The reason the lithium battery can't charge quickly

For pure electric vehicles with lithium-ion batteries, charging difficulties are still a big problem, so "quick charge" has become a gimmick for many manufacturers. The author thinks that the fast charge problem of lithium electricity needs to be analyzed from two levels.

At the core level, the doubling performance of lithium-ion batteries is limited by the intrinsic transmission characteristics of positive/electrolyte/negative electrode materials, and on the other hand, the electrode process and core structure design also have a greater impact on the doubling performance..

However, in terms of the most essential carrier conduction and transport behavior, lithium electricity is not suitable for "fast charging." The characteristic carrier conduction and transport behavior of lithium electric system depends on the conductivity of positive and negative electrode materials, the diffusion coefficient of lithium ions, and the conductivity of organic electrolytes. Based on the embedded reaction mechanism, lithium-ion positive material(olivine of one-dimensional ion channel, The diffusion coefficients in the two-dimensional channel layered material and the three-dimensional channel spinel positive material) and the negative graphite negative electrode material(layered structure) are generally several orders of magnitude lower than the rate constant of the heterogeneous Redox reaction in water secondary batteries..

Moreover, the Ionic conductivity of organic electrolytes is two orders of magnitude lower than that of water secondary batteries(strong acids or strong bases). The negative electrode surface of lithium has a layer of SEI membrane. In fact, the doubling performance of lithium is largely controlled by the diffusion of lithium ions in the SEI membrane. Since the polarization of powder electrodes in organic electrolytes is much more serious than that of water systems, lithium can be easily analyzed on negative surfaces under high magnification or low temperature conditions, which brings serious safety hazards.

In addition, under the condition of high-power charging, the lattice of the positive electrode material is vulnerable to destruction, and the negative electrode graphite sheet layer may also be damaged. These factors will accelerate the attenuation of the capacity, which will seriously affect the service life of the power cell. Therefore, the essential characteristics of the embedded reaction determine that lithium ion batteries are not suitable for high power charging. The results have confirmed that the cycle life of single battery will decrease significantly in fast charging and fast discharging mode, and the performance of battery will decrease significantly in the later stage of use.

Of course, some readers may say that lithium titanate(LTO) batteries can be charged and discharged at a large rate?

The doubling performance of lithium titanate can be explained by its crystal structure and ion diffusion coefficient. However, the energy density of lithium titanate batteries is very low, and its power type use is based on the sacrifice of energy density. This leads to a high cost per unit energy($/ Wh) of lithium titanate batteries, and the low value for money determines the lithium titanate battery. It is impossible to become the mainstream of lithium electricity development. In fact, Japan's sluggish sales of Toshiba's SCiB batteries over the years have shown the problem.

At the core level, doubling performance can be improved from the point of view of electrode technology and core structure design. For example, measures such as making electrodes thinner or increasing the proportion of conductive agents are commonly used technical measures. What is more, some manufacturers even use extreme methods such as removing the thermistor in the core and thickening the set fluid. In fact, many power battery companies in China use their LFP power battery data at a high rate of 30C or even 50C as a technical highlight.

What I want to point out here is that as a test method is not wrong, but what changes have taken place inside the core is the key. For a long period of high magnification, perhaps the structure of the positive and negative poles has been destroyed, and the negative Poles have already analyzed lithium. These problems require the use of in-situ detection methods(such as SEM, XRD, and neutron diffraction, etc.). Can be clear. Unfortunately, these in-situ testing methods have little application in domestic battery companies.

I would also like to remind readers of the difference between the charge process of lithium electric discharge and the discharge process. Unlike the charging process, the damage caused by the discharge of lithium electric discharge(external work) to the battery is not as serious as the rapid charge. This is similar to other water secondary batteries. However, for the actual use of electric vehicles, the need for high power charging(fast charging) is undoubtedly more urgent than large current discharge.

Rising to the battery level will be more complicated. The charging voltage and charging current of different individual cells are not consistent during the charging process, and the charging time of the power cell will inevitably exceed that of the single cell. This means that although conventional charging technology can also charge a single cell to half its capacity within 30 minutes, the battery pack will certainly exceed this time, which in part means that the advantages of fast charging technology are not very obvious.

In addition, during the use(discharge) of lithium-ion batteries, the consumption and discharge time of their capacity are not linear but accelerate over time. For example, when an electric car has a full mileage of 200 kilometers, when it is 100 kilometers normal, the power battery may have 80 % of the capacity left. When the battery capacity is 50 %, the electric car may only be able to travel 50 kilometers.

This characteristic of lithium-ion batteries tells us that simply charging power cells to half or 80 % is not enough to meet the actual needs of electric vehicles. For example, Tesla's promotion of fast charging technology is actually more gimmick than practical in the author's view, and fast charging will seriously deteriorate the service life and performance of the battery, and it will bring safety risks.

Since lithium is not essentially suitable for fast charging, theoretically, the switching mode can make up for its fast charging shortcomings. Although designing a power battery as a pluggable type will cause problems with the structural strength of the vehicle and electrical insulation, and there are also super problems with battery standards and excuses, but I personally believe that, This model is a technically(and only technically) feasible solution to the problem of rapid charge of lithium.

In my opinion, the "battery lease + switch model" has no successful precedent on a global scale, except for the problem of consumer habits(owners believe that batteries are their private property like cars). The main obstacle lies in the huge distribution of benefits hidden behind technical standards. In highly market-oriented Western countries, solving this problem is much more difficult than in China. Personally, the author believes that in the future, there may be more room for the development of electric vehicle in the field of bus and taxi in China.

The page contains the contents of the machine translation.