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Morphological evolution of Zn-sponge electrodes monitored by in situ X-ray computed microtomography

The steady increase of electrical energy demand brings into the spotlight ethical, environmental, societal and political concerns, related to the usage of fossil fuels. Renewable sources could replace hydrocarbons, but sustainability imposes the integration with reliable and efficient electrical energy storage (EES) facilities. Currently, only 30 ppm of the globally produced electrical energy is stored and 95% of this is through hydroelectric methods that are essentially saturated. Other viable EES technologies are mechanical (compressed air, flywheels), thermal (molten salts) and electrochemical (batteries, fuel cells and supercapacitors). In terms of flexibility and cost, electrochemistry is playing a key role in the quest for the definitive EES device. Among diverse electrochemical concepts, rechargeable batteries with metal anodes exhibit storage potentialities ranging from low-power portable consumer electronics, to automotion, home and grid applications. The key appeal of metal anodes is that their energy density is ca. x2÷10 that of carbon, used in Li-ion technologies. The candidate metal of choice is Li, but for a series of availability, cost and safety reasons, post-Li options (Na, K, Ca, Mg, Zn, Al) are strong prospective competitors. In particular, use of Zn would be in strong favour of safety (non-pyrophoric metal, non-flammable electrolytes, enables use of fully non-toxic and recyclable battery components) and low cost.
Although disposable Zn-based batteries have been developed into widespread commercial devices decades ago, successful applications of rechargeable Zn batteries are still hindered by various technical drawback, the single most crucial one being limited cycle-life, due to uncontrolled morphology changes upon cycling. In order to cope with structural and morphological cycling instabilities of the Zn anode, several solutions have been proposed, among which, recently, the use of Zn-sponge electrodes. Their structure consists in a monolithic, porous architecture, exhibiting connected metallic Zn branches, covered with a layer of ZnO, of thickness depending on the depth of discharge (DOD). The inner, connected core of electron-conductive metallic Zn branches, is meant to persists even down to deep DODs, in principle allowing extensive electrochemical cycling without formation of macroscale dendrites or loose particles. Research in the field of Zn-sponge anodes is very active and in a few years, notable results have been achieved, regarding material fabrication and electrochemical testing. Nevertheless, in situobservations of these electrodes are still lacking and could contribute highly valuable information for a more insightful mechanistic understanding, leading to further performance improvement. Our research presented here, has used for the first time in situ X-ray computed microtomography (XMCT) to follow and quantify changes in shape and distribution of the Zn and ZnO phases, as well as porosity and connectivity of the metallic framework, during electrochemical cycling. XMCT data have been obtained through measurements performed at the TomoLab station of  Elettra. In particular, we have concentrated on the initial charging of an anode fabricated in the discharged state, and on subsequent extensive cycling and deep discharge. Relative variations of metal and oxide contents and their distribution, as well as the evolution of: porosity, metal agglomeration and metal connectivity were followed as a function of electrochemical operating conditions (Figure 1). The metal-to-oxide ratio, the framework porosity and continuity, as well as the pore distribution, are affected by shrinking and expansion of the external shell of the sponge network, resulting from the alternation of oxidation and reduction processes, but they are quite reversible and only prolonged cycling brings about an increase in metal content and connectivity, accompanied by some degree of pore closure.

The availability of this information and the possibility of applying this measurement protocol systematically, on the one hand will enable knowledge-based material and cell design, and, on the other hand, will provide rational guidelines for the definition of optimal charge/discharge policies.

Figure 1.    In situ XMCT of graphite-supported Zn-sponge anodes in 6 M KOH solution. (A) Reconstructed axial slices, (B) volume renderings and (C, D) Zn backbone connectivity of graphite-supported Zn-sponge anodes at indicated electrochemical conditions: (E) initial charge, (F) deep discharge, (G) 64 charge−discharge cycles. (H) Fractional porosity, ZnO/Zn volume ratio and (I) pore size extracted from in situ XMCT data.

 

This research was conducted by the following research team:

Benedetto Bozzini1, Claudio Mele2, Alessio Veneziano3, Nicola Sodini3, Gabriele Lanzafame3, Antonietta Taurino4, Lucia Mancini3,5 

 

1 Department of Energy, Politecnico di Milano Milano, Italy
2 Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Lecce, Italy
3 Elettra Sincrotrone Trieste, Trieste, Italy
4 Institute for Microelectronics and Microsystems, IMM-CNR, Lecce, Italy
5 LINXS – Lund Institute for advanced Neutron and X-ray Science, Lund, Sweden


Contact persons:

Lucia Mancini, email: lucia-mancini@elettra.eu
Benedetto Bozzini, email: benedetto.bozzini@unisalento.it

 

Reference

Benedetto Bozzini, Claudio Mele, Alessio Veneziano, Nicola Sodini, Gabriele Lanzafame, Antonietta Taurino, and Lucia Mancini,“Morphological evolution of Zn-sponge electrodes monitored by in situ X-ray computed microtomography”, ACS Applied Energy Materials 3, 4931–4940 (2020); DOI:10.1021/acsaem.0c00489



Last Updated on Wednesday, 17 June 2020 12:16