radiolysis of carbonates: a tool to determine reaction

1 Journée Jacqueline Belloni

Radiolysis of carbonates: a tool to determine reaction mechanisms at stake in electrolytes of

lithium-ion batteries

Sophie Le Caër

CEA, DRF, IRAMIS, NIMBE, UMR 3685, Gif Sur Yvette, France


Most common battery technologies

LIBs market is projected to cross the figure of US $ 40 Billion by the end of year 2024

The commercial use of LIBs is increasing drastically due to many electronic applications

J.-M. Tarascon & M. Armand, Nature 414, 359 (2001)


Degradation processes in lithium-ion batteries

Lithium ion batteries (LIBs)

H2, HF…SafetySlow electrolyte decomposition

Progressive decrease of the battery performance

Here we will focus onthe stability ofelectrolytes

Ageing in LIBs

× Reduce the cycle life of LIBs× Production of hazardous compounds


Systems under study: carbonates

Commercial electrolytes: solutions of lithium salts (LiPF6, 1 mol.dm-3) in mixtures of carbonates.

Main goals:(1) Identify and quantify decomposition products

in irradiated diethyl carbonate with LiPF6

(2) Compare the degradation products with those obtained by electrolysis(3) Compare the reactivity of cyclical and non-cyclical carbonates

Full understanding of the reaction mechanisms (from ps to minutes!)

J. Phys. Chem. A, 2013, 117, 10801

Diethyl carbonate

Dimethyl carbonateEthylene carbonate

EC PCDMC

DEC Propylene carbonate


The case of a linear carbonate: diethyl carbonate in the presence of LiPF6


Comparing gas phase results for the DEC/LiPF6 system

0 2 4 6 8 10 12 14 16 18 200.0

3.0x105

6.0x105

9.0x105

Inte

nsity

(a.u

)Time (min)

0 2 4 6 8 10 12 14 16 18 200.0

5.0x104

1.0x105

1.5x105

2.0x105

Inte

nsity

(a.u

)

Time (min)

Ar

H2

CO

CO2

CH4

a)

Ethylene

Ethane

Ethanol

1,1-difluoroethane

Butane

b)

1.0 1.5 2.0 2.5 3.00

1x107

2x107

3x107

Inte

nsity

(a.u

.)

Time (min)

Molecules identifiedalso by EI-MS:H2, CH4, CO, HF

6 8 10 12 14 16 180.0

2.0x105

4.0x105

6.0x105

8.0x105

Inte

nsity

(a.u

.)

Time (min)

CO2

C2H2

C2H4

C2H6

C3H8

C2H5OC2H5

C4H10

Si(CH3)2F2

C2H5F

CH3CHO

HCOOH

a)

b)

ElectrolysisRadiolysis

HF is indirectly identified

Similar decompositionproducts are detected inthe gas phase by radiolysisand electrolysis

Radiolysis allows thedetection of minordecomposition products

GC-EI/MS20 kGy

GC-FTIR1 month at 4.9 V / 55°C

DEC/LiPF6


Comparison between radiolysis and electrolysis

ChemSusChem, 2015, 8, 3605

In an electrolyte, the speciesgenerated during thecharge/discharge cycles ofbatteries and the speciesgenerated by radiolysis aresimilar.

Advantages of radiolysis:possibility of acceleratedaging (minutes / hours vsweeks and months) and alsopossibility of determiningreaction mechanisms

DEC


Picosecond time-resolved spectra of DEC/LiPF6 at ELYSE (Laboratoire de Chimie Physique, Orsay)

0.4 0.6 0.8 1.0 1.2

0.00

0.02

0.04

0.06

400 ps

100 ps20 ps

400 ps

100 ps

Abso

rban

ce /

0.5

cm

Wavelength (µm)

20 ps DEC/LiPF6 0.1 M

DEC/LiPF6 1 M

Intensity increasesmonotonously until the detection limit: attributedto the solvated electronBroad band around 600 nm:

attributed to DEC(-H)•

DEC


Pulse radiolysis experiments on the DEC/LiPF6 mixture: the electron decay kinetics at 1200 nm

LiPF6concentration increases

Precursors of solvated electronsare scavenged at high LiPF6concentration

e- (solvated or beforesolvation) reacts with (Li+, PF6-) ion pairs

k = 7.5 109 mol-1.dm3.s-1

e-DEC + (Li+, PF6

-) à Li+ + PF5●- + F-

ChemSusChem, 2015, 8, 3605


Li+ + F- à LiF(s)

Proposed reaction pathways

(1) First: ionize and excite.DECvvvv à DEC+., e-, DEC*

(3) Radical recombination

F. + C2H5.à C2H5F

(4) DEC hydrogen abstraction19 JUIN 2019F. + DEC HF(DEC-H). +

(2) Reactivity with LiPF6

e- + PF6- à F- + PF5-.

DEC+. + PF6- à F. + PF5

The solution darkens: successive substitutions reactions to form for instance POF(OH)(OC2H5), oligomerizations….

PF6-

vvv à PF6-*, PF6

●, e-

PF6-* à PF5 + F-

PF6● à PF5 + F●

PF5 + H2O à POF3

DEC + POF3 à POF2(OCOOC2H5) à POF2(OC2H5) + CO2C2H5F


The case of a a cyclical carbonate: propylene carbonate


The electron in propylene carbonate: a very reactive species

Transient optical spectra

Broad band assigned to the solvated electron

e-PC + PC à PC●-

Ultrafast decay of the electronin PC due to the formation of the radical anion PC●-

Decay kinetics of the electron in varioussolvents

J. Phys. Chem. Lett., 2016, 7, 186

εs = 66


Formation of the radical anion PC•-

50 ps

Major channel

solvatedelectronin PC

PC•-

Calibrated spectrum of PC•-

Streak camera!

J. Phys. Chem. B, 2016, 120, 2388


Influence of the salt in the PC degradation

Radiolytic yields G (µmol J-1)

H2 CH4 CO CO2

PC 0.13 0.04 0.31 0.31PC/LiPF6 0.14 0.04 0.30 0.59

PC/LiClO4 0.13 0.03 0.28 0.35

PC/LiBF4 0.13 0.03 0.29 0.36PC/LiN(CF3)2(SO2)2 0.12 0.05 0.27 0.35

The presence of the LiPF6changes the reaction pathways. The other salts have globally no effect on the reactivity.

This effect is striking in the case of LiPF6 due to the reaction of the electron with PF6-. The other anions do no react with the electron.

Decay kinetics of the electron in PC atincreasing LiPF6 concentration

J. Power Sources, 2016, 326, 285


Reaction mechanisms accounting for the strong influence of LiPF6 on the reactivity

J. Power Sources, 2016, 326, 285


Conclusion

• Radiolysis allows accelerated ageing studies of electrolytes in lithium-ion batteries as compared to electrolysis (minutes/hours versusweeks/months)→ Similar products obtained by radiolysis and electrolysis and possibilityto get kinetic data and reaction mechanisms

• Radiolysis approach→ Promising tool to simulate the ageing behavior of different anode materialsand facilitate the development of next generation batteries

• Radiolysis allows the formation of aggregation products near the surfaceof the active material (Solid Electrolyte Interface topic, not shown here)

Accelerated and global vision of the phenomena at stake in LIBs


Félicitations Jacqueline pour cette médaille amplement méritée


Quantification is possible….

DEC-H* à Hl

0 5 10 15 20 250

2

4

6

Dose (kGy)

Gas

pro

duce

d pe

r mas

s un

it (m

mol

/kg)

CO2

CH4

CO

C2H6

H2

The same products were identified andquantified in the gas phase

Quantitatively different...

Reaction mechanisms explaining the quantification difference

Electrolysis

RadiolysisDEC

H2 + DEC(-H)l

DEC + e-DEC à Hl + DEC(-H)-

DEC H2 + DEC(-H)lElectrode surface

Radiolysis results

32

471011

32


High resolution mass spectrometry results of compounds due to the presence of the salt

FT-ICR 7 T

m/z120 150 180 210 240

b) Radiolysis

119.076

125.083

135.025 145.041163.057

197.106214.098 220.106 243.140

a) Electrolysis

119.076

125.083

139.100151.099

185.115183.083

214.098 220.106 243.140

183.083

Inte

nsity

(a.u

.)

10 µL of sample in 1mL H2O/MeOH

10 µL of sample in 1mL MeOH/MeCNSimilar set of products

Decomposition products containing Li, P or F atoms

100 kGy

Very good comparison

Just a few molecules are method-specific

Radiolysis

Electrolysis

(R1)(R2)(R3)P=O family products!


Unravelling the complex liquid phase

Liquid phase is much more complex

Dynamic behaviour due to LiPF6 à from colourless to brown during one weekstorage under Argon atmosphere

Analytical techniquesused in our study

Radiolysis

Wilken et al. R. Soc. Chem. Adv. 2013, 3

Electrolysis

GC-EI/MSHigh Resolution Mass SpectrometryInfrared spectra (FT-IR)Ion mobility/ Infrared Multiphoton dissociation19F and 31P NMR

Ortiz et al., ChemSusChem, 2015, 8, 3605

1 week


Dynamic behavior studied by 19F NMR

(R1)(R2)(R3)P=O family products

Excellent comparison with literatureDynamic behaviour depending on time/dose

Broadening and shift of bands

Disappearance of products

hd-sto100 kGy

hd100 kGy

ld20 kGy

SamplesLow irradiation dose (20 kGy)

High irradiation dose (100 kGy)

High irradiation dose (100 kGy) + one week storage

Proposed molecule

PF6-

(OH)2(F)P=O

(F)2(OCO2C2H5)P=O

(F)2(OC2H5)P=O

HF

X-F

Disappearswith time

CO2

formationDecreases with dose

radiolysis of carbonates: a tool to determine reaction

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