2. Results and Discussion
A Simple and extensible chemical vaporization-assisted synthesis method
was executed to uniformly cover the 2D
g-C3N4-thin layer on NCM811 using
dicyandiamide as the precursor at 500 °C (Figure 1a) . Rietveld
refinement shows that the diffraction peaks of both NCM811 and
NCM811@CN-3wt% have α-NaFeO2 layered structure (space
group: R-3m), with no additional peaks from related impurities(Figure 1b and 1c) . As the relative amount of
g-C3N4 increases above 10 wt%, the
peaks of g-C3N4 can be observed, as
shown in Figure S1 . Table S1 shows the
Li+/Ni2+ cation mixing of
NCM811@CN-3wt% is slightly reduced, which is due to the formation of
strong chemical bonds between the unsaturated bonds on the surface of
g-C3N4 and the NCM oxide
cathode.[34,35] The reduction of
Li+/Ni2+ disorder should be
consistent with the improved structural stability. Based on the TGA
analysis, the coating amount of NCM811@CN-3wt% was determined(Figure 1d) . g-C3N4 coated on
the surface of NCM811 particles decomposes at 600 °C, and the mass loss
of NCM811@CN-3wt% is about 3.31 wt%. As shown in Figure 1e ,
except for that of pristine NCM811, the Fourier transform infrared
spectrometer (FT-IR) curves of NCM811@CN-3wt% and
g-C3N4 show characteristic peaks
corresponding to the triazine unit (810 cm-1) as well
as the C-N heterocycle (1200-1700 cm-1). This result
indicates that g-C3N4 is successfully
coated.
The surface chemical and valence states of NCM811 and NCM811@CN-3wt%
were determined by X-ray photoelectron spectroscopy (XPS). The N 1s of
NCM811@CN-3wt% show three peaks
at 397.55 eV, 398.52 eV, and 401.25 eV, respectively corresponding to
C-N-C/C-N=C, N-(C)3 and C-N-H in
g-C3N4(Figure 1f) . Meanwhile, two unique peaks can be found in C 1s
of NCM811@CN-3wt% at 287.18 eV and 288.78 eV, corresponding to
N-(C)3 and N-C-N/N-C=N, respectively (Figure
1g) . In contrast, the spectrum of NCM811 does not display N 1s peaks
and the unique N-(C)3 and N-C-N/N-C=N in C 1s peaks.
This result demonstrates the existence of the
g-C3N4 coating on
NCM811. The relative intensity of
Ni3+ for NCM811@CN-3wt% is greater than that of
uncoated NCM811, as shown in Figure S2a . In addition, C 1s of
the NCM811 and NCM811@CN-3wt% exhibit two peaks at 283.86 eV and 288.78
eV, belonging to hydrocarbon and the
Li2CO3/LiOH, which are mostly probably
generated from exposure in air. Compared with NCM811, the relative
intensity of Li2CO3/LiOH on the surface
of NCM811@CN-3wt% decreases from 29.80% to 19.34% (Figure 1g). O 1s
further indicates that the relative of
Li2CO3 decreases after
g-C3N4 modification (Figure
S2b) , in agreement with improvement of NCM811 surface stability by
g-C3N4 coating.
The morphologies of NCM811 and NCM811@CN-3wt% were observed using
scanning electron microscopy (SEM), as shown in Figure 2 . All
the samples exhibit spherical morphology with a particle size of
approximately 13 μm, aggregated from parent crystal particles. Compared
to NCM811 (Figure 2a and 2b) , the surface of NCM811@CN-3wt%
exhibits a relatively fuzzy roughness, which suggests that it contains a
thin coating layer (Figure 2d and 2e) . The chemical composition
of its coating is examined by EDS-mappings, and the elements Ni, Co, Mn,
O, and N are uniformly distributed on the surface of NCM811@CN-3wt%,
confirming the uniform coverage of
g-C3N4 (Figure S3) .
Transmission electron microscopy (TEM) further revealed the
surface/interfacial structures of NCM811 and NCM811@CN-3wt%. As a
result, it is observed that the NCM811 particles in NCM811@CN-3wt% are
uniformly encapsulated by a crystalline coating of approximately 7 nm(Figure 2e and 2f) . Meanwhile, the relatively smooth surface of
the original NCM811 can be found in Figure 2c . According to the
TEM-EDS spectra (Figure 2g) , the Ni, Co, Mn, O, and N elements
of NCM811@CN-3wt% remain uniformly distributed.
The cycling performance of NCM811 and the
g-C3N4-coated ones are evaluated at 1 C
and the voltage range of 3-4.3 V, as shown in Figure 3a and
Figure S4 . The NCM811@CN-3wt%
cathodes show the best
performance of NCM811@CN with different CN content. After 300 cycles,
the reversible capacity of
NCM811@CN-3wt% cathodes remains
150.6 mAh g-1 with a capacity retention of 81.8% at
30 °C. In contrast, the NCM811 specific capacity decreases to 114.1 mAh
g-1 after 300 cycles with 62.1% capacity retention.
Furthermore, the uncoated NCM811 exhibits severe voltage decay during
cycling. It can be found inFigure 3b and 3c that the NCM811 discharge voltage decreases by
284 mV from the 1st to the 300th, whereas the NCM811@CN-3wt% decreases
by only 98 mV in the first 300 cycles, indicating the improvement effect
of g-C3N4 coating on mitigation voltage
degradation. A key issue facing Ni-rich cathodes is that charge and
discharge at elevated temperatures accelerate the decomposition of
LiPF6, which leads to severe side reactions on the
cathode surface. Therefore, the electrochemical stability of NCM811 and
NCM811@CN-3wt% is comparatively studied at 55 °C. Figure S5displays the charge-discharge characteristic curves of NCM811 and
NCM811@CN-3wt% in the 3 V to 4.3 V range at 0.5 C and 55 °C.Figure 3d shows the specific capacity of NCM811@CN-3wt% after
400 cycles is 161.3 mAh g-1 and the capacity retention
of 84.6%. After 220 cycles, the specific capacity of NCM811 is only
129.3 mAh g-1 with 67.4% capacity retention. Another
issue with Ni-rich cathodes is that increased charge voltage causes
irreversible phase transition, thereby deteriorating the cycle
performance. Figure 3g shows that the capacity of the NCM811
cathode degrades rapidly within the voltage range of 3-4.5 V at 0.5 C,
only retaining 165.6 mAh g-1 and 74.8% capacity
retention after 200 cycles (Figure S7a) . In contrast, the
NCM811@CN-3wt% cathode still shows the ideal specific capacity as high
as 195.2 mAh g-1 and 88.6% capacity retention after
200 cycles (Figure S7b) . Furthermore, when the loading mass of
the cathode increases, the capacity of NCM811 decreases rapidly, and the
capacity retention rate after 100 cycles is only 75.1%, while the
capacity retention rate of NCM811@CN-3wt% maintains 82.1%(Figure S8) . Figure 3h and Figure S6 display the rate
capability of NCM811 and the
g-C3N4-coated
ones, tested from 1 C to 10 C
between 3–4.3 V, then followed by switching to 0.5 C for 165 cycles.
Notably, the
g-C3N4-coated NCM811 cathodes have a
higher specific capacity than the uncoated NCM811. This difference
becomes more pronounced as the density of the current increases.
Meanwhile, the NCM811@CN-3wt% cathode exhibits a specific capacity of
119.2 mAh g-1 at 5 C and 95.7 mAh
g-1 at 10 C, which is superior to that of the NCM811
cathode (99.5 mAh g-1 at 5 C and 30.1 mAh
g-1 at 10 C), owing to the interfacial stabilization
of the g-C3N4 coating facilitating the
diffusion of lithium ions.
Furthermore, when switching to 0.5 C for cycling, the specific capacity
of the NCM811@CN-3wt% cathode is as high as 180.1 mAh
g-1 after 165 cycles with 95.2% capacity retention.
Whereas the capacity of the NCM811 cathode decreases to 119.8 mAh
g-1 after 165 cycles with a capacity retention of
65.2%
The equivalent differential capacity (dQ dV-1) curves
of NCM811 and NCM811@CN-3wt% are detailed to investigate the
electrochemical behaviors associated with phase transition during long
cycling between 3-4.3 V and 3-4.5 V, the results being shown inFigure 3e and 3f , and Figure S7c and S7d . NCM811 and
NCM811@CN-3wt% undergo the same phase transition (H1→M→H2→H3). The
H2→H3 phase transition causes the crystal structure to shrink sharply
along the c-axis, resulting in uneven stress distribution and local
stress concentration, thus forming microcracks. The peak intensity of
the unfavorable H2→H3 phase transition in NCM811@CN-3wt% is smaller
than that in NCM811. Compared with NCM811@CN-3wt%, the redox of three
pairs peaks of NCM811 undergo a larger shift from the 2nd to 200th
cycle, indicating that the g-C3N4coating reduces the degree of polarisation and improves the
reversibility of the H2→H3 phase transition. These results indicate that
the NCM811@CN-3wt% cathodes have better cycle performance than NCM811,
which can be attributed to the reduction of side reactions as well as
the improvement of crystallinity stability.
To understand how the
g-C3N4 modification improves the cycling
capability and structural stability, in-situ XRD measurements were
carried out during lithiation/delithiation processes of NCM811,
NCM811@CN-3wt% at 0.1 C in the voltage region from 3 V to 4.3 V. As
previously reported hexagonal the LiCoO2, the peak shift
of (003) indicates the change in the lattice parameter of c, while (101)
represents changes in a and b.[36,37] Although
NCM811 and NCM811@CN-3wt% have similar crystal structures, significant
differences in the crystalline phase evolution of the cells during
charging and discharging can be observed in Figure 4a and 4b .
The (003) peak of NCM811
indicates a greater variation of 0.98° than that of NCM811@CN-3wt%
(i.e. 0.82°). The different evolutionary behavior of the (003) peaks
during charging and discharging shows that the volume change of NCM811
is larger than that of NCM811@CN-3wt%. It is known that the large
volume change causes irreversible phase transition and layered structure
collapse, especially in the case of elevated temperatures and increased
cut-off voltages. Therefore, the above results clearly indicate that the
modification of g-C3N4 is related to
suppression of the lattice distortion and stabilization of the crystal
structure.
The de-/lithiation kinetics of
the cathodes investigated by cyclic voltammetry (CV) in the voltage of
3-4.3 V at the scan speeds from 0.1 to 0.5 mV s-1 are
shown in Figure 4c and
4d . Both cathodes undergo a
series of phase transitions, which are consistent with the dQ
dV-1 curves. As the scanning speed increases, the
redox potential of NCM811 shifts in a greater extent than that of
NCM811@CN-3wt% does. Figure S9a displays the CV curves of
NCM811 and NCM811@CN-3wt% at 0.5 mV s-1. The
potential difference between the oxidation and reduction peaks of NCM811
(i.e., ΔE = 0.15 V) is much greater than that of NCM811@CN-3wt% (i.e.,
ΔE = 0.07 V), indicating that the g-C3N4coating helps to reduce the polarization. Meanwhile, the redox coupling
becomes more sensitive, indicating that the electrochemical reaction is
faster. The lithium-ion diffusion coefficient is calculated by the
Randles-Sevcik formula based on the relationship between the maximum
current intensity and the square root of the scan rate (Figure
4e) .[38,39]As a result, NCM811@CN-3wt%
shows the high apparent lithium-ion coefficients of
2.07×10-6 cm2 s-1and 4.67×10-7 cm2s-1 for the delithiation and lithiation process,
respectively. The lithium-ion
diffusion coefficient of NCM811@CN-3wt% is approximately a factor of
two greater than that of NCM811 (Figure S9b) . This indicates
that the g-C3N4 coating helps to
increase the adsorption and diffusion of lithium ions, in agreement with
the fact that the NCM811@CN-3wt% anode has excellent rate capability.
To understand how the g-C3N4 coating on
the surface of NCM811 particles affects the interfacial characteristics
and thus optimizes the cycle performance, EIS studies were performed and
the results are shown in Figure 5a-c . The EIS data can be
quantitatively analysed by fitting the spectra to the corresponding
equivalent circuit model as shown in Figure S10 . The fitting
results are shown in Figure 5c . Notably, the charge transfer
resistance (Rct) of the exposed NCM811 cathode increased significantly
from 92.3 Ω in the 2nd to 646.4 Ω after 300 cycles. In contrast, the Rct
for NCM811@CN-3wt% cathode is only increased from 50.7 Ω to 259.4 Ω
(after 300 cycles). This is due to the accumulation of side reactions
during long-term cycling, leading to surface passivation and increased
impedance. This indicates that such problem can be effectively solved by
the g-C3N4 coating modification. In
addition, the change of surface film resistance (Rf) further confirms
this result. By comparison, changes of Rf and Rct for NCM811@CN-3wt%
are smaller than that for NCM811 during 300 cycles. Importantly, Rf of
NCM811 changes with cycling, while it remains nearly constant for
NCM811@CN-3wt% (Figure 5c). This indicates that NCM811@CN-3wt% forms
the more stable interface during cycles with the help of the
g-C3N4 coating.
The surface chemistry of NCM811 and NCM811@CN-3wt% was determined by
XPS analysis. As shown in Figure 5d , the peaks at 285.8 eV and
288.5 eV on the C 1s spectrum can be attributed to ethers and
carbonates, which are derived from the decomposition of electrolytes.
Notably, the peak intensities of carbon oxides in NCM811@CN-3wt% are
smaller than that of NCM811, which indicates that the electrolyte
decomposition on the surface of the
g-C3N4-coated cathode is less. For the F
1s spectrum shown in Figure 5e , the peaks at 685.00 eV and
687.10 eV are LiF and
LixPOyFz (i.e.,
hydrolysis products of LiPF6), respectively. The weaker
peak intensities of LiF and LixPOyFz on NCM811@CN-3wt% compared to
NCM811 implies that the g-C3N4 coating
attenuates the dissolution of actives and decomposition of
LiPF6. The improved stability of surface is also
verified by the M-O bonds in the O 1s spectra with the higher intensity
for NCM811@CN-3wt% than that of NCM811 (Figure 5f) . Moreover,
the analysis of N 1s shows that Li3N and
LiNxOy with the high ionic conductivity
appeare on NCM811@CN-3wt%. g-C3N4 is
reduced to inorganic with high ionic conductivity, which further
confirms that the g-C3N4 coating
contributes to the increase of lithium-ion adsorption and diffusion(Figure S11) . The cross-section SEM images of NCM811 and
NCM811@CN-3wt % are shown in Figure 5h and 5j . The NCM811
shows obvious cracks from center to surface, while the NCM811@CN-3wt%
exhibits no obvious microcracks. Such microcracks can lead to
electrolyte infiltration, which can lead to side reactions. The
secondary microspheres of NCM811@CN-3wt% are well preserved without
apparent cracks, as can be seen in Figure 5g and 5i, and Figure
S12 , verifying that the g-C3N4 coating
can effectively inhibit the generation of microcracks. The above results
indicate that NCM811@CN-3wt% has excellent interfacial, structural, and
mechanical stability, which contributes to its excellent electrochemical
performance and long-term cycle life.
To enhance the safety of lithium batteries and evaluate the
compatibility of g-C3N4 coated NCM811
with solid-state electrolyte systems, the PVDF:LLZTO membranes with
small amount of ionic liquid as the wetting materials for
cathode/electrolyte interfaces have been used for the solid-state-hybrid
Li batteries. Meanwhile, a series of characterization and
electrochemical tests on PVDF:LLZTO electrolytes are carried out. The
particle sizes of LLZTO, i.e., D10, D50, and D90 are 0.096 μm, 0.302 μm,
and 0.636 μm, respectively (Figure
S13a) . The thickness of the
PVDF:LLZTO electrolyte membranes is approximately 50 µm (Figure
S13b) . The small thickness endows the PVDF:LLZTO membranes with
excellent flexibility, as shown in the inset of Figure S13b. PVDF:LLZTO
membranes show the dense arrangement of small grains, which form the
continuous texture (Figure
S13c) . The electrochemical
window for measuring the PVDF:LLZTO electrolyte using linear scanning
voltammetry (LSV) is approximately 4.8 V, as shown in Figure
S13d . The Li-ion conductivities
of PVDF:LLZTO at 30 °C and 45 °C are 1.19×10-4 S
cm-1 and 2.18×10-4 S
cm-1, respectively (Figure S13e) .
The Li-ion transference number
(tLi+)
of PVDF:LLZTO is approximately 0.32 (Figure S13f) . The
symmetric battery composed of Li|PVDF:LLZTO|Li is
cycled stably for more than 4000 hours at 30 °C, 0.1 mA
cm-2 and 0.1 mAh cm-2, as shown inFigure S14a . Increasing the current density to 0.2 mA
cm-2 led to stable operation for over 700 hours, as
shown in Figure S14b .
The NCM811|PVDF:LLZTO|Li and
NCM811@CN-3wt%|PVDF:LLZTO|Li cells are investigated,
the results of which are shown in Figure 6a .Figure 6b shows the rate
performance. When the current density increases from 0.1 C to 1 C, the
capacity of the NCM811@CN-3wt% changes from 188.9 mAh
g-1 to 95.3 mAh g-1, whereas the
capacity of the NCM811 changed from 182.9 mAh g-1 to
only 34.7 mAh g-1. Figure 6c illustrates the
cycle performance of NCM811 and NCM811@CN-3wt% under 0.1 C between
3–4.3 V at 30 °C. The reversible capacity of NCM811@CN-3wt% is still
163.8 mAh g-1 after 200 cycles with a capacity
retention of 88.1%, as shown in Figure 6d. In contrast, the
specific capacity of NCM811 decreases to 130.7 mAh g-1after 200 cycles with 71.8% capacity retention, as shown inFigure 6e .Figure 6f shows the
cycle performance at the elevated temperature of 45 °C. The capacity of
NCM811 capacity is reduced from 189.6 mAh g-1 to 131.7
mAh g-1 after 200 cycles with a capacity retention of
69.5% (Figure 6g) . Nevertheless, the NCM811@CN-3wt% capacity
drops from 189.6 mAh g-1 to 159.5 mAh
g-1 with 84.2% capacity retention (Figure
6h) . These results indicate that g-C3N4coating is also beneficial to the cycle performance of lithium batteries
with solid-state-hybrid electrolytes.
A comparison of cycle performance
between this work and other coated NCM cathodes for liquid and
solid-liquid-hybrid cells has been summarized in Table S2 and
Table S3 . This comparison confirms that the prepared material has
better cycle performance than other coated NCM cathodes for liquid and
solid-liquid hybrid batteries.