Redox Mediator Effects of LiNO3 in Tetraglyme-Sulfone Dual Solvent Electrolyte Solution on Li-O2 Battery Performance
Authors: Hayashi Yoshiya , Sohmiya Minoru , Otsuka Hiromi , Ito Kimihiko , Kubo Yoshimi , Horiba Tatsuo , Saito Morihiro 
Published: December 2020

Lithium-oxygen (LOB) batteries still have many issues, although they possess a large theoretical energy density of 3500 Wh kg-1. Among the issues, we focused on the large overvoltage on charging to cause serious energy loss and short cycle life. Recently, redox mediator (RM) has been attracting much attention and studied intensively. The RM can chemically oxidize discharge product, Li2O2, at a lower potential compared to that without RM. LiNO3 is a unique RM among various kinds of RM, because it works not only as a RM for the positive electrode (PE), but also as a surface modifier for the lithium negative electrode (NE).

In our recent paper we demonstrated that dimethylsulfoxide (DMSO) revealed a preferable effect to reduce overvoltage as the second solvent of tetraglyme (G4)-based LiNO3 electrolyte solution [1]. This result is attributed to the strong solvation of DMSO due to its high dielectric constant, while the complicated behavior of NO3⁻/NO2⁻ and other nitrogen oxide chmical species can be elucidaed on the basis of a reaction scheme summaried as follows [2].

2Li + NO3⁻ → NO2⁻ + Li2O (1)

NO2⁻ →NO2 + e⁻ (2)

NO2 + Li2O2 → NO2⁻ + Li+ + LiO2 (3)

Therefore, we further studied on the effect of the second solvent for G4-based LiNO3 eletrolyte solution by comparing some of sulfone solvents with DMSO for the purpose to understand the working mechanism of LiNO3 as the RM and to enhance the LOB performance.

Four kinds of solvents were used in this study: G4 (dielectric constant ε=7.7), DMSO (ε=48), ethyl methyl sulfone (EMS, ε=55), and tetramethylene sulfone (TMS, ε=43). G4 was mixed with one of other solvents in the volume ratio of 7:3 or 5:5 for 1 M LiNO3/G4-based electrolyte.

Swagelok cells were used for LOB cell test, consisting of a PE with Ketjen Black (KB) and PVDF binder coated on a carbon paper, a Li foil NE, Celgard 2400 separator intervening the two electrodes, and the electreolyte solution prepared as dscribed above. Simple galvanostatic mode was applied to the cell tests between 2.0 and 4.5 V.

All of the LOB cells with the dual solvent electrolyte solution showed suppressed charging voltage plateaus at around 3.5 V, suggesting the reacion of equation (2) followed by equation (3) to oxidize Li2O2. These results evidenced the critical effect of dissociation of LiNO3 and diffusivity of NO2⁻ ions in the electrolyte solutions on the performance of the LOB. Moreover, the cell temperature elevation up to 50℃ amplified the effect and its difference among DMSO, EMS and TES, which comfirmed us the importance of the diffusivity of NO2⁻ and the suppression of NO2 consuming side reactions. We will discuss on the whole picture of the working mechanism of the LiNO3 RM referring other data such as physicochemical properties of the electrolyte, diffuivity of Li+ by NMR, analysis of the electrodes by SEM-EDS and XRD.

This study was supported by JST “Next Generation Batteries Area in Advanced Low Carbon Technology Research and Development Program (ALCA)” from MEXT, Japan

Reference
s
:

[1] Y. Hayashi, M. Sohmiya, Y. Kubo, and M. Saito, et al., J. Electrochem. Soc.,167, 020542 (2020).

[2] V. S. Bryantsev, J. Uddin, G. V. Chase, and Dan Addison, et al., J. Am. Chem. Soc., 136, 3087 (2014).