Highly reversible Zn metal anode enabled by sustainable hydroxyl chemistry

Abstract

Rechargeable Zn metal batteries (RZMBs) may provide a more sustainable and lower-cost alternative to established battery technologies in meeting energy storage applications of the future. However, the most promising electrolytes for RZMBs are generally aqueous and require high concentrations of salt(s) to bring efficiencies toward commercially viable levels and mitigate water-originated parasitic reactions including hydrogen evolution and corrosion. Electrolytes based on nonaqueous solvents are promising for avoiding these issues, but full cell performance demonstrations with solvents other than water have been very limited. To address these challenges, we investigated MeOH as an alternative electrolyte solvent. These MeOH-based electrolytes exhibited exceptional Zn reversibility over a wide temperature range, with a Coulombic efficiency > 99.5% at 50% Zn utilization without cell short-circuit behavior for > 1,800 h. More important, this remarkable performance translates well to Zn || metal-free organic cathode full cells, supporting < 6% capacity decay after > 800 cycles at -40 °C.

Keywords: Zn metal batteries; high reversibility; solid electrolyte interphase; sustainable electrolyte design.

Fig. 1.

Bulk and transport properties of MeOH-based electrolytes. (A and B) The Raman spectra of the S–O symmetric stretching band of OTF⁻ in (A) 111:1, (B) 23:1, and (C) 14:1 electrolytes, respectively. Experimental spectra (dots) were deconvoluted using Gaussian function (solid line). Red peak: SSIP; blue peak: CIP. (D) The experimental real permittivity spectra (dots) for selected electrolytes measured using DRS at room temperature and their corresponding fitted lines (solid line). (E) The FTIR spectra of the O–H band. The yellow square indicates the region with higher wavenumber (∼3500 cm⁻¹). (F) The experimental synchrotron X-ray scattering curve (solid line) and the MD simulation (at 298K) line (dashed line). (G and H) Snapshots of the MD simulation box (at 333K) for (G) the 14:1 electrolyte and (H) its clusters collected from MD. Note that the cluster of (Zn(OTf)₂(MeOH)₄) is less frequent than the cluster of (ZnOTf(MeOH))⁺. (I) Temperature-dependent conductivities for MeOH-based electrolytes. (J) Summary of Zn²⁺ transference number in MeOH-based electrolytes measured using the steady-state galvanostatic polarization method. Open squares indicate MD calculated values (at 333K) using the Rolling method.

Fig. 2.

Zn metal anode electrochemical reversibility in MeOH-based electrolytes. (A–C) Zn plating/stripping profiles and corresponding CE cycled in (A) 111:1, (B) 23:1, and (C) 14:1 electrolyte, respectively, using a Cu|Zn (10 µm) cell setup at room temperature and 2.93 mA cm⁻² to an areal capacity of 2.93 mAh cm⁻² per cycle. (D and E) Galvanostatic Zn plating/stripping cycled in (D) 111:1, (E) 23:1, and (F) 14:1 electrolyte, respectively, using a Zn|Zn(100 µm) cell setup at room temperature with 2.5 mA cm⁻² to an areal capacity of 2.5 mAh cm⁻² per cycle. (G) Zn plating/stripping profiles and corresponding CE cycled in 111:1 electrolyte using Cu|Zn(10 µm) cell setup at −40 °C and 1.17 mA cm⁻² to an areal capacity of 1.17 mAh cm⁻² per cycle.

Fig. 3.

Postcycling analysis of Zn metal anode. (A and B) FIB-SEM images of Zn metal anode obtained from Zn|Zn (100 μm) symmetric cells at a zero state of charge after (A) 20 h and (B) 200 h cycling (2.5 mA cm⁻², 2.5 mAh cm⁻² per cycle) with 111:1 electrolyte at room temperature. (C and D) FIB-SEM images of Zn metal anode obtained from Zn|Zn (100 μm) symmetric cells at a zero state of charge after (C) 20 h and (D) 200 h cycling (2.5 mA cm⁻², 2.5 mAh cm⁻² per cycle) with 14:1 electrolyte at room temperature. (E and F) TEM images of Zn metal anodes corresponding to (A) and (B), respectively. (G and H) TEM images of Zn metal anodes corresponding to (C) and (D), respectively. The interphasial region is labeled by blue dashed lines. (I and J) XPS spectra of O 1s for Zn metal anodes corresponding to (A) and (B), respectively. (K and L) XPS spectra of O 1s for Zn metal anodes corresponding to (C) and (D), respectively. The O 1s peak position assignment can be referred to C–O (∼533 eV) (33, 34), ZnCO₃ (∼532 eV) (16), and Zn(OH)₂ (∼531 eV) (35). (M) wB97XD/6–31+G(d,p) DFT calculation results on the decomposition process of MeOH on the nanoscale ZnO cluster distributed on the Zn metal surface. H: white; O: red; Zn: blue gray; C: light gray.

Fig. 4.

Full cell performance demonstration of MeOH-based electrolytes. (A) CE and corresponding specific discharge capacity vs. cycle number for 111:1 electrolyte tested with PANI (∼7 mg cm⁻²)|Zn(10 µm) cell setup at 60 mA g⁻¹ (based on the mass of PANI) between 0.4 and 1.4 V at 30 °C. (B) CE and corresponding specific discharge capacity vs. cycle number for 14:1 electrolyte using the same cell setup and condition as in (A). (C and D) Charge–discharge profile (seventh cycle) corresponding to the cell setup and electrolytes in (A) and (B), respectively, at 60 mA g⁻¹ and 360 mA g⁻¹ at 30 °C. (E) CE and corresponding specific discharge capacity vs. cycle number for 111:1 electrolyte tested with PANI (∼7 mg cm⁻²)|Zn(10 µm) cell setup at 60 mA g⁻¹ (based on the mass of PANI) between 0.4 and 1.8 V at −40 °C.