Abstract
AbstractHerein, an eco‐friendly and high safety aqueous Mg‐ion electrolyte (AME) with a wide electrochemical stability window (ESW) ≈3.7 V, containing polyethylene glycol (PEG) and low‐concentration salt (0.8 m Mg(TFSI)2), is proposed by solvation structure reorganization of AME. The PEG agent significantly alters the Mg2+ solvation and hydrogen bonds network of AMEs and forms the direct coordination of Mg2+ and TFSI‐, thus enhancing the physicochemical and electrochemical properties of electrolytes. As an exemplary material, V2O5 nanowires are tested in this new AME and exhibit initial high discharge/charge capacity of 359/326 mAh g‐1 and high capacity retention of 80% after 100 cycles. The high crystalline α‐V2O5 shows two 2‐phase transition processes with the formation of ε‐Mg0.6V2O5 and Mg‐rich MgxV2O5 (x ≈1.0) during the first discharge. Mg‐rich MgxV2O5 (x ≈1.0) phase formed through electrochemical Mg‐ion intercalation at room temperature is for the first time observed via XRD. Meanwhile, the cathode electrolyte interphase (CEI) in aqueous Mg‐ion batteries is revealed for the first time. MgF2 originating from the decomposition of TFSI‐ is identified as the dominant component. This work offers a new approach for designing high‐safety, low‐cost, eco‐friendly, and large ESW electrolytes for practical and novel aqueous multivalent batteries.
Highlights
Glycol (PEG) and low-concentration salt (0.8 m Mg(TFSI)2), is proposed by solvation structure reorganization of aqueous Mg-ion electrolyte (AME)
In contrast to theoretical predictions, the V2O5 nanoclusters/carbon composites show an initial discharge capacity of 300 mAh g−1 in the voltage range of 0.5–2.8 V versus Mg2+/Mg.[2i]. Despite this, the cycle stability of this kind of high voltage cathode material has been seriously limited by the low electrochemical stability window (ESW, 1.3–2 V) of aqueous Mg-ion electrolyte (AME).[1b,3] The ESW of
Compared to PEG-free 0.8 m Mg(TFSI)2-100%H2O, the ESW of the AME expands with increasing the amount of PEG, where both the O2 and H2 evolution potentials are pushed beyond the thermodynamic stability of water (Figure 1a)
Summary
PEG 400 is considered an ideal crowding agent for AMEs due to its unique advantages such as miscibility with water, inertness, low toxicity, and low cost. Nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR) were carried out to reveal how the PEG crowding agent impacts the physicochemical properties of the AMEs. 1H spectrum of 0.8 m Mg(TFSI)2-100%H2O exhibits a chemical shift at 4.84 ppm (Figure 2a), corresponding to H2O solvent.[10] Upon adding PEG from 75% to 85%, the 1H peak of H2O weakens and shifts to higher values, indicating the decrease of electron density around H atoms and the shortening of H–O bond in H2O. The lifetime of HBs increases from 50.86 ps (0.8 m Mg(TFSI)2-100%H2O) to 79.33 ps (0.8 m Mg(TFSI)2-85%PEG-15%H2O), which demonstrates that the PEG crowding agent improves the stability of the AMEs. Figure 2b,c shows the FTIR spectra of different electrolytes. In 0.8 m Mg(TFSI)2-85%PEG-15%H2O (Figure S4e, Supporting Information), demonstrating the “‘quasi-water-in-salt”’ property of this electrolyte due to the direct coordination of Mg2+ and TFSI–. The addition of PEG breaks the water networking and allows direct contacts between Mg2+ and TFSI–, while part of water molecules are replaced and PEG and TFSI– do form a new network through the bonding with H2O and Mg2+
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