Electronic properties of Ir₃Li and ultra-nanocrystalline lithium superoxide formation

Highlights

• Bulk synthesis and characterization of Ir₃Li as Li-O2 battery cathode material.

• Reduced Li-O₂ battery charge overpotentials using Ir₃Li-rGO cathodes.

• Formation of stable, > 200 nm particles of lithium superoxide as discharge product.

Abstract

Current lithium-oxygen (Li-O₂) batteries suffer from large charge overpotentials related to electronic resistivity of the insulating lithium peroxide (Li₂O₂) discharge product. One potential solution to this challenge is the stabilization of the lithium superoxide (LiO₂) discharge intermediate, which has much higher electronic conductivity compared to Li₂O₂. Cathodes based on small iridium (Ir) nanoparticles have been recently used in a Li-O₂ battery to successfully stabilize the LiO₂ product, however, the LiO₂ had a short lifetime. In the previous study, researchers found that the LiO₂ was stabilized on Ir₃Li surfaces which were formed from Ir nanoparticles during battery operation. Little is known about the electronic properties of Ir₃Li and its role in stabilizing LiO₂ product formation. This work provides the first study of the electronic properties of Ir₃Li, which was thermally synthesized in bulk prior to implementation on the reduced graphene oxide (rGO) cathode of a Li-O₂ cell. The bulk Ir₃Li was found to have comparable electrical conductivity to Ir metal, possess metal-like magnetic properties, and has an affinity towards O₂ adsorption. The LiO₂ discharge product formed from the Li-O₂ battery discharge was characterized using Raman spectroscopy, titration, along with a comprehensive transmission electron microscopy (TEM) study. This analysis revealed the formation of ultra-nanocrystalline LiO₂ particles greater than 200 nm. This result was attributed to the use of large micron sized Ir₃Li particles, which could stabilize larger LiO₂ particles compared to previous cathodes that utilized Ir nanoparticles that partially converted to Ir₃Li during cycling. These results demonstrate that cathode properties can be modified to stabilize the bulk LiO₂ discharge product, which can be useful for the further development of LiO₂-based Li-O₂ batteries.

Introduction

Developing batteries with comparable energy densities to internal combustion technology is essential in order to fully transition to an electrified transportation-based society. Li-O₂ batteries, are seen as the ‘holy grail’ of battery technologies since they have the highest theoretical energy density of all current battery technologies [1]. Current state-of-the-art Li-O₂ batteries suffer from limited practical capacity and large charge overpotentials resulting from the build-up of Li₂O₂ on the cathode surface [2]. The insulating nature of bulk Li₂O₂ with a large band gap of 2.02 eV can prevent efficient charge transport from the cathode to the reaction interface, leading to parasitic reactions and reduced cycling efficiency [3]. Among the solutions to high overpotentials on charge are the use of soluble catalysts or use of cathodes that result in discharge products with good electronic conductivity. Soluble catalysts, known as redox mediators, oxidize the surface of the Li₂O₂ so electronic conductivity is not an issue and they can avoid side reactions as well [4], [5]. One disadvantage of redox mediators is that they may degrade with cycling due to reactions with the cathode or the discharge product. Discharge products with good electronic conductivity can include Li₂O₂ with amorphous grain boundaries, Li₂O₂ that is amorphous, and LiO₂. The LiO₂ based Li-O₂ batteries can also potentially provide longer cycle life compared with Li₂O₂ based Li-O₂ batteries since oxidation of LiO₂ during charging will likely not produce singlet oxygen since LiO₂ is a doublet and decomposition will lead to a triplet O₂ not singlet O₂. Singlet O₂ is produced during oxidation of Li₂O₂, and causes significant parasitic reactions that limit cell longevity [6].

The stabilization of the LiO₂ discharge product, which limits Li₂O₂ formation, has been the subject of recent research. Experimental and theoretical studies have indicated that LiO₂ has a much higher electronic conductivity compared with Li₂O₂, and this difference has been correlated to lower charge overpotentials [7], [8], [9], [10]. Lu et al. employed an Ir nanoparticle-based rGO cathode in a Li-O₂ battery and demonstrated the formation of LiO₂ rods as the sole discharge product, which could be cycled for up to 40 cycles with low charge cell potentials (3.3–3.5 V). The LiO₂ was identified by TEM, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), differential electrochemical mass spectrometry (DEMS), and Raman spectroscopy [7]. LiO₂ was identified by titration in a similar, subsequent study [11]. Surprisingly, TEM analysis identified the formation of Ir₃Li on Ir surfaces during electrochemical discharge of the Li-O₂ cell [7]. Density functional (DFT) studies demonstrated a strong lattice match between LiO₂ (111) and epitaxial Ir₃Li (121), which provided explanation for the stabilization mechanism of LiO₂ on Ir₃Li [7].

In subsequent work Halder et al. employed TEM to investigate the formation of Ir₃Li layers on Ir nanoparticles, which were supported on an rGO cathode [12]. The researchers observed a core-shell structure comprised of an Ir core and Ir₃Li shell of a few nanometers thickness, which was subsequently covered by LiO₂ during discharge in a Li-O₂ cell. Furthermore, it was shown through TEM analysis of cycled cathodes that Ir₃Li was electrochemically stable once formed in an Li-O₂ environment. TEM results demonstrated that after the initial formation of Ir₃Li surfaces during the initial discharge, the Ir₃Li surfaces remained inert over 10 discharge/charge cycles. Discharge tests were also performed in an argon atmosphere to demonstrate that Ir₃Li did not form from the direct intercalation of lithium (Li) into Ir. DFT calculations indicated that the Ir₃Li formation mechanism during discharge in a Li-O₂ cell (reaction (1)) was thermodynamically favorable (−5.9 eV at 0 K) [12].

LiO₂(s) + 3Ir(s) → Ir₃Li(s) + O₂(g),

The synthesis of intermetallic Ir₃Li particles was first reported by Donkersloot et al., in 1976 [13]. The authors heated a mixture of Ir powders and Li metal at 800 °C for seven days in order to produce a phase pure Ir₃Li product that was characterized using single crystal XRD. The material was found to have a body-centered orthorhombic unit cell structure, with lattice constants of a = 2.6726 Å, b = 8.6949 Å and c = 4.6703 Å. Sangster and Pelton used thermodynamic calculations and experimental data to describe the phase diagram of the Ir-Li binary system. The authors described two Ir-Li phases: a Li rich IrLi phase with a layered structure and the Ir₃Li phase with stacked Li columns [14].

Aside from identification of the crystal structure and calculated Ir₃Li phase diagram [13], [14], little is known about the physical properties of the material especially regarding its efficiency in serving as a template for LiO₂ in Li-O₂ batteries [12]. Experimental characterization and a comprehensive understanding of the Ir₃Li electronic properties is necessary in order to optimize the material for effective implementation in LiO₂-based Li-O₂ batteries. More importantly, understanding Ir₃Li key material properties may provide guidance for the design of non-precious-metal-based cathodes for economically viable LiO₂ based batteries. Further investigation of the formation of LiO₂ on iridium-based particles is also essential in order to develop an understanding of properties of LiO₂, which is difficult to study due to its prominent disproportionation to Li₂O₂ and O₂.

In the current study, we demonstrate the first implementation of ex-situ synthesized Ir₃Li particles in a Li-O₂ battery. Comprehensive spectroscopic and microscopic analysis including, XRD, SEM, conductive atomic force microscopy (CAFM), X-ray photoelectron spectroscopy (XPS), Raman, superconducting quantum interference device (SQUID) measurements, and electron paramagnetic resonance (EPR) were performed on the Ir₃Li particles in order to better understand key properties of this novel material. The cathode based on bulk Ir₃Li successfully led to formation of a LiO₂ discharge product with much larger particle size and stability than previously found. TEM studies were carried out on the LiO₂ discharge product to better understand the morphology of the LiO₂. DFT studies provided evidence for the LiO₂ stabilization mechanism.