The working principle of lithium sulfur batteries and the existing problems

Jan 11, 2019   Pageview:130

Lithium-sulfur battery is a kind of lithium battery. It uses sulfur as the positive electrode of the battery and a kind of metallic lithium as the negative electrode of the battery. The elemental sulfur is abundant in the earth and has the characteristics of low price and environmental friendliness.


Lithium-sulfur battery introduction


A lithium-sulfur battery is a type of lithium battery, which uses a sulfur element as a positive electrode of the battery and a lithium metal as a negative electrode. Elemental sulfur is abundant in the earth and has the characteristics of low price and environmental friendliness. Lithium-sulfur batteries using sulfur as a positive electrode material have higher theoretical specific capacity and battery theoretical energy, reaching 1675mah/g and 2600wh/kg, respectively, which is much higher than the capacity of commercially available lithium cobalt oxide batteries (< 150mah/g). And sulfur is an environmentally friendly element that has no pollution to the environment and is a very promising lithium battery.


Lithium sulfur batteries with sulfur as the anode reaction substance, the cathode for lithium. Discharge when the cathode reaction for lithium electron into lithium ion, the positive reaction for sulfur and lithium ions and electrons generated sulfide, the positive and negative response provided by the potential difference is the lithium sulfur batteries discharge voltage. Under the action of the applied voltage, the positive and negative electrodes of the lithium-sulfur battery react in reverse, which is the charging process. According to the unit mass of elemental sulfur fully into S2 - can provide electricity sulfur can be concluded that the theory of quality of discharge specific capacity is 1675mah/g, the same can be concluded that elemental lithium theoretical quality of discharge specific capacity is 3860mah/g. The theory of lithium sulfur batteries discharge voltage of 2.287 V, when sulfur and lithium reactions to generate lithium sulfide (Li2S) in full. The corresponding theory of lithium sulfur batteries discharge quality than the energy of 2600wh/kg.


Below we'll look at some of the lithium battery charge and discharge performance:


From the point of discharge curve, lithium sulfur batteries are two discharge platform, platform high voltage 2.4 V, 2.1 V low voltage platform, but the capacity is very high, easy to + 1000mah/g. There are a lot in the process of the intermediate, Li2S8, Li2S6, Li2S4.These intermediate products are often out of the way, their existence to sulfur positive brings many problems, such as shuttle effect, solubility, and the final product is electrical insulators, this reduces the rate of the reaction kinetics, lowered the rate performance of the battery, the density of sulphur is bigger than the product Li2S, i.e. Li2S than S more fluffy pile up, so the expansion of the volume is inevitable, it is also an inevitable problem.


Here, we should be able to understand the problems encountered in lithium-sulfur batteries. The current research is basically focused on these issues, to achieve high energy density, improve the content of positive S, enhance cycle stability, and safety.


The problems of lithium sulfur batteries


Main problems of lithium battery: the lithium sulfur compounds dissolve in the electrolyte; Sulfur as a non-conductive material, electrical conductivity is very bad, bad for battery high ratio of performance; Sulfur in the process of charging and discharging, the volume expansion of narrow is very big, may cause damage to the battery.


First, the poor electronic conductivity and ionic conductivity of elemental sulfur, sulfur materials at room temperature, the electrical conductivity of very low (5.0 x 10 to 30 s. cm - 1), Li2S2 and the product of Li2S also be electrical insulators, is not conducive to high rate performance of the battery.


Second, the intermediate discharge product of the lithium-sulfur battery dissolves into the organic electrolyte, increasing the viscosity of the electrolyte and reducing the ionic conductivity. Polysulfide ions can migrate between the positive and negative electrodes, resulting in loss of active material and waste of electrical energy (Shuttle effect). The dissolved polysulfide diffuses across the separator to the negative electrode and reacts with the negative electrode to destroy the solid electrolyte interface film (SEI film) of the negative electrode.


Third, lithium sulfur batteries discharge product Li2Sn (n = 1 ~ 2) electronic insulation and does not dissolve in the electrolyte, deposited on the surface of the conductive frame; Part of lithium sulfide from conductive frame, by no means the charging process of reversible reaction into sulfur or higher-order polysulphide, caused the great attenuation of capacity.


Fourth, the density of lithium sulfur and sulfide were 2.07 and 1.66 g · cm - 3, in the process of charging and discharging is up to 79% of the volume expansion/contraction, this expansion will lead to the positive changes in the morphology and structure, cause from sulfur and conductive frame, resulting in the capacity of the attenuation; The volume effect under the button batteries was not significant, but in large volume effect will be amplified in the battery, will have a significant capacity attenuation, may cause the damage of the battery, the huge volume change of the electrode structure would be destroyed.


Fifth, the lithium-sulfur battery uses metallic lithium as the negative electrode. In addition to the high activity of the metallic lithium itself, the metallic lithium negative electrode undergoes volume change during charging and discharging, and dendrites are easily formed.


Sixth, the research on the laboratory scale of lithium-sulfur batteries is carried out. The sulfur loading per unit area is generally below 3.0mg·cm-2. The research on high-load polar spectrometers is of great value for obtaining high-performance lithium-sulfur batteries.


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