Research and decomposition of internal resistance characteristics of power lithium-ion batteries -Lithium - Ion Battery Equipment
In the actual use process, the change of the internal resistance of the battery is also affected by various factors such as temperature and SOC. This article is an overview of the research results of internal resistance characteristics in recent years by relevant technical staff of Guoxuan Hi-Tech.
Electric vehicles have the advantages of no pollution, low noise, high energy efficiency, and simple structure, and have become an important development direction of the automobile industry. In recent years, new energy vehicles in the market have higher and higher requirements for high-rate charge and discharge performance of power lithium batteries, and internal resistance is an important factor affecting battery power performance and discharge efficiency. Its initial size is mainly determined by the structural design of the battery, raw materials Performance and process technology decision.
With the use of lithium-ion batteries, the battery performance continues to decline, mainly manifested as capacity decay, internal resistance increase, power drop, etc. The change of battery internal resistance is affected by various conditions such as temperature and discharge depth. Therefore, this paper mainly expounds the factors affecting the internal resistance of batteries from the aspects of battery structure design, raw material performance, process technology and service conditions.(Lithium - Ion Battery Equipment)
1. 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 tabs directly affect the internal resistance of the battery. To a certain extent, increasing the number of tabs can effectively reduce the internal resistance of the battery. The position of the tabs can also affect the internal resistance of the battery. The internal resistance of the coiled battery with the tab position at the head of the positive and negative electrodes is the largest. Compared with the coiled battery, the laminated battery is equivalent to dozens of small batteries. Connected in parallel, the internal resistance is smaller.
2. Effect of raw material performance
1. Positive and negative active materials
In lithium-ion batteries, the cathode material is the lithium storage side, which determines the performance of the lithium-ion battery more. The cathode material mainly improves the electronic conductivity between particles through coating and doping. For example, after doping Ni, the strength of the P-O bond is strengthened, the structure of LiFePO4/C is stabilized, the volume of the unit cell is optimized, and the charge transfer resistance of the positive electrode material can be effectively reduced.
However, through the simulation decomposition of the electrochemical thermal coupling model, it is known that under high-rate discharge conditions, the significant increase in activation polarization, especially the activation polarization of the negative electrode, is an important 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, in terms of battery design, research on the improvement of the positive and negative electrode materials themselves is also essential.
2. Conductive agent
Graphite and carbon black are widely used in lithium-ion batteries because of their good properties. Compared with graphite-based conductive agents, the battery rate performance of adding carbon black-based conductive agents to the positive electrode is better, because graphite-based conductive agents have a flaky particle shape, which causes a large increase in the pore meandering coefficient at high magnifications, and is prone to Li liquid phase A phenomenon in which the diffusion process limits the discharge capacity. The internal resistance of the battery added with CNTs is smaller, because compared to the point contact between graphite/carbon black and the active material, the fibrous carbon nanotube and the active material are in line contact, which can reduce the interface impedance of the battery.
3. Collector
Reducing the interfacial resistance between the current collector and the active material and improving the bond strength between the two are critical means to improve the performance of lithium-ion batteries. Coating conductive carbon coating on the surface of aluminum foil and corona treatment of aluminum foil can effectively reduce the interface resistance 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 internal resistance of the battery during use.
The AC internal resistance of aluminum foil treated by corona can be reduced by about 20%. In the commonly used range of 20%-90% SOC, the overall DC internal resistance is relatively small, and the increase is gradually smaller with the increase of the discharge depth.
4. Diaphragm
The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the pores of the separator. The liquid absorption and wetting ability of the separator is the key to forming a good ion flow channel. When the separator has a higher liquid absorption rate and porous structure, it can be improved. The conductivity reduces the battery impedance and improves the rate performance of the battery. Compared with the general base film, the ceramic diaphragm and the rubber-coated diaphragm can not only greatly improve the high temperature shrinkage resistance of the diaphragm, but also enhance the liquid absorption and wetting ability of the diaphragm. Adding SiO2 ceramic coating to the PP diaphragm can make the diaphragm absorb more Liquid volume increased by 17%. Coating 1μm PVDF-HFP on the PP/PE composite diaphragm, the liquid absorption rate of the diaphragm increased from 70% to 82%, and the internal resistance of the cell decreased by more than 20%.
3. Influence of process factors
1. Combine pulp
The uniformity of slurry dispersion during slurry mixing 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 adding high-speed dispersion, the uniformity of slurry dispersion can be improved, and the internal resistance of the battery is smaller. By adding a surfactant, the uniform distribution of the conductive agent in the electrode can be improved, the electrochemical polarization can be reduced, and the median discharge voltage can be improved.
2. Coating
Surface density is one of the key parameters of battery design. When the battery capacity is increased, the surface density of the new electrode sheet will inevitably reduce the total length of the current collector and separator, and the ohmic internal 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 detachment of solvent molecules during coating and drying are closely related to the temperature of the oven, which directly affects the distribution of the binder and conductive agent in the pole piece, and then affects the formation of the conductive grid inside the pole piece. Therefore, the process of coating and drying Temperature is also a critical process for optimizing battery performance.
3. Rolling
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 raw material particles decreases, the more contact between the particles, the more conductive bridges and channels, the battery Impedance is reduced. The control of compaction density is mainly achieved by rolling thickness. Different rolling thicknesses have a greater 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 failure of the active material to be rolled tightly, and the internal resistance of the battery increases. And after the battery is cycled, cracks will appear on the surface of the positive electrode of the battery with a large rolling thickness, which will further increase the contact resistance between the active material on the surface of the electrode sheet and the current collector.
4. Pole piece turnaround time
The different storage time of the positive electrode has a great influence on the internal resistance of the battery. When the storage time is short, the internal resistance of the battery increases slowly due to the influence of the carbon coating layer on the surface of the lithium iron phosphate and the force of the lithium iron phosphate. When the storage time is longer (greater than 23h), the internal resistance of the battery increases significantly due to the joint influence of the reaction between lithium iron phosphate and water and the bonding application of the adhesive. Therefore, the turnaround time of pole pieces needs to be strictly controlled in actual processing.
5. Injection
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 range of the solvent, and is also affected by the concentration of lithium salt and the size of the anion. In addition to the optimization research on conductivity, the amount of liquid injection and the soaking time after liquid injection also directly affect the internal resistance of the battery. A small amount of liquid injection or insufficient soaking time will cause the internal resistance of the battery to be too large, thus affecting the battery. capacity play.
4. Influence of conditions of use
1. Temperature
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 the ionic conductivity of the electrolyte, and the change trend at low temperature is consistent with the change trend of the electrolyte conductivity. Compared with the increase of bulk phase resistance and SEI film resistance at low temperature, the charge reaction resistance increases more significantly with the decrease of temperature. Below -20°C, the proportion of charge reaction resistance to the total internal resistance of the battery reaches almost 100%.
2. SOC
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 of the battery in the actual state: the DC internal resistance of the lithium-ion battery varies with the battery discharge depth When DOD is added and added, the internal resistance is basically unchanged in the 10%-80% discharge interval, and the internal resistance increases significantly at a deeper discharge depth.
3. 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 vary in their internal resistance. After long-term 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. Stroe et al. quantified the relationship between them through the storage research of LFP/C batteries for 24-36 months (as follows):
Wherein, the unit of temperature is K, the unit of SOC is percentage, and the unit of time is month.
4. Cycle
Whether it is storage or cycling, the effect of temperature on the internal resistance of the battery is the same. The higher the cycle temperature, the greater the rate of increase in internal resistance. Different cycle intervals have different effects on the internal resistance of the battery. The internal resistance of the battery accelerates as the depth of charge and discharge increases, and the increase in internal resistance is proportional to the depth of charge and discharge.
In addition to the influence of the depth of charge and discharge in the cycle, the charge cut-off voltage also has an effect: too low or too high upper limit of the charging voltage will increase the interface impedance of the electrode. Zheng et al. think that the optimal upper limit charge voltage of the LFP/C battery in the cycle is 3.9 - 4.3V, the test found that the passivation film cannot be formed well under the upper limit voltage that is too low, and the upper limit voltage that is too high will cause the electrolyte to oxidize and form products with low conductivity on the surface of the LiFePO4 electrode.
5. Other
Car lithium-ion batteries will inevitably experience poor road conditions in actual use, but research has found that the vibration environment of lithium-ion batteries has little effect on the internal resistance of lithium-ion batteries during use.
5. Outlook
Internal resistance is an important parameter to weigh the power performance of lithium-ion and evaluate the battery life. The larger the internal resistance, the worse the rate performance of the battery, and the faster it will be added during storage and cycle use. The internal resistance is related to the battery structure, battery material characteristics and manufacturing process, and changes with changes in ambient temperature and state of charge. Therefore, the development of low internal resistance batteries is the key to improving the power performance of batteries. At the same time, mastering the change law of battery internal resistance has very important practical significance for battery life prediction.
Electric vehicles have the advantages of no pollution, low noise, high energy efficiency, and simple structure, and have become an important development direction of the automobile industry. In recent years, new energy vehicles in the market have higher and higher requirements for high-rate charge and discharge performance of power lithium batteries, and internal resistance is an important factor affecting battery power performance and discharge efficiency. Its initial size is mainly determined by the structural design of the battery, raw materials Performance and process technology decision.
With the use of lithium-ion batteries, the battery performance continues to decline, mainly manifested as capacity decay, internal resistance increase, power drop, etc. The change of battery internal resistance is affected by various conditions such as temperature and discharge depth. Therefore, this paper mainly expounds the factors affecting the internal resistance of batteries from the aspects of battery structure design, raw material performance, process technology and service conditions.(Lithium - Ion Battery Equipment)
1. 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 tabs directly affect the internal resistance of the battery. To a certain extent, increasing the number of tabs can effectively reduce the internal resistance of the battery. The position of the tabs can also affect the internal resistance of the battery. The internal resistance of the coiled battery with the tab position at the head of the positive and negative electrodes is the largest. Compared with the coiled battery, the laminated battery is equivalent to dozens of small batteries. Connected in parallel, the internal resistance is smaller.
2. Effect of raw material performance
1. Positive and negative active materials
In lithium-ion batteries, the cathode material is the lithium storage side, which determines the performance of the lithium-ion battery more. The cathode material mainly improves the electronic conductivity between particles through coating and doping. For example, after doping Ni, the strength of the P-O bond is strengthened, the structure of LiFePO4/C is stabilized, the volume of the unit cell is optimized, and the charge transfer resistance of the positive electrode material can be effectively reduced.
However, through the simulation decomposition of the electrochemical thermal coupling model, it is known that under high-rate discharge conditions, the significant increase in activation polarization, especially the activation polarization of the negative electrode, is an important 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, in terms of battery design, research on the improvement of the positive and negative electrode materials themselves is also essential.
2. Conductive agent
Graphite and carbon black are widely used in lithium-ion batteries because of their good properties. Compared with graphite-based conductive agents, the battery rate performance of adding carbon black-based conductive agents to the positive electrode is better, because graphite-based conductive agents have a flaky particle shape, which causes a large increase in the pore meandering coefficient at high magnifications, and is prone to Li liquid phase A phenomenon in which the diffusion process limits the discharge capacity. The internal resistance of the battery added with CNTs is smaller, because compared to the point contact between graphite/carbon black and the active material, the fibrous carbon nanotube and the active material are in line contact, which can reduce the interface impedance of the battery.
3. Collector
Reducing the interfacial resistance between the current collector and the active material and improving the bond strength between the two are critical means to improve the performance of lithium-ion batteries. Coating conductive carbon coating on the surface of aluminum foil and corona treatment of aluminum foil can effectively reduce the interface resistance 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 internal resistance of the battery during use.
The AC internal resistance of aluminum foil treated by corona can be reduced by about 20%. In the commonly used range of 20%-90% SOC, the overall DC internal resistance is relatively small, and the increase is gradually smaller with the increase of the discharge depth.
4. Diaphragm
The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the pores of the separator. The liquid absorption and wetting ability of the separator is the key to forming a good ion flow channel. When the separator has a higher liquid absorption rate and porous structure, it can be improved. The conductivity reduces the battery impedance and improves the rate performance of the battery. Compared with the general base film, the ceramic diaphragm and the rubber-coated diaphragm can not only greatly improve the high temperature shrinkage resistance of the diaphragm, but also enhance the liquid absorption and wetting ability of the diaphragm. Adding SiO2 ceramic coating to the PP diaphragm can make the diaphragm absorb more Liquid volume increased by 17%. Coating 1μm PVDF-HFP on the PP/PE composite diaphragm, the liquid absorption rate of the diaphragm increased from 70% to 82%, and the internal resistance of the cell decreased by more than 20%.
3. Influence of process factors
1. Combine pulp
The uniformity of slurry dispersion during slurry mixing 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 adding high-speed dispersion, the uniformity of slurry dispersion can be improved, and the internal resistance of the battery is smaller. By adding a surfactant, the uniform distribution of the conductive agent in the electrode can be improved, the electrochemical polarization can be reduced, and the median discharge voltage can be improved.
2. Coating
Surface density is one of the key parameters of battery design. When the battery capacity is increased, the surface density of the new electrode sheet will inevitably reduce the total length of the current collector and separator, and the ohmic internal 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 detachment of solvent molecules during coating and drying are closely related to the temperature of the oven, which directly affects the distribution of the binder and conductive agent in the pole piece, and then affects the formation of the conductive grid inside the pole piece. Therefore, the process of coating and drying Temperature is also a critical process for optimizing battery performance.
3. Rolling
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 raw material particles decreases, the more contact between the particles, the more conductive bridges and channels, the battery Impedance is reduced. The control of compaction density is mainly achieved by rolling thickness. Different rolling thicknesses have a greater 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 failure of the active material to be rolled tightly, and the internal resistance of the battery increases. And after the battery is cycled, cracks will appear on the surface of the positive electrode of the battery with a large rolling thickness, which will further increase the contact resistance between the active material on the surface of the electrode sheet and the current collector.
4. Pole piece turnaround time
The different storage time of the positive electrode has a great influence on the internal resistance of the battery. When the storage time is short, the internal resistance of the battery increases slowly due to the influence of the carbon coating layer on the surface of the lithium iron phosphate and the force of the lithium iron phosphate. When the storage time is longer (greater than 23h), the internal resistance of the battery increases significantly due to the joint influence of the reaction between lithium iron phosphate and water and the bonding application of the adhesive. Therefore, the turnaround time of pole pieces needs to be strictly controlled in actual processing.
5. Injection
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 range of the solvent, and is also affected by the concentration of lithium salt and the size of the anion. In addition to the optimization research on conductivity, the amount of liquid injection and the soaking time after liquid injection also directly affect the internal resistance of the battery. A small amount of liquid injection or insufficient soaking time will cause the internal resistance of the battery to be too large, thus affecting the battery. capacity play.
4. Influence of conditions of use
1. Temperature
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 the ionic conductivity of the electrolyte, and the change trend at low temperature is consistent with the change trend of the electrolyte conductivity. Compared with the increase of bulk phase resistance and SEI film resistance at low temperature, the charge reaction resistance increases more significantly with the decrease of temperature. Below -20°C, the proportion of charge reaction resistance to the total internal resistance of the battery reaches almost 100%.
2. SOC
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 of the battery in the actual state: the DC internal resistance of the lithium-ion battery varies with the battery discharge depth When DOD is added and added, the internal resistance is basically unchanged in the 10%-80% discharge interval, and the internal resistance increases significantly at a deeper discharge depth.
3. 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 vary in their internal resistance. After long-term 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. Stroe et al. quantified the relationship between them through the storage research of LFP/C batteries for 24-36 months (as follows):
Wherein, the unit of temperature is K, the unit of SOC is percentage, and the unit of time is month.
4. Cycle
Whether it is storage or cycling, the effect of temperature on the internal resistance of the battery is the same. The higher the cycle temperature, the greater the rate of increase in internal resistance. Different cycle intervals have different effects on the internal resistance of the battery. The internal resistance of the battery accelerates as the depth of charge and discharge increases, and the increase in internal resistance is proportional to the depth of charge and discharge.
In addition to the influence of the depth of charge and discharge in the cycle, the charge cut-off voltage also has an effect: too low or too high upper limit of the charging voltage will increase the interface impedance of the electrode. Zheng et al. think that the optimal upper limit charge voltage of the LFP/C battery in the cycle is 3.9 - 4.3V, the test found that the passivation film cannot be formed well under the upper limit voltage that is too low, and the upper limit voltage that is too high will cause the electrolyte to oxidize and form products with low conductivity on the surface of the LiFePO4 electrode.
5. Other
Car lithium-ion batteries will inevitably experience poor road conditions in actual use, but research has found that the vibration environment of lithium-ion batteries has little effect on the internal resistance of lithium-ion batteries during use.
5. Outlook
Internal resistance is an important parameter to weigh the power performance of lithium-ion and evaluate the battery life. The larger the internal resistance, the worse the rate performance of the battery, and the faster it will be added during storage and cycle use. The internal resistance is related to the battery structure, battery material characteristics and manufacturing process, and changes with changes in ambient temperature and state of charge. Therefore, the development of low internal resistance batteries is the key to improving the power performance of batteries. At the same time, mastering the change law of battery internal resistance has very important practical significance for battery life prediction.
