visualizing the growth process of sodium microstructures ...10.1038/s41565-020-074… · the sum...
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Articleshttps://doi.org/10.1038/s41565-020-0749-7
Visualizing the growth process of sodium microstructures in sodium batteries by in-situ 23Na MRI and NMR spectroscopyYuxuan Xiang1, Guorui Zheng1, Ziteng Liang1, Yanting Jin 2, Xiangsi Liu1, Shijian Chen1, Ke Zhou1, Jianping Zhu1, Min Lin1, Huajin He1, Jiajia Wan1, Shenshui Yu1, Guiming Zhong 3 ✉, Riqiang Fu 4, Yangxing Li5 and Yong Yang 1 ✉
1State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, China. 2Department of Chemistry, University of Cambridge, Cambridge, UK. 3Xiamen Institute of Rare Earth Materials, and Fujian Institute of Research on the Structure of Matter, Haixi Institutes, Chinese Academy of Sciences, Xiamen, China. 4National High Magnetic Field Laboratory, Tallahassee, FL, USA. 54135 Belle Meade Circle, Belmont, NC, USA. ✉e-mail: [email protected]; [email protected]
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Supplementary Information
Visualizing the Growth Process of Sodium Microstructures in sodium
batteries by in-situ 23Na MRI and NMR Spectroscopy
Yuxuan Xiang†, Guorui Zheng†, Ziteng Liang†, Yanting Jin§, Xiangsi Liu†,
Shijian Chen†, Ke Zhou†, Jianping, Zhu†, Min Lin†, Jiajia Wan†, Shenshui Yu†,
Guiming Zhong‡,*, Riqiang Fu△, Yangxing Li^, Yong Yang†,*
Supplementary Information includes:
1. Supplementary Figures 1-14
2. Supplementary Tables 1-2
3. Supplementary discussion
Supplementary Fig. 1 | Coulombic efficiency of Na||Cu cells using electrolytes of 1
M NaClO4 in various solvents, at a current density of 0.5 mA cm-2 and a fixed area
capacity of 0.5 mAh cm-2.
Supplementary Fig. 2 | The deposition overpotential of Na||Cu cells cycled in F0
electrolyte with different volumes.
Supplementary Fig. 3 | The schematics of in-situ MRI and operando NMR set-
ups. (a1/b1) The schematics and (a2/b2) the assembly drawings of in-situ MRI and
operando cells in the NMR spectrometer. Both cells are made of polyether-ether-
ketone (PEEK) material. (a3/b3) The partially enlarged views show the schematic
diagrams of the Na||Cu MRI cell and Na||Na operando cell as well as their orientation
to the external magnetic field B0. (a4/b4) The digital images of in-situ MRI and
operando cells.
Supplementary Fig. 4 | One-dimensional 23Na spectra extracted from the 23Na
MRI experiment for F0 electrolytes shown in Figure 3 in the main text. 1st C and
1st D indicate the charged and discharged states in the first cycle. The distance, d, is
the separation between the measured slice and the Cu foil.
Supplementary Fig. 5 | (a) The voltage profiles of Na||Cu in-situ cell with F0
electrolyte and (b) corresponding median overpotential.
Supplementary Fig. 6 | Detailed 23Na NMR spectra and the 3D views of the
operando results for Na metal cycled in (a1-a4) F2 and (b1-b4) F0 electrolytes.
The NMR dataset corresponds to the NMR data shown in Figure 4 in the main text.
The line shape and intensity of the Na metal signals in F2 electrolyte barely changes
during cycling. In contrast, sample in F0 electrolyte has an asymmetric broadening at
downfield, which can be well-fitted by two peaks at ~1142 ppm and ~1162 ppm that
are highly relevant to the different morphologies of SMSs. Of note, the SMSs peak
centered at around 1162 ppm (denotes as grey) can be observed in both electrolytes
at OCV state, which is ascribed to the fact that sodium metal electrodes are not
completely flat. The phenomenon is also observed in the previous-reported 7Li MRI
results. 1 The integral analysis is presented in Figure 4.
Supplementary Fig. 7| (a) 1D MRI 23Na spectra at d=0.09, d=0.18, d=0.27 mm, and
their sum spectrum, which includes the total SMSs signal in the Na||Cu cell. (b)
Deconvolution results of the sum spectrum. The raw data is in grey; the overall fitted
spectrum is colored in red; each component is shaded. (c) Voltage-capacity profiles of
operando Na||Cu and Na||Na cells in the first cycle. The operando 23Na NMR spectra
of (d) Cu||Na and (e) Na||Na cell after 15 cycles.
It’s worth mentioning that the operando NMR spectra displayed in Figure 4 represent
the sum signal of deposition SMSs, dead SMSs, and bulk sodium metal, while a 1D
MRI spectrum shown in Figure 3 only present the SMSs signal at that position. We
added up the 1D MRI spectra from 0.09 mm to 0.27 mm which belong to the signal of
SMSs from Figure S4 to obtain a sum spectrum, where we clearly observe the
asymmetric broadening at 1160 ppm and 1138 ppm, as shown in Figure S7a. The
sum spectra can be well-fitted by two peaks centered at 1160 and 1138 ppm, as shown
in Figure S7b. The deconvolution results are in good agreement with the operando
NMR spectra in Figure S6-b2, demonstrating the credibility of the MRI measurement.
One of the differences between the MRI and operando NMR experiments is their
relative peak intensities, which leads to slightly different contour plots presented in
Figure 3 and 4. We notice that the relative intensity of the SMSs signal in the sum
spectrum obtained from MRI experiment (Figure S7b) is smaller in comparison to the
operando NMR spectra (Figure S7e), which can be ascribed to the following
experimental variables of MRI and operando NMR.
1. Cell configuration. Less capacity is involved in the formation of SMSs in the
Na/Cu MRI cell for every single cycle, as shown in Figure S7c. To verify the
influence of cell configuration, we acquired the operando NMR on the Cu/Na
cell and found that the depressed SMSs signal is similar to the sum spectra in
MRI results, as shown in Figure S7d.
2. The pulse sequence. In the MRI pulse sequence used in this experiment, an
additional ramp down time of 1 ms is employed to switch off the gradient field,
during which the magnetization aligns at the B0 direction and would decay
according to the spin-lattice relaxation:
�(�) = ��exp (−�/��)
The T1 value of sodium metal is around 10 ms as measured by saturation
recovery. Thus, during the ramp down time, there will be approximately 10%
signal loss, which may contribute to the damped SMSs signal.
In summary, the cell configuration and pulse sequence employed in the MRI
measurement lead to a relatively weaker SMSs signal. Moreover, the MRI spectrum
only shows the sliced signal of the entire battery, which makes the weak signal at 1162
ppm difficult to be observed. It is worth noting that despite such differences between
MRI and operando NMR, both results have revealed a similar three-stage failure
mechanism and electrochemical response, which suggests that the failure mechanism
we observed is rational and universal.
Supplementary Fig. 8 | 23Na MAS NMR spectra of SEI species collected from Cu foil
after 50 cycles, as well as the spectra of reference compounds: NaF, NaH, NaHCO3,
NaOH and Na2CO3.
Supplementary Fig. 9 | The thickness of dead SMSs measured by (a) SEM and (b)
23Na MRI after 15 cycles.
The thickness of dead Na measured by SEM and MRI is very similar, confirming the
spatial resolution of the MRI imaging method.
Supplementary Fig. 10 | XPS spectra of the surface species on the Na metal. The
Na||Na cells were cycled with 0.5 mAh cm-2 at 0.5 mA cm-2 for 50 cycles in 0% FEC
and 2% FEC electrolytes.
Supplementary Fig. 11 | Pulse program designed for 23Na MRI experiment. The
ramp-up time and stable time of gradient field are set to be 1500 us and 500 us,
respectively. A Hahn-echo pulse was executed to excited the 23Na MRI signal,
followed by a 90° pulse to switch the magnetization to the B0 direction for avoiding the
fast decay of magnetization due to the spin-spin relaxation process. The ramp down
time of 1000 us was optimized to avoid any interference of eddy currents.
Supplementary Fig. 12 | The Density of state (DOS) for NaF and NaH.
Supplementary Table 1| T1 relaxation time of surface species in F0 and F2
electrolytes.
18.8 ppm (NaH) 7.2 ppm (NaF) -11.0 ppm
F0 6.01 s \ 0.0103 s
F2 7.42 s 5.04 s 0.0105 s
Supplementary Table 2 | Possible reaction process of NaH and correspond
normalized reaction energy.
Reaction Equation Normalized reaction
energy(ev)
0.667Na+0.333NaOH = 0.333NaH+0.333Na2O -0.01
0.667Na+0.333CH3ONa = 0.667NaH+0.333NaOH+0.333C -1.01
0.857Na+0.143NaHCO3 = 0.143NaH+0.429Na2O+0.143C -0.33
0.909Na+0.091CH3CH2OCOONa =
0.455NaH+0.273Na2O+0.273C -0.48
0.667Na+0.333H2 = 0.667NaH -0.26
0.8Na+0.2CH4 = 0.8NaH+0.2C -0.06
The increasing signal of SMSs
The key to explain the increasing signal of SMSs is to compare the NMR signal
generated by bulk sodium metal (�����)and SMSs (�����), which can be calculated by
the following equations:
����� = ����� × �� (1)
����� = ����� × �� (2)
Where ����� and ����� are the volume of bulk sodium metal and SMSs, respectively.
�� represent the signal per unit volume of the sodium metal.
(i) Calculation of �����.
According to the skin effects, which describe the radio frequency (�(�)) could undergo
an exponential decay as a function of depth (�) in the bulk metal,
�(�) = ������ (3)
The radio frequency can only penetrate a certain depth (�) of bulk metal. This depth is
so-called “skin depth”, which can be determined by the following equation2:
����� =1
�����
�
����
(4)
Where �� represents the permeability of the vacuum, � represents resistivity of sodium,
�� is the relative permeability of sodium and �� is the frequency of the RF field (105.8
MHz in this study). In our experiment, the ����� is calculated to be 10.7 µm for sodium
metal, indicating only the surface part with depth around 10 µm of the bulk sodium
metal can be excited by the radio frequency (marked as orange) in this experiment, as
shown in the following schematic figure.
In this case, the ����� is not the actual volume of sodium metal disk, while it should
be calculated by the method developed by Bhattacharyya et al (Nature Material 9,
504–510 (2010).) and Chandrashekar (Nature Material 11, 311–315 (2012))
����� =��
��� ��
�
�
sin���(�)��� �� (5)
Where, �� is the surface area of sodium metal disk, �� is the strength of the applied
radio frequency and �� is the duration of the radio frequency pulse. In our work, a 90°
flip angle was optimized to maximize the NMR signal of sodium metal. Thus, the
product of �� and �� is �/2. And we used 5 mm ✕ 5 mm ✕ 0.1 mm sodium metal disk
in this study. The surface area of bulk sodium metal can be calculated:
�� = 2��� + 2��ℎ = 40.82 ��� (6)
Thus, ����� and ����� are obtained:
����� = ����� × �� = 0.64 ����� = 2.79 × 10�� �� (7)
(ii) Calculation of �����.
Supplementary Fig. 13| SME images of SMSs after 20 cycles in the Na||Na cell.
We first performed SEM measurements on the surface of Na metal after 20 cycles in
the Na/Na cell. The SMSs show a diameter of around 0.2 um, which is much smaller
than the skin depth �����. Therefore, we believe that the SMSs can be fully penetrated
by the radio frequency. In this case, the ����� is the actual volume of SMSs. which can
be calculated by:
����� =�����
�(8)
where ����� is the mass of SMSs and � is the density of sodium metal. However, the
mass of SMSs in the battery is difficult to determine accurately, as we presented in the
introduction part of main text.
Here, we consider a special case, that is, the same quantity of SMSs and bulk sodium
metal, to compare the NMR signal of the two:
����� = ����� (9)
����� =�����
�=
�����
�=
�������
�= �����
� (10)
Where the ������ is the actual volume of 5 mm ✕ 5 mm ✕ 0.1 mm sodium metal disk:
������ = ℎ��� = 0.1 × 3.14 × 2.5 × 2.5 = 1.96 × 10�� ���� (11)
Thus,
����� = ����� × �� = 1.96 × 10�� �� (12)
According to the Equ.7 and Equ.12,
����� = 7.0 ����� (13)
Same quantity of SMSs will lead to 7 times NMR signal over bulk sodium metal.
Therefore, when a large amount of bulk sodium metal is converted into SMSs with the
progress of cycling, the signal of SMSs is reasonable to be observed over bulk sodium
metal.
In addition, Bhattacharyya et al (Nature Material 9, 504–510 (2010).) and
Chandrashekar (Nature Material 11, 311–315 (2012)) have proposed a method to
calculate the signal intensity of Li metal microstructures using 7Li NMR. The we applied
the same method to derive the 23Na NMR signal from Na metal microstructure here
denotes as the ������
������ = 0.64 ��� �����
�(�) − �(0)
�(0)(14)
Where � is the surface area of sodium metal, ����� is the skin depth, �(�) is the NMR
signal of sodium metal at time � and �(0) is the initial signal of sodium metal. Take
t=40 hours as an example to calculate the ������ . From Figure 5c, we can calculate
the value of �(�)��(�)
�(�) , which is equal to 0.83. Thus, the �����
� is accessible:
������ = 0.64 ��� �����
�(�) − �(0)
�(0)
= 0.64 × 40.82 × 10.7 × 0.83 × 10�� ��
= 2.30 × 10�� �� (15)
The comparable value of ������ and ����� also demonstrate the credibility of our
operando NMR results and related discussions.
In addition, the dominating signal of metal microstructures has been reported in the
lithium counterpart by 7Li operando NMR and MRI.1-3 In the research of lithium metal
anode by MRI, Chandrashekar et al thought that :” The total NMR signal acquired after
several charging cycles was shown to be proportional to the acquired mass of metallic
microstructure”.3 we also performed the operando NMR experiment of lithium metal
and compared it with the reported results (Nature Material 9, 504–510 (2010).).2
Similarly, in Figure S13, we found such the signal (denotes as dot line) ascribed to the
lithium metal microstructures is increasing constantly. At the end of cycles, this
intensity of the signal even exceeds that of the bulk lithium metal signal. This indicates
that the dominant signal generated by the metal microstructure should be a general
phenomenon for Li and Na metal electrodes.
Supplementary Fig. 14| a Operando 7Li NMR spectra of Li/Li symmetric cells
using LiPF6/ EC:EMC (3:7 by volume). b operando 7Li NMR spectra of Li/Li
symmetric cells using an ionic–liquid electrolyte (C2mim BF4 CLiBF4) and a VC
additive.2 Copy right by Nature publishing group.
In summary, the above discussions clearly demonstrate that the signal intensity of
the continuously formed of SMSs can be comparable with the signal from the bulk
sodium metal on account of the “skin effect”, which support our observations and
explanations.
Reference
1 Chang, H. J. et al. Correlating Microstructural Lithium Metal Growth with
Electrolyte Salt Depletion in Lithium Batteries Using 7Li MRI. Journal of the
American Chemical Society 137, 15209-15216 (2015).
2 Bhattacharyya, R. et al. In situ NMR observation of the formation of metallic
lithium microstructures in lithium batteries. Nature Materials 9, 504-510 (2010).
3 Chandrashekar, S. et al. 7Li MRI of Li batteries reveals location of microstructural
lithium. Nature Materials 11, 311-315 (2012).