With the use of lithium batteries, the battery performance decays continuously, mainly in terms of capacity decay, internal resistance increase, power decrease, etc. The change of battery internal resistance is influenced by various usage conditions such as temperature and depth of discharge. Therefore, the factors affecting the internal resistance of the battery are described in relation to the structural design of the battery, the performance of the raw materials, the manufacturing process and the use conditions.
The resistance is the resistance to the flow of current through the interior of the battery when the lithium battery is in operation. Generally, the internal resistance of lithium batteries is divided into ohmic internal resistance and polarized internal resistance. Ohmic internal resistance consists of electrode material, electrolyte, diaphragm resistance and the contact resistance of each part. Polarization internal resistance is the resistance caused by polarization during electrochemical reaction, including electrochemical polarization internal resistance and concentration difference polarization internal resistance. The ohmic internal resistance of a cell is determined by the total conductivity of the cell, and the polarised internal resistance of a cell is determined by the solid phase diffusion coefficient of lithium ions in the electrode active material.
Ohmic internal resistance
The ohmic internal resistance is divided into three main components, one is the ionic impedance, the other is the electronic impedance and the third is the contact impedance. If we want the internal resistance of a lithium battery to be as small as possible, then we need to take specific measures to reduce the ohmic internal resistance for these three components.
Ionic impedance
Lithium battery ionic impedance is the resistance to the transfer of lithium ions within the battery. In lithium batteries, lithium ion migration rate and electron conduction rate play an equally important role, ionic impedance is mainly affected by the positive and negative electrode materials, diaphragm and electrolyte. To reduce ionic impedance, the following points need to be addressed.
Ensure good wettability of the cathode and anode materials and electrolyte
If the compaction density is too high, the electrolyte will not be easily infiltrated and the ionic impedance will be increased. For the negative electrode, if the SEI film formed on the surface of the live material during the first charge and discharge is too thick, it will also increase the ionic impedance, which needs to be solved by adjusting the cell formation process.
Influence of the electrolyte
The electrolyte should have the right 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. The electrolyte also needs to be of a low concentration, too high a concentration is also detrimental to its flow and wetting. The conductivity of the electrolyte is the most important factor affecting the ionic impedance, which determines the migration of ions.
Effect of diaphragm on ionic impedance
The main factors influencing the ionic impedance of diaphragms are: electrolyte distribution in the diaphragm, diaphragm area, thickness, pore size, porosity and flexural coefficient. In the case of ceramic diaphragms, it is also necessary to prevent ceramic particles from blocking the pores of the diaphragm to the detriment of ion passage. While ensuring that the electrolyte is sufficiently wetted into the diaphragm, there must not be a residual amount of electrolyte remaining in it, which reduces the efficiency of the use of the electrolyte.
Electronic impedance
Electronic impedance is influenced by many factors and can be improved by materials and processes.
Positive and negative electrode plates
The main factors affecting the electronic impedance of positive and negative electrode plates are: the contact between the live substance and the collector, the live substance itself and the plate parameters. The live material has to be in full contact with the collector decent, which can be considered from the collector copper foil, aluminium foil substrate, and the bonding of the positive and negative electrode paste. Live matter itself porosity, particle surface by-products, and uneven mixing with conductive agents can cause changes in electronic impedance. Pole plate parameters such as too small a density of live matter and large particle gaps are not conducive to electron conduction.
Diaphragm
The main factors affecting the electronic impedance of the diaphragm are: diaphragm thickness, porosity and by-products in the charging and discharging process. The first two are easy to understand. After disassembly of the cell, the diaphragm is often found to be covered with a thick layer of brown material, including graphite negative electrode and its reaction by-products, which can cause blockage of the diaphragm pores and reduce battery life.
Collector substrates
The material, thickness and width of the collector and the degree of contact with the lugs all affect the electrical impedance. The collector needs to be made of a base material that is not oxidised and passivated, otherwise the impedance will be affected. Poor soldering of the copper and aluminium foil to the lugs will also affect the electrical impedance.
Contact impedance
Contact resistance is formed between the contact of the copper and aluminium foil with the live material and requires a focus on the bonding of the positive and negative pastes.
Internal resistance to polarisation
The deviation of the electrode potential from the equilibrium electrode potential when current is passed through the electrode is called the polarisation of the electrode. Polarisation includes ohmic polarisation, electrochemical polarisation and differential concentration polarisation. The polarisation resistance is the internal resistance caused by the polarisation of the positive and negative electrodes of the battery during the electrochemical reaction. It reflects the internal consistency of the battery, but is not applicable in production due to the influence of the operation and method. The internal resistance of polarisation is not a constant and changes over time during charging and discharging due to the composition of the active material, the concentration of the electrolyte and the temperature. The ohmic internal resistance obeys Ohm's law, which states that the polarised internal resistance increases with increasing current density, but is not linear. It often increases linearly with the logarithm of the current density.
Structural design influence
In the structural design of the battery, in addition to the riveting and welding of the structural parts of the battery itself, the number, size and position of the battery lugs directly affect the internal resistance of the battery. To a certain extent, increasing the number of lugs can effectively reduce the internal resistance of the battery. The position of the lugs also affects the internal resistance of the battery. The internal resistance of a wound battery is greatest when the lugs are positioned at the head of the positive and negative electrodes, and compared to a wound battery, a stacked battery is equivalent to several dozen small cells in parallel, and its internal resistance is smaller.
Influence of raw material performance
Anode and cathode active materials
The anode material is the lithium storage side of the lithium battery and determines the performance of the lithium battery. The anode material is mainly coated and doped to improve the electron conduction between the particles. For example, the doping of Ni enhances the strength of the P-O bond, stabilizes the structure of LiFePO4/C and optimizes the cell volume, which can effectively reduce the charge transfer impedance of the cathode material. The large increase in activation polarisation, especially in the negative electrode, is the main reason for the severe polarisation. Reducing the particle size of the cathode can effectively reduce the activation polarisation of the cathode. When the particle size of the cathode solid phase is reduced by half, the activation polarisation can be reduced by 45%. Therefore, in terms of battery design, research into the improvement of the cathode and anode materials themselves is also essential.
Conducting agents
Graphite and carbon black are widely used in the field of lithium batteries due to their good performance. Compared to graphite based conductive agents, carbon black based conductive agents are added to the positive electrode for better battery multiplication performance, as graphite based conductive agents have a flaky particle shape, which causes a larger increase in the pore curvature coefficient at large multiplication rates and is prone to Li liquid phase diffusion process limiting the discharge capacity. The cells with CNTs have a lower internal resistance because the fibrous carbon nanotubes have a linear contact with the active material compared to the point contact between graphite/carbon black and the active material, which reduces the interfacial impedance of the cell.
Collectors
Reducing the interfacial resistance between the collector and the active material and increasing the bond strength between the two is an important means of improving the performance of lithium batteries. Conductive carbon coating on the surface of aluminium foil and corona treatment of aluminium foil can effectively reduce the interfacial impedance of the battery. The use of carbon-coated aluminium foil can reduce the internal resistance of the battery by approximately 65% compared to plain aluminium foil, and can reduce the increase in internal resistance during use of the battery. The AC internal resistance of corona-treated aluminium foil can be reduced by around 20%, and the overall DC internal resistance is low and increases less with depth of discharge in the commonly used 20% to 90% SOC range.
Diaphragm
The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the pores of the diaphragm, and the wettability of the diaphragm is the key to forming a good ion flow channel. Compared to ordinary base films, ceramic and adhesive coated diaphragms not only significantly improve the high temperature shrinkage resistance of the diaphragm, but also enhance the wettability of the diaphragm, adding SiO2 ceramic coating to PP diaphragms can increase the wicking capacity of the diaphragm by 17%. Coating 1μm PVDF-HFP on PP/PE composite diaphragms increases the diaphragm wicking rate from 70% to 82% and reduces the internal resistance of the cell by more than 20%.
In terms of process and usage conditions, the main factors affecting the internal resistance of the battery include
Process factors influence
Pulping
The homogeneity of the paste dispersion during pasting affects whether the conductive agent can be evenly dispersed in the active material and in close contact with it, which is related to the internal resistance of the battery. By increasing high speed dispersion, the uniformity of paste dispersion can be improved and the lower the internal resistance of the cell. The addition of surfactants improves the uniformity of the distribution of the conductive agent in the electrode, which reduces electrochemical polarisation and increases the median discharge voltage.
Coating
The surface density is one of the key parameters in battery design. At a certain capacity, increasing the surface density of the electrodes will inevitably reduce the total length of the collector and the diaphragm, which will reduce the ohmic internal resistance of the battery, so within a certain range, the internal resistance of the battery decreases as the surface density increases. The migration and detachment of solvent molecules during coating and drying is closely related to the oven temperature, which directly affects the distribution of binder and conductive agent within the cell, and therefore the formation of a conductive grid within the cell, so the temperature of coating and drying is also an important process for optimising the performance of the cell.
Roller pressing
To a certain extent, the internal resistance of the cell decreases as the compaction density increases because the distance between the raw material particles decreases as the compaction density increases, the more contact between the particles, the more conductive bridges and channels, and the lower the cell impedance. The control of compaction density is mainly achieved by the thickness of the roll. Different roll thickness has a greater impact on the internal resistance of the battery. When the roll thickness is larger, the contact resistance between the active material and the collector fluid increases due to the failure of the active material to roll tightly, and the internal resistance of the battery increases. The contact resistance between the active material and the fluid collector increases when the active material is not rolled tightly enough. The surface of the positive electrode is cracked after cycling, which further increases the contact resistance between the active material and the fluid collector.
Cell turnover time
When set aside for a short period of time, the internal resistance of the battery increases more slowly due to the influence of the carbon coating layer on the surface of lithium iron phosphate and lithium iron phosphate; when set aside for a longer period of time (more than 23h), the internal resistance of the battery increases more significantly due to the reaction between lithium iron phosphate and water and the bonding effect of the adhesive. Therefore, the actual production needs to strictly control the turnover time of the pole piece.
Liquid injection
The ionic conductivity of the electrolyte determines the internal resistance and multiplicity of the cell. In addition to the optimization of conductivity, the amount of liquid injection and the wetting time after liquid injection also directly affect the internal resistance of the battery.
Use conditions affect
Temperature
The lower the temperature, the slower the ion transport within the battery, and the greater the internal resistance of the battery. Cell impedance can be divided into bulk phase impedance, SEI film impedance and charge transfer impedance. Bulk phase impedance and SEI film impedance are mainly influenced by the ionic conductivity of the electrolyte and the trend at low temperatures is the same as the trend of the electrolyte conductivity. Compared to the increase in bulk phase impedance and SEI membrane impedance at low temperatures, the charge response impedance increases more significantly with decreasing temperature, and below -20°C, the charge response impedance accounts for almost 100% of the total internal resistance of the cell.
SOC
When the battery is in different SOC, its internal resistance size is also different, especially the DC internal resistance directly affects the power performance of the battery, which in turn reflects the battery performance in the actual state: the DC internal resistance of Li-ion battery increases with the increase of the battery discharge depth DOD, and the internal resistance size is basically unchanged in the discharge interval of 10%~80%, and generally increases significantly in the deeper discharge depth.
Storage
As the storage time of lithium-ion batteries increases, the batteries continue to age and their internal resistance increases. Different types of lithium-ion batteries have different degrees of internal resistance change. After a long storage period of 9-10 months, the rate of increase in internal resistance is higher for LFP cells than for NCA and NCM cells. The rate of increase in internal resistance is related to storage time, storage temperature and storage SOC.
Cycling
The effect of temperature on the internal resistance of a battery is the same whether it is stored or cycled, the higher the cycling temperature the greater the rate of increase in internal resistance. The effect on internal resistance varies between different cycling intervals. The increase in internal resistance accelerates with increasing depth of charge and discharge, and the increase in internal resistance is proportional to the strengthening of the depth of charge and discharge. In addition to the influence of the depth of charge and discharge in the cycle, the voltage as of charge also has an influence: too low or too high an upper charge voltage will make the interfacial resistance of the electrode increase, too low an upper voltage will not be able to form a passivation film well, while too high an upper voltage will cause the electrolyte to oxidise and decompose on the LiFePO4 electrode surface to form a product with low conductivity.
Other
Inevitably, automotive lithium batteries will experience poor road conditions in practice, but research has found that the vibration environment of lithium batteries has little effect on the internal resistance of lithium batteries in the application process.
Outlook
Internal resistance is an important parameter for measuring Li-ion power performance and assessing battery life. The higher the internal resistance, the poorer the multiplier performance of the battery, and the faster it increases in storage and cyclic use. Internal resistance is related to cell structure, cell material properties and manufacturing process, and varies with ambient temperature and charge state. Therefore, the development of low internal resistance batteries is the key to improve the power performance of batteries, and at the same time, the knowledge of the variation of internal resistance of batteries is of great practical importance for battery life prediction.