For lithium-ion battery-powered electric vehicles, charging difficulties remain a significant problem, making "fast charging" a selling point for many manufacturers. In my opinion, the fast charging issue for lithium batteries needs to be analyzed from two perspectives.
From the perspective of the battery cell, the rate performance of lithium-ion batteries is constrained by the intrinsic transport characteristics of the positive electrode/electrolyte/negative electrode material combination system, and the electrode process and cell structure design also have a significant impact on the rate performance.
However, from the perspective of intrinsic carrier conduction and transport behavior, lithium batteries are not suitable for "fast charging". The intrinsic carrier conduction and transport behavior of lithium battery systems depends on several key factors, including the conductivity of the positive and negative electrode materials, the lithium-ion diffusion coefficient, and the conductivity of the organic electrolyte. Based on the embedded reaction mechanism, the diffusion coefficient of lithium ions in positive electrode materials (olivine with one-dimensional ion channels, layered materials with two-dimensional channels, and spinel positive electrode materials with three-dimensional channels) and negative electrode graphite negative electrode materials (layered structure) is generally several orders of magnitude lower than the rate constant of heterogeneous redox reactions in aqueous secondary batteries.
Furthermore, the ionic conductivity of organic electrolytes is two orders of magnitude lower than that of aqueous secondary battery electrolytes (strong acids or strong bases). The negative electrode surface of a lithium battery has an SEI film, and the rate performance of a lithium battery is largely controlled by the diffusion of lithium ions within this SEI film. Because the polarization of powder electrodes in organic electrolytes is much more severe than in aqueous electrolytes, lithium deposition on the negative electrode surface is prone to occur at high rates or low temperatures, posing a serious safety hazard.
Furthermore, under high-rate charging conditions, the crystal lattice of the positive electrode material is easily damaged, and the graphite layers of the negative electrode may also be damaged. These factors will accelerate capacity decay, thus severely affecting the lifespan of the power battery. Therefore, the inherent characteristics of embedded reactions determine that lithium-ion batteries are not suitable for high-rate charging. Research results have confirmed that the cycle life of a single battery cell will decrease significantly under fast charging and discharging modes, and battery performance will degrade significantly in the later stages of use.
Of course, some readers might say, "Don't lithium titanate (LTO) batteries have high charge and discharge rates?"
The rate performance of lithium titanate can be explained by its crystal structure and ion diffusion coefficient. However, lithium titanate batteries have very low energy density, and their power applications rely on sacrificing energy density. This results in a high cost per unit energy ($/Wh), and the low cost-effectiveness means that lithium titanate batteries cannot become the mainstream of lithium battery development. In fact, the sluggish sales of Toshiba's SCiB batteries in recent years have already illustrated this problem.
At the cell level, rate performance can be improved through electrode manufacturing processes and cell structure design. Common techniques include making the electrodes thinner or increasing the proportion of conductive agent. Some manufacturers have even gone so far as to eliminate thermistors from the cell and thicken the current collector. In fact, many domestic power battery companies highlight the high rate performance of their LFP power batteries at 30C or even 50C.
The author points out that while testing methods are acceptable, the key is understanding what changes actually occur inside the battery cell. Prolonged high-rate charging and discharging may damage the structure of the positive and negative electrode materials, and lithium plating may have already occurred on the negative electrode. These issues require in-situ detection methods (such as SEM, XRD, and neutron diffraction) to clarify. Unfortunately, there are almost no reports of these in-situ detection methods being used by domestic battery companies.
The author would also like to remind readers of the difference between the charging and discharging processes of lithium batteries. Unlike the charging process, the damage to lithium batteries caused by discharging (doing work) at a higher rate is not as severe as that caused by fast charging, similar to other aqueous rechargeable batteries. However, for the actual use of electric vehicles, the need for high-rate charging (fast charging) is undoubtedly more urgent than high-current discharging.
The situation becomes more complex at the battery pack level. During charging, the charging voltage and current of different individual cells are not consistent, inevitably causing the charging time of the entire battery pack to exceed that of individual cells. This means that while conventional charging technology can charge an individual cell to half its capacity within 30 minutes, the battery pack will definitely take longer. This, to some extent, suggests that the advantages of fast charging technology are not entirely significant.
Furthermore, during the use (discharge) of lithium-ion batteries, the capacity consumption is not linearly related to the discharge time but decreases at an accelerated rate over time. For example, if a fully charged electric vehicle has a range of 200 kilometers, after driving 100 kilometers, the battery may still have 80% capacity remaining. When the battery capacity drops to 50%, the electric vehicle may only be able to travel 50 kilometers.
This characteristic of lithium-ion batteries tells us that simply charging the battery to half or 80% is completely insufficient to meet the actual needs of electric vehicles. For example, Tesla's much-touted fast-charging technology, in my opinion, is more hype than practicality. Moreover, fast charging will inevitably and severely degrade battery life and performance, and also pose safety hazards.
Since lithium batteries are inherently unsuitable for fast charging, theoretically, battery swapping can compensate for this shortcoming. Although designing the power battery as pluggable would bring problems with the structural strength of the entire vehicle, as well as electrical insulation, and there are also major challenges related to battery standards and interfaces, I personally believe that this model is a technically (only from a technical perspective) feasible solution to the problem of fast charging of lithium batteries.
In my view, the reason why the "battery leasing + battery swapping model" has not yet had a successful precedent globally, besides the issue of consumer habits (car owners consider batteries to be their private property, just like the car), is mainly due to the huge profit distribution issues hidden behind technical standards. In highly market-oriented Western countries, solving this problem is much more difficult than in China. Personally, I believe that the battery swapping model has significant potential for development in my country's public transportation and taxi sectors, where pure electric vehicles are prevalent.
