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Challenge the future
DelftUniversity ofTechnology
Space charge solid electrolytesPrinciples and perspectives
Lucas Haverkate
TNW / R3 / FAME 15-10-2012
2Space charge solid electrolytes: principles and perspectives
Motivation
Principles
Perspectives
Outline:Outline
2
3Space charge solid electrolytes: principles and perspectives
Motivation
• Batteries
� Less space required for electrolyte
� Li-metal (and sulfur) battery possible
• Fuel cells
� Water free conduction mechanism
� Operate at higher temperatures
3
The advantages of solid electrolytes
4Space charge solid electrolytes: principles and perspectives
Motivation
• Batteries
� Less space required for electrolyte
� Li-metal (and sulfur) battery possible
3
The advantages of solid electrolytes
Li-metal dendrite growth within liquid electrolyteTarascon, Nature 2001
5Space charge solid electrolytes: principles and perspectives
Motivation
3
The advantages of solid electrolytes
• Fuel cells
� Water free conduction mechanism
� Operate at higher temperatures
�Higher overall efficiency
�Easier cooling
�Less poisoning of expensive catalyst
�No humidification systems needed
6Space charge solid electrolytes: principles and perspectives 4
Norby, Nature 2001 Haile et. al., Nature 2001
Solid acids as fuel cell electrolytes
• Acids in solid crystalline phase at room temperature
• Examples: CsHSO4 , CsH2PO4
• Superprotonic phase transition
CsHSO4
7Space charge solid electrolytes: principles and perspectives 4
Solid acids as fuel cell electrolytes
1000/T / K-12.2 2.4 2.6 2.8
-6
-5
-4
-3
-2
Log (
σ/ S/cm)
bulk CsHSO4
Phase I:superprotonic
Wang et. al. Solid State Ionics 176 (2005) 755
Only high conductivity in superprotonic phase
Problem!Phase II
8Space charge solid electrolytes: principles and perspectives
Solution: nanostructuring
5
Wang et. al. Solid State Ionics 176 (2005) 755
1000/T / K-12.2 2.4 2.6 2.8
-6
-5
-4
-3
-2
Log (
σ/ S/cm)
bulk (CsHSO4)
x100
Nanocomposite(SiO2 – CsHSO4)
Large conductivity increase in nanocomposites
Nanoparticles
(TiO2, SiO2; 7-40nm)
+
Solid acids
(CsHSO4,CsH2PO4)
9Space charge solid electrolytes: principles and perspectives
Motivation
Principles
Perspectives
Outline:
6
10Space charge solid electrolytes: principles and perspectives
Proton mobility in nanocomposites
bulk CsHSO4;
CsHSO4+TiO
2 24nm;
CsHSO4+SiO
2 7nm;
CsHSO4+SiO
2 40nm
From Quasi Elastic Neutron Scattering + NMR
Chan, Haverkate et. al., Advanced Functional Materials 2011
7
11Space charge solid electrolytes: principles and perspectives
Proton mobility in nanocompositesFrom Quasi Elastic Neutron Scattering + NMR
7
bulk CsHSO4;
CsHSO4+TiO
2 24nm;
CsHSO4+SiO
2 7nm;
CsHSO4+SiO
2 40nm
Chan, Haverkate et. al., Advanced Functional Materials 2011
12Space charge solid electrolytes: principles and perspectives
ND on liquid D2SO4–TiO2 system
D+ position: [0.0 0.75 0.43]
• TiO2 has been well investigated with Li-ion insertion*
*M. Wagemaker et. al., Nature 2002W.K.Chan et. al. Chemical Communications 2008
8
13Space charge solid electrolytes: principles and perspectives
ND on solid CsDSO4-TiO2 composite
CsDSO4-TiO2 15 nm
CsDSO4-TiO2 24 nm
Bulk CsDSO4
D+ position: [0.0 0.75 0.43]
9
14Space charge solid electrolytes: principles and perspectives
Size dependent D insertion in TiO2
10
Large mobility increase due to proton relocation
Chan, Haverkate et. al., Advanced Functional Materials 2011
15Space charge solid electrolytes: principles and perspectives
Explaining the size effectsSpace charge layers in TiO2-CsHSO4 composites?
Haverkate et. al., Advanced Functional Materials 2010
11
16Space charge solid electrolytes: principles and perspectives 11
• More interface → larger effect
• No blue coloration of composites →neutral intercalation excluded
• H+ concentrations (~ 1021 cm-3) are extremely high for space charge effects
Large space charge effects?
Explaining the size effectsSpace charge layers in TiO2-CsHSO4 composites?
17Space charge solid electrolytes: principles and perspectives
What is the space charge effect?
Donating phase (CsHSO4)
= + (H = H+ + e-)
Accepting phase (TiO2)
available sitesfor (H+)
Chemical potential
interface
12
18Space charge solid electrolytes: principles and perspectives
Donating phase (CsHSO4)
= + (H = H+ + e-) interface
Accepting phase (TiO2)
available sitesfor (H+)
Electrical potential Chemical potential
12
19Space charge solid electrolytes: principles and perspectives
Particle size dependence
13
Smaller sizes -> larger influence of interfacial region
20Space charge solid electrolytes: principles and perspectives
Small vs. large space charge effects
• Small: Boltzmann approximation
low defect concentrations
Examples:
Si p-n junctions
BaF2/CaF2 nanocomposites*
*Maier, Nature Materials 2005
00
lnB
ck T
cµ µ= +
00
lnB
ck T
cµ µ= +
14
high defect concentrations
00
lnB
ck T
c cµ µ= +
−0
0lnB
ck T
c cµ µ= +
−
• Large: Fermi-Dirac distribution
21Space charge solid electrolytes: principles and perspectives 15
Formation enthalpy + particle sizeLow ∆GF + nanoparticles = large space charge effects
DDD DDD DDDAAA AAA AAA
Layer thickness
phase center
T = 300 K; c0 (A,D)= 10 nm-3 ; εA = εD = 10
22Space charge solid electrolytes: principles and perspectives
Application to TiO2-CsHSO4 system
16
DFT calculations: proton insertion energetically
favorable
23Space charge solid electrolytes: principles and perspectives
Theory vs. experiment: CsHSO4-TiO2Space charge model explains observed intercalation
17
7-40 nm
Space charge layer thickness: ~ 0.3 nm
(24% of 7 nm particle!)Haverkate et. al., Advanced Functional Materials 2010
24Space charge solid electrolytes: principles and perspectives
DFT calculations of formation energy Fourier difference maps from neutron diffraction
reflection lossdeuterium
bulk CsD2PO4
1CsD2PO4 - 3TiO2 16 nm
Theory vs. experiment: CsH2PO4-TiO2Space charge model explains less D-reflection in acid
18
25Space charge solid electrolytes: principles and perspectives
Amorphization of the solid acid phaseDepleted interfacial regions relax over time
19
1,5 2,0 2,5 3,0
a.u.
d-space
* * *
*
* *
XRD on 1:2 CsHSO4 – TiO2 (7 nm)
1st day
2 weeks
2 months
ND on CsD2PO4 – TiO2
Calculated grain size CDP (nm)
Amorphous fraction
26Space charge solid electrolytes: principles and perspectives
Amorphization of the solid acid phaseDepleted interfacial regions relax over time
19
Calculated space charge profilesND on CsD2PO4 – TiO2
Calculated grain size CDP (nm)
Amorphous fraction
27Space charge solid electrolytes: principles and perspectives
Apparent morphology: “Swiss cheese”Nanoparticles embedded in solid acid matrix
20
28Space charge solid electrolytes: principles and perspectives
Motivation
Principles
Perspectives
Outline:
21
29Space charge solid electrolytes: principles and perspectives
The promise of nanostructuringPercolation of space charge regions improves ionic
charge carrier transport
22
30Space charge solid electrolytes: principles and perspectives
Tuning the materials parameters
23
Example: optimization of conductivity in solid
acid nanocomposites
Data CDP: Otomo, Electrochimica Acta 2008
31Space charge solid electrolytes: principles and perspectives
Solid acid proton conductors
24
For fuel cell application
*Uda, Solid state ionics 2005
� Vacancies in nanostructured acids solve the ‘traffic jam’ in low
temperature phase
� Large conductivity increase -> RT-300 0C operation possible
� Questions: water solubility, mechanical and chemical stability*
32Space charge solid electrolytes: principles and perspectives
Li-ion solid electrolytes
25
For battery application
� Similar space charge layer principles
� Examples: LATP Ceramics*, Halides (LiX) **
� Insulators can become ionic conductors
*Kumar et. al., Journal of the Electrochemical Society 2009; **Maier, PCCP 2009
LiI composite** LiTi2(PO4)3 ceramics*
Li
LiI
Al2O3
33Space charge solid electrolytes: principles and perspectives
Space charge solid electrolytes
26
For fuel cell and battery application
New solid nanocomposites can be rationally designed for future application
34Space charge solid electrolytes: principles and perspectives
Acknowledgements
2
• TU Delft: Prof. Fokko Mulder (supervisor); Winkee Chan,
Jouke Heringa, Prof. Ignatz de Schepper
• RU Nijmegen: Prof. Arno Kentgens and Ernst van Eck
• ISIS (Oxford, UK): Ron Smith
• ILL (Grenoble, France): Prof. Mark Johnson
• ANSTO (Menay, Australia): Prof. Don Kearley
• Financial support:
26
35Space charge solid electrolytes: principles and perspectives
The role of the formation enthalpy
18
• Free formation enthalpy
• Defect concentrations near interface
0 0FG µ µ+ −∆ = +
0 0FG µ µ+ −∆ = +
0 0
(0) (0)
(0) (0)exp[ ]F
B
c c
c c c c
G
k T+ −
+ + − −− −
∆= −0 0
(0) (0)
(0) (0)exp[ ]F
B
c c
c c c c
G
k T+ −
+ + − −− −
∆= −
-1.0 0.0 1.00.00.20.40.60.81.0
∆GF (eV)
c(0)
/ c0
-1.0 0.0 1.00.00.20.40.60.81.0
∆GF (eV)
c(0)
/ c0
Negative ∆GF = large effects
…of an interfacial defect pair
36Space charge solid electrolytes: principles and perspectives
1D solution for large effects
17
0
0
( )( ) ln ( )
( )B
c xx k T ze x
c c xµ µ ϕ= + +
−%
0
0
( )( ) ln ( )
( )B
c xx k T ze x
c c xµ µ ϕ= + +
−%
0
( ) ( )x zec x
x
ϕε ε
∂= −
∂ 0
( ) ( )x zec x
x
ϕε ε
∂= −
∂
( )0
x
x
µ∂=
∂
%( )
0x
x
µ∂=
∂
%
DDD DDD DDDAAA AAA AAA
• Electrochemical potential
• Transport equilibrium
• Poisson equation
• Boundary conditions 1D model
1. Zero E field in phase center
2. No charge at surface
3. No space between phases
1D periodic multilayer model
1
2+3
37Space charge solid electrolytes: principles and perspectives
1D defect profiles for TiO2-CsHSO4
21
-0.4 -0.2 0.0 0.2 0.4 0.60.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
x (nm)
4CsHSOTiO
SO- fraction Defect fraction
Ele
ctric
pot
entia
l (eV
)
H+ fraction
2
4
-0.4 -0.2 0.0 0.2 0.4 0.60.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
x (nm)
4CsHSOTiO
SO- fraction Defect fraction
Ele
ctric
pot
entia
l (eV
)
H+ fraction
2
4
Region of about 0.3 nm with large defect fraction
TiO2: particle size 7 nm, ε=37.6, c0= 58.6 nm-3 ; CsHSO4: 4.2 nm, ε=10 , c0= 8.8 nm-3