18650 li ion rechargeable battery

　　Research and analysis of internal resistance characteristics of 18650 li ion rechargeable battery

　　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 on the market have increasingly higher requirements for high-rate charge and discharge performance of power batteries. 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 and the performance of raw materials. and process technology decisions.

　　With the use of lithium batteries, battery performance continues to decay, which is mainly manifested by capacity attenuation, internal resistance increase, power drop, etc. Changes in battery internal resistance are affected by various usage conditions such as temperature and discharge depth. Therefore, this article mainly explains the factors that affect the internal resistance of the battery from the aspects of battery structure design, raw material performance, manufacturing process and usage conditions.

　　1. Influence of structural design

　　In the battery structure design, in addition to the riveting and welding of the battery structural parts themselves, 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 coiled battery with the tabs positioned at the heads of the positive and negative electrodes has the highest internal resistance. Compared with the coiled battery, the stacked battery is equivalent to dozens of small batteries. When connected in parallel, the internal resistance is smaller.

　　2. Impact of raw material performance

　　1. Positive and negative active materials

　　In lithium batteries, the cathode material is the Li storage side and determines more the performance of the lithium battery. The cathode material mainly improves the electronic conductivity between particles 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 unit cell volume, and can effectively reduce the charge transfer resistance of the cathode material.

　　Through electrochemical thermal coupling model simulation analysis, it is known that under high-rate discharge conditions, the significant increase in activation polarization, especially the activation polarization of the negative electrode, is the main reason for serious polarization. Reducing the particle size of the negative electrode can effectively reduce the activation polarization of the negative electrode. When the solid particle size of the negative electrode is reduced by half, the activation polarization can be reduced by 45%. Therefore, in terms of battery design, research on improving the positive and negative electrode materials themselves is also essential.

　　2. Conductive agent

　　Graphite and carbon black are widely used in the field of lithium batteries due to their good properties. Compared with graphite conductive agents, batteries with carbon black conductive agents added to the positive electrode have better rate performance because graphite conductive agents have a flaky particle morphology, which causes a large increase in the pore tortuosity coefficient at high rates and is prone to Li liquid phase diffusion. The process limits the discharge capacity. The internal resistance of the battery with CNTs added is smaller, because compared with the point contact between graphite/carbon black and the active material, the fibrous carbon nanotubes and the active material are in line contact, which can reduce the interface impedance of the battery.

　　3. Current collector

　　Reducing the interface resistance between the current collector and the active material and improving the bonding strength between the two are important means to improve the performance of lithium batteries. Coating the surface of aluminum foil with a conductive carbon coating and corona treatment of the aluminum foil can effectively reduce the interfacial 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 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 DC internal resistance is overall small and its increase rate gradually becomes smaller as the discharge depth increases.

　　4. Diaphragm

　　Ion conduction inside the battery relies on the diffusion of Li ions in the electrolyte through the porous 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 improve Conductivity reduces 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 the liquid absorption and wetting ability of the diaphragm. Adding SiO2 ceramic coating to the PP diaphragm can improve the liquid absorption of the diaphragm. Volume increased by 17%. When 1 μm PVDF-HFP is coated on the PP/PE composite separator, the liquid absorption rate of the separator increases from 70% to 82%, and the internal resistance of the battery core decreases by more than 20%.

　　3. Influence of process factors

　　1. Mix 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, and is related to the internal resistance of the battery. By increasing high-speed dispersion, the uniformity of slurry dispersion can be improved, and the internal resistance of the battery is smaller. By adding surfactant, the distribution uniformity of the conductive agent in the electrode can be improved, and the electrochemical polarization can be reduced to increase the discharge median voltage.

　　2. Coating

　　Areal density is one of the key parameters of battery design. When the battery capacity is certain, increasing the areal density of the electrodes 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 within the pole piece, and then affects the formation of the conductive grid inside the pole piece. Therefore, the coating and drying process Temperature is also an important process for optimizing battery performance.

　　3.Rolling

　　To a certain extent, the internal resistance of the battery decreases as the compaction density increases. Because the compaction density increases, the distance between raw material particles decreases. The more contacts between particles, the more conductive bridges and channels, the battery Impedance decreases. 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 larger, the contact resistance between the active material and the current collector increases because the active material cannot be rolled tightly, and the internal resistance of the battery increases. In addition, cracks will occur on the surface of the battery positive electrode with a large roll thickness after battery cycling, which will further increase the contact resistance between the active material on the surface of the electrode piece and the current collector.

　　4. Pole piece turnaround time

　　Different storage times of the positive electrode sheet have a greater impact 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 lithium iron phosphate. When the battery is left for a long time (more than 23 hours), the internal resistance of the battery increases significantly due to the combined effects of the reaction between lithium iron phosphate and water and the bonding effect of the adhesive. Therefore, the turnaround time of pole pieces needs to be strictly controlled in actual production.

　　5.Inject liquid

　　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 anions. In addition to the optimization research on conductivity, the amount of liquid injected and the infiltration time after liquid injection also directly affect the internal resistance of the battery. A small amount of liquid injection or insufficient infiltration time will cause the internal resistance of the battery to be too large, thus affecting the battery. capacity.

　　4. Influence of usage conditions

　　1.Temperature

　　The effect 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 film impedance and charge transfer impedance. The bulk impedance and SEI film impedance are mainly affected by the ionic conductivity of the electrolyte. The changing trend at low temperature is consistent with the changing trend of the electrolyte conductivity. Compared with the increase in bulk impedance and SEI film resistance at low temperatures, the charge reaction impedance increases more significantly as the temperature decreases. Below -20°C, the charge reaction impedance accounts for almost 100% of the total internal resistance of the battery.

　　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, which in turn reflects the battery performance in the actual state: the DC internal resistance of the lithium battery changes with the battery discharge depth DOD The internal resistance increases with the increase of the discharge range. The internal resistance is basically unchanged in the discharge range of 10% to 80%. Generally, the internal resistance increases significantly at deeper discharge depths.

　　3.Storage

　　As the storage time of lithium-ion batteries increases, the batteries continue to age and their internal resistance continues to increase. Different types of lithium batteries have different internal resistance changes. After a long period of storage of 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 24-36 months (as follows):

　　Among them, the temperature unit is K, the SOC unit is percentage, and the time unit is month.

　　4. Loop

　　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 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 as the depth of charge and discharge increases, and the increase in internal resistance is proportional to the increase in the depth of charge and discharge.

　　In addition to the influence of the depth of charge and discharge in the cycle, the charging cut-off 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. Zheng et al. believe that the optimal upper limit charging voltage of the LFP/C battery in the cycle is 3.9-4.3V. Experiments have found that the passivation film cannot be formed well under too low upper limit voltage, and too high upper limit voltage will cause the electrolyte to oxidize and decompose on the surface of the LiFePO4 electrode to form products with low conductivity.

　　5.Others

　　Vehicle-mounted lithium batteries will inevitably experience poor road conditions in practical applications, but research has found that the vibration environment of lithium batteries during application has little impact on the internal resistance of lithium batteries.

　　5. Outlook

　　Internal resistance is an important parameter to measure lithium-ion power performance and evaluate battery life. The greater the internal resistance, the worse the rate performance of the battery, and the faster it increases during storage and recycling. 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, developing low internal resistance batteries is the key to improving battery power performance. At the same time, grasping the changing rules of battery internal resistance has very important practical significance for battery life prediction.

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