The charge/discharge rate of a lithium-ion battery determines how quickly we can store energy in the battery or how quickly we can release energy from it. Of course, this storage and release process is controllable, safe, and will not significantly affect the battery's lifespan or other performance indicators.
The battery capacity ratio is particularly important when batteries serve as the energy carrier for electric tools, especially electric vehicles. Imagine you're driving an electric car to run errands, and halfway there you realize the battery is almost dead. You find a charging station, but after an hour it's still not fully charged, and you're likely to miss your errands. Or perhaps your electric car is climbing a steep hill, and no matter how hard you press the accelerator, the car moves as slowly as a tortoise, giving you the urge to push it.
Clearly, these are scenarios we don't want to see, but they represent the current state of lithium-ion batteries. Charging takes a long time, and discharging can't be too rapid, otherwise the battery will age quickly and may even cause safety issues. However, in many applications, we need batteries with high charge-discharge rates, so we're once again stuck on the "battery" itself. For lithium-ion batteries to achieve better development, we need to understand what factors are limiting their rate performance.
The charge/discharge rate performance of lithium-ion batteries is directly related to the migration ability of lithium ions at the positive and negative electrodes, the electrolyte, and the interfaces between them. Any factor affecting the migration speed of lithium ions (which can also be considered equivalent to the battery's internal resistance) will affect the charge/discharge rate performance of lithium-ion batteries. Furthermore, the heat dissipation rate inside the battery is also a crucial factor affecting rate performance. If the heat dissipation rate is slow, the heat accumulated during high-rate charge/discharge cannot be dissipated, severely impacting the safety and lifespan of the lithium-ion battery. Therefore, research and improvement of the charge/discharge rate performance of lithium-ion batteries mainly focus on improving the lithium-ion migration speed and the heat dissipation rate inside the battery.
1. Improve the lithium-ion diffusion capability of both positive and negative electrodes.
The rate at which lithium ions are inserted and extracted within the positive/negative electrode active materials—that is, the speed at which lithium ions escape from the positive/negative electrode active materials, or the speed at which they enter the active materials from the positive/negative electrode surface to find a place to "settle down"—is an important factor affecting the charge/discharge rate.
For example, there are many marathon races around the world every year. Although everyone starts at roughly the same time, the road width is limited, but the number of participants is so large (sometimes tens of thousands), causing congestion. In addition, the participants' physical fitness levels vary, and the race ends up being an extremely long line. Some people reach the finish line quickly, some arrive several hours late, and some run until they faint and give up halfway.
The diffusion and movement of lithium ions at the positive and negative electrodes is essentially similar to a marathon. There are slow runners and fast runners, and the varying lengths of the paths they choose significantly limit the time it takes for everyone to finish. Therefore, we don't want to run a marathon; ideally, everyone should be running 100-meter sprints—a short enough distance for everyone to reach the finish line quickly. Furthermore, the track should be wide enough to avoid crowding, and the path should be straight rather than winding to reduce the difficulty of the race. In this way, at the referee's signal, everyone rushes towards the finish line, the race ends quickly, and the performance is excellent.
At the cathode material, we want the electrode sheet to be thin enough, meaning the active material should be thin. This shortens the "race distance," so we aim to maximize the compaction density of the cathode material. Inside the active material, there needs to be sufficient porosity to provide pathways for lithium ions. Simultaneously, these "tracks" must be evenly distributed, avoiding uneven distribution. This requires optimizing the cathode material structure, altering the distance and structure between particles to achieve uniform distribution. These two points are actually contradictory: increasing compaction density, while reducing thickness, decreases interparticle spacing, making the "tracks" more crowded. Conversely, maintaining a certain interparticle spacing hinders the thinning of the material. Therefore, we need to find a balance to achieve the optimal lithium-ion migration rate.
Furthermore, different cathode materials have a significant impact on the lithium-ion diffusion coefficient. Therefore, selecting a cathode material with a relatively high lithium-ion diffusion coefficient is also an important direction for improving rate performance.
The approach to processing anode materials is similar to that of cathode materials, primarily focusing on aspects such as material structure, size, and thickness to reduce the concentration gradient of lithium ions and improve their diffusion capabilities. Taking carbon-based anode materials as an example, recent research on nanomaterials (nanotubes, nanowires, nanospheres, etc.) to replace traditional layered anode structures can significantly improve the specific surface area, internal structure, and diffusion channels of anode materials, thereby substantially enhancing their rate performance.
2. Improve the ionic conductivity of the electrolyte.
Lithium ions are racing in positive/negative electrode materials, but swimming in electrolytes.
In swimming competitions, reducing water (electrolyte) resistance is crucial for increasing speed. In recent years, swimmers have commonly worn sharkskin suits, which significantly reduce water resistance on the body surface, thus improving performance and becoming a highly controversial topic. Lithium ions must travel between the positive and negative electrodes, much like swimming in a "swimming pool" formed by the electrolyte and battery casing. The ionic conductivity of the electrolyte, like water resistance, greatly influences the speed at which lithium ions "swim." Currently, the organic electrolytes used in lithium-ion batteries, whether liquid or solid, do not have very high ionic conductivity. The electrolyte's resistance is a significant component of the overall battery resistance, and its impact on the high-rate performance of lithium-ion batteries cannot be ignored.
Besides improving the ionic conductivity of the electrolyte, it is also crucial to focus on its chemical and thermal stability. During high-rate charge and discharge, the electrochemical window of the battery varies over a very wide range. If the electrolyte's chemical stability is poor, it is prone to oxidative decomposition on the surface of the positive electrode material, affecting the electrolyte's ionic conductivity. The thermal stability of the electrolyte has a significant impact on the safety and cycle life of lithium-ion batteries. This is because the decomposition of the electrolyte when heated generates many gases, which pose a safety hazard to the battery and, in addition, some of these gases can damage the SEI film on the negative electrode surface, affecting its cycle performance.
Therefore, selecting an electrolyte with high lithium-ion conductivity, good chemical and thermal stability, and compatibility with electrode materials is an important direction for improving the rate performance of lithium-ion batteries.
3. Reduce the battery's internal resistance
This involves several different substances and interfaces between them, and the resistance values they create, all of which affect the conduction of ions/electrons.
Conductive agents are typically added inside the positive electrode active material to reduce the contact resistance between active materials and between the active material and the positive electrode substrate/current collector, thereby improving the conductivity (ionic and electronic conductivity) of the positive electrode material and enhancing rate performance. Different materials and shapes of conductive agents will affect the battery's internal resistance, and thus its rate performance.
The current collectors (tabs) at the positive and negative electrodes are the carriers through which lithium-ion batteries transfer electrical energy to the outside world, and the resistance of the current collectors has a significant impact on the rate performance of the battery. Therefore, by changing the material, size, lead-out method, and connection process of the current collectors, the rate performance and cycle life of lithium-ion batteries can be improved.
The degree of wetting between the electrolyte and the positive and negative electrode materials affects the contact resistance at the electrolyte-electrode interface, thus impacting the battery's rate performance. Factors such as the total amount of electrolyte, viscosity, impurity content, and porosity of the positive and negative electrode materials all alter the contact impedance between the electrolyte and the electrodes, representing an important research direction for improving rate performance.
During the first cycle of a lithium-ion battery, as lithium ions embed into the negative electrode, a solid electrolyte intercalation (SEI) film forms. While the SEI film exhibits good ionic conductivity, it still somewhat hinders lithium ion diffusion, especially during high-rate charge-discharge cycles. With increasing cycle count, the SEI film continuously detaches, peels off, and deposits on the negative electrode surface, leading to a gradual increase in the negative electrode's internal resistance and becoming a factor affecting rate performance. Therefore, controlling the changes in the SEI film can improve the rate performance of lithium-ion batteries during long-term cycling.
In addition, the liquid absorption rate and porosity of the separator also have a significant impact on the permeability of lithium ions, and will also affect the rate performance of lithium-ion batteries to a certain extent (relatively small).
