Opportunities and Limitations of Ionic Liquid‐ and Organic Carbonate Solvent‐Based Electrolytes for Mg‐Ion‐Based Dual‐Ion Batteries

Abstract Dual‐ion batteries (DIBs) offer a great alternative to state‐of‐the‐art lithium‐ion batteries, based on their high promises due to the absence of transition metals and the use of low‐cost materials, which could make them economically favorable targeting stationary energy storage applications. In addition, they are not limited by certain metal cations, and DIBs with a broad variety of utilized ions could be demonstrated over the last years. Herein, a systematic study of different electrolyte approaches for Mg‐ion‐based DIBs was conducted. A side‐by‐side comparison of Li‐ and Mg‐ion‐based electrolytes using activated carbon as negative electrode revealed the opportunities but also limitations of Mg‐ion‐based DIBs. Ethylene sulfite was successfully introduced as electrolyte additive and increased the specific discharge capacity significantly up to 93±2 mAh g−1 with coulombic efficiencies over 99 % and an excellent capacity retention of 88 % after 400 cycles. In addition, and for the first time, highly concentrated carbonate‐based electrolytes were employed for Mg‐ion‐based DIBs, showing adequate discharge capacities and high coulombic efficiencies.

S-2  Table S2a. Description of different cycling procedures used within this work. Before the first step, an open circuit potential (OCP) step of 10 h was applied for every cycling procedure. The electrolytes used in cells cycled with this procedure are indicated in brackets. The device used for all cells with this procedure is enclosed in square brackets.

Constant current cycling at high cut-offs and currents (Mg-Pyr+ES) [VMP3]
Step Specific current / mA g −1 Cut-off potential vs. Li|Li + / V Repetition S-4 Table S2b. Description of different cycling procedures used within this work. Before the first step, an OCP step of 10 h was applied for every cycling procedure. The electrolytes used in cells cycled with this procedure are indicated in brackets. The device used for all cells with this procedure is enclosed in square brackets.

Procedure 4:
Constant current cycling at varying cut-offs potentials For an easier comparison of the cell performance depending on the cut-off potential, the SDCs and CEff of the 10 th cycle of step 2-12 are shown in Figure 4a, 7a and 10b, and Table S6.
Step Specific current / mA g −1 Cut-off potential vs. Li|Li + / V Repetition S-5 Table S2c. Description of different cycling procedures used within this work. Before the first step, an OCP step of 10 h was applied for every cycling procedure. The electrolytes used in cells cycled with this procedure are indicated in brackets. The device used for all cells with this procedure is enclosed in square brackets.

Procedure 5:
Constant current cycling with varying specific currents (Mg-Pyr, Li-Pyr, Mg-Pyr+ES, Mg-DMC, Mg-DEC) [MACCOR] For an easier comparison of the cell performance depending on the specific current, the SDCs and CEff of the 5 th cycle of step 3-11 are shown in Figure 4b, 7b and 10c, and Table S7.
Step Specific current / mA g −1 Cut-off potential vs. Li|Li + / V Repetition  Table S2d. Description of different cycling procedures used within this work. Before the first step, an OCP step of 10 h was applied for every cycling procedure. The electrolytes used in cells cycled with this procedure are indicated in brackets. The device used for all cells with this procedure is enclosed in square brackets.

Mg-Pyr
Charge  Dominant stage (n) 2, 3 1 2 1 *Graphite ‖ AC cells cycled with 10 mA g −1 did not reach the cut-off potential of 5.3 V vs Li|Li + within 10 h, similar to the cells cycled at 60 °C, likely resulting from side reactions, why a current of 100 mA g −1 was used for high cut-off potentials.           in Pyr14TFSI (red), 0.5 M Mg(TFSI)2 in Pyr14TFSI + 2 wt.% ES (blue) at 100 mA g −1 (1 pre-cycle at 10 mA g −1 ) and 0.5 M Mg(TFSI)2 in Pyr14TFSI with pre-cycled graphite and a) pristine AC (grey) or b) pre-cycled AC (black) at 100 mA g −1 (3 pre-cycles with Mg-Pyr+ES at 10 mA g −1 ) with cut-off potentials of 3.4 V and 5.0 V vs. Li|Li + . c) The corresponding differential capacity vs. potential plots of the 50 th cycle (Mg-Pyr and Mg-Pyr+ES) and the 3 rd precycle with Mg-Pyr+ES (cyan) and first and 50 th subsequent cycle with Mg-Pyr using pristine AC (grey) and precycled AC (black).