What are the differences between lithium-air batteries and lithium-ion batteries? Current research on lithium-ion batteries mainly focuses on lithium-air batteries and lithium-sulfur batteries. Lithium-air batteries have a higher energy density than lithium-ion batteries because their cathodes (mainly composed of porous carbon) are very light, and oxygen is obtained from the environment instead of being stored in the battery. This article will discuss the differences between the two.
What are the differences between lithium-air batteries and lithium-ion batteries?
Lithium-ion batteries
In lithium-ion batteries, the negative electrode is carbon, and the positive electrode is a different transition metal oxide, such as cobalt, manganese, or iron. Both are immersed in an electrolyte containing dissolved lithium salts. During charging, lithium ions move from the positive electrode (cathode) to the porous carbon at the negative electrode (anode), embedding themselves in the carbon material. External current flows from the negative electrode to the positive electrode (electrons move from the positive to the negative electrode), forming a closed circuit. During discharging, lithium ions are de-intercalated from the negative electrode and return to the positive electrode, and external current flows from the positive electrode to the negative electrode (electrons move from the negative to the positive electrode). Ultimately, the battery capacity depends on how much material can accommodate lithium ions, i.e., it is determined by the volume and mass of the electrodes.
Lithium-ion battery principle
When a battery is charged, lithium ions are generated at the positive electrode. These lithium ions then move through the electrolyte to the negative electrode. The carbon used as the negative electrode has a layered structure with many micropores. The lithium ions that reach the negative electrode are embedded in these micropores. The more lithium ions embedded, the higher the charging capacity.
Similarly, when a battery is discharged (i.e., when we use the battery), lithium ions embedded in the carbon layer of the negative electrode are released and move back to the positive electrode. The more lithium ions return to the positive electrode, the higher the discharge capacity. The battery capacity we usually refer to is the discharge capacity.
It's easy to see that during the charging and discharging process of a lithium-ion battery, lithium ions are in a state of motion, moving from the positive electrode to the negative electrode and back to the positive electrode. If we figuratively compare a lithium-ion battery to a rocking chair, with the two ends of the chair representing the battery's electrodes, the lithium ions are like excellent athletes running back and forth between the two ends. Therefore, experts have given lithium-ion batteries another endearing name: rocking chair batteries.
Lithium-air batteries
A lithium-air battery is a type of battery that uses lithium as the negative electrode and oxygen from the air as the positive electrode reactant. Lithium-air batteries have a higher energy density than lithium-ion batteries because their cathode (mainly composed of porous carbon) is very light, and the oxygen is obtained from the environment instead of being stored in the battery.
Unlike lithium-ion batteries, which require lithium compounds and graphite as electrodes, lithium-air batteries can use elemental lithium (Li) and oxygen (O2) from the air directly as electrodes. In the ideal scenario, during discharge, oxygen oxidizes lithium to form lithium peroxide (Li2O2), generating current in the external circuit; during charging, lithium peroxide decomposes back into lithium and oxygen. The entire process requires no other elements with significant mass, and the cathode can even use air, whose weight and cost are negligible!
Therefore, lithium-air batteries can achieve a much higher energy density than lithium-ion batteries. In fact, because lithium is the lightest metallic element in the periodic table in terms of relative atomic mass, and oxygen comes from the air, lithium-air batteries have the highest theoretical energy density among electrochemical batteries—in other words, a unit mass of lithium-air battery can store and release more energy than any other electrochemical energy storage medium.
The theoretical energy density of non-liquid lithium-air batteries can reach 12 kWh/kg, which is 5 to 10 times that of existing lithium-ion batteries and almost comparable to gasoline's approximately 13 kWh/kg. If lithium-air batteries can eventually reach the market, electric vehicles will have the same driving range as gasoline vehicles, completely breaking the range bottleneck caused by the low energy density of lithium-ion batteries. This will be of great significance to the future development of clean energy.
Working principle of lithium-air batteries
The concept of lithium-air batteries was first proposed by Lockheed, using an alkaline aqueous solution as the electrolyte. Oxygen undergoes an oxygen reduction reaction at the air electrode, forming hydroxides. The discharge reaction equation is as follows:
4Li + O₂ + 2H₂O → 4LiOH (1-1)
During discharge, Li, H₂O, and O₂ are consumed, forming a protective film on the Li surface that hinders the rapid progress of the electrochemical reaction. Under open-circuit or low-power conditions, Li exhibits a high self-discharge rate, accompanied by Li corrosion.
Li + H₂O → LiOH + 1/2H₂ (1-2)
In aqueous electrolytes, metallic lithium reacts readily with water, thus requiring highly water-resistant lithium-ion separators. Currently, there are no commercially available products with this feature. Considering both practicality and safety, aqueous lithium-air batteries are not the preferred choice for ultimate practical use.
In conclusion, the above summarizes the differences between lithium-air batteries and lithium-ion batteries. Lithium-ion batteries have experienced rapid development, with secondary battery systems, represented by lithium-ion batteries, becoming the power source for various small portable electronic devices, greatly promoting the development of electronic products. Currently, research on lithium-air batteries and lithium-sulfur batteries is still in the development stage. In addition to further improving the specific capacity and stability of the battery cathode materials, key issues such as battery safety also urgently need to be addressed.
