Invited: Novel Cell Design and Inexpensive Electrocatalysts for Hybrid Lithium-Air Batteries

Growing environmental concerns and increasing demand for energy have stimulated extensive interest in electrical energy storage, particularly in the form of batteries. Li-air batteries are appealing in this regard as they offer much higher energy density than the current Li-ion batteries. However, the nonaqueous Li-air batteries are hampered by electrolyte degradation and clogging of the air electrodes by the insoluble discharge products. Interestingly, hybrid Li-air batteries in which a solid electrolyte separates the lithium metal anode in an aprotic electrolyte from the air electrode in an aqueous catholyte could overcome these problems. However, there are many challeges that need to be overcome, e.g.,poor stability of the solid electrolyte in strongly acidic or alkaline catholytes, high internal resistance, and lack of less expensive, efficient, durable oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts. Much effort has been focused on developing efficient bifunctionl catalysts to lower the overpotential and improve the stability of hybrid Li-air batteries in recent years, but the cycle life is still limited. This presentation will focus on advanced cell configurations and high-performance catalysts for hybrid lithium-air batteries.

We have developed an advanced hybrid Li-air cell with decoupled, mesoporous nanocatalysts, which could be cycled for over 100 cycles (400 hours) in air with only 0.08 V increase in round-trip overpotential based on discharge and charge end voltages. With this cell configuration, mesoporous NiCo₂O₄ nanoflakes directly grown onto a nickel foam (NCONF@Ni) serve as the OER electrode. The catalytic active surface area increased from < 1 m² g^-1 for nickel foam to more than 80 m² g^-1. The spinel NiCo₂O₄ catalyst exhibits much higher catalytic activity and stability than Ni metal at high potentials, leading to a fairly low OER overpotential that is comparable to that seen with the noble-metal IrO₂ catalyst. A nitrogen-doped mesoporous carbon (NMC) with extremely high surface area (1520 m² g^-1) and optimized nitrogen doping content (3.9 wt. %) was loaded onto a hydrophobic carbon fiber paper to act as the ORR electrode. The highly mesoporous NMC exhibits activity similar to the noble-metal Pt/C catalyst, but with much better stability. With this configuration, a variety of other non-noble-metal or metal-free catalysts could play the role of ORR without worrying about their stability in the high-voltage charge process. For example, we have developed a 3-D O- and N-doped carbon nanoweb as a highly active metal-free catalyst for ORR in hybrid Li-air cells. The 3-D nanoweb structure provides an ideal backbone support for catalytically active sites due to the fast electron and mass transport properties compared with the 1-D or 2-D structure. In addition, the synergistic effect between the O and N groups creates highly active pyridone groups all over the nanoweb surface, which significantly improves the catalytic activity toward ORR.   

Furthermore, a phosphate buffer catholyte with a moderate pH has been developed to protect the solid electrolyte in hybrid Li-air batteries. It contains phosphoric acid and supporting salts, which reduce the internal resistance and overpotential. A high energy density could be achieved by utilizing all three protons in phosphoric acid. Further increase in power density and efficiency has also been realized by increasing the solid-electrolyte conductivity and the operating temperature to 40 ^oC.