3. Conclusion
2D g-C3N4 coating on NCM811 has been achieved by in-situ surface engineering. The g-C3N4-coated NCM811 contributes to the formation of a thin and homogeneous cathode/electrolyte interface, thereby greatly reducing adverse side reactions. Structural stability is improved and no micro-cracking occurs after long-term cycling. Furthermore, the g-C3N4 coating promotes lithium ion transport and improves reaction kinetics. As a result, the NCM811@CN-3wt% cathode displays 161.3 mAh g-1 and capacity retention of 84.6% at 0.5 C and 55 °C after 400 cycles and 95.7 mAh g-1 at 10 C, much better than NCM811 (i.e. 129.3 mAh g-1 and capacity retention of 67.4% at 0.5 C and 55 °C after 220 cycles and 28.8 mAh g-1 at 10 C). The performance improvement of the NCM811@CN-3wt% cathode is also applicable to solid-liquid-hybrid electrolyte systems composed of PVDF:LLZTO with ionic-liquid electrolytes, which show 163.8 mAh g-1 and the capacity retention of 88.1% at 0.1 C and 30 °C after 200 cycles and 95.3 mAh g-1 at 1 C. This work sparks new ideas on constructing the interface layers with excellent comprehensive performance for high-nickel ternary metal oxide cathodes.
4. Experimental Section
4.1. Synthesis of g-C3N4 coating on NCM811. A series of dicyandiamides corresponding to 1 wt%, 2 wt%, 3 wt%, and 4 wt% of LiNi0.8Co0.1Mn0.1O2(denoted as NCM811) were dissolved in 50 mL of anhydrous ethanol. Subsequently, 3 g of commercial NCM811 cathode was added to each solution and ball milled for 12 hours at a 10:1 ball-to-powder ratio and 100 rpm. Finally, the mixture was dried at 80 °C for 12 h and calcined under Ar at 500°C for 1 h to synthesize g-C3N4-coated NCM811 samples (denoted as NCM811@CN-1wt% for 1 wt%, NCM811@CN-2wt% for 2 wt%, NCM811@CN-3wt% for 3 wt%, and NCM811@CN-4wt% for 4 wt%, respectively).
4.2. Fabrication of PVDF:LLZTO electrolytes. The fabrication process of the electrolyte was carried out in a dry room (dew point of -40 ℃). Typically, 600 mg polyvinylidene fluoride (PVDF) and 600 mg Lithium bis((trifluoromethyl)sulfonyl)azanide (LiTFSI) were dissolved in 6 mL Li6.4La3Zr1.4Ta0.6O12(LLZTO) dispersed N,N-Dimethylformamide (DMF) solvent and mixed for 12 h to obtained a uniform slurry (Content of LLZTO is 15 wt%). Afterwards, the slurry was scraped onto the surface of the glass plate with a scraper and vacuum dried at 80 ° C for 24 h. The PVDF:LLZTO electrolyte membranes were obtained when the temperature was reduced to room temperature.
4.3. Electrode Preparation. The composite electrodes were fabricated by using NCM811 and g-C3N4-modified NCM811 as active material (80 wt%), PVDF as the adhesive (10 wt%), and super-p (10 wt%) as a conductive agent. The homogeneous paste was applied to the Al foil and then vacuum-dried overnight at 80 °C to remove residual solvents. The mass loading of NCM811 and NCM811@CN was ~2.0 mg cm-2. Moreover, NCM811 and NCM811@CN cathodes were also prepared using the same method, with a mass loading of up to ~7.5 mg cm-2.
4.4. Battery assembly. For electrochemical characterization, CR2032 button cells were assembled in a glove box filled with argon ( O2 < 0.1 ppm, H2O < 0.1 ppm ). Li metal anodes and 1 M LiPF6 in EC/DEC (1:1 in volume) were used as the counter electrodes and electrolytes, respectively. The electrolyte was infiltrated into a single-layer Polypropylene (PP) membrane, serving as the separator. For solid-liquid-hybrid batteries, Li metal anodes and PVDF:LLZTO electrolyte membranes were used as the counter electrodes and electrolytes, respectively. 1.5 μL cm-21-ethyl-3-methylimidazolium bis(trifluoromethanesulfo-nyl)imide (EMIM-TFSI) ionic liquid as the wetting materials for cathode/electrolyte interfaces. Li/Li symmetric cells were assembled by sandwiching the electrolyte membranes between two pieces of Li foils.
4.5. Materials characterization. The crystalline structure was analysed in the 2θ range of 10°-80° by smartlab SE X-ray diffractometer (XRD) using Cu-Kα. The microstructure and morphology were characterized by transmission electron microscopy (TEM, JEM-2100F) and scanning electron microscopy (SEM, Hitachi, FE-SU4800). The particle size distribution was analysed using a (Brookhaven Zeta Plus) laser particle size analyser. Fourier transform infrared spectroscopy (FTIR, Necchi X70) was used to analyse the composition of the samples. Thermogravimetric analysis (TGA) (TA, SDT-650) was performed in an argon atmosphere in the range of 25-900 °C. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to characterize the surface chemical environment of materials.
4.6. Electrochemical Analysis. The battery charge-discharge experiments were performed at various rates using the NETWARE (CT-4008Tn) battery analyser. Cyclic voltammetry (CV, 0.1 mV s-1-0.5 mV s-1 and 3-4.3 V) characterization was performed using a CHI660E electrochemical workstation, while the electrochemical workstation in Princeton, NJ, USA, was used to acquire electrochemical impedance spectra (EIS, frequency 100,000-0.1 Hz, signal amplitude 5 mV). The electrochemical window of the PVDF:LLZTO electrolyte was tested between 0 and 6 V by linear sweep voltammetry (LSV, 1 mV s-1) and the lithium-ion mobility number (tLi+) was measured by direct current (DC) polarization of Li/Li symmetric batteries with a polarization voltage of 10 mV using an electrochemical workstation (in Princeton, NJ, USA).