Researchers gain fundamental insight into key reaction for Li-air batteries

8 November 2014

A new study from Delft University and the University of Waterloo finds a different OER mechanism for electrochemically-generated Li₂O₂ than for commercial Li₂O₂. Credit: ACS, Ganapathy et al. Click to enlarge.

A team from Delft University in The Netherlands and the University of Waterloo in Canada has used operando X-ray diffraction to show that oxidation of electrochemically-generated Li₂O₂ in high energy density Li-air batteries occurs in two stages, but in only one step for commercial (crystalline) Li₂O₂. This discovery reveals a fundamental difference in the OER depending upon the nature of the peroxide.

In a paper published in the Journal of the American Chemical Society, the authors conclude that their findings not only reveal the fundamental nature of the charge reaction in Li−air batteries but also show the impact that the nature of the lithium peroxide (size, shape, and crystallinity) has on the oxidation mechanism. Controlling this process may be the key to high performance Li−air batteries, they suggest.

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Research into nonaqueous Li−air/Li−O₂ batteries has
exploded over the past few years. This is primarily due to their high theoretical specific energy in the charged state (∼11500 Wh/kg), making them extremely attractive for electric automobiles. The electrochemical processes that drive this battery are represented by the total chemical reaction 2Li + O ⇄ Li₂O₂. Several bottlenecks that impede the functioning of this battery system need to be addressed before it can become viable.

These include the high (dis)charge overpotential resulting in a lower round trip efficiency, slow kinetics, electrolyte instability (side product formation) leading to poor cyclability, and the requirement of high purity O₂. Of fundamental importance is the understanding of the mechanism of lithium peroxide [Li₂O₂] formation and oxidation and the governing factors. Over the past years there has been significant progress in the understanding of the Li₂O₂ formation process during discharge. Clear correlations have been established between the solvent donor number, discharge voltage, current density, and composition of the gas diffusion electrode on the morphology of Li₂O₂ formed and the mechanism of their formation, be it via solution or on the electrode surface. But one of the many challenges of the Li−O₂ system includes the mechanism of Li₂O₂ oxidation which remains less well understood, with the elusive LiO₂ superoxide intermediate remaining experimentally very difficult to observe.

…In this article, we combine operando X-ray diffraction
(XRD), Rietveld refinement, calculations, and online electrochemical mass spectrometry (OEMS) to elucidate the mechanism of the oxidation evolution reaction, comparing electrochemically generated Li₂O₂ (E-Li₂O₂ ) and bulk crystalline (commercial) Li₂O₂ (C-Li₂O₂ ) during the charge reaction in a Li−O₂ cell. A clear difference is observed between the oxidation of E-Li₂O₂ and C-Li₂O₂ which can be explained by the difference in the nature of the particles and crystallites. The OER mechanism, however, appears similar for both E-Li₂O₂ and C-Li₂O₂.

Schematic of the Li-O₂ cell used for in-situ XRD measurements. To conduct the study, the team built a Li−O₂ cell that would allow X-ray diffraction measurements to be conducted in reflection mode during battery cycling. The top half of the cell comprises a Kapton window enclosed in a stainless steel casing with a small inlet to allow the intake of O₂ gas. An Al mesh current collector was integrated into this top half.

The bottom half consists of a stainless steel coin attached to a spring, also enclosed in stainless steel, akin to that of a typical Swagelok cell design. This formed the current collector on the anode side.

The top and bottom halves of the cell were separated with a Teflon spacer to prevent a short circuit, after which they were clamped together from the outside with a nonconductive clamp. The battery stack was assembled within, with the cathode on the side of the Kapton window. Credit: ACS, Ganapathy et al. Click to enlarge.

The researchers fabricated Li-O₂ cell gas diffusion electrodes (cathodes) by casting a mixture of activated carbon and a lithiated Nafion binder on carbon paper. They also fabricated preloaded Li₂O₂ cathodes combining Vulcan XC72 carbon, Li₂O₂ and PTFE; these were used in charge-only XRD experiments to probe the oxygen evolution mechanism.

The battery, comprising the cathode, a glass microfiber separator soaked with electrolyte, and a Li-metal anode, was assembled in the XRD cell.

They collected operando X-ray diffraction patterns on Li−O₂ cells run for a complete (dis)charge cycle at low and intermediate current densities of
25 and 50 μA/cm². They determined that smaller average crystallite sizes for E-Li₂O₂ are generated at a current density of 50 μA/cm² and larger crystallite sizes are formed at 25 μA/cm² at the end of discharge.

They observed different oxidation processes were observed for E-Li₂O₂ and C-Li₂O₂ associated with the differences in their nature.

• A two-stage oxidation for E-Li₂O₂ in which oxidation proceeds first through a noncrystalline (i.e., amorphous) lithium peroxide component, followed at higher potential by the crystalline peroxide via a Li-deficient solid solution (Li₂–xO₂) reaction.

  They confirmed a platelet crystallite shape for the E-Li₂O₂, and speculated that the toroid particles are deconstructed one platelet at a time, starting with the smallest sizes that expose more peroxide surface.

• A single stage oxidation for bulk crystalline Li₂O₂ with an isotropic crystallite shape and larger crystallite dimensions. The observation of substoichiometric Li₂−xO₂ at the early stage of oxidation and the gradual decreasing average crystallite size suggests a small active fraction that also evolves oxygen via a Li deficient solid solution reaction. However, in this case the oxidation process gradually consumes the larger C-Li₂O₂ crystallites.

Cartoon showing the mechanism of (a) E-Li₂O₂ and (c) C-Li₂O₂ oxidation during the charge process as determined from X-ray diffraction. Scanning electron microscopy images (SEM) recorded at different stages of oxidation of E-Li₂O₂ and C-Li₂O₂ are depicted in panels (b) and (d), respectively. Credit: ACS, Ganapathy et al.Click to enlarge.

The fundamental insight thus gained in the OER charge mechanism and its relation to the nature of the Li₂O₂ particles is essential for the design of future electrodes with lower overpotentials, one of the key challenges for high performance Li–air batteries.

Resources

• Swapna Ganapathy, Brian D. Adams, Georgiana Stenou, Maria S. Anastasaki, Kees Goubitz, Xue-Fei Miao, Linda F. Nazar, and Marnix Wagemaker (2014) “Nature of Li₂O₂ Oxidation in a Li–O2 Battery Revealed by Operando X-ray Diffraction” Journal of the American Chemical Society doi: 10.1021/ja508794r