Structure and rechargeability of a room-temperature Li-N2 battery. (A) Structure of a Li-N2 battery with a Li-foil anode, ether-based electrolyte, and CC cathode. (B) N2 fixation (blue) and N2 evolution (red) curves of a Li-N2 battery with a CC cathode at a current density of 0.05 mA cm-2 (C) CV curves of a Li-N2 battery at a scan rate of 0.05 mV s-1 in N2-saturated (black)
and Ar-saturated (red) atmospheres. (D) Cyclic performance of a Li-N2 battery at a current density of 0.05 mA cm-2. Ma et al. Click to enlarge.
As the most abundant gas in Earth’s atmosphere, nitrogen has been an attractive option as a source of renewable energy. But nitrogen gas—which consists of two nitrogen atoms held together by a strong, triple covalent bond—doesn’t break apart under normal conditions, presenting a challenge to scientists who want to transfer the chemical energy of the bond into electricity.
Now, researchers in China have developed a rechargeable lithium-nitrogen (Li-N–) battery with the proposed reversible reaction of 6Li + N– ? 2Li–N. The assembled N– fixation battery system, consisting of a Li anode, ether-based electrolyte, and a carbon cloth cathode, shows a promising electrochemical faradic efficiency (59%).
The “proof-of-concept” design, described in an open-access paper in the journal Chem, works by reversing the chemical reaction that powers existing lithium-nitrogen batteries. Instead of generating energy from the breakdown of lithium nitride (2Li3N) into lithium and nitrogen gas, the researchers’ battery prototype runs on atmospheric nitrogen in ambient conditions and reacts with lithium to form lithium nitride. Its energy output is brief but comparable to that of other lithium-metal batteries.
Although it constitutes about 78% of Earth’s atmosphere, N2 in its molecular form is unusable in most organisms because of its strong nonpolar N=N covalent triple-bond energy, negative electron affinity, high ionization energy, and so on. In terms of energy efficiency, the honorable Haber-Bosch process, which was put forward more than 100 years ago, is the most efficient process for producing the needed N2 fertilizers from atmospheric N2 in industrial processes. However, the energy-intensive Haber-Bosch process is inevitably associated with major environmental concerns under high temperature and pressure, leaving almost no room for further improvement by industry optimization.
Inspired by rechargeable metal-gas batteries such as Li-O2, Li-CO2, Li-SO2, Al-CO2, and Na-CO2 (which have attracted much attention because of their high specific energy density and ability to reduce gas constituent), research on Li-N2 batteries has not seen any major breakthroughs yet. Although Li-N2 batteries have never been demonstrated in rechargeable conditions, the chemical process is similar to that of the previously mentioned Li-gas systems. During discharging reactions, the injected N2 molecules accept electrons from the cathode surface, and the activated N2 molecules subsequently combine with Li ions to form Li-containing solid discharge products. From the results of theoretical calculations, the proposed Li-N2 batteries show an energy density of 1,248 Wh kg-1, which is comparable to that of rechargeable Li-SO2 and Li-CO2 batteries.
The research team demonstrated that a rechargeable Li-N2 battery is possible under room temperature and atmospheric pressure with the following reversible battery reactions:
The team investigated the use of Ru-CC and ZrO2-CC composite cathodes to improved the N2 fixation efficiency. Li-N2 batteries with catalyst cathodes showed higher fixation efficiency than pristine CC cathodes.
- (Equation 1) anode: 6Li = 6Li+ + 6e-
- (Equation 2) cathode: 6Li+ + N2 + 6e- = 2Li3N
- (Equation 3) overall: 6Li+ + N2 = 2Li3N
This promising research on a nitrogen fixation battery system not only provides fundamental and technological progress in the energy storage system but also creates an advanced N2/Li3N (nitrogen gas/lithium nitride) cycle for a reversible nitrogen fixation process. The work is still at the initial stage. More intensive efforts should be devoted to developing the battery systems.
This work was financially supported by the Ministry of Science and Technology of China and the National Natural Science Foundation of China.