Reasons why lithium batteries cannot be charged quickly

　　For lithium-ion battery pure electric vehicles, the difficulty of charging is still a big problem, so "fast charging" has become a gimmick of many manufacturers.

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

　　From the cell level, the rate performance of lithium-ion batteries is restricted by the intrinsic transmission characteristics of the positive electrode/electrolyte/negative electrode material collocation system on the one hand, and on the other hand, the pole piece technology and cell structure design also have a greater impact on the rate performance. Big impact.

　　But in terms of the most intrinsic carrier conduction and transport, lithium batteries are not suitable for "fast charging". The intrinsic carrier conduction and transport behavior of the lithium battery system are determined by the main factors of the electrical conductance of the positive and negative electrode materials, the lithium ion diffusion coefficient, and the electrical conductivity of the organic electrolyte. Based on the embedded reaction mechanism, the diffusion of lithium ions in the cathode material (one-dimensional ion channel olivine, two-dimensional channel layered material and three-dimensional channel spinel cathode material) and anode graphite anode material (layer structure) The coefficient is generally several orders of magnitude lower than the rate constant of the heterogeneous redox reaction in water-based secondary batteries.

　　Moreover, the ionic conductivity of organic electrolyte is two orders of magnitude lower than that of aqueous secondary battery electrolyte (strong acid or alkali). There is a layer of SEI film on the surface of the negative electrode of the lithium battery. In fact, the rate performance of the 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, lithium is likely to be deposited on the surface of the negative electrode under high-rate or low-temperature conditions, which brings serious safety hazards.

　　In addition, under the condition of high-rate charging, the crystal lattice of the positive electrode material is easily damaged, and the graphite sheet layer of the negative electrode may also be damaged. These factors will accelerate the attenuation of the capacity and seriously affect the service 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 research results have confirmed that the cycle life of the single battery in the fast charge and fast release mode will be greatly reduced, and the battery performance will be significantly degraded in the later period of use.

　　Of course, some readers may say, can’t lithium titanate (LTO) batteries be charged and discharged at a high rate?

　　"The rate performance of lithium titanate can be explained from its crystal structure and ion diffusion coefficient. However, the energy density of lithium titanate batteries is very low, and its power use is achieved by sacrificing energy density, which leads to a high cost per unit energy ($/Wh) of lithium titanate batteries, and low cost performance determines lithium titanate Batteries cannot become the mainstream of lithium battery development. In fact, Japan's Toshiba SCiB battery's sluggish sales in recent years has already explained the problem.

　　At the cell level, the rate performance can be improved from the perspective of pole piece technology and cell structure design. Measures such as making the electrode thinner or increasing the proportion of conductive agent are commonly used technical means. What's more, some manufacturers even use extreme methods such as canceling the thermistor in the battery cell and thickening the current collector. In fact, many domestic power battery companies regard the high rate data of their LFP power batteries at 30C or even 50C as a technical highlight.　　 What I want to point out here is that there is nothing wrong with it as a test method, but what changes happen inside the battery is the key. Long time high-rate charging and discharging, maybe the structure of the positive and negative electrodes has been destroyed, and the negative electrode has already separated out lithium. These problems need to use some in-situ (In-Situ) detection methods (such as SEM, XRD and neutron diffraction, etc.) to solve these problems. clear. It is a pity that these in-situ detection methods have hardly been reported in domestic battery companies.

　　The author also reminds readers to pay attention to the difference between the charging and discharging process of lithium battery. The difference from the charging process is that the damage to the battery caused by the discharge of lithium battery at a higher rate (external work) is not as serious as that of fast charging. The water-based secondary battery is similar. But for the actual use of electric vehicles, the demand for high-rate charging (fast charging) is undoubtedly more urgent than high-current discharge.

　　When 　　 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 batteries are not consistent, which will inevitably cause the charging time of the power battery to exceed the single battery. This means that although the single battery can be charged to half the capacity within 30 minutes using conventional charging technology, the battery pack will definitely exceed this time, which to a certain extent means that the advantages of fast charging technology are not very good. obvious.

　　In addition, during the use (discharge) of a lithium-ion battery, the consumption of its capacity and the discharge time are not linearly related but decrease with time. For example, if an electric vehicle has a full-charged driving range of 200 kilometers, when it travels normally for 100 kilometers, the power battery may have 80% of its capacity left. When the battery capacity is 50%, the electric vehicle may only be able to travel. 50 kilometers away.

　　This characteristic of lithium-ion batteries tells us that just charging the power battery to half or 80% is completely unable to meet the actual needs of electric vehicles. For example, Tesla’s popular fast charging technology is actually more gimmick than practical in my opinion, and fast charging will seriously deteriorate the battery’s service life and performance, and bring safety hazards.

　　Since lithium batteries are inherently not suitable for fast charging, in theory, the battery swap mode can make up for the shortcomings of fast charging. Although the design of the power battery as a pluggable type will bring about the structural strength of the vehicle and the problems of electrical insulation, and there are also super problems of battery standards and excuses, but I personally believe that this mode can be a solution to the problem of fast charging of lithium batteries. A technically (only technically speaking) method is more feasible.

　　In the author’s opinion, the reason why the “battery leasing + battery exchange model” has no successful precedent in the world, apart from the problem of consumption habits (car owners think that the battery is the same as the car’s private property), the main obstacle lies in the hidden technology The huge benefit distribution problem behind the standard. In highly market-oriented Western countries, it is much more difficult to solve this problem than in China. The author personally believes that in the future, the battery swap mode may have more room for development in the two areas of concentrated use of pure electric vehicles in my country's buses and taxis.