Increased temperature accelerates battery aging
During fast charging, the charging device charges the battery with a current several times or even tens of times higher than the standard charging current. This high current generates a large amount of heat inside the battery, causing its temperature to rise rapidly. As we know, the rate of chemical reactions generally increases with temperature, and the chemical reactions inside a battery are no exception. When the battery temperature rises, the various chemical reactions inside the battery accelerate, essentially pressing the "fast forward" button on battery aging. In high-temperature environments, the battery's active materials decompose more rapidly, and the structure of the electrode materials is gradually damaged, thus affecting the battery's performance and lifespan.
For example, when the battery temperature remains high for an extended period, the electrolyte may decompose, producing gas and increasing internal pressure, which in turn affects the battery's seal. Furthermore, high temperatures can alter the crystal structure of the electrode materials, reducing the efficiency of lithium-ion insertion and extraction, ultimately leading to a decrease in battery capacity. Moreover, if the battery temperature exceeds its normal operating range and the cooling system cannot dissipate heat effectively, thermal runaway may occur, potentially causing serious safety incidents such as battery fires or explosions.
Cell polarization intensifies
During fast charging, the large current flowing through the battery exacerbates the polarization phenomenon within the battery cells. Polarization refers to the phenomenon where, during battery charging and discharging, the electrode potential deviates from its equilibrium potential due to the presence of current. When polarization occurs, the battery's internal resistance increases, meaning that under the same current, more heat is generated inside the battery, further accelerating the temperature rise.
As polarization intensifies, the battery's charge and discharge efficiency decreases, and its actual usable capacity also diminishes. The greater the deviation of the electrodes from the equilibrium potential, the higher the energy barrier the battery needs to overcome during charge and discharge. This means that some electrical energy cannot be effectively converted into chemical energy for storage, or cannot be fully released during discharge, leading to a loss of battery capacity. Furthermore, the performance of the electrode materials in batteries that are under high polarization for extended periods will gradually deteriorate, ultimately affecting the overall lifespan of the battery.
Lithium plating phenomenon occurs in battery cells
Fast charging can also easily lead to lithium plating within the battery cells. During normal charging, lithium ions are orderly embedded into the negative electrode material. However, during fast charging, due to the excessive current, lithium ions cannot be evenly embedded into the negative electrode material in time, resulting in the direct deposition of metallic lithium on the negative electrode surface. This is because during fast charging, the temperature distribution inside the battery is uneven, with the positive electrode temperature higher than the negative electrode, causing the negative electrode temperature to be too low, thus triggering the lithium plating effect.
Lithium plating poses multiple threats to batteries. First, it reduces the number of lithium ions in the battery, which are crucial for charging and discharging; a decrease in lithium ions directly translates to a reduction in battery capacity. Second, the deposited metallic lithium is a highly reactive metal that can chemically react with the electrolyte and other substances inside the battery, further disrupting its internal structure and chemical balance. Moreover, as lithium plating accumulates, it can form lithium dendrites. These dendrites can grow to a certain size and eventually puncture the battery separator, causing a short circuit between the positive and negative electrodes, triggering thermal runaway, and seriously threatening battery safety.
Electrode material structure changes
A joint study by Argonne National Laboratory and the University of Illinois at Urbana-Champaign has revealed the underlying reasons for the performance degradation of lithium-ion batteries during fast charging in electric vehicles. The study found that at high charging rates, not only electroplating effects but also the chemical reactions at the anode during charging and discharging play a crucial role. In graphite electrodes, even small amounts of high-rate cycling can cause significant disorder, affecting lithium-ion intercalation and the electroplating process. This disorder is not uniformly distributed within the particles but exhibits reactive heterogeneity, posing a challenge to the long-term reliability of the battery.
Researchers used scanning electron nanodiffraction (SEM) to observe that the atomic-level structural distortion of graphite particles hinders lithium-ion insertion. The faster the battery charges, the more severe the distortion of the graphite atomic network, further impeding lithium-ion movement—a key factor leading to performance loss. During fast charging, not only does electroplating occur on the anode surface, but reaction products also accumulate in the internal pores, causing anode expansion and thus damaging battery performance. These changes in electrode material structure directly affect battery capacity and performance.
While fast charging brings great convenience to electric vehicle users, the above analysis shows that it does negatively impact battery capacity. To protect the battery and extend its lifespan, owners should adhere to the principle of "slow charging for daily use and fast charging for emergencies" in daily use. If a home charging station is available, slow charging should be prioritized, as it provides approximately 15% more range at the same 100% charge level compared to fast charging. This is due to the higher current, increased battery temperature, and incomplete chemical reactions during fast charging, resulting in a "phantom charge." Furthermore, charging should be performed according to the vehicle's instruction manual, avoiding overcharging and frequent fast charging to ensure the battery maintains optimal performance and health. With continuous advancements in battery technology, it is believed that better solutions will emerge in the future to balance the relationship between fast charging and battery life, making electric vehicle use more convenient and reliable.
