temperature is, the greater the increase rate of internal resistance will be. The influence of different cycle intervals on the internal resistance of the battery is also different. The internal resistance of the battery accelerates with the increase of the charge and discharge depth, and the increase of internal resistance is proportional to the increase of the charge and discharge depth.

In addition to the effect of charging and discharging depth in the cycle, the charging cutoff voltage also has an effect: Low or high upper limit of charging voltage will increase the interface impedance of the electrode. Zheng et al. believed that the optimal upper limit of charging voltage in the cycle of LFP/C battery was 3.9~4.3V. Experiments found that the passivation film could not be formed well under the low upper limit of charging voltage. However, a high voltage upper limit will lead to the oxidation and decomposition of electrolyte on the surface of LiFePO4 electrode, resulting in products with low electrical conductivity.

5. Others

The on-board lithium battery will inevitably experience poor road conditions in practical application, but the research found that the vibration environment of lithium battery in the application process has almost no effect on the internal resistance of lithium battery.

Internal resistance is an important parameter to measure the power performance of lithium ion and evaluate the battery life. The larger the internal resistance is, the worse the multiplier performance of the battery will be, and the faster it will increase in storage and recycling use. The internal resistance is related to the battery structure, battery material characteristics and manufacturing process, and changes with the change of ambient temperature and state of charge. Therefore, the development of low internal resistance battery is the key to improve the power performance of the battery, and it is of great practical significance to grasp the variation law of the internal resistance of the battery for the prediction of battery life.