PEG solid electrolyte narrow voltage window, what\’s the pot?

All-solid-state lithium batteries with high energy density requires the development of a solid electrolyte comprising a high voltage stability (i.e., voltage window width) of metal lithium negative electrode, and a high stability. However, the moment the most common solid polymer electrolyte prepared polymer material – polyethylene oxide (the PEO) – unbearable voltage higher than 4 V, which greatly limits the energy density of the battery. Although PEO bottleneck has been very clear, but the bottleneck caused by the \”culprit\” rarely appeared. University of Western Ontario and University of Toronto professor Sun Xueliang team Professor Chandra Veer Singh this team has identified the \”culprit.\” Selecting researchers PEO having the same molecular structure polyethylene glycol (PEG) for the study, found that when the end of the PEG -OH methylation to -OCH3, The electrochemical stability window of 4.05 V to 4.3 broadening V, while compatibility with the lithium negative electrode is also improved . Studies have been published in Energy & Environmental Science on. PEG固态电解质电压窗口窄,孰之锅? Since the C-C bond can be high, the authors argue that PEO decomposed at a high voltage to an oxygen atom to be related to the molecular structure: C-O bonds in a main chain and a terminal -OH. In order to confirm the cause of instability group, the authors compared the end group is -OCH3 polyethylene glycol dimethyl ether (PEGDME, FIG. 1a) and electrochemical stability of PEG (FIG. 1b) of the solid electrolyte. Two solid electrolyte inclusive of LiTFSI (bis (trifluoromethanesulfonyl) imide sulfonamide) and LiFSI (lithium bis fluorosulfonylimide salt) lithium salt supplies lithium ions. Experimental results show that, Li (-) / PEGDME / C65 (+) of the battery electrochemical stability than Li (-) / PEG / C65 (+) high ~ 250 mV (FIG. 1c). Further, the authors also found that by increasing the molecular weight of PEG in order to reduce the density of -OH end groups may also be extended electrochemical stability. Thus, the instability factor of -OH end groups are limited voltage window of PEG. The authors speculated that because of instability is -OH can be oxidized to -COOH (Li), and further generates Li2O. C-O bond of the main chain oxidation will occur only when a voltage higher than 4.3 V voltage.

FIG. 1. (a) -OCH3 terminated polyethylene glycol dimethyl ether (PEGDME) both high voltage stability and compatibility with lithium; and (b) -OH-terminated polyethylene glycol (PEG) and a voltage stability are poor compatibility with lithium. (C) Li (-) / PEGDME / C65 (+) and Li (-) / PEG / C65 (+) cell line scan voltammogram. Showed a sudden rise in the current electrolytic chemical reaction, is no longer stable. FIG PEGDME-4, and PEG-4. 4 represents LiFSI and LiTFSI than four substances.

PEGDME with respect to PEG have superior compatibility with metallic lithium negative electrode. Show scanning electron micrographs (FIG. 2), a lithium negative electrode are deposited continuously PEGDME – 1000 hours after peeling still maintain a smooth surface (FIGS. 2a-c). And continuously deposited in PEG – 300 hours after the release, a large number of deposits adhered to the surface of the lithium negative electrode as metal lithium end groups -OH reaction product of (2E and FIG. 2d), a thickness of about 70 μm (FIG. 2f).

Figure 2. SEM image of the lithium anode for use in solid polymer electrolyte circulation: In the continuous deposition PEGDME-4 – release surface of the lithium negative electrode after 1000 hours (ab) and transverse cross section (C); PEG-4 is deposited continuously in – the surface of the release lithium negative (de) after 300 hours and a cross-section (f). Deposition – release a current density: 0.2 mA / cm2.

The experimental result is further explained in FIG. 2, the authors studied the stability of the PEG in the presence of an external electric field and PEGDME lithium anode surface is calculated using the density functional theory (DFT). Studied the adsorption sites comprises an upper (100) crystal plane in Li (FIGS. 3a and 3d), bridged (FIG. 3b and 3e) and parallel configuration (Fig. 3c and 3f). Adsorption PEGDME three configurations are stronger than PEG, thus PEGDME in Li (100) surface is more stable, and can improve the surface wettability of Li + conduction and speed. Moreover, due to molecular changes occur under the electric energy, it can be judged indirectly by the stability of the molecular structure of energy variation difference. PEGDME (FIG. 3g, red) in the charge (from 0 to 0.2 V / Å) and the discharge energy difference changes (from 0 to -0.2 V / Å) is less than during PEG (FIG. 3g, blue)The energy change difference. Thus, PEGDME whether in reducing conditions or in the oxidizing conditions stable than PEG.

FIG 3. Li (100) crystal planes PEGDME (ac) and PEG (df) is a schematic view of the adsorption site site: an upper (a, d), (b, c) bridge even, (c, f) in parallel. Adsorption energy value of the upper right corner. (G) (red) and molecular energy PEG (blue) changes caused by the electric field PEGDME.

FIG. Source: Energy & Environmental Science. In order to study the influence of the battery electrochemical stability performance, PEG-4 to author PEGDME-4, and electrolyte assembled Li (-) / LiNi0.5Mn0.3Co0.2O2 (NMC532, +) of the electrochemical charge and discharge cycle performance and after five laps were characterized by the positive electrode. The results show, Li / PEGDME-4 / NMC532 battery voltage range of 2.5-4.3 V in the stable ring 100 charge-discharge capacity was maintained at 121 mAh / g, an average coulombic efficiency greater than 99%. After the charge-cutoff voltage increased to 4.5 V, the battery cycle stability and coulombic efficiency decreased. Li / PEG-4 / NMC532 2.5-4.3 V battery capacity fade significantly. After 100 cycles the charge and discharge cycle, remaining capacity 19 mAh / g (FIG. 4a). XPS results showed that the positive electrode Li / PEGDME-4 / NMC532 2.5-4.3 V in the battery charge, the ratio of the front and rear discharge (Metal-O + -O-C = O) / CO is no significant change in this voltage indicates that the range PEGDME stable structure (FIG. 4b-d). The positive electrode Li / PEG-4 / NMC532 2.5-4.3 V in the cell and the Li / positive electrode PEGDME-4 / NMC532 of the battery in the charging voltage range of 2.5-4.5 V, the front and rear discharge (Metal-O + -O-C = O ) / CO ratio was significantly increased, indicating that PEG-C-OH, or -C-O-C- are oxidized to -O-C = O and of Li2O (FIG. 4f). Binding electrochemical stability test results, we can infer that the oxidation potential of the -C-O-C- in 4.3V or more, and 4.05 V or more -OH i.e. oxidation . The latter group be limiting factors limit PEO electrolyte electrochemical stability windows.

Fig 4. (a) Li / PEGDME-4 / LiNi0.5Mn0.3Co0.2O2 (NMC532) and Li / PEG-4 / NMC532 button cell cycle at various charge and discharge section stability. (B-d) Li / PEGDME-4 / NMC532 and Li / PEG-4 / NMC532 button cell charge and discharge cycles 5 laps the XPS spectra of the positive electrode. (E) oxidation at high voltage -OH and -O-C-O- group exploded view.

Finally, the authors demonstrated the Li (-) / PEGDME-4 / LiFePO4 (+) and Li (-) / PEGDME-4 / NMC532 (+) of the soft pack battery having excellent cycle stability. Still holding 158.3 mAh / g and 155.1 mAh rear / PEGDME-4 / LiFePO4 (+) of the battery at 0.1C (Figure 5a) and 0.33C (FIG. 5c) continuous current density charge-discharge cycle ring 210 and ring 100 – Li () / g, the discharge capacity, capacity retention rate of 98% and 97%, respectively. Li (-) / PEGDME-4 / NMC532 (+) at a current density of 0.1C discharge point of the continuous charging and discharging capacity after 110 cycles to maintain the ring at 120 mAh / g (FIG. 5d), the capacity retention rate of about 90%. Further, after the assembled battery 100 at a current density of 0.1C continuous loop of polarization charge-discharge charge-discharge curve did not increase (FIG. 5b and FIG. 5E), reflecting the good performance PEGDME and potential applications in lithium batteries .

FIG. 5. Performance PEGDME electrolyte lithium ion battery. Li / PEGDME-4 LiFePO4 (LFP, loading: 3.3 mg / cm2) / charge-discharge cycle stability of the battery at a current density of 0.1C (a), charge-discharge curves (b) and the charge current density at 0.33C discharge cycle stability (c). 1C = 170 mA / g. Li / PEGDME-4 / NMC532 (loading: 3.5mg / cm2) of the battery charge and discharge cycling stability at 0.1C current density (d) and the charge-discharge curves (e) 1C = 160 mA / g. FIG. Source: Energy & Environmental Science.

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