Why lithium sulfur/lithium empty battery does not have the power battery application prospect?

Improving the specific energy has become the core technology direction of the electric vehicle range.

Although in theory also can be stacked battery load to a substantial increase in range, but also increase the cost and weight accordingly, from the Angle of economy and safety, is actually do more harm than good.

Says ai, a professor at the university of Wuhan to existing vehicle technical conditions, the most reasonable design should be monomer 300 wh/kg corresponding to the range of 300 km; Monomer 400 wh/kg corresponding to the range of 400 km; And if monomer do 500 wh/kg, the life will be able to reach 500 km.

In fact, the industry is generally accepted that Lithium electricity Technology of short-term goals is through the three yuan high nickel anode, silicon carbon negative achieve 300 wh/kg; Medium-term target (2025) is based on the rich lithium manganese base/high capacity anode, Si C achieve monomer 400 wh/kg;The forward is the development of lithium sulfur, lithium air batteries, to realize the energy 500 wh/kg of the monomer ratio.

To this, ai said, in 2020 to reach 2020 wh/kg, in addition to the security is not sure, there is no technical risk. As for the medium-term target, according to the calculation results, the positive capacity 400 wh/kg requirements 250 mah/g, the cathode capacity of 800 mah/g, the request to the present material system is feasible.

And long-term goals, lithium sulfur, empty theoretical value far more than 500 wh/kg (2600 wh/kg, lithium sulfur gas 11000 wh/kg), but its feasibility to be consideration.

Of lithium metal lithium is used as the cathode, the oxygen in the air as the anode system of a battery, of course, need oxygen electrode porous carbon as a reaction to the carrier. Although these years in catalyst selection, mechanism research and choice of electrolyte, rechargeable has made great progress, but as a product, lithium air batteries have four fatal flaw:

First, moisture control problem. Lithium air batteries is an open system, it is and Lithium ion batteries Lithium, empty to use oxygen in the air, and water is contained in the air, lithium will react with water. Want both air and water proof, this is a difficult to solve the problem.

Second, is the catalytic reduction of oxygen. Oxygen reaction speed is very slow, to improve oxygen must adopt efficient catalyst reactivity, now are precious metal catalyst, therefore, must develop effective cheap catalyst, which has been restricting the development of fuel cell.

Third, is the metal anode of lithium rechargeable. Also is the problem of the industry has been the lithium dendrite, over the past 60 years, numerous researchers advance wave upon wave, still no progress.

Fourth, is a product discharge of decomposition. Lithium battery discharge empty product is lithium oxide, solid lithium oxide catalytic decomposition again into oxygen and lithium, how hard.

To gather so many century difficult problem of lithium air batteries, its feasibility has been slim, so to speak. Look at lithium sulfur batteries and its negative use metal lithium, the positive use of sulfur, sulfur capacity is very high, 1600 mah/g, which is why everyone is a study of the causes of it.

Yet lithium sulfur batteries have many pain points, when the electrode cycle performance is poor. Sulfur electrode discharge when not directly generate lithium sulfide, but gradually be restored, accompanied by the generation of polysulfide lithium intermediate; Polysulfide lithium dissolve in the electrolyte, dissolve erosion. Dissolution of polysulfide lithium on the other hand will spread to the cathode reduction, again in the anode oxidation, shuttle effect, lead to low coulombic efficiency and high self-discharge; Lithium dissolved polysulfide, on the other hand, in the process of charging in the anode preferably deposit on the surface, resulting in table of the electrode face jam for inactivation, as a result, the electrode cycle performance is poor.

At present, the research community method, is used to block, porous carbon materials much sulfur ions adsorption, reduce the loss of dissolved it. This strategy seems to be very effective in academic, but the actual role is very limited. The main difference between laboratory research work is based on a small button cell, the electrode is very thin, sulfur capacity is not high, the total sulfur content in about a few mg level; Sulfur content is bigger and the actual battery (g), and the electrode is very thick, sulfur unit loads is very high.

Such as professor ai in lithium sulfur batteries of 863 projects, the laboratory can cycle on 1000 times of sulfur/carbon composite material, in the actual battery can only cycle several times, sometimes even a power not to come out and it is for this reason.

Lithium sulfur batteries of the second question is anode of lithium rechargeable, it is also difficult to solve the problem of short time. Process of electrochemical reaction must contain several series, the first process is from the bulk solution of reactants to the electrode on the surface of the transmission, is called liquid mass transfer; The second process for the reactants in the electrode surface gained or lost electrons, the process of forming product, called the electrochemical reaction steps. Which speed is slow, the electrode reaction is controlled by which steps.

For lithium electrode, the electron exchange process is very fast, so the liquid transmission is its control step response, which is the transmission of lithium ion ontology from the solution to the electrode surface of this step is relatively slow. It brings some problems, liquid phase transfer is actually affected by convection, as long as there is gravity, convection will exist, and the electrode surface convection velocity of each point is not the same, therefore, the reaction rate of each is different also. Where a long fast, lithium ion transmission distance is shorter, lithium deposition rate is faster and faster, and this is the reason why lithium dendrite growth.

Of course, is negative, the distance between current distribution is different also, this is also the important cause leading to the lithium dendrite growth. Obviously, these factors in actual battery is difficult to avoid, as a result, dendrite growth caused by lithium rechargeable of can't say there is no way, but it is hard to find the effective solutions.

The third problem is the volume of lithium sulfur batteries energy density is lower, probably only lithium iron phosphate batteries quite.Because of sulfur is an insulator, let it conductive, let it, let it spread out, we must adopt a large number of carbon on the surface of the aspect ratio, lead to the density of sulfur/carbon composites is very small; In addition, the reaction of sulfur is first dissolved deposit again, so the electrode must exist a large amount of liquid transport channel.

And now most of the sulfur lithium-sulfur battery electrode pole piece is not pressure, what kind of is what kind of, particularly high porosity, so the volume energy density is very low. For car, especially for passenger cars, as the energy density reaches a certain value, the volume energy density is more important, because there is no so many places passenger car battery. So from this meaning, at least in the field of vehicle dynamics, lithium sulfur batteries is no hope.

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