With the growing demand for lithium-ion batteries with higher energy densities, the use of flammable organic electrolytes raises safety concerns. As a solution, the use of aqueous electrolytes is not only safe, but also reduces costs and improves environmental friendliness. However, its application is limited by the narrow electrochemical window (EW). High concentrations of lithium saline aqueous electrolytes can extend the electrochemical window to 2.7 V or even larger, inhibiting hydrogen evolution by forming water-in-salt complexes. However, over-reliance on expensive lithium salts increases costs and environmental burdens. One compromise is to add an organic solvent to the aqueous electrolyte, reducing the water content to promote the formation of a solid electrolyte middle layer at the anode, but this comes at the expense of safety. Another strategy is to introduce a flowable hydrogel electrolyte made of polymers, but there are still disadvantages at high water content and pressure. In addition, although the LiF-based solid electrolyte mesolayer is capable of being formed from specific anions, the moisture of the electrolyte can still pass through the mesolayer, resulting in excessive electrolyte depletion and reduced energy density.
Here, Academician Liqun Zhang and Professor Weidong Zhou of Beijing University of Chemical Technology demonstrate a water-in-polymer electrolyte that maximizes water usage but has an operating voltage range as wide as that of a highly concentrated electrolyte. At the heart of this formulation is the introduction of a polyacrylamide network that immobilizes and tames otherwise active water molecules. Thus, polymerized solid water electrolytes containing 4.1 m (18 wt% water) and 7.6 m (11 wt% water) bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) showed extended electrochemical windows of 2.7 V and 3.7 V, comparable to 21 m and 40 m saturated electrolytes, respectively. Solid-state Li4Ti5O12//LiMn2O4 cells achieve stable cycling even at high cathode loads (16 mg cm-2) and poor electrolyte conditions of 7 g Ah-1. In addition, up to 80% of LiTFSI salts can be recycled and polymer matrices can be regenerated. This electrolyte design represents a big step towards a more sustainable aqueous battery. The results were published in Nature Sustainability under the title of "Water-in-polymer electrolyte with a wide electrochemical window and recyclability", and the first author is Associate Professor Hao Shumeng, and Jianxun Zhu, Shuang He and Le Ma are co-authors.
EW of Polymer Water-in-Polymer Solid Electrolytes and F-containing Polymer Water-in-Polymer Solid Electrolytes (WIPSE) membranes were produced by polymerizing acrylamide (AM) aqueous solutions with different concentrations of LiTFSI. Compared to the pure LiTFSI aqueous solution, the solution supplemented with AM slightly extended the EW in the range of 3.2 m to 7.6 m LiTFSI concentrations, especially at 7.6 m LiTFSI concentrations, which extended the EW by 0.7 V. This phenomenon is attributed to the hydrogen bonding between AM and water molecules and its restriction effect on water molecules. Through in-situ polymerization and curing, the EW of the WIPSE membrane was further extended and the HER voltage was significantly reduced, indicating successful inhibition of the hydrogen evolution reaction (HER). In addition, the introduction of fluorine-containing tetrafluoropropyl methacrylate (TFMA)-modified WIPSE (i.e., FWIPSE) demonstrated a wider EW, similar to the HER voltage of a high-concentration LiTFSI aqueous solution system, demonstrating the inhibition of HER by curing and introducing active fluorine-containing segments.
Figure 1: EW test of WIPSE and FWIPSE Coordination structure of lithium ions and water in WIPSE In order to explore the electrochemical properties of specific water systems, the authors first investigated the interaction between water and LiTFSI at different ratios. (Fig. 2) Through the analysis of single crystal structure, it was found that with the increase of water content, the distance between Li and O=S in TFSI increased from 1.93-1.99 Å to 2.01-2.27 Å, while the distance between Li and O-H in water molecules was shorter, indicating that the interaction between Li+ and water was more close. In the case of further increase of water content, the degree of dissociation of LiTFSI varies, and some Li+ forms a complex solvation structure with more water molecules and less TFSI-. In particular, when AM was added to 10 m LiTFSI aqueous solution and polymerized, amide partially replaced part of the water in the formed WIPSE, which participated in the main solvation of Li+, significantly changed the solvation structure of Li+ and enhanced the dissociation of LiTFSI. Further analysis showed that the PAM chain not only binds to water and Li+ through hydrogen bonding and coordination, but also anchors the excess water molecules that are not coordinated with Li+, effectively limiting their diffusion. This structure is particularly important for the reduction of HER voltage, as it reduces the proportion of free water and increases the degree of solvation of Li+. In addition, the shift of the O-H absorption peak in 4.1 m and 7.6 m WIPSE further demonstrated the enhancement of Li+- water coordination. All these results suggest that through the formation of polymer networks and effective confinement of water, LiTFSI dissociation in WIPSE is higher than in pure aqueous solution, which is essential for improving the electrochemical window of aqueous electrolytes and reducing HER voltages.
Figure 2: Interaction of water with Li ions and PAM in WIPSE Effects of hydrogen bonding and Li+ coordination To inhibit the reduction reaction of water, thermodynamically strengthen the O-H bond of water and kinetically limit the diffusion of water or passivate the anode surface. It has been found that the hydrogen bonding between acrylamide (AM) and water can replace the hydrogen bonding between water molecules and change the chemical properties of water. The O-H bond of water is strengthened by the coordination of Li+ with water, as can be seen from the high-field shift of the 1H signal of water in the 1H NMR spectrum (Figure 3). In addition, increasing the concentration of LiTFSI can significantly reduce the diffusion coefficient of water and limit the movement of water molecules. After the addition of AM and the formation of WIPSE, the diffusion coefficient of water was further reduced, indicating that the polymer network had an effective limit on the movement of water molecules. Especially in high concentrations of WIPSE, it helps to reduce the HER voltage by enhancing the Li+-O interaction and limiting the movement of water. LF-NMR experiments also confirmed that the tight binding of water molecules to polymer chains in WIPSE further limits the movement of water. In conclusion, by strengthening the O-H bond of water and restricting the movement of water molecules, the reduction reaction of water can be effectively inhibited and the stability of the electrochemical system can be improved.
Figure 3: Hydrogen bonding and Li+-O interaction in WIPSE Electrochemical performance of WIPSE and FWIPSE In order to accommodate the electrochemical windows (EW) of 4.1 and 7.6 m WIPSE, Mo6S8 and Li4Ti5O12 were selected as the anode and LiMn2O4 as the cathode. Ion-to-particle conduction is facilitated by in-situ polymerization of the WIPSE precursor solution on the electrode. Experiments show that this method can effectively prolong the starting voltage of HER and improve battery performance. In order to increase the degree of polymerization of PAM on the Mo6S8 anode and limit the activity of water, the in-situ polymerization of crosslinker and catalyst was added, which improved the discharge capacity and cycling stability of the solid-state Mo6S8//LiMn2O4 battery. In addition, the FWIPSE generated by TFMA copolymerization further stabilized the battery performance by constructing a LiF-rich solid-state electrolyte interface, and maintained high initial capacity and cycling stability at low temperatures. These results show that the reduction reaction of water can be effectively inhibited by restricting water activity and passivating the anode interface, which is very beneficial for the development of low-cost and environmentally friendly lithium-ion batteries (LIBs). When higher voltage batteries are required, a strategy of building a double-layer solid-state electrolyte compatible with the more challenging lithium metal anode may be feasible.
Figure 4: Electrochemical performance of WIPSE and FWIPSE in solid-state batteriesLiTFSI recovery and FWIPSE regenerationIn order to recover expensive LiTFSI, the authors first treated the FWIPSE membrane with a Soxhlet extractor and ethanol as an eluent, leaving a light yellow powder. After dichloromethane cleaning to remove organic impurities and dissolving with isopropyl ether, insoluble substances such as LiF and Li2CO3 are filtered out, and finally the solution is evaporated to obtain white powder. This process turns the otherwise transparent film into an opaque white color. The recovered LiTFSI was consistent with commercial LiTFSI by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermogravimetric analysis, and Fourier transform infrared spectroscopy (FT-IR), with recoveries of 77% to 80%, exceeding 71% in aqueous solutions, thanks to the limitation of interfacial side reactions in FWIPSE and the reduction in LiTFSI consumption. The recovered LiTFSI maintained stable electrochemical performance, demonstrating its successful recovery. FWIPSE membranes can also be regenerated by absorbing LiTFSI solutions after LiTFSI extraction, and the conductivity of the regenerated membranes is similar to that of the new sample. This efficient LiTFSI recovery and polymer electrolyte regeneration strategy, combined with a reduction in LiTFSI concentration, significantly reduces production costs.
Fig. 5 Recovery of LiTFSI and regeneration of FWIPSE Summary: The coordination structure of water is critical for the electrochemical window of the electrolyte. From a thermodynamic point of view, hydrogen bonding lengthens and weakens the O-H bond, while Li+-O interaction strengthens the O-H bond and dominates it. Kinetically, hydrogen bonding and Li+-O interaction work together to limit the movement of water molecules, which helps to inhibit the HER reaction. By dynamically limiting water molecules by hydrogen bonds formed with the polymer network and thermodynamically strengthening O-H bonds through Li+-O interaction, the aqueous electrolyte with low concentration and high water content can limit the interfacial side reactions and achieve efficient and stable cycling of solid-state batteries. In particular, by constructing a robust polymer network and low concentrations of WIPSE (4.1 m and 7.6 m), the extended EW comparable to that of high concentrations of LiTFSI was exhibited, while the cycling stability of the battery was improved. By introducing TFMA copolymer, the hydrophobic F-SEI layer on the cathode is enhanced, which further limits the electrode interface reaction and keeps the battery stable for a long time at low P/N ratios. At the same time, by optimizing the Li salt concentration and moisture fixation, the 7.6 m FWIPSE also showed good battery performance under high cathodic loads. In addition, 77 to 80 percent of LiTFSI can be recycled, while FWIPSE membranes can also be regenerated, opening up new avenues for sustainable lithium-ion battery technology to reduce costs and improve environmental friendliness. Source: Frontiers of Polymer Science