The current lithium-ion battery research hotspots mainly revolve around lithium-air batteries and lithium-sulfur batteries, both of which are considered to be the most promising new generation of lithium batteries. They are very different from the previous lithium-ion battery cathode materials in terms of structure and reaction mechanism.
1 Lithium-air battery
Lithium-air battery is one of the metal-air batteries, and its theoretical specific energy is very high because it uses the lowest molecular weight of lithium metal as the active material. Without calculating the mass of oxygen, it is 11140 Wh/kg, and the actual energy density available is up to 1700 Wh/kg, which is much higher than other battery systems. The basic structure and working mechanism of lithium-air batteries are shown in the figure below.
Lithium-air batteries can be divided into water system, organic system, water-organic hybrid system and all-solid lithium-air battery according to the state of the electrolyte used. When the organic system lithium air battery works, the raw material O2 enters the inside of the battery through the porous air electrode and is catalyzed to O2- or O22- on the electrode surface, then combined with Li+ in the electrolyte to produce lithium peroxide (Li2O2) or lithium oxide (Li2O), and the product is deposited on the air electrode surface. When all the air pores in the air electrode are blocked by the products, the cell discharge is terminated. The electrode reaction is shown as follows.
Positive electrode: O2+ 2e- + 2Li+ ↔ Li2O2 ; O2 + 4e- + 4Li+ ↔ 2Li2O
Negative electrode: Li ↔ Li+ + e-
Total reaction: 2Li + O2 ↔ Li2O2 (2.96V) ; 4Li + O2 ↔ 2Li2O (2.91V)
Lithium-air batteries have incomparable advantages such as ultra-high energy density, environmental friendliness and low price, but their research is still in its infancy and there are very many difficult problems, mainly.
(1) catalyst is needed for the positive reaction. During the discharge process, oxygen reduction is very slow without the presence of a catalyst; during the charging process, the voltage plateau is about 4V, which can easily cause side reactions such as decomposition of the electrolyte. A proper catalyst is needed to help the battery reaction.
(2) Lithium-air batteries are open systems, which can cause a series of fatal problems such as electrolyte volatilization, electrolyte oxidation, and the reaction of moisture and CO2 in the air with lithium metal.
(3) Air electrode orifice clogging problem. Li2O and Li2O2, which are insoluble in electrolyte, will accumulate in the air electrode and block the air aperture, leading to air electrode deactivation and discharge termination.
In summary, there are many problems in lithium-air batteries that need to be solved, including the catalysis of oxygen reduction reaction, oxygen-permeable hydrophobicity of air electrode, and air electrode deactivation. Although lithium-air batteries have made some progress, there is still a long way to go for real applications.
2 Lithium-sulfur batteries
In 2009, Linda F. Nazar's group reported that the sulfur-carbon complex as the cathode material for lithium-sulfur batteries had good cyclability and very high discharge capacity, which set off a boom in lithium-sulfur battery research. Lithium-sulfur batteries mainly use monolithic sulfur or sulfur-based compounds as the battery cathode material, and the negative electrode mainly uses lithium metal, and its battery structure is shown in Fig.
The theoretical specific capacity is 1675 mAh/g, the theoretical discharge voltage is 2.287 V, and the theoretical energy density is 2600 Wh/kg, calculated with the monomeric sulfur (mainly in the form of S8 ring) as the cathode material.
Anode: S8(s) + 2e- + 2Li+ ↔ Li2S8 ;
Li2S8+ 2e- + 2Li+ ↔ 2Li2S4 ;
Li2S4+ 2e- + 2Li+ ↔ 2Li2S2(s) ;
Li2S2(s) + 2e- + 2Li+ ↔ 2Li2S(s)
Negative: Li ↔ Li+ + e-
Total reaction: S8(s) + 16e- + 16Li+ ↔ 8Li2S(s)
In lithium-sulfur batteries, the reaction of the cathode material is a multi-electron, multi-step step-by-step reaction, as shown in the figure.
Taking the sulfur discharge process as an example, it can be simply divided into two stages, firstly solid monolithic sulfur S8 and Li+ generate liquid Li2S8, with the depth of discharge will go through soluble Li2S6 eventually generating soluble Li2S4, corresponding to the voltage platform 2.4 V - 2.1V, this process is faster due to the generation of liquid material. Then with further discharge, at the voltage plateau of 2.1 V, soluble Li2S4 is converted into insoluble solid phase Li2S2, and finally further solid phase Li2S is generated as the end product, and the reaction speed is slower because the solid starts to be generated in this stage, which makes the ion diffusion slower. Unlike traditional lithium-ion battery materials, lithium-sulfur batteries are charged and discharged with lithium polysulfide Li2Sx (x=2-8) rather than through the embedding and de-embedding of lithium ions back and forth between the cathode and anode materials to achieve charging and discharging, so the performance of lithium-sulfur batteries is less affected by the ability to de-embed lithium ions in the cathode material.
The advantages of lithium-sulfur batteries are obvious: very high theoretical capacity; no oxygen in the material, no oxygen precipitation reaction, thus good safety performance; abundant sulfur resources and extremely low price of single-quality sulfur; environmentally friendly, low toxicity. However, the real application of lithium-sulfur batteries still faces some problems, mainly including.
(1) poor electrical conductivity and lithium conductivity: monomeric sulfur molecules are connected to 8 S to form a crown S8, which is a typical electronic, ionic insulator, its electrical conductivity at room temperature is only 5 × 10-30 S/cm. and the product Li2S2 and Li2S are also electronic insulators. Thus, the utilization rate of the active material is not high and the multiplicity performance is not good. At present, the problem of electrical conductivity and lithium conductivity of lithium-sulfur battery cathode materials is mainly solved by preparing small-size sulfur-carbon composites.
(2) Lithium polysulfide shuttle effect: During the charging and discharging process of lithium-sulfur batteries, the long-chain lithium polysulfide Li2Sx(4
(3) volume expansion problem: sulfur in the fully charged into lithium sulfide, volume expansion of 76%, easy to cause the structure of the cathode material is destroyed, affecting the stability of the active material, resulting in capacity decay.
(4) lithium metal negative electrode: because sulfur itself does not contain lithium atoms, it is necessary to use lithium metal monomers as the negative electrode material, but this will inevitably produce the problem of lithium metal dendrites, posing a safety hazard.
Although lithium-sulfur batteries still have some problems, in recent years, with the in-depth research on lithium-sulfur batteries, a lot of progress has been made in improving the capacity and cyclability of sulfur materials by reducing the size of sulfur particles, coating sulfur materials, preparing sulfur-carbon composites, adsorption of lithium polysulfide, improving electrolytes, and many other measures.
In the past three decades, lithium batteries have experienced rapid development, in which the secondary battery system represented by lithium-ion batteries has become the power source of various small portable electronic devices, greatly promoting the development of electronic products, making smart phones, tablet PCs, digital cameras, notebook computers and other portable devices widely popular. With the continuous development of society, the demand for secondary batteries in large electric drive equipment is increasing day by day, however, the theoretical specific capacity limit of the cathode material in lithium-ion batteries is low, which seems to be stretched in the power supply system of large electric drive equipment. Lithium-air and lithium-sulfur batteries, as a new generation of secondary battery systems, have very high theoretical specific capacity values and are of keen interest to researchers and the secondary battery market, but the research on lithium-air and lithium-sulfur batteries is still in the R&D stage. The understanding of the working principle of lithium battery cathode materials can help to grasp the core issues of such battery research and the development of battery cathode materials.
