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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
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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
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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
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Mercier structure: Space Group P63/mcm (#193)Disorder in P-P placements:
S
P
Li
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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
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Structural comparison – c-axis projection NaPS
a
a x c
Results
Mercier structures Kuhn structure
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Structural comparison – view including c-axis
Na4P2S6
NaPS
c
a x c
Mercier (�3�1m) Kuhn (C2/m)
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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
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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
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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
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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