Superior Mg2+ storage properties of VS2 nanosheets by using an APC-PP14Cl/THF electrolyte
Graphical abstract
Introduction
Magnesium ion batteries (MIBs) have become an attractive alternative to lithium ion batteries due to the abundance of magnesium resource, high specific capacity (2205 mA h g−1), and high volumetric capacity (3833 mA h cm−3) [1,2]. In particular, the operation of Mg metal anodes is not plagued by dendritic deposition, which offers excellent battery safety. Despite these promising advantages, Mg metal anodes are difficult to use with conventional carbonate electrolytes due to the formation of passivation layers. Moreover, the strong polarity and slow diffusivity of Mg2+ cause sluggish electrode kinetics and poor cyclability. Thus far, the development of high performance Mg2+ storage materials and electrolytes has been the biggest challenge for MIBs [[3], [4], [5]].
Significant improvements of MIBs have been gained in the past few years by using complex Grignard reagents dissolved in ethereal solvents as the electrolyte [6,7]. In particular, a second generation electrolyte composed of two equivalents of phenylmagnesium chloride and one equivalent of aluminum trichloride dissolved in tetrahydrofuran, termed APC/THF, has shown near 100% Mg dissolution/deposition efficiency and high anodic stability up to 3.0 V vs. Mg [8]. Very recently, some new electrolytes with broader voltage windows and better cell compatibility have been developed, such as a non-nucleophilic electrolyte composed of Mg-bis(hexamethyldisilazide) and AlCl3 dissolved in glymes [9,10]. However, the complex electrolytes have been shown to be tightly bonded to Cl−, forming a series of THF coordinated complexes, such as MgCl+·5THF, MgCl2·4THF, and Mg2Cl3+·6THF. Mg2+ must overcome a high desolvation energy of ∼3.0 eV to break the MgCl bond before intercalating into the host material. This energy is much higher than the Mg2+ diffusion barriers of many intercalation materials, such as V2O5, MoS2, and Mo6S8 (1.69, 1.12, and 0.50 eV, respectively) [[11], [12], [13]]. Hence, compared to Mg2+ diffusion in the electrode, Mg2+ desolvation becomes the rate-limiting step in MIBs. Due to the difficult electrolyte desolvation, Yoo et al. have suggested that the intercalation species in MIBs is MgCl+ rather than Mg2+ [14]. They have shown that the MgCl+ diffusion could be greatly improved in an interlayer expanded TiS2 by electrolyte regulation. However, the theoretical specific capacities associated with monovalent MgCl+ intercalation is only half of that expected from divalent Mg2+ intercalation [14]. Thus far, a drastically decreased Mg2+ desolvation energy is vital for improving MIB's electrochemical performance [15,16].
Apart from the electrolyte, the cathode material also plays a key role in MIB electrochemical performance [17]. The Chevrel phase of Mo6S8 has been regarded as the most successful MIB cathode, which benefits from large diffusion channels and moderate Mg2+ polarity with an anionic sulfur (S) framework [18]. Moreover, Mg2+ desolvation in APC/THF is relatively easy under the catalyst effect of Mo cations, thus favoring Mg2+ insertion [13]. However, the application of Mo6S8 in MIBs has been halted by its small capacity and low working voltage, being 120 mA h g−1 and 1.0 V, respectively. To overcome the large polarity of Mg2+, shielding species have been used to screen the intense Mg2+ charge density. For instance, it has been shown that Mg2+(H2O)n is effectively inserted into MnO2 under the shielding effects of water [19]. However, this requires large interstitial spaces in the host material to accommodate hydrated cations. Also, water is typically incompatible with Mg anodes and most nonaqueous electrolytes, which limits MIB's cyclability. Recently, two-dimensional transition metal chalcogenides (TMCs), such as MoS2 [20], TiS2 [21], and WS2 [22], have attracted intensive interests in rechargeable batteries because the weak van der Waals bonding between interlayers is very favorable for insertion of different guest cations, such as Li+, Na+, and Zn2+ [23,24]. However, to date, the electrochemical performance of TMCs in MIBs has remained unsatisfying. For example, the MoS2/graphene hybrid with an enlarged interlayer spacing of 0.98 nm only showed a capacity of 115 mA h g−1 at the current density of 20 mA·g−1 and a poor capacity retention of 71% after 50 cycles [25].
To decrease the Mg2+ desolvation energy in APC/THF electrolyte and overcome the large polarity of Mg2+, in this study we developed an APC-PP14Cl/THF electrolyte using 1-buty-1-methylpiperidinium chloride (PP14Cl, C10H22NCl) as an electrolyte additive. The electrolyte was combined with Mg metal and VS2 nanosheets to construct a VS2‖APC-PP14Cl/THF‖Mg MIB cell. Experiments combined with first-principles calculations confirmed that Mg2+ in APC-PP14Cl/THF easily desolvated under the catalytic effects of PP14+. In addition, large-sized PP14+ was inserted into the VS2 interlayer spaces during the initial discharge and remained in the material during subsequent cycling. As a result, the interlayer spacing of VS2 was permanently expanded by PP14+, which significantly reduced the Mg2+ diffusion barrier. Thanks to the reduced Mg2+ desolvation barrier and improved Mg2+ diffusivity, the present MIB cells showed superior electrochemical performance in terms of large specific capacities, excellent rate capabilities, and long term cyclability.
Section snippets
Material preparation
VS2 nanosheets were prepared by a facile hydrothermal process. First, 1.0 g polyvinylpyrrolidone (PVP; Sigma-Aldrich) was dissolved in a mixed solution of 30 mL of deionized water and 2.0 mL of ammonium hydroxide. Next, 2.0 mmol of NH4VO3 and 20 mmol of thioacetamide (TAA, Sigma-Aldrich) were added to the solution in sequence with constant stirring. Then, the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 20 h. The obtained suspension was
Physiochemical properties of APC-PP14Cl/THF electrolyte
The physiochemical properties of an electrolyte are critical for the electrochemical performance of rechargeable batteries. Dissolution experiments here showed that PP14Cl was barely soluble in THF solvent. However, with the addition of PP14Cl, the APC/THF electrolyte turned transparent under constant stirring (Fig. 1a), which suggested that a solution reaction had occurred between PP14Cl and APC/THF. CV analysis showed that Mg deposition/dissolution efficiency of APC-PP14Cl/THF electrolyte was
Conclusions
The effects of PP14Cl as an electrolyte additive for the APC/THF electrolyte were systematically investigated. Mg2+ in APC-PP14Cl/THF was confirmed to be easily desolvated under the catalytic effects of PP14+. In addition, large-sized PP14+ was inserted into VS2 interlayer spaces during the initial discharge, which permanently expanded the interlayer spacing of this layered electrode material. Based on the above synergetic effects, the challenges of poor Mg2+ diffusivity and large desolvation
Acknowledgements
This work was supported by the Ministry of Science and Technology of China (No. 2015CB251103), the National Natural Science Foundation of China (Nos. 51472104, 21473075, and 21773091), and Science and Technology Department of Jilin Province (No. 20180414004GH).
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These authors contributed equally to this work.