Insights into nanoscale phase stability and charging mechanisms in alkali o2 batteries from first principles calculations

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First Principles Insights into
Nanoscale Phase Stability and
Charging Mechanisms
inAlkali-O2 Batteries

ShinYoung Kang,Yifei Mo, Shyue Ping Ong,
Gerbrand Ceder
Aug 12, 2014
ACS 248th National Meeting

The promise of alkali-air batteries

A+ + O2 + e− à AxOy AxOy è A+ + O2 + e−
Oxygen
Reduction
Reaction
Oxygen
Evolution
Reaction
Equilibrium potential
(V)
Theoretical specific
energy* (kWh/kg)
Theoretical energy
density* (kWh/L)
Li / Li2O2 2.96 3.46 7.99
Na / Na2O 1.96 1.70 3.86
Na / Na2O2 2.33 1.60 4.48
Na / NaO2 2.27 1.10 2.43
metal
anode
air cathode
*based on the mass and volume of discharge product only
Aug 12, 2014 ACS 248th National Meeting

Outline

1.  Facile topotatic delithiation of
Li2O2 in Li-O2 batteries
2.  Nanoscale Phase Stability of
NaxOy

Mizuno, Nakanishi, Kotani,Yokoishi, Iba,
50th Battery Symposium in Japan (2009)
T. Ogasawara,A. Debart, M. Holzapfel, P. Novak, P.G. Bruce, J.Am. Chem.
Soc. 2006
G. Girishkumar, B. McCloskey,AC. Luntz, S. Swanson,W.Wilcke, J. Phys.
Chem. Lett. 2010
K. Xu, Chem. Rev. 2004
Poor reversibility (~50 cycles)
Side reactions with electrolyte
(up to 99% Li2CO3)
Low power density
Low cyclic efficiency (~60%)
High charging overpotential (~1.1-1.5V)
Safety of Li metal anode
Challenges in Li-
Air Batteries

Recent experimental results reveal highly improved
performance

Improved cyclability (~ 100 cycles)4,5
Higher rate (~ 3 mA/cm2)5
Lower discharging overpotential
Low charging overpotential at the initial stage of charging 4,5,6
More stable electrolyte (no carbonate!!)à less by-products4,5
McCloskey et al. JPCL (2012)
Potential
vs.
Li/Li+
(V)

Capacity
(mAh)

Peng et al. Science (2012)
Discharge
capacity

(mAh/ggold)

Cycle

Evidence of LiO2 formation during discharge

Peng et al. 8 observed
the formation of
metastable LiO2 using
in-situ surface
enhanced Raman
spectroscopy (SERS)
h
w
e
is
s,
2]
e
er
+
ct
À
n
is
e
e
V
of in situ SERS measurements are presented in Figure 3. A
background spectrum was collected before application of a
potential to the cell (OCV; open circuit voltage). The
Figure 3. In situ SERS during O2 reduction and re-oxidation on Au in
O2-saturated 0.1m LiClO4-CH3CN. Spectra collected at a series of
times and at the reducing potential of 2.2 V versus Li/Li+
followed by
other spectra at the oxidation potentials shown. The peaks are
assigned as follows: 1) CÀC stretch of CH3CN at 918 cmÀ1
, 2) OÀO
stretch of LiO2 at 1137 cmÀ1
, 3) OÀO stretch of Li2O2 at 808 cmÀ1
,
4) ClÀO stretch of ClO4
À
at 931 cmÀ1
.
Li2O2 LiO2
O2 + e−
Li+ + O2
−
2LiO2
*
→  O2
−,
→  LiO2
*,
→ Li2O2 + O2
(* indicates surface sites)
Proposed discharge mechanism

Is there a non-equilibrium,kinetically favored
pathway for delithiation with low overpotential?

Li2O2 (LiLiO2) is isostructural
with P2 NaCoO2!
P2 NaCoO2 LiLiO2
De-sodiation
Na1-xCoO2 Li1-xLiO2
(Li2-xO2)
Topotactic
de-lithiation
Co
Na
O
Liinterlayer
O
Li2O2
Li2-xO2
Li, O2
Liintralayer
Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G.A Facile Mechanism for Recharging Li2O2 in Li–O2 Batteries, Chem. Mater., 2013, 25, 3328–3336

Determining the structure and energy of
LiO2

Candidates: Known superoxides, XO2 peroxides, Li2O2 deriv., and NaCoO2
polymorphs
a b
c
a b
c
a b
c
a b
c
a b
c
P63/mmc
layered
P63/mmc
monomers
Li2O2
(P63/mmc = P2)
a b
c
P3m
disproportionated
R3m
(P3 layered)
Pnnm
I4/mmmC2/m PbcaPa3
Pyrite OrthorhombicLayered Bi-pyramidal
arrangement of
(LiO2)2
Marcasite

-2.7
-2.5
-2.3
-2.1
-1.9
ΔGform(eV/O2)
P3m
disproportionated
I4/mmm
Pa3
P bca
R3m
(P3 layered)
Pnnm
P63
/mmc layered
P63
/mmc monomers
C2/m
Calculated formation free energy of LiO2

Derived from Li2O2
a b
c
Pnnm
−2.68 eV/O2
P3m
disproportionated
−2.63 eV/O2
1.50 Å
1.21 Åa b
c
P63/mmc-layered
−2.61 eV/O2
a b
c
Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G.A Facile Mechanism for Recharging Li2O2 in Li–O2 Batteries,
Chem. Mater., 2013, 25, 3328–3336

Overpotential required for topotactic delithiation
of Li2O2 at the initial stage of charging

0
−0.5
−1.0
−1.5
−2.0
−2.5
Mole fraction of Li
O2 Li
ΔHform(eV/atom)
LiO2
Li2O
Li2O2
Source: materialsproject.org
0 0.5 1.0
Equilibrium path:
Li2O2 2 Li+ + 2 e− + O2
φeq = −
ΔGf (Li2O2 )
2e
= 2.97 V
Non-equilibrium topotactic
delithiation path:
Li2O2 Li2-xO2 + x Li
+
φ =
ΔGf (Li2−x1
O2 )− ΔGf (Li2−x2
O2 )
(x1 − x2 )e

Delithiated Li2-xO2 x = 0.25,0.5,0.75

Three intermediate states between Li2O2 and LiO2 are considered:
Li1.25O2, Li1.5O2, and Li1.75O2
…
…
Superoxide
Peroxide
2×1×1 supercell orderings 1×1×2 supercell orderings
“Layered”
configurations
Peroxide Superoxide
“Channel”
configurations

The
lowest
energy
structures
are

layered
structures
for
all
Li2-‐xO2

Formation free energy of off-stoichiometric
phases Li2-xO2 referencing to the equil. path

0.0
0.1
0.2
0.3
0.4
0.5
0.0 0.2 0.4 0.6 0.8 1.0
ΔGform–ΔGform(eV/O2)
x in Li2-xO2
equil
ΔGform-ΔGform(eV/
O2)

equil

x in Lix-2O2

Li2O2
LiO2

Pnnm LiO2

½ Li2O2 + ½ O2

P63/mmc
layered LiO2

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.1

0.2

0.3

0.4

0.5

à
Potential
continuous
topotactic

delithiation
path
from
Li2O2
to
LiO2

Li1.5O2

Li1.75O2

Li1.25O2

Li2O2

P63/mmc
layered LiO2

Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G.A Facile Mechanism for Recharging
Li2O2 in Li–O2 Batteries, Chem. Mater., 2013, 25, 3328–3336

Voltage profile of kinetically favored non-
equilibrium topotactic delithiation path
2.5
2.7
2.9
3.1
3.3
3.5
0.0 0.5 1.0 1.5 2.0
3.34 3.34
3.27
3.40
2.61
Equil. decomposition path
(Li2O2 à 2Li+ + 2e− + O2)
Φeq= 2.97V
Voltagevs.Li/Li+(V)
x in Lix-2O2
Overpotential as low as
~0.3–0.4V
Predicted metastable voltage of 3.34V
consistent with experimentally observed
charging voltage plateau at 3.1−3.4V
Li2-xO2 can further decompose
through oxygen evolution reaction
or the ion dissolution in electrolyte
Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G.A Facile Mechanism for Recharging Li2O2 in Li–O2 Batteries, Chem. Mater., 2013, 25, 3328–3336

Conclusions
1.  Low-energy topotatic delithiation pathway exists
for Li2O2èLiO2
2.  Delithiation pathway likely to be kinetically favored
3.  Predicted overpotential of 0.3-0.4V consistent
with experimental observations
Li2O2 Li2-xO2 +
x(Li+ + e−)
2Li+ + 2e− + O2
Li+
O2 or
O2
−
Li+
Charging Mechanism 1:
Topotactic delithiation
Charging Mechanism 2:
??

Discharge product formed has huge impact
on Na-O2 battery performance

Kim et al. PCCP 2013; Liu et al., ChemComm 2013; Li et al., ChemComm 2013
NaClO4/TEGDME
Not rechargeable
In NaPF6 or NaClO4/DME
Cathode: carbon or GNS
NaSO3CF3/DEGDME
Cathode: n-doped graphene
nanosheet (GNS)
Na2O2 as the dominant discharge product è
i.  High charging overpotentials (cf. ϕeq = 2.33V)
ii.  Negligible cyclability
When NaO2 is formed, charging overpotentials is
only 0.2V (cf. ϕeq = 2.27V)
Hartmann et al. Nature Mat. 2012

Question: Under what conditions (temperature,
oxygen partial pressure, particle size, etc.) would
NaO2 preferentially form instead of Na2O2?
To answer this question, we need to construct phase
diagram of Na-O system as a function of temperature,
pO2 and particle size.
(d) Pnnm NaO2
a
b
c
a
b
c
(a) Im3m Na
(c) P62m Na2O2
c
a b
a
b
c
(b) Fm3m Na2O
(g) Imm2 NaO3
(e) Pa3 NaO2
a
c
b
(f) R3m NaO2
b
c
a
a
c
b

Oxidation energy corrections for oxides,
peroxides,and superoxides

Li2O
MgO
Al2O3
Na2O
K2O Li2O2, SrO2
K2O2
Na2O2
CaO
KO2
NaO2
RbO2
Correction
E (eV/O2)
Oxides 1.33
Peroxides 0.85
Superoxides 0.23
O=O bond is broken to
different degrees when
forming different oxides,
requiring different corrections
for DFT binding energy error.

Phase diagram of bulk Na-O compounds
as a function of temperature and pO2

Disordered Pa-3 NaO2
Phase transition from Pnnm
NaO2 to Na2O2 at PO2= 1
atm, 230-240 K
Phase transition from
Fm-3m NaO2 to Na2O2 at
T= 300 K, 8.5 atm.
Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G. Nanoscale stabilization of
sodium oxides: implications for Na-O2 batteries., Nano Lett.,
2014, 14, 1016–20

Calculated surface energy of Na2O2 as a
function of oxygen chemical potential

O2

Na2O2

Na2O

μO

NaO2

298 K, 1 atm

Na

~30−45 meV/Å2
Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G. Nanoscale stabilization of sodium oxides: implications for Na-O2 batteries., Nano Lett., 2014, 14, 1016–20

Calculated surface energy of Pa-3 NaO2 as
a function of oxygen chemical potential

[010]
[001]
[100]
{100}
O2

Na2O2

Na2O

μO

NaO2

298 K, 1 atm

Na

Stoichiometric {100} surface
has the lowest surface energy
of 12 meV/Å2

Wulff shapes of Na2O2 and Pa-3 NaO2

Na2O2

Pa3 NaO2

μNa

O2

Na2O2

Na2O

Na

μO

NaO2

10
15
20
25
30
35
40
45
O2 limit

{1100}

{1120}

{0001}

O2 and Na2O2 limits

10
15
20
25
30
35
40
45
{100}

γ

(meV/Å2)

10
15
20
25
30
35
40
45
Na2O limit

10
15
20
25
30
35
40
45
{1100}

{1120}

{0001}

Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G. Nanoscale stabilization of sodium oxides:
implications for Na-O2 batteries., Nano Lett., 2014, 14, 1016–20

Phase diagram of Na-O nanoparticles as a
function of PO2

Surface energy + bulk energy à particle size-dependent ΔGform
* Particle size d = (V0)1/3,
where V0 is the total volume of the particle
Due to the low surface
energies, NaO2 nanoparticles
are stable over Na2O2 at
small particle size
When particle size bigger
than 6 nm, the low bulk
formation energy stabilizes
Na2O2 over NaO2

Critical nucleation parameters of Na-O
nanoparticles as a function of pO2 and ϕ

As a function of voltage at pO2 = 1atm As a function of pO2 at voltage = 2.1V
NaO2 particles are more likely to nucleate due to smaller
nucleation energy barrier and critical nucleus size

Conclusions

Bulk Na2O2 is stable
and NaO2 is metastable
at standard conditions.
NaO2 has significantly
lower surface energy
compared to Na2O2
O2 partial pressure
determine formation
and growth of a
particular sodium oxide
phase
Thermodynamic
equilibrium path leads
to Na2O2 formation
NaO2 stabilized in the
nanometer regime
where nucleation takes
place.
At higher O2 pressure,
NaO2 nucleation
barrier reduced and
remains stable up to
larger particle sizes

Acknowledgements and Publications

Grant No.
EDCBEE,
DE-FG02-96ER45571
FE-PI0000012
Grant No.
TG-DMR97008S
Publications
i.  Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G.A Facile Mechanism for Recharging Li2O2
in Li–O2 Batteries, Chem. Mater., 2013, 25, 3328–3336, doi:
10.1021/cm401720n.
ii.  Kang, S.; Mo,Y.; Ong, S. P.; Ceder, G. Nanoscale stabilization of sodium oxides:
implications for Na-O2 batteries., Nano Lett., 2014, 14, 1016–20, doi:
10.1021/nl404557w.

materiaIs
virtuaLab
Thank you.

Aug 12, 2014
ACS 248th National Meeting

Insights into nanoscale phase stability and charging mechanisms in alkali o2 batteries from first principles calculations

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