Why does the Internal Resistance of Lithium Batteries Become Larger?

In recent years, the requirements for the high magnification charge and discharge performance of power batteries have become higher and higher, and the internal resistance is an important factor affecting the power performance and discharge efficiency of the battery. Its initial size is mainly determined by the structural design of the battery, the performance of raw materials and the process technology.

With the use of lithium batteries, the performance of the battery continues to decay, mainly manifested as capacity attenuation, internal resistance increase, power decrease, etc. , The change of the internal resistance of the battery is affected by various conditions of use such as temperature and discharge depth. Therefore, combining the aspects of battery structure design, raw material performance, process technology and use conditions, the factors affecting the internal resistance of the battery are expounded.

Resistance is the resistance of current flowing through the battery when the lithium battery is working. Generally, the internal resistance of lithium batteries is divided into ohmic internal resistance and polarized internal resistance. The ohmic internal resistance is composed of electrode material, electrolyte, diaphragm resistance and the contact resistance of each part. The internal resistance of polarization refers to the resistance caused by polarization during electrochemical reactions, including the internal resistance of electrochemical polarization and the internal resistance of concentration polarization. The ohmic internal resistance of the battery is determined by the total conductivity of the battery, and the polarization internal resistance of the battery is determined by the solid phase diffusion coefficient of lithium ions in the electrode active material.

Ohm Internal Resistance

The ohmic internal resistance is mainly divided into three parts, one is the ion impedance, the other is the electronic impedance, and the third is the contact impedance. We hope that the smaller the internal resistance of lithium batteries, the smaller the internal resistance, so we need to take specific measures for these three contents to reduce the ohmic internal resistance.

Ion Impedance

Lithium battery ion impedance refers to the resistance of lithium ions to be transmitted inside the battery. In lithium batteries, the migration speed of lithium ions and the electron conduction speed play an equally important role. The ion impedance is mainly affected by the positive and negative electrode materials, the diaphragm, and the electrolyte. If you want to reduce the ion impedance, you need to do the following:

Ensure that the positive and negative electrode materials and electrolytes have good wettability. When designing the pole piece, it is necessary to select the appropriate compaction density. If the compaction density is too large, the electrolyte is not easy to infiltrate, which will increase the ion impedance. For the negative electrode sheet, if the SEI film formed on the surface of the living substance is too thick during the first charge and discharge, the ion impedance will also be increased. At this time, the battery formation process needs to be adjusted to solve it.

Effect of Electrolyte

The electrolyte must have the appropriate concentration, viscosity and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration between it and the positive and negative active substances. At the same time, the electrolyte also needs a lower concentration, and the concentration is too high, which is also not conducive to its flow infiltration. The conductivity of the electrolyte is the most important factor affecting the ion impedance, which determines the migration of ions.

Effect of Diaphragm on Ion Impedance

The main influencing factors of the diaphragm on the ion impedance are: the distribution of electrolyte in the diaphragm, the area of the diaphragm, the thickness, the pore size, the porosity, and the tortuous coefficient. For ceramic diaphragms, it is also necessary to prevent ceramic particles from clogging the pores of the diaphragm, which is not conducive to the passage of ions. While ensuring that the electrolyte fully infiltrates the diaphragm, no excess electrolyte can remain in it, reducing the efficiency of the electrolyte.

ELECTRONIC IMPEDANCE

There are many influencing factors of electronic impedance, which can be improved from materials, processes, etc.

POSITIVE AND NEGATIVE PLATES

The main factors that affect the electronic impedance of the positive and negative electrode plates are: the contact between the living substance and the collector fluid, the factors of the living substance itself, the plate parameters, etc. The living substance should be in full contact with the collector surface, and the adhesion of the positive and negative slurry can be considered from the collector copper foil and aluminum foil substrate. The porosity of the living substance itself, the by-products of the particle surface, and the uneven mixing with the conductive agent will all cause changes in electronic impedance. Plate parameters such as the density of living matter is too small, and the particle gap is large, which is not conducive to electron conduction.

DIAPHRAGM

The main influencing factors of the diaphragm on the electronic impedance are: the thickness of the diaphragm, the porosity, and the by-products in the charge and discharge process. The first two are easy to understand. After the battery is disassembled, it is often found that the diaphragm is stained with a thick layer of brown material, which includes the graphite negative electrode and its reaction byproducts, which can cause clogging of the diaphragm hole and reduce the battery life.

COLLECTOR SUBSTRATE

The material, thickness, width, and degree of contact between the collector fluid and the pole ear all affect the electronic impedance. The collector needs to choose a substrate that is not oxidized and passivated, otherwise it will affect the impedance. Poor welding of copper, aluminum foil and pole ears can also affect the electronic impedance.

CONTACT IMPEDANCE

The contact resistance is formed between the contact of copper aluminum foil and the living substance, and it is necessary to pay attention to the adhesion of the positive and negative electrode slurry.

POLARIZATION INTERNAL RESISTANCE

When current passes through the electrode, the phenomenon of the electrode potential deviating from the equilibrium electrode potential is called the polarization of the electrode. Polarization includes ohmic polarization, electrochemical polarization, and concentration difference polarization. Polarization resistance refers to the internal resistance caused by the polarization of the positive and negative electrodes of the battery during the electrochemical reaction. It can reflect the consistency inside the battery, but due to the influence of operation and method, it is not suitable for production. The internal resistance of polarization is not constant, and it changes continuously over time during the charge and discharge process. This is because the composition of the active substance, the concentration and temperature of the electrolyte are constantly changing. Ohm’s internal resistance obeys Ohm’s law, and the polarization internal resistance increases with the increase of current density, but it is not a linear relationship. It often increases linearly with the increase of the logarithm of the current density.

STRUCTURAL DESIGN IMPACT

In the design of the battery structure, in addition to the riveting and welding of the battery structure itself, the number, size, and position of the battery pole ears directly affect the internal resistance of the battery. To a certain extent, increasing the number of pole ears can effectively reduce the internal resistance of the battery. The position of the pole ear also affects the internal resistance of the battery. The internal resistance of the winding battery with the pole ear position at the head of the positive and negative pole piece is the largest, and compared with the winding battery, the laminated battery is equivalent to dozens of small batteries in parallel, and its internal resistance is smaller.

Impact of raw material performance Positive and negative active materials The cathode material of lithium batteries is the lithium storage party, which determines the performance of lithium batteries more. The cathode material mainly improves the electron conduction ability between the particles through coating and doping. For example, after Ni is doped, the strength of the P-O bond is enhanced, the structure of LiFePO4/C is stabilized, the unit cell volume is optimized, and the charge transfer impedance of the cathode material can be effectively reduced. The activation polarization, especially the significant increase in the activation polarization of the negative electrode, is the main reason for the serious polarization. Reducing the particle size of the negative electrode can effectively reduce the activation polarization of the negative electrode. When the particle size of the negative electrode solid phase is reduced by half, the activation polarization can be reduced by 45%. Therefore, as far as battery design is concerned, the improvement research of the positive and negative electrode materials themselves is also essential.

CONDUCTIVE AGENT

Graphite and carbon black are widely used in the field of lithium batteries because of their good properties. Compared with graphite conductive agents, the battery magnification performance of carbon black conductive agents added to the positive electrode is better, because graphite conductive agents have a sheet-like particle morphology, which causes a large increase in pore torsion coefficient at large magnification, and it is prone to the phenomenon of limiting the discharge capacity during the diffusion of the liquid phase. The internal resistance of a battery with CNTs added is smaller, because compared with the point contact between graphite/carbon black and the active material, the fibrous carbon nanotubes are in line contact with the active material, which can reduce the interface impedance of the battery.

COLLECTOR

Reducing the interface resistance between the collector fluid and the active substance and improving the bond strength between the two is an important means to improve the performance of lithium batteries. Coating the surface of the aluminum foil with a conductive carbon coating and corona treatment of the aluminum foil can effectively reduce the interface impedance of the battery. Compared with ordinary aluminum foil, the use of carbon-coated aluminum foil can reduce the internal resistance of the battery by about 65%, and can reduce the increase in the internal resistance of the battery during use. The AC internal resistance of aluminum foil treated with corona can be reduced by about 20%. In the range of 20% to 90% SOC commonly used, the DC internal resistance as a whole is too small and the increase gradually decreases with the increase of the discharge depth.

DIAPHRAGM

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

From the perspective of process technology and use conditions, the factors that affect the internal resistance of the battery mainly include:

PROCESS FACTORS AFFECT COMBINED PULP

When the slurry is combined, the uniformity of the dispersion of the slurry 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 slurry dispersion can be improved, and the smaller the internal resistance of the battery. The uniformity of the distribution of conductive agents in the electrode can be improved by adding surfactants, and the electrochemical polarization can be reduced and the discharge median voltage can be increased.

COATING

Surface density is one of the key parameters of battery design. When the battery capacity is certain, increasing the surface density will inevitably reduce the total length of the collector and diaphragm, and the ohmic internal resistance of the battery will decrease accordingly. Therefore, within a certain range, the internal resistance of the battery decreases 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 polar sheet, thereby affecting the formation of the conductive grid inside the polar sheet. Therefore, the temperature of coating and drying is also an important process to optimize battery performance.

ROLL PRESSURE

To a certain extent, the internal resistance of the battery decreases with the increase of the compaction density. Because the compaction density increases, the distance between the particles of the raw material decreases. The more contact between the particles, the more conductive bridges and channels, and the battery impedance decreases. The control of compaction density is mainly achieved by rolling thickness. Different roll pressure thicknesses have a greater impact on the internal resistance of the battery. When the roll pressure thickness is large, the contact resistance between the active substance and the collector fluid increases due to the failure of the active substance to roll tightly, and the internal resistance of the battery increases. Moreover, cracks occur on the surface of the positive electrode of the battery with a larger rolling thickness after the battery cycle, which will further increase the contact resistance between the active substance on the surface of the electrode sheet and the collector fluid.

Pole Piece Turnaround Time

The different shelving times of the positive electrode sheet have a greater impact on the internal resistance of the battery. When the shelving time is short, the internal resistance of the battery increases more slowly due to the influence of the carbon coating on the surface of lithium iron phosphate and the force of lithium iron phosphate; when the shelving time is long (greater than 23h), the internal resistance of the battery increases more significantly due to the reaction of lithium iron phosphate with water and the adhesion of the adhesive. Therefore, the turnover time of the pole piece needs to be strictly controlled in actual production.

Injection

The ionic conductivity of the electrolyte determines the internal resistance and magnification characteristics of the battery. The size of the electrolyte conductivity is inversely proportional to the viscosity of the solvent, and it is also affected by the lithium salt concentration and the size of the anion. In addition to the optimization research on electrical conductivity, the injection volume and the infiltration time after the injection also directly affect the internal resistance of the battery. A small injection volume or insufficient infiltration time will cause the internal resistance of the battery to be too large, thereby affecting the battery’s capacity.

Impact of use conditions

TEMPERATURE

The influence of temperature on the size of 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. The battery impedance can be divided into bulk phase impedance, SEI membrane impedance, and charge transfer impedance. The bulk phase impedance and SEI membrane impedance are mainly affected by the ion conductivity of the electrolyte, and the trend of change at low temperatures is consistent with the trend of change of electrolyte conductivity. Compared with the increase of bulk phase impedance and SEI membrane impedance at low temperatures, the charge reaction impedance increases more significantly with the decrease of temperature. Below -20℃, the charge reaction impedance accounts for almost 100% of the total internal resistance of the battery.

SOC

When the battery is in different SoCs, the size of its internal resistance is not the same, especially the DC internal resistance directly affects the power performance of the battery, which in turn reflects the battery performance of the battery in the actual state: the DC internal resistance of the lithium battery increases with the increase of the battery discharge depth DOD, and the internal resistance is basically the same in the discharge interval of 10%~80%, and generally the internal resistance increases significantly at the deeper discharge depth.

STORAGE

With the increase of the storage time of lithium-ion batteries, the batteries continue to age and their internal resistance continues to increase. The internal resistance of different types of lithium batteries varies to different degrees. After a long period of storage from September to October, 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.

CYCLE

Regardless of whether it is storage or circulation, the influence of temperature on the internal resistance of the battery is the same. The higher the circulation 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 accelerates with the increase of the depth of charge and discharge, and the increase of the internal resistance is proportional to the increase of the depth of charge and discharge. In addition to the influence of the depth of charge and discharge in the cycle, the upper limit of the charging voltage also has an impact: too low or too high the upper limit of the charging voltage will increase the interface impedance of the electrode, and the upper limit of the voltage is too low to form a passivation film well, and the upper limit of the voltage is too high will cause the electrolyte to oxidize and decompose on the surface of the LiFePO4 electrode to form a product with low conductivity.

The internal resistance is an important parameter for measuring the power performance of lithium-ion batteries and evaluating battery life. The greater the internal resistance, the worse the magnification performance of the battery, and the faster it increases in storage and recycling. The internal resistance is related to the battery structure, battery material characteristics and manufacturing process, and changes with the changes of ambient temperature and state of charge. Therefore, the development of low internal resistance batteries is the key to improving battery power performance. At the same time, mastering the law of changes in battery internal resistance is of great practical significance for battery life prediction.