According to the point of view of Ai Xinping of Wuhan University, the most reasonable design of the whole vehicle should be that the single 300h / kg corresponds to the battery life of 300 km; the single Body 400wh / kg corresponding battery life 400km; and if the monomer to do 500wh / kg, then the battery will reach 500km.
In fact, it is generally accepted in the industry that the immediate goal of lithium-ion technology is to achieve 300 Wh / kg through a high-nickel ternary positive and silicon carbon negative; the medium-term (2025) goal is based on a lithium-rich manganese based / high capacity Si-C negative Monomer 400wh / kg; long-term goal is to develop lithium-sulfur, lithium battery, to achieve the specific energy than 500wh / kg monomer.
The following figure lists the battery life of various types of battery electric vehicles, the current lithium-ion battery is 160 km, continue to optimize the lithium-ion battery is expected to reach 200 km in the future.
Looking at the present research progress of various types of batteries, lithium-air and lithium-sulfur batteries are generally considered to have great potential, but the author is not optimistic about the prospects of their batteries. The following figure shows the lithium-air battery (with water and anhydrous electrolytes) and lithium Sulfur battery works.
01 Why lithium batteries can not?
Of course, the oxygen electrode requires porous carbon as a reaction support, although catalyst selection, mechanism studies, electrolyte selection, chargeability, etc., have been the major culprit in the past few years because of the use of lithium metal as the negative electrode and oxygen in the air as the positive electrode Great progress has been made, but as a product, lithium-ion battery has four fatal flaws:
1. Moisture control
Lithium-ion battery is an open system, which is not the same as lithium-ion batteries, lithium-air use of oxygen in the air, and the air contains water, lithium will react with water .Once both oxygen and water, which is a Difficult to solve the problem.
2. Catalytic reduction of oxygen
Lithium is not stable enough, currently only used in pure oxygen environment
The reaction speed of oxygen is very slow. To improve the reactivity of oxygen, an efficient catalyst must be used. Nowadays the catalysts are all precious metals. Therefore, it is necessary to develop efficient and cheap catalysts, which have always been the shortcomings restricting the development of fuel cells.
3. Lithium metal anode chargeability
That is, the industry has been studying lithium dendrites, 60 years ago, numerous scientific and technological personnel one after another, still not the slightest progress.
Discharged product decomposition
Lithium-ion battery discharge product is lithium oxide, the solid lithium oxide re-catalyzed decomposition of oxygen and lithium, how difficult.
Gathering so many years of problems in a lithium-ion battery, its feasibility can be said to have been very slim.
02 Why lithium sulfur battery does not work?
Lithium-air batteries have only recently attracted attention, while lithium-sulfur batteries were studied as early as 1940. Lithium-sulfur batteries use lithium metal for the negative electrode and sulfur for the positive electrode, with a very high capacity of 1600 mAh / g, which is also Why do you study the reasons for it, but there are many lithium-sulfur battery pain points.
Electrode cycle performance is poor
The first is that the electrode cycle performance is poor.Sulfur electrode discharge is not directly generated lithium sulfide, but gradually be reduced, with the formation of lithium polysulfide intermediate products; lithium polysulfide will dissolve in the electrolyte, the dissolution of the loss. On the one hand, dissolved lithium polysulfide diffuses to the negative electrode and then oxidizes at the positive electrode, resulting in a shuttle effect resulting in low coulombic efficiency and high self-discharge; on the other hand, dissolved lithium polysulfide also has a positive charge on the positive electrode surface during charging Deposition, resulting in the electrode surface due to blockage inactivation, therefore, the electrode cycle performance is poor.
At present, the scientific research method is to use porous carbon material to block, to adsorb polysulfide ions and reduce its loss of dissolution.This strategy seems academic very effective, but the actual effect is very limited.The main difference between the two is Laboratory research is based on very small button cells, very thin electrodes, low sulfur loading, and a total sulfur content of about a few milligrams; while real cells have a higher sulfur content (grams) and Electrode is very thick, high sulfur loading unit.
For example, in Professor Ai Xinping lithium-sulfur battery 863 project, the laboratory can cycle 1000 times the sulfur / carbon composite material in the actual battery can only be recycled several times, and sometimes even a power can not be released, it is This reason.
2. Lithium negative chargeability
The chargeability of lithium anode is also a difficult problem to solve in short time.Electrochemical reaction must contain several processes in series, the first one is the transfer of reactants from the bulk solution to the electrode surface, which is called liquid mass transfer. The second process is the reaction of the electrode surface to get or lose the electron, the formation of the product process, called the electrochemical reaction step, which slow, the electrode reaction is controlled by which step.
For the lithium electrode, the electron exchange process is very fast, so the liquid phase transfer is the reaction control step, that is, the transfer of lithium ions from the bulk of the electrode to the electrode surface is a relatively slow step.This brought some problems, the liquid phase The transfer is actually affected by the convection, as long as there is gravity, there will be convection, and the electrode surface convection speed is not the same at each point, therefore, the reaction speed of each point is different.Where place is fast, lithium-ion The shorter the transmission distance, the faster the deposition rate of lithium, which is the reason for the growth of lithium dendrites.
Of course, the distance between positive and negative is not the same, the current distribution is not the same, which is also led to the growth of lithium dendrite important reason.Obviously, these factors in the actual battery is difficult to avoid, therefore, dendritic growth Caused by the lithium rechargeable problem can not say there is no solution, but it is still difficult to find an effective solution.
Volumetric energy density is lower
Lithium-sulfur batteries have relatively low volumetric energy densities and may be comparable to lithium iron phosphate batteries because sulfur is an insulator that allows it to react, allowing it to react and disperse it, necessitating the use of large amounts of high specific surface carbon, resulting in sulfur / carbon The density of the composite material is very small; in addition, the sulfur reaction is first dissolved and then deposited, so the electrode must have a large number of liquid-phase transport channels.
And now most of the lithium-sulfur battery sulfur electrode pole piece can not be pressure, what kind of coated what kind of, the porosity is particularly high, so the volume energy density is very low for cars, especially passenger cars When the energy density reaches a certain value, the volume energy density is even more important, because the passenger car is not so much place batteries.
So in this sense, at least in the field of automotive power, lithium-sulfur battery is no hope.
summary
From the current perspective, there is no technical risk to achieve the recent target of 300wh / kg by 2020, except for less certainty of safety. As for the medium-term target, according to the calculation, 400wh / kg requires a positive capacity of 250mAh / g, with a negative capacity of 800 mAh / g, this requirement is also feasible with current material systems.
However, the long-term goal, lithium sulfur, lithium empty theoretical value far more than 500wh / kg (lithium-sulfur 2600wh / kg, lithium gas 11000wh / kg), lithium-sulfur batteries may be closer to the market than the lithium-air batteries. The problem has not been solved for decades and the study of lithium air is still short. As a whole, chemical problems have not yet been clearly understood and are currently difficult to commercialize. It is difficult to say which one will ultimately be commercialized in the future, and its feasibility To be considered.
