Operando Liquid-Electrochemical TEM for Monitoring Growth and Dissolution Steps of NaO 2 Cubes in Na-O 2 Battery

• Published in: Meeting abstracts (Electrochemical Society) Vol. MA2020-01; no. 45; p. 2576
• Main Authors: Dachraoui, Walid, Lutz, Lukas, Johnson, Lee, Bruce, Peter G., Tarascon, Jean-Marie, Grimaud, Alexis, Demortière, Arnaud
• Format: Journal Article
• Language: English
• Published: 01.05.2020
• ISSN: 2151-2043; 2151-2035
• DOI: 10.1149/MA2020-01452576mtgabs

Compared to conventional Li/Na-ion batteries, in which reversible energy storage relies on the use of redox active transition metal oxides as positive electrodes, the metal-O 2 battery systems would theoretically offer greater energy density owing to the use of the redox of gaseous oxygen using conductive and light carbon electrodes. Although in sodium−oxygen (Na−O 2 ) batteries show promise as high-energy storage systems, this technology is still the subject of intense fundamental research, owing to the complex reaction by which it operates. To understand the formation mechanism of the discharge product, sodium superoxide (NaO 2 ), liquid- electrochemical TEM holder has been used (Poseidon 510) [7]. Here we present for the first time the use of a "Na-O 2 microbattery" using a liquid aprotic electrolyte (liquid cell) coupled with fast imaging TEM (Oneview camera) and HAADF-STEM to visualize, in real time, the mechanism of NaO 2 nucleation/growth. The electrolyte (NaPF 6 /DME) was saturated with ultrapure O 2 , prior to inject into the liquid TEM cell, to induce the reaction Na + + O 2 + e - -> NaO 2 using electrochemical echips with a glassy carbon electrode (in the observation window) as working electrode (with two others Pt electrodes). Indeed, the recent development of liquid cell for TEM [1,2] enables an exciting opportunity to explore [3,4] and even quantify [5,6] complex electrochemical reactions occurring inside batteries during operation. Indeed, the miniaturization of new electrochemistry echips (Protochips) [7] with complex electrode designs (3-electrodes cell) adds the capability to perform true in situ/operando quantitative electrochemical reactions, which can be followed using TEM multi-techniques. We observe that the formation of NaO 2 cubes [8] onto the glassy carbon electrode during reduction occurs by a solution-mediated nucleation process. The cyclic voltammetry response obtained during discharge and charge of the Na-O 2 microbattery exhibits two peaks corresponding to the reduction and oxydation processes. We visualized the solution-mediated growth of NaO 2 in real-time and identified that the 3D growth process is governed by the equilibrium between NaO 2 (solv) ↔ NaO 2 (solid). Based on the fast imaging acquisition (30 frames/s) of this phenomenon, we quantified the kinetics of growth of NaO 2 cubes and demonstrated the strong impact of O 2 diffusion in the liquid electrolyte on the NaO 2 cube growth. By HAADF-STEM imaging the charge process, we demonstrated that the same solvation−desolvation equilibrium is responsible for the dissolution of the NaO 2 discharge product, which consumes the NaO 2 cubes from the NaO 2 −electrolyte interface toward the electrode. Finally, we observed the formation of parasitic shell around NaO 2 cubes during the cycling of a Na-O 2 battery, in which the fast imaging TEM revealed the chemical reactivity of NaO 2 at the interface with the electrolyte. Parasitic products continuously accumulate on the cube surface to form a thick shell surrounding the NaO 2 cubes, which passivate the electrode surface. This information is vital for optimization of the battery, since this parasitic shell is responsible for its poor cyclability by preventing crucial O 2 redox and further nucleation of NaO 2 . The assessment of the discharge−charge mechanistic in Na-O2 batteries through operando liquid-electrochemical TEM visualization should facilitate the development of this battery technology [8]. References: [1] Ross, F. M. Science 2015, 350. [2] Dachraoui, W.;Kurkulina, O.;Hadermann, J.;Demortiere, A. Microsc. Microanal. 2016, 22, 24−25. [3] Holtz, M. E et al. A. Nano Lett. 2014, 14, 1453−1459. [4] Mehdi, B. L et al. Nano Lett. 2015, 15, 2168−2173. [5] Sacci, R. L. et al. Nano Lett. 2015, 15, 2011−2018. [6] Luo, L.; Liu, B.; Song, S.; Xu, W.; Zhang, J. G.; Wang, C. Nat. Nanotechnol. 2017, 12, 535−540. [7] http://www.protochips.com/products/poseidon-select/ [8] Lutz, L. et al. Nano Lett., 2018, 18, 1280-1289. Figure Caption: (a,b) electron diffraction pattern of NaO2 cubes and NaOx nano-cubes. (c) Time-resolved and animated operando TEM images illustrate the morphological evolution of the NaO2 product during discharge, featuring a nucleation event (10 s) and the subsequent growth of NaO2 cubes, following a solution-mediated growth-process (5−30 s). (d,f) TEM overview images in the presence of liquid electrolyte showing NaO2 cubes covering the GC electrode after discharge (d) and after charge exhibiting cube dissolution and parasitic shell (f). (e) Curve of voltammetry cyclic measured inside the in situ electrochemical cell in which peaks of oxidation and reduction of Na-air micro-battery system is exhibited. (g) Fast and animated operando TEM images showing the evolution of the parasitic shell at the cube−electrolyte interface. Figure 1