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In recent years, there have been great interest in alkali-O2 batteries with extremely high specific energies. Li-O2 batteries offer the greatest theoretical specific energy, but currently suffer from large charging overpotentials and low power densities. Na-O2 offers a somewhat lower theoretical specific energy compared to Li-O2, but still a substantial improvement over today’s lithium-ion batteries. In this talk, we will demonstrate how first principles calculations can provide crucial insight into the workings of alkali-O2 batteries. We will elucidate a facile mechanism for recharging Li2O¬¬2 that is accessible at relatively low overpotentials of ~0.3-0.4V and is likely to be kinetically favored over Li2O2 decomposition. We will also demonstrate that sodium superoxide (NaO2) is predicted to be considerably more stable than sodium peroxide (Na2O2) at the nanoscale. Using first principles calculations, we derive the specific electrochemical conditions to nucleate and retain NaO2 and comment on the importance of considering the nanophase thermodynamics when optimizing an electrochemical system.
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materiaIsvirtuaLab
First Principles Insights into Nanoscale Phase Stability and Charging Mechanisms ���in Alkali-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
Aug 12, 2014 ACS 248th National Meeting
Outline
1. Facile topotatic delithiation of Li2O2 in Li-O2 batteries
2. Nanoscale Phase Stability of NaxOy
Aug 12, 2014 ACS 248th National Meeting
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
Aug 12, 2014 ACS 248th National Meeting
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
Aug 12, 2014 ACS 248th National Meeting
McCloskey et al. JPCL (2012)
Potential vs. Li/Li+ (V)
Capacity (mAh)
Peng et al. Science (2012)
Discharge capacity
(mAh/g
gold)
Cycle
Evidence of LiO2 formation during discharge
Aug 12, 2014 ACS 248th National Meeting
Peng et al. 8 observed the formation of metastable LiO2 using in-situ surface enhanced Raman spectroscopy (SERS)
were obtained by fitting the current response to a potentialstep at an Au microelectrode (Figure S3) following theprocedure described previously[7] (see Experimental Section).It is known that O2
! can form ion pairs with molecular cationssuch as organic ammonium ions, however, such interactionsare weak compared with those involving Li+ ions.[2,26]
The reaction between O2! and Li+ was investigated as a
function of Li+ concentration (Figure 2). Addition of a 1 mmconcentration of Li+ resulted in the appearance of a new
reduction peak at higher potentials (2.35 V) compared withthe original O2 reduction peak. The magnitude of the newpeak grows with increasing Li+ concentration and at theexpense of the area under the original O2 reduction peak. Thisbehavior is consistent with an EC mechanism, that is,electrochemical reduction followed by a chemical step.[32]
Such following chemical reactions severely deplete theconcentration of O2
! thus shifting the potential to highervoltages, as observed here.[32] When the concentration of Li+
ions is lower than O2, then there is insufficient Li+ to reactwith all the O2
! that is generated, hence “unbond” O2!
persists and two peaks are apparent. When the concentrationof Li+ exceeds that of O2 (in this case the O2 concentration is6.8 mm) then all the O2
! is consumed by reacting with Li+. Thelow voltage reduction peak disappears leaving only onereduction peak. For a similar reason the peak at 2.75 Vassociated with O2
! oxidation disappears when the Li+
concentration exceeds that of O2. The shift of the reductionpotentials to lower voltages and the lowering of the reductioncurrent with increasing Li+ concentration are consistent withpartial blockage of the electrode surface by the insulatingreduction products, which becomes more severe at higher Li+
concentrations. Such a phenomenon has been observedbefore.[27] As stated above, Au was used because it permitsSERS studies of the electrode surface. The same electro-chemical reactions occur on glassy carbon electrodes, asshown in Figure S4.
Although these and previous electrochemical studies arevery valuable, they cannot identify directly the species formedon reduction. This is illustrated by the fact that differentauthors have proposed different mechanisms for O2 reductionbased on electrochemical measurements;[25–29] two examplesare given here:
O2 þ e! ! O2! ð1aÞ
2 O2! $ O2 þO2
2! !2 LiþLi2O2
ð1bÞ
or
O2 þ e! ! O2! ð2aÞ
O2! þ Liþ ! LiO2 ð2bÞ
2 LiO2 ! Li2O2 þO2 ð2cÞ
Spectroscopic methods can identify directly the reactionproducts and their intermediates, and therefore are invalu-able in investigating the O2 reduction mechanism. The resultsof in situ SERS measurements are presented in Figure 3. A
background spectrum was collected before application of apotential to the cell (OCV; open circuit voltage). Thespectrum is consistent with that expected for CH3CN; thepeak (1) at 918 cm!1 is assigned to the C!C symmetric stretchin CH3CN. Data were then collected at a potential of 2.2 V,that is, within the reduction peak in Figure 2. Spectra areshown at this potential for successive time intervals. Within ashort time, two new peaks (2 and 3) appear that were notpresent at OCV. The most prominent occurs at 1137 cm!1 andis associated with the O!O stretch of LiO2.
[33, 34] The smallerpeak at 808 cm!1 corresponds to the O!O stretch of adsorbedLi2O2.
[35, 36] With the passage of time the LiO2 peak diminishesuntil only the Li2O2 peak remains. The LiO2 peak occurs some
Figure 3. In situ SERS during O2 reduction and re-oxidation on Au inO2-saturated 0.1m LiClO4-CH3CN. Spectra collected at a series oftimes and at the reducing potential of 2.2 V versus Li/Li+ followed byother spectra at the oxidation potentials shown. The peaks areassigned as follows: 1) C!C stretch of CH3CN at 918 cm!1, 2) O!Ostretch of LiO2 at 1137 cm!1, 3) O!O stretch of Li2O2 at 808 cm!1,4) Cl!O stretch of ClO4
! at 931 cm!1.
Figure 2. Cyclic voltammetry at an Au electrode in O2-saturated 0.1mnBu4NClO4-CH3CN containing various concentrations of LiClO4 asindicated. The scan rate was 1.0 Vs!1 because at this rate thereduction to O2
! and LiO2 as a function of Li+ concentration can beseen most clearly.
Communications
6352 www.angewandte.org ! 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 6351 –6355
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!
Aug 12, 2014 ACS 248th National Meeting
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
Aug 12, 2014 ACS 248th National Meeting
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/mmm C2/m Pbca Pa3 Pyrite Orthorhombic Layered Bi-pyramidal
arrangement of (LiO2)2
Marcasite
-2.7
-2.5
-2.3
-2.1
-1.9
ΔGfo
rm (e
V/O
2)
P3m disproportionated
I4/mmm
Pa3 P bca
R3m
(P3 layered)
Pnnm P6 3/m
mc layered
P6 3/mmc monomers
C2/m
Calculated formation free energy of LiO2
Aug 12, 2014 ACS 248th National Meeting
Derived from Li2O2
a b
c
Pnnm −2.68 eV/O2
P3m disproportionated −2.63 eV/O2
1.50 Å
1.21 Å a 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
Aug 12, 2014 ACS 248th National Meeting
0
−0.5
−1.0
−1.5
−2.0
−2.5
Mole fraction of Li
O2 Li
ΔH
form
(eV
/ato
m) 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−x1O2 )− ΔGf (Li2−x2O2 )
(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
Aug 12, 2014 ACS 248th National Meeting
… …
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
ΔGfo
rm – ΔG
form
(eV/
O2)
x in Li2-xO2
equi
l ΔG
form
- ΔG
form
(eV
/O
2) equi
l
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
Aug 12, 2014 ACS 248th National Meeting
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
Aug 12, 2014 ACS 248th National Meeting
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= 2.97 V
Volta
ge v
s. Li
/Li+
(V
)
x in Lix-2O2
Overpotential as low as ~0.3–0.4 V
Predicted metastable voltage of 3.34 V consistent with experimentally observed charging voltage plateau at 3.1−3.4 V
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 Li2O2èLiO2
2. Delithiation pathway likely to be kinetically favored
3. Predicted overpotential of 0.3-0.4V consistent with experimental observations
Aug 12, 2014 ACS 248th National Meeting
Li2O2 Li2-xO2 + x(Li+ + e−)
2Li+ + 2e− + O2
Li+
O2 or O2−
Li+
Charging Mechanism 1: Topotactic delithiation
Charging Mechanism 2: ??
Outline
1. Facile topotatic delithiation of Li2O2 in Li-O2 batteries
2. Nanoscale Phase Stability of NaxOy
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
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)
Aug 12, 2014 ACS 248th National Meeting
Na2O2 as the dominant discharge product è i. High charging overpotentials (cf. ϕeq = 2.33 V) ii. Negligible cyclability
When NaO2 is formed, charging overpotentials is only < 0.2 V (cf. ϕeq = 2.27 V)
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.
Aug 12, 2014 ACS 248th National Meeting
(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
Aug 12, 2014 ACS 248th National Meeting
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
Aug 12, 2014 ACS 248th National Meeting
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
Aug 12, 2014 ACS 248th National Meeting
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
Aug 12, 2014 ACS 248th National Meeting
[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
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
Wulff shapes of Na2O2 and Pa-3 NaO2
Aug 12, 2014 ACS 248th National Meeting
Na2O2 Pa3 NaO2 μNa
O2
Na 2
O2
Na 2
O
Na
μO
NaO2
10
15
20
25
30
35
40
45
O2 limit
{110
0}
{112
0}
{0001}
O2 and Na2O2 limits
10
15
20
25
30
35
40
45
{100}
γ (meV/Å2)
1015202530354045
Na2O limit
10
15
20
25
30
35
40
45{1
100}
{112
0}
{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
Aug 12, 2014 ACS 248th National Meeting
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
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
Critical nucleation parameters of Na-O nanoparticles as a function of pO2 and ϕ
Aug 12, 2014 ACS 248th National Meeting
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
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
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
Aug 12, 2014 ACS 248th National Meeting
Acknowledgements and Publications
Grant No.
EDCBEE,
DE-FG02-96ER45571
FE-PI0000012
Aug 12, 2014 ACS 248th National Meeting
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.
materiaIsvirtuaLab
Thank you.
Aug 12, 2014
ACS 248th National Meeting