Articleshttps://doi.org/10.1038/s41563-020-0673-0

Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytesDmitrii A. Rakov1,2, Fangfang Chen   1,2 ✉, Shammi A. Ferdousi1, Hua Li3,4, Thushan Pathirana1, Alexandr N. Simonov   5, Patrick C. Howlett   1,2, Rob Atkin3 and Maria Forsyth   1,2 ✉

1Institute for Frontier Materials, Deakin University, Geelong, Victoria, Australia. 2ARC Centre of Excellence for Electromaterials Science (ACES), Deakin University, Burwood, Victoria, Australia. 3School of Molecular Sciences, University of Western Australia, Crawley, Western Australia, Australia. 4Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley, Western Australia, Australia. 5School of Chemistry and the ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria, Australia. ✉e-mail: fangfang.chen@deakin.edu.au; maria.forsyth@deakin.edu.au

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Supplementary Information for

Engineering high energy density Na battery anodes for improved

cycling with superconcentrated ionic liquid electrolytes

Dmitrii A. Rakov1,2, Fangfang Chen1,2*, Shammi A. Ferdousi1, Hua Li3, Thushan

Pathirana1, Alexandr N. Simonov4, Patrick C. Howlett1,2, Rob Atkin3, Maria Forsyth1,2*

1Institute for Frontier Materials, Deakin University, Geelong, VIC 3217, Australia. 2ARC

Centre of Excellence for Electromaterials Science (ACES), Deakin University, Burwood, 3125,

Australia. 3School of Molecular Sciences, University of Western Australia, 35 Stirling

Highway, Crawley, 6009, Australia. 4School of Chemistry, Monash University and the ARC

Centre of Excellence for Electromaterials Science, Clayton, 3800, Australia. Correspondence

and request for materials should be addressed to M. F. (email: maria.forsyth@deakin.edu.au)

and F. C. (email: fangfang.chen@deakin.edu.au).

Computational Details

The neat IL system consist of 216 C3mpyrFSI ion pairs (IPs) which are packed in a cubic

simulation box randomly using Packmol code1. The IL/salt systems are prepared by replacing

22 and 108 C3mpyrFSI with NaFSI salt for two salt concentrations of 10 and 50 mol%,

respectively. All IL systems are equilibrated firstly at both 293, 348 and 393 K for more than

10s ns using the NPT ensemble and the Nose-Hoover and Parrinello-Rahman methods for

temperature and pressure coupling. The densities were calculated through additional 7 ns MD

trajectory. The pressure is set at 1 bar. The electrostatic interactions were computed using PME

methods. A FTT grid spacing of 0.1 nm and cubic interpolation for charge distribution were

used to compute electrostatic interaction in reciprocal space. The cut-off distance of 1.2 nm

was adopted for electrostatics and Van der Waals interactions. The LINCS algorithm was used

to constrain the C-H bond. The Velocity Verlet integrator was adopted with a time step of 1 fs.

Trajectories were analysed by MDAnalysis code whenever the respective tool was unavailable

in GROMACS2. The simulated bulk phase densities are given in Supplementary Table 1 and

are compared with available experimental data (values in parentheses), and the errors are within

0.07–1.6% for both neat IL and IL/salt systems, suggesting the good consistency between the

simulation and experimental results.

AFM Measurements and Water Effect Discussions

Atomic force microscopy (AFM) measurements were conducted at ambient condition. Each IL

system was measured at least three times to ensure the results are reproducible. The ILs for

AFM studies were kept in degassed desiccator, and water content was also measured and did

not exceed 300 ppm in 10 days.

Previously, it was reported in AFM experiments that a large amount of water will affect both

the position and number of interfacial layered nanostructures3,4,5,6. Thereby, we also

investigated the impact of low water content (< 300 ppm) on the interfacial nanostructure of

current systems by mean of MD simulations. The force field of water adopted is the TIP3P

model7. The wet electrolytes were simulated at both PZC and -0.5 V vs. PZC voltage based on

the same procedure used for the dry system simulation. The 393 K was simulated to allow the

system to gain sufficient dynamics, and it was found previously that the temperature does not

significantly change the density profile when using the flat electrode surface through either the

non-polarisable force field with constant charge electrode8 or polarizable force field with

constant potential electrode9. The number density profiles of both cation, anion and Na+ were

calculated and compared with the results from dry system simulation in Supplementary Figure

1. It is found that the presence of 300 ppm of water does not significantly affect either the

position or the number of peaks of the number density profile of all ion species. Therefore, this

can eliminate concerns about a significant effect from a small amount of water on the interfacial

nanostructure. The shape of the innermost peaks was almost unaffected for C3mpyr. The peak

intensity of anions changes slightly, but the main characteristics of the peak in the dry system

remain in the wet system. The further discussion of water effect is our future work.

Supplementary Table 1. The average number of C3mpyr+, FSI– and Na+ in the innermost

layer within 0.61 nm from Au(111) surface, calculated over the 40 ns of MD trajectory file.

Systems Chemical composition

PZC -0.5 V vs. PZC -1.25 V vs. PZC

0 mol% C3mpyr'

FSI+=3027

= 1.11 C3mpyr'

FSI+=31.824

= 1.325 C3mpyr'

FSI+=

3618.3

= 1.96

10 mol% C3mpyr'

FSI+=29.631

= 0.95

Na+ = 3.6

C3mpyr'

FSI+=32.828.6

= 1.146

Na+ = 2.3

C3mpyr'

FSI+=3024

= 1.25

Na+ = 9

50 mol% C3mpyr'

FSI+=

1940.2

= 0.47

Na+ = 25

C3mpyr'

FSI+=

2236.3

= 0.60

Na+ = 22.1

C3mpyr'

FSI+=

1545.1

= 0.33

Na+ = 43.1

Supplementary Table 2. Simulated mass density (g/cm3) for C3mpyrFSI IL with 0, 10 and 50

mol% salt at 298 K, 348 K, and 393 K. The experimental results are given in brackets. and

compared with simulation results. The error of density is given in D= (DMD-Dexp)/Dexp ´ 100%.

T, K

Density, g cm-3

0 mol% ∆, % 10 mol% ∆, % 50 mol% ∆, %

393 1.236 − 1.271 − 1.473 −

348 1.278 (1.299)*

-1.61 1.316 (1.328)*

-0.90 1.523 (1.532)*

-0.58

298 1.337 (1.338)*

-0.07 1.366 (1.367)*

-0.07 1.575 (1.577)*

-0.12

*experimental value from reference 10,11

Supplementary Table 3. The charge densities ascribed to top layer of gold electrode used for

different electrostatic potential for both neat IL and IL/salt systems.

Salt concentration 0 mol% 10 mol% 50 mol%

Surface charge density, e/nm2 -0.13 -0.34 -0.14 -0.36 –0.23 -0.57

Voltage vs. PZC, V -0.5 -1.25 -0.5 -1.25 -0.5 -1.25

Supplementary Figure 1. Comparison of number density profiles of ions in both dry

system and wet system with 300 ppm of water. Number density profiles were calculated on

a), c), e) cation (through Nitrogen atom) and b), d), f) Na and anion (through Nitrogen atoms)

for the a), b) 0 mol%, c), d)10 mol%, and e), f) 50 mol% NaFSI systems

Supplementary Figure 2. Radial distribution function (RDF) of Na-NFSI calculated at

different Z intervals from the Au(111) surface. a)–c), simulation conducted at PZC. d)–f),

simulation conducted at – 0.5 V vs. PZC.

Supplementary Figure 3. Variation of the number of Na+ in the innermost layer near the

Au(111) surface with different applied potentials over time. The less noisy lines of the 10

mol% system suggest the low frequencies in the change of the Na+ number, i.e. the less hopping

events, which is in contrast with the 50 mol% system.

Supplementary Figure 4. 2D (x, y) snapshots of the innermost layer within 0.61 nm of

the Au(111) surface. The snapshots were taken from all simulated systems with three different

salt concentrations of 0, 10 and 50 mol% at different applied surface polarization.

Supplementary Figure 5. Angular distribution analysis of [C3mpyr]+ in the inner most

layer next to the Au(111) surface. a) Representation of the selected normal vector (𝑛) which

is vertical to the pyrrolidinium ring. b) Angle orientation distribution of vector (𝑛) relative to

the z axis ( z axis is vertical to the gold surface). The peaks close to 90° and 0° / 180° stands

for the [C3mpyr]+ ring perpendicular or parallel to the gold surface, respectively.

Supplementary Figure 6. Galvanostatic stripping-plating cycling of Na|50 mol% NaFSI

in C3mpyrFSI|Na cell at 50 °C at different current densities from 0.1 to 1.0 mAcm-2 and

to 5 mAcm-2 with a charge of 0.1, 1.0 and 1.0 mAhcm2. Prior to this cycling, cells were

preconditioning at a) 0.1 mAcm-2/0.1 mAhcm-2, b) 1.0 mAcm-2/0.1 mAhcm-2, and c) 5.0

mAcm-2/0.1 mAhcm-2 for 5 cycles and the polarization potential is shown in inset figures.

Supplementary Figure 7. Visual representation of electrolyte species and simulation box

for 50 mol% NaFSI in C3mpyrFSI confined by two Au(111) electrodes.

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