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).