Space charge solid electrolytes · 2017. 9. 12. · Space charge solid electrolytes. Principles and...

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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