Kimihiko Ito

Highly Efficient Br–/NO3– Dual-Anion Electrolyte for Suppressing Charging Instabilities of Li–O2 Batteries

The main issues with Li-O2 batteries are the high overpotential at the cathode and the dendrite formation at the anode during charging. Various types of redox mediators (RMs) have been proposed to reduce the charging voltage. However, the RMs tend to lose their activity during cycling owing to not only decomposition reactions but also undesirable discharge (shuttle effect) at the Li metal anode. Moreover, the dendrite growth of the Li metal anode is not resolved by merely adding RMs to the electrolytes. Here we report a simple yet highly effective method to reduce the charge overpotential while protecting the Li metal anode by incorporating LiBr and LiNO3 in a tetraglyme solvent as the electrolyte for Li-O2 cells. The Br-/Br3- couple acts as an RM to oxidize the discharge product Li2O2 at the cathode, whereas the NO3- anion oxidizes the Li metal surface to prevent the shuttle reaction. In this work, we found that both anions work synergistically in the mixed Br-/NO3- electrolyte to dramatically suppress both parasitic reactions and dendrite formation by generating a solid Li2O thin film on the Li metal anode. As a result, the charge voltage was reduced to below 3.6 V over 40 cycles. The O2 evolution during charging was more than 80% of the theoretical value, and CO2 emission during charging was negligible. After cycling, the Li metal anode showed smooth surfaces with no indication of dendrite formation. These observations clearly demonstrate that the Br-/NO3- dual-anion electrolyte can solve the problems associated with both the overpotential at the cathode and the dendrite formation at the anode.

CNT Sheet Air Electrode for the Development of Ultra-High Cell Capacity in Lithium-Air Batteries

Lithium-air batteries (LABs) are expected to provide a cell with a much higher capacity than ever attained before, but their prototype cells present a limited areal cell capacity of no more than 10 mAh cm−2, mainly due to the limitation of their air electrodes. Here, we demonstrate the use of flexible carbon nanotube (CNT) sheets as a promising air electrode for developing ultra-high capacity in LAB cells, achieving areal cell capacities of up to 30 mAh cm−2, which is approximately 15 times higher than the capacity of cells with lithium-ion battery (LiB) technology (~2 mAh cm−2). During discharge, the CNT sheet electrode experienced enormous swelling to a thickness of a few millimeters because of the discharge product deposition of lithium peroxide (Li2O2), but the sheet was fully recovered after being fully charged. This behavior results from the CNT sheet characteristics of the flexible and fibrous conductive network and suggests that the CNT sheet is an effective air electrode material for developing a commercially available LAB cell with an ultra-high cell capacity.

Factors influencing fast ion transport in glyme-based electrolytes for rechargeable lithium–air batteries

To elucidate the determination factors affecting Li-ion transport in glyme-based electrolytes, six kinds of 1.0 M tetraglyme (G4) electrolytes were prepared containing a Li salt (LiSO3CF3, LiN(SO2CF3)2, or LiN(SO2F)2) or different concentrations (0.5, 2.0, or 2.7 M) of LiN(SO2CF3)2. In addition to conventional bulk parameters such as ionic conductivity (σ), viscosity (η), and density, self-diffusion coefficients of Li+, anions, and G4 were measured by pulsed-gradient spin-echo nuclear magnetic resonance method. Interaction energies (ΔE) were determined by density functional theory calculations based on the supermolecule method for Li+–anion (salt dissociation) and G4–Li+ (Li+ solvation) interactions. The ΔE values corresponded to ion diffusion radii formed by solvation and/or ion pairs. The order of dissociation energies ΔE was LiSO3CF3 > LiN(SO2CF3)2 > LiN(SO2F)2, which agreed well with the dissociation degree of these salts in the electrolytes. From the obtained knowledge, we also demonstrated that increasing the mobility and number of carrier ions are effective ways to enhance σ of glyme-based electrolytes by using 1,2-dimethoxyethane with lower η and similar dielectric constant to those of G4.

Operando structural study of non-aqueous Li–air batteries using synchrotron-based X-ray diffraction

Non-aqueous lithium–air batteries (LABs) attract attention as a candidate technology for next-generation energy storage devices. It is crucial to understand how the discharge product Li2O2 is formed and decomposed by the electrochemical reactions to improve the cycle performance and decrease the charge voltage, which are the most important subjects for LAB development. Here, operando X-ray diffraction with high-brilliant X-rays in a transmission mode was used to observe the intensity and structural changes of crystalline Li2O2 in an operating non-aqueous LAB in real time, and the Li–O2 electrochemical reaction involving Li2O2 formation and decomposition was clearly demonstrated. The electrochemically formed Li2O2, which had an anisotropic domain size of 10 nm in the c-direction and 40–70 nm in the ab-plane, grew due to the increase of the number of domains during the discharge process. No other reaction products with a crystalline phase such as LiOH were found in either the cathode or anode of the LAB, whereas the accelerated decomposition rate of the domains was accompanied with the change of the domain shape and lattice constant of the c-axis in the latter half of the charge process with voltage higher than 4 V.