With the use of lithium batteries, battery performance continuously degrades, mainly manifested in capacity decay, increased internal resistance, and decreased power. Changes in battery internal resistance are affected by various usage conditions such as temperature and depth of discharge. Therefore, this article mainly discusses the factors influencing battery internal resistance from the aspects of battery structure design, raw material performance, manufacturing process, and usage conditions.

Structural design influence

   In battery structure design, besides the riveting and welding of the battery structure itself, the number, size, and position of the battery tabs directly affect the battery's internal resistance. To a certain extent, increasing the number of tabs can effectively reduce the battery's internal resistance. The position of the label also affects the battery's internal resistance. Wire-wound batteries with the electrodes located at the ends of the positive and negative electrodes have high internal resistance. Compared to wire-wound batteries, stacked batteries are equivalent to dozens of smaller batteries. 

Parallel connection results in lower internal resistance.

Impact of raw material properties

1. Positive and negative electrode active materials

     The cathode material in a lithium-ion battery is responsible for lithium storage and significantly determines the battery's performance. Cathode materials primarily improve interparticle electronic conductivity through coating and doping. For example, Ni doping enhances the strength of the PO bond, stabilizes the LiFePO4/C structure, optimizes battery volume, and effectively reduces the charge transfer resistance of the cathode material. Simulation analysis using an electrochemical-thermal coupling model reveals that under high-rate discharge conditions, activation polarization increases significantly, especially at the negative electrode, which is the main cause of severe polarization. Reducing the particle size of the negative electrode can effectively reduce its active polarization. When the solid-phase particle size of the negative electrode is reduced by half, the active polarization can be reduced by 45%.

Therefore, in battery design, research on improvements to the positive and negative electrode materials themselves is also essential.

2. Conductive agent

Graphite and carbon black are widely used in lithium-ion batteries due to their excellent properties. Compared with graphite-based conductive agents, cathodes using carbon black-based conductive agents exhibit better battery rate performance because the particle morphology of graphite-based conductive agents is plate-like, resulting in a high porosity and facilitating Li liquid-phase diffusion.

The phenomenon of process limitations on discharge capacity. Batteries with added carbon nanotubes have lower internal resistance because, compared to the point contact between graphite/carbon black and the active material, the fibrous carbon nanotubes form a line contact with the active material, which can reduce interfacial impedance.

3. Collective fluid

Reducing the interfacial resistance between the current collector and the active material, and improving the bonding strength between them, are important means to enhance the performance of lithium batteries. Coating the aluminum foil with a conductive carbon coating and subjecting the aluminum foil to corona treatment can effectively reduce the battery's interfacial impedance. Compared to ordinary aluminum foil, using carbon-coated aluminum foil can reduce the battery's internal resistance by about 65%, and can also reduce the increase in internal resistance during battery use.

The AC internal resistance of aluminum foil treated with corona discharge can be reduced by about 20%. In the commonly used range of 20% to 90% SOC, the DC internal resistance is generally small and the increase gradually decreases with the increase of discharge depth.

4. Diaphragm

Ion conduction within the battery relies on the diffusion of Li ions from the electrolyte through the porous membrane. The membrane's liquid absorption and wetting ability is crucial for forming good ion flow channels. Higher liquid absorption and a porous structure in the membrane enhance conductivity, reduce battery impedance, and improve rate performance. Compared to ordinary base membranes, ceramic and coated membranes significantly improve high-temperature shrinkage resistance and enhance liquid absorption and wetting ability. Adding a SiO2 ceramic coating to a PP membrane increases liquid absorption by 17%. Coating a 1μm thick PVDF-HFP layer onto a PP/PE composite membrane increases liquid absorption from 70% to 82% and reduces cell internal resistance by over 20%.

Process factors

1. Mixing paste

The uniformity of slurry dispersion during mixing affects whether the conductive agent can be uniformly dispersed in the active material and in close contact with it, which is related to the battery's internal resistance. Increasing high-speed dispersion can improve the uniformity of slurry dispersion, resulting in lower battery internal resistance. Adding surfactants can improve the uniformity of conductive agent distribution in the electrode, reducing electrochemical polarization and increasing the median discharge voltage.

2. Coating

Areal density is one of the key parameters in battery design. With a constant battery capacity, increasing the areal density of the electrodes will inevitably reduce the total length of the current collector and separator, and the ohmic resistance of the battery will decrease accordingly. Therefore, within a certain range, the internal resistance of the battery decreases with increasing areal density. 

The migration and separation of solvent molecules during coating and drying are closely related to the oven temperature, directly affecting the distribution of binders and conductive agents in the electrode, and consequently influencing the formation of the conductive mesh within the electrode. Therefore, the coating and drying process temperature is also a crucial factor in optimizing battery performance.

3 Roller pressing

To a certain extent, the internal resistance of a battery decreases with increasing compaction density. This is because increased compaction density reduces the distance between raw material particles, resulting in more contact between particles, more conductive bridges and channels, and thus lower battery impedance. Controlling compaction density is primarily achieved through rolling thickness. Different rolling thicknesses have a significant impact on battery internal resistance. With a larger rolling thickness, the active material is not rolled tightly enough, increasing the contact resistance between the active material and the current collector, thereby increasing the battery's internal resistance. Furthermore, after cycling, cracks may appear on the surface of the positive electrode of a battery with a larger rolling thickness, further increasing the contact resistance between the active material and the current collector on the electrode surface.

4. Electrode Turnover Time

Different storage times for the positive electrode have a significant impact on the battery's internal resistance. When the storage time is short, the battery's internal resistance increases slowly due to the effect of lithium iron phosphate and its carbon coating layer. When the storage time exceeds 23 hours, the battery's internal resistance increases significantly due to the combined effects of the reaction between lithium iron phosphate and water and the bonding effect of the binder. Therefore, in actual production, it is necessary to strictly control the electrode turnover time.

5. Injection solution

The ionic conductivity of the electrolyte determines the battery's internal resistance and rate performance. The electrolyte conductivity is inversely proportional to the solvent viscosity and is also affected by the lithium salt concentration and anion size. Besides optimizing conductivity, the injection volume and subsequent wetting time directly impact the battery's internal resistance. Insufficient injection volume or wetting time will result in excessive internal resistance, thus affecting battery capacity.

Usage conditions affect:

1. Temperature

The effect of temperature on internal resistance is obvious; the lower the temperature, the slower the ion transport inside the battery, and the greater the internal resistance. Battery impedance can be divided into bulk impedance, SEI film impedance, and charge transfer impedance. Bulk impedance and SEI film impedance are mainly affected by the electrolyte ionic conductivity, and their changing trends at low temperatures are consistent with those of the electrolyte conductivity.

2 SOC

When a battery is at different SOC (State of Charge), its internal resistance is also different. In particular, the DC internal resistance directly affects the battery's power performance, and thus reflects the battery's performance under actual conditions: the DC internal resistance of a lithium battery increases with the increase of the depth of discharge (DOD). In the discharge range of 10% to 80%, the internal resistance remains basically unchanged, while the internal resistance increases significantly at deeper discharge depths.

3. Storage

As lithium-ion batteries age, their internal resistance increases. The degree of internal resistance change varies among different types of lithium batteries. After 9-10 months of storage, the internal resistance increase rate of LFP batteries is higher than that of NCA and NCM batteries. The rate of internal resistance increase is related to storage time, storage temperature, and storage state of charge (SOC). Stroe et al. quantified this relationship through a 24-36 month storage study of LFP/C batteries (see below):

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Temperature is measured in Kelvin (K), SOC is measured as a percentage, and time is measured in months.

4 cycles

Whether in storage or cycling, temperature has the same effect on battery internal resistance. The higher the cycling temperature, the greater the rate of increase in internal resistance. Different cycle intervals have different effects on battery internal resistance. Battery internal resistance increases with increasing depth of charge/discharge, and the increase in internal resistance is directly proportional to the increase in depth of charge/discharge.

Besides the influence of the depth of charge and discharge during cycling, the charging cut-off voltage also has an impact: both excessively low and excessively high upper limits of the charging voltage will increase the interfacial impedance of the electrodes. Zheng et al. believed that the upper limit of the cyclic charging voltage for LFP/C batteries is 3.9~4.3V. Experiments showed that an excessively low upper limit voltage cannot effectively form a passivation film, and the upper limit voltage will cause the electrolyte to oxidize and decompose on the LiFePO4 electrode surface, forming products with low conductivity.

5. Other

In practical applications, automotive lithium batteries inevitably encounter poor road conditions. However, research has found that the vibration environment of lithium batteries has almost no impact on their internal resistance during use. Internal resistance is an important parameter for measuring the dynamic performance of lithium-ion batteries and evaluating battery life. The higher the internal resistance, the worse the battery's rate performance, and the faster it increases during storage and recycling. Internal resistance is related to battery structure, battery material characteristics, and manufacturing process, and varies with ambient temperature and state of charge. 

Therefore, developing low internal resistance batteries is key to improving battery power performance, and understanding the variation law of battery internal resistance is of great practical significance for predicting battery life.