Why can't lithium battery have high power and high energy density

For pure electric vehicles with lithium-ion batteries, the difficulty of charging is still a big problem, so "quick charging" has become a gimmick for many manufacturers. In the author's opinion, the quick charging problem of lithium battery needs to be analyzed from two levels.

On the cell level, the multiplier performance of lithium ion battery is restricted by the intrinsic transmission characteristics of the anode/electrolyte/negative electrode material collocation system on the one hand, and on the other hand, the chip technology and cell structure design also have a great influence on the multiplier performance. However, in terms of carrier conduction and transport operation, lithium is not suitable for "quick charge". The intrinsic carrier conduction and transport of lithium system depend on the conductivity of anode and cathode materials, the lithium ion diffusion coefficient and the conductivity of organic electrolyte.

Based on embedded reaction mechanism of lithium ion in anode materials (one-dimensional ion channels of olivine, two-dimensional channel layered materials and three-dimensional spinel cathode material) and negative graphite anode materials (layers) of the diffusion coefficient is generally water system of the secondary battery out phase REDOX reaction rate constant low several orders of magnitude. Moreover, the ionic conductivity of organic electrolyte is two orders of magnitude lower than that of water system secondary battery electrolyte (strong acid or strong base).

The negative electrode of lithium battery has an SEI film on its surface. The polarizing of powder electrode in organic electrolyte is much more serious than that of water system. In addition, under the condition of high-magnification charging, the lattice of the positive material is easily damaged, and the negative graphite sheet layer may also be damaged. All these factors will accelerate the attenuation of the capacity, thus seriously affecting the service life of the power battery.

Therefore, the intrinsic 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 single battery will be greatly reduced in the mode of quick charge and quick release, and the battery performance will decline significantly in the later period of use.

Of course, one reader might say, isn't it possible to charge and discharge lithium titanate (LTO) batteries at high rates? The multiplicities of lithium titanate can be explained by its crystal structure and ion diffusion coefficient. However, the energy density of lithium titanate battery is very low, and its power type use is achieved at the expense of energy density, which leads to the high unit energy cost ($/Wh) of lithium titanate battery, and the low cost performance determines that lithium titanate battery cannot become the mainstream of lithium battery development. Indeed, the sluggish sales of Toshiba's SCiB batteries over the past few years are telling.

At the cell level, the multifactor performance can be improved from the perspective of chip technology and cell structure design, such as making the electrode thinner or increasing the ratio of conductive agent. What is more, some manufacturers have resorted to extreme measures such as removing the thermistors from the cells and thickening the collector fluid. In fact, many domestic power battery companies have their LFP power battery in 30C or even 50C high power data as a technical highlight.

What I want to point out here is that as a test, yes, but what happens inside the cell is the key. With long time and high rate of charge and discharge, the structure of positive and negative materials may have been destroyed, and lithium has already been separated from the negative electrode. These problems need to be identified with some in-situ detection methods (such as SEM,XRD and neutron diffraction, etc.). Unfortunately, there are few reports about the application of these in-situ detection methods in domestic battery enterprises.

Here the author also reminds the reader to pay attention to the difference between the charging and discharging process of lithium battery. Different from the charging process, the damage caused by the discharge of lithium battery at a high rate of power (external work) is not as serious as that caused by quick charging, which is similar to other secondary batteries in water systems. However, for the practical use of electric vehicles, the need for high rate charging (quick charging) is undoubtedly more urgent than large current discharge.

When it comes to the level of battery pack, the situation will be more complicated. In the charging process, the charging voltage and current of different single batteries are not consistent, which will inevitably lead to the charging time of power batteries exceeding that of single batteries. That means the battery pack will surely surpass the half-capacity of a single battery in 30 minutes with conventional charging, partly because the advantages of rapid charging are not so obvious.

In addition, in the use (discharge) process of lithium ion battery, its capacity consumption and discharge time are not linear relationship, but with the time of accelerating decline. For example, if an electric car has a full range of 200km, the power battery may still have 80% of its capacity after a normal range of 100km. When the battery capacity is 50%, the electric car may only be able to travel 50km. This feature of lithium ion battery tells us that charging only half or 80% of the power of power battery cannot meet the actual use needs of electric cars. For example, Tesla's much-publicized rapid charging technology is actually more gimmick than practical in the author's opinion, and frequent quick charging will definitely worsen the service life and performance of the battery, and bring serious safety risks.

Since lithium battery is not suitable for quick charging in essence, then theoretically speaking, the mode of electrical change can make up for its shortcoming of quick charging. Although the power battery design into pluggable type leads to the vehicle structure strength and electrical insulation technical difficult problem, but also the battery interface standard and super difficult problem, but I personally believe that this model can yet be regarded as a technology for li-ion battery fast charge problems on (also only is technically more practical way.

In the author's opinion, the reason why the "battery rental + power exchange model" has no successful precedent in the world, besides the problem of consumption habits (the owner thinks that the battery is his private property just like the car), the main obstacle lies in the huge profit distribution problem hidden behind the technical standards. In the highly market-oriented west, the problem is much harder to solve than in China. Personally, the author believes that in the future, there may be a large space for the development of the electric-change-for-electricity model in the areas where pure electric vehicles, such as buses, taxis or Shared vehicles, are concentrated.

2.3.2 high power characteristics of fuel cells: compared with the problem of quick charging of lithium ion power cells, the problem of filling hydrogen into fuel cells is much easier. Almost all fc-evs today can be filled with hydrogen in three minutes. While three minutes is a bit longer than a regular refueling session, it's nothing compared to tesla's six-hour pure-charge/half-hour fast-charge. However, it is not appropriate to compare the quick charging problem of lithium with the hydrogenation of fuel cells. Because it is easy to combine electric vehicle charging with the power grid, and the hydrogenation of fuel cells, infrastructure is much harder to build than charging stations.

When it comes to the power performance, the author will discuss the power density of lithium battery and fuel cell again, because the power is actually the power problem. Technically, lithium-ion batteries can be charged and discharged at a higher rate using a process that involves making the electrodes very thin or increasing the amount of conductive material in the battery.

In other words, it is fundamentally impossible for a single lithium cell to have both high energy density and high power density. For example, A123's AHR32113 single cell has excellent magnification performance, and its power density can be as high as 2.7kw /Kg under the ultra-high magnification test condition of 40C, but its energy density is only 70Wh/Kg. For another example, the energy density of i-phone7 soft-pack cell has reached the level of 250Wh/Kg, but its power performance is relatively poor, and it can only charge and discharge at a low power rate below 0.5c.

But I want to emphasize here that fuel cells can easily be both high energy and high power, which is precisely because of their unique open working principle. PEMFC reactor is the site of electrochemical generation. Its unique heterogeneous electrocatalytic reaction process enables high exchange current density on Pt/C catalyst surface regardless of the electrochemical oxidation of hydrogen or the electrochemical reduction of oxygen.

In fact, the current density of Toyota and GM's new generation PEMFC reactors is generally close to the level of 1A/cm2 under actual working conditions (0.6-0.7v for a single battery), which is about two orders of magnitude higher than the current density of the LFP power batteries widely used in China at the rate of 1C.

ToyotaMirai's PEMFC system has an energy density of over 350Wh/Kg and a power density of 2.0KW/Kg. By contrast, TeslaModelS 'lithium-ion battery system has an energy density of 156Wh/Kg, while the power density is only 0.16kw /Kg, an order of magnitude lower than Mirai! The PEMFC stack is assembled as a single cell filter press, and its power can be increased by increasing the number of single cells (non-linearity). The energy density of PEMFC depends on the amount of hydrogen stored in the hydrogen storage system, which can also be increased by increasing the volume or number of hydrogen storage tanks.

In other words, PEMFC system can have both high energy density and high power density, which is impossible for any kind of secondary battery. The fundamental reason lies in the essential difference between closed system and open working mode. But simultaneously has the high energy and the high power condition characteristic, exactly is the modern automobile to the power system most basic technical request.

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