Carbon nanospheres for improved sodium-sulfur batteries

Sodium-sulfur batteries are promising electrical energy storage technologies that can serve as a key solution to intermittency problems and can be integrated with renewable forms of energy generation. An international research team has reported the synthesis of micro-mesoporous carbon nanospheres with continuous pore distribution as an efficient sulfur host for sodium-sulfur batteries. The work sheds new light on the progress of the sulfur cathode in sodium-sulfur batteries and provides a promising strategy for the viable design of other metal–sulfur batteries. Experiments at the CLAESS beamline in ALBA allowed determining the sulfur species during charge/discharge processes.

Solar and wind power are useful resources for energy generation but they are intermittent (at night or on cloudy days solar panels do not work, for example). Electrical energy storage technologies serve as a key solution to these intermittency problems and can be integrated with renewable forms of energy generation. Among these technologies, room-temperature sodium-sulfur (Na–S) batteries are deemed to be one of the most promising candidates, owing to their high theoretical energy density – the amount of energy they can store – and low cost. Nonetheless, this battery system suffers from a slow reaction rate at room temperature, which radically limits battery performance and makes difficult its practical commercialization.

An efficient strategy to deal with this challenge is the use of porous carbon material as a host to encapsulate molecular sulfur, significantly enhancing its conductivity. This system acts as the cathode of the battery, which is the electrode where reduction occurs. To make the battery work, sodium ions have to migrate from the anode to the cathode. However, in these systems, it is a challenge to provide fully accessible sodium ions that do not obstruct the sub-nanosized pores of the carbon host.

In a publication in the Advanced Materials journal, an international research team made up of Australian and Chinese institutions in collaboration with the ALBA Synchrotron has reported the synthesis of micro-mesoporous carbon nanospheres (MMPCS) with continuous pore distribution as an efficient sulfur host for sodium-sulfur batteries. This unique feature creates continuous channels that allow the movement of sodium ions without channels being obstructed. This enables a high conductivity, leading to fast sulfur reduction-oxidation reaction during the charge/discharge processes.

Read more on the ALBA website

Probing the Structure of a Promising NASICON Material

As physicists, materials scientists, and engineers continue striving to enhance and improve batteries and other energy storage technologies, a key focus is on finding or designing new ways to make electrodes and electrolytes.  One promising avenue of research involves solid-state materials, making possible batteries free of liquid electrolytes, which can pose fire and corrosion hazards.  An international group of researchers joined with scientists at Argonne National Laboratory to investigate the structure of crystalline and amorphous compounds based on the NASICON system, or sodium super-ion conductors. The work (using research carried out at the U.S. Department of Energy’s Advanced Photon Source [APS] and published in the Journal of Chemical Physics) reveals some substantial differences between the crystalline and glass phases of the NAGP system, which affect the ionic conductivity of the various materials.  The investigators note that the fraction of non-bridging oxygen (NBO) atoms appears to play a significant role, possibly altering the Na+ ion mobility, and suggest this as an area of further study.  The work provides fresh insights into the process of homogeneous nucleation and identifying superstructural units in glass ― a necessary step in engineering effective solid-state electrolytes with enhanced ionic conductivity. 

Because of their high ionic conductivity, materials with a NASICON structure are prime candidates for a solid electrolyte in sodium-ion batteries.  They can be prepared by a glass-ceramic route, which involves the crystallization of a precursor glass, giving them the usefulness of moldable bulk materials.  In this work, the research team specifically studied the NAGP system [Na1+xAlxGe2-x(PO4)3] with x = 0, 0.4 and 0.8 in both crystalline and glassy forms. Working at several different facilities, they used a combination of techniques, including neutron and x-ray diffraction, along with 27Al and 31P magic angle spinning and 31P/23Na double-resonance nuclear magnetic resonance spectroscopy.  The glassy form of NAGP materials was examined both in its as-prepared state and after thermal annealing, so that the changes on crystal nucleation could be studied.

Neutron powder diffraction measurements were performed at the BER II reactor source, Helmholtz-Zentrum Berlin, using the fine resolution powder diffractometer E9 (FIREPOD), followed by Rietveld analysis.  Further neutron diffraction observations were conducted at the Institut Laue-Langevin using the D4c diffractometer and at the ISIS pulsed neutron source using the GEM diffractometer.  X-ray diffraction studies were performed at X-ray Science Division Magnetic Materials Group’s beamline 6-ID-D of the APS, an Office of Science user facility at Argonne National Laboratory. 

Read more on the APS website

Image: Fig. 1. NASICON crystal structure showing the tetrahedral P(4) phosphate motifs (purple), octahedral GeO6 motifs (cyan) and Na+ ions (green). Oxygen atoms are depicted in red.

Catalytic role of oxygen-containing groups on carbon electrodes

The electrochemical reduction of oxygen plays a significant role in many critical applications such as gas sensors, hydrogen peroxide electrosynthesis, and electrochemical energy storage. Oxygen reduction reaction (ORR) drives the operation of fuel cells and metal-air batteries. The latter potentially can provide the highest specific energy among energy storage devices.

To increase the ORR efficiency, a catalyst immobilized on (or mixed with) conductive support is introduced to the positive electrode composition. Usually, porous sp2-carbon materials, like graphene, serve as such supporting materials. Its electronic configuration (sp2) provides the sufficient electric conductivity to the positive electrode. Nevertheless, ORR proceeds too slowly on the neat surface of ideal sp2-carbon in the absence of a catalyst.

The role of graphene imperfections (vacancies, impurity atoms, and functional groups) on catalyzing ORR (mainly in aqueous media) has been under intense investigation during the last decades. However, little is known about the effect of oxygen functionalization of carbon onORR in aprotic media (lacking the acidic protons). The interest in this process, especially in the presence of metal ions in the electrolyte, is relevant for various aprotic metal-oxygen batteries (lithium, sodium, magnesium, etc.) which are now considered as the most promising electrochemical power sources due to their outstanding theoretical performance. For such devices carbon electrodes are highly attractive due to their light weight and low cost, and the effect of carbon surface chemistry on the processes occurring upon battery operation is of great importance.

The present research shows for the first time that oxygenation of carbon electrode surface does not affect the rate of one-electron oxygen reduction in aprotic media. At the same time, in Li+-containing electrolytes, oxygen groups enhance both the rate of electrochemical Li2O2 formation and carbon electrode degradation due to faster oxidation by lithium superoxide (LiO2) intermediate yielding carbonate species as a product.

The research is led by scientists from Lomonosov Moscow State University and the Semenov Institute of Chemical Physics, in collaboration with FriedrichAlexanderUniversität Erlangen-NürnbergIFW DresdenSaint Petersburg State UniversityDonostia International Physics Center and Massachusetts Institute of Technology. 

Read more on the ALBA website

Image: C 1s core level spectra of a) pristine and b) oxidized graphene electrodes before and after discharge. C) Model spectroelectrochemical Li-O2 cell. D) Evolution of C 1s components’ ratios upon discharge for pristine and oxidized graphene.