UNICELL Research progress in UN lithium battery formation technology
Abstract: In the production process of lithium batteries, formation is necessary to achieve wetting of the electrodes and full activation of the electrode materials. At the same time, during the initial charging process, with the insertion of lithium ions in the negative electrode, the electrolyte composition undergoes a reduction reaction in the negative electrode to form a stable solid electrolyte interface facial mask (SEI film) to prevent the irreversible consumption of electrolyte and lithium ions in the subsequent cycle. Therefore, this technology has an extraordinary significance for battery performance. The formation effect directly affects the subsequent performance of lithium batteries, including storage performance, cycle life Multiplication performance and safety, etc. However, each individual battery in the electric vehicle battery pack requires several days or even weeks of formation and aging processes, resulting in lower battery production efficiency; A large number of charging and discharging equipment, temperature control equipment, and environmental space have increased the production cost of batteries; Traditional transformation methods cannot fully meet high-performance requirements such as capacity, lifespan, and safety. At present, many studies have optimized the formation technology of lithium batteries to improve battery performance, reduce formation time, and thus reduce battery production costs. This review focuses on the optimization of lithium-ion formation technology, introduces the significance of battery formation, cost analysis, various technical parameters and formation methods, and looks forward to future research and improvement directions.
UNICELL Keywords: Lithium battery; Formation technology; Solid electrolyte interface facial mask; Production efficiency; production cost
UNICELL With the widespread use of lithium batteries, electrochemical energy storage has put forward higher requirements for the cost, energy density, cycle life, and safety of lithium batteries. Nowadays, researchers have made great efforts to improve the comprehensive performance of batteries, with the positive electrode, negative electrode, separator, and electrolyte all developing towards high energy density, high service life, high magnification, and high safety. However, the production cost of lithium batteries remains high, and controlling battery costs without sacrificing battery performance has become a major challenge for current electric vehicle battery pack manufacturers. The starting point of this review will be to break away from the internal materials of batteries and introduce the essential formation/aging technologies after battery preparation, reducing formation and aging time while achieving high battery performance, which is particularly important in the battery production process.
UNICELLA s is well known, in the production process of lithium batteries, there is a continuous process of liquid injection, formation, and aging after the completion of battery cell preparation, which has a significant impact on the performance and service life of lithium batteries. After aging, defective batteries will be selected through capacity testing (sub capacity), and some manufacturers will also grade their batteries to reduce the differences in their subsequent use. Therefore, all battery manufacturers will optimize the formation technology, and there are differences in technology among manufacturers to achieve the optimal cost-effectiveness. Because in addition to the high cost of battery materials, the battery formation/aging process is the most expensive process in battery manufacturing, accounting for up to 6.4% of the total cost.
UNICELL Wood et al. mentioned that if a low rate is used for charging and discharging, the formation/aging process is a long and costly process. The first step is the electrode wetting process, and sufficient and balanced cell wetting is particularly important during the formation/aging process. The porosity of the separator in the battery (the surface energy of polyethylene is 35-36mN/m, while that of polypropylene is only 30.1mN/m), and the electrode bonding agent and conductive carbon black both hinder the wettability of the electrode. The basic electrode wetting process takes a 12-24 hour cycle, and there is still a large amount of small pore space that cannot be fully wetted. The infiltration rate is closely related to the properties of the electrolyte (such as viscosity and surface tension), temperature, and porous electrodes (such as porosity and microstructure). Therefore, in order to ensure capacity consistency, safety, and long service life of high-quality lithium batteries, the electrode infiltration and formation process will consume 3-7 days and the subsequent aging time of nearly 2 weeks, greatly increasing the production cost of the battery. In addition to time cost, a large number of battery testing systems and temperature control systems need to be installed, resulting in large-scale land occupation, heat and power consumption and loss. Therefore, some studies have focused on reducing formation time to reduce battery production costs.
On the other hand, achieving better battery performance under the same preparation and processing techniques is also an important factor in reducing battery costs. Graphite has stable cycling performance and relatively high safety characteristics compared to lithium metal, making it the most common negative electrode material for lithium batteries. During the first charge, some solvents in the electrolyte, such as alkyl carbonate or electrolyte film forming additives, can preferentially reduce and decompose on the negative electrode surface and form a layer of solid electrolyte interface facial mask (SEI film). This passivation film has high ionic conductivity and electronic insulation, and Li+can freely embed and detach through this passivation layer. This SEI film can inhibit the continuous decomposition of electrolyte on the electrode and protect the electrode material structure. However, if a dense and stable SEI film cannot be formed during this process, exposure of the fresh electrode surface to the electrolyte will lead to continuous decomposition of the electrolyte during subsequent cycles and co embedding of solvent molecules, resulting in graphite peeling. Therefore, how to form a stable SEI film during the first charging process, minimize the loss of active lithium and electrolyte decomposition, effectively improve the cycling stability and safety during battery use, is a major challenge to the battery formation/aging process.
After the electrolyte fully infiltrates the electrode, the SEI film at the electrode/electrolyte interface forms during the formation stage and undergoes further chemical rearrangement adjustment during the aging stage. During this process, various technical parameters such as temperature, external mechanical pressure, charging and discharging current, charging and discharging voltage, state of charge, electrolyte composition and properties, and battery chemical characteristics affect the formation effect of the battery, resulting in differences in the formation time and performance of the battery, directly determining the production cost of the battery, as shown in Figure 1. Although major battery manufacturers now have standardized formation/aging processes, there is relatively little research and analysis on the mechanism of this process. Currently, the optimization of the conditions for lithium battery formation/aging is not comprehensive enough. This review focuses on optimizing the technical parameters and methods for lithium battery formation/aging, improving production efficiency, and reducing production costs. Firstly, an overview of the lithium battery formation/aging process and its cost analysis are introduced; Secondly, analyze the formation/aging technology and its impact on battery performance; Finally, a summary of the entire article and an outlook on the future research trends of lithium battery formation/aging processes are presented, aiming to provide the foundation and ideas for further reducing the production cost of lithium batteries and developing an efficient and convenient formation method.
Figure 1 Lithium Battery Formation Technology
Overview and Cost Analysis of Lithium Battery Formation Technology
Lithium batteries need to form a SEI film with excellent properties during the formation stage, which plays an irreplaceable role in the battery's service life and safety. The electrode undergoes an active material infiltration stage after liquid injection, and the wettability of the electrolyte to the electrode is an important factor that must be considered in the development of high-performance lithium batteries. The uneven distribution of electrolyte in the electrode can lead to uneven current density and SEI film formation. Insufficient electrolyte infiltration will lead to a sharp decline in battery performance and lithium deposition on the graphite negative electrode. A method for measuring the liquid absorption rate of lithium batteries based on the permeability coefficient (COP) and solid permeability coefficient (SPC) as important parameters has been reported. The larger the COP of the electrolyte, the easier it is for the electrolyte to soak the electrode; The larger the electrode SPC, the easier it is to be soaked. The results indicate that the addition of lithium salt concentration in the electrolyte can lead to a decrease in electrode wetting rate. The EC-EMC solvent system is more conducive to electrode wetting than the EC-DEC solvent system. The graphite negative electrode without rolling has higher wetting ability than the LiNi0.5Co0.2Mn0.3O2 without rolling. In addition, a Lattice Boltzmann model was used to simulate the distribution and wetting characteristics of the positive and negative electrode electrolytes in lithium batteries, and a porous electrode model was used to demonstrate the influence of electrode wetting on battery performance and the dependence of capacity on wetting degree. The simulation results show that there are still widely distributed areas of non wetting after the electrode wetting step is completed, and this incomplete wetting comes from the electrolyte and the air wrapped in the electrode particles. Among them, the air in the electrolyte can be removed during the high-temperature aging stage, and the air in the negative electrode can be removed by repeated charging and discharging, causing the graphite negative electrode to expand/contract. Low positive electrode saturation leads to low battery capacity, while low negative electrode saturation can cause metal lithium deposition, thereby affecting battery safety and cycle life.
In addition to the influence of battery material properties on electrode wettability, the formation/aging processes currently used by most manufacturers are not consistent. Technical parameters include temperature, external mechanical pressure, charge discharge current, charge discharge voltage, state of charge, etc. Among them, charging methods include traditional constant current charging, constant voltage charging, commonly used step charging and high-temperature pressure charging, as well as the latest report of "high rate charging+high voltage shallow cycling". In charging methods, constant current charging generally involves charging from the initial low current to the later high current, which takes a long time and wastes resources seriously. The initial current of constant voltage charging is relatively high, and then gradually decreases until the charging is completed. The current is zero, which is less time-consuming but difficult to control. Improper selection of charging voltage can have an impact on the battery. The existing technology of step charging is achieved by injecting electrolyte and sealing it in stages through vacuum pumping, activating it through small current step charging, and applying intermittent pulse discharge. Although this method can shorten the formation time, it may also cause incomplete electrochemical reactions. The high-temperature pressure formation method involves clamping the battery cell into a fixture cabinet, heating it to form a fixture, and then placing the battery cell in place for constant current charging. The gas inside the battery cell is then pressurized to extrude the gas, which can adjust the temperature and applied pressure to complete the formation process. This method enhances the formation effect of lithium batteries and shortens the formation time through the advantages of controllable temperature, external mechanical pressure, and charging and discharging current, but the cost is relatively high. Therefore, it is necessary to further develop new formation methods, save time and cost, and improve battery performance. Given the importance of formation in battery production costs, Wood et al. conducted a statistical analysis of the cost of electrode infiltration and formation processes. They believe that in industry, these two processes take 1.5 to 3 weeks and will result in an unacceptable processing bottleneck and production costs. The typical formation process starts with a 2-3 day soaking process at room temperature, followed by the first charge and discharge (a very low charge and discharge rate such as 0.05C/-0.05C), and then another 1-2 day soaking process; Subsequently, a slightly faster charging and discharging process (such as 0.1C/-0.1C) and a high-temperature (50-60 ℃) soaking process; The third one is faster to charge and discharge (approximately 0.25C/-0.25C). These formation cycles are often carried out at high temperatures, and the long cycle greatly increases the production cost of batteries. The article conducts cost statistics on each link of the formation process, and the results are shown in Table 1. The formation cost per kilowatt hour of energy is 22.6 US dollars, accounting for 6.4% of the total manufacturing cost of the battery. This shows the importance of the formation process in battery production. If scientific and efficient formation technology is used to shorten the formation time by 60% to 75%, it can save $13.6-17 per kilowatt hour.
Table 1 Cost Distribution of Lithium Batteries for Basic Electrode Processing Conditions (Estimated Energy Available per kWh Assuming 70% Cyclic Discharge Depth)
2. Technical parameters/methods of formation and their impact on battery performance
2.1 Temperature
The formation and aging temperature play a decisive role in the characteristics of the electrode SEI film. There are two opposing research results regarding the formation temperature. On the one hand, high-temperature formation has been reported to have serious capacity loss. German et al. studied the effect of temperature on capacity loss and subsequent electrochemical performance during the formation process of Lix (Ni1/3Co1/3Mn1/3) yO2 (NCM)/graphite full cell, NCM half cell, and graphite half cell. They found that the sources of capacity loss were different between the positive and negative electrodes, with the negative electrode capacity loss being important due to the formation of SEI film on the graphite surface, while the positive electrode capacity loss was attributed to the inhibition of NCM kinetics. As the formation temperature increases, the irreversible capacity loss of the positive and negative electrodes increases. However, due to the increase in lithium diffusion coefficient of the NCM electrode, the capacity loss ratio of the positive and negative electrodes decreases, which seriously degrades the performance of the graphite battery. As shown in Table 2, the capacity loss of the electrodes during the formation and subsequent charging and discharging processes at different temperatures is determined. The increase in formation temperature and the loss of graphite negative electrode capacity are attributed to the intensification of the decomposition degree of electrolyte components. Due to the accelerated kinetics, the capacity loss of the NCM positive electrode decreases, but the capacity loss caused by low-temperature formation can be partially recovered during subsequent room temperature cycling. Therefore, high-temperature formation has no advantage for the entire battery, as both positive and negative electrodes exhibit severe lithium loss and a decrease in the cycling stability of graphite electrodes. Similarly, Yan et al. compared the difference in formation behavior between graphite half cells and NCM/graphite full cells at different temperatures and concluded that high-temperature formation leads to serious side reactions, while low-temperature formation will form SEI films with low ion conductivity. SEI films formed at room temperature have the best ion conductivity and stability. The difference is that Huang et al. demonstrated that after high-temperature formation, batteries have higher discharge capacity and better capacity retention rate. By exploring the cycling performance of LiNi1/3Co1/3Mn1/3O2/artificial graphite batteries at different formation temperatures (25 ℃ and 45 ℃), the results showed that the irreversible capacity loss decreased from 18.4% at 25 ℃ to 10.5% at 45 ℃. The high-temperature formation at 45 ℃ is beneficial for reducing the impedance and irreversible capacity loss of the SEI film, and the irreversible capacity loss under high current formation conditions of 1.077mA/cm2 is only 12.8%. Similarly, it has a higher transfer rate under high-temperature formation, and a more uniform SEI film can be formed on the graphite negative electrode.
Table 2: Capacity losses corresponding to (a) transformation into the state of charge during the charging phase and (b) subsequent charging cycle to 100% state of charge
Regarding the aging process, Lopez et al. believe that the most suitable temperature depends on the conditions of the previous formation. For example, if formed at room temperature, a long cycle performance can be achieved at an aging temperature of 5 ℃; When formed at a low temperature of 5 ℃, high-temperature aging at 45 ℃ can achieve the best cycling performance. By characterizing the composition of negative electrode SEI films under different aging conditions, we can improve battery performance by optimizing battery aging conditions. After aging at an ambient temperature of 5 ℃, the SEI film has a thinner thickness, stronger graphite signal peaks, and weaker signal peaks for oxygen and fluorine; The SEI film formed at an aging environment temperature of 45 ℃ is thicker, and as the aging cycle prolongs, the signal peaks of phosphorus and fluorine become stronger.
2.2 External mechanical pressure
The advantages and disadvantages of applying external mechanical pressure to lithium batteries have been reported in existing literature. Advantages include better electrode contact, less lithium deposition, and less gas appearance and distribution. Disadvantages include the possibility of graphite expansion caused by lower mechanical pressure and deformation caused by uneven pore closure of the diaphragm under higher pressure, which hinders the internal dynamics of the battery. Heimes et al. explored the time required for three stages by setting different pressures and temperatures in a synthesis program that included constant current charging, constant voltage charging, and constant current discharge. The results are shown in Figure 2. When the external mechanical pressure increases from 0.05kN to 1.70kN, the constant voltage charging stage time significantly decreases, while the constant current charging and discharging stage time does not have a significant difference. The entire process can save 14.7% of the formation time by adding external mechanical pressure. The article also proves that high external mechanical pressure has more potential to reduce battery formation time than high ambient temperature, thus increasing the possibility of saving battery costs. Additionally, when high temperatures and