Therefore, charging speed has become a significant obstacle to the promotion and application of new energy vehicles. Shortening charging time can greatly improve the user experience of electric vehicles, which is of great significance to the promotion of electric vehicles.
There are generally two ways to reduce the charging time of electric vehicles: 1) One is to increase the charging speed and shorten the charging time, which is the strategy adopted by most new energy vehicles at present; 2) The second is to quickly replace the empty battery pack with a fully charged battery pack by replacing the battery pack, which is usually used in commercial vehicles such as taxis.
While increasing charging speed can shorten charging time, excessively high charging speeds can also accelerate battery degradation, affecting the lifespan of electric vehicles. Recently, Anna Tomaszewska (first author, corresponding author) and Xuning Feng (corresponding author) from Imperial College London, along with Academician Ouyang Minggao from Tsinghua University, conducted a comprehensive review and summary of the influencing factors, degradation mechanisms, and solutions for lithium-ion battery fast charging, from materials to system levels.
Lithium-ion batteries are mainly composed of a positive electrode, a negative electrode, a separator, and an electrolyte. During charging, Li+ ions are released from the positive electrode and diffuse through the electrolyte to the negative electrode. Therefore, there are three main factors that can affect the charging speed of lithium-ion batteries: 1) Li+ diffusion in the solid phase; 2) Li+ reaction at the solid/liquid interface; and 3) Li+ diffusion in the electrolyte, including solvation and desolvation. During fast charging, positive electrode degradation and the growth of the positive electrode interphase (CEI) film are usually not limiting factors. However, due to the relatively poor kinetic conditions at the negative electrode, lithium plating is more likely to occur during fast charging, thus reducing the effective area available for Li+ insertion and causing battery performance degradation.
The impact of fast charging on lifespan degradation
The impact of heat generated by fast charging on the battery
Fast charging causes heat to be generated inside the lithium battery. This heat generation in lithium batteries is mainly of two types: reversible and irreversible. Irreversible heat generation is shown in the following formula, where U is the open-circuit voltage of the battery, V is the battery voltage, and I is the current.
A significant portion of the irreversible heat generated above comes from the ohmic resistance of the battery, as shown in the following formula. The amount of heat generated is proportional to the square of the current. Therefore, the battery generates more ohmic heat during fast charging.
The reversible heat generated during battery charging primarily originates from the entropy change within the battery; the reversible heat can be calculated based on this entropy change.
Related studies have shown that reversible heat is an important source of battery heat at lower rates, while irreversible heat is an important source of battery heat at higher rates. Battery temperature has a significant impact on the lifespan of lithium batteries. Therefore, the battery temperature changes caused by fast charging have an important impact on the lifespan of lithium batteries.
Lithium batteries can be mainly divided into three categories according to their structure and shape: 1) cylindrical; 2) prismatic; 3) pouch. Different battery structures have different heat dissipation efficiencies in different directions. For example, in the diameter direction of cylindrical batteries, due to the presence of materials with poor thermal conductivity such as separators, the high temperature inside the battery is mainly concentrated in the middle of the cell. In contrast, for prismatic and pouch lithium batteries, due to the higher current density at the tabs, the high temperature area is also mainly concentrated near the tabs, and the temperature near the positive tab is usually higher than that near the negative tab.
Uneven temperature distribution within a battery leads to uneven current distribution. Improper tab placement can also contribute to this uneven current distribution. This uneven current distribution can cause localized overcharging or over-discharging during charging and discharging, as well as inconsistent side reaction rates, resulting in inconsistent battery degradation rates. Uneven temperature distribution isn't limited to the battery level; at the system level, the arrangement of individual cells within the battery module and the design of the cooling system can also create significant temperature gradients between different cells. Excessive temperature on the positive electrode side exacerbates binder decomposition, irreversible phase transitions, and the dissolution of transition metals, while on the negative electrode side, it accelerates SEI film growth, consuming the limited active lithium within the battery, leading to irreversible capacity loss and causing gas generation.
Lithium plating on the negative electrode caused by fast charging
Under normal circumstances, Li+ ions migrate from the positive electrode to the surface of the negative electrode and then embed themselves into the negative electrode. However, when the negative electrode surface is subjected to excessive current or excessively low temperature, large polarization will occur. When the polarization potential of the negative electrode surface is lower than that of metallic Li, Li+ ions will be deposited on the negative electrode surface in the form of metallic Li, causing a decrease in the coulombic efficiency of the battery and capacity loss. In severe cases, it may even puncture the separator and cause serious safety accidents.
To improve the lifespan and safety of lithium batteries, it is essential to prevent lithium plating during use. Therefore, various methods for detecting lithium plating in lithium batteries have been developed, such as optical microscopy, scanning electron microscopy, transmission electron microscopy, and nuclear magnetic resonance (NMR). However, these methods require dissection of the battery or design of a special battery structure during manufacturing. Therefore, various non-destructive methods for detecting lithium plating on the negative electrode have also been developed, such as decay rate methods, voltage plateau methods, and model methods.
Taking the rate decay method as an example, metallic Li has high reactivity. After lithium is deposited on the negative electrode surface, metallic Li will continue to react with the electrolyte, thereby consuming the limited active Li and accelerating the decay of the lithium battery. Therefore, we can judge whether the battery has lithium deposited during the cycle by the change in the battery decay rate.
Lithium plating usually leads to a slight decrease in the coulombic efficiency of a battery. Therefore, a high-precision coulombic efficiency meter can also determine whether lithium plating has occurred in a lithium battery by detecting minute changes in the coulombic efficiency of the lithium battery.
Some of the metallic Li deposited at the negative electrode can be re-intercalated into the graphite negative electrode during the resting phase after battery charging. Therefore, we can observe a plateau on the voltage curve during the battery resting process. We can determine whether lithium plating has occurred in the lithium battery by observing whether this plateau appears.
Electrode pulverization and breakage caused by fast charging
Electrode pulverization and breakage are common phenomena in lithium-ion batteries, observed in NCM, NCA, and Si anodes. The loss of active material due to electrode pulverization and breakage is a common mechanism of lithium-ion battery degradation. Based on the scale changes from micro to macro, the authors categorize pulverization and breakage phenomena into the following types: 1) cracks within the active material particles; 2) separation of active material particles from conductive agents and binders; 3) delamination between the electrode and the current collector.
The main reason for electrode pulverization and breakage is the change in Li concentration inside the battery caused by fast charging. During fast charging, due to the rapid rate of Li de-intercalation and Li insertion, a significant Li concentration gradient will appear inside both the positive and negative electrodes, resulting in uneven stress distribution inside the lithium battery. This leads to the breakage of active material particles, electrode peeling, and other phenomena, causing loss of active material.
How to improve the fast charging performance of batteries
Selection of positive and negative electrode active materials
Traditional lithium-ion batteries use graphite as the negative electrode active material. Graphite's lithium intercalation potential is close to that of metallic Li, making it highly susceptible to lithium plating during high-current charging. Studies have shown that coating the graphite negative electrode with a 1% Al₂O₃ layer can increase its capacity to 337.1 mAh/g at a high current density of 4000 mA/g. Furthermore, while Li₄Ti₅O₁₂ has a lower capacity, it exhibits excellent fast-charging performance and very good cycle stability. Its high potential also virtually eliminates the risk of lithium plating, making it highly suitable as a negative electrode material for fast-charging lithium-ion batteries.
Besides the selection of anode materials, modifying the anode/electrolyte interface is also an effective way to improve the fast-charging performance of lithium batteries. Coating graphite with amorphous carbon, metal coating, and doping (such as Cu and Sn) are all effective methods to improve the fast-charging performance of graphite anodes. At the same time, the crystal structure of graphite materials also has a significant impact on its rate performance. Studies have shown that the fast-charging performance of mesophase soft carbon is significantly better than that of mesophase graphite and hard carbon materials.
Electrode structure design
In addition to material selection, electrode design also has a significant impact on the fast-charging performance of the battery. For example, studies have shown that increasing the porosity of the electrode can effectively improve the fast-charging performance of the battery, while increasing the N/P ratio can also effectively reduce the risk of lithium plating on the negative electrode and improve the rate performance of the battery.
Battery structure design
Besides the electrode structure design, the structure design of lithium batteries also has a significant impact on their fast charging performance. The position, material, structure, and welding method of the tabs will affect the distribution of current inside the battery. At the same time, the shape of the battery will also affect the distribution of internal temperature, which in turn affects the distribution of current inside the lithium battery. Uneven current distribution is more likely to cause new polarization in the battery, leading to local lithium plating, thus affecting the battery's fast charging performance.
Battery pack design
Although there is a lot of research on the fast charging performance of lithium batteries, there is still relatively little research on the fast charging performance of battery packs. Some studies have shown that the battery pack of the Nissan Leaf electric vehicle degrades much faster at a 2C charging rate than individual cells using the same charging speed. The research shows that this is mainly due to the accumulated deviations between individual cells within the battery pack. Therefore, improving the fast charging performance of battery packs not only requires high-performance individual cells, but also places very high demands on the charging management and thermal management system of the battery pack.
Charging strategy selection
Constant current constant voltage charging (CCCV) is the most traditional charging method. This method initially uses a large current for constant current charging, and after reaching the cutoff voltage, it switches to constant voltage charging, continuously reducing the charging current to minimize battery polarization. Therefore, for this traditional charging method, the most effective way to shorten charging time is to increase the charging current. However, increasing the charging current can lead to increased polarization and longer constant voltage charging time. Furthermore, high-current charging can also cause lithium plating on the negative electrode. Therefore, choosing a suitable charging strategy is crucial for fast charging.
Multi-step constant current charging method
As the amount of lithium intercalated in a graphite anode increases, the solid-phase diffusion coefficient of Li+ continuously decreases. Based on this characteristic of graphite anodes, the multi-step constant-current charging method has emerged. This method involves multiple constant-current values during the constant-current charging phase. Initially, a larger charging current is typically chosen, and towards the end of the charging process, the constant-current value decreases to prevent lithium deposition on the anode. After multiple (at least two) constant-current charging steps, the battery enters the constant-voltage charging phase. By applying a large current initially, charging time can be effectively shortened, and this method is currently used in most electric vehicle fast-charging strategies.
Pulse charging strategy
Pulse charging is a method that uses a large current to charge the battery for a short time, followed by a period of rest or even discharge, and then performs a large current pulse charging again. The main purpose of this method is to eliminate polarization by resting and reduce the risk of lithium plating on the negative electrode. Some strategies add a discharge process to eliminate the metallic Li deposited on the surface of the negative electrode by discharging, thereby shortening the charging time and improving the cycle life of the lithium battery.
Accelerated Start-up Charging Strategy
This charging strategy is similar to the multi-step constant current charging strategy, but its initial charging current is much higher than that of the multi-step charging strategy. Studies have shown that adding a 5-minute acceleration charging current at the start of charging can shorten the charging time by 30-40% (compared to 1CCCV charging) without significantly affecting the battery life.
The impact of thermal management on fast charging of lithium batteries
Lithium-ion battery systems are highly sensitive to temperature. Excessive temperature can lead to a sharp decline in battery life, while excessively low temperature can easily cause lithium plating during charging, which can also seriously affect the battery's lifespan and, in severe cases, even cause safety accidents. Therefore, proper thermal management is of great significance for improving the fast-charging performance of lithium-ion batteries.
Heat dissipation
Based on the heat dissipation medium, heat dissipation systems can generally be divided into air cooling, liquid cooling, and phase change cooling. Air cooling has the lowest cost and simplest structure, but its heat dissipation effect is poor, making it unsuitable for fast charging systems. Liquids have a high heat capacity, so their heat dissipation effect is far superior to air cooling. In some studies, to maximize heat dissipation, the battery is even directly immersed in coolant. To prevent short circuits, non-electronic conductor liquids, such as deionized water and mineral oil, are usually used. Phase change cooling mainly utilizes the latent heat of phase change of materials to absorb the heat generated during battery charging and discharging. This strategy also has significant drawbacks. For example, when the ambient temperature is too high, the material undergoes a phase change prematurely, thus failing to absorb the heat released by the battery. Furthermore, once the material undergoes a phase change from solid to liquid, its thermal conductivity is low, making it difficult to dissipate the heat inside the battery in a timely manner.
preheating
Besides addressing the heat generation issue caused by fast charging, fast charging for lithium batteries also needs to solve the problem of lithium plating that can easily occur at low temperatures. In northern winters, temperatures typically drop significantly. To prevent lithium plating during charging, the battery must be preheated before starting charging to quickly raise its temperature to a rechargeable level, thus shortening charging time. There are many methods for preheating lithium batteries, with internal heating being the most efficient. Common internal heating methods include discharge heating, alternating pulse heating, and AC voltage heating. Studies have shown that an AC current with a 10mV amplitude can raise the temperature of an 18650 battery from -20°C to 20°C within 80 seconds. In recent years, researchers have proposed pre-installing heating elements inside the battery to achieve rapid internal heating as well.
With the continuous increase in the driving range of new energy vehicles, the range anxiety problem has been basically solved. The next challenge is to shorten the charging time. Fast charging technology is an effective way to shorten the charging time, but doing fast charging well is not a simple matter. We need to consider the selection of materials, electrode design, battery design, and the design of battery packs and thermal management systems to improve the charging speed of power lithium batteries without affecting the cycle life of power lithium batteries.
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