Why can't lithium battery be balanced with high power and high energy density?

The PEMFC system can combine both high energy density and high power density, which is impossible for any secondary battery. The fundamental reason is the essential difference between closed system and open working mode. At the same time, the characteristics of high energy and high power are precisely the most basic technical requirements of modern automobiles for power systems.

Lithium-ion battery rate and fuel cell power problem

Objectively analyze the fast charge problem of lithium-ion batteries: 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. Great impact. 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 can not 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. However, the lithium titanate battery has a very low energy density, and its power-type use is achieved by sacrificing energy density, which results in a high cost per unit of energy ($/Wh) for lithium titanate battery, and low cost performance determines lithium titanate. The battery is unlikely to 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, the negative electrode has been precipitated lithium, these problems need to use some in-situ (In-Situ) detection means (such as SEM, XRD and neutron diffraction, etc.) 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 charging only half or 80% of the power of a power battery does not meet 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 frequent fast charging will definitely deteriorate the battery life and performance, and bring serious 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 technical problems of electrical insulation, as well as the super problem of the battery standard and the interface, the author personally believes that this mode is a solution to the problem of fast charging of lithium battery. A technically (and 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 technology hidden The huge interest distribution problem behind the standard. 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 pure electric vehicles such as bus, taxi or shared car in China, there may be a large room for development.

High power characteristics of fuel cells

Compared with the fast charging problem of lithium ion power batteries, the problem of fuel cell filling with hydrogen is much easier. Almost all FC-EVs can now be filled with hydrogen in three minutes. Although three minutes is longer than the regular refueling time, compared to Tesla's 6-hour general charge/half-hour fast charge, three minutes is obviously not worth mentioning. However, comparing the fast charge problem of lithium battery with the hydrogenation of fuel cells is not appropriate to the author. Because the combination of electric vehicle charging and power grid is easy, and the fuel cell hydrogenation problem, infrastructure construction is much more difficult than building a charging station.

When it comes to rate performance, I will discuss the power density of lithium batteries and fuel cells here, because the rate is actually a power problem. Technically, lithium batteries can use some process measures (such as making the electrodes thin or increasing the amount of conductive agent, etc.) to achieve a large rate of charge and discharge, but these technical measures must sacrifice the energy density of the battery.

That is to say, fundamentally, it is impossible for a lithium battery cell to have both high energy density and high power density. For example, AHR32113 A123 single core power rate performance is excellent, the power density at a high rate of 40C test conditions may be up to 2.7 KW / Kg, but with only the energy density of 70 Wh / Kg only. For example, the energy density of the soft-package cell of i-Phone 7 has reached 250 Wh/Kg, but its rate performance is poor, and it can only charge and discharge at a low rate lower than 0.5 C.

But what I want to emphasize here is that the fuel cell can easily combine both high energy and high power characteristics, which is determined by its unique open working principle. The PEMFC stack is a place where electrochemistry occurs. Its unique heterogeneous electrocatalytic reaction process enables high exchange current density on the surface of the Pt/C catalyst, whether it is electrochemical oxidation of hydrogen or electrochemical reduction of oxygen.

In fact, a new generation of Toyota and GM PEMFC stack, under actual operating conditions (0.6-0.7 unit cell) generally substantially horizontal current density 1A / cm2, the LFP battery power than the current widely used in the domestic 1C rate The current density is about two orders of magnitude higher.

Toyota Mirai's PEMFC system has an energy density of over 350Wh/Kg and a power density of 2.0 kW/Kg. In contrast, the Tesla Model S's lithium-ion battery system has an energy density of 156 Wh/Kg, while the power density is only 0.16 KW/Kg an order of magnitude lower than Mirai! The PEMFC stack is assembled by a single cell in accordance with a filter press, and its power can be increased by increasing the number of cells (non-linear relationship). The energy density of PEMFC depends on the hydrogen storage capacity of the hydrogen storage system, and can also be improved by increasing the volume or quantity of the hydrogen storage tank.

That is to say, the PEMFC system can simultaneously have both high energy density and high power density, and this feature is impossible for any secondary battery. The fundamental reason is the essential difference between the closed system and the open working mode. At the same time, the characteristics of high energy and high power are precisely the most basic technical requirements of modern automobiles for power systems.