First principles investigation of the structural and electrochemical properties of Na4P2S6 and Li4P2S6

Larry E. Rush Jr., N.A.W. HolzwarthDepartment of Physics, Wake Forest University, Winston-Salem, NC, 27109

Supported by DMR-1105485 and DMR-1507942

3/17/2016 APS 2016 March Meeting 1

Outline

I. Rationale

II. Methodology

III.Results

IV.Conclusions

3/17/2016 APS 2016 March Meeting 2

3/17/2016 APS 2016 March Meeting 3

Underlying Questions to Answer

I. What is the structure of Na4P2S6 compared to Li4P2S6?

II. Kuhn structure vs. Mercier structure

III. Is Na4P2S6 a good solid electrolyte for Na-ion batteries?

Rationale

Interest in Na-ion conductors (abundant, inexpensive, and air-stable)

Stability of Li4P2S6 compared to Li3PS4 (Solid State Ionics, Vol. 284(2016), pgs. 61-70)

Mercier analyzed structure for Li4P2S6 (Journal of Solid-State Chem., Vol.43(1982), pgs.151-162)

Kuhn analyzed structure for Na4P2S6 (ZAAC, Vol.640 (2014), pgs.689-692)

Solve structural puzzle

3/17/2016 APS 2016 March Meeting 4

Methodology

Density Functional Theory with Local-density Approximation (LDA)

Projector Augmented-Wave (PAW) Formalism (Phys. Rev. B, Vol. 50(1994), pgs.17953-17979)

Datasets generated with ATOMPAW code (http://www.pwpaw.wfu.edu)

Electronic Structure calculations performed using Quantum Espresso (http://www.quantum-espresso.org)

Plane Wave Expansion for wave functions with |k+G|2 ≤ 64 Ry

3/17/2016 APS 2016 March Meeting 5

3/17/2016 APS 2016 March Meeting 6

Mercier structure: Space Group P63/mcm (#193)Disorder in P-P placements:

S

P

Li

3/17/2016 APS 2016 March Meeting 7

Examples:

DE = 0

DE = 0

DE = 0.03 eV

100% P

50% P50% P

50% P50% P

S

P

Li

31P m

Pnnm

Pnma

Two model configurations of disordered ground state structure

3/17/2016 APS 2016 March Meeting 8

Structural comparison – c-axis projection NaPS

a

a x c

Results

Mercier structures Kuhn structure

3/17/2016 APS 2016 March Meeting 9

Structural comparison – view including c-axis

Na4P2S6

NaPS

c

a x c

Mercier (�3�1m) Kuhn (C2/m)

3/17/2016 APS 2016 March Meeting 10

Results for Na4P2S6:Calculated heats of formation (eV per formula unit) for Na4P2S6 and Li4P2S6 in 4 structural models

Na4P2S6 Li4P2S6

Kuhn structure -11.47 eV -12.07 eV

-11.47 eV -12.42 eV

-11.56 eV -12.46 eV

-11.56 eV -12.46 eV

Models of Mercier structure

The calculations indicate the most stable structure for both Na4P2S6 and Li4P2S6 to be the disordered Mercier structure, which suggests that the Kuhn structure is meta-stable.

31P m

Pnnm

Pnma

3/17/2016 APS 2016 March Meeting 11

Comparison of vacancy migration of Na4P2S6 and Li4P2S6

NaPS

“Nudged Elastic Band Method”

G.Henkleman, B.P. Uberuaga, H.Jόnsson

J. Chem. Phys 113, pgs. 9901-9904 (2000)

Mercier (�3�1m)

Kuhn (C2/m)

c

b

c

a

3/17/2016 APS 2016 March Meeting

Minimum ion vacancy migration energies

Na4P2S6 0.3 eV 0.3 eV

Li4P2S6 0.5 eV 0.1 eV

31P m 2 /C m

NaPS

12

Mercier (�3�1m) Kuhn (C2/m)

c

a

c

b

3/17/2016 APS 2016 March Meeting 13

Conclusions

Kuhn structure is meta-stable

Kuhn structure might have favorable conductivity (Ef≈0.2eV & Ea≈0.4eV)

Mercier structure is the ground-state structure for Na4P2S6 & Li4P2S6

3/17/2016 APS 2016 March Meeting 14

“In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual”

- Galileo Galilei

Questions?

*Cartoon made by Maxwell Turner