High‐Voltage Aqueous Mg‐Ion Batteries Enabled by Solvation Structure Reorganization

Abstract

Herein, 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.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

“…In addition, H 2 O molecules can connect to each other and form a network through H bonds. The H 2 O–H 2 O interaction network will be disrupted upon the addition of salts or organic solvents, which can be attributed to the additional interaction of H 2 O with anions or solvent molecules. − For instance, Chang et al proposed an aqueous hybrid electrolyte containing ethylene glycol (EG) solvents. EG can not only interact with Zn 2+ but also form the H bond with H 2 O to weaken the Zn 2+ –H 2 O interaction, affording the electrolyte with a low freezing point and reversible Zn deposition/stripping …”

Aqueous Supercapacitors with Low Cost and High Voltage Enabled by Co‐Solute Crowding Effect of Electrolyte

“…[23,27] Similarly, as seen in Figure 2c, the 1 H NMR chemical shift of H 2 O decreased in the presence of PEG, which can be attributed to the increase in electron density around the H atom due to the hydrogen bonding between the O atom in PEG and the H atom in H 2 O. [24] The above results indicate that PEG is an excellent molecular crowding agent for reducing the activity of water molecules, and the P50 electrolyte has the best physical and chemical properties and, as such, may be a promising electrolyte for low-cost SCs devices. As Figure S4 shows, through DFT-MD simulations, we found that there were a large number of free water molecules in 8 m NaClO 4 , which form a hydrogen bonding network with each other.…”

“…The PEG agent was able to significantly alter the Mg 2 + solvation and hydrogen bonds networks of Mg-ion batteries, improving the OVW and other physical and chemical properties of the Mg-ion electrolyte. [24] This was because the molecular crowding effect of PEG was able to change the hydrogen-bonding structure of water, reducing its activity and inhibiting its decomposition. The above studies demonstrated the great potential of PEG in aqueous Li-ion, Zn-ion, and Mgion batteries and also inspired us to hypothesize that PEG must have a similar effect when acting as an electrolyte additive for SCs.…”

“…found that PEG, as an electrolyte additive for aqueous Zn‐ion batteries, can be used to prepare a Zn(OTf) 2 ‐H 2 O‐PEG electrolyte that can not only improve the OVW of a device but also simultaneously enables smooth Zn deposition. The PEG agent was able to significantly alter the Mg 2+ solvation and hydrogen bonds networks of Mg‐ion batteries, improving the OVW and other physical and chemical properties of the Mg‐ion electrolyte [24] . This was because the molecular crowding effect of PEG was able to change the hydrogen‐bonding structure of water, reducing its activity and inhibiting its decomposition.…”

Switchable and Strain‐Releasable Mg‐Ion Diffusion Nanohighway Enables High‐Capacity and Long‐Life Pyrovanadate Cathode

“…After 110 cycles, the capacity retention ratio of the CVOH microspheres is 75%, which is quite high for V‐bearing cathode materials storing Mg‐ion in aqueous three‐cathode configuration; the capacity loss is due to the slight dissolution of cathode material into the excessive electrolyte solution. [ 4,19 ] The coulombic efficiencies are stable at ≈100% during these 110 cycles. These results indicate that the insertion/extraction process of Mg 2+ ions in CVOH microspheres is highly reversible while the CVOH cathode is highly stable during the discharging/charging process.…”