Magnesium Borohydride: From Hydrogen Storage to Magnesium Battery

氢气储存 硼氢化 电池(电) 化学 无机化学 材料科学 冶金 有机化学 催化作用 物理 热力学 功率(物理)
作者
Rana Mohtadi,Masaki Matsui,Timothy S. Arthur,Son‐Jong Hwang
出处
期刊:Angewandte Chemie [Wiley]
卷期号:51 (39): 9780-9783 被引量:418
标识
DOI:10.1002/anie.201204913
摘要

Beyond hydrogen storage: The first example of reversible magnesium deposition/stripping onto/from an inorganic salt was seen for a magnesium borohydride electrolyte. High coulombic efficiency of up to 94 % was achieved in dimethoxyethane solvent. This Mg(BH4)2 electrolyte was utilized in a rechargeable magnesium battery. Since Bogdanović and Schwickardi illustrated the possibility of reversibly storing hydrogen in sodium alanate,1 extensive research efforts have been dedicated to investigating the hydrogen storage potential of complex metal hydrides. In particular, borohydrides have attracted great interest because of their superior gravimetric hydrogen content.2 Of these, magnesium borohydride Mg(BH4)2, first reported in 19503 and more recently studied for hydrogen storage, has attracted attention because of its relatively low hydrogen-release temperature and reversibility.2a, 4 Furthermore, borohydrides are strong reducing agents that are widely used in organic and inorganic syntheses. This reducing power translates to high stability against electrochemical reduction; this stability could be exploited in highly reductive environments, such as battery anodes. Therefore, for the first time, we have conducted research towards harnessing this property of borohydrides for their use in rechargeable batteries. In particular, we have been focusing on utilizing a Mg(BH4)2 based electrolyte in a rechargeable magnesium battery. Recently, magnesium batteries have received increased attention as alternatives to the lithium-based battery because of the high volumetric capacity (3832 mA h cm−3), improved safety (nondendritic), and abundance of Mg metal.5 Despite the potential of Mg batteries, several key challenges need to be overcome for this technology to become viable. For instance, current state-of-the-art electrolytes use organomagnesium salts and complexes as they are the only ones known to be compatible with the Mg anode that allow for reversible electrochemical Mg deposition and stripping.5b, 6 Although some of these electrolytes have shown impressive stability against electrochemical oxidation, they were also found to be corrosive.6 This property was attributed to the presence of chlorides in either/both their cations and anions.6 Conventional inorganic and ionic salts such as Mg(ClO4)2 were found to be incompatible with the Mg anode as a result of the formation of an ion-blocking layer formed by their electrochemical reduction.6 Hence, the discovery of halide-free electrolytes with high reductive stabilities is crucial for realizing a practical rechargeable Mg battery system. Herein, we propose a new class of electrolytes based on Mg(BH4)2 for a Mg battery. We show the first example of electrochemical reversible Mg deposition/stripping in a halide-free inorganic salt in both tetrahydrofuran (THF) and dimethoxyethane (DME) solvents. An increase of several orders of magnitude in the current densities, and high coulombic efficiencies of up to 94 % are observed in DME when LiBH4 is used as an additive. Furthermore, we use this electrolyte in a rechargeable Mg battery, thus giving the first example of a borohydride electrolyte in a battery system. This work also illustrates the unique properties of borohydrides and opens the door for designing a whole new class of electrolytes for Mg batteries. Mg deposition/stripping was studied for Mg(BH4)2 in ether solvents. Figure 1 a shows the cyclic voltammogram obtained for 0.5 M Mg(BH4)2/THF where a reversible reduction–oxidation process took place with onsets at −0.6 V/0.2 V and a 40 % coulombic efficiency (Figure 1 a, inset), thus indicating reversible Mg deposition and stripping. X-ray diffraction (XRD) confirmed that the deposited product from the galvanostatic reduction of the above solution (Figure 1 b) was hexagonal Mg, hereby establishing the compatibility of Mg(BH4)2 with Mg metal. The electrochemical oxidative stabilities measured on platinum, stainless steel, and glassy carbon electrodes were 1.7, 2.2, and 2.3 V, respectively (Figure S7). These results showed that for the first time: 1) Mg(BH4)2 is electrochemically active in THF, that is, ionic conduction is possible, and 2) reversible magnesium deposition/stripping from an inorganic, relatively ionic (Mg Bader charge is +1.67)7 and halide-free salt is feasible. Although these results are promising, to make this electrolyte more practical for use in batteries the electrochemical performance needs to be improved by lowering the overpotentials, and achieving higher current density and coulombic efficiency. In addition, the demonstration of this performance in less-volatile solvents would make Mg(BH4)2 based electrolytes even more practical. Therefore, DME was selected (its boiling temperature is 19 °C higher than that of THF) for further investigations. The cyclic voltammogram obtained for 0.1 M Mg(BH4)2/DME is shown in Figure 1 c where a substantial improvement in the electrochemical performance compared to Mg(BH4)2/THF was evident from: 1) a 10-fold increase in the current density, 2) a reduction in the overpotentials (deposition/stripping onsets at −0.34 V/0.03 V versus −0.6 V/0.2 V in THF), and 3) a higher coulombic efficiency of 67 % (40 % in THF). These findings suggested that the Mg electroactive species was present in higher concentration and had increased mobility in DME despite the lower solubility of Mg(BH4)2 in DME versus THF. For 0.5 M Mg(BH4)2/THF: a) Cyclic voltammogram (8 cycles), inset shows deposition/stripping charge balance (third cycle), and b) XRD results following galvanostatic deposition of Mg on a Pt working electrode. c) Cyclic voltammogram for 0.1 M Mg(BH4)2/DME compared to 0.5 M Mg(BH4)2/THF. Inset shows deposition/stripping charge balance for Mg(BH4)2/DME. All experiments used Pt working electrode and Mg reference/counter electrodes. For Mg(BH4)2 in THF (red line) and in DME (black line): a) IR spectra, b) 11B NMR spectra, and c) 1H NMR spectra. For the spectrum of Mg(BH4)2/DME, although the main features present in the spectrum of Mg(BH4)2/THF were retained, the νBHt band is broader and shifted to a lower value and the νBHb intensity is relatively weaker. Although νBHt band broadening suggests a pronounced presence of a species similar to that found in THF, the shift in the band maximum indicates a more-ionic BH bond (the νBHt shift is similar to those resulting from BH4− ions that have enhanced ionic character, such as in stabilized covalent borohydrides).8 In addition, the relative weakening in νBHb intensity suggests that there is more free BH4−. The NMR spectrum of BH4− in DME (Figure 2 b and c) indicates that there is increased boron shielding as the associated signal is shifted by about 0.5 ppm (quintet in 11B NMR spectrum), and slightly reduced proton shielding (0.01 ppm, quartet in 1H NMR spectrum); these results are consistent with BH bonds that have a higher ionic character than those in BH4− in THF (distinguishing BHt from BHb is not possible likely because of rapid hydrogen exchange). These findings are evidence of weaker interactions between Mg2+ and BH4− within the ion pair and an enhanced dissociation in DME [Eq. (1) and (2)]. So despite the fact that DME has a slightly lower dielectric constant (7.2) compared to THF (7.4), its chelation properties (owing to the presence of two oxygen sites per molecule)11 resulted in an enhanced dissociation and thus an improved electrochemical performance. Based on the understanding gained of the nature of Mg(BH4)2 in solution, we hypothesized that electrochemical performance would be enhanced when the association within the ion pair is weakened. To achieve this, an additive that has an acidic cation with the following characteristics is desirable: 1) reductive stability comparable to Mg(BH4)2, 2) nonreactive, 3) halide free, and 4) soluble in DME. Hence, LiBH4 was selected as it fulfils all of the above criteria. Mg deposition and stripping was studied in DME using different molar ratios of LiBH4 to Mg(BH4)2; Figure 3 a shows the cyclic voltammogram obtained for 3.3:1 molar LiBH4 to Mg(BH4)2 (Figure S8a and S8b show the cyclic voltammograms for different concentrations). The use of LiBH4 resulted in an increase of two orders of magnitude in the current density (i.e. oxidation peak current Jp=26 mA cm−2), and in a higher coulombic efficiency of up to 94 %. We attribute the deposition/stripping currents solely to Mg because of the absence of Li after galvanostatic deposition (Figure 3 b), and also the lack of electrochemical activity in a LiBH4/DME solution (Figure S8a). The ionic character of BH4− was enhanced, as evidenced by lower νBHt and higher νBHb bands in the IR spectrum (Figure 3 c), thus implying that LiBH4 has a role in increasing Mg(BH4)2 dissociation (the BH bands for LiBH4/DME occur at lower values, Figure S9). The coulombic efficiency was proportional to the molar ratios of LiBH4/Mg(BH4)2 (Figure S10). A rechargeable Mg battery with a Chevrel phase Mo6S8 cathode, an Mg metal anode, and this optimized electrolyte (Figure 4) demonstrated reversible cycling capabilities at a 128.8 mA g−1 rate (capacity retention and cathode magnesiation are shown in Figure S11 and Figure S12). We are currently investigating the sources of the overcharge and capacity fade. For LiBH4 (0.6 M)/Mg(BH4)2 (0.18 M) in DME: a) Cyclic voltammogram (inset shows deposition/stripping charge balance). b) XRD results following galvanostatic deposition of Mg on a Pt disk. c) IR spectra (red | indicates band maxima for Mg(BH4)2/DME). Charge/discharge profiles with Mg anode/Chevrel phase cathode for 3.3 molar LiBH4/Mg(BH4)2 in DME. Cycle 1 (blue), cycle 2 (red), cycle 20 (black), cycle 40 (green). In summary, unprecedented reversible Mg deposition and stripping from an inorganic and relatively ionic salt was obtained in THF and DME. Higher current density and lower overpotentials were achieved in DME compared to those in THF. Substantial enhancement in the coulombic efficiency and the current density was accomplished by the addition of LiBH4. Battery performance was demonstrated using a Chevrel phase cathode. Although the oxidative stability (1.7 V vs. Mg on platinum) currently limits Mg(BH4)2 utilization with high voltage cathodes, reversibility in the absence of halides and THF makes this salt extremely unique and these findings very important for designing a whole new class of Mg(BH4)2 based electrolytes. Currently, we are investigating improving the oxidative stability of Mg(BH4)2. In addition, the exact nature of the electroactive species in the presence and the absence of the additive is being studied to guide the design of Mg(BH4)2 based electrolytes. This work provides a stepping stone for extending the applications of Mg(BH4)2 and underscores the beauty and versatility of the chemistry of borohydrides. Magnesium borohydride (Mg(BH4)2, 95 %) lithium borohydride (LiBH4, 90 %), anhydrous tetrahydrofuran (THF), and dimethoxyethane (DME) were purchased from Sigma–Aldrich. Cyclic voltammetry was conducted in a three-electrode cell with Mg wire/ribbon as reference/counter electrodes. The electrochemical testing was conducted in an argon filled glovebox with O2 and H2O amounts kept below 0.1 ppm. Details of the analyses and battery testing conducted are described in the Supporting Information. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
最长约 10秒,即可获得该文献文件

科研通智能强力驱动
Strongly Powered by AbleSci AI
更新
PDF的下载单位、IP信息已删除 (2025-6-4)

科研通是完全免费的文献互助平台,具备全网最快的应助速度,最高的求助完成率。 对每一个文献求助,科研通都将尽心尽力,给求助人一个满意的交代。
实时播报
Popeye应助单纯血茗采纳,获得10
刚刚
淡然冬灵发布了新的文献求助10
刚刚
Popeye应助单纯血茗采纳,获得10
刚刚
荔枝的油饼iKun完成签到,获得积分10
1秒前
Bosen完成签到,获得积分10
1秒前
Astraeus完成签到 ,获得积分10
2秒前
fengyuenanche完成签到,获得积分10
3秒前
五虎完成签到,获得积分10
4秒前
Akim应助Rollei采纳,获得10
5秒前
hoshi1018完成签到,获得积分10
6秒前
友好曲奇完成签到,获得积分10
6秒前
dongdong完成签到 ,获得积分10
7秒前
CR7完成签到,获得积分0
8秒前
左丘忻完成签到,获得积分10
8秒前
凤迎雪飘完成签到,获得积分10
8秒前
8秒前
FashionBoy应助云轩采纳,获得10
9秒前
领导范儿应助伏伏雅逸采纳,获得10
9秒前
10秒前
Rondab应助悦耳荟采纳,获得10
10秒前
liqian完成签到,获得积分10
10秒前
易安发布了新的文献求助100
11秒前
12秒前
完美世界应助科研通管家采纳,获得10
12秒前
852应助科研通管家采纳,获得10
12秒前
今后应助科研通管家采纳,获得10
12秒前
cdh1994应助科研通管家采纳,获得10
12秒前
英姑应助科研通管家采纳,获得10
12秒前
沉默的谷秋完成签到,获得积分10
12秒前
大个应助科研通管家采纳,获得10
12秒前
过时的热狗完成签到,获得积分10
13秒前
13秒前
上官若男应助科研通管家采纳,获得10
13秒前
13秒前
13秒前
星辰大海应助科研通管家采纳,获得10
13秒前
13秒前
科目三应助科研通管家采纳,获得10
13秒前
14秒前
14秒前
高分求助中
【提示信息,请勿应助】关于scihub 10000
Les Mantodea de Guyane: Insecta, Polyneoptera [The Mantids of French Guiana] 3000
徐淮辽南地区新元古代叠层石及生物地层 3000
The Mother of All Tableaux: Order, Equivalence, and Geometry in the Large-scale Structure of Optimality Theory 3000
Global Eyelash Assessment scale (GEA) 1000
Picture Books with Same-sex Parented Families: Unintentional Censorship 550
Research on Disturbance Rejection Control Algorithm for Aerial Operation Robots 500
热门求助领域 (近24小时)
化学 材料科学 医学 生物 工程类 有机化学 生物化学 物理 内科学 纳米技术 计算机科学 化学工程 复合材料 遗传学 基因 物理化学 催化作用 冶金 细胞生物学 免疫学
热门帖子
关注 科研通微信公众号,转发送积分 4038446
求助须知:如何正确求助?哪些是违规求助? 3576149
关于积分的说明 11374627
捐赠科研通 3305875
什么是DOI,文献DOI怎么找? 1819354
邀请新用户注册赠送积分活动 892680
科研通“疑难数据库(出版商)”最低求助积分说明 815048