Composite Anodes for Secondary Magnesium Ion Batteries Prepared via Electrodeposition of Nanostructured Bismuth on Carbon Nanotube Substrates

Abstract

Magnesium-ion batteries are attractive in part due to the high environmental abundance and low cost of magnesium metal. Anode materials other than Mg metal can provide access to new electrochemistries in non-corrosive Mg 2+ electrolytes. A cyclic voltammetric method for the preparation of bismuth (Bi) based anodes was developed by systematically exploring electrodeposition using a quartz crystal microbalance. Controlled deposition of Bi on carbon nanotubes substrates could be achieved, enabling the first electrochemical investigation of bismuth-carbon nanotube (Bi-CNT) composite electrodes. Quasi-reversible Mg electrochemistry of Bi-CNT composite electrodes in non-corrosive magnesium-based electrolyte was demonstrated, with an initial delivered capacity exceeding 180 mAh/g. While the initial capacities were high, significant capacity decreases were observed with repeated cycling, indicating that additional development is warranted to further optimize this system. Interest in magnesium-ion batteries as possible replacements for lithium-ion batteries remains high, in part due to the low cost and high earth abundance of magnesium.1-5 Much of the research to date employs magnesium metal as the anode, given the high theoretical volumetric capacity of 3832 mAh/cm 3 and non-dendritic electrochemical behavior associated with magnesium metal.3,6 However, several challenges remain for full implementation of a magnesium anode battery technology. The surface of magnesium metal when exposed to conventional electrolytes based on carbonates does not generate a surface that is readily electrochemically stripped and plated, which has been attributed to the formation of a non-ion conducting layer on the metal surface. [6][7][8] In order to address the surface issues of the magnesium new classes of electrolytes have been investigated. [8][9][10] However, these electrolytes are typically corrosive and may display significant volatility.11-14 Thus, an alternative anode to magnesium metal that may enable use of less corrosive electrolytes is desirable.Recent reports have investigated the use of bismuth metal as an anode material in magnesium based batteries. 15,16 Assuming six electron equivalents of bismuth oxidation to form magnesium bismuthide, (3Mg 2+ + 2Bi + 6e − Mg 3 Bi 2 ) translates to a theoretical capacity of 385 mAh/g Bi, Figure 1. As bismuth is a dense material (9.8 g/cm 3 ), a significant volume expansion (25-40% increase in Bi-Bi bond length for Mg 3 Bi 2 ) is required to accommodate Mg 2+ insertion and thus loss of contact to the current collecting electrode substrate upon repeated cycling may occur.Matsui and coworkers demonstrated capacities of ∼240 mAh/g for electrodeposited bismuth in corrosive alkylmagnesium/ alkylaluminum chloride based electrolyte.15 Proof of concept quasi-reversible electrochemistry for electrodeposited bismuth in acetonitrile/Mg(TFSI) 2 based electrolyte was also provided, however, only one cycle was shown and the capacity under this condition was not reported. 15 Liu and coworkers reported ∼3...

Unexpected Behavior of the InSb Alloy in Mg-Ion Batteries: Unlocking the Reversibility of Sb

“…Such Mg-ion cells could be a reliable alternative to lithium-based systems. Recently, some studies showed that p-block elements (Sn, [11][12][13][14][15][16][17] Sb, 18,19 In, 17,20,21 Pb, 22 Bi, 5,18,[23][24][25][26][27] ...) can electrochemically alloy with Mg and can be stable with conventional electrolyte such as Mg(TFSI)2 dissolved in a glyme solvent. 5,26 Despite their interesting electrochemical reactivity, these alloys present lower theoretical gravimetric capacities than Mg metal (300 to 900 mAh g-1 ) [11][12][13][14][15][16][17][18][19][20][21] .…”

High Energy Density Electrode Materials for Rechargeable Magnesium Batteries

“…An alternative Mg(BH)4 -based electrolyte was also examined and similar results were achieved. From these results we conclude that Bi-CNT composite electrodes are quasireversible for magnesium electrochemistry, and worthy of further improvement studies (29). …”

Ionic liquid hybrids: Progress toward non-corrosive electrolytes with high-voltage oxidation stability for magnesium-ion based batteries

“…Here, we measure conductivities and oxidative stabilities for ionic liquids blended with di(propylene glycol) dimethyl ether (DPDGME, regarded as a "green" glyme) and acetonitrile, which has found success in high voltage applications. [31,32] Our findings indicate a cosolvent-induced conductivity enhancement as high as 12-fold with the mixing of IL with acetonitrile and oxidative stability up to an estimated 5.5 V vs Mg/Mg 2+ . Additionally, we used these electrolytes to successfully insert and remove Mg-ion cathodes that can deliver >175 mAh/g discharge capacities.…”

“…Of the known Mg-ion anode materials, is envisioned that a bismuth based anode (Mg 3 Bi 2 ) with a potential of +0.23 V versus magnesium may be most appropriate. [31,32,59] Since acetonitrile reduces at a voltage of -0.2 V vs Mg, [60] a magnesium ion battery based on an Mg 3 Bi 2 anode should be compatible with the hybrid electrolytes described herein. Table 1.…”