Solid electrolytes for battery applications –
a theoretical perspective a
Natalie Holzwarth and Nicholas Lepley
Yaojun Du (previously)
Department of Physics, Wake Forest University, Winston-Salem, NC, USA
• Introduction and motivation for solid electrolytes
• What can computation do for this project?
• Specific examples – LiPON, thio phosphates, other solid electrolytes
• Suggestions for collaboration between theory and experiment
a Supported
by NSF grants DMR-0427055, 0705239, and 1150501; WFU’s DEAC computer cluster.
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Materials components of a Li ion battery
Li
+
−
+
Cathode Electrolyte Anode
−
e
V = IR
I
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Solid vs liquid electrolytes in Li ion batteries
Solid electrolytes
Disadvantages
Advantages
1. Excellent chemical and physical stability.
2. Perform well as thin film (≈ 1µ).
3. Li+ conduction only (excludes electrons).
1. Thin film geometry provides poor contact area
for high capacity electrodes.
2. Subject to interface stress if electrodes change
size during charge and discharge cycles.
3. Relatively low conductivity per unit area.
Liquid electrolytes
Advantages
Disadvantages
1. Excellent contact area with high capacity
electrodes.
2. Can accomodate size changes of electrodes
during charge and discharge cycles.
3. Relatively high conductivity per unit area.
1. Relatively poor physical and chemical stability.
2. Relies on the formation of “solid electrolyte
interface” (SEI) layer.
3. May have both Li+ and electron conduction.
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Current solid state usage
Ref: Presentation by Prof. Kevin S. Jones, Department of Materials
Science, and Engineering, University of Florida
• < 1% of the battery business is invested in all solid state batteries at
the present time.
• Several companies are involved in all solids state battery
development: Cymbet, Excellatron, Front Edge, Infinite Power,
Sakti3, Seeo, Toyota/AIST, Planar Energy.
• Micro battery applications are in production; larger applications are
perhaps possible??
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Example: http://www.frontedgetechnology.com/gen.htm
R
“NanoEnergy
is a miniature power source designed for highly space limited micro devices such as smart
card, portable sensors, and RFID tag. ”
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Another example: http://www.planarenergy.com
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Conductivity profiles of electrolytes
Ref: N. Kamaya et al Nature Materials 10 682-686 (2011)
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Comments on Nature Materials article
Ref: N. Kamaya et al Nature Materials 10 682-686 (2011)
• Li10 GeP2 S12 ; crystal structure determined by X-ray and neutron diffraction
• Measured the highest lithium ion conductivity ever measured for a solid:
σ = 0.012 S/m at room temperature and σ = 0.001 S/m at -20o F. (Exceeding
performance of organic liquid electrolytes.)
• Comment by Christian Masquelier: “The results of Kamaya et al demonstrate how
continuous investment into energy storage systems – such as the 30-year-long effort
that Japanese authorities and companies in particular have made in lithium-ion
technology – can lead to significant breakthroughs arising from previously
disregarded chemistries.”
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How can computer modeling help?
• Given a class of materials, can find optimal structures and estimate their heats of
formation. ⇒ Can predict new materials
• For each stable structure, can estimate phonon spectra.
• Using Nudged Elastic Band approximation (NEB), can estimate activation energy
for ion mobility.
Combined experiment and computer modeling
• Validation and refinement of materials prediction and analysis
• Provide a framework for understanding ion migration mechanisms
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Computational methods – (brief summary)
Em from “Nudged elastic band”a
estimate of minimal energy path:
Quantities derived from
min{Ra } E({Ra }):
• Stable and meta-stable structures
• Lattice lattice vibration modes and frequencies (ν)
• Heats of formation (∆H)
Em
• Migration energies (Em )
• Energies for interstitial-vacancy pair
formation (Ef )
a Jónsson et al in Classical and Quantum Dynam-
ics in Condensed Phase Simulations, edited by Berne
et al (World Scientific, 1998), p. 385; Henkelman et al,
J. Chem. Phys. 113 9901, 9978 (2000).
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LiPON family of materials – Lix POy Nz
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Li3 PO4
7
7
1
43
5
2
6
8
2c
3
5
2
4
2c
6
1
2a
2b
a
γ-Li3 PO4
2b
β-Li3 PO4
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Li3 PO4 (different view)
c
a
b
γ-Li3 PO4
β-Li3 PO4
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Raman spectra – Experiment & Calculation
Exp. A
Exp. B
Exp. C
Exp. D
LDA
LDA
(PAW)
(PAW)
LDA
LDA
(USPP)
(USPP)
GGA
GGA
(USPP)
(USPP)
200
400
600
-1
ν (cm )
800
1000
200
400
γ-Li3 PO4
600
-1
ν (cm )
800
1000
β-Li3 PO4
Exp A: Room temp.: B. N. Mavrin et al, J. Exp. Theor. Phys. 96, 53 (2003)
Exp B: Room temp. & Exp C: Liquid N2 temp.: F. Harbach et al, Phys. Stat. Sol. (b), 66 237 (1974)
Exp D: Liquid N2 temp.: L. Popović et al, J. Raman Spec. 34 77 (2003)
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Some heats of formation
Material
Li2 O
Li2 O2
β-Li3 PO4
γ-Li3 PO4
LiPO3
Li4 P2 O6
Li4 P2 O7
Li7 P3 O11
α-Li3 N
γ-Li3 N
s1 -Li2 PO2 N
s2 -Li2 PO2 N
Li5 P2 O6 N
Li8 P3 O10 N
LiNO3
h-P2 O5
α-P3 N5
Structure
F m3̄m (#225)
P 63 /mmc (#194)
P mn21 (#31)
P nma (#62)
P 2/c (#13)
P 3̄1m (#162)
P 1̄ (#2)
P 1̄ (#2)
P 6/mmm (#191)
F m3̄m (#225)
P bcm (#57)
Aem2 (#39)
P 1̄ (#2)
P 1̄ (#2)
R3̄c (# 167)
R3c (#161)
C2/c (#15)
∆H (eV per formula unit)
(USPP)
(EXP)
-6.18
-6.20
-6.54
-6.57
-21.41
-21.38
-21.72
-12.85
-30.03
-34.26
-55.32
-1.64
-1.71
-1.19
-12.48
-12.51
-33.49
-54.85
-5.49
-5.01
-15.54
-15.53
-3.24
-3.32
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Phosphate chain materials: LiPO3 plus N
LiPO3 in P 2/c structure; 100 atom unit cell
s1 -Li2 PO2 N in P bcm structure; 24 atom unit cell
Chain direction perpendicular to plane of diagram
Chain direction perpendicular to plane of diagram
2a
c−a
2b
Ball colors:
•
a+c
=Li, =P,
• •=O.
2c
Ball colors:
•=Li, •=P, •=O, •=N.
Single chain view
Single chain view
b
c−a
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Two forms of Li2 PO2 N
a
a
b
b
c
s1 -Li2 PO2 N
c
s2 -Li2 PO2 N
Possible exothermic reaction pathways:
1
5 P2 O5
+ 15 P3 N5 + Li2 O → Li2 PO2 N + 2.5 eV.
Li2 O2 + 15 P3 N5 + 25 P → Li2 PO2 N + 5.3 eV.
LiNO3 + Li + P → Li2 PO2 N + 12 O2 + 7.0 eV.
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Summary of measured and calculated conductivity
parameters in Lix POy Nz materials
exp
Measured activation energies EA
compared with calculated migration energies
cal
cal
for vacancy (Em
(vac.)) and interstitial (Em
(int.)) mechanisms and
vacancy-interstitial formation energies (Efcal ). All energies are given in eV.
Form
exp
EA
γ-Li3 PO4
single crystala
1.23, 1.14
Li2.88 PO3.73 N0.14
poly cryst.
0.97
Li3.3 PO3.9 N0.17
amorphous
0.56
Li1.35 PO2.99 N0.13
amorphous
0.60
LiPO3
poly cryst.
1.4
LiPO3
amorphous
0.76-1.2
s1 -Li2 PO2 N
single crystal
LiPN2
poly cryst.
0.6
Li7 PN4
poly cryst.
0.5
Material
cal (vac.)
Em
cal (int.)
Em
Efcal
0.7, 0.7
0.4, 0.3
1.7
1.3, 1.1
0.6, 0.7
0.7
1.2
1.1-1.2
0.5, 0.6
1.7
1.3-1.5
0.4
2.5
1.7
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cal
EA
18
Conductivity profiles of electrolytes
Ref: N. Kamaya et al Nature Materials 10 682-686 (2011)
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LiPON and LiS2 -P2 S5 conductivities
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Structure of Li7 P3 S11
2c
4b
2a
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Structure of Li7 P3 S11 – another view
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Some heats of formation
Material
Li2 S
Li2 S2
β-Li3 PS4
γ-Li3 PS4
Li4 P2 S6
Li4 P2 S7
Li7 P3 S11
Li7 P3 S∗11
SO3
Li2 SO4
Structure
F m3̄m (#225)
P 63 /mmc (#194)
P mn21 (#31)
P nma (#62)
P 3̄1m (#162)
P 1̄ (#2)
P 1̄ (#2)
P 1̄ (#2)
P na21 (#33)
P 21 /a (#14)
∆H (eV per formula unit)
(USPP)
(EXP)
-4.29
-4.57
-4.09
-8.36
-8.17
-12.41
-11.58
-20.00
-19.93
-4.83
-4.71
-14.74
-14.89
Some decomposition reactions
Reaction
Li7 P3 S11 → Li3 PS4 + Li4 P2 S7
Li7 P3 S11 → Li3 PS4 + Li4 P2 S6 + S
Li7 P3 S∗11 → Li3 PS4 + Li4 P2 S7
Li7 P3 O11 → Li3 PO4 + Li4 P2 O7
Li8 P3 O10 N → Li3 PO4 + Li5 P2 O6 N
∆H (eV)
0.06
-0.69
-0.01
-0.35
-0.06
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Some energy path diagrams for Li vacancy migration
Epath (eV)
0.4
Along c axis
0.3
0.2
0.1
0
1
Epath (eV)
0.4
2
5
6
3
1
2
7
4
3
1
3
4
Along a axis
0.3
0.2
0.1
0
1
4
1
2c
1
3
2
6
5
2
Epath (eV)
0.4
1
5 4
6
0.3
0.2
0.1
7
7
Along b axis
0
4
3
3
1
4b
2a
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Possible structure of Li8 P3 O10 N
a
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Possibilities for collaboration
• Experimental verification (or otherwise) of the predicted Li2 PO2 N structure
• Iterative refinement of other LiPON and LiPS materials
• Investigation of new materials such as the supersuper ionic conductor Li10 GeP2 S12 .
General comments
• From “theory” perspective – there is a need to know the range of predictive power of
“first-principles” calculations
• From the renewal energy perspective – there is an incentive to make progress on
solid state electrolytes
• From the prospective of possible projects – there seem to be many possibilities
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