Abstract
Lithium-oxygen (Li-O2) batteries are a promising class of rechargeable Li batteries with a potentially very high achievable energy density. One of the major challenges for Li-O2 batteries is the high charge overpotential, which results in a low energy efficiency. In this work size-selected subnanometer Ir clusters are used to investigate cathode materials that can help control lithium superoxide formation during discharge, which has good electronic conductivity needed for low charge potentials. It is found that Ir particles can lead to lithium superoxide formation as the discharge product with Ir particle sizes of ∼1.5 nm giving the lowest charge potentials. During discharge these 1.5 nm Ir nanoparticles surprisingly evolve to larger ones while incorporating Li to form core-shell structures with Ir3Li shells, which probably act as templates for growth of lithium superoxide during discharge. Various characterization techniques including DEMS, Raman, titration, and HRTEM are used to characterize the LiO2 discharge product and the evolution of the Ir nanoparticles. Density functional calculations are used to provide insight into the mechanism for formation of the core-shell Ir3Li particles. The in situ formed Ir3Li core-shell nanoparticles discovered here provide a new direction for active cathode materials that can reduce charge overpotentials in Li-O2 batteries.
Highlights
The following conclusions can be drawn from this investigation of cathodes for Li−O2 batteries based on size-specific Ir clusters (n = 2, 4, 8) deposited on an reduced graphene oxide (rGO) support: (1) The performance of a Li−O2 battery based on the clusters was found to depend on whether they were annealed to larger nanoparticles
(3) On the basis of HRTEM results, the Li becomes incorporated into the outer region of the Ir nanoparticles to form in situ an Ir3Li intermetallic giving nanoparticles with Ir3Li shell and a Ir core
The only previous synthesis of an Ir3Li intermetallic has been by high temperature synthesis, and these new results suggest that there is a low temperature synthesis method for Ir3Li based on an electrochemical reaction involving LiO2
Summary
The use of lithium−oxygen bonds for storage of electrical energy is of much interest because of the possibility for Received: July 18, 2019 Revised: September 25, 2019 Published: October 28, 2019. The cell is ML Ir8−rGO, operated with (c) 23% ML Ir8−rGO, and (d) discharge limited to 1000 mAh annealed 23% g−1 capacity at achieving very high energy densities needed for long-range electric vehicles.[1−5] The development of batteries based on formation and decomposition of covalent lithium−oxygen bond requires fundamental understanding of Li−O2 electrochemistry whereby lithium peroxide (Li2O2) is usually formed during discharge and decomposed during charge. In the work presented here, we have used size-specific subnanometer Ir clusters on a rGO support to probe the evolution of Ir particles in a Li−O2 cell and the role of this evolution in stabilizing of LiO2, which can be important for reducing charge potentials in Li−O2 batteries.
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