Aqueous zinc ion batteries: focus on zinc metal anodes

Abstract

Despite the prevalence of lithium ion batteries in modern technology, the search for alternative electrochemical systems to complement the global battery portfolio is an ongoing effort. The search has resulted in numerous candidates, among which mildly acidic aqueous zinc ion batteries have recently garnered significant academic interest, mostly due to their inherent safety. As the anode is often fixed as zinc metal in these systems, most studies address the absence of a suitable cathode for reaction with zinc ions. This has led to aggressive research into viable intercalation cathodes, some of which have shown impressive results. However, many investigations often overlook the implications of the zinc metal anode, when in fact the anode is key to determining the energy density of the entire cell. In this regard, we aim to shed light on the importance of the zinc metal anode. This perspective offers a brief discussion of zinc electrochemistry in mildly acidic aqueous environments, along with an overview of recent efforts to improve the performance of zinc metal to extract key lessons for future research initiatives. Furthermore, we discuss the energy density ramifications of the zinc anode with respect to its weight and reversibility through simple calculations for numerous influential reports in the field. Finally, we offer some perspectives on the importance of optimizing zinc anodes as well as a future direction for developing high-performance aqueous zinc ion batteries.

Fig. 1. (a) Pourbaix diagram of a Zn/H₂O system. Reprinted with permission from ref. 6. Copyright 1996 Elsevier. (b) Schematic illustration of the reaction pathways and potential problems of Zn anodes in alkaline environments. Reprinted with permission from ref. 7. Copyright 2016 Wiley-VCH. (c) Pourbaix diagram of a Zn/H₂O system with HER overpotential considerations. Reprinted with permission from ref. 26. Copyright 1990 Elsevier. (d) A schematic illustration of the Zn plating/stripping process in a mildly acidic environment (symmetric cell configuration).

Fig. 2. (a) Galvanostatic voltage profiles of Zn symmetric cells with 3 M ZnSO₄ (red) and 3 M Zn(CF₃SO₃)₂ (blue). (b) Ex situ SEM images and elemental mapping results of Zn deposited on Ti foil at 0.2 V vs. Zn/Zn²⁺ in a CV test conducted at 0.5 mV s^–1 for 3 M Zn(CF₃SO₃)₂. (a and b) Reprinted with permission from ref. 31. Copyright 2016, American Chemical Society. (c) SEM images of the Zn anode after 1000 cycles in a TEP-containing 0.5 M Zn(CF₃SO₃)₂ electrolyte at a current density of 0.5 mA cm^–2. (d) Galvanostatic voltage profiles of Zn symmetric cells with the TEP additive at 0.5 mA cm^–2 and (e) plating/stripping tests on stainless steel electrodes with the TEP-containing electrolyte (left) and without TEP (right). (c–e) Reprinted with permission from ref. 33. Copyright 2019, Wiley-VCH.

Fig. 3. (a) A schematic illustration of Zn plating on Cu mesh with a normal aqueous electrolyte and that with a PAM additive. (b) Galvanostatic voltage profiles of Zn symmetric cells with (red) and without (black) the PAM additive at 1 mA h cm^–2. (a and b) Reprinted with permission from ref. 34. Copyright 2019, Wiley-VCH. (c) Molecular dynamics simulations of Zn²⁺ coordination in different LiTFSI concentrations (20, 10, and 5 M). (d) SEM image of plated Zn in a high concentration electrolyte (20 M LiTFSI & 1 M ZnTFSI); inset: ex situ XRD profile of pristine (black) and cycled Zn (red). (e) Plating test results on Cu in a high concentration electrolyte. (c–e) Reprinted with permission from ref. 43. Copyright 2018, Springer Nature.

Fig. 4. (a) A schematic illustration of Zn anode stabilization with ultrathin TiO₂ coating. (b) Ex situ XRD pattern of cycled Zn electrodes (top left) and SEM images of cycled pristine Zn and TiO₂-coated Zn (top right). Galvanostatic cycling results for pristine and TiO₂-coated Zn at 1 mA h cm^–2 (bottom). (a and b) Reprinted with permission from ref. 51. Copyright 2018, Wiley-VCH. (c) A schematic illustration of the anticipated effects of CaCO₃-coating on Zn plating. Reprinted with permission from ref. 50. Copyright 2018, Wiley-VCH. (d) A schematic illustration of the expected difference in Zn plating behavior between bare (left) and Au-sputtered Zn (right). (e) SEM images of bare (top left) and Au-sputtered (top right) Zn electrodes for 2000 cycles at 0.5 A g^–1 paired with a CNT/MnO₂ cathode. Initial galvanostatic voltage profiles of bare (black) and Au-sputtered (red) Zn electrodes showing a difference in the initial nucleation overpotential (bottom left). Long-term cycling performance of symmetric Zn cells for bare (black) and Au-sputtered (red) Zn electrodes (bottom right). (d and e) Reprinted with permission from ref. 45. Copyright 2019, American Chemical Society.

Fig. 5. (a) A schematic illustration of Zn plating with bare (left) and PA-coated Zn (right). (b) SEM images of bare Zn (left) and PA-coated Zn (right) electroplated with Zn at a current density of 0.2 mA cm^–2 (3.0 mA h cm^–2) on Ti foil. (c) Galvanostatic voltage profiles for bare Zn (blue) and PA-coated Zn (red) at a current density of 0.5 mA cm^–2 (0.25 mA h cm^–2). (a–c) Reprinted with permission from ref. 49. Copyright 2019, The Royal Society of Chemistry. (d) Experimental scheme for designing a Zn-graphite fiber host by means of annealing and electrodeposition. (e) Galvanostatic voltage profiles of a Zn symmetric cell with graphite fiber host anodes (top) and bare Zn foil (bottom). (d and e) Reprinted with permission from ref. 53. Copyright 2017, Elsevier.

Fig. 6. (a) Coulombic efficiency test results with annealed ZIF-8 host anodes and Cu foil at a fixed areal capacity of 1 mA h cm^–2 (left) and a fixed current density of 20 mA cm^–2 (right). (b) SEM images of Zn deposits at a current density of 1 mA cm^–2 for different areal capacities. (a and b) Reprinted with permission from ref. 54. Copyright 2019, Cell Press. (c) XRD results of synthesized host anodes (red: Zn-deposited Cu with the PAM additive, blue: Zn-deposited Cu, black: Cu mesh). (d) Calculated binding affinity between Zn and various materials. (c and d) Reprinted with permission from ref. 34. Copyright 2019, Wiley-VCH. (e) A schematic illustration of the design principle for epitaxial metal electrodeposition. (f) Grazing incident XRD (GIXRD) results of Zn electrodeposited on bare (left) and graphene-coated (right) stainless steel electrodes. (g) Coulombic efficiency levels at high current densities on epitaxially grown anodes. (h) SEM image of layered, homoepitaxially deposited Zn on graphene-steel for 12 min at a current density of 4 mA cm^–2. (e–h) Reprinted with permission from ref. 57. Copyright 2019, American Association for the Advancement of Science.

Fig. 7. (a) A schematic illustration of the “electro-healing” strategy. (b) Galvanostatic voltage profiles of Zn symmetric cells with (magenta) and without (dark green) a healing step at different current densities (top: 7.5 mA cm^–2, bottom: 10 mA cm^–2). Reprinted with permission from ref. 58. Copyright 2019, Wiley-VCH.