Factors Affecting the Internal Resistance of Lithium Batteries
The performance of battery keep decaying over time. It is mainly manifested as capacity decay, internal resistance increase, power drop, etc. The change of battery internal resistance is affected by various usage conditions like temperature and depth of discharge.
This article will discuss the factors that affect the battery internal resistance from battery structure design, raw material performance, manufacturing process and use conditions.
1. The influence of structural design
In the battery structure design, besides the riveting and welding, the number, size, and location of the battery tabs directly affect the internal resistance. To a certain extent, increasing the number of tabs can effectively reduce the internal resistance of the battery. The position of the tab can also affect the internal resistance of the battery. The internal resistance of the winding battery with the tab position at the head of the positive and negative pole pieces is the largest. Compared with the winding battery, the laminated battery is equivalent to dozens of small batteries in parallel. Its internal resistance is smaller.
2. The impact of raw material performance
(1) Positive and negative active materials
The positive electrode material is the one that stores Li, which determines the performance of the lithium battery. The positive electrode material improves the electronic conductivity between particles mainly through coating and doping. For example, doping with Ni enhances the strength of the P-O bond, stabilizes the structure of LiFePO4/C, optimizes the cell volume. What’s more, it can effectively reduce the charge transfer resistance of the positive electrode material.
The simulation analysis of electrochemical thermal coupling model tells us the following conclusion. Under high-rate discharge conditions, the significant increase in activation polarization, especially of the negative electrode, is the main reason for the serious polarization. Reducing the particle size of the negative electrode can effectively reduce the active polarization of the negative electrode. When the solid phase particle size of the negative electrode is reduced by half, the active polarization can be reduced by 45%. Therefore, in terms of battery design, research on the improvement of the positive and negative materials themselves is also indispensable.
coating and doping. For example, doping with Ni enhances the strength of the P-O bond, stabilizes the structure of LiFePO4/C, optimizes the cell volume. What’s more, it can effectively reduce the charge transfer resistance of the positive electrode material.
(2) Conductive agent
Graphite and carbon black are widely used in the field of lithium batteries because of their good properties. Compared with graphite-based conductive agent, the positive electrode with carbon black-based conductive agent has better battery rate performance. Because graphite-based conductive agent has a flaky particle morphology, which causes a large increase in pore tortuosity at a large rate. Then, it’s likely that the diffusion process of Li liquid will limit the discharge capacity. The battery added CNTs has lower internal resistance. And it’s because compared to the point contact between graphite/carbon black and the active material, the fibrous carbon nanotubes are in line contact with the active material. And that can reduce the interface impedance of the battery.
To improve the performance of lithium batteries, we can reduce the interface resistance between the current collector and the active material. And then improve the bonding strength between the two. Covering a conductive carbon coating on the surface of the aluminum foil and corona treatment on the aluminum foil can effectively reduce the interface impedance of the battery. Compared with ordinary aluminum foil, using carbon-coated aluminum foil can reduce the internal resistance by about 65%. Besides, it can reduce the increase in the internal resistance of the battery during use.
The AC internal resistance of corona treated aluminum foil can be reduced by about 20%. Within the commonly used 20%~90% SOC range, the overall DC internal resistance is relatively small and the increase is gradually smaller as the depth of discharge increases.
The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the porous diaphragm. The liquid absorption and wetting ability of the diaphragm is the key to forming a good ion flow channel. When the diaphragm has a higher liquid absorption rate and porous structure, it can improve the conductivity. Also, it can reduce the battery impedance and improves battery rate performance. Compared with ordinary base membranes, ceramic diaphragms and rubber-coated diaphragms can not only greatly improve the high temperature shrinkage resistance of the diaphragm, but also enhance its liquid absorption and wetting ability. The addition of SiO2 ceramic coating on the PP diaphragm can increase the liquid imbibition of diaphragm by 17%. Coating 1μm PVDF-HFP on the PP/PE composite diaphragm, the liquid absorption rate of the diaphragm is increased from 70% to 82%. While the internal resistance of the cell is reduced by more than 20%.
3. The influence of process factors
The uniformity of the slurry dispersion during mixing affects whether the conductive agent can be uniformly dispersed in the active material and closely contact with it. What’s more, it’s related to the internal resistance of the battery. By increasing the high-speed dispersion, the uniformity of the slurry dispersion can be improved with the internal resistance of the battery getting smaller. Adding a surfactant can improve the uniformity of the distribution of the conductive agent in the electrode. Besides, it also can reduce the electrochemical polarization and increase the median discharge voltage.
Areal density is one of the key parameters of battery design. With the constant battery capacity, increasing the surface density of the pole pieces will inevitably reduce the total length of the current collector and the diaphragm. And the ohmic resistance of the battery will decrease accordingly. Therefore, within a certain range, the internal resistance of the battery decreases as the areal density increases. The migration and separation of solvent molecules during coating and drying is closely related to the temperature of the oven. And that directly affects the distribution of binder and conductive agent in the pole piece. Then, it affects the formation of the conductive grid inside the pole piece. Therefore, the temperature of coating and drying process is also an important process for optimizing battery performance.
To a certain extent, the internal resistance of the battery decreases as the compaction density increases. Because if the compaction density increases, the distance between the raw material particles will decrease. Then, there will be more contact between the particles, more conductive bridges and channels. And finally, the impedance is reduced. The control of compaction density is mainly achieved by rolling thickness. Different rolling thicknesses have a great impact on the internal resistance of the battery. When the rolling thickness is large, the contact resistance between the active material and the current collector increases due to the tight rolling failure of the active material. Also, 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. That will further increase the contact resistance between the active substance on the electrode surface and the fluid collector.
(4) Pole piece turnaround time
The different deposited time of the positive electrode can greatly influence the internal resistance of the battery. When the deposited time is short, the internal resistance will increase slowly due to the interaction between the surface carbon coating and LiFePO4. When the battery is left for a long time (more than 23h), the internal resistance of the battery increases significantly due to the combined effect of the reaction between lithium iron phosphate and water and the adhesion of the binder. Therefore, it is necessary to strictly control the turnaround time of pole pieces in actual production.
The ionic conductivity of the electrolyte determines the internal resistance and rate characteristics of the battery. The conductivity of the electrolyte is inversely proportional to the viscosity of the solvent. At the same time, it’s also affected by the lithium salt concentration and the size of anion. In addition to the optimization research on the conductivity, the injection volume and the infiltration time after injection also directly affect the internal resistance. Small injection volume or insufficient infiltration time will cause the overly large internal resistance, thereby affecting the capacity of battery.
4. The influence of use conditions
The influence of temperature on the internal resistance is obvious. The lower the temperature, the slower the ion transmission inside the battery and the greater the internal resistance of the battery. Battery impedance can be divided into bulk impedance, SEI membrane impedance, and charge transfer impedance. The bulk impedance and SEI membrane impedance are mainly affected by electrolyte ionic conductivity. The change trend at low temperature is consistent with the change trend of electrolyte conductivity. Compared with the increase of bulk impedance and SEI film resistance at low temperatures, the charge reaction impedance increases more significantly with the decrease in temperature. Below -20°C, the charge reaction impedance accounts for almost 100% of the battery’s total internal resistance.
When the battery is in different SOC, its internal resistance is also different. Especially the DC internal resistance directly affects the power performance of the battery, and then reflects the battery performance in the actual state. The DC internal resistance of the lithium battery varies with the depth of discharge DOD of the battery. The internal resistance is basically unchanged in the 10%~80% discharge interval. Generally, the internal resistance increases significantly at a deeper discharge depth.
As the storage time of lithium-ion batteries increases, the batteries continue to age with their internal resistance increasing. Different types of lithium batteries have different degrees of change in internal resistance. After a long period of storage for 9-10 months, the internal resistance increase rate of LFP batteries is higher than that of NCA and NCM batteries. The increase rate of internal resistance is related to storage time, storage temperature and storage SOC. Stroe et al. quantified the relationship between them through a storage study of LFP/C batteries for 24 to 36 months:
Among them, the temperature unit is K, the unit of SOC is percentage, and time unit is month.
Whether it is storage or cycling, the temperature has the same effect on the internal resistance of the battery. The higher the cycle temperature, the greater the increase rate of internal resistance. Different cycle intervals have different effects on the internal resistance of the battery. The internal resistance of the battery increases with the increase of the depth of charge and discharge. Besides, the increase of the internal resistance is proportional to the increase of the depth of charge and discharge.
In addition to the impact of the depth of charge and discharge in the cycle, the charge cut-off voltage also has an impact. A overly low or overly high upper limit of the charging voltage will increase the interface impedance of the electrode. Some believe that the optimal upper limit of the charging voltage for the LFP/C battery in the cycle is 3.9~4.3V. Experiments have shown that the passivation film cannot be formed well under the upper limit voltage that is too low. While the upper limit voltage will cause the electrolyte to oxidize and decompose on the surface of the LiFePO4 electrode to form products with low conductivity.
Vehicle-mounted lithium batteries will inevitably experience poor road conditions in practical applications. But studies have found that the vibration environment has almost no effect on the internal resistance of the lithium battery during the application.