Electrolyte Transport Parameters and Interfacial Effects in Calcium Metal Batteries: Analogies and Differences to Magnesium and Lithium Counterparts

Abstract

Magnesium and calcium metal batteries are promising emerging technologies. Their high capacity and low redox potential translate to a high theoretical energy density, making them attractive candidates for future energy storage solutions. Owing to their neighboring position and the diagonal relationship in the periodic table to lithium, Mg²⁺, Ca²⁺, and Li⁺ ions feature commonalities in terms of ionic radius, carried charge, and charge density. The present study aims to shed light on the similarities but also differences of Ca electrolytes and metal anodes in comparison to their Mg and Li counterparts in terms of transport properties and processes at the anode/electrolyte interface, respectively. To ensure comparability, an electrolyte comprising B(hfip)₄ ^- anions in monoglyme is applied in either case. By executing galvanostatic polarization and pulsing with different separator materials, the separator tortuosity, diffusion coefficient, and transference number are determined. Further, the charge transfer characteristics as well as the adsorption layer and solid electrolyte interphase formation are investigated by electrochemical impedance spectroscopy. The cation charge density was found to be crucial for diffusion and desolvation processes, yet surprisingly, also a cation-dependent separator tortuosity was observed. The study concludes with a recommendation on suitable separators for each metal battery system.

Keywords: EIS; electrolyte transport; metal anode battery; multivalent metals; separator tortuosity.

Figure 1

a,b) Raman spectra of the 0.2 m Mg[B(hfip)₄]₂/G1, 0.2 m Ca[B(hfip)₄]₂/G1 and 0.2 m Li[B(hfip)₄]/G1 electrolytes. c) Ionic conductivity and viscosity as well as d) the molecule structure and number of coordinating G1 molecules in M[B(hfip)₄]_n / G1 electrolytes and salts, respectively.

Figure 2

Separator morphology of a) Whatman GF/C (glass fiber), b) Celgard 2500 (PP), c) Solupor 3P07A (PE), d) NKK TBL4620 (cellulose), and e) Dreamweaver Silver 25 (cellulose).

Figure 3

Calculated separator tortuosity in dependence on the cation system.

Figure 4

Determination of the binary diffusion coefficient D_± (top) and transference number t₊ (bottom) from DC pulse polarization of symmetrical cells with Celgard 2500 separator (10 layers).

Figure 5

Polarization of a) Mg|Mg, b) Ca|Ca, and c) Li|Li cells comprising a 0.2 m M[B(hfip)₄]_n / G1 electrolyte at 1.0 mA cm⁻² applying different separators. In case of GF/C 150 µl, in other cases 50 µl electrolyte volume was used.

Figure 6

Polarization of symmetrical a) Mg, b) Ca, and c) Li cells comprising a 0.2 m M[B(hfip)₄]_n / G1 electrolyte at 0.1 and 1 mA cm⁻² applying a C2500 separator.

Figure 7

Impedance spectra evolution during 50 h OCV in Mg|Mg, Ca|Ca and Li|Li symmetrical cells with two layers of GF/C ((a–c) 250 µl) and one layer of C2500 ((d,e) 50 µl), respectively. The corresponding Bode plots are depicted in Figure S14 (Supporting Information).

Figure 8

Comparison of the final impedance spectra after 50 h OCV and the first impedance spectra during polarization with 0.1 mA cm⁻² in Mg|Mg, Ca|Ca and Li|Li symmetrical cells with two layers of GF/C ((a–c) 250 µl) and one layer of C2500 ((d—f) 50 µl), respectively. The corresponding Nyquist plots are depicted in Figure S15 (Supporting Information).

Figure 9

Impedance spectra of Ca|Ca cells comprising a) pellet electrodes and different separators and b) different Ca electrodes and C2500 separator (pot.stat., 5 mV).

Figure 10

Impedance spectra and corresponding ECM of Mg|Mg, Ca|Ca and Li|Li cells during a–c) polarization at different current densities and d–f) polarization at 0.1 mA cm⁻² (2x GF/C, 250 µl).

Figure 11

Fitted resistance values corresponding to the spectra in Figure 10. The fit errors were included yet too small be visible herein.