Prevention of dendrite growth and volume expansion to give high-performance aprotic bimetallic Li/Na–O2 batteries
Despite great progress, current lithium-ion intercalation technology, even when they are fully developed, are difficult to satisfy society’s rapidly increasing demand of high-energy-density power sources for electric vehicles and electronics. In such a context, non-aqueous alkali metal-oxygen (AM–O2: AM = Li, Na, etc.) batteries become the promising candidates to replace conventional lithium-ion battery due to their ultrahigh theoretical energy density, deriving from the use of air cathode and AM (high theoretical specific capacity and low electrochemical potential) anode. However, AM is extremely reactive towards air and almost all nonaqueous electrolytes, thus resulting in significant parasitic reactions. Furthermore, uncontrollable Li (Na) deposition during plating/stripping, generally emerging as dendrites, easily induces cells short circuit accompanying by fire/explosion events, plaguing AM anodes towards practical applications. Therefore, to achieve a safe and stable AM-O2 cell, it is important to solve the dendrite coupled with oxidation/corrosion issues of AM anode.
Various attempts have been carried out to suppress AM dendrites. Among them, there is an interesting report wherein the Li/Na deposit had a rougher but non-dendritic surface while the lithium- or sodium-only deposit showed the preferential growth of dendrites (J. Electrochem. Soc., 2006, 153, A1353; J. Electrochem. Soc., 2011,158, A1100; J. Electrochem. Soc., 1995, 142, 3632). Subsequent studies reported by J-G. Zhang et el elucidated the reason for the above non-dendritic surface Li/Na deposit. It reveals that adding electrostatic shield additive such as a cation with a reduction potential lower than Li+ or Na+ help eliminate Li or Na dendrites (J. Am. Chem. Soc., 2013, 135, 4450). Further studies revealed that the nonuniform and porous SEI results in Li metal reacting with electrolyte leads to the low Coulombic efficiency (Nano Lett., 2014, 14, 6889). Therefore, a good combination of electrolyte solvent, salt, and additives are proposed to obtain a robust and compact SEI. Even so, it should be noted that the high reactivity of AM towards air and nonaqueous electrolytes is also detrimental to cycle life of AM. Alloying is an effective method to improve the resistance of alkali metal to air and nonaqueous electrolytes. However, Li- or Na-based alloys such as Li- or Na-Sb and Li- or Na-Sn alloys introduce inactive components (Adv. Energy Mater., 2015, 5, 1400317), thus compromising the specific capacity of alkali metal electrode.
To this end, Li-Na alloy is the optimum project for improving the resistance of Li and Na to air and nonaqueous electrolytes without sacrificing the specific capacity of anode because Li and Na metals exhibit similar reaction activities as well as fine electrolyte compatibility of their ions. Besides, the stripping Li+ can prevent Na+ concentrating, thus suppressing dendrites. Our study elucidates the critical role of Li-Na alloy anode and electrolyte additive, DOL, in stabilizing Li or Na plating/stripping electrochemistry. By optimizing Na/Li value of alloy, a dendrite-suppressed, oxidation-resistant, and crack-free Li-Na alloy anode is obtained. When the Li-Na alloy with the optimal ratio is used, a proposed aprotic bimetallic Li/Na-O2 battery with high cycling stability is realized. Furthermore, with the help of electrocatalyst such as Co nanoparticles embedded in N-doped carbon fibers, the overall performance of the above metal–O2 batteries can be significantly improved. This study provides guidance for developing other bimetal batteries such as bimetal ion batteries, and bimetal-S/Se batteries, which may possess new chemistries and exhibit much better electrochemical performance than mono-metal batteries.
You can read more about this research here <https://www.nature.com/articles/s41557-018-0166-9>