the challenge of on-hybridfordonscentrum.se/wp-content/uploads/2014/03/... · the challenge of...

Göran Lindbergh | SHC 2

The challenge of on-board energy storage

Göran Lindbergh, Thema1c area 3

SHC Swedish Centre of Excellence

for Electromobility


SHC is a national centre of excellence for research !and development of electric and hybrid vehicles. It is an !

arena where Sweden’s automotive industry, universities and government agencies meet and collaborate to generate !new technology, insights and competence for the future.!

SHC

Göran Lindbergh | SHC

On-board energy storage

Gasoline/diesel: §  High energy density §  Low efficiency of the ICE §  Emissions (CO2, NOx, …) §  Not reversible Batteries: §  Low energy density §  High efficiency of energy conversion §  No emissions §  Reversible energy storage


Conflicting demands on batteries

§  High energy density §  High power density §  Long lifetime §  Low cost §  Safety Furthermore: §  Engineering tools that predict battery behaviour, and

that radically reduce the need for testing are not fully developed today.


What determines the behaviour of the energy storage system?

§  Materials §  Design of the cell §  Design of the battery module §  System design §  Usage and load profile


How does the electrochemical performance change when adding the flame retardant?

§  SHC funded project between KTH, Chalmers and Uppsala University

§  4 PhD students contributing with different tools §  TPP flame retardant additive in the context of a high

power application (Like HEV)


Lithium-ion Batteries

•  Electrolyte sandwiched between the two electrodes

•  Research: “Pure” electrolyte Solvent + Li-salt

•  Commercial cells: Electrolyte with additive

cocktail Content is a trade

secret


Additives for Lithium-Ion Battery Electrolytes

Used to enhance specific properties: §  Low temperature performance §  Passivating films §  Over-charge protection §  Flammability


Flammability of Lithium-ion Battery Electrolytes

Reduced electrolyte flammability would be a huge leap towards LiB safety §  What are the options? §  Switch to non-flammable

solvents §  Ionic liquids, solid state

electrolytes etc… §  Additives to suppress

flammability

Tesla Model S in a recent fire


Triphenyl Phosphate (TPP) Flame Retardant Additive

Believed to prevent fires by two mechanisms: §  Formation of a shielding layer of char protecting the

liquid phase §  Radical scavenger in the gaseous phase inhibiting

combustion chain reactions §  Lowers the self-extinguising time §  Does not change the flash point


Performance of TPP as Flame Retardant

High amounts (>20 wt%) of additive needed to reduce flammability significantly Flashpoint: Temperature at which the material forms a ignitable mixture with air. SET (Self-Extinguishing Time): The time the material can support a flame after the source of the flame is withdrawn.

TPP content / wt% Flashpoint / ºC SET / s g-1

0 36.0 62.0 5 37.4 59.0 10 37.0 51.5 20 37.9 40.0


Performance in simulated HEV operation Hybrid Pulse Power Characterization test

TPP content / wt%

Energy efficiency / %

0 87 1 87 3 87 5 84 10 84 15 81


What causes these performance changes?

Likely candidates: §  Ionic conductivity §  Diffusion resistivity §  Viscosity §  Lithium-ion solvation/de-solvation §  Electrode surface changes


Viscosity and Ionic Conductivity

Ionic conductivity (σ) Viscosity (η) Walden product (σ*η)


Raman spectroscopy Lithium-ion solvation/de-solvation

à LiPF6 coordinates to TPP rather than to EC:DEC


Diffusion Resistivity

Diffusion resisitivity Ohmic resistivity


XPS Surface changes


Conclusions TPP

§  High amounts (>20 wt%) of TPP are needed to achieve significant decrease in flammability

§  Both instantaneous and time-dependent polarization increase with TPP content:

- Increased viscosity - Decreased ionic conductivity - Increased diffusion resistivity - Thicker electrode surface films

§  Not suitable for HEV


Summery

§  Our mastery, as users and system integrators, of the currently available lithium-ion batteries is far from sufficient.

§  Better characterisation techniques and engineering tools that can be used to understand and predict battery behaviour would be immensely valuable.

§  This cannot be obtained without a profound knowledge about all limiting processed in the battery, from system perspective down to molecular level in the individual cells.


[email protected]

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