An efficient organic magnesium borate-based electrolyte with non-nucleophilic characteristics for magnesium–sulfur battery

Author(s): Aobing Du ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Zhonghua Zhang ⁶ ^, ⁷ ^, ⁸ ^, ⁵ , Hongtao Qu ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Zili Cui ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Lixin Qiao ⁶ ^, ⁷ ^, ⁸ ^, ⁵ , Longlong Wang ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Jingchao Chai ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Tao Lu ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Shanmu Dong ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Tiantian Dong ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Huimin Xu ⁶ ^, ⁷ ^, ⁸ ^, ⁵ , Xinhong Zhou ⁶ ^, ⁷ ^, ⁸ ^, ⁵ , Guanglei Cui ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵

Publication date (Electronic): 2017

Journal: Energy & Environmental Science

Publisher: Royal Society of Chemistry (RSC)

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Abstract

An efficient organic magnesium borate-based electrolyte with non-nucleophilic characteristics for magnesium–sulfur battery.

Abstract

Two-electron transfer chemistry based on earth-abundant Mg and S offers great possibilities of delivering higher energy density than current Li-ion technology. The development of non-nucleophilic electrolytes that reversibly and efficiently plate and strip Mg is believed to be a major obstacle to the implementation of this divalent battery technology. In this study, we present a new type of organic magnesium borate-based electrolyte that primarily comprises tetrakis(hexafluoroisopropyl)borate anions [B(HFP) ₄] ⁻ and solvated cations [Mg ₄Cl ₆(DME) ₆] ²⁺, which was synthesized via a facile in situ reaction of tris(hexafluoroisopropyl)borate [B(HFP) ₃], MgCl ₂ and Mg powder in 1,2-dimethoxyethane (DME). Rigorous analyses including NMR, mass spectroscopy and single-crystal XRD were conducted to identify the equilibrium species in the abovementioned solution. The as-prepared Mg-ion electrolyte exhibited unprecedented Mg plating/stripping performance, such as high anodic stability up to 3.3 V ( vs. Mg/Mg ²⁺), high ionic conductivity of 5.58 mS cm ⁻¹, a low overpotential of 0.11 V for plating processes and Coulombic efficiencies greater than 98%. By virtue of the non-nucleophilic nature of this electrolyte, a fully reversible Mg/S battery was constructed that displayed an extremely low overpotential of 0.3 V and a high discharge capacity of up to 1247 mA h g ⁻¹ and yielded a specific energy of approximately 1200 W h kg ⁻¹ (10 times higher that of the Chevrel benchmark) based on the weight of active sulfur. More significantly, commonly used sulfur-carbon nanotube (S-CNTs) cathodes with S contents of 80 wt% and S loadings of 1.5 mg cm ⁻² were demonstrated to withstand more than 100 cycles without obvious capacity decay and to enable fast conversion processes, which achieved a charging current rate of up to 500 mA g ⁻¹. Our findings convincingly validate the pivotal role of the newly designed non-nucleophilic Mg-ion electrolyte for practical Mg/S battery chemistry.

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Most cited references 36

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Prototype systems for rechargeable magnesium batteries.

D Aurbach, Z. Lu, A. Schechter … (2000)

The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, because it may provide a considerably higher energy density than the commonly used lead-acid and nickel-cadmium systems. Moreover, in contrast to lead and cadmium, magnesium is inexpensive, environmentally friendly and safe to handle. But the development of Mg batteries has been hindered by two problems. First, owing to the chemical activity of Mg, only solutions that neither donate nor accept protons are suitable as electrolytes; but most of these solutions allow the growth of passivating surface films, which inhibit any electrochemical reaction. Second, the choice of cathode materials has been limited by the difficulty of intercalating Mg ions in many hosts. Following previous studies of the electrochemistry of Mg electrodes in various non-aqueous solutions, and of a variety of intercalation electrodes, we have now developed rechargeable Mg battery systems that show promise for applications. The systems comprise electrolyte solutions based on Mg organohaloaluminate salts, and Mg(x)Mo3S4 cathodes, into which Mg ions can be intercalated reversibly, and with relatively fast kinetics. We expect that further improvements in the energy density will make these batteries a viable alternative to existing systems.