MOF/Poly(Ethylene Oxide) Composite Polymer Electrolyte for Solid-state Lithium Battery

MOF/Poly(Ethylene Oxide) Composite Polymer Electrolyte for Solid-state Lithium Battery

LIANG Fengqing, WEN Zhaoyin

1. CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Abstract

Keywords： composite electrolyte ; poly(ethylene oxide) ; metal-organic framework material ; lithium metal battery

Lithium-batteries technology can be enhanced by replacing the liquid electrolytes currently in use with solid polymer electrolytes (SPEs), enabling the fabrication of flexible, compact, laminated solid-state structures free from leaks and available in varied geometries. The SPEs explored for these purposes are ionically conducting polymer membranes formed by complexes between lithium salt (LiX) and high molecular weight polymer containing Li+ coordinating groups, such as poly(ethylene oxide) (PEO). In PEO polymer electrolytes, with the polymer in amorphous state, Li+ is fast transported along with local relaxation and segmental motion of polymer chain, but the PEO tends to crystallize below 60 ℃. So the conductivity of PEO polymer electrolytes reaches practically useful values (of the order of 10-4 S/cm) only at the temperature above 60 ℃. Numerous attempts for diminishing the polymer crystallinity were made to improve the conductivity of the polymer electrolytes, including mixing with other co-polymers, adding plasticizers and doping inorganic particles. Incorporating inorganic materials into polymer matrix is the most successful approach, which improves ionic conductivity as well as electrochemical stability and mechanical properties. These inorganic materials mainly include nonconductive materials, such as SSZ-13, Al2O3, SiO2, and conductive materials, such as Li0.33La0.57TiO3, Li6.75La3Zr1.75Ta0.25O12, and Li1.5Al0.5Ge1.5(PO4)3. Investigations showed that nanoparticles with Lewis acidic surface properties can more efficiently boost the dissociation of lithium salt and reduce the crystallinity of PEO, thus improving the ionic conductivity. However, the poor contact between inorganic nanoparticle and PEO for the surface energy gap usually leads to inhomogeneous dispersion. Ceramic fillers grafted with molecular brushes and modified with dopamine are endowed with inorganic-organic properties. They are expected to enhance the miscibility with PEO, future improving the ionic conductivity and stability of polymer electrolytes.

Metal-organic frameworks (MOFs) consisting of metal ion clusters and organic linkers are typical nanoporous materials, which possess inorganic-organic hybrid property and high specific surface area, thus being ideal fillers to polymer electrolytes. In 2013, Yuan, et al. used Zn4O(1,4-benzenedicarboxylate)3 metal-organic framework(MOF-5) as filler for PEO electrolyte obtaining high ionic conductivity of 3.16×10-5 S∙cm-1 (25 ℃) due to the uniformly dispersion. But the weak metal-organic coordination bonds of MOF-5 are easy to be attacked, leading to crystal transition or structure collapse and poor stability for lithium battery.

In this work, nano-sized UiO-66, one of the extensive investigated MOFs, was introduced as filler into PEO electrolyte. The UiO-66 with outstanding hydrothermal and chemical stability does not contain transition metals which provide redox-active centers, so the electronic conduction can be avoided when contact with metallic Li.

1 Experimental

1.1 Synthesis of nano-sized UiO-66

Nano-sized UiO-66 was synthesized according to the reported two-step synthesis. (1) 207 mg ZrCl4 (98%, Aladdin) was dissolved in 40 mL N,N-dimethylformamide (DMF) (99.9%, Aladdin) under stirring, and the solution was heated to about 120 ℃ for 2 h. Then 1 mL acetic acid was added and stirred for additional 0.5 h at 120 ℃. (2) 147 mg 1,4-benzenedicarboxylic acid (H2BDC) (99%, Aladdin) was added into the solution. And the resulting mixture was introduced into a 50 mL Teflon-lined stainless-steel autoclave and placed in an oven at 120 ℃ for 24 h. After cooling to room temperature, the resulting precipitates were centrifuged, washed with DMF, purified in methanol and then dried at 60 ℃ under vacuum for 24 h.

1.2 Preparation of UiO-66/PEO composite polymer electrolytes (CPEs)

PEO (Mw = ~600,000, 99.9%, Aladdin) was dried at 50 ℃, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (99%, Aladdin) was dried at 100 ℃ for 24 h under vacuum and stored in an Ar-filled glove box. Firstly, LiTFSI was dissolved in anhydrous acetonitrile, and UiO-66 and PEO were added under magnetic stirring to afford homogeneous solution, in which the molar ratio of EO : Li+ was kept 16 : 1, and the content of nano-sized UiO-66 fillers was designed to be 0, 5%, 10%, 15%, 20%, 25%, naming the corresponding electrolytes as SPE, CPE-(5%, 10%, 15%, 20%, 25%). Afterwards, the solution was cast on polytetrafluoroethylene template to volatilize the solvent at ambient temperature. Finally, the membranes were dried at 60 ℃ for 12 h under vacuum to volatilize the residual solvent.

1.3 Sample characterization

1.4 Electrochemical measurement and cells assembly

The ionic conductivity was measured at temperature from 25 to 80 ℃ in symmetric cell with stainless steel (SS) electrodes by the AC impedance analysis (Autolab, Model PGSTAT302N) in the frequency range from 1 Hz to 1 MHz and at an amplitude of 50 mV. Linear sweep voltammetry (LSV) was employed to examine the electrochemical window in SS/electrolyte/Li cells, conducting from 3 to 5.5 V at a scan rate of 10 mV/s. The transference number of Li+ (t+) was tested in Li/electrolyte/Li cells and calculated according to t+ = I∞( Δ V−I0R0)I0( Δ V−I∞R∞), where ΔV is the applied DC polarization voltage (10 mV), I0 and I∞ are the initial and steady current values during polarization, respectively. R0 and R∞ are the resistance values before and after polarization, respectively. For inhibition ability of lithium dendrites growth test, a symmetric cell with solid electrolyte sandwiched between two lithium metal electrode was assembled, and the test was carried out at 60 ℃。

2 Results and discussion

It is clear that higher ionic conductivity is obtained with the addition of the nano-sized UiO-66 into PEO electrolyte. As the coordination of [Zr6O4(OH)4]12+ with oxygen in PEO reduces the crystallinity of PEO chain to promote the segmental motion of polymer chain, which is proved by the XRD pattern of CPE-10% compared with PEO (Fig. 2(a)). Moreover, the interaction between [Zr6O4(OH)4]12+ and TFSI- promotes the dissociation of lithium salt. The increase of UiO-66 fillers content below a certain value is accompanied by the promotion of ionic conductivity. However, the further increase of fillers reduces ionic conductivity owing to dilution and block effects. The CPE-10% shows the highest ionic conductivity (3.0×10-5 S/cm at 25 ℃, 5.8×10-4 S/cm at 60 ℃), while the ionic conductivity of SPE is only 5.0×10-6 S/cm at 25 ℃ and 1.7×10-4 S/cm at 60 ℃. The conducting properties of CPE-10% at the temperature from 25 to 80 ℃ were also investigated by AC impedance spectroscopy, and the Nyqiust plots are presented in Fig. 2(c). It shows that the impedance value decreases with the temperature increasing.

The effect of UiO-66 on the electrochemical window of PEO electrolyte was investigated by LSV at 60 ℃. As shown in Fig. 2(d), the steady platform of CPE-10% at about 4.9 V is higher than that of SPE, owing to the coordination of UiO-66 with oxygen which promotes the oxidation voltage of PEO and the fact that Zr(IV) in UiO-66 is difficult to be reduced. Thus, it is expected that the CPE is suitable for a lithium battery matching with high-voltage positive cathode. The transference number of Li+ is an important parameter providing information about the rate capability contribution of Li+ in solid state electrolyte. Time-current curves following 10 mV of DC polarization for SPE and CPE-10% are presented in Fig. 2(e-f). The t+ of CPE-10% is 0.36 and higher than that of SPE (0.25). It is due to the fact that the coordination of [Zr6O4(OH)4]12+ with the oxygen of PEO in CPEs weakens the coordination of oxygen with Li+ deriving the transfer of Li+, and a fraction of anions are immobilized by [Zr6O4(OH)4]12+.

3 Conclusion