Lithium-air and lithium-sulfur batteries -Lithium - Ion Battery Equipment
The current research hotspots of lithium batteries mainly focus on lithium-air batteries and lithium-sulfur batteries, both of which are considered to be the most promising new-generation lithium batteries. They are very different from previous lithium battery cathode materials in structure and reaction mechanism.
1. Lithium-air battery
Lithium-air battery is a kind of metal-air battery, and its theoretical specific energy is very high due to the use of lithium metal with the lowest molecular weight as the active material. If the oxygen mass is not calculated, it is 11140Wh/kg, and the available energy density can actually reach 1700Wh/kg, which is much higher than other battery systems. The basic structure and working mechanism of the lithium-air battery are shown in the figure below.
Lithium-air batteries can be mainly divided into water systems, organic systems, water-organic hybrid systems and all-solid-state lithium-air batteries according to the state of the electrolyte used. When the organic system lithium-air battery works, the raw material O2 enters the battery through the porous air electrode, is catalyzed into O2- or O22- on the electrode surface, and then combines with Li+ in the electrolyte to generate lithium peroxide (Li2O2) or lithium oxide ( Li2O), the product was deposited on the surface of the air electrode. When all the air channels in the air electrode are blocked by the product, the discharge of the cell is terminated. Its electrode reaction is as follows:(Lithium - Ion Battery Equipment)
Positive electrode: O2+2e-+2Li+↔Li2O2; O2+4e-+4Li+↔2Li2O
Negative electrode: Li↔Li++e-
Overall reaction: 2Li+O2↔Li2O2(2.96V); 4Li+O2↔2Li2O(2.91V)
Li-air batteries have the advantages of incomparable ultra-high energy density, environmental friendliness and low price, but their research is still in the initial stage, and there are many difficult problems, the important ones are:
(1) The positive reaction requires a catalyst. During the discharge process, in the absence of a catalyst, the oxygen reduction is very slow; during the charging process, the voltage plateau is about 4V, which is likely to cause side reactions such as the decomposition of the electrolyte. Appropriate catalysts are used to aid the battery reaction.
(2) The lithium-air battery is an open system, which can cause a series of fatal problems such as electrolyte volatilization, electrolyte oxidation, moisture in the air, and the reaction of CO2 and metallic lithium.
(3) Blockage of air electrode pores. The Li2O and Li2O2 that are insoluble in the electrolyte generated by the discharge will accumulate in the air electrode, blocking the air pores, resulting in the deactivation of the air electrode and the termination of the discharge.
To sum up, there are many problems in Li-air batteries that need to be solved urgently, including the catalysis of oxygen reduction reaction, the oxygen permeability and hydrophobicity of the air electrode, and the deactivation of the air electrode. Although lithium-air batteries have made some progress, there is still a long way to go before they can be used in real life.
2. Lithium-sulfur battery
The research on lithium-sulfur batteries first originated in the 1970s, but the actual capacity of lithium-sulfur batteries is not high and the attenuation is serious, so it has not been paid attention to. In 2009, Linda F. Nazar's research group reported that sulfur-carbon composites were used as cathode materials for lithium-sulfur batteries to obtain good cyclability and very high discharge capacity, setting off an upsurge in lithium-sulfur battery research. Lithium-sulfur batteries mainly use elemental sulfur or sulfur-based compounds as the positive electrode material of the battery, and the negative electrode mainly uses metallic lithium. The battery structure is shown in the figure.
Among them, the cathode material is calculated as elemental sulfur (mainly in the form of S8 ring), and its theoretical specific capacity is 1675mAh/g, the theoretical discharge voltage is 2.287V, and the theoretical energy density is 2600Wh/kg. When charging and discharging, the electrode reaction is as follows:
Positive electrode: S8(s)+2e-+2Li+↔Li2S8;
Li2S8+2e-+2Li+↔2Li2S4;
Li2S4+2e-+2Li+↔2Li2S2(s);
Li2S2(s)+2e-+2Li+↔2Li2S(s)
Negative electrode: Li↔Li++e-
Overall reaction: S8(s)+16e-+16Li+↔8Li2S(s)
In a lithium-sulfur battery, the reaction of the cathode material is a multi-electron, multi-step step-by-step reaction, as shown in the figure.
1. Lithium-air battery
Lithium-air battery is a kind of metal-air battery, and its theoretical specific energy is very high due to the use of lithium metal with the lowest molecular weight as the active material. If the oxygen mass is not calculated, it is 11140Wh/kg, and the available energy density can actually reach 1700Wh/kg, which is much higher than other battery systems. The basic structure and working mechanism of the lithium-air battery are shown in the figure below.
Lithium-air batteries can be mainly divided into water systems, organic systems, water-organic hybrid systems and all-solid-state lithium-air batteries according to the state of the electrolyte used. When the organic system lithium-air battery works, the raw material O2 enters the battery through the porous air electrode, is catalyzed into O2- or O22- on the electrode surface, and then combines with Li+ in the electrolyte to generate lithium peroxide (Li2O2) or lithium oxide ( Li2O), the product was deposited on the surface of the air electrode. When all the air channels in the air electrode are blocked by the product, the discharge of the cell is terminated. Its electrode reaction is as follows:(Lithium - Ion Battery Equipment)
Positive electrode: O2+2e-+2Li+↔Li2O2; O2+4e-+4Li+↔2Li2O
Negative electrode: Li↔Li++e-
Overall reaction: 2Li+O2↔Li2O2(2.96V); 4Li+O2↔2Li2O(2.91V)
Li-air batteries have the advantages of incomparable ultra-high energy density, environmental friendliness and low price, but their research is still in the initial stage, and there are many difficult problems, the important ones are:
(1) The positive reaction requires a catalyst. During the discharge process, in the absence of a catalyst, the oxygen reduction is very slow; during the charging process, the voltage plateau is about 4V, which is likely to cause side reactions such as the decomposition of the electrolyte. Appropriate catalysts are used to aid the battery reaction.
(2) The lithium-air battery is an open system, which can cause a series of fatal problems such as electrolyte volatilization, electrolyte oxidation, moisture in the air, and the reaction of CO2 and metallic lithium.
(3) Blockage of air electrode pores. The Li2O and Li2O2 that are insoluble in the electrolyte generated by the discharge will accumulate in the air electrode, blocking the air pores, resulting in the deactivation of the air electrode and the termination of the discharge.
To sum up, there are many problems in Li-air batteries that need to be solved urgently, including the catalysis of oxygen reduction reaction, the oxygen permeability and hydrophobicity of the air electrode, and the deactivation of the air electrode. Although lithium-air batteries have made some progress, there is still a long way to go before they can be used in real life.
2. Lithium-sulfur battery
The research on lithium-sulfur batteries first originated in the 1970s, but the actual capacity of lithium-sulfur batteries is not high and the attenuation is serious, so it has not been paid attention to. In 2009, Linda F. Nazar's research group reported that sulfur-carbon composites were used as cathode materials for lithium-sulfur batteries to obtain good cyclability and very high discharge capacity, setting off an upsurge in lithium-sulfur battery research. Lithium-sulfur batteries mainly use elemental sulfur or sulfur-based compounds as the positive electrode material of the battery, and the negative electrode mainly uses metallic lithium. The battery structure is shown in the figure.
Among them, the cathode material is calculated as elemental sulfur (mainly in the form of S8 ring), and its theoretical specific capacity is 1675mAh/g, the theoretical discharge voltage is 2.287V, and the theoretical energy density is 2600Wh/kg. When charging and discharging, the electrode reaction is as follows:
Positive electrode: S8(s)+2e-+2Li+↔Li2S8;
Li2S8+2e-+2Li+↔2Li2S4;
Li2S4+2e-+2Li+↔2Li2S2(s);
Li2S2(s)+2e-+2Li+↔2Li2S(s)
Negative electrode: Li↔Li++e-
Overall reaction: S8(s)+16e-+16Li+↔8Li2S(s)
In a lithium-sulfur battery, the reaction of the cathode material is a multi-electron, multi-step step-by-step reaction, as shown in the figure.
