Why not lithium batteries charge fast?

For lithium-ion battery pure electric vehicles, charging is still a big problem, so "fast charge" has become the gimmick of many manufacturers. The author personally believes that the fast charge problem of lithium battery needs to be analyzed from two levels.

From the level of the cell, the rate performance of the lithium-ion battery is limited by the intrinsic transmission characteristics of the cathode/electrolyte/negative electrode material system. On the other hand, the pole piece process and the cell structure design also have higher rate performance.

However, from the most intrinsic carrier conduction and transmission operation, lithium battery is not suitable for "fast charge". The intrinsic carrier conduction and transport operation of the lithium battery system is mainly dependent on the conductance of the positive and negative materials and the lithium ion diffusion coefficient and the conductivity of the organic electrolyte. Based on the embedded reaction mechanism, lithium ions diffuse in the positive electrode material (one-dimensional ion channel olivine, two-dimensional channel layered material and three-dimensional channel spinel cathode material) and negative electrode graphite anode material (layered structure) The coefficient is generally several orders of magnitude lower than the rate constant of the heterogeneous redox reaction in the aqueous secondary battery.

Moreover, the ionic conductivity of the organic electrolyte is two orders of magnitude lower than that of the aqueous secondary battery electrolyte (strong acid or strong base). The surface of the negative electrode of lithium battery has a layer of SEI film. In fact, the rate performance of lithium battery is largely controlled by the diffusion of lithium ions in the SEI film. Since the polarization of the powder electrode in the organic electrolyte is much more serious than that of the water system, the surface of the negative electrode is prone to lithium deposition under high-rate or low-temperature conditions, which poses a serious safety hazard.

In addition, under large-rate charging conditions, the lattice of the positive electrode material is easily damaged, and the negative graphite sheet may also be damaged. These factors will accelerate the attenuation of the capacity, thereby seriously affecting the life of the power battery . Therefore, the essential characteristics of the embedded reaction determine that lithium-ion batteries are not suitable for high-rate charging. The results of the study have confirmed that the cycle life of the single cell in the fast charge and fast release mode will be greatly reduced, and the battery performance is significantly degraded after use.

Of course, some readers may say that lithium titanate (LTO) batteries cannot be charged and discharged at a large rate.

The rate performance of lithium titanate can be explained by its crystal structure and ion diffusion coefficient. But lithium titanate battery energy density is very low, which is relying on the use of power-type sacrifice energy density achieved, which led titanate lithium battery unit of energy ($ / Wh) high cost, low-cost lithium titanate decided not May become the mainstream of lithium battery development. In fact, the downturn in sales of Toshiba SCiB batteries in Japan has already explained the problem.

At the cell level, it is possible to improve the rate performance from the perspective of the pole piece process and the cell structure design. For example, measures such as making the electrode thinner or increasing the proportion of the conductive agent are common technical means. What's more, even manufacturers have adopted extreme methods such as eliminating the thermistor in the cell and thickening the current collector. In fact, many domestic power battery companies have made high-magnification data of their LFP power batteries at 30C or even 50C as a technical highlight.

What I want to point out here is that it is understandable as a test method, but what changes have occurred inside the cell is the key. Long-term high-rate charge and discharge, perhaps the structure of the positive and negative materials has been destroyed, and the negative electrode has already been precipitated with lithium. These problems require some in-situ (In-Situ) detection methods (such as SEM, XRD and neutron diffraction). clear. Unfortunately, these in-situ detection methods have rarely been reported in domestic battery companies.

The author here also reminds the reader to pay attention to the difference between the charging and discharging process of lithium battery. Unlike the charging process, the lithium battery is discharged at a higher rate (external work) and the damage caused to the battery is not as severe as the fast charge. The water secondary battery is similar. However, for the actual use of electric vehicles, the demand for high-rate charging (fast charging) is undoubtedly more urgent than high-current discharging.

When it rises to the level of the battery pack, the situation will be more complicated. During the charging process, the charging voltage and charging current of different single cells are inconsistent, which inevitably causes the charging time of the power battery to exceed the single battery. This means that although conventional charging technology can charge a single battery to half the capacity in 30 minutes, the battery pack will definitely exceed this time, which means that the advantage of fast charging technology is not very obvious.

In addition, during the use (discharge) of a lithium ion battery, the consumption of the capacity and the discharge time are not linear but accelerate with time. For example, if an electric car has a full range of 200 kilometers, then when it runs 100 kilometers normally, the power battery may still have 80% capacity. When the battery capacity is 50%, the electric car may only be able to drive 50 kilometers.

This characteristic of lithium-ion batteries tells us that simply charging half or 80% of the power battery is completely incapable of meeting the actual needs of electric vehicles. For example, Tesla promotes more rapid charging technology, which in fact seems to be more practical than the author, and fast charging will seriously deteriorate the battery life and performance, and bring security risks.

Since lithium battery is not suitable for fast charging in essence, then theoretically, the power-switching mode can make up for its fast charging shortcomings. Although the design of the power battery into a pluggable type will bring about the structural strength problem of the whole vehicle and the problem of electrical insulation, and there are also super problems in battery standards and excuses, I personally believe that this mode is a solution to the problem of lithium battery fast charging. A technically (only technically) approach is more feasible.

In my opinion, the reason why “battery rental + power exchange mode” has not been successful in the world, except for the problem of consumption habits (the owner thinks that the battery is the same as the private property of the car), the main obstacle lies in the huge interest distribution standard behind the technology. In highly marketed Western countries, it is much more difficult to solve this problem than in China. The author personally believes that in the future, in the field of centralized use of two pure electric vehicles such as buses and taxis in China, there may be a large room for development.