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: gmzhong@fjirsm.ac.cn; yyang@xmu.edu.cn

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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).