Theoretical Exploration of Various Lithium Peroxide Crystal Structures in a Li-Air Battery

Kah Lau,Jun Lu,Dantong Qiu,Larry Curtiss,Xiangyi Luo,Jeffrey Greeley,Khalil Amine

doi:10.3390/en8010529

Kah Lau, Jun Lu + Show 5 more

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https://doi.org/10.3390/en8010529

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Abstract

We describe a series of metastable Li2O2 crystal structures involving different orientations and displacements of the O22− peroxy ions based on the known Li2O2 crystal structure. Within the vicinity of the chemical potential ΔG ~ 0.20 eV/Li from the thermodynamic ground state of the Li2O2 crystal structure (i.e., Föppl structure), all of these newly found metastable Li2O2 crystal structures are found to be insulating and high-k materials, and they have a common unique signature of an O22− O-O vibration mode (ω ~ 799–865 cm−1), which is in the range of that commonly observed in Li-air battery experiments, regardless of the random O22− orientations and the symmetry in the crystal lattice. From XRD patterns analysis, the commercially available Li2O2 powder is confirmed to be the thermodynamic ground state Föppl-like structure. However, for Li2O2 compounds that are grown electrochemically under the environment of Li-O2 cells, we found that the XRD patterns alone are not sufficient for structural identification of these metastable Li2O2 crystalline phases due to the poor crystallinity of the sample. In addition, the commonly known Raman signal of O22− vibration mode is also found to be insufficient to validate the possible existence of these newly predicted Li2O2 crystal structures, as all of them similarly share the similar O22− vibration mode. However considering that the discharge voltage in most Li-O2 cells are typically several tenths of an eV below the thermodynamic equilibrium for the formation of ground state Föppl structure, the formation of these metastable Li2O2 crystal structures appears to be thermodynamically feasible.

Highlights

Of particular interest in the context of future transportation needs for high energy density in energy storage, Li-O2 electrochemical cells are often touted as a potentially promising technological successor to current Li-ion batteries
It is reasonable to assume that lithium peroxide might exhibit several metastable structures, in addition to the commonly known ground state Föppl structure, which can be found in commercial Li2O2 powders
In the exploration of the potential energy surface, we found that altering the orientation of O22− peroxy ions can be a simple, yet powerful atomic displacement parameter in determining in the search for metastable Li2O2 crystalline structures

Summary

Introduction

Of particular interest in the context of future transportation needs for high energy density in energy storage, Li-O2 (or Li-air) electrochemical cells are often touted as a potentially promising technological successor to current Li-ion batteries. Through directly reacting with the O2 gas at the cathode, the reactions involving Li and O2 are thermodynamically favorable and can potentially yield a high energy density and high specific energy [1,2,3,4,5]. Many possible side reactions have been observed in Li-air cells when in operation [3,4,5,6,7], the fundamental reaction should ideally involve only Li and O2 that yields Li2O2. With the appropriate electrolyte (e.g., ether-based electrolytes) [7,8,9,10,11,12,13], the formation of Li2O2 as the main reaction product of Li-air cell is highly probable during the oxygen reduction reaction (ORR). The Li2O2 discharge product morphology and crystallization can be a function of several parameters that are dictated by the electrolyte, Li- and

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