For pure electric vehicles with lithium-ion batteries, the difficulty of charging is still a big problem, so “fast charging” has become a gimmick of many manufacturers. Xiaobian personally believes that the rapid charging of lithium power needs to be analyzed from two levels.
From the core level, the rate performance of lithium-ion batteries is restricted by the intrinsic transmission characteristics of the cathode/electrolyte/negative electrode material collocation system on the one hand, and on the other hand, the process of the plate and the structure design of the core also have a great impact on the rate performance. However, from the most intrinsic carrier conduction and transport operation, lithium is not suitable for “fast charging”. The intrinsic carrier conduction and transport of lithium system depend on the conductivity of cathode material, diffusion coefficient of lithium ion and conductivity of organic electrolyte.
Based on the embedded reaction mechanism, the diffusion coefficients of lithium ions in cathode materials (olivine in one-dimensional ion channels, layered materials in two-dimensional channels and Spinel Cathode Materials in three-dimensional channels) and graphite negative materials (layered structures) are generally several orders of magnitude lower than the rate constants of heterogeneous redox reactions in water-based secondary batteries. Moreover, the ionic conductivity of organic electrolyte is two orders of magnitude lower than that of water-based secondary battery electrolyte (strong acid or alkali).
Lithium batteries are batteries with lithium metal or lithium alloy as anode materials and non-aqueous electrolyte solutions. Lithium metal batteries were first proposed and studied by Gilbert N. Lewis in 1912. In the 1970s, M.S. Whittingham proposed and began to study lithium-ion batteries. Because the chemical characteristics of lithium metal are very active, the processing, preservation and use of lithium metal require very high environmental requirements. With the development of science and technology, lithium batteries have become the mainstream.
There is a SEI film on the surface of lithium electrodes. In fact, the rate performance of lithium electrons is largely controlled by the diffusion of lithium ions in the SEI film. Because the polarization of powdered electrodes in organic electrolyte is much more serious than that in water system, it is easy to precipitate lithium on the surface of negative electrodes at high rate or low temperature, which brings serious security risks. In addition, the lattice of cathode material is vulnerable to damage and the graphite sheet of negative electrode is also likely to be damaged under the condition of high rate charging. These factors will accelerate the capacity decay, thus seriously affecting the service life of power batteries.
Therefore, the intrinsic characteristics of embedded reactions determine that lithium-ion batteries are not suitable for high-rate charging. The research results have confirmed that the cycle life of single cell battery will decrease significantly under fast charge and fast release mode, and the battery performance will decrease significantly in the later period of service.
Of course, some readers may say that lithium titanate (LTO) batteries can not be charged and discharged at a high rate? The rate 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 their power-type uses are achieved by sacrificing energy density, which leads to high cost per unit energy ($/Wh) of lithium titanate batteries. Low cost performance ratio determines that lithium titanate batteries can not become the mainstream of lithium power development. In fact, the downturn in sales of Toshiba SCiB batteries in recent years has illustrated the problem.
At the core level, measures such as making the electrode thinner or increasing the proportion of conductive agent are commonly used to improve the rate performance from the viewpoint of chip technology and structure design. What’s more, some manufacturers even adopt such extreme methods as eliminating the thermistor in the core and thickening the collector. In fact, many domestic power battery companies regard the high rate data of their LFP power batteries at 30C or even 50C as the technical highlight.
What Xiaobian wants to point out here is that, as a means of testing, there is no criticism, but what changes have taken place inside the core is the key. For a long time, the structure of cathode and cathode materials has been destroyed and lithium has been precipitated. These problems need in-situ (in-situ) detection methods (such as SEM, XRD and neutron diffraction) to clarify. Unfortunately, there are few reports about the application of these in-situ detection methods in domestic battery enterprises.
Here, the editor also reminds readers of the difference between lithium charging and discharging. Unlike the charging process, the damage to the battery caused by lithium discharging (external work) at a higher rate is not as serious as that caused by fast charging, which is similar to other secondary batteries in water system. However, for the actual use of electric vehicles, the need for high rate charging (fast charging) is undoubtedly more urgent than high current discharge.
Up to the battery level, the situation will be more complex. In the charging process, the charging voltage and current of different single batteries are not the same, which will inevitably cause the charging time of power batteries to exceed that of single batteries. This means that although conventional charging technology can charge a single battery to half its capacity in 30 minutes, the battery pack will certainly exceed this time, which to some extent means that the advantages of fast charging technology are not very obvious.
In addition, the capacity consumption of lithium-ion batteries is not linear with the discharge time, but decreases with time. For example, an electric vehicle has a full mileage of 200 kilometers. When it is running 100 kilometers normally, the power battery may have 80% capacity left. When the battery capacity is 50%, the electric vehicle may only be able to drive 50 kilometers. This characteristic of lithium-ion batteries tells us that only half or 80% of the power battery can not meet the actual needs of electric vehicles. than