Although Li-ion batteries have been considered as promising power sources of electric vehicles, their energy and power densities should be improved to satisfy the demanded electrochemical performance of the electric vehicles^{1, 2}. In this regard, lithium-oxygen batteries with high energy densities have attracted a great deal of attention as an alternative to replace conventional Li-ion batteries.

However, there are several challenging issues for the development of Li-O₂ batteries. First, the large polarization of an oxygen electrode observed during charging should be improved by developing appropriate catalysts. Second, Li dendrite growth leads to poor cycle performance and safety. Third, stable electrolytes against both superoxide radicals and Li metal should be introduced. Many electrolytes are easily decomposed by nucleophilic attack of superoxide radicals, forming lithium alkyl carbonates such as Li₂CO₃^3-5. In this respect, high donor number solvents such as N,N-dimethylacetamide (DMA) are appropriate for Li-O₂ batteries because they are chemically and electrochemically stable against superoxide radicals. However, most high donor number solvents cannot be used as electrolytes because they are known to be decomposed with Li metal^6-8. Therefore, Li metal protection should be demanded to use high donor number solvents for Li-O₂batteries. Many efforts have been made to protect Li metal, leading to improved electrochemical performance. However, many problems still remain unsolved.

There are at least three requirements for Li metal protection in Li-O₂ batteries, as follows: i) The electrolyte should be rarely decomposed on a protective layer, ii) Li dendrite growth on a protective layer should be thermodynamically unfavorable, and iii) the mechanical strength of a protective layer should be sufficiently high to suppress Li dendrite growth (> 3.4 GPa)⁹. In this study, as a promising protective layer satisfying these requirements, we introduce a two-dimensional phosphorene-derived Li₃P protective layer for the first time. An electrochemically active phosphorene-derived Li₃P protective layer thermodynamically suppresses electrolyte decomposition because the redox potential of Li₃P is higher than the lowest unoccupied molecular orbital level (LUMO) of electrolyte solvents. In addition to the suppressed electrolyte decomposition, the phosphorene-derived Li₃P protective layer thermodynamically inhibits Li dendrite growth because Li plating is thermodynamically unfavorable on Li₃P surfaces compared to Li metal surfaces. This is supported by density functional theory (DFT) calculations that compared Li addition and removal energies on Li metal and Li₃P surfaces. Because of these properties, the phosphorene-coated Li metal anode shows excellent electrochemical performance including stable cycle performance of Li symmetric cells over 500 cycles with a variety of electrolytes and no capacity fading of Li-O₂ batteries over 50 cycles even with 1 MLiTFSI in DMA.

References

1. N.-S. Choi, et al., Angewandte Chemie International Edition, 2012, 51, 9994.

2. D. Larcher, et al., Nat. Chem., 2015, 7, 19.

3. H.-D. Lim, et al., Electrochim. Acta, 2013, 90, 63.

4. S. A. Freunberger, et al., J. Am. Chem. Soc., 2011, 133, 8040.

5. F. Mizuno, et al., Electrochemistry, 2010, 78, 403.

6. V. S. Bryantsev, et al., J. Electrochem. Soc., 2013, 160, A160.

7. W. Walker, et al., J. Am. Chem. Soc., 2013, 135, 2076.

8. V. Giordani, et al., J. Electrochem. Soc., 2013, 160, A1544.

9. G. M. Stone, et al., J. Electrochem. Soc., 2012, 159, A222.