A universal strategy towards high-energy aqueous multivalent-ion batteries

Abstract

Rechargeable multivalent metal (e.g., Ca, Mg or, Al) batteries are ideal candidates for large-scale electrochemical energy storage due to their intrinsic low cost. However, their practical application is hampered by the low electrochemical reversibility, dendrite growth at the metal anodes, sluggish multivalent-ion kinetics in metal oxide cathodes and, poor electrode compatibility with non-aqueous organic-based electrolytes. To circumvent these issues, here we report various aqueous multivalent-ion batteries comprising of concentrated aqueous gel electrolytes, sulfur-containing anodes and, high-voltage metal oxide cathodes as alternative systems to the non-aqueous multivalent metal batteries. This rationally designed aqueous battery chemistry enables satisfactory specific energy, favorable reversibility and improved safety. As a demonstration model, we report a room-temperature calcium-ion/sulfur| |metal oxide full cell with a specific energy of 110 Wh kg^-1 and remarkable cycling stability. Molecular dynamics modeling and experimental investigations reveal that the side reactions could be significantly restrained through the suppressed water activity and formation of a protective inorganic solid electrolyte interphase. The unique redox chemistry of the multivalent-ion system is also demonstrated for aqueous magnesium-ion/sulfur||metal oxide and aluminum-ion/sulfur||metal oxide full cells.

Fig. 1. The electrochemical stability window of electrolytes.

a Linear voltammetry curves recorded at 1 mV s^–1 in 1 m, 2 m, 5 m, saturated (8.37 m) Ca(NO₃)₂ electrolytes and aqueous gel electrolyte. The insets are the magnified views of the regions marked near anodic and cathodic extremes. b The electrochemical stability windows of electrolytes, and the redox voltages of Ca_0.4MnO₂ cathode and sulfur anode obtained from experimental data.

Fig. 2. Molecular dynamics simulations and characterization of electrolytes.

Snapshots of local structure evolution for a 1 m Ca(NO₃)₂ electrolyte, b saturated Ca(NO₃)₂ electrolyte, and c aqueous gel electrolyte based on MD simulation at 10 ns. d The hydrogen bonds and the percentage of water molecular coordinated with Ca²⁺ for three electrolyte samples based on MD simulation at 10 ns. The hydrogen bond between the Ca²⁺–H₂O complex and PVA repetitive unit is shown in the inset. The green, red, white, and gray balls represent Ca, O, H, and C, respectively. e Raman spectra of the 1 m, 2 m, 5 m and saturated Ca(NO₃)₂ aqueous electrolytes, and aqueous gel electrolyte.

Fig. 3. Reaction mechanism of elemental sulfur in the aqueous gel electrolyte.

a Local structure evolutions of CaS₄ (yellow) diffusions in 1 m, 8.37 m Ca(NO₃)₂ electrolytes, and aqueous gel electrolyte based on MD simulation. The yellow balls represent S atoms, and other symbols are same with those in Fig. 2a–c. b Visual observation of calcium polysulfides diffusion in 1 m, 8.37 m Ca(NO₃)₂ electrolyte, and gel electrolyte. c The initial CV curves of S/C electrode collected at a scan rate of 0.2 mV s^–1 in the aqueous gel electrolyte. The CV curves of Ca metal||S/C cell with 0.5 m Ca(OTf)₂ in TEGDME or 0.5 m Ca(OTf)₂ in PC electrolytes at 0.2 mV s^–1 are shown in inset of Fig. 3c. d The Young’s Modulus of S/C anodes tested in 1 m Ca(NO₃)₂ and aqueous gel electrolytes. The points are selected from the AFM scanning images in Supplementary Fig. 16. The inset is HADDF–STEM images of S/C anodes after cycling in aqueous gel electrolyte, scale bar: 10 nm. e The DFT calculations of reduction of Ca²⁺(NO₃^–)₃(H₂O)_x aggregate. The green, blue, red, and white balls represent Ca, N, O, and H atoms, respectively. f In-depth Ca 2p XPS of the S/C electrode after cathodic CV scanning.

Fig. 4. Characterization of the Ca_0.4MnO₂ cathode.

a Schematic illustration of the atomic structure change during the in situ electrochemical conversion. The HADDF–STEM images of b the Mn₃O₄ precursor and c The Ca_0.4MnO₂ cathode material. Blue, orange, green, and red balls represent Mn³⁺, Mn²⁺, Ca, and O atoms, respectively. Scale bars are 2 nm in Fig. 4b and c. d Synchrotron powder diffraction patterns of the Mn₃O₄, birnessite MnO₂, and Ca_0.4MnO₂. The peaks marked with star symbol correspond to the stainless-steel mesh current collector. e CV curves of the Ca_0.4MnO₂ cathode collected at a scan rate of 0.3 mV s^–1.

Fig. 5. The electrochemical performances of the as–assembled ACSBs.

a The CV curves of the S/C|aqueous gel electrolyte|Ca_0.4MnO₂ full cell collected at a scan rate of 0.2 mV s^–1. b The rate performance of full cells with different electrolytes. c The voltage profiles of full cell assembled with aqueous gel electrolyte at different specific currents. d Comparison of specific energies of various aqueous energy storage devices based on the total mass of the electrode active materials. Color code: cyan, brown, orange, blue, black, and violet represent Li–, Na–, K–, Mg–, Al–, Ca–based aqueous batteries, respectively. e The cycling performances and corresponding Coulombic efficiencies of S/C||Ca_0.4MnO₂ full cells at 0.2 C. The mass ratio of Ca_0.4MnO₂ cathode to S/C anode is about 1.6:1. f Water–soaking test of charged S/C|aqueous gel electrolyte|Ca_0.4MnO₂ pouch cell after corner cut.