Amorphous Li2O2: Chemical Synthesis and Electrochemical Properties

Article abstract of DOI:10.1002/anie.201605228

When aprotic Li–O₂batteries discharge, the product phase formed in the cathode often contains two different morphologies, that is, crystalline and amorphous Li₂O₂. The morphology of Li₂O₂impacts strongly on the electrochemical performance of Li–O₂cells in terms of energy efficiency and rate capability. Crystalline Li₂O₂is readily available and its properties have been studied in depth for Li–O₂batteries. However, little is known about the amorphous Li₂O₂because of its rarity in high purity. Herein, amorphous Li₂O₂has been synthesized by a rapid reaction of tetramethylammonium superoxide and LiClO₄in solution, and its amorphous nature has been confirmed by a range of techniques. Compared with its crystalline siblings, amorphous Li₂O₂demonstrates enhanced charge-transport properties and increased electro-oxidation kinetics, manifesting itself a desirable discharge phase for high-performance Li–O₂batteries.

Full text of DOI:10.1002/anie.201605228

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201605228
German Edition: DOI: 10.1002/ange.201605228
Li–O Batteries
2
Amorphous Li O : Chemical Synthesis and Electrochemical Properties
2
2
Yelong Zhang, Qinghua Cui, Xinmin Zhang, William C. McKee, Ye Xu, Shigang Ling, Hong Li,
Guiming Zhong, Yong Yang, and Zhangquan Peng*
Abstract: When aprotic Li–O batteries discharge, the product
identified at the end of discharge, which include crystalline
(typically toroid-shaped) and amorphous (typically film-like)
2
phase formed in the cathode often contains two different
morphologies, that is, crystalline and amorphous Li O . The
[
14]
Li O . The morphology of Li O has been recognized to
2
2
2
2
2
2
morphology of Li O impacts strongly on the electrochemical
impact strongly on the electrochemical performance (partic-
2
2
performance of Li–O cells in terms of energy efficiency and
ularly charging) of Li–O cells in terms of energy efficiency
2
2
[7]
rate capability. Crystalline Li O is readily available and its
and rate capability, see recent works by Adams et al. and Lu
2
2
[
13]
properties have been studied in depth for Li–O2 batteries.
However, little is known about the amorphous Li O because
et al. As a result, considerable research efforts have been
devoted to the understanding of the formation and growth,
charge transport, and defect chemistry of Li O in the past few
2
2
of its rarity in high purity. Herein, amorphous Li O has been
2
2
2
2
[3–14]
synthesized by a rapid reaction of tetramethylammonium
years.
So far, it is generally accepted that crystalline Li O , which
superoxide and LiClO in solution, and its amorphous nature
4
2
2
has been confirmed by a range of techniques. Compared with
is detectable by powder X-ray diffraction (PXRD), can be
electro-deposited at low current densities or overpotentials,
and usually has a toroid-shaped appearance with dimensions
its crystalline siblings, amorphous Li O2 demonstrates
2
enhanced charge-transport properties and increased electro-
oxidation kinetics, manifesting itself a desirable discharge
[
5]
of hundreds of nanometers. The formation of large Li O
2
2
phase for high-performance Li–O batteries.
particles suggests that unconventional charge-transport path-
ways or diffusion of soluble intermediates may operate in the
discharge process. For example, charge transport via sur-
2
B
y replacing the conventional lithium-based intercalation
chemistry with highly exergonic conversion reactions, a step-
change in energy density can be achieved for the resultant
[
15]
[16]
[17]
face, grain boundaries, or defects in bulk of crystalline
Li O have been proposed for the growth of large particulate
2
2
ꢀ
energy-storage devices, among which the aprotic Li–O
Li O . In addition, dissolution of O and LiO promoted by
2 2 2 2
additives (e.g. H O)
solvents have been suggested to drive solution-mediated
2
[1,2]
[18]
battery has attracted much attention.
Operation of the
or high-donor-number electrolyte
2
[
9]
aprotic Li–O battery relies on the O reduction reaction
2
2
forming solid Li O in the positive electrode on discharge, and
Li O2 formation, which is not limited by the transport
2
2
2
[3–5]
the reverse Li O oxidation releasing O upon recharge.
properties of Li O . Meanwhile, solid-state studies on nom-
2
2
2
2
2
[
19,20]
Depending on the Li–O2 cellꢀs operating conditions (dis-
inally pure, crystalline Li O (i.e., chemical Li O )
found
2
2
2
2
[6–8]
charging rate and overpotential)
ode material, electrolyte, and catalyst),
morphologies of the product phase have been frequently
and configurations (cath-
that the electronic conductivity of crystalline Li O is very
2
2
[
9–13]
two distinct
limited and cannot support electrons traveling long distances
to reach the reaction sites that have been presumed to be at
+
[21–23]
the Li O j Li electrolyte interface.
Therefore, a solu-
2
2
tion-mediated pathway is more likely for the formation of
[
8,24,25]
[
*] Y. Zhang, Dr. Q. Cui, Dr. X. Zhang, Prof. Z. Peng
State Key Laboratory of Electroanalytical Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Science
large Li O .
Because of its poor conductivities, crystal-
2
2
line Li O has been found to be difficult to decompose and
2
2
usually a large overpotential (h > 1 V) is required for the
charging of Li–O cells containing electro-deposited crystal-
2
Changchun, Jilin, 130022 (China)
E-mail: zqpeng@ciac.ac.cn
[
26]
[27,28]
line Li O2
or commercial Li O .
2 2
2
Although the discharge product of aprotic Li–O batteries
Y. Zhang
2
University of Chinese Academy of Science
Beijing 100049 (China)
was at one time presumed to be crystalline Li O , mounting
2
2
evidence has suggested the presence of amorphous Li O ,
2
2
[
7,12]
Dr. W. C. McKee, Prof. Y. Xu
Department of Chemical Engineering, Louisiana State University
Baton Rouge, LA 70803 (USA)
which can be promoted by higher discharging rates,
overpotentials, and certain electro-catalysts.
larger
Amor-
[
7]
[10,11,13]
phous Li O has been theoretically studied by Tian et al. using
2
2
S. Ling, Prof. H. Li
Institute of Physics, Chinese Academy of Sciences
Beijing, 100190 (China)
first-principles “melt-and-quench” molecular dynamics cou-
pled with percolation theory, and enhanced charge transport
properties, compared with crystalline Li O , have been
2
2
Dr. G. Zhong, Prof. Y. Yang
[24]
predicted. Until now, experimental studies on amorphous
Li O are limited to Li–O cathodes electrochemically dis-
State Key Lab of Physical Chemistry of Solid Surfaces and College of
Chemistry & Chemical Engineering, Xiamen University
Xiamen, 361005 (China)
2
2
2
charged at higher rates (presumably forming amorphous
Li O ), which, however, could be complicated by the side
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
2
2
[
29]
reaction products during discharge
tiously co-formed crystalline Li O . Moreover, the intrinsic
and by the adventi-
[
12]
1002/anie.201605228.
2
2
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1
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properties (e.g., charge transport and
oxidation kinetics) of amorphous Li O2
2
are largely unknown because of the
unavailability of these compounds in
high purity.
Herein, we report the chemical syn-
thesis and electrochemical properties of
amorphous Li O . The amorphous Li O
2
2
2
2
was prepared by a rapid disproportiona-
tion reaction of tetramethylammonium
superoxide (TMAO ) and LiClO4 in
2
acetonitrile, and its amorphous nature
has been confirmed by a range of
complementary techniques including
powder X-ray diffraction (PXRD),
transmission
electron
microscopy
(
TEM) coupled with selected area elec-
tron diffraction (SAED), and Raman
spectroscopy. Solid-state studies by con-
ductivity measurement, nuclear mag-
+
ꢀ
Figure 1. a) Reaction of 1 mm TMA O and 100 mm LiClO in acetonitrile studied with
2
4
netic resonance (NMR), and electron stopped-flow UV/Vis spectrophotometry, b) PXRD patterns of synthesized, annealed, and
spin resonance (ESR) techniques, commercial Li , c) TEM image and SAED (inset) of synthesized Li , and d) Raman spectra
of the various Li O compounds.
O
2
O
2 2
2
together with electrochemical studies
by differential electrochemical mass
spectrometry (DEMS), indicated that
2
2
amorphous Li O , compared with its crystalline siblings,
TEM coupled with SAED, and Raman spectroscopy (Fig-
ure 1b–d). PXRD analysis showed a diffraction pattern that
was essentially featureless (Figure 1b red curve). However,
annealing the product at 1208C overnight led to the appear-
ance of a set of diffraction peaks that can be indexed to the
crystalline Li O (Figure 1b green curve). For comparison,
2
2
possesses enhanced charge-transport properties (higher
ionic and electronic conductivities) and increased oxidation
kinetics (lower oxidation overpotentials). Combined, these
results suggest that amorphous Li O is a desirable discharge
2
2
phase for high-performance aprotic Li–O batteries, and that
2
2
2
pursuing the combinations of cathode material, electrolyte,
and catalyst that can promote the formation of amorphous
Li O at a broad range of discharging rates will contribute
PXRD result of the freshly ball-milled commercial Li O was
2 2
also included (Figure 1b black curve).
Another evidence for the amorphous nature of the Li O
2
2
2
2
greatly to the future development of aprotic Li–O batteries.
was from a TEM study coupled with SAED, in which a porous
structure was observed (Figure 1c) and no electron diffrac-
tion pattern of any periodic structures was observed (Fig-
ure 1c, inset), while more compact structures (Figure S1 and
S2 in the Supporting Information) and well-defined electron
diffraction patterns (rings and spots in the insets of Figure S1
and S2) were obtained for the annealed and commercial
Li O , suggesting the improvement in the crystallinity upon
2
Amorphous Li O was synthesized by a disproportionation
2
2
reaction of TMAO2 (1.102 g) and LiClO4 (1.404 g) in
acetonitrile at room temperature. TMAO2 was used as
a precursor because of its excellent solubility in common
[
30]
organic solvents. The detailed synthesis and characteriza-
tion of TMAO can be found in Ref. [8]. The reaction of
2
TMAO and LiClO producing solid Li O and gaseous O has
2
4
2
2
2
2
2
been quantitatively studied in dimethyl sulfoxide (DMSO)
annealing at elevated temperatures. In addition, a scanning
electron microscopy (SEM) study provided the complemen-
tary morphological information of the three Li O samples
including their particle size, agglomeration extent, and sur-
face area (amorphous > annealed > commercial), see Fig-
ure S3.
ꢀ1
ꢀ1
before, in which a rate constant of k = 24.6m
s
was
2
measured and nano-crystalline Li O with an average size of
2
2
2
2
[
8]
about 9 nm was obtained.
In this work, we use acetonitrile as the reaction medium
because of its lower solvating ability (donor number 14 vs. 30
of DMSO), which could essentially increase the reaction
kinetics because of the less buffered reactivity (more exactly,
A
Raman spectroscopic study of the as-prepared,
annealed and commercial Li O (Figure 1d) demonstrated
2
2
+
ꢀ
acidity) of Li ions toward O2 , favoring the formation of
that they were all peroxides having a Raman band of around
[
31,32]
ꢀ1
Li O with even smaller size or amorphous phase.
The
788 cm assigned to the stretching vibration of LiO–OLi.
2
2
rate constant of the reaction of TMAO2 and LiClO4 in
However, the much decreased Raman signal intensity,
ꢀ
1
ꢀ1
ꢀ1
acetonitrile has been quantified to be 560m
s
by a stopped-
broadened full wave at half maximum (FWHM, ca. 36 cm
ꢀ1
flow UV/Vis spectrophotometer, which is more than 20 times
faster than in DMSO, see Figure 1a.
The reaction product, after washing copiously with
acetonitrile and drying overnight under vacuum at room
temperature, was subjected to the examination by PXRD,
vs. 16 cm of commercial Li O ) of LiO–OLi stretching, and
2 2
particularly the absence of the Li–O lattice vibration
ꢀ1
(258 cm ) for the as-prepared Li O indicated that the
2 2
synthesized Li O2 was amorphous (Figure 1d red curve).
2
The recovery of Li–O vibration by annealing at 1208C
2
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[
19]
(
Figure 1d green curve) also suggested the improvement in
It has been independently identified by Gerbig et al.
[
17]
the crystallinity. Similar Raman spectroscopic features have
been observed for the lattice vibrations (M–O) of other metal
oxides with amorphous structures (or poor crystallinity), such
and Radin and Siegel that the ionic conductivity of Li O is
2 2
via lithium vacancies and the electronic conductivity via
electron holes, that is, localized O2 ions. To understand the
enhanced sion and seon of amorphous Li O , solid state studies
ꢀ
[33]
[34]
[35]
[36]
as TiO2, ZrO2, CuO, and Fe O . We did not exclude
2
3
2
2
the possibility that the grain size of the synthesized Li O was
using NMR spectroscopy (Figure S4 and Table S2) to probe
2
2
+
so small that it could not be detected by PXRD and SAED,
with the SAED having the ability to decompose its struc-
the Li mobility and ESR spectroscopy (Figure S5) to probe
ꢀ
the density of localized O2 , respectively, have been con-
[37]
ture.
ducted. NMR spectroscopic results indicated that the dom-
After confirming the amorphous nature of the synthesized
Li O , electrochemical studies have been conducted to get the
inating spin-lattice relax time (T ), for example at 298 K, was
1
in the following order of amorphous Li O (1.17 s) < annealed
2
2
2
2
fundamental information of its charge-transport properties
and oxidation kinetics. Transport properties were investigated
by a combination of AC impedance and DC polarization
techniques at room temperature, according to a procedure
reported by Gerbig et al., see also Experimental Section in
the Supporting Information for details of the pellet prepara-
Li O (6.87 s) < commercial Li O (122 s), in which a smaller
2 2 2 2
+
T suggests a higher Li mobility. More T values at various
1
1
temperatures are compiled in Table S2, and were qualitatively
consistent with the ionic conductivity measurements
(Table S1). The ESR study of the various Li O compounds
[
19]
2
2
at 77 K indicated that the amorphous Li O has the highest
2
2
ꢀ
tion and data analysis. The ionic (sion) and electronic (seon
)
density (40.8 ppm) of O2 defects (Figure S5), accounting for
its relatively high electronic conductivity. However, annealing
amorphous Li O at 1208C greatly annihilated the O defects
conductivities of the amorphous Li O were measured to be
2
2
ꢀ
8
ꢀ1
ꢀ9
ꢀ1
ꢀ
7
.10 ꢁ 10 Scm and 5.02 ꢁ 10 Scm , respectively (see
2
2
2
Figure 2a,b and Table S1), which are 2–3 orders of magnitude
higher than those of nano-Li O at room temperature by
(13.6 ppm) resulting in decreased electronic conductivity
(Figure 2c,d, and Table S1).
2
2
[20]
Dunst et al. For comparison, conductivity measurements
have also been conducted on annealed and commercial Li O ,
To shed light on the effects of the morphologies and
conductivities on the oxidation kinetics of various Li O ,
2
2
2
2
and much smaller values were obtained, see Figure 2c–f and
Table S1.
a DEMS study has been conducted, where galvanostatic
charging voltage profile and gaseous O (and CO ) evolution
2
2
rate were recorded, simultane-
ously. For the preparation of the
cathodes in discharged state,
Li O , PVDF binder and
2
2
Super P carbon (1:1:8 m/m)
were dispersed in acetonitrile
and applied to a carbon fiber
paper current collector. The total
mass loading of the cathode was
about 30 mg. For the charging of
the cathodes, a constant current
ꢀ1
of 0.2 mA (ca. 8 mAg carbon) was
applied, and to avoid excess para-
sitic reactions during charging
only about 10% of the total
capacity of the cathodes was
charged.
Figure 3a–c show the DEMS
results, in which an average
charging plateau of 3.54 V
+
versus Li/Li was obtained for
the oxidation of amorphous
Li O , while plateaus of 3.82 and
2
2
4
.04 V were observed for the
annealed and commercial Li O ,
2
2
respectively. Extended Li O oxi-
2
2
dation (i.e., > 10% of the packed
capacity) showed that O evolu-
2
tion was accompanied by CO2
release at the late stage of charg-
ing, indicating the decomposition
of cathode materials and electro-
Figure 2. AC impedance (left) and DC polarization (right) measurements of a,b) amorphous, c,d) an-
nealed, and e,f) commercial Li O . Insets in (a,c,e) show the equivalent circuits used to fit the
2
2
[
28]
impedance spectra.
lytes at high voltages, see Fig-
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Angewandte
Communications
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complementary techniques. Conductivity measurements indi-
cated that the amorphous Li O , compared with its crystalline
2
2
siblings, possesses improved charge-transport properties
+
ꢀ
rising from the enhanced Li mobility and increased O2
concentration within the amorphous Li O , results which are
2
2
supported by solid-state studies using NMR and ESR
spectroscopy techniques. In addition, a DEMS study provided
direct evidence that amorphous Li O can be decomposed at
2
2
a much lower potential than crystalline Li O under the same
2
2
charging current density. These results suggest that amor-
phous Li O is a desirable discharge phase for high-perfor-
2
2
mance aprotic Li–O batteries, and that pursuing the combi-
2
nations of cathode material, electrolyte, and catalyst that can
promote the formation of amorphous Li O at a broad range
2
2
of discharging rates will contribute greatly to the future
development of Li–O batteries.
2
Acknowledgements
Z.P. is indebted to the National Natural Science Foundation
of China (Grant No. 21575135 and 91545129), the “Strategic
Priority Research Program” of the CAS (Grant No.
XDA09010401), the Ministry of Science and Technology of
China (Grant No. 2016YBF0100100), the “Recruitment
Program of Global Youth Experts” of China, the Science
and Technology Development Program of the Jilin Province
(Grant No. 20150623002TC and 20160414034GH) and the
Natural Science Foundation of Jiangsu Province (Grant No.
BK20131139).
Keywords: amorphous Li O · aprotic Li–O batteries ·
2
2
2
charge-transport properties · oxidation kinetics
Figure 3. DEMS results of charging a Li–O cathode containing
2
a) amorphous, b) annealed, and c) commercial Li O with a constant
2
2
ꢀ1
current of 0.2 mA (ca. 8 mAg carbon). d) Tafel plot of galvanostatic
charging voltage versus carbon true surface area normalized current
for electrodes containing various Li O .
[
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[
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[
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4
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2528.
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Li–O Batteries
Better off amorphous: Amorphous Li O
2 2
2
has been synthesized by a rapid reaction
of TMAO and LiClO in solution, and its
Y. Zhang, Q. Cui, X. Zhang, W. C. McKee,
Y. Xu, S. Ling, H. Li, G. Zhong, Y. Yang,
2
4
amorphous nature has been confirmed by
a range of techniques. Compared with its
crystalline siblings, amorphous Li O
Z. Peng*
&&&& — &&&&
2
2
Amorphous Li O : Chemical Synthesis
and Electrochemical Properties
demonstrates increased charge-transport
capabilities and enhanced oxidation
kinetics, manifesting itself a desirable
discharge phase for high-performance Li–
2
2
O batteries.
2
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!