Iodine Boosted Fluoro-Organic Borate Electrolytes Enabling Fluent Ion-conductive Solid Electrolyte Interphase for High-Performance Magnesium Metal Batteries

Time:2024-11-14Viewed:13

Due to the high abundance of magnesium metal on Earth, its high theoretical volumetric capacity (3833 mAh cm−3), and its high safety advantages, such as the reduced likelihood of dendrite formation during charging and discharging, magnesium batteries are expected to replace lithium-ion batteries in the post-lithium era. However, the development of magnesium batteries has been significantly hindered by the lack of ideal electrolytes. The electrolytes for magnesium batteries have entered a new era of boron-based electrolytes, with various synthesis routes being gradually developed for magnesium salts with boron as the coordination center and terminal fluorinated substituents as the anions. Nevertheless, more straightforward, and universal synthesis methods are still needed. Additionally, the passivation issue on the surface of magnesium sheets remains a challenge in boron-based electrolytes, and novel strategies are required to understand and address these problems.

The team led by Prof. Zhong Jin from the School of Chemistry and Chemical Engineering at Nanjing University has proposed a new and simple method for synthesizing boron-based magnesium salts with terminal fluorinated anions (Mg[B(ORF)4]2, RF = fluorinated alkyl) to overcome these obstacles in magnesium ion battery energy storage technology. Using the synthesis of Mg[B(HFIP)4]2/DME as an example, 1.0 mmol of B(HFIP)3 and 0.5 mmol of elemental iodine are added to 2 mL of DME solvent, followed by the addition of excess magnesium powder. After stirring for more than 24 hours and filtering, a clear, colorless, transparent solution of Mg[B(HFIP)4]2/DME electrolyte is obtained. However, the original Mg[B(HFIP)4]2/DME electrolyte still causes passivation issues on the magnesium anode surface. To address this, leveraging the fact that MgI2 is an excellent magnesium ion-conductive component in the SEI (solid electrolyte interphase) on magnesium anodes, the magnesium powder in the formulation was replaced with a magnesium metal anode, and B(HFIP)3 and I2 were directly dissolved in DME to serve as the electrolyte. The resulting Mg[B(HFIP)4]2/DME-MgI2 electrolyte not only forms magnesium salts in situ during charging and discharging but also forms an SEI interface primarily composed of MgI2 on the magnesium sheet surface. As a result, the symmetric and half-cell performance of the battery using this new electrolyte is significantly superior to that of the original Mg[B(HFIP)4]2/DME electrolyte. When paired with the Mo6S8 cathode, the battery demonstrates excellent cathode compatibility, superior kinetic performance, and enhanced cycling stability, surpassing all previously reported boron-based electrolytes. This research, titled “Iodine Boosted Fluoro-Organic Borate Electrolytes Enabling Fluent Ion-conductive Solid Electrolyte Interphase for High-Performance Magnesium Metal Batteries,” has been published in Angewandte Chemie International Edition (https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202417450). The authors are affiliated with the School of Chemistry and Chemical Engineering, Institute of Green Chemistry and Engineering, Tianchang Advanced Materials and Energy Technology Research Center, State Key Laboratory of Coordination Chemistry, Key Laboratory of Mesoscopic Chemistry of Ministry of Education, Key Laboratory of Polymer Materials and Technology of Ministry of Education, and Jiangsu Key Laboratory of Advanced Organic Materials at Nanjing University. The co-first authors are Xinmei Song and Dr. Jingjie Sun. This work was supported by the National Natural Science Foundation of China, the Jiangsu Province Carbon Peak and Carbon Neutrality Science and Technology Innovation Special Project, and the Fundamental Research Funds for the Central Universities.

Figure 1. (a) The reaction equation for the preparation of pristine Mg[B(HFIP)4]2/DME electrolyte. (b-c) The ESI-MS results of pristine Mg[B(HFIP)4]2/DME electrolyte in (b) the negative sweep mode at the regions of [676, 682] and the positive sweep mode at the regions of (c) [239, 244], respectively. (d) The ESI-MS results of Mg[B(TFE)4]2/DME electrolyte in the regions of [390, 420]. The inset in (b) is the schematic structure illustration of the anions in pristine Mg[B(HFIP)4]2/DME electrolyte (Atoms: white, hydrogen; fray, carbon; red, oxygen; yellow, fluorine; dark yellow, boron). (e) The calculated LUMO and HOMO energy levels of B(HFIP)3, [B(HFIP)4]-, [B(HFIP)3I]-, and DME molecules. (f) In-situ preparation procedure of Mg[B(HFIP)4]2/DME-MgI2 electrolyte and its bifunctional properties in generating the Mg[B(HFIP)4]2 and the MgI2/MgF2-based SEI layer.

Figure 2. (a, b) CV curves of Mg||Mo half battery with Mg[B(HFIP)4]2/DME-MgI2 electrolyte at a scan rate of 5 mV s-1 for (a) the initial 6 cycles and (b) the subsequent 7-20 cycles. (c) CV curves of Mg||Mo half battery with pristine Mg[B(HFIP)4]2/DME electrolyte at a scan rate of 5 mV s-1. (d) XRD curve, (e) SEM and corresponding EDS mapping images, and (f) EDS curve of the Mo collector after discharge for 15 h in Mg||Mo half battery with Mg[B(HFIP)4]2/DME-MgI2 electrolyte. (g) LSV curves of various working electrodes (Mo, SS, and Cu) in Mg[B(HFIP)4]2/DME-MgI2 electrolyte (Scan rate: 1 mV s-1). (h) DC polarization curve of Mg||Mg symmetrical battery with Mg[B(HFIP)4]2/DME-MgI2 electrolyte at 25 , with a total applied potential difference of ~0.005 V. (i) AC complex impedance plot before and after DC polarization of Mg||Mg symmetrical battery with Mg[B(HFIP)4]2/DME-MgI2 electrolyte.

Figure 3. Mg plating/stripping cycling performance of Mg||Mg symmetrical batteries with pristine Mg[B(HFIP)4]2/DME (purple lines) and Mg[B(HFIP)4]2/DME-MgI2 (orange lines) electrolytes at room temperature with the current densities of (a) 0.1 mA cm-2 for 0.1 mAh cm-2 and (b) 0.5 mA cm-2 for 0.25 mAh cm-2, respectively. (c) Overpotentials of Mg||Mg symmetrical batteries with different electrolytes at different current densities varying from 0.1 mA cm-2 to 0.5 mA cm-2 for 1.0 mAh cm-2. (d, e) EIS results of Mg||Mg symmetrical batteries with (d) pristine Mg[B(HFIP)4]2/DME and (e) Mg[B(HFIP)4]2/DME-MgI2 electrolytes at the initial state and after the 20th cycle. (f) Long-term cycling performance of Mg||Mo half battery with Mg[B(HFIP)4]2/DME-MgI2 electrolyte cycled at 0.1 mA cm-2 with a cutoff voltage of 1.0 V. (g) Voltage profile of Mg||Mo half battery with Mg[B(HFIP)4]2/DME-MgI2 electrolyte measured in the 50th cycle and 100th cycle. (h) Coulombic efficiency versus cycle number profiles of Mg||Mo half batteries with Mg[B(HFIP)4]2/DME-MgI2 electrolyte. (i, j) Electro-plating/stripping curves of Mg||Mo half battery with Mg[B(HFIP)4]2/DME-MgI2 electrolyte (i) at different current densities and (j) with cutoff voltages changing from 1.0 V to 3.0 V, respectively.

Figure 4. (a, c) SEM and (b, d) corresponding EDS mapping images of Mg anodes in (a, b) Mg[B(HFIP)4]2/DME-MgI2 and (c, d) pristine Mg[B(HFIP)4]2/DME electrolytes after 100 cycles, respectively. (e, f) I 3d, F 1s, O 1s, C 1s, and B 1s XPS spectra after various Ar+ sputtering times on Mg anodes in (e) Mg[B(HFIP)4]2/DME-MgI2 and (f) pristine Mg[B(HFIP)4]2/DME electrolytes after 100 cycles, respectively. Schematic representation of the surface passivation layer formed by (g) pristine Mg[B(HFIP)4]2/DME and (h) Mg[B(HFIP)4]2/DME-MgI2 electrolyte on Mg anode.

Figure 5. COMSOL Multiphysics simulated (a, c) electric field and (b, d) Mg2+ ion concentration distributions on Mg anodes in (a, b) Mg[B(HFIP)4]2/DME-MgI2 and (c, d) pristine Mg[B(HFIP)4]2/DME electrolytes, respectively. The bottom areas with different morphologies represent the deposited metal electrode, where Mg2+ is reduced to Mg metal. During deposition, the bottom electrode potential is set to 0 V, Mg2+ concentration is 2 M, current density is 0.5 mA cm-2, and electrolyte conductivity is 11 mS cm-1.

Figure 6. CV curves of Mo6S8 cathodes assembled with (a) Mg[B(HFIP)4]2/DME-MgI2 and (b) pristine Mg[B(HFIP)4]2/DME electrolytes, respectively. (c) Rate performances of Mg||Mo6S8 batteries using pristine Mg[B(HFIP)4]2/DME and Mg[B(HFIP)4]2/DME-MgI2 electrolytes. (d) Galvanostatic charge/discharge profiles of Mg||Mo6S8 battery using Mg[B(HFIP)4]2/DME-MgI2 electrolyte at various current densities of 100, 200, 300, and 500 mA g-1. (e) Corresponding cycling stability of Mg||Mo6S8 batteriesutilizing Mg[B(HFIP)4]2/DME-MgI2 and pristine Mg[B(HFIP)4]2/DME electrolytes at a current density of 300 mA g-1. (f) Charge/discharge profiles of Mg||Mo6S8 batteries using Mg[B(HFIP)4]2/DME-MgI2 and pristine Mg[B(HFIP)4]2/DME electrolytes at a current density of 100 mA g-1. (g, h) CV curves of Mg||Mo6S8 batteries using (g) Mg[B(HFIP)4]2/DME-MgI2 and (h) pristine Mg[B(HFIP)4]2/DME electrolytes at scan rates varying from 0.2 to 2.0 mV s-1, respectively. (i) As-calculated capacitive/diffusion contributions of Mo6S8 cathodes in Mg[B(HFIP)4]2/DME-MgI2 and pristine Mg[B(HFIP)4]2/DME electrolytes. (j) GITT curves measured under a repeating constant current pulse of 50 mA g-1 for 10 min followed by a relaxation period of 30 min. (k) Corresponding diffusion coefficients of Mg2+ calculated from the GITT results in (j).