XPS chemical shifts calculations: confrontation of...

Germain VallverduUPPA / IPREM Roscoff, May 21-27, 2016

XPS chemical shifts calculations : confrontation of experimentaland theoretical investigations

Germain VALLVERDU

XPS chemical shiftsGermain Vallverdu, UPPA / IPREM

Roscoff, May 21-27, 2016 – 2/38

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1 X-Ray Photoemission spectroscopy

2 Computational approach of core level binding energies

3 Application to the study of Li-ion battery materials

Core level calculations on LiPON models

Surface reactivity of layered lithium oxides

4 Conclusion


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4 Conclusion

XPS chemical shiftsGermain Vallverdu, UPPA / IPREM Roscoff, May 21-27, 2016 – 4/22

XPS spectra

Computationalapproach

Li-ionLiPON ElectrolyteSurface reactivity

Conclusion

X-Ray Photoemission Spectroscopy (XPS)Basics of the method

The binding energy (BE) of an electron into a core level iscomputed from the photon energy and the kinetic energy ofthe emitted electron

BE = hν − Ekinet = EcN−1 − EN

BE depends on• Orbital of the emitted electron• Chemical environment

Valence bands : P2O5 O1s core level of P2O5


XPS spectra



Conclusion

Valence band spectraA trivial approach

• Calculation from the density of states of valence electrons• Atomic contributions are weighted by photoionisation cross-sections

VB =∑A,o

σA,o × DOSA,o

γ-Li3PO4 valence band photoionisation cross-sectionAtom Orbital σA,o

Li 2s 0,0008

O2s 0,1405

2p(1/2) 0,00652p(3/2) 0,0128

P3s 0,1116

3p(1/2) 0,01243p(3/2) 0,0244

J.H. Scofield, Journal of Electron Spectroscopyand Related Phenomena, 1976, 8, 129-137


XPS spectra



Conclusion

Core level binding energy and chemical shiftsEmpirical point of view and first approach

Core level shifts, initial and final state effects• Core level shifts (CLS) depends on the chemical environment.• CLS splits in a final state and initial state effects.

CLS(ψc) = BEmat, c − BEref, c

CLS(ψci ) =

{Emat, c

N−1 − Eref, cN−1

}−

{Emat

N − ErefN

}final state shift initial state shift

Empirical approachBased on initial state effect :

• Linear behavior of the BE with atomiccharge

• BE decreases when electronic populationincreases

LiCoO2 surface

XPS spectra

L. Dahéron et al., J. Phys. Chem. C, 2009, 113, 5843-5852


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4 Conclusion


XPS spectra



Conclusion

How to compute core level binding energiesConstraints and key issues

System and formalism we would like• Periodic systems• Density functional theory calculations• Plane waves basis set

Janak theorem

Considering fractional occupation numbers, η, the Janak theorem reads

∂E∂ηi

= ϵi BEc = EN−1 − EN =

∫ 0

1

ϵ(ηi)dηi

Slater-Janak transition state approximation

Assuming εi is a linear function of the occupation number η∫ 0

1

ε(ηi)dηi = −εi(0)−1

2[εi(1)− εi(0)] ≈ −εi(1/2)

J.F. Janak, Phys. Rev. B 1978, 18, 7165.C. Göransson, Phys. Rev. B 2005, 72, 134203.


XPS spectra



Conclusion

VASP implementation of core level eigenstates calculationsPAW pseudo-potentials and Z+1 approximation

Computational issues• Core electrons are frozen• Calculations on charged cells are not affordable• Core electrons are not described by atomic orbitals

Z+1 approximation and PAW implementation

all electrons pseudo functionpseudo functionin PAW sphere

all electron inPAW sphere

= - +

|ψ⟩ =∣∣∣ ψ ⟩

+∑

i

(|ϕi⟩ −

∣∣∣ϕi⟩)⟨

pi

∣∣∣ψ⟩• A fraction of electron is extracted from the core and put in the conduction band• The charge nucleus is increased of the same fraction• Assume valence electrons screen the core hole treated as a default• Khon Sham equations are solved in the PAW sphere


XPS spectra



Conclusion

Validity of the computational approachIonic compounds and linearity

Core eigenstates as a functionof occupation number

-220

-200

-180

-160

-140

-120ei

gen

ener

gy /

e

V

P3N

5

P2O

5

Li3PO

4

Li2PO

2N

-570

-555

-540

-525

-510

-495

eigen

ener

gy /

e

V

00.20.40.60.81Occupation number

-435

-420

-405

-390

-375

eigen

ener

gy /

e

Va) P2p

b) O1s

c) N1s

E. Guille, G. Vallverdu et al., J. Chem. Phys.,2014, 141, 244703

System ϵi(1) ϵi(1/2) integralP2O5 O1s -502.33 -536.76 -536.30P3N5 N1s -370.92 -400.87 -400.64γ-Li3PO4 O1s -502.36 -537.35 -535.92Li2PO2N O1s -501.67 -536.06 -534.83

N1s -371.63 -402.20 -401.45

CLScmat = ϵc

mat − ϵcref

• Variation of chemical shifts with respect of P2O5or P3N5 are within 500meV

• γ − Li3PO4 example :

CLSO1s(1/2)− CLSO1s(int) = 0.59− 0.38

= 0.21eV

Main cavits• Janak theorem is limited to the HOMO• Linearity is achieve for deep core state• No spin-orbit coupling for 2p core state


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4 Conclusion


XPS spectra



Conclusion

Interface phenomena in Li-ions batteriesSubstantial proportion of interfaces generated during the electrochemical cycles

Li-ions battery

Electrode + Electrode -

Electrolyte

Electron

ElectronElectron

Electron

SEI

Microbattery

Substrate : 1 mm2 to 1 cm2

electrode 1Electrolyte

electrode 2collector collector

10µ

m

Liquid electrolyte cycling• Formation of a Solid Electrolyte

Interphase (SEI)• Result of reductive electrolyte

degradation processes

Solid électrolyte cycling (microbatteries)• solid stacking of electrodes and

electrolyte (LixPOyNz for example)

Conversion materials (higher capacity)• MaXb + (bn)Li −−→ aM + b LinX• Redox processes lead to a composite

electrode (metallic nano sized particlesembedded into a LinX matrix)


XPS spectra



Conclusion

Commonly used solid electrolyte in commercial cellsLiPON, a LixPOyNz glass

Assumed structure and migration model of LixPOyNz

Key issues

• What are the recurent patterns in LixPOyNz glasses ?• Nitrogen coordination and role in migration mechanism• Strucutre of LixPOyNz at the interface

R. Marchand et al., J. Non-Crystalline Solids, 1988, 103, 35E. Guille, G. Vallverdu et al., J. Chem. Phys., 2014, 141, 244703


XPS spectra



Conclusion

LiPON electrolyte : periodic modelsNitrogen doped systems from lithium phosphate compounds

Dim. LixPOyNz Chain LixPOyNz Li2PO2N

Y.A. Du et al, Phys. Rev. B 2010, 81, 184106

Computed O1s, N1s and P2p Bending energiesO1s (nb) O1s (b) N1s P2p

Dim. LixPOyNz 532.6 395.6 131.6Chain LixPOyNz 532.0 533.2 394.8 130.6

s1-Li2PO2N 533.3 395.7 132.3s2-Li2PO2N 533.3 395.7 132.4s3-Li2PO2N 532.1 394.7 131.3

Exp.(∗) 532.3 533.5 397.9 132.8–133.8(∗)B. Fleutot et al., Solid State Ionics, 2011, 186, 29-36

All experimental dataare not simultaneouslyreproduced.

E. Guille, G. Vallverdu et al., J. Chem. Phys., 2014, 141, 244703


XPS spectra



Conclusion

Structural modification of nitrogen dopped structurePossible existence of non-bridging nitrogen

P

O

O

-

-

N

P

O

O

-

-

NP

O

O

-

-

a) Li2PO2N

P

O

O

-

-

O

P

N

O

-

-

NP

O

O

-

-

b) n(Li2PO2N)mod

P

O

O

-

-

N

P

O

O

-

-

NP

O

O

-

-

a) Li2PO2N

P

O

O

-

-

O

P

O

O

-

-

NP

O

O

-

-

b) Li1.5PO2.5N0.5

N1s Bending energies

Conclusion• Presence of non-bridging nitrogen shifts up BE toward experimental value• Available periodic models all fail to reproduce XPS data

Relevant patterns identified from MD amorphisation followed by XPS and Ramancalculations

E. Guille, G. Vallverdu et al., J. Phys. Chem. C, 2015, 119, 23379


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4 Conclusion


XPS spectra



Conclusion

Surface reactivity of layered lithium oxidesA coupled experimetal-theoretical study

Synthesis and elaboration : Manage surface and morphology

Experimental sample

Computational approach• model compounds• relevant surface model

Surface reactivity : Indirect approach from gaz probes adsorption

XPS characterisation

Identification of active sites(type/number)

DFT periodic calculations

• Electronic processes• Electronic density analyses


XPS spectra



Conclusion

SO2 adsorption on (101) and (001) surfaces of Li2MnO3XPS scale of S2p chemical shifts

Slater-Janak transition state approximation

Assuming εi is a linear function of the occupation number η

EN−1 − EN =

∫ 0

1

ε(ηi)dηi = −εi(0)−1

2[εi(1)− εi(0)] ≈ −εi(1/2)

Li2MnO3 surfaces used to investigatereactivity aginst SO2

• (110) and (001) surfaces• Sulfate (bridge) and sulfite (top) sites

CLS of S2p


XPS spectra



Conclusion

SO2 adsorption on (101) and (001) surfaces of Li2MnO3Comparison with experimental results

Comp.Exp. 169 eV 167 eV 162 eV

M

SO2

Dissociative

Redox

OS O

OSulfite AX3E1

Ac/Base

O OS

O OSulfate

RedoxOS

O OSulfite AX3

Redox

sulfate (110)qS=3.60

sulfite (001)qS=3.35



• Identification of adsorbed species• Electronic processes taking place• Lack of spin orbit coupling for S2p core level


XPS spectra



Conclusion

SO2 adsorption on (101) and (001) surfaces of Li2MnO3Comparison with experimental results

Comp.Exp. 169 eV 167 eV

167.1 eV

162 eV

168.8 eV 168 eV

M

SO2

Dissociative

Redox

OS O

OSulfite AX3E1

Ac/Base

O OS

O OSulfate

RedoxOS

O OSulfite AX3

Redox

?





• Identification of adsorbed species• Electronic processes taking place• Lack of spin orbit coupling for S2p core level


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4 Conclusion


XPS spectra



Conclusion

Conclusion and perspectivesMainly perspectives ...

VASP implementation of the Janak theorem

• The Slater-Janak transition state approximation may be relevant for insulators

• We applied the method to• better understand CLS and corel level structure• the identification of reccuring patterns in an amorphous solid electrolyte• the investigation of surface reactivity of layered lithium oxides• the identification of oxydation degrees of transition metals

Theoretical challenges about core level bending energy calculations

• Core electrons relaxations with reasonnable computational cost• Core excited state of cations in periodic calculations : multideterminental approach ?• Spin-orbit coupling for p and d core states• Cross sections of ionization processes


Émilie Guille Isabelle Baraille Didier Bégué Yann Tison

A. Quesne-Turin Delphine Flahaut L. Croguennec Michel Ménétrier

Thank you for your attention

CONTACT

Germain Vallverdu

UPPA / IPREM

[email protected]

http://gvallver.perso.univ-pau.fr

@gvallverdu

http://gvallver.perso.univ-pau.fr


XPS spectra



Conclusion

LixPOyNz agregates obtained from molecular dynamics simulationsRecurent pattern and chain lengths

Amorphisation of � –Li3PO4 : LiPON in silicoComputational details

• DL_POLY• Born-Mayer pair potential• NPT simulations (1-2 ns)• box 32×32×26 Å (2400 atoms)

Simulation box before and after amorphisation

Conclusion• PO4

3– tetrahedron form short chains of 2 to 3 units.• Agree with HPLC measurements (Wang et al J. Non-Crystalline Solids, 1995, 183, 297)



XPS spectra



Conclusion

Vibrational characteristics of LiPON agregatesSimulation of Raman spectra

Raman experimental spectra Simulated Raman spectra

600 700 800 900 1000 1100 1200

wavenumber (cm-1

)

0

0.5

1

1.5

Rel

ativ

e in

ten

sity

LiPON-40 (exp)

C0, C2, C4

C0, C2, C3

C0, C1, C2

695 cm-1

800 cm-1

950 cm-1

1025 cm-1

case 3

case 2

case 1

• Raman spectra are correctly reproduced by superimposing individual spectra of agregates• 695 - 800−1 region is described by a combination mode involving non-bridging nitrogen

atomsRelevant patterns

C0 C3 C4

XPS chemical shifts calculations: confrontation of...

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