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Journal Pre-proof Construction of the Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 Sodium-Ion Cathode with Balanced High-Power/Energy-Densities Changchun Sun, Shaowen Li, Miao Bai, Weiwei Wu, Xiaoyu Tang, Wenyu Zhao, Min Zhang, Yue Ma PII:
S2405-8297(20)30046-5
DOI:
https://doi.org/10.1016/j.ensm.2020.02.007
Reference:
ENSM 1089
To appear in:
Energy Storage Materials
Received Date: 12 August 2019 Revised Date:
23 December 2019
Accepted Date: 5 February 2020
Please cite this article as: C. Sun, S. Li, M. Bai, W. Wu, X. Tang, W. Zhao, M. Zhang, Y. Ma, Construction of the Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 Sodium-Ion Cathode with Balanced HighPower/Energy-Densities, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2020.02.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
1
Construction of the Na0.92Li0.40Ni0.73Mn0.24Co0.12O2
2
Sodium-Ion Cathode with Balanced
3
High-Power/Energy-Densities
4 5
Changchun Sun, Shaowen Li, Miao Bai, Weiwei Wu, Xiaoyu Tang, Wenyu Zhao, Min
6
Zhang and Yue Ma*
7 8
State Key Laboratory of Solidification Processing, Center for Nano Energy Materials,
9
School of Materials Science and Engineering, Northwestern Polytechnical University
10
and Shaanxi joint Laboratory of Graphene (NPU), Xi’an 710072, China
11
*Corresponding Author
12
E-mail: [email protected] (Y. Ma)
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
1
1
Construction
of
the
2
Sodium-Ion
3
High-Power/Energy-Densities
Cathode
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 with
Balanced
4 5
Abstract
6
Layered P2 type transition metal oxides (TMOs) are considered as the promising
7
cathode candidates for the sodium ion batteries (SIBs). However, the high operating
8
voltage of the P2 cathodes always involves the irreversible phasic transition, which thus
9
compromises the structural stability and practical applications. Through the sustainable
10
recycling of biomass carbon as the sacrificial precursor framework, herein, a
11
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 cathode with the coexisting P2/O3 phases is reported.
12
By the aid of transmission-mode operando X-ray diffraction, the real-time phasic
13
transition upon the solid-state reaction is precisely tracked. Furthermore, a full cell
14
prototype by pairing the as-fabricated cathode with the anode that developed via a
15
similar sacrificial templating strategy is established. The full cell model renders the
16
simultaneous robust stability, the high energy density of ~218.5 Wh kg-1 at a power
17
density of 83 W kg-1 (0.5C). This biomass-templated strategy demonstrates a precise
18
control over the structural and compositional features of electrodes for the SIBs.
19 20
Keywords
21
Sodium ion batteries; Hybrid layered cathode structure; Operando X-ray diffraction;
22
Full cell prototype; High operating voltage
23 24
1. Introduction
25
To cater to the market needs of the emerging electric vehicles and utility-scale
26
energy storage applications, the key performance metrics of the rechargeable batteries,
27
for instance, the raw material cost, energy/power densities and operation safety become
1
increasingly demanding that far beyond current lithium ion batteries (LIBs)
2
technologies.[1–6] Besides, the finite lithium reserve and its spatially localized
3
distribution mismatch with the rapid growing requirements for the sustainable
4
deployment of energy storage systems.[7] In this context, the development of the
5
alternative sodium ion batteries (SIBs), attracts more attention due to the merits of
6
unlimited sodium resources, low environment foot-print and the low operating voltage
7
(-2.71 V vs. standard hydrogen electrode (SHE)). [8–14] Among the cathode
8
candidates for SIBs, transitional metal oxides (TMOs) with layered host for sodium
9
ions, such as the binary (NaNixMnyO2, NaFexMnyO2) and ternary layered derivatives,
10
i.e. NaNixMnyCozO2 (NaNMC) have been extensively investigated due to the high
11
gravimetric capacities of ~120-130 mAh g-1 and the possible operating voltage up to 4.5
12
V, which render the SIBs comparable energy density of ~250-300 Wh/kg as
13
LIBs.[1,12,15,16] However, the performance optimization of the layered TMOs
14
cathodes is rather challenging, especially in consideration of the precise control over
15
the compositional and structural parameters, i.e. crystallinity, particle size, cation
16
distribution as well as the synergistic coupling of coexisting phases.
17
The Delmas’ notation designates two main types of polymorph for the TMOs
18
derivatives as SIBs cathodes: The O3 type structure arranges the crystal layers in a
19
manner of ABCABC stacking, which possesses the higher reversible capacity upon the
20
high voltage charge. However, the Na+ migration in the O3 structure requires more
21
energy during the high voltage range, which unfortunately induces the retarded Na+
22
migration and the compromised rate behaviors. While P2 type structure possesses the
23
close-packed oxygen atoms with ABBA stacking, the enlarged interplanar spacing of
24
which favors the fast diffusion of sodium in the structures with a limited reversible
25
capacity.[10,17–21]
26
O3-NaNi1/3Mn1/3Co1/3O2 cathode delivered a high capacity ~120 mAh g−1 at 0.1C, yet
27
only ~80 mAh g−1 at 1C.[22] The P2-Na2/3Ni1/3Mn2/3O2 layered cathode, as reported in
28
pioneer studies by Li. Y et.al and Liu. Y et.al, delivered an enhanced level of rate
29
behaviors ~100 mAh g−1 at 1C and ~120 mAh g−1 at 0.2C within the voltage charge
For
instance,
M.Sathiya
et.al
reported
that
1
range of 2.0-4.4 V.[23,24] However, the complete extraction of the Na ions during the
2
high voltage range would induce the lattice distortion of P2 structure and thus the
3
irreversible phasic transition to O2 phase, preventing the reinsertion of Na+ with the
4
deteriorated cycling reversibility.[17,24–28] Thus the rational design of cathode
5
materials and the facile electrode kinetics of the P2 type composites will be the feasible
6
route to realize the simultaneous high energy/power densities and long cycle life,
7
especially upon the high operating voltage. Although the similar design rationale of the
8
intergrowth biphasic structure has been proposed, the precise compositional
9
manipulation of the layered cathodes as well as their practical use in the full cell models
10
are yet to be further explored.
11
The elucidation of the dynamic phasic transition of the NaNMC phase is
12
considered as the prerequisite for the rational optimization of the layered cathodes.
13
Thus far, pioneer studies have investigated the feasible modification strategies to
14
improve the reversibility of the sodiation process. Chen et.al introduced the Cu doping
15
to enhance the lattice spacing, thereby reducing the internal resistance of the electrode
16
and improving the cycle performance within the voltage range of 1.5-4.0 V. [29] Dang
17
et.al. investigated the suppression effect of the oxide coating layer which effectively
18
avoided the unfavorable side reaction and the exfoliation of the transition metal (TM)
19
layers.[25] Luo et.al proposed the quasi-solid-solution reaction for the sodium
20
extraction process within the voltage range of 2.2–3.9 V.[30] Xu et.al investigated the
21
effect
22
Na0.80[Li0.12Ni0.22Mn0.66]O2, stabilizing the P2 structure through inhibition of the Mn
23
dissolution above 4.0 V.[31] However, the dynamic phasic transition of the ternary
24
oxide derivatives, especially during the high operating voltage, is yet to be further
25
elucidated, so as to simultaneously realize the high operating voltage and structural
26
stabilization.
of
lithium
substitution
of
Mn
sites
in
the
P2
structure
of
27
Herein, we propose a coherent modification strategy to ameliorate the
28
electrochemical performance of the layered P2/O3 NaLiNMC structure at the high
29
operating voltage range. The deliberate control over the compositional stoichiometry
1
(Li occupancy) and microstructural features (P2/O3 mixture nanocrystallines with
2
optimal particle size) was realized via the sustainable recycling of the biomass as the
3
sacrificial framework. The transmission-mode operando X-ray diffraction (XRD)
4
documented the dynamic phasic transformation in the voltage region of 2.5-4.4 V,
5
suggesting the trace lithium doping has introduced the intergrowth O3 phase, and thus
6
enhanced the reversibility of the P2-O2 phasic transition upon the continued cycling.
7
The as-fabricated Fatsia Japonica- Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 (FJ-NaLiNMC, the
8
default calcining time set as 10h) cathode exhibited a reversible capacity of ~120.6
9
mAh g-1 at 0.5C, high rate behavior and average coulombic efficiency (CE) of ~99% for
10
100 cycles. To deliver a more informative demonstration, the FJ-NaLiNMC cathode
11
was paired with an as-developed ternary metal oxide anode and assembled into a full
12
cell prototype: a stable reversible capacity, high energy density of ~152.65 Wh kg-1 at a
13
power density of ~827 W kg-1 (5C) could be realized. Noted that the biomass carbon
14
(Fatsia Japonica) was introduced as a generic sacrificial framework for both the cathode
15
and anode synthesis with the precise control over the stoichiometric ratio, particle size,
16
phase purity, uniform cation distribution and microstructure. We hope this synthetic
17
strategy will promote the low cost, scalable production of multinary oxide electrode
18
materials in the practical application of SIBs.
19 20
2. Experimental Section
21
2.1 Synthesis of FJ-NaLiNMC hybrid composite
22
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 precursors were synthesized by co-precipitation
23
and calcination method. The cation ratio of Na/Li/Ni/Mn/Co in the as-designated
24
product of Fatsia Japonica- Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 (FJ-NaLiNMC, the default
25
calcining time set as 10h) was maintained as 0.92:0.40:0.73:0.24:0.12. Briefly,
26
Ni(NO3)2 (99%, Sigma-Aldrich), Co(NO3)2 (99%, Sigma-Aldrich) and Mn(NO3)2
27
(99%, Sigma-Aldrich) were titrated into a 2 mol L-1 Na2CO3 (99%, Sigma-Aldrich)
28
solution. At the same time, the Fatsia Japonica (FJ) biomass carbon template was
1
added. After stirring for 12h, the resultant product was collected, thoroughly washed
2
and dried at 60°C overnight. The precursor was then grounded with Na2CO3 and LiNO3
3
(99%, Sigma-Aldrich) at the predetermined stoichiometric ratios. The mixture was
4
annealed in a muffle furnace at 530°C for 5h and then annealed 900°C for 10h with a
5
ramping rate of 5°C min-1. The reaction is depicted in Equation (1).
6
0.39 Na2CO3+0.40 LiNO3+ 1.22Na0.2Ni0.6Mn0.2Co0.1CO3+O2
(1)
7
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2+1.61CO2+0.2N2+1.40O2
8
In the control experiments, the samples were fabricated without sacrificial
9
template framework (denoted as Pure-NaLiNMC) and without any lithium dopant
10
(denoted as FJ-NaNMC) while kept all other calcination parameters identical as
11
FJ-NaLiNMC Additionally, the samples fabricated at the different calcination time (5h,
12
10h and 20h) also were developed to investigate the influence on the morphology.
13
2.2 Synthesis of FJ-FeNiMnOx composite
14
The Fatsia Japonica-FeNiMnOx electrode was synthesized by a very simple
15
immersion-calcining process. The precursor metal nitrate was dissolved in deionized
16
water to obtain 0.5mol L-1 solution, then biomass template obtained in the previous step
17
was soaked in this solution for 12h. After being taken out, it was placed in a tube
18
furnace under argon atmosphere with 550°C calcined 2h, the final product designated
19
as FJ-FeNiMnOx.
20
2.3 Characterizations
21
The cation ratios in the composite was measured by inductively coupled plasma-optical
22
emission spectroscopy (ICP-OES), which was performed on Agilent 5110. Powder
23
X-ray diffraction (XRD) patterns were tested by transmission-mode X-ray
24
diffractometer (STADIP STOE) with a position-sensitive detector and Cu Kα1
25
radiation (1.5405 Å) scanning in the 2-Theta range of 10-70°, operating at 40 kV and 40
26
mA. Transmission-mode Operando XRD analysis was carried out by the same
27
equipment. Rietveld refinement was performed using GSAS-II. The particle size and
28
morphology of the samples were tested by field emission scanning electron microscopy
29
(SEM, Quanta 600 FEG) at 15 kV with energy-dispersive X-ray spectroscopy (EDS).
30
Transmission electron microscope (TEM, FEI Talos F200X) characterizations were
1
conducted at 200 kV. The X-ray photoelectron spectroscopy (XPS) spectra were
2
recorded with a spectrometer (Thermo escalab 250XI) having Al Kα radiation
3
(hv=1486.6 eV), the peak areas were integrated by a weighed least-squares fitting of
4
model curves. 7Li MAS NMR (I) 3 /2, 92.6% abundance) spectra were recorded on a
5
Chemagnetics Infinity operating at 155.4 and 233.2 MHz, respectively. The ex-situ XPS
6
measurements were performed using a PHI5500 spectrometer operated using a
7
monochromatic Al Kα radiation and an electron emission angle of 45°. For the ex-situ
8
XPS characterization of the FJ-NaLiNMC electrode at different potential stages,
9
Ar+-ion beam sputtering was performed for 20 mins for each sample at pressure of 20 ×
10
10−3 Pa with an emission current of 25 µA and beam energy of 4 kV. Raman spectrums
11
were taken on a Raman spectrometer (HORIBA, France) with 532 nm line of a
12
helium-neon as the excitation beam.
13
2.4 Electrochemical Measurements
14
Both the cathode and anode slurries were made by mixing the electrode material (90
15
wt.%) with acetylene black (5 wt.%) and polyvinylidene fluoride (PVDF) binder (5
16
wt.%) in an appropriate amount of 1-methyl-2-pyrrolidinone (NMP). After
17
homogenous mixing, the cathode and anode slurries were spread onto an aluminum and
18
copper foil separately, and then dried at 120°C overnight in a vacuum oven. Circular
19
slices with a diameter of 12 mm were punched from the dried foil as the working
20
electrode. CR2016 type half coin cells were assembled in a glove box with argon gas,
21
using sodium metal and glass fiber (GF/D Whatman) as reference electrode and
22
separator respectively. The electrolyte is 1M NaClO4 in propylene carbonate (PC) with
23
2 vol% fluorinated ethylene carbonate (FEC) as additive. The galvanostatic
24
charge/discharge tests were performed by Neware instrument (Shenzhen, China) at
25
room temperature within the voltage of 2.5-4.4 V (V versus Na+/Na). The Cyclic
26
voltammograms (CV) were performed on a CHI660e electrochemical workstation
27
(ChenHua Instrument Co., China) at a scan rate of 0.1 mV s−1 from 2.0 V to 4.4 V.
28
Electrochemical impedance spectroscopy (EIS) was tested on the same electrochemical
29
workstation in the frequency range between 100 kHz and 0.1 Hz. CR2016 type full coin
30
cells were prepared under the same condition, the mass ratio between cathode and
1
anode is about 1.2:1. Electrolyte and separator were the same as those used in the half
2
cells. The test was conducted between 4.4 V and 1.4 V (V versus Na+/Na).
3
Galvanostatic intermittent titration technique (GITT) was conducted on the Gamry
4
instruments (Interface 1000) with the cell charged/discharged at the current density of
5
10 mA g-1 for a pulse of 30 mins followed by a relaxation of 60 mins to approach the
6
steady state value. 1C corresponds to 110 mA g-1 for all the cell tests.
7 8
3. Results and Discussion
9
It has been a hot direction to synthesize the anode and cathode materials of SIBs
10
with excellent performance. We use biomass templating strategy to synthesize the
11
excellent cathode materials for SIBs. To validate the generic applicability of this
12
biomass templating strategy for regulating the pre-designed structural and
13
compositional features, we intentionally developed the Fatsia Japonica-FeNiMnOx
14
(FJ-FeNiMnOx) anode by optimizing the cation ratios in the precursor solution. We
15
further established a full cell prototype by paring FJ-NaLiNMC and FJ-FeNiMnOx. The
16
Schematic illustration was shown in Figure 1. The synthesis of the hybrid P2/O3
17
structure of FJ-NaLiNMC cathode began with the co-precipitation process of the
18
cation precursors with the aid of macroporous biomass framework (Figure S1). The
19
biomass precursor was sustainably recycled from the Fatsia Japonica (FJ) stem.
20
Acting as the sacrificial template, the hydroxyl groups that abundantly present on the
21
biomass framework anchored the various metal cations onto the surface, forming
22
“metal hydroxides” in a hydrocarbon matrix. During the ensuing solid-state reactions,
23
the metal hydroxides were converted into the sodiated ternary oxide with trace lithium
24
incorporation, at the same time the oxidative decomposition of the biomass
25
framework occurred. The morphology of the FJ- NaLiNMC hybrid composite was
26
approximately the reverse replication of the microporous structure of FJ biomass
27
carbon whereas the particle growth of the sodiated oxide was restrained by the gas
28
released from the thermal decomposition of the biomass template, which enabled the
29
particle size control of NaLiNMC under 800 nm with good crystallinity.
1
The cation ratio of Na/Li/Ni/Mn/Co in the as-prepared FJ-NaLiNMC hybrid
2
composite was determined as 0.90:0.40:0.72:0.24:0.12 by Inductively Coupled Plasma-
3
Optical Emission Spectroscopy (ICP-OES), which is in good accordance with the target
4
stoichiometry ratio. ICP-OES results of the control samples are shown in Table S1. The
5
X- ray diffraction (XRD) pattern of the as-synthesized FJ-NaLiNMC hybrid composite
6
is shown in Figure S2, in which most of the diffraction peaks clearly display the P2
7
phase. As compared to the sample prepared without lithium doping, namely the
8
FJ-NaNMC, the FJ-NaLiNMC hybrid composite shows the additional peak located in
9
18.4° that indexed to the O3 structure due to the lithium incorporation into the
10
lattice.[31,32] By comparing the crystallization degree and particle size (Figure S3),
11
we optimized the time duration of the calcination process as 10h. On the other hand, the
12
transmission- mode in-situ XRD was also conducted to document the real time phasic
13
transition of the solid-state reaction of the FJ-NaNMC composite upon the pre-set
14
ramping program, the result shows the onset crystallization temperature of the P2 phase
15
occurred at around ~530°C (Figure S4), in this sense, we purposely extended the
16
duration of the nucleation process at 530°C for 5h, thereby precisely promoting the
17
uniform distribution of the P2 nanocrystallines.
18
The layered P2 and O3 structures are schematically illustrated in Figure 2a-b.
19
Noted the MO2 (M=Ni, Mn, Co and Li) stacked layers arrange in the ABBA order of P2
20
structure, while the Na ions locate at the 2b sites in the trigonal prism of oxygen ions,
21
sharing the faces with two MO6 octahedrons.[33] However, O3 structure consists of a
22
cubic close-packed oxygen array in which sodium ions are accommodated at distinct
23
octahedral sites.[34] Based on the Rietveld refinement results of the XRD pattern of the
24
P2 FJ-NaNMC composite and the P2/O3 FJ-NaLiNMC hybrid composite (Figure 2c-d
25
and corresponding crystallographic lattice parameters in Table S2), the majority of the
26
doped lithium ions are positioned at the transition metal (TM) sites in the crystal
27
structure to maintain the P2 majority.[35] In comparison with the lattice parameters of
28
the P2 FJ-NaNMC, the parameters of both a and c decrease to the lower values (a from
29
2.8735(2) Å to 2.8639(1) Å, c from 11.0735(8) Å to 11.0336(3) Å), generating reduced
30
TMO2 units in c axis. As a result, the stabilization effect for the TM cations maintains the
1
structural robustness of the P2 phase. This observation is consistent with previous studies
2
including both the XRD refinement results, neutron diffraction and lithium nuclear
3
magnetic resonance.[31,34] Meanwhile, the trace amount of lithium ions occupies the
4
Na sites to generate a coexisting O3 structure (the relative mass content is estimated as
5
4.33% based on the refinement results), thereby slightly improving the retrievable
6
specific capacity for the hybrid composite without sacrificing the electrode kinetics.
7
To elucidate the doping effect on the structural features, the morphology of FJ-
8
NaLiNMC and FJ-NaNMC composites were compared in Figure 3. The transmission
9
electron microscope (TEM) image of the FJ-NaLiNMC hybrid composite, shown in
10
Figure 3a as an example, demonstrates the uniformly distributed nanoparticles ranging
11
from 500 to 800 nm. A scrutiny of the representative particle, as revealed by high
12
resolution transmission electron microscope (HRTEM) in Figure 3b-c, suggests the
13
interplanar lattice fringes spaced by 0.23 nm and 0.56 nm apart, corresponding to the
14
d-spacing values of (012) and (002) planes respectively, in the P2 structure. The
15
NaLiNMC composite synthesized without FJ as the sacrificial template (Pure-
16
NaLiNMC), in stark contrast, shows the obviously enlarged particles of ~5 um in
17
diameter (Figure S3c). This comparison clearly indicates the oxidative decomposition
18
of the biomass framework has delayed the particle growth rate. Energy dispersive
19
spectroscopy (EDS) mapping of the FJ-NaLiNMC hybrid composite, as shown in
20
Figure 3d-f, demonstrates the uniform distribution of Na, Ni, Mn and Co elements
21
across the particles, implying that the hydroxyl functional groups that abundant present
22
on the sacrificial biomass framework provide an effective anchoring effect for the
23
multinary cations.
24
The survey X-ray photoelectron spectroscopy (XPS) spectrum in Figure 4a reveals
25
the coexistence of O, Co, Na, Li, Ni and Mn elements in the FJ-NaLiNMC hybrid
26
composite. The core-level Na 1s spectrum exhibits the peak at the binding energy of
27
1071.0 eV that assignable to Na-O bond (Figure 4b). For the core-level O 1s spectrum as
28
shown in Figure 4c, the peak could be deconvoluted into the bimodal envelopes at 528.67
29
eV and 531.0 eV, assignable to the lattice oxygen and the coordinative interfacial oxygen
30
atoms, respectively.[36] As shown in the core-level Ni 2p (Figure 4d), the binding
1
energies of Ni 2p3/2 and Ni 2p1/2 position at 854.0 eV and 871.8 eV respectively,
2
corresponding to the existence of Ni2+.[25] The Mn 2p3/2 and Mn 2p1/2 peaks (Figure
3
4e) are fitted to the characteristic peaks of the tetravalent Mn with binding energy values
4
positioned at 641.6 eV and 653.4 eV, respectively.[37] The core-level Co 2p spectrum
5
exhibits the Co 2p3/2 and Co 2p1/2 (Figure 4f) peaks at the binding energies of 780.8 eV
6
and 795.1 eV, suggesting the existence of trivalent Co in the as-synthesized oxide
7
cathode.[33]
8
Figure 5a exhibits the cyclic voltammograms (CV) curves of FJ-NaLiNMC
9
cathode during the 1st, 2nd (pristine electrode after battery assembly) and 80th
10
(post-cycled electrode after 80th galvanostatic charge/discharge cycles) at a scan rate of
11
0.1 mV s-1 within the voltage range of 2.5-4.4 V. The CV curves show multiple redox
12
peaks and superimposable curves upon the cycling. The redox peak couple, within the
13
range of 2.5-3.2 V (Square I), involves the reversible faradic reaction of Mn4+/Mn3+
14
couple.[38,39] Reversible peaks between 3.2 and 4.1 V can be attributed to the redox
15
reaction of Ni2+/Ni3+ and Ni3+/Ni4+ couples (Square Ⅱ), at the same time the partially
16
vacancy ordering process via the Na+ occupancy is also involved within this
17
process.[25,27,30] Whereas the faradic process at the high-voltage range (4.1-4.4 V,
18
Square ⅡI) can be attributed to the oxygen loss and structure rearrangement process
19
from P2 to O2 phase.[39] Meanwhile, some Li ions migrate from the TM layer to the
20
Na layer at high voltage, and yet this process is highly reversible.[31] In order to ensure
21
the electrochemical reaction during charge/discharge process, ex-situ XPS and ex-situ
22
MAS NMR were applied to evaluate the different charge/discharge voltage. Ex-situ
23
MAS NMR spectra was shown in Figure S5, we can assign the resonances of ca. 1688
24
ppm and 1450 ppm to Li sites within the TMO2 layer; while the NMR resonances
25
positioned at 543, 563 and 715 ppm can be assigned to octahedrally distorted sites in
26
the Na layer. Therefore , the 7Li MAS NMR spectrum of the FJ-NaLiNMC cathode at
27
pristine state confirm the two types of the spatial occupancy of Li+; Upon the charge
28
process till 4.4 V, the resonances which represent the Li sites within the TMO2 layer
29
disappear, while the dominant resonances at ca. 0 and 95 ppm indicate the migration of
30
Li ions to the tetrahedrally distorted sites in the Na layer instead. (Chemistry of
1
Materials, 2014, 26, 1260-1269; Journal of the American Chemical Society, 2001, 123,
2
11454-11461.) [31,41] Figure S6 exhibits the ex-situ XPS spectra of Mn 2p and Ni 2p
3
at the selected discharge and charge potentials. The electrochemically reduced trivalent
4
manganese appears (BE value negatively shifted from 641.8 to 641.1 eV) when the
5
electrode was reduced from 2.8 V (Figure S6, point Ⅱ) to 2.0 V (Figure S6, point
6
Ⅱ).Afterwards, the Mn3+ species in the electrode were re-oxidized to tetravalent
7
valence during the anodic sweep from 2.0 V (Figure S6, point Ⅱ) to 2.9 V (Figure S6,
8
point Ⅱ) and maintained till 3.9 V (Figure S6, point Ⅱ). On the other hand, tetravalent
9
nickel only appears (BE value positioned at 854.8 eV) during the anodic sweep to 3.9 V
10
(from point Ⅱ to point Ⅱ), while the divalent nickel retains the valence state from point
11
Ⅱ to point Ⅱ. These reversible Mn3+/Mn4+ and Ni2+/Ni4+ redox reactions in the different
12
voltage ranges are consistent with the above CV analysis results.
13
The rate capability of FJ-NaLiNMC and FJ-NaNMC cathode were evaluated at
14
the rates ranging from 0.2C to 5C with the sodium foil as the reference electrode. As
15
shown in Figure 5b, the FJ-NaLiNMC cathode exhibits the reversible capacities are
16
133.5, 121.7, 104.4, 92.5 and 80.7 mAh g-1, at 0.2C, 0.5C, 1C, 2C and 5C, respectively.
17
Even as the C rate returns to 0.2C, the discharge capacity could deliver the initial value
18
of 129.3 mAh g-1 and maintain stable capacity in the subsequent cycles, suggesting the
19
satisfactory rate behaviors of the FJ-NaLiNMC cathode. In comparison, the
20
FJ-NaNMC cathode exhibits the reversible capacities as 120.5, 105.7, 84.8, 59.0 and
21
32.3 mAh g-1, at 0.2C, 0.5C, 1C, 2C and 5C, respectively. On the contrary, the
22
FJ-NaNMC cathode exhibits the reversible capacities are 120.5, 105.7, 84.8, 59.0 and
23
32.3 mAh g-1, at 0.2C, 0.5C, 1C, 2C and 5C, respectively. Figure 5c shows the
24
overlapped charge/discharge curves of the FJ-NaLiNMC cathode during the 1st, 50th
25
and 100th cycles, suggesting the structural robustness of the cathode upon the cycling.
26
As indicated in Figure 5d, the FJ-NaLiNMC cathode maintains the discharge capacity
27
of ~120.6 mAh g-1 for 100 cycles at 0.5C, with the CE kept higher than 98% from the 3rd
28
cycle onwards. The FJ-NaNMC cathode delivers an initial capacity of 110.6 mAh g-1
29
with the capacity retention maintained as ~54% only after 50 cycles. The lower
30
reversible capacity and serious the capacity fading can be ascribed to the irreversible
1
P2-O2 phasic transition during high operating voltage, as the integrated O3 phase is
2
absent. For comparison purpose, the as-fabricated samples under the different synthetic
3
parameters (at the different calcination durations, varied compositional ratios and
4
synthesis without FJ biomass as the sacrificial precursor) were electrochemically
5
compared in Figure S7 and Figure S8. As shown in Figure S7, the FJ-NaLiNMC-5h
6
cathode (5h represents the calcination duration at 900°C) with the poor crystallinity
7
only shows the limited initial discharge capacity of 77.8 mAh g-1 with 43% capacity
8
retention after 100 cycles. FJ-NaLiNMC-20h cathode (20h represents the calcination
9
duration at 900°C) with the enlarged particle size, in stark contrast, also demonstrates
10
the interior retrievable capacity of 82.3 mAh g-1 and unsatisfactory capacity retention
11
for 100 cycles. Figure S8 shows the cycling performance of the Pure-NaLiNMC
12
cathode at 0.5C. Without FJ biomass as the sacrificial framework, the Na+ transport is
13
retarded due to the increased solid-state diffusion range that derived from the large
14
particle size, therefore the Pure-NaLiNMC cathode shows the initial discharge capacity
15
of 81.4 mAh g-1 with a compromised capacity retention of 48% after 100 cycles.
16
Previous studies investigated the effect of the upper limit voltage on the cycling
17
stability of the electrode. As shown in Figure S9, the cycling performance of both the
18
FJ-NaLiNMC and Pure-NaLiNMC cathodes were also evaluated within the voltage
19
range of 2.5-3.9 V at 0.5C to avoid the high voltage operation. The result shows that the
20
FJ-NaLiNMC cathode demonstrates a stable reversible capacity of 90 mAh g-1 for 100
21
cycles with an initial CE value of 93%. And the Pure-NaLiNMC cathode also remains a
22
stable capacity of 80 mAh g-1 for 100 cycles with an initial CE value of 87%.
23
Obviously, the reduced upper limit voltage has ruled out the capacity contribution that
24
stems from the P2-O2 phasic transition at the voltage higher than 4 V. Therefore, the
25
better cycling stability was realized at the penalty of the reduced retrievable capacity. In
26
this sense, the FJ-NaLiNMC cathode realizes the robust cycling performance and rate
27
behaviors at the high operating voltage range, suggesting the harmony balance of the
28
optimal particle size, crystallinity and mutually beneficial phasic coupling that derived
29
from the deliberate optimization of the calcination time and the compositional
30
stoichiometric ratio of the hybrid composite.
1
The improved rate capability in FJ-NaLiNMC electrode suggests faster charge
2
transport kinetics, thus, we conducted Galvanostatic intermittent titration technique
3
(GITT) to investigate the Na+ diffusion coefficient of cycled FJ-NaLiNMC and
4
FJ-NaNMC electrodes. During the single galvanostatic titration process (Figure S10a
5
and S10c, Figure S11a and S11c), the obvious linear relationship can be observed
6
between the voltage and the square root of the galvanostatic time (Figure S10b and S10d,
7
Figure S11b and S11d). The plot of Na+ diffusivity as a function of voltage during the
8
charge and discharge process is shown in Figure 6 (Details of GITT analysis can be found
9
in the Supporting Information). Overall, the values of D for FJ-NaLiNMC is higher than
10
FJ-NaNMC in general. These results reveal the lithium incorporation will facilitate
11
electrode kinetics and enhance the rate capability performance.
12
Figure S12 shows the electrochemical impedance spectra (EIS) results of
13
FJ-NaLiNMC and Pure-NaLiNMC cathodes both after the initial cycle and
14
post-mortem (after 50 cycles). The Nyquist plots consist of 1) a semicircle which
15
suggests the charge transfer resistance (Rct) and 2) a quasi-straight line, with the slope
16
positively correlating with the Warburg diffusion coefficient in the solid state.[42] As
17
shown in Figure S12a, the Pure-NaLiNMC cathode demonstrates the obvious enlarged
18
semicircle after 50 cycles, suggesting the increase of Rct, while the reduced slope of
19
Warburg diffusion region also indicates the retarded ion diffusion in the solid-state.
20
Compared to the Pure-NaLiNMC electrode, the FJ-NaLiNMC cathode does not
21
demonstrate the obvious change for either the Rct or the slope of Warburg diffusion
22
region, as illustrated in Figure S12b. This comparison indicates the lithium
23
incorporation in the structural lattice has enhanced the charge transfer kinetics at the
24
electrode/electrolyte interface and eased the solid-state lithium diffusion.
25
To track the dynamic structural evolution of the FJ-NaLiNMC cathode upon the
26
sodiation and desodiation process, transmission-mode operando XRD tests were
27
conducted to document the real-time phasic changes of the cathode structures,
28
especially upon the high operating voltage as shown in Figure 7. The FJ-NaLiNMC
29
cathode exhibits the (002) diffraction peak of the P2 phase positioned at 15.3°. Upon
30
the initial desodiation process until the high voltage of 4.2 V, this diffraction peak does
1
not exhibit obvious variation in terms of the peak intensity and the 2-Theta position
2
(Figure 7a-b), with the further sodium extraction from the structural lattice above 4.2
3
V, the peak intensity of (002) diffraction peak suddenly declined, involving the
4
structural transition from P2 phase to O2 phase. Upon the sodiation process, the peak
5
intensity of (002) diffraction peak gradually restored as the electrode discharge below
6
4.2 V. This phasic transition involving the P2-O2 phasic transition remained reversible
7
even after 60 cycles with the reappearance of the identifiable (002) diffraction peak
8
(Figure 7c). The transmission-mode operando XRD results suggest the highly
9
reversible P2-O2 phasic transition upon the high operating voltage of the FJ-NaLiNMC
10
cathode. In comparison, the FJ-NaNMC cathode without the lithium doping
11
demonstrates the obvious mitigated (002) peak upon the cycling. As shown in Figure
12
7d-e, this diffraction peak almost disappeared as the FJ-NaNMC cathode was charged
13
higher than 4.0 V. This observation clearly implies the compromised structural
14
robustness upon the continuous cycling, accounting for the drastic structural distortion
15
of P2 phase upon the high voltage cycling. After 60 cycles within the range of 2.5-4.4
16
V, we clearly observe the mitigated (002) diffraction peak with the gradually reduced
17
peak intensity (Figure 7f). Therefore, the FJ-NaNMC cathode demonstrates the
18
irreversibility of the P2-O2 phasic transition upon the high operating voltage, and this
19
reversible process accompanies with the compromised capacity retention for the
20
cathode.
21
The post-cycling TEM analysis was also conducted as the FJ-NaLiNMC cathode
22
was disassembled from the half cell at the fully discharged state (2.5 V). As shown in
23
Figure 8a, the formation of a well-crystallized region of the fully discharged electrode
24
(2.5 V) was observed on the cathode particle with the lattice fringes spaced by 0.333
25
nm, corresponding to (002) planes of the P2 structure. As shown in Figure 8b, the
26
selected area electron diffraction (SAED) pattern further demonstrates the diffraction
27
points that are indexed to (100), (010) and (110) of the P2 structure. This observation
28
suggests the well preservation of the robust P2 structure after 60 cycles of reversible
29
P2-O2 phasic transition, which is consistent with the operando XRD results. In
30
comparison, the HRTEM and SAED pattern of FJ-NaNMC cathode at the fully
1
discharged state (2.5 V) are shown in Figure 8c-d. Noted the spacing in-between the
2
crystal faces are not discernable, demonstrating an irreversible P2-O2 phasic transition
3
occurred after 60 cycles. Meanwhile, the SAED pattern of the post-cycled FJ-NaNMC
4
cathode indicates its polycrystalline characteristic due to the structure collapse.
5
Obviously, the heterophane was irreversibly generated after 60 cycles, leading to the
6
deterioration of the electrochemical performance.
7
The full cell prototype model through pairing the FJ-FeNiMnOx anode and
8
FJ-NaLiNMC cathode. The characterization and electrochemical measurements of the
9
FJ-FeNiMnOx oxide anode via half cells were elaborated in Figure S13-S14. Figure 9a
10
records the galvanostatic charge-discharge curves for the full cell measured at 0.5C.
11
The charge-discharge curves during the 1st, 50th, 100th cycles almost overlapped, while
12
the reversible charge and discharge capacities maintained at ~100 mAh g-1 for 100
13
cycles. The cycling performance of the full cell was evaluated at 0.5C (Figure 9b), the
14
first discharge and charge capacities were recorded as 105.4 mA h g−1 and 112.1 mA h
15
g−1, leading to an initial CE value as ~95%. Afterwards, the CE value maintained higher
16
than 98% during the subsequent 100 cycles. After a total 100 cycles, the reversible
17
capacity still recovered to 102.97 mAh g-1 with 98% capacity retention. As displayed in
18
Figure 9c, the full cell demonstrates an impressive rate behavior of 123.2, 103.4, 92.6,
19
83.5 and 72.3 mAh g-1 at 0.2, 0.5, 1, 2 and 5C, respectively. In sharp contrast, the full
20
cell assembled by paring the FJ-NaNMC cathode and FJ-FeNiMnOx anode
21
demonstrates an inferior cycling stability and rate capability, the capacity decayed from
22
107.8 to 29.8 mAh g-1 only after 30 cycles (Figure S15). Based on the calculation of the
23
supplementary note, the FJ-FeNiMnOx and FJ-NaLiNMC full cell prototype realizes a
24
gravimetric energy density of ~152.65 Wh kg-1 at a power density of ~827 W kg-1
25
(5C).[43] In this sense, our as-developed full cell model which derived from the
26
biomass framework realizes the simultaneous structural robustness, balanced
27
high-power/energy densities at a low production cost.
28 29
4. Conclusions
1
In summary, we have synthesized a layered oxide cathode of the P2/O3 hybrid
2
composite through the co-precipitation and subsequent solid reaction process. By the
3
aid of transmission-mode operando XRD technique, we could precisely document the
4
real-time phasic transition of the solid-state reaction and the dynamic peak shift upon
5
the galvanostatic cycling process, enabling our deliberate compositional engineering of
6
the lithium incorporation into both the TM sites and the Na sites to facilitate the
7
intergrowth of O3 phase. Electrochemical evaluations of the FJ-NaLiNMC cathode
8
prove the simultaneous realization of the high retrievable capacity, structural robustness
9
and rate behaviors in both the half and full cell models, validating the pivotal role of the
10
enhanced reversibility of the phasic transition upon the high operating voltage.
11
Moreover, the sustainable recycling of the biomass framework for the cathode/anode
12
designs in this study demonstrates the generic applicability of our synthetic protocol to
13
regulate the structural and compositional features of the multinary oxide, and thus
14
exhibits the tremendous potential in the developments of high-performance SIBs.
15 16 17
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version
18 19
at……
20 21
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1
Figures and Figure Caption
2 3
Figure 1. Schematic illustration of (a) the design FJ-NaLiNMC and FJ-FeNiMnOx
4
composite and (b) the FJ-NaLiNMC||FJ-FeNiMnOx full cell prototype model.
5
1 2
Figure 2. Schematic illustration of the layered structure of (a) P2 and (b) O3. The
3
Rietveld refinement of the reference phases for the (c) FJ-NaNMC composite and (d)
4
FJ-NaLiNMC hybrid composite.
1 2
Figure 3. (a) TEM image, (b-c) HRTEM images of FJ-NaLiNMC hybrid
3
composite. (d) SEM images of FJ-NaLiNMC hybrid composite. The
4
energy-dispersive X-ray spectroscopy of FJ-NaLiNMC particle with the elemental
5
maps of (e) Na, (f) Ni, (g) Mn and (h) Co respectively.
6
1 2
Figure 4. (a) Survey XPS spectrum of FJ-NaLiNMC hybrid composite. (b) Na 1s and
3
(c) O 1s core-level XPS spectrum of FJ-NaLiNMC hybrid composite. (d) Ni 2p,
4
(e) Mn 2p and (f) Co 2p core-level XPS spectrum of FJ-NaLiNMC hybrid
5
composite fitted with multiple envelopes.
6
1 2
Figure 5. (a) The 1st, 2nd and 80th CV curves of the FJ-NaLiNMC cathode at a scan
3
rate of 0.1 mV s-1 within the voltage range of 2.5-4.4 V. (b) The rate capability at
4
different rates of 0.2C, 0.5C, 1C, 2C and 5C for FJ-NaLiNMC and FJ-NaNMC
5
cathodes. (c) The 1st, 50th, and 100th charge–discharge curves of the FJ-NaLiNMC
6
cathode at 0.5C within the voltage range of 2.5-4.4 V. (d) The cycling performance
7
and the CE of the FJ-NaLiNMC and FJ-NaNMC cathodes at 0.5C.
8 9 10 11 12 13 14
1 2
Figure 6. GITT profiles and Na+ diffusivity of FJ-NaLiNMC cathode for (a) charge and
3
(b) discharge process; GITT curves and Na+ diffusivity of FJ-NaNMC cathode for (c)
4
charge and (d) discharge process.
5
1 2
Figure 7. (a) The electrochemical load curves during operando XRD measurement of
3
the phasic changes of the FJ-NaLiNMC cathode and colour maps of the 2nd (after initial
4
charge/discharge on the battery testing system) complete electrochemical cycle. (b)
5
Operando XRD patterns of the 2nd and (c) 60th (after 60 cycles charge/discharge on the
6
battery testing system) of the FJ-NaLiNMC cathode during galvanostatic charge and
7
discharge within the range of 2.5-4.4 V at 0.2C. (d) The corresponding electrochemical
8
load curves during operando XRD measurement of the phasic changes of the FJ-
9
NaNMC cathode and color maps of the 2nd complete electrochemical cycle. (e)
10
Operando XRD patterns collected during the 2nd and (f) 60th of the FJ-NaNMC cathode
11
during galvanostatic charge and discharge within the range of 2.5-4.4 V at 0.2C.
1 2
Figure 8. (a) The HRTEM image and (b) SAED pattern of FJ-NaLiNMC cathode
3
at the fully discharge state (2.5 V) after 60 cycles. (c) The HRTEM image and (d)
4
SAED pattern of FJ-NaNMC cathode at the fully discharge state (2.5 V) after 60
5
cycles.
1 2
Figure 9. (a) The charge–discharge profiles of the FJ-NaLiNMC||FJ-FeNiMnOx
3
full cell in different 1st, 50th, and 100th cycles at 0.5C within the voltage range of
4
1.4–4.4 V. (b) The cycling performance at 0.5C and (c) The rate capability at
5
different rates of 0.2C, 0.5C, 1C, 2C and 5C of FJ-NaLiNMC||FJ-FeNiMnOx full
6
cell. (d) Ragone plot of the SIBs full cell models based on the cathode and anode
7
mass.
8 9
Conflict of interest statement The authors declare no competing financial interest.
Acknowledgements We acknowledged the financial support of this work by the National Natural Science Foundation of China (51602261 and 51711530037), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.160-QP2016), the Natural Science Foundation of Shaanxi Province (2018JM5116), the Fundamental Research Funds for the Central Universities (3102019JC005), and Young Talent fund of University Association for Science and Technology in Shaanxi, China. Furthermore, we would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for providing some test equipment.
S2405-8297(20)30046-5
DOI:
https://doi.org/10.1016/j.ensm.2020.02.007
Reference:
ENSM 1089
To appear in:
Energy Storage Materials
Received Date: 12 August 2019 Revised Date:
23 December 2019
Accepted Date: 5 February 2020
Please cite this article as: C. Sun, S. Li, M. Bai, W. Wu, X. Tang, W. Zhao, M. Zhang, Y. Ma, Construction of the Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 Sodium-Ion Cathode with Balanced HighPower/Energy-Densities, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2020.02.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
1
Construction of the Na0.92Li0.40Ni0.73Mn0.24Co0.12O2
2
Sodium-Ion Cathode with Balanced
3
High-Power/Energy-Densities
4 5
Changchun Sun, Shaowen Li, Miao Bai, Weiwei Wu, Xiaoyu Tang, Wenyu Zhao, Min
6
Zhang and Yue Ma*
7 8
State Key Laboratory of Solidification Processing, Center for Nano Energy Materials,
9
School of Materials Science and Engineering, Northwestern Polytechnical University
10
and Shaanxi joint Laboratory of Graphene (NPU), Xi’an 710072, China
11
*Corresponding Author
12
E-mail: [email protected] (Y. Ma)
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
1
1
Construction
of
the
2
Sodium-Ion
3
High-Power/Energy-Densities
Cathode
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 with
Balanced
4 5
Abstract
6
Layered P2 type transition metal oxides (TMOs) are considered as the promising
7
cathode candidates for the sodium ion batteries (SIBs). However, the high operating
8
voltage of the P2 cathodes always involves the irreversible phasic transition, which thus
9
compromises the structural stability and practical applications. Through the sustainable
10
recycling of biomass carbon as the sacrificial precursor framework, herein, a
11
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 cathode with the coexisting P2/O3 phases is reported.
12
By the aid of transmission-mode operando X-ray diffraction, the real-time phasic
13
transition upon the solid-state reaction is precisely tracked. Furthermore, a full cell
14
prototype by pairing the as-fabricated cathode with the anode that developed via a
15
similar sacrificial templating strategy is established. The full cell model renders the
16
simultaneous robust stability, the high energy density of ~218.5 Wh kg-1 at a power
17
density of 83 W kg-1 (0.5C). This biomass-templated strategy demonstrates a precise
18
control over the structural and compositional features of electrodes for the SIBs.
19 20
Keywords
21
Sodium ion batteries; Hybrid layered cathode structure; Operando X-ray diffraction;
22
Full cell prototype; High operating voltage
23 24
1. Introduction
25
To cater to the market needs of the emerging electric vehicles and utility-scale
26
energy storage applications, the key performance metrics of the rechargeable batteries,
27
for instance, the raw material cost, energy/power densities and operation safety become
1
increasingly demanding that far beyond current lithium ion batteries (LIBs)
2
technologies.[1–6] Besides, the finite lithium reserve and its spatially localized
3
distribution mismatch with the rapid growing requirements for the sustainable
4
deployment of energy storage systems.[7] In this context, the development of the
5
alternative sodium ion batteries (SIBs), attracts more attention due to the merits of
6
unlimited sodium resources, low environment foot-print and the low operating voltage
7
(-2.71 V vs. standard hydrogen electrode (SHE)). [8–14] Among the cathode
8
candidates for SIBs, transitional metal oxides (TMOs) with layered host for sodium
9
ions, such as the binary (NaNixMnyO2, NaFexMnyO2) and ternary layered derivatives,
10
i.e. NaNixMnyCozO2 (NaNMC) have been extensively investigated due to the high
11
gravimetric capacities of ~120-130 mAh g-1 and the possible operating voltage up to 4.5
12
V, which render the SIBs comparable energy density of ~250-300 Wh/kg as
13
LIBs.[1,12,15,16] However, the performance optimization of the layered TMOs
14
cathodes is rather challenging, especially in consideration of the precise control over
15
the compositional and structural parameters, i.e. crystallinity, particle size, cation
16
distribution as well as the synergistic coupling of coexisting phases.
17
The Delmas’ notation designates two main types of polymorph for the TMOs
18
derivatives as SIBs cathodes: The O3 type structure arranges the crystal layers in a
19
manner of ABCABC stacking, which possesses the higher reversible capacity upon the
20
high voltage charge. However, the Na+ migration in the O3 structure requires more
21
energy during the high voltage range, which unfortunately induces the retarded Na+
22
migration and the compromised rate behaviors. While P2 type structure possesses the
23
close-packed oxygen atoms with ABBA stacking, the enlarged interplanar spacing of
24
which favors the fast diffusion of sodium in the structures with a limited reversible
25
capacity.[10,17–21]
26
O3-NaNi1/3Mn1/3Co1/3O2 cathode delivered a high capacity ~120 mAh g−1 at 0.1C, yet
27
only ~80 mAh g−1 at 1C.[22] The P2-Na2/3Ni1/3Mn2/3O2 layered cathode, as reported in
28
pioneer studies by Li. Y et.al and Liu. Y et.al, delivered an enhanced level of rate
29
behaviors ~100 mAh g−1 at 1C and ~120 mAh g−1 at 0.2C within the voltage charge
For
instance,
M.Sathiya
et.al
reported
that
1
range of 2.0-4.4 V.[23,24] However, the complete extraction of the Na ions during the
2
high voltage range would induce the lattice distortion of P2 structure and thus the
3
irreversible phasic transition to O2 phase, preventing the reinsertion of Na+ with the
4
deteriorated cycling reversibility.[17,24–28] Thus the rational design of cathode
5
materials and the facile electrode kinetics of the P2 type composites will be the feasible
6
route to realize the simultaneous high energy/power densities and long cycle life,
7
especially upon the high operating voltage. Although the similar design rationale of the
8
intergrowth biphasic structure has been proposed, the precise compositional
9
manipulation of the layered cathodes as well as their practical use in the full cell models
10
are yet to be further explored.
11
The elucidation of the dynamic phasic transition of the NaNMC phase is
12
considered as the prerequisite for the rational optimization of the layered cathodes.
13
Thus far, pioneer studies have investigated the feasible modification strategies to
14
improve the reversibility of the sodiation process. Chen et.al introduced the Cu doping
15
to enhance the lattice spacing, thereby reducing the internal resistance of the electrode
16
and improving the cycle performance within the voltage range of 1.5-4.0 V. [29] Dang
17
et.al. investigated the suppression effect of the oxide coating layer which effectively
18
avoided the unfavorable side reaction and the exfoliation of the transition metal (TM)
19
layers.[25] Luo et.al proposed the quasi-solid-solution reaction for the sodium
20
extraction process within the voltage range of 2.2–3.9 V.[30] Xu et.al investigated the
21
effect
22
Na0.80[Li0.12Ni0.22Mn0.66]O2, stabilizing the P2 structure through inhibition of the Mn
23
dissolution above 4.0 V.[31] However, the dynamic phasic transition of the ternary
24
oxide derivatives, especially during the high operating voltage, is yet to be further
25
elucidated, so as to simultaneously realize the high operating voltage and structural
26
stabilization.
of
lithium
substitution
of
Mn
sites
in
the
P2
structure
of
27
Herein, we propose a coherent modification strategy to ameliorate the
28
electrochemical performance of the layered P2/O3 NaLiNMC structure at the high
29
operating voltage range. The deliberate control over the compositional stoichiometry
1
(Li occupancy) and microstructural features (P2/O3 mixture nanocrystallines with
2
optimal particle size) was realized via the sustainable recycling of the biomass as the
3
sacrificial framework. The transmission-mode operando X-ray diffraction (XRD)
4
documented the dynamic phasic transformation in the voltage region of 2.5-4.4 V,
5
suggesting the trace lithium doping has introduced the intergrowth O3 phase, and thus
6
enhanced the reversibility of the P2-O2 phasic transition upon the continued cycling.
7
The as-fabricated Fatsia Japonica- Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 (FJ-NaLiNMC, the
8
default calcining time set as 10h) cathode exhibited a reversible capacity of ~120.6
9
mAh g-1 at 0.5C, high rate behavior and average coulombic efficiency (CE) of ~99% for
10
100 cycles. To deliver a more informative demonstration, the FJ-NaLiNMC cathode
11
was paired with an as-developed ternary metal oxide anode and assembled into a full
12
cell prototype: a stable reversible capacity, high energy density of ~152.65 Wh kg-1 at a
13
power density of ~827 W kg-1 (5C) could be realized. Noted that the biomass carbon
14
(Fatsia Japonica) was introduced as a generic sacrificial framework for both the cathode
15
and anode synthesis with the precise control over the stoichiometric ratio, particle size,
16
phase purity, uniform cation distribution and microstructure. We hope this synthetic
17
strategy will promote the low cost, scalable production of multinary oxide electrode
18
materials in the practical application of SIBs.
19 20
2. Experimental Section
21
2.1 Synthesis of FJ-NaLiNMC hybrid composite
22
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 precursors were synthesized by co-precipitation
23
and calcination method. The cation ratio of Na/Li/Ni/Mn/Co in the as-designated
24
product of Fatsia Japonica- Na0.92Li0.40Ni0.73Mn0.24Co0.12O2 (FJ-NaLiNMC, the default
25
calcining time set as 10h) was maintained as 0.92:0.40:0.73:0.24:0.12. Briefly,
26
Ni(NO3)2 (99%, Sigma-Aldrich), Co(NO3)2 (99%, Sigma-Aldrich) and Mn(NO3)2
27
(99%, Sigma-Aldrich) were titrated into a 2 mol L-1 Na2CO3 (99%, Sigma-Aldrich)
28
solution. At the same time, the Fatsia Japonica (FJ) biomass carbon template was
1
added. After stirring for 12h, the resultant product was collected, thoroughly washed
2
and dried at 60°C overnight. The precursor was then grounded with Na2CO3 and LiNO3
3
(99%, Sigma-Aldrich) at the predetermined stoichiometric ratios. The mixture was
4
annealed in a muffle furnace at 530°C for 5h and then annealed 900°C for 10h with a
5
ramping rate of 5°C min-1. The reaction is depicted in Equation (1).
6
0.39 Na2CO3+0.40 LiNO3+ 1.22Na0.2Ni0.6Mn0.2Co0.1CO3+O2
(1)
7
Na0.92Li0.40Ni0.73Mn0.24Co0.12O2+1.61CO2+0.2N2+1.40O2
8
In the control experiments, the samples were fabricated without sacrificial
9
template framework (denoted as Pure-NaLiNMC) and without any lithium dopant
10
(denoted as FJ-NaNMC) while kept all other calcination parameters identical as
11
FJ-NaLiNMC Additionally, the samples fabricated at the different calcination time (5h,
12
10h and 20h) also were developed to investigate the influence on the morphology.
13
2.2 Synthesis of FJ-FeNiMnOx composite
14
The Fatsia Japonica-FeNiMnOx electrode was synthesized by a very simple
15
immersion-calcining process. The precursor metal nitrate was dissolved in deionized
16
water to obtain 0.5mol L-1 solution, then biomass template obtained in the previous step
17
was soaked in this solution for 12h. After being taken out, it was placed in a tube
18
furnace under argon atmosphere with 550°C calcined 2h, the final product designated
19
as FJ-FeNiMnOx.
20
2.3 Characterizations
21
The cation ratios in the composite was measured by inductively coupled plasma-optical
22
emission spectroscopy (ICP-OES), which was performed on Agilent 5110. Powder
23
X-ray diffraction (XRD) patterns were tested by transmission-mode X-ray
24
diffractometer (STADIP STOE) with a position-sensitive detector and Cu Kα1
25
radiation (1.5405 Å) scanning in the 2-Theta range of 10-70°, operating at 40 kV and 40
26
mA. Transmission-mode Operando XRD analysis was carried out by the same
27
equipment. Rietveld refinement was performed using GSAS-II. The particle size and
28
morphology of the samples were tested by field emission scanning electron microscopy
29
(SEM, Quanta 600 FEG) at 15 kV with energy-dispersive X-ray spectroscopy (EDS).
30
Transmission electron microscope (TEM, FEI Talos F200X) characterizations were
1
conducted at 200 kV. The X-ray photoelectron spectroscopy (XPS) spectra were
2
recorded with a spectrometer (Thermo escalab 250XI) having Al Kα radiation
3
(hv=1486.6 eV), the peak areas were integrated by a weighed least-squares fitting of
4
model curves. 7Li MAS NMR (I) 3 /2, 92.6% abundance) spectra were recorded on a
5
Chemagnetics Infinity operating at 155.4 and 233.2 MHz, respectively. The ex-situ XPS
6
measurements were performed using a PHI5500 spectrometer operated using a
7
monochromatic Al Kα radiation and an electron emission angle of 45°. For the ex-situ
8
XPS characterization of the FJ-NaLiNMC electrode at different potential stages,
9
Ar+-ion beam sputtering was performed for 20 mins for each sample at pressure of 20 ×
10
10−3 Pa with an emission current of 25 µA and beam energy of 4 kV. Raman spectrums
11
were taken on a Raman spectrometer (HORIBA, France) with 532 nm line of a
12
helium-neon as the excitation beam.
13
2.4 Electrochemical Measurements
14
Both the cathode and anode slurries were made by mixing the electrode material (90
15
wt.%) with acetylene black (5 wt.%) and polyvinylidene fluoride (PVDF) binder (5
16
wt.%) in an appropriate amount of 1-methyl-2-pyrrolidinone (NMP). After
17
homogenous mixing, the cathode and anode slurries were spread onto an aluminum and
18
copper foil separately, and then dried at 120°C overnight in a vacuum oven. Circular
19
slices with a diameter of 12 mm were punched from the dried foil as the working
20
electrode. CR2016 type half coin cells were assembled in a glove box with argon gas,
21
using sodium metal and glass fiber (GF/D Whatman) as reference electrode and
22
separator respectively. The electrolyte is 1M NaClO4 in propylene carbonate (PC) with
23
2 vol% fluorinated ethylene carbonate (FEC) as additive. The galvanostatic
24
charge/discharge tests were performed by Neware instrument (Shenzhen, China) at
25
room temperature within the voltage of 2.5-4.4 V (V versus Na+/Na). The Cyclic
26
voltammograms (CV) were performed on a CHI660e electrochemical workstation
27
(ChenHua Instrument Co., China) at a scan rate of 0.1 mV s−1 from 2.0 V to 4.4 V.
28
Electrochemical impedance spectroscopy (EIS) was tested on the same electrochemical
29
workstation in the frequency range between 100 kHz and 0.1 Hz. CR2016 type full coin
30
cells were prepared under the same condition, the mass ratio between cathode and
1
anode is about 1.2:1. Electrolyte and separator were the same as those used in the half
2
cells. The test was conducted between 4.4 V and 1.4 V (V versus Na+/Na).
3
Galvanostatic intermittent titration technique (GITT) was conducted on the Gamry
4
instruments (Interface 1000) with the cell charged/discharged at the current density of
5
10 mA g-1 for a pulse of 30 mins followed by a relaxation of 60 mins to approach the
6
steady state value. 1C corresponds to 110 mA g-1 for all the cell tests.
7 8
3. Results and Discussion
9
It has been a hot direction to synthesize the anode and cathode materials of SIBs
10
with excellent performance. We use biomass templating strategy to synthesize the
11
excellent cathode materials for SIBs. To validate the generic applicability of this
12
biomass templating strategy for regulating the pre-designed structural and
13
compositional features, we intentionally developed the Fatsia Japonica-FeNiMnOx
14
(FJ-FeNiMnOx) anode by optimizing the cation ratios in the precursor solution. We
15
further established a full cell prototype by paring FJ-NaLiNMC and FJ-FeNiMnOx. The
16
Schematic illustration was shown in Figure 1. The synthesis of the hybrid P2/O3
17
structure of FJ-NaLiNMC cathode began with the co-precipitation process of the
18
cation precursors with the aid of macroporous biomass framework (Figure S1). The
19
biomass precursor was sustainably recycled from the Fatsia Japonica (FJ) stem.
20
Acting as the sacrificial template, the hydroxyl groups that abundantly present on the
21
biomass framework anchored the various metal cations onto the surface, forming
22
“metal hydroxides” in a hydrocarbon matrix. During the ensuing solid-state reactions,
23
the metal hydroxides were converted into the sodiated ternary oxide with trace lithium
24
incorporation, at the same time the oxidative decomposition of the biomass
25
framework occurred. The morphology of the FJ- NaLiNMC hybrid composite was
26
approximately the reverse replication of the microporous structure of FJ biomass
27
carbon whereas the particle growth of the sodiated oxide was restrained by the gas
28
released from the thermal decomposition of the biomass template, which enabled the
29
particle size control of NaLiNMC under 800 nm with good crystallinity.
1
The cation ratio of Na/Li/Ni/Mn/Co in the as-prepared FJ-NaLiNMC hybrid
2
composite was determined as 0.90:0.40:0.72:0.24:0.12 by Inductively Coupled Plasma-
3
Optical Emission Spectroscopy (ICP-OES), which is in good accordance with the target
4
stoichiometry ratio. ICP-OES results of the control samples are shown in Table S1. The
5
X- ray diffraction (XRD) pattern of the as-synthesized FJ-NaLiNMC hybrid composite
6
is shown in Figure S2, in which most of the diffraction peaks clearly display the P2
7
phase. As compared to the sample prepared without lithium doping, namely the
8
FJ-NaNMC, the FJ-NaLiNMC hybrid composite shows the additional peak located in
9
18.4° that indexed to the O3 structure due to the lithium incorporation into the
10
lattice.[31,32] By comparing the crystallization degree and particle size (Figure S3),
11
we optimized the time duration of the calcination process as 10h. On the other hand, the
12
transmission- mode in-situ XRD was also conducted to document the real time phasic
13
transition of the solid-state reaction of the FJ-NaNMC composite upon the pre-set
14
ramping program, the result shows the onset crystallization temperature of the P2 phase
15
occurred at around ~530°C (Figure S4), in this sense, we purposely extended the
16
duration of the nucleation process at 530°C for 5h, thereby precisely promoting the
17
uniform distribution of the P2 nanocrystallines.
18
The layered P2 and O3 structures are schematically illustrated in Figure 2a-b.
19
Noted the MO2 (M=Ni, Mn, Co and Li) stacked layers arrange in the ABBA order of P2
20
structure, while the Na ions locate at the 2b sites in the trigonal prism of oxygen ions,
21
sharing the faces with two MO6 octahedrons.[33] However, O3 structure consists of a
22
cubic close-packed oxygen array in which sodium ions are accommodated at distinct
23
octahedral sites.[34] Based on the Rietveld refinement results of the XRD pattern of the
24
P2 FJ-NaNMC composite and the P2/O3 FJ-NaLiNMC hybrid composite (Figure 2c-d
25
and corresponding crystallographic lattice parameters in Table S2), the majority of the
26
doped lithium ions are positioned at the transition metal (TM) sites in the crystal
27
structure to maintain the P2 majority.[35] In comparison with the lattice parameters of
28
the P2 FJ-NaNMC, the parameters of both a and c decrease to the lower values (a from
29
2.8735(2) Å to 2.8639(1) Å, c from 11.0735(8) Å to 11.0336(3) Å), generating reduced
30
TMO2 units in c axis. As a result, the stabilization effect for the TM cations maintains the
1
structural robustness of the P2 phase. This observation is consistent with previous studies
2
including both the XRD refinement results, neutron diffraction and lithium nuclear
3
magnetic resonance.[31,34] Meanwhile, the trace amount of lithium ions occupies the
4
Na sites to generate a coexisting O3 structure (the relative mass content is estimated as
5
4.33% based on the refinement results), thereby slightly improving the retrievable
6
specific capacity for the hybrid composite without sacrificing the electrode kinetics.
7
To elucidate the doping effect on the structural features, the morphology of FJ-
8
NaLiNMC and FJ-NaNMC composites were compared in Figure 3. The transmission
9
electron microscope (TEM) image of the FJ-NaLiNMC hybrid composite, shown in
10
Figure 3a as an example, demonstrates the uniformly distributed nanoparticles ranging
11
from 500 to 800 nm. A scrutiny of the representative particle, as revealed by high
12
resolution transmission electron microscope (HRTEM) in Figure 3b-c, suggests the
13
interplanar lattice fringes spaced by 0.23 nm and 0.56 nm apart, corresponding to the
14
d-spacing values of (012) and (002) planes respectively, in the P2 structure. The
15
NaLiNMC composite synthesized without FJ as the sacrificial template (Pure-
16
NaLiNMC), in stark contrast, shows the obviously enlarged particles of ~5 um in
17
diameter (Figure S3c). This comparison clearly indicates the oxidative decomposition
18
of the biomass framework has delayed the particle growth rate. Energy dispersive
19
spectroscopy (EDS) mapping of the FJ-NaLiNMC hybrid composite, as shown in
20
Figure 3d-f, demonstrates the uniform distribution of Na, Ni, Mn and Co elements
21
across the particles, implying that the hydroxyl functional groups that abundant present
22
on the sacrificial biomass framework provide an effective anchoring effect for the
23
multinary cations.
24
The survey X-ray photoelectron spectroscopy (XPS) spectrum in Figure 4a reveals
25
the coexistence of O, Co, Na, Li, Ni and Mn elements in the FJ-NaLiNMC hybrid
26
composite. The core-level Na 1s spectrum exhibits the peak at the binding energy of
27
1071.0 eV that assignable to Na-O bond (Figure 4b). For the core-level O 1s spectrum as
28
shown in Figure 4c, the peak could be deconvoluted into the bimodal envelopes at 528.67
29
eV and 531.0 eV, assignable to the lattice oxygen and the coordinative interfacial oxygen
30
atoms, respectively.[36] As shown in the core-level Ni 2p (Figure 4d), the binding
1
energies of Ni 2p3/2 and Ni 2p1/2 position at 854.0 eV and 871.8 eV respectively,
2
corresponding to the existence of Ni2+.[25] The Mn 2p3/2 and Mn 2p1/2 peaks (Figure
3
4e) are fitted to the characteristic peaks of the tetravalent Mn with binding energy values
4
positioned at 641.6 eV and 653.4 eV, respectively.[37] The core-level Co 2p spectrum
5
exhibits the Co 2p3/2 and Co 2p1/2 (Figure 4f) peaks at the binding energies of 780.8 eV
6
and 795.1 eV, suggesting the existence of trivalent Co in the as-synthesized oxide
7
cathode.[33]
8
Figure 5a exhibits the cyclic voltammograms (CV) curves of FJ-NaLiNMC
9
cathode during the 1st, 2nd (pristine electrode after battery assembly) and 80th
10
(post-cycled electrode after 80th galvanostatic charge/discharge cycles) at a scan rate of
11
0.1 mV s-1 within the voltage range of 2.5-4.4 V. The CV curves show multiple redox
12
peaks and superimposable curves upon the cycling. The redox peak couple, within the
13
range of 2.5-3.2 V (Square I), involves the reversible faradic reaction of Mn4+/Mn3+
14
couple.[38,39] Reversible peaks between 3.2 and 4.1 V can be attributed to the redox
15
reaction of Ni2+/Ni3+ and Ni3+/Ni4+ couples (Square Ⅱ), at the same time the partially
16
vacancy ordering process via the Na+ occupancy is also involved within this
17
process.[25,27,30] Whereas the faradic process at the high-voltage range (4.1-4.4 V,
18
Square ⅡI) can be attributed to the oxygen loss and structure rearrangement process
19
from P2 to O2 phase.[39] Meanwhile, some Li ions migrate from the TM layer to the
20
Na layer at high voltage, and yet this process is highly reversible.[31] In order to ensure
21
the electrochemical reaction during charge/discharge process, ex-situ XPS and ex-situ
22
MAS NMR were applied to evaluate the different charge/discharge voltage. Ex-situ
23
MAS NMR spectra was shown in Figure S5, we can assign the resonances of ca. 1688
24
ppm and 1450 ppm to Li sites within the TMO2 layer; while the NMR resonances
25
positioned at 543, 563 and 715 ppm can be assigned to octahedrally distorted sites in
26
the Na layer. Therefore , the 7Li MAS NMR spectrum of the FJ-NaLiNMC cathode at
27
pristine state confirm the two types of the spatial occupancy of Li+; Upon the charge
28
process till 4.4 V, the resonances which represent the Li sites within the TMO2 layer
29
disappear, while the dominant resonances at ca. 0 and 95 ppm indicate the migration of
30
Li ions to the tetrahedrally distorted sites in the Na layer instead. (Chemistry of
1
Materials, 2014, 26, 1260-1269; Journal of the American Chemical Society, 2001, 123,
2
11454-11461.) [31,41] Figure S6 exhibits the ex-situ XPS spectra of Mn 2p and Ni 2p
3
at the selected discharge and charge potentials. The electrochemically reduced trivalent
4
manganese appears (BE value negatively shifted from 641.8 to 641.1 eV) when the
5
electrode was reduced from 2.8 V (Figure S6, point Ⅱ) to 2.0 V (Figure S6, point
6
Ⅱ).Afterwards, the Mn3+ species in the electrode were re-oxidized to tetravalent
7
valence during the anodic sweep from 2.0 V (Figure S6, point Ⅱ) to 2.9 V (Figure S6,
8
point Ⅱ) and maintained till 3.9 V (Figure S6, point Ⅱ). On the other hand, tetravalent
9
nickel only appears (BE value positioned at 854.8 eV) during the anodic sweep to 3.9 V
10
(from point Ⅱ to point Ⅱ), while the divalent nickel retains the valence state from point
11
Ⅱ to point Ⅱ. These reversible Mn3+/Mn4+ and Ni2+/Ni4+ redox reactions in the different
12
voltage ranges are consistent with the above CV analysis results.
13
The rate capability of FJ-NaLiNMC and FJ-NaNMC cathode were evaluated at
14
the rates ranging from 0.2C to 5C with the sodium foil as the reference electrode. As
15
shown in Figure 5b, the FJ-NaLiNMC cathode exhibits the reversible capacities are
16
133.5, 121.7, 104.4, 92.5 and 80.7 mAh g-1, at 0.2C, 0.5C, 1C, 2C and 5C, respectively.
17
Even as the C rate returns to 0.2C, the discharge capacity could deliver the initial value
18
of 129.3 mAh g-1 and maintain stable capacity in the subsequent cycles, suggesting the
19
satisfactory rate behaviors of the FJ-NaLiNMC cathode. In comparison, the
20
FJ-NaNMC cathode exhibits the reversible capacities as 120.5, 105.7, 84.8, 59.0 and
21
32.3 mAh g-1, at 0.2C, 0.5C, 1C, 2C and 5C, respectively. On the contrary, the
22
FJ-NaNMC cathode exhibits the reversible capacities are 120.5, 105.7, 84.8, 59.0 and
23
32.3 mAh g-1, at 0.2C, 0.5C, 1C, 2C and 5C, respectively. Figure 5c shows the
24
overlapped charge/discharge curves of the FJ-NaLiNMC cathode during the 1st, 50th
25
and 100th cycles, suggesting the structural robustness of the cathode upon the cycling.
26
As indicated in Figure 5d, the FJ-NaLiNMC cathode maintains the discharge capacity
27
of ~120.6 mAh g-1 for 100 cycles at 0.5C, with the CE kept higher than 98% from the 3rd
28
cycle onwards. The FJ-NaNMC cathode delivers an initial capacity of 110.6 mAh g-1
29
with the capacity retention maintained as ~54% only after 50 cycles. The lower
30
reversible capacity and serious the capacity fading can be ascribed to the irreversible
1
P2-O2 phasic transition during high operating voltage, as the integrated O3 phase is
2
absent. For comparison purpose, the as-fabricated samples under the different synthetic
3
parameters (at the different calcination durations, varied compositional ratios and
4
synthesis without FJ biomass as the sacrificial precursor) were electrochemically
5
compared in Figure S7 and Figure S8. As shown in Figure S7, the FJ-NaLiNMC-5h
6
cathode (5h represents the calcination duration at 900°C) with the poor crystallinity
7
only shows the limited initial discharge capacity of 77.8 mAh g-1 with 43% capacity
8
retention after 100 cycles. FJ-NaLiNMC-20h cathode (20h represents the calcination
9
duration at 900°C) with the enlarged particle size, in stark contrast, also demonstrates
10
the interior retrievable capacity of 82.3 mAh g-1 and unsatisfactory capacity retention
11
for 100 cycles. Figure S8 shows the cycling performance of the Pure-NaLiNMC
12
cathode at 0.5C. Without FJ biomass as the sacrificial framework, the Na+ transport is
13
retarded due to the increased solid-state diffusion range that derived from the large
14
particle size, therefore the Pure-NaLiNMC cathode shows the initial discharge capacity
15
of 81.4 mAh g-1 with a compromised capacity retention of 48% after 100 cycles.
16
Previous studies investigated the effect of the upper limit voltage on the cycling
17
stability of the electrode. As shown in Figure S9, the cycling performance of both the
18
FJ-NaLiNMC and Pure-NaLiNMC cathodes were also evaluated within the voltage
19
range of 2.5-3.9 V at 0.5C to avoid the high voltage operation. The result shows that the
20
FJ-NaLiNMC cathode demonstrates a stable reversible capacity of 90 mAh g-1 for 100
21
cycles with an initial CE value of 93%. And the Pure-NaLiNMC cathode also remains a
22
stable capacity of 80 mAh g-1 for 100 cycles with an initial CE value of 87%.
23
Obviously, the reduced upper limit voltage has ruled out the capacity contribution that
24
stems from the P2-O2 phasic transition at the voltage higher than 4 V. Therefore, the
25
better cycling stability was realized at the penalty of the reduced retrievable capacity. In
26
this sense, the FJ-NaLiNMC cathode realizes the robust cycling performance and rate
27
behaviors at the high operating voltage range, suggesting the harmony balance of the
28
optimal particle size, crystallinity and mutually beneficial phasic coupling that derived
29
from the deliberate optimization of the calcination time and the compositional
30
stoichiometric ratio of the hybrid composite.
1
The improved rate capability in FJ-NaLiNMC electrode suggests faster charge
2
transport kinetics, thus, we conducted Galvanostatic intermittent titration technique
3
(GITT) to investigate the Na+ diffusion coefficient of cycled FJ-NaLiNMC and
4
FJ-NaNMC electrodes. During the single galvanostatic titration process (Figure S10a
5
and S10c, Figure S11a and S11c), the obvious linear relationship can be observed
6
between the voltage and the square root of the galvanostatic time (Figure S10b and S10d,
7
Figure S11b and S11d). The plot of Na+ diffusivity as a function of voltage during the
8
charge and discharge process is shown in Figure 6 (Details of GITT analysis can be found
9
in the Supporting Information). Overall, the values of D for FJ-NaLiNMC is higher than
10
FJ-NaNMC in general. These results reveal the lithium incorporation will facilitate
11
electrode kinetics and enhance the rate capability performance.
12
Figure S12 shows the electrochemical impedance spectra (EIS) results of
13
FJ-NaLiNMC and Pure-NaLiNMC cathodes both after the initial cycle and
14
post-mortem (after 50 cycles). The Nyquist plots consist of 1) a semicircle which
15
suggests the charge transfer resistance (Rct) and 2) a quasi-straight line, with the slope
16
positively correlating with the Warburg diffusion coefficient in the solid state.[42] As
17
shown in Figure S12a, the Pure-NaLiNMC cathode demonstrates the obvious enlarged
18
semicircle after 50 cycles, suggesting the increase of Rct, while the reduced slope of
19
Warburg diffusion region also indicates the retarded ion diffusion in the solid-state.
20
Compared to the Pure-NaLiNMC electrode, the FJ-NaLiNMC cathode does not
21
demonstrate the obvious change for either the Rct or the slope of Warburg diffusion
22
region, as illustrated in Figure S12b. This comparison indicates the lithium
23
incorporation in the structural lattice has enhanced the charge transfer kinetics at the
24
electrode/electrolyte interface and eased the solid-state lithium diffusion.
25
To track the dynamic structural evolution of the FJ-NaLiNMC cathode upon the
26
sodiation and desodiation process, transmission-mode operando XRD tests were
27
conducted to document the real-time phasic changes of the cathode structures,
28
especially upon the high operating voltage as shown in Figure 7. The FJ-NaLiNMC
29
cathode exhibits the (002) diffraction peak of the P2 phase positioned at 15.3°. Upon
30
the initial desodiation process until the high voltage of 4.2 V, this diffraction peak does
1
not exhibit obvious variation in terms of the peak intensity and the 2-Theta position
2
(Figure 7a-b), with the further sodium extraction from the structural lattice above 4.2
3
V, the peak intensity of (002) diffraction peak suddenly declined, involving the
4
structural transition from P2 phase to O2 phase. Upon the sodiation process, the peak
5
intensity of (002) diffraction peak gradually restored as the electrode discharge below
6
4.2 V. This phasic transition involving the P2-O2 phasic transition remained reversible
7
even after 60 cycles with the reappearance of the identifiable (002) diffraction peak
8
(Figure 7c). The transmission-mode operando XRD results suggest the highly
9
reversible P2-O2 phasic transition upon the high operating voltage of the FJ-NaLiNMC
10
cathode. In comparison, the FJ-NaNMC cathode without the lithium doping
11
demonstrates the obvious mitigated (002) peak upon the cycling. As shown in Figure
12
7d-e, this diffraction peak almost disappeared as the FJ-NaNMC cathode was charged
13
higher than 4.0 V. This observation clearly implies the compromised structural
14
robustness upon the continuous cycling, accounting for the drastic structural distortion
15
of P2 phase upon the high voltage cycling. After 60 cycles within the range of 2.5-4.4
16
V, we clearly observe the mitigated (002) diffraction peak with the gradually reduced
17
peak intensity (Figure 7f). Therefore, the FJ-NaNMC cathode demonstrates the
18
irreversibility of the P2-O2 phasic transition upon the high operating voltage, and this
19
reversible process accompanies with the compromised capacity retention for the
20
cathode.
21
The post-cycling TEM analysis was also conducted as the FJ-NaLiNMC cathode
22
was disassembled from the half cell at the fully discharged state (2.5 V). As shown in
23
Figure 8a, the formation of a well-crystallized region of the fully discharged electrode
24
(2.5 V) was observed on the cathode particle with the lattice fringes spaced by 0.333
25
nm, corresponding to (002) planes of the P2 structure. As shown in Figure 8b, the
26
selected area electron diffraction (SAED) pattern further demonstrates the diffraction
27
points that are indexed to (100), (010) and (110) of the P2 structure. This observation
28
suggests the well preservation of the robust P2 structure after 60 cycles of reversible
29
P2-O2 phasic transition, which is consistent with the operando XRD results. In
30
comparison, the HRTEM and SAED pattern of FJ-NaNMC cathode at the fully
1
discharged state (2.5 V) are shown in Figure 8c-d. Noted the spacing in-between the
2
crystal faces are not discernable, demonstrating an irreversible P2-O2 phasic transition
3
occurred after 60 cycles. Meanwhile, the SAED pattern of the post-cycled FJ-NaNMC
4
cathode indicates its polycrystalline characteristic due to the structure collapse.
5
Obviously, the heterophane was irreversibly generated after 60 cycles, leading to the
6
deterioration of the electrochemical performance.
7
The full cell prototype model through pairing the FJ-FeNiMnOx anode and
8
FJ-NaLiNMC cathode. The characterization and electrochemical measurements of the
9
FJ-FeNiMnOx oxide anode via half cells were elaborated in Figure S13-S14. Figure 9a
10
records the galvanostatic charge-discharge curves for the full cell measured at 0.5C.
11
The charge-discharge curves during the 1st, 50th, 100th cycles almost overlapped, while
12
the reversible charge and discharge capacities maintained at ~100 mAh g-1 for 100
13
cycles. The cycling performance of the full cell was evaluated at 0.5C (Figure 9b), the
14
first discharge and charge capacities were recorded as 105.4 mA h g−1 and 112.1 mA h
15
g−1, leading to an initial CE value as ~95%. Afterwards, the CE value maintained higher
16
than 98% during the subsequent 100 cycles. After a total 100 cycles, the reversible
17
capacity still recovered to 102.97 mAh g-1 with 98% capacity retention. As displayed in
18
Figure 9c, the full cell demonstrates an impressive rate behavior of 123.2, 103.4, 92.6,
19
83.5 and 72.3 mAh g-1 at 0.2, 0.5, 1, 2 and 5C, respectively. In sharp contrast, the full
20
cell assembled by paring the FJ-NaNMC cathode and FJ-FeNiMnOx anode
21
demonstrates an inferior cycling stability and rate capability, the capacity decayed from
22
107.8 to 29.8 mAh g-1 only after 30 cycles (Figure S15). Based on the calculation of the
23
supplementary note, the FJ-FeNiMnOx and FJ-NaLiNMC full cell prototype realizes a
24
gravimetric energy density of ~152.65 Wh kg-1 at a power density of ~827 W kg-1
25
(5C).[43] In this sense, our as-developed full cell model which derived from the
26
biomass framework realizes the simultaneous structural robustness, balanced
27
high-power/energy densities at a low production cost.
28 29
4. Conclusions
1
In summary, we have synthesized a layered oxide cathode of the P2/O3 hybrid
2
composite through the co-precipitation and subsequent solid reaction process. By the
3
aid of transmission-mode operando XRD technique, we could precisely document the
4
real-time phasic transition of the solid-state reaction and the dynamic peak shift upon
5
the galvanostatic cycling process, enabling our deliberate compositional engineering of
6
the lithium incorporation into both the TM sites and the Na sites to facilitate the
7
intergrowth of O3 phase. Electrochemical evaluations of the FJ-NaLiNMC cathode
8
prove the simultaneous realization of the high retrievable capacity, structural robustness
9
and rate behaviors in both the half and full cell models, validating the pivotal role of the
10
enhanced reversibility of the phasic transition upon the high operating voltage.
11
Moreover, the sustainable recycling of the biomass framework for the cathode/anode
12
designs in this study demonstrates the generic applicability of our synthetic protocol to
13
regulate the structural and compositional features of the multinary oxide, and thus
14
exhibits the tremendous potential in the developments of high-performance SIBs.
15 16 17
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version
18 19
at……
20 21
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1
Figures and Figure Caption
2 3
Figure 1. Schematic illustration of (a) the design FJ-NaLiNMC and FJ-FeNiMnOx
4
composite and (b) the FJ-NaLiNMC||FJ-FeNiMnOx full cell prototype model.
5
1 2
Figure 2. Schematic illustration of the layered structure of (a) P2 and (b) O3. The
3
Rietveld refinement of the reference phases for the (c) FJ-NaNMC composite and (d)
4
FJ-NaLiNMC hybrid composite.
1 2
Figure 3. (a) TEM image, (b-c) HRTEM images of FJ-NaLiNMC hybrid
3
composite. (d) SEM images of FJ-NaLiNMC hybrid composite. The
4
energy-dispersive X-ray spectroscopy of FJ-NaLiNMC particle with the elemental
5
maps of (e) Na, (f) Ni, (g) Mn and (h) Co respectively.
6
1 2
Figure 4. (a) Survey XPS spectrum of FJ-NaLiNMC hybrid composite. (b) Na 1s and
3
(c) O 1s core-level XPS spectrum of FJ-NaLiNMC hybrid composite. (d) Ni 2p,
4
(e) Mn 2p and (f) Co 2p core-level XPS spectrum of FJ-NaLiNMC hybrid
5
composite fitted with multiple envelopes.
6
1 2
Figure 5. (a) The 1st, 2nd and 80th CV curves of the FJ-NaLiNMC cathode at a scan
3
rate of 0.1 mV s-1 within the voltage range of 2.5-4.4 V. (b) The rate capability at
4
different rates of 0.2C, 0.5C, 1C, 2C and 5C for FJ-NaLiNMC and FJ-NaNMC
5
cathodes. (c) The 1st, 50th, and 100th charge–discharge curves of the FJ-NaLiNMC
6
cathode at 0.5C within the voltage range of 2.5-4.4 V. (d) The cycling performance
7
and the CE of the FJ-NaLiNMC and FJ-NaNMC cathodes at 0.5C.
8 9 10 11 12 13 14
1 2
Figure 6. GITT profiles and Na+ diffusivity of FJ-NaLiNMC cathode for (a) charge and
3
(b) discharge process; GITT curves and Na+ diffusivity of FJ-NaNMC cathode for (c)
4
charge and (d) discharge process.
5
1 2
Figure 7. (a) The electrochemical load curves during operando XRD measurement of
3
the phasic changes of the FJ-NaLiNMC cathode and colour maps of the 2nd (after initial
4
charge/discharge on the battery testing system) complete electrochemical cycle. (b)
5
Operando XRD patterns of the 2nd and (c) 60th (after 60 cycles charge/discharge on the
6
battery testing system) of the FJ-NaLiNMC cathode during galvanostatic charge and
7
discharge within the range of 2.5-4.4 V at 0.2C. (d) The corresponding electrochemical
8
load curves during operando XRD measurement of the phasic changes of the FJ-
9
NaNMC cathode and color maps of the 2nd complete electrochemical cycle. (e)
10
Operando XRD patterns collected during the 2nd and (f) 60th of the FJ-NaNMC cathode
11
during galvanostatic charge and discharge within the range of 2.5-4.4 V at 0.2C.
1 2
Figure 8. (a) The HRTEM image and (b) SAED pattern of FJ-NaLiNMC cathode
3
at the fully discharge state (2.5 V) after 60 cycles. (c) The HRTEM image and (d)
4
SAED pattern of FJ-NaNMC cathode at the fully discharge state (2.5 V) after 60
5
cycles.
1 2
Figure 9. (a) The charge–discharge profiles of the FJ-NaLiNMC||FJ-FeNiMnOx
3
full cell in different 1st, 50th, and 100th cycles at 0.5C within the voltage range of
4
1.4–4.4 V. (b) The cycling performance at 0.5C and (c) The rate capability at
5
different rates of 0.2C, 0.5C, 1C, 2C and 5C of FJ-NaLiNMC||FJ-FeNiMnOx full
6
cell. (d) Ragone plot of the SIBs full cell models based on the cathode and anode
7
mass.
8 9
Conflict of interest statement The authors declare no competing financial interest.
Acknowledgements We acknowledged the financial support of this work by the National Natural Science Foundation of China (51602261 and 51711530037), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.160-QP2016), the Natural Science Foundation of Shaanxi Province (2018JM5116), the Fundamental Research Funds for the Central Universities (3102019JC005), and Young Talent fund of University Association for Science and Technology in Shaanxi, China. Furthermore, we would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for providing some test equipment.