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 ��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




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/g

gold)

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)

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!


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/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


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


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

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


… …

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



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= 2.97 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


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


Li2O2 Li2-xO2 + x(Li+ + e−)

2Li+ + 2e− + O2

Li+

O2 or O2−

Li+

Charging Mechanism 1: Topotactic delithiation

Charging Mechanism 2: ??

Outline




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)


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.


(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


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

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}


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.

materiaIsvirtuaLab

Thank you.

Aug 12, 2014

ACS 248th National Meeting