Masedi M.C , Sithole H.M and Ngoepe P.E1 Materials Modelling Centre, School of Physical and Mineral Sciences

University of Limpopo, Private Bag x 1106, Sovenga, 0727, South Africa

2 CSIR, Meraka Institute, Meiring Naude, Brummeria, P. O. Box 395, Pretoria

0001, South Africa

CHPC National Meeting 2012

Outline

• Introduction

• Comparison between some rechargeable batteries

• Lithium-air battery

• Discharging and Charging phases

• Structures

• Methodology

• Results and Discussions

• Conclusion

• Future study

• Acknowledgements

Energy storage will be more important in the future than at any time in the past. Among the myriad energy-storage technologies, lithium batteries will play an increasingly important role because of their high specific energy and energy density. Li-ion batteries have transformed portable electronic devices [1–4]. New generations of such batteries will electrify transport and find use in stationary electricity storage. However, even when fully developed, the highest energy storage that Li-ion batteries can deliver is too low to meet the demands of key markets, such as transport. Reaching beyond the horizon of Li-ion batteries is a formidable challenge; it requires the exploration of new chemistry, especially electrochemistry and new materials [2]. Here we consider two: Li-O and Li-S batteries. Lithium-air (Li-O) batteries are potentially viable ultrahigh energy density chemical power sources, which could potentially offer specific energy up to 3000 Wh/kg being rechargeable. In the current work we present a comparative study on stability, structural and electronic

properties of discharge products of sulphur and oxygen formed in Li-O and Li-S batteries.

1. Nagaura, T. & Tozawa, K. Lithium ion rechargeable battery. Prog. Batteries Sol. Cells 9, 209–217 (1990). 2. Tarascon, J. M. & Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 414, 359–367 (2001). 3. Schalkwijk, W.v. & Scrosati, B. Advances in Lithium-Ion Batteries (Kluwer Academic/Plenum, 2002). 4. Nazri, G-A. & Pistoia, G. Lithium Batteries: Science and Technology (Springer, 2003).

Introduction

‘Our Energy Future’.

5. US Advanced Battery Consortium USABC Goals for Advanced Batteries for Evs (2006)

6. Freunberger, S. A. et al. Reactions in the rechargeable lithium–O2 battery with alkyl carbonate electrolytes. J. Am. Chem. Soc. 133, 8040–8047 (2011).

Lithium-air battery

Li

Li Li

O

O

nanostructured

Cathode

Li+

e- e- e-

e-

e-

e-

O -

O

Li+

Li+

Li+

Lithium

anode

Discharge phase

Li+

Li+

O -

O -

Li+ e-

– +

Aprotic electrolyte

O2+e -> O2-

O2- + Li+ ->LiO2

LiO2 + Li+ + e->Li2O2

Li -> Li++ e

Di-Lithium

Peroxide

Li2O2

Operations Model: Aprotic Li/Air battery

Li

Li Li

e-

e- e- e-

Li+

Li+

Li+

Charging phase

Li+

Li+

Li+ O -

O - Li+

O -

O - Li+ Li+

catalyst

particle e-

e-

e-

O

O - Li+

Li2O2 –> LiO2- + Li+

LiO2- –> LiO2 + Li+ +

e

Li+ + e –> Li 0

LiO2 –> O2 + Li+ + e

Li+ + e –> Li 0

O

O

e-

e- e- e- e-

e-

Li

e-

Operations Model: Aprotic Li/Air battery

– +

Structures

(a) Li2S (b) Li2O

(c) Li2O2 (d) Li2S2

Li

S

S

O

O

Li Li

Li Li2O and Li2S have a cubic anti-fluorite structure with Fm-3m symmetry

Li2O2 have an hexagonal structure with P63/mmc symmetry, Li2S2 have an hexagonal structure with symmetry P2_1

Active materials in lithium batteries

W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, 91st edition (2010)

Methodology

The calculations were carried out using ab initio density functional theory (DFT) formalism as implemented in the VASP total energy package [7] with the projector augmented wave (PAW) [8]. An energy cut-off of 500 eV was used, as it was sufficient to converge the total energy of all the systems and k-points of 8x8x8 was used. For the exchange-correlation functional, the generalized gradient approximation of Perdew and Wang (GGA-PBE) [9] was chosen. The elastic constants were calculated for the strains ranging from 0.001 to 0.005. Phonon dispersions calculations the interaction range of 10.0Å and displacement of atoms of +/- 0.02Å were used.

[7] P. E. Blöchl, Phys. Rev. B 50, 17953 (1994); G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999). [8] H.J. Monkhorst and J.D. Pack, Phys. Rev. B 13, 5188 (1976). [9] J. P. Perdew and Y. Wang, Phys. Rev. B 45, 13244 (1992).

Results and Discussion

Structure and Heats of Formation

[10] J.M Osollo-Guillen, B. Holm, R. Ahuja and B.A Johonsson. Journal 167, 221-227 (2004)[14] [11’] Pandit et al Ind J Pure and Appl Phys 47,804 (2009) [11’’] Bertheville J Phys. Cond Matt 10, 2155 -2169 (1998) (Exp thermal exp) [12] A. Golffon, J.C. Dumas and E. Phillippot. Journal 1, 1-123 (2002) [13] E. Zintl, A. Harder and B. Dauth, Z Elektorchem, 40 588 (1934)

Structure Calc

a

(Å)

Exp

a

(Å)

Calc

c

(Å)

Exp

c

(Å)

Volume

(Å3)

ΔH

(kJ/mol)

Li2O 4.63 4.57 [10] 99.42 -541.57

Li2S 5.72 5.71[11’]

5.71[11’’]

187.15 -378.02

Li2O2 3.16 3.18 [12] 7.69 7.72 [12] 66.67 -277.16

Li2S2 4.19 4.06 145.28 -168.34

There is a good agreement between the experimental and calculated lattice constants. From ΔH it is observed that all structures are generally stable because of low values.

Phonon Dispersions of Li2O and Li2S

W L G X W K

-1

4

9

14

19

24

Fre

qu

en

cy (

TH

z)

Brillouin Zone Direction

Li2O

Phonon dispersion calculations for Li2O and Li2S structures, indicates that the structures are stable.

W L G X W K

-1

4

9

14

19

24

Fre

qu

en

cy (

TH

z)

Brillouin Zone Direction

Li2S ΔH = -541.57 kJ/mol

ΔH = -378.02 kJ/mol

Γ K W X Γ X W L

Phonon Dispersion for Li2S

Calculated Experimental – Bill et al (1991)

Good agreement of calculated and experimental, especially on acoustic modes and lower optical modes.

Optical

Acoustic

Optical

Acoustic

LA

TA

LA

TA

Experimental-M Wilson et al (2004)

Phonon Dispersion for Li2O

Calculated W X Г

Acoustic

Optical

LA

TA

Good agreement of calculated and experimental, especially on acoustic modes and lower optical modes.

15

A H L A G K M G

-1

4

9

14

19

24

Fre

qu

en

cy (

TH

z)

Brillouin Zone Direction

Phonon Dispersion for Li2O2 and Li2S2

Li2O2

Calculated phonon dispersions for Li2O2 and Li2S2 structures suggest that the structures are generally stable. Experimental results not yet available.

Li2S2

C Y G B A E Z

0

2

4

6

8

10

12

14

Fre

qu

en

cy (

THz)

Brillouin Zone Direction

ΔH = -277.16 kJ/mol

ΔH = -168.34 kJ/mol

Elastic Properties

Structures Li2O Li2S Li2O2 Li2S2 C11 C12 C13 C33 C44

Calc Exp [8] 200.6 217.0 19.39 25.00 50.65 68.00

Calc Exp [9] 82.14 83.90 18.43 18.10 34.19 32.20

Calc Exp [10]

169.0 207.3 49.77 33.80 -0.13 21.50 158.5 358.1 38.13 46.40

Calc 72.90 24.81 1.18 36.63 4.43

Bulk Voigt Reuss Hill

63.15 63.15 63.15

37.55 37.55 37.55

60.61 98.00 60.60 60.60

26.31 21.51 23.91

Elastic constants values are in good agreement with the experimental values especially for Li2O and Li2S structures.

Mechanical stability (Elastic constants) The cubic structures Li2O and Li2S must satisfy the necessary conditions for stability. C11>0, C11-C12>0, C44>0

The hexagonal structures Li2O2 and Li2S2 must satisfy the necessary conditions for stability. C11>0, C11-C12>0, C44>0, (C11-C12) C33-2C13

2 >0 Li2O and Li2S satisfy the necessary conditions for stability. C11>0, C11-C12>0, C44>0 • Hence Li2O and Li2S are mechanically stable. Li2O2 and Li2S2 satisfy the necessary conditions for stability. C11>0, C11-C12>0, C44>0, (C11-C12) C33-2C13

2>0 • Hence Li2O2 and Li2S2 are mechanically stable.

• All discharge products of oxygen and sulphur in Li–O2 and Li–S batteries are stable because of low values of the heats of formations.

• Lattice parameters and elastic constants values are in good agreement with the experimental values especially for Li2O and Li2S structures.

• The elastic constants suggest mechanical stability of all discharge products .

• Our phonon dispersion calculations shows that all the discharge products are generally stable with the absence of vibrations in the negative frequency.

• Phonon dispersion calculations is in good agreement with the experimental values especially for Li2O and Li2S structures.

• We were successfully able to determine stability of discharge products formed in the Li–O2 and Li–S batteries.

Conclusion

FUTURE WORK

• Create Lithium cells using Battery Design Studio (BDS) to check properties such as cycle, charging and discharging, voltages, prices.

• Work on other metal-air batteries such as : Zn-air and Na-air.

Acknowledgements

©2011 Optimal Energy

Research Success

If we succeed in developing this technology, we are facing the ultimate breakthrough for electric cars, because in practice, the energy density of Li-air batteries will be comparable to that of petrol and diesel.

Thank you all for your attention.

“The only battery chemistries that have a chance of achieving energy densities in the 1,000 Wh/kg range are rechargeable metal-air “ Energy Secretary Steven Chu addressed the United Nations Climate Change Conference in Cancun.

23

Dendrite Formation on Charge

• In all nonaqueous lithium batteries, the anode is covered by a thin film called a Solid Electrolyte Interphase (SEI) [4].

• As a result, on charge, lithium deposits through the SEI in the form of lithium dendrites and mossy (sponge) lithium.

• This raises safety issues – the formation of internal short circuits by lithium dendrites.

• For these reasons, efforts to develop rechargeable lithium-metal batteries have failed and today only rechargeable lithium-ion batteries, which do not contain metallic lithium, are in use.

[4]E. Peled

The Electrochemical Behavior

of Alkali and Alkaline Earth

Metals in

Nonaqueous Battery Systems

-The Solid Electrolyte

Interphase (SEI) Model.

J. Electrochem. Soc. 126,

2047-2051 (1979).

Charge

24

Molten sodium as an anode for a rechargeable air cell

• Sodium is much cheaper and more abundant than lithium.

• The surface tension of the liquid sodium anode is expected to prevent the formation of sodium dendrites on charge. Any sodium dendrites that might be formed would be absorbed into the liquid phase.

• The higher operating temperature accelerates electrode kinetics and reduces electrolyte resistance, thus enabling running the cell at higher power.

• Sodium peroxide is less stable and more reactive than lithium peroxide and can be decomposed by a manganese dioxide.