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The factor of affecting lithium-ion cell's internal resistance
The factor of affecting lithium-ion cell includes, design of cell’s cathode piece and tab, raw material, manufacturing process, applications, etc. The article mainly expound that raw material’s effect on lithium-ion cell’s internal resistance of lithium battery.
Active material for positive and negative electrodes: the positive electrode of lithium cell stores the li-ion, it mainly determines the performance of the cell. Positive electrode material mainly improve the electronic conductivity between particles through coating and doping. Doping the Ni enhance the strength of P-O chemical bond, stabilize the structure of LiFePO4/C, optimize the volume of unit cell, effectively reduce the electron transfer resistance of the positive electrode material. Through the simulation analysis of the electrochemical thermal coupling model, it is known that under the condition of high-rate discharge, 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 active polarization of the negative electrode. When the solid phase particle size of the negative electrode is reduced by half, the active polarization can be reduced by 45%. Therefore, in terms of battery design, research and improvement of positive and negative materials are essential.
Conductive agent: Graphite and carbon black are widely used in the field of lithium batteries because of their good properties. Compared with graphite-based conductive agent, the positive electrode with carbon black-based conductive agent has better battery rate performance, because graphite-based conductive agent has a flaky particle morphology, which causes a large increase in pore tortuosity at a large rate, and Li liquid phase diffusion is easy to occur The phenomenon that the process limits the discharge capacity. The battery with added CNTs has lower internal resistance, because compared to 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.
Current collector: Reducing the interface resistance between the current collector and the active material and improving the bond strength between the two are important means to improve the performance of lithium batteries. Coating a conductive carbon coating on the surface of the aluminum foil and corona treatment on 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 corona treated aluminum foil can be reduced by about 20%. In the commonly used 20%~90% SOC range, the overall DC internal resistance is relatively small and the increase is gradually smaller as the depth of discharge increases.
Diaphragm: The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the porous diaphragm. The liquid absorption and wetting ability of the diaphragm is the key to forming a good ion flow channel. When the diaphragm has a higher liquid absorption rate and porous structure, It can improve the conductivity, reduce the battery impedance, and improve the rate performance of the battery. 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. The addition of SiO2 ceramic coating on the PP diaphragm can make the diaphragm absorb liquid The volume increased by 17%. Coating 1μm PVDF-HFP on the PP/PE composite diaphragm, the liquid absorption rate of the diaphragm is increased from 70% to 82%, and the internal resistance of the cell is reduced by more than 20%.
PLB has deeply researched the mechanism of the influence of raw materials on the internal resistance of the cells, and optimized the design. The 26650 cells produced have the excellent characteristics of low internal resistance and high consistency.
Statement of the internal resistance of influencing of the lithium cell(electrode and tabs)
1. The influencing factors that affect the internal resistance of lithium-ion batteries include: cell electrode and tabs, raw materials, manufacturing processes, applications, and many other aspects. This article mainly discusses the influence on the internal resistance of the lithium battery from the design of the cell electrode and tab.
2. In the cell structure design, in addition to the riveting and welding of the cell structure itself, the length of the cell electrode, the material, quantity, size, and location of the tabs directly affect the internal resistance of the cell. To a certain extent, increasing the number of tabs can effectively reduce the internal resistance of the battery. The position of the tab can also affect the internal resistance of the cell. The internal resistance of the winding battery with the tab position at the head of the cathode and anode electrode is the largest, and compared to the winding battery, the laminated battery is equivalent to dozens of small battery in parallel, and its internal resistance is smaller.
3. The 26650 battery cells designed and manufactured by Power Long Battery are optimized in terms of the material, quantity and location of the tabs. The battery cells have obvious advantages of low internal resistance and good consistency.
Explanation of Factors Affecting the Internal Resistance of Lithium ion Batteries (Manufacturing Process)
The influencing factors of the internal resistance of lithium-ion batteries include electrode and tab design, raw materials, manufacturing process, application and many other aspects. This article mainly discusses the impact on the internal resistance of the battery from the aspects of cell manufacturing process and process control.
Slurry mix process: The uniformity of the slurry dispersion when mixing the slurry affects whether the conductive agent can be evenly dispersed in the active material 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 internal resistance of the battery will be smaller. By adding a surfactant, the uniformity of the distribution of the conductive agent in the electrode can be improved, and the electrochemical polarization can be reduced and the median discharge voltage can be increased.
Coating process: Area density is one of the key parameters of battery design. When the battery capacity is constant, increasing the electrode area density will inevitably reduce the total length of the current collector and the separator, and the ohmic internal resistance of the battery will decrease accordingly. Within a certain range, the internal resistance of the battery decreases as the areal density increases. The migration and separation of solvent molecules during coating and drying is closely related to the temperature of the oven, which directly affects the distribution of binder and conductive agent in the electrode, and then affects the formation of the conductive grid inside the electrode. Therefore, the coating and drying process temperature is also an important process for optimizing battery performance.
Rolling process: 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 the contact between the particles, the more the conductive bridges and channels, the lower the battery impedance. 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. In addition, cracks are generated on the positive electrode surface of the battery with a large thickness after the battery is rolled, which will further increase the contact resistance between the surface active material of the electrode and the current collector.
Electrode turnaround time control: different shelf time of the positive electrode has a greater impact on the internal resistance of the battery. When the shelf time is short, the internal resistance of the battery is affected by the force of the carbon coating on the surface of the lithium iron phosphate and the lithium iron phosphate itself. The increase is slower. When the storage time is longer (more than 23h), the internal resistance of the battery increases significantly due to the combined effect of the reaction between lithium iron phosphate and water and the adhesion of the adhesive. Therefore, it is necessary to strictly control the turnaround time of electrode in actual production.
Liquid injection process: 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 of the solvent, and is also affected by the concentration of lithium salt and the size of anions. In addition to the optimization research on the conductivity, the injection volume and the infiltration time after injection also directly affect the internal resistance of the battery. Small injection volume or insufficient infiltration time will cause the internal resistance of the battery to be too large, thereby affecting the battery capacity performance.
The 26650 battery cell designed and manufactured by Power Long Battery is subject to rigorous process design and strict manufacturing process control. The battery cell has obvious advantages of low internal resistance and good consistency.
Category and Characteristic of Li-ion batteries
In the industry of Li-ion batteries, rechargeable lithium iron batteries ((hereinafter referred to as "Li batteries") are mainly divided into the following categories according to the different cathode materials: LiCoO2 lithium cobalt oxide (LCO); LiFePO4 lithium iron phosphate (LFP) ; LiMn2O4 Lithium Manganese Oxide (LMO); LiNixCoyMn(1 -xy)O2 Nickel Cobalt Manganese/ LiNixCoyAl(1-xy)O2 Nickel Cobalt Aluminum Ternary Lithium (NCM/NCA).
1. LCO battery: Lithium cobalt oxide (LCO) as the cathode material of Li batteries has been rapidly commercialized and widely used in various portable electronic devices such as mobile phones, MP3/MP4, notebook computers, Bluetooth devices, tablet computers, and power tools etc.
2. LFP Battery: LFP with olivine structure is more stable than other cathode materials. In 2004, LFP material first input in China. Due to its unique thermal stability and safety, it is especially suitable for electronic vehicles. Has been vigorously promoted and applied by BYD, and has been followed up by Guoxuan Hi-Tech, Lishen and CATL etc.
3. LMO Battery: As with characteristics of low cost, thermal stability， great over-charge performance and over operation voltage, become the hotpots of Li batteries research.
4. NCM/NCA Battery: Combines the advantages of cycle performance of LCO, high capacity of LNO, low cost and safety performance of LMO. At the beginning of this century, as the demand of high-energy density and long-life batteries, NCM and NCA batteries have been widely used in commercial applications. Especially for electronic vehicles pursuing long lfie, NCM/NCA Li batteries have been widely popularized. Tesla uses Panasonic’s NCA batteries, and BMW, Volkswagen and other companies use Samsung and LG’s NCM. For ternary lithium batteries, domestic companies generally follow the NCM ternary lithium route.
Comparison among LMO, LFP and NCM/NCA Among 4 main materials of Li batteries, LCO with highest metallic content and cobalt as a rare metal with concentrated production areas and expensive, so the cost of LCO Battery is also the highest, and only suitable for It is a small battery to meet the needs of portable electronic devices, but it is not suitable for larger batteries to be used in the power and energy storage applications. LMO, NCM, LAP have large-scale applications in huge power (four-wheel vehicles) and small power (two-wheeler/three-wheeler). We can grasp their future development trend from serious analyzing.
（1） The outstanding advantages of LMO materials are safety and low cost. LTO does not contain metal cobalt, so the material cost is lower, it’s very helpful to promote the LTO batteries. Secondly, LTO with great performance in thermal management, is not easy to decompose and thermally runaway at high temperature, and has awesome safety performance, which is especially suitable for the power system with high safety requirements. The main disadvantages of LTO batteries is poor high-temperature performance, short life cycle, and the dissolution of the anode material at high temperatures, which leads to a rapid decline in battery capacity and life cycle. Corresponding to this, the low temperature performance of LTO is great, it can keep in good activity at minus 30 ℃, and can be used for charging and discharging. Therefore, LTO batteries are not suitable for the southern regions with higher temperatures, but more suitable for the northern regions with low long-term temperatures and cold winters. In addition, the rate performance of LTO is very suitable for fast charging products. Of course, the heat dissipation problem must be solved, otherwise the cycle life decays too fast, which will lead to the faster the charging speed and shorten the life.
(2) NCM/NCA Battery: The outstanding advantage of ternary lithium material is its high energy density. With 3 kinds of materials, the specific energy density of ternary lithium is the highest. For example, the NCM battery of 811 material has a single energy density of 300Wh/kg, which is almost twice LTO and LFP. Therefore, NCM battery is particularly suitable for small and light application scenarios, which can obtain the most energy storage and the longest driving mileage, such as pure EV and electric motorcycles. The low-temperature characteristics of NCM are similar to those of LMO, and it can also maintain good activity at low temperatures, and its high-temperature performance is significantly better than LTO. It works at high temperatures and its cycle life will not rapidly decline. Therefore, the operating temperature range NCM is wdier, which can meet various climate conditions in the south and north. The obviously shortcomings of NCM are high cost and poor safety. Because the prices of cobalt, nickel, and aluminum are relatively high, the price of NCM always stay in high, which is not good for promotion in the civilian market. Moreover, NCM with poor stability at high temperatures, and will easily decompose and release oxygen when exposed to heat, causing thermal runaway of the battery, burning or explosion. With higher nickel content of the NCM, the higher energy density, but the lower the safety, so there with great contradictory between high energy density and safety for NCM batteries. Due to the consideration of cost and safety, the application of NCM battery with limited used.
（3） LFP Battery: LFP battery with outstanding advantage in lower cost, long life cycle and high safety.Thanks to the excellent thermal stability and structural stability, the safety LFP batteries is significantly better than NCM batteries. Because of with sufficient supplement, the price of LFP is the cheapest among various cathode materials. Refer to cycle life, LFP also has obvious advantages, with a cycle life of 6000 to 8000 cycles, and power type LFP batteries can also reach 3000 cycles. Up and down, this point is incomparable from LMO and NCM.
PLB is focus on R&D and manufacturing of 26650 LFP Batteries, with significant technical and performance advantages such as high safety, high consistency, long cycle, and very great cost performance ratio, is widely used by first-class international and domestic clients.
What is "lithium precipitation" in lithium ion batteries?
When a lithium ion battery is charged, lithium ions are intercalated from the positive electrode and inserted into the negative electrode; but when there are some abnormal conditions: such as insufficient space for inserting lithium in the negative electrode, too much resistance to inserting lithium ions into the negative electrode,lithium ions fall off from the positive electrode too quickly, but cannot be embedded into the negative electrode equally.When an abnormalityoccurs, such as a large amount of insertion into the negative electrode, the lithium ions that cannot be inserted into the negative electrode. So that they can only get electrons on the surface of the negative electrode, thereby forming a silver-white metallic lithium element, which is often called "lithium precipitation"
The main manifestations of lithium precipitation are:
Under low temperature conditions, the ionic conductivity of the electrolyte will decrease, the resistance of lithium ions from the positive electrode and the insertion of the negative electrode will be greatly increased, and the increase in the resistance of the negative electrode will be greater, which will cause lithium precipitation.
When the battery is charged at a high rate, a large amount of lithium ions are extracted from the positive electrode and come to the negative electrode. However, because the impedance of lithium ions inserted into the negative electrode is much greater than that of the positive electrode, the rush of lithium ions cannot be 100% guaranteed. If it is too late to insert into the negative electrode, it will get electrons on the surface of the negative electrode and form metallic lithium.
When the positive electrode coating is too heavy or the negative electrode coating is too light, there will be insufficient space for lithium insertion in the negative electrode, so that after lithium ions are extracted from the positive electrode and reach the negative electrode, electrons will be obtained on the surface of the negative electrode and metal lithium will be formed.
After the negative electrode is compacted beyond its limit, it will destroy the body structure of the material and increase the resistance during the insertion of lithium ions, thereby triggering lithium precipitation. If the negative electrode is exposed to the foil, lithium ions will directly gain electrons from the copper foil and carry out lithium precipitation during charging.
A core with a large thickness or an internally tightly wound core is easily deformed after volume separation and will cause poor contact of the pole pieces. The poor contact area will be filled with gas inside the cell, thereby losing the lithium ion migration channel. Eventually, a strip-shaped area with no lithium intercalation is formed, which may be accompanied by lithium precipitation.
The electrolyte acts as a channel for conducting lithium ions. If the amount is small or the pole piece is not sufficiently infiltrated, the knife will cause lithium precipitation.
During the design or manufacturing process of 26650, PLB fully considers the above-mentioned lithium precipitation phenomenon and mechanism, and ensures that the battery does not have lithium precipitation from multiple dimensions such as design, material, and technology, so as to ensure the 26650 cells with the long cycle, high capacity, and high capacity, stable electrochemical performance and other advantages.
How to improve the low temperature performance of lithium batteries?
The low-temperature performance of lithium-ion batteries is one of the key factors that restrict the widespread use of lithium batteries. How to improve the low-temperature performance of lithium batteries is still a hot and difficult point of current research.
The battery system reaction process mainly includes four steps: Li+ transport in the electrolyte, crossing the electrolyte/electrode interface membrane, charge transfer, and Li+ diffusion in the active material body. At low temperatures, the rate of each step decreases, which causes the impedance of each step to increase, which leads to aggravation of electrode polarization, and causes problems such as a decrease in low- temperature discharge capacity and lithium precipitation in the negative electrode. The poor low-temperature performance of lithium-ion batteries is mainly due to the following three factors:
1. At low temperatures, the viscosity of the electrolyte increases, and the conductivity decreases;
2. The electrolyte/electrode interface membrane impedance and charge transfer impedance increase;
3. The migration rate of lithium ions in the active material body is reduced. As a result, the electrode polarization is increased at low temperatures and the charge and discharge capacity is reduced.
To improve the low temperature performance of lithium batteries, the influence of the positive electrode, negative electrode, electrolyte and other comprehensive factors in the battery should be comprehensively considered. The conductivity of the electrolyte should be improved by optimizing the composition of electrolyte solvent, additives and lithium salt, while reducing the film-forming resistance; The electrode material undergoes modification treatments such as doping, coating, and granulation to optimize the material structure and reduce the interface resistance and the diffusion resistance of Li+ in the active material body. Through the overall optimization of the battery system, the polarization of the lithium battery at low temperatures is reduced, and the low temperature performance of the battery is further improved.
The mainstream ways to improve the ion diffusion performance of cathode materials at low temperatures are as follows: First, use materials with excellent conductivity to coat the active material body to improve the conductivity of the cathode material interface, reduce the interface impedance, and reduce the positive electrode material and The side reaction of the electrolyte stabilizes the material structure. The second is to do bulk- doping of the material body with Mn, Al, Cr, Mg, F and other elements to increase the layer spacing of the material to increase the diffusion rate of Li+ in the body, reduce the diffusion resistance of Li+, and increase the low temperature of the battery. performance. The third is to reduce the particle size of the material and shorten the Li+ migration path. It should be pointed out that this method will increase the specific surface area of the material and increase the side reaction with the electrolyte.
As an important part of the lithium ion battery, the electrolyte not only determines the migration rate of Li+ in the liquid phase, but also participates in the formation of the SEI film, which plays a key role in the performance of the SEI film. At low temperatures, the viscosity of the electrolyte increases, the conductivity decreases, the impedance of the SEI film increases, and the compatibility with the positive and negative materials deteriorates, which greatly deteriorates the energy density and cycle performance of the battery. At present, there are two ways to improve the low-temperature performance of the electrolyte: one is to improve the low-temperature conductivity of the electrolyte by optimizing the composition of the solvent and using new electrolyte salts; the second is to use new additives to improve the properties of the SEI membrane, so that It is conducive to Li+ conduction at low temperatures.
Dongguan Power Long Battery Technology Co., Ltd. (PLB) specializes in the R&D and manufacturing of 26650 cylindrical lithium- ion batteries. PLB effectively improves the low-temperature performance of the battery through in-depth research on the low-temperature mechanism of lithium ion, and optimizes and improves materials that affect the low-temperature performance of the battery. So that 26650 lithium iron phosphate can meet application scenarios or projects with low temperature requirements.
What is the effect of the SEI film on the battery performance?
The use of THE SEI should be analyzed from its own characteristics: THE SEI is an interface layer between the electrode material and electrolyte, which separates the two.Has the characteristics of solid electrolyte.Li+ can pass, but electrons can't.
1. The degree of corrosion caused by the reaction between lithium metal and electrolyte components is determined by the passivation performance of the SEI film on its surface.
2. In the dissolution-deposition process of lithium, it must occur through THE SEI film, and the migration of lithium ions through the SEI film is the rate control step of the lithium deposition-dissolution process, which determines the uniformity of the lithium deposition-dissolution process.When the lithium deposition is uniform, the metal lithium can largely prevent corrosion during the cycling process, so that the lithium electrode can achieve better cycling efficiency.
3. lithium, lithium ion can occur through the SEI film deposition and dissolution process, but in the process of dissolution and deposition, because of the important composed of ionic components of SEI is difficult to adapt to the process of lithium surface morphology change, therefore the SEI film ruptures, lead to the emergence of the nude lithium and its further reaction of the electrolyte, so repeated deposition in lithium - solvent thermal process, lithium and electrolyte composition is consumed, lead to lithium electrode cycle performance attenuation.
4, the SEI film breakdown can lead to form some highly active, so as to accelerate the deposition of these sites lithium and solvent speed, leading to lithium electrode surface current distribution inhomogeneity, inhomogeneity and dendrite formation leads to a series of security problems, this is the commercialization of metallic lithium battery application was the important reason for the failure.
What effect does charging time have on the battery?
Charging mode is generally fast charge, slow charge.Fast charging Adopts large current to directly charge the battery through the non-vehicle charger, which can be fully charged with 80% capacity in half an hour;Slow charge refers to ac charging, charging process takes 6 hours to 8 hours.
What's wrong with the battery charge?
This is the existence of safety redundancy in the battery, reserve a part of the power to prevent the battery overcharge and excessive discharge.For example, the original battery capacity is 100%, but the actual battery capacity is about 80%-90%, and the value after the power redundancy is the actual power that the car owner can use.Because during the charging process, the battery can't be too full, nor can it be discharged into space.If the battery is fully charged every time, it will greatly reduce its service life;Charging too full, once the charging system has a fault, it is possible to cause safety accidents because of "overcharging".