Background Introduction
Traditional electrolytes are usually prepared by dissolving salt into a solvent (aqueous or non-aqueous) to produce solvated ions as charge carriers. Solvented ions promote ion transfer between electrodes and through interfaces. In addition, the decomposition of the solvated structure creates a solid electrolyte interface (SEI), allowing the operation of the electrode to exceed the thermodynamic stabilization window of the electrolyte. Although the traditional solvents in electrolyte have many advantages, there are some limitations. Non-aqueous electrolytes usually contain highly toxic and flammable solvents, which causes safety issues in lithium-ion batteries (LIBs). The boiling and freezing points of the solvent also limit the operating temperature range of LiBs. The use of organic solvents containing H2O can solve the safety problem. The narrow window of water limits the working voltage of water battery , thereby limiting the energy density of the battery.
2. Text part
1. Introduction to the results
Recently, Professor Wang Fei and Professor Xia Yongyao and Professor Wang Chunsheng of the University of Maryland, USA, published in the internationally renowned journal Nature Communications. The article introduces solvent-free proton liquid electrolytes, which show excellent electrochemical performance in a wide temperature range.
2, research highlights
This article reports that polyphosphate, as a solvent-free proton liquid electrolyte, eliminates the shortcomings of the solvent and shows unprecedented advantages, including non-flammability, a wider window of electrochemical stability (2.5 V) than water electrolyte , low volatility and a wide operating temperature range (400°C). The proton conductive electrolyte enables the MoO3/LiVPO4F rocking chair battery to operate well over a wide temperature range of 0°C to 250°C and provides a high power density of 4975 W kg−1 at high temperatures of 100°C. Solvent-free electrolytes can provide a feasible way for stable and safe batteries that work under harsh conditions, opening up a path for the design of wide-temperature electrolytes.
3, picture and text introduction
PPA and aqueous H3PO4 electrolyte
[Figure 1] Comparison of various H3PO4 electrolytes. aATR-FTIR and b1H NMR spectra. c Arrhenius diagram of PPA electrolyte. d 1 M H3PO4 and DSC of PPA electrolyte. eElectrochemical stability window.
Without solvent, it will deviate from the liquid structure of the electrolyte. To prove its unique physical and chemical properties, solvent-free PPA was compared with phosphoric acid aqueous solution at different concentrations. In combination with the above characterization, solvent-free characteristics impart special properties to PPA, including high thermal stability, non-flammability, medium plasma conductivity and extended electrochemical windows.
inhibits the dissolution of MoO3 negative electrode
[Figure 2] Characterization and electrochemical properties of MoO3 negative electrode. aXRD diagram of original MoO3 electrode and MoO3 electrode soaked in 1M H3PO4 for 5 days. b DSC of a mixture of MoO3 membrane and PPA (or 1 M H3PO4). c. Comparison of d voltage curve and cycling performance. eUsage ICP and corresponding digital images to measure dissolved molybdenum concentrations in different electrolytes after 5 days of circulation. fMoO3 electrochemical properties of cycling using PPA electrolyte at 60°C.
In order to verify the superiority of PPA, the model material MoO3 was selected as the active working electrode of the proton battery. Combined with the above results, it can be concluded that eliminating the water solvent can stabilize the MoO3 electrode and allow reversible insertion/extraction of protons, otherwise MoO3 will dissolve in water containing 1 M H3PO4 electrolyte and deteriorate at high temperatures.
inhibits LVPF positive electrode dissolution
[Figure 3] Characterization and electrochemical properties of LVPF positive electrode. a. b Comparison of voltage curves and cycling performance of LVPF circulated in 1 M H3PO4 and PPA electrolyte. c Electrochemical properties of LVPF cycle. d dissolved V concentration. eXRD diagram
charged LVPF undergoes severe dissolution and structural collapse in water containing 1 M H3PO4, resulting in poor circulation performance. In contrast, no dissolved V was detected in the cyclic PPA, indicating that LVPF lattice is stable.The 22.5° characteristic peak of LVPF disappeared after initial charging and did not appear during subsequent discharge, indicating that Li+ was not inserted into deliquent VPO4F, mainly because the proton concentration was one order of magnitude higher than the Li+ concentration produced by deliquent. In the subsequent cycle, no significant changes were observed in the XRD graph, confirming that the reversible insertion of H+ (from PPA) had little effect on the VPO4F lattice frame. Due to the almost zero strain insertion mechanism, structural collapse and further dissolution can be avoided by replacing the H3O+ carrier ions with non-solvent protons (H+) in PPA.
DFT simulation
[Figure 4] DFT simulation diagram. a ion size of various solvated proton structures. Schematic comparison of proton embedded reduction MoO3 and c VPO4F lattice frames in diluted 1 M H3PO4 (left) and PPA (right) electrolytes.
The dissolution of electrodes in diluted 1M H3PO4 and PPA electrolyte was studied using DFT calculations. The small size of the solvated proton structure makes it possible to co-calibrate water molecules. Intercalated water is easily solvated with Mo and V atoms with larger solvation energy. The dissolution process not only occurs on the surface of the particles, but also occurs inside the particles through co-intercalation of water molecules. However, for proton clusters in PPA electrolytes, the intercalation is blocked due to steric hindrance. Protons must be desoluble before insertion, speeding up migration. The solvation energy of Mo3+ and VO2+ with H3PO4 molecules shows that MoO3 and VPO4F are easily dissolved into PPA. However, dissolution is limited to the interface between the electrode and the PPA electrolyte.
Electrochemical properties of proton whole battery
[Figure 5] Electrochemical properties of MoO3||LVPF whole battery. aSchematic diagram of the battery. bCycling performance at a rate of 0.5 at room temperature. cVoltage curves at different temperatures. d. Reversible specific capacitance at different temperatures. eComparison of operating temperature and capacity. f-cell rate performance from 1 C to 100 C at 100°C. gThe cycle performance of the battery at 100°C. h Optical image of LED powered in tandem by two beaker batteries under baking.
To prove the feasibility of PPA electrolytes in a wide operating temperature range, a rocking chair-type full-proton battery was assembled using MoO3 anode and LVPF cathode, with an N/P ratio of 2/3, as shown in Figure 5a. The above electrochemical results successfully prove that solvent-free PPA electrolytes enable MoO3|| LVPF proton cells to work well in the ultra-wide temperature range of 0°C to 250°C, and are particularly suitable for high temperature applications requiring high power density and high reliability, such as fire rescue inspection robots and space exploration.
4, summary and prospect
polyphosphate (PPA) has been proven to be a solvent-free proton liquid electrolyte that harmonizes the advantages of traditional liquid electrolytes and solid electrolytes while solving their respective disadvantages. Thanks to solvent-free properties, PPA has non-flammability, wide electrochemical stability window (2.5 V), low volatility and a wide operating temperature range (400°C). At the same time, combined with experimental results and DFT simulations, it was shown that the solvent-free proton interpolation in PPA can inhibit the degradation of the electrode. In addition, a battery with MoO3 anode and LVPF cathode was designed, which not only operates in the ultra-wide temperature range of 0–250°C, but also has high rate performance (1–100C), and can even light up the LED under the flame of alcohol lamp , which is better than the current liquid battery . The exploration of solvent-free electrolytes provides a viable pathway for highly stable and high safety batteries that can operate under harsh conditions, especially in scenarios where high power density and high safety are required, such as rescue/check robots and space exploration. Methods to eliminate solvents to avoid their inherent defects are expected to open up a new field of research.
References
Mochou Liao, Xiao Ji, Yongjie Cao, Jie Xu, Xuan Qiu, Yi hua Xie, Fei Wang*, Chunsheng Wang * Yongyao Xia *Solvent-free protic liquid enabling batteries operation at an ultra-wide temperature range, Nature Communications, DOI: 10.1038/s41467-022-33612-2
https://www.nature.com/articles/s41467-022-33612-2
