Stabilizing Anode/Electrolyte Interface in Li-O₂ Batteries by Molecular Sieves

Lithium-oxygen (Li-O₂) batteries with an ultra-high theoretical specific energy (3500 Wh kg^-1) has recently attracted enormous attention. Due to the difficulty in decomposing discharge product Li₂O₂, the charging overpotential of Li-O₂ batteries is high, which provokes parasitic reactions and worsens the cycling stability of the batteries. Redox mediators (RMs) are widely used to decrease the charging overpotential by altering the electrochemical processes in Li-O₂ batteries. However, unwanted side reactions between RMs and Li metal anode may severely damage both the anode and the RMs; therefore, it is necessary to suppress the shuttle effect of RMs to extend the lifespan of Li-O₂ batteries. To improve the interfacial stability of the Li anode, 4A zeolite, a molecular sieve with a narrow aperture size of ~ 0.4 nm, was used to restrain the diffusion of RMs in this work. A polymer membrane containing 4A zeolite was synthesized to evaluate the performance of the molecular sieve. With the membrane, the Li-O₂ batteries with 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) as the RM showed greatly improved performance. The charging overpotentials were kept at low levels and the cycle life of the batteries was extended eight times. The results from Raman spectroscopy indicated that the small pores of 4A zeolite physically blocked the movement of TEMPO, thus stabilizing the interface between the Li anode and the electrolyte. Furthermore, lithiated 4A zeolite was directly coated on the surface of the Li anode to study its effectiveness in suppressing the shuttle effect of RMs. The protective layer composed of lithiated 4A zeolite not only suppressed the shuttle effect of TEMPO, but also reduced the formation of Li dendrites. As a consequence, the cycle life of Li-O₂ batteries was prolonged more than ten times and the full-discharge capacity of the batteries at room temperature was greatly enhanced. In summary, with the assistance of 4A zeolite-typed molecular sieves, it is promising to fabricate interface-stable, high-energy-density, and long-cycle-life Li-O₂ batteries.