Supporting Information

Metal-oxygen bonds: stabilizing the intermediate species

towards practical Li-air batteries

Yuyang Houa, Yuqing Liua, Zhen Zhoub, Lili Liuc, Haipeng Guoc, Huakun Liuc, Jiazhao Wangc,*,

and Jun Chena,*

a ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute,

Australian Institute of Innovative Materials, Innovation Campus, University of Wollongong,

Squires Way, North Wollongong, NSW 2500, Australia.

b Key Laboratory of Advanced Energy Material Chemistry (Ministry of Education), Institute of

New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and

Engineering (Tianjin), School of Materials Science and Engineering, National Institute for

Advanced Materials, Nankai University, Tianjin 300350, China.

c Institute for Superconducting and Electronic Materials, Australian Institute of Innovative

Materials, Innovation Campus, University of Wollongong, Squires Way, North Wollongong,

NSW 2500, Australia.

* Corresponding Author: jiazhao@uow.edu.au and junc@uow.edu.au

S1

Fig. S1. TGA curves of CNT (blue) and Mo2C/CNT (red) at increasing temperature of 10 °C

min-1 in air.

The initial weight gain from 150 °C to 490 °C is attributed to the gradual oxidation of Mo2C to

MoO3, followed by a slight weight loss caused by the combustion of CNTs. When Mo2C/CNT is

heated to 650 °C, the composite material totally transformed to MoO3, followed by the

thermolysis of MoO3 above 700 °C.

According to these results, the Mo2C content is estimated to be 94.8 wt.% in Mo2C/CNT based

on the following equation: m (Mo2C) = 133.8 wt.% * M(Mo2C)/2M(MoO3) = 94.8 wt.%, where

m is the wt.% and M is the number of moles, and the CNT content is calculated to be 5.2 wt. %

S2

Fig. S2. Discharge and charge voltage curves under different atmospheres.

This test was intended to exclude the de-intercalation of lithium ions in the charge process, and

the cell was tested as follows: 1) purge with oxygen for 3 hours, then discharge and charge for 20

hours separately; 2) purge with nitrogen, then discharge to 2.35 V; 3) purge with oxygen, then

discharge and charge for 20 hours separately for the second time. There are corresponding

plateaus in oxygen at steps 1) and 3), while there is no plateau, but only rapid fading to 2.35 V in

nitrogen at step 2), suggesting that lithium ion de-intercalation and intercalation of the discharge

product be excluded.

S3

Fig. S3. a) Voltage profiles (left) and photographs (right) of glass fiber separators corresponding

to different galvanostatic discharge and charge stages using Mo2C/CNT as cathode under N2. b)

Voltage profiles (left) and photographs (right) of glass fiber separators corresponding to different

galvanostatic discharge and charge stages using Mo2C/CNT as cathode under O2.

Only overcharging the Mo2C/CNT electrode made the glass fiber turn dark blue, which shows

that the blue color was not due to dissolution of the discharge product or lithium extraction from

the discharge product, but due to Mo2C self-decomposition at a high voltage.

S4

Fig. S4. XPS survey spectra of pristine, discharged, and recharged Mo2C/CNT electrode. The

survey spectra clearly show the intensity change of O1s and C1s peaks, indicating deposition of

oxygen reduction species in discharge, and releasing oxygen reduction species in charge.

S5

Fig. S5. Energy dispersive spectroscopy (EDS) of pristine (top row), discharged (middle row),

and charged (bottom row) Mo2C/CNT electrodes. The element mapping clearly shows that

oxygen reduction species are deposited on the surface of the Mo2C/CNT electrode during the

discharge process and released from the surface of the Mo2C/CNT electrode during the charge

process.

S6

Fig. S6. a) Raman spectra, and b) XRD patterns of Mo2C/CNT electrodes at different stages after

exposure in ambient air for several days; c) SEM image of discharged Mo2C/CNT electrode, and

d) SEM image of charged Mo2C/CNT electrode, both were characterized after exposure in

ambient air for several days.

As Raman spectra, XRD and SEM shown, only discharge Mo2C/CNT electrode could generate

crystalline Li2MoO4, indicating amorphous discharge product may react with moisture in

ambient air to generate Li2MoO4. According to this, we confirmed the formation of Mo-O bonds

in discharge and proposed a possible reaction mechanism of the formation of Li2MoO4.

LixO+ H2O → LiOH

MoxO3 + H2O → H2MoO4

H2MoO4 + LiOH → Li2MoO4 + H2O

S7

Electrode Species Peak position for

Mo 3d5/2 (former);

Mo 3d3/2 (latter) / eV

FWHM for

Mo 3d5/2 (former);

Mo 3d3/2 (latter) / eV

Species

percentage

/ %

Pristine Mo2C/CNT

Mo2+ 228.41; 231.89 1.60; 1.50 18.4

Mo3+ 229.38; 232.52 1.60; 1.57 13.2

Mo5+ 231.33; 234.89 1.69; 1.72 15.0

Mo6+ 233.05; 236.09 1.81; 1.96 53.4

Discharged

Mo2C/CNT

Mo2+ - - -

Mo3+ - - -

Mo5+ 231.78;234.92 1.47; 1.49 17.4

Mo6+ 233.26; 236.48 1.72; 1.75 82.6

charged Mo2C/CNT

Mo2+ 228.28; 231.79 1.35; 1.38 16.1

Mo3+ 229.58; 232.81 1.35; 1.37 12.0

Mo5+ 231.17; 234.68 1.31; 1.33 15.0

Mo6+ 233.09; 236.13 1.83; 1.90 56.9

Table S1. Fitting parameters (peak position, full width at half maximum (FWHM), and species

percentages) for both Mo 3d3/2 and Mo 3d5/2 spectra collected from Mo2C/CNT electrodes at

different stages (pristine, discharged, charged)

S8