Supercapacitors for Automotive Applications Peter J Hall, Anthony J R Rennie, Vitor L Martins, Roberto M Torresi* Chemical Engineering, U Sheffield, UK * Chemistry, U Sao Paulo, Brazil

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Three Questions:

1. What are supercapacitors?

2. Why are they not routinely used in automotive power trains?

3. What are we doing about it?

WHAT ARE SUPERCAPACITORS?

1st Question

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Supercapacitors • Store energy

electrostatically through the separation of ions at a surface (~1nm)

• Colossal electrode surface area ~1000’s m2: ~kF devices are possible

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Supercaps can reduce the size of battery packs

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WHY ARE SUPERCAPS NOT USED MORE BROADLY?

2nd Question:

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Problems with Supercaps

• Typical Maxwell Unit (BCAP0350): • 350F, 9.5 kW kg-1 but 5.9 Wh kg-1 (ESR 3.2 mΩ)

• Acetonitrile electrolyte (metabolised to HCN), volatile

• Energy density is too low for many practical automotive applications

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WHAT ARE WE DOING ABOUT IT?

Question 3:

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Improvements:

• Use electrolytes with higher ESW • E = ½ C (Vmax – Vmin)2

• Vmax acetonitrile = 2.85 V

• Vmax ionic liquid = 3.9 V (240% increase in specific energy if Vmin = 0.9 V)

• Use controlled porosity polymer-derived carbons

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[Pyr14] [Tf2N]

N-n-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide

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Operating voltage 3.6-3.9V Specific energy up to 30 Whkg-1

Typically operated at 60ºC

Sulfonium based cations

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Triethylsulfonium

[S222]

Diethylmethylsulfonium

[S221]

Diethylpropylsulfonium

[S223]

Physical characteristics

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Ionic Liquid Td [ºC]

Tm [ºC]

δ [g cm-3]

η [mPa s-1]

σ [mS cm-1]

[Pyr14][Tf2N] 445 -6.5 1.40 78 3.0

[S221][Tf2N] 295 -11.7 1.50 41 6.7

[S222][Tf2N] 290 -17.6 1.46 34 7.3

[S223][Tf2N] 280 -6.4 1.42 38 5.5

H2O < 10ppm

Performance

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J. Phys. Chem. C 2015, 119, 23865-23874

Conclusions

• Ionic liquids are ideal bases for supercapacitors for automotive applications

• Carbon porosity needs to be tailored to match electrolyte

• Latest research shows that poly ionic liquid binders can increase energy density

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