Graphite Intercalation with Large Fluoroanions

Dept. of Chemistry and Center for Advanced Materials, Oregon State University

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Intercalation

http://www.cem.msu.edu/~pinnweb/research-na.htm

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Intercalation HostsIon exchange: (fixed charge density)

smectite clay Nax+y[Al2-yMgySi4-xAlxO10(OH)2]

layered double hydroxide [Mg3Al(OH)8]Clmetal phosphorous sulfide K0.4[Mn0.80.2PS3]

Redox reaction: (variable charge density)

metal dichalocogenide Lix[MoS2]

layered oxides Lix[CoO2], Nax[MoO3]

graphite K[C8], [C24]HF2

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EnergeticsFor clays – reaction is ion-exchange:Na+ Mont- + N(R)4

+ Cl- (aqu) -> N(R)4+ Mont- + NaCl (aqu)

For graphite – reaction is redox:Cx + A -> Cx

+ A-

ΔHrxn = I (Cx) - Ea (A) - ΔHL

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Graphite structure C-C in-plane = 1.42 Å Usually (AB)n hexgonal

stacking Interlayer distance

= 3.354 Å

Source: http://www.ccs.uky.edu/~ernst/

A

B

A

Graphite is a semi-metal, chemically stable, light, strong

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Graphite Intercalation

-+

-+

-+

oxidant

This is an acceptor-type GICDonor-type reduces layers and intercalates cations

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GIC typesReduction M+Cx

-

 Group 1 except Na  Oxidation Cx

+An-

F, Br3-, O (OH)

BF4-, P BiF6

- , GeF62- to PbF6

2-, MoF6-, NiF6

2-, TaF6-, Re PtF6

-

SO4-, NO3

-, ClO4-, IO3

-, VO43-, CrO4

2-

AlCl4-, GaCl4-,FeCl4-, ZrCl6-,TaCl6-

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Staging and dimensions

Ic = di + (n - 1) (3.354 Å)

For fluoro, oxometallates di ≈ 8 A, for chlorometallates di ≈ 9-10 A

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Graphite oxidation potentials

H2O oxidation potential vs Hammett acidity

Colored regions show the electrochemical potential for GIC stages.

0.9

1.1

1.3

1.5

1.7

-7 -5 -3 -1 1 3 5

H0

E /

V

stage 1 GIC

stage 2

stage 3

high stage

no intercalation

49% hydrofluoric acidAll GICs are

unstable in ambient atmosphere , they oxidize H2O

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GIC special issues Anions must be oxidatively stable Larger anions could give larger

galleries, wider range of chemistry GICs that rapidly decompose in air

or aqueous acid are hard to process further

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CxPFOS - preparationCx+ K2Mn(IV)F6 + KSO3C8F17

CxSO3C8F17 + K3Mn(III)F6

(CxPFOS)

Solvent = aqueous HF3.35 A

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CxPFOS intercalate structure

Anions self-assemble as bilayers within graphite galleries

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PFOS twist angle Chain twist

defined by FC-CF tortion

angle

          

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CxPFOS thermal stability

0

20

40

60

80

100

0 200 400 600

temp / °C

mas

s pe

rcen

t

-2.0

-1.0

0.0

1.0

2.0

pow

er /

mW

0

20

40

60

80

100

0 200 400 600

temp / °C

-1.0

2.0

5.0

8.0

11.0

14.0

KPFOSCxPFOS

New syntheses: chemical method

   

N SO

OCF3S

O

OF3C ..

Cx + K2MnF6 + LiN(SO2CF3)2 CxN(SO2CF3)2 + K2LiMnF6oxidant anion source GIC

1,2

1. 48% hydrofluoric acid, ambient conditions2. hexane, air dry

Oxidant and anion source are separate and changeable. Surprising stability in 50% aqueous acid.

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CxN(SO2CF3)2 chem prepn

5 15 25 35 45 55

2 / deg

Inte

n / a

rb u

nits

15 sec

1 min

2 min

4 min

8 min

12 min

15 min

4 wks

graphite

   

0

20

40

60

80

100

120

0.1 1 10 100 1000 10000reaction time (h)

x

New syntheses: N(SO2CF3)2 orientation

F

F

F

F

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CxN(SO2CF3)2 echem prepn

2.0

3.0

4.0

5.0

6.0

0 100 200 300 400 500 600

Capacity (mAh/g)

V v

s Li

+ /Li

charge discharge

ab c

d e

100

4.30 4.70 5.10V vs Li+/Li

dQ/d

V

2 13 2

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CxN(SO2CF3)2 - echem prepn

4.0

4.5

5.0

5.5

6.0

0 100 200 300 400

Charge (mAh/g)

V vs

Li+ /L

i

(a)(b)

a

b c

d eb

dCxPFOS

CxN(SO2CF3)2

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CxN(SO2CF3)2 anion orientation

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CxN(SO2CF3)2 thermal stability

0

20

40

60

80

100

0 200 400 600 800

t / °C

Mas

s lo

ss /

pct (a)(b)

(d)

(e)

(c)

LiN(SO2CF3)2

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Imide (NR2-) intercalates

Anion MW di / Å

N(SO2CF3)2 280 8.1

N(SO2C2F5)2 380 8.2

N(SO2CF3) 430 8.3

(SO2C4F9)

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Other intercalated anionsAnion MW di / Å

C(SO2CF3)3 411 12.3

SO3C8F17 499 29.5

SO3C10F21 599 33.7

SO3C6F10(C2F5) 461 24.4

1.12

0.78 nm

CxB(O2C2O(CF3)2)2

Stage 2

1.13

0.85 nm

Stage 1

CxB(O2C2(CF3)4)2

Borate chelate GIC’s

Blue: obsPink: calc

Unexpected anion orientation - long axis to sheets

T

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Intercalation rates   Intercalate Temp / °C Reaction Anion  half-life / h

SO3C8F17 20 10N(SO2CF3)2 20 0.01N(SO2CF3)(SO2C4F9) 70 100N(SO2CF2CF3)2 70 500C(SO2CF3)3 70 > 1000 SO3C6F5 20, 70 no reaction 

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GIC ambient stabilities

0.0

0.1

0.2

0.3

0.4

0.5

0.01 1 100 10000time / h

1 / n

'

TFMI

PFOS

PFEI

nitrate (HF)

sulfate (HF)

bifluoride

stage2

345610

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Application - IRP strategy

1. Intercalation2. Removal3. Optional cycle

Targets1. increase internal volume and disorder not surface

area2. low residual content

Parameters: intercalate anion, reduction method (thermolysis, hydrolysis, hydrogenation)

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IRP charge-discharge

• GIC is CxPFOS stage 2

• removal is by

heating under N2 for 3 h

• rate = C / 20

irrev reversible

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IRP for Li ion battery anodes

0

100

200

300

400

500

600

0 200 400 600 800 1000

temp / °C

capa

city

, mA

h/g

NG, rev

NG, irrev

rev. after IRP

irrev. after IRP

e- + Cx + Li+ = CxLi