2. MATERIAL AND METHODS
2.1. Materials: Cellulose acetate (CA) was obtained from Aladdin Biochemical Technology Co., Ltd. CaCO3 nanoparticle was purchased from Qianhai JYSA Technology Co., Ltd. Lithium hydroxide, N,N-Dimethylacetamide and acetone were provided via Kelong Chemical Reagents. N-Methylpyrrolidone (NMP), LiFePO4, LiNi0.6Co0.2Mn0.2O2, graphite, super P and poly(vinylidene fluoride) (PVDF) was supplied from Canrd New Energy Technology Co., Ltd. 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (v:v:v=1:1:1) was purchased from Songjing New Energy Technology Co., Ltd. Polypropylene (PP) separator (Celgard 2500) was used as a control.
2.2. Nanofiber synthesis: Firstly, the cellulose acetate (15% w/w) and calcium carbonate (0, 0.5%, 1% w/w) were dissolved in DMAc/acetone (at a mass ratio of 3:7) to form a uniform mixture. After ultrasound and defoaming, the mixture was transferred into a syringe equipped with a Φ19mm needle. During electrospinning, the injection speed was 0.15mm/min and the voltage was 20 V. The nanofibers which accumulated on the collector were collected. The residual solvent of nanofibers was removed by drying at 80 °C to obtain white membranes, and subsequently immersed into 0.05M LiOH aqueous solution for hydrolysis for 12 h. Finally, the cellulose and cellulose/CaCO3 membranes were obtained by vacuum drying.
2.3. Characterization: The morphologies characterizations were recorded via scanning electron microscope (SEM, JSM-7500F) at 15 kV. All samples were sprayed with gold before test. Energy dispersive X-ray (EDX) spectroscopy was used to test the element distribution mapping. X-ray diffraction (XRD) patterns were recorded by X‘Pert Pro MPD diffractometer (Cu-Kα) in a scattering angle (2θ) range of 10º-60º. Thermogravimetric Analysis (TGA) was carried out by a NETZSCH TG 209 F1 in N2 flowing at a heating rate of 10 °C min–1.
\(Electrolyte\ uptake\ =\ \frac{W_{t}-W_{0}}{W_{0}}\ \times\ 100\%\)(1)
where W0 and Wt represent the weight of the membrane before and after liquid electrolyte saturation, respectively.
Ionic conductivity of the membrane was measured with a coin cell by a CHI 760E electrochemical station from 1 Hz to 100 kHz. The membrane disk with a diameter of 19 mm was sandwiched between two stainless steels and then assembled in a CR2032 coin cell. The ionic conductivity was obtained by the equation (2):
\(\sigma\ =\ \frac{l}{SR}\) (2)
where \(l,\ R\) and S refer to the thickness of the membrane, resistance, and the contact area between membrane and stainless steel, respectively.
The electrochemical stability window was determined by linear sweep voltammetry. The stainless steel and lithium metal were used as electrode and counter electrode, respectively. The voltage was swept at a scan rate of 1 mV s–1 over a potential range of 0 V to 6 V (vs. Li/Li+).
Cycling performance of batteries was measured by battery test system (Shenzhen Neware Electronics Co., Ltd). The voltage ranges of cells are 2.5-4.2 V. The conventional slurry-coating method was used to prepare electrodes. The active material (LiFePO4), acetylene black and PVDF were mixed with a weight ratio of 8:1:1. The active material weight is around 1.6 mg cm–2. The weight of Li anode is 45.9 mg (15.6 mm in diameter and 0.45 mm in thickness).