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Göran Lindbergh | SHC 2
The challenge of on-board energy storage
Göran Lindbergh, Thema1c area 3
SHC Swedish Centre of Excellence
for Electromobility
Göran Lindbergh | SHC 3
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
Göran Lindbergh | SHC
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.
Göran Lindbergh | SHC
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
Göran Lindbergh | SHC
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)
Göran Lindbergh | SHC
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
Göran Lindbergh | SHC
Additives for Lithium-Ion Battery Electrolytes
Used to enhance specific properties: § Low temperature performance § Passivating films § Over-charge protection § Flammability
Göran Lindbergh | SHC
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
Göran Lindbergh | SHC
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
Göran Lindbergh | SHC
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
Göran Lindbergh | SHC
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
Göran Lindbergh | SHC
What causes these performance changes?
Likely candidates: § Ionic conductivity § Diffusion resistivity § Viscosity § Lithium-ion solvation/de-solvation § Electrode surface changes
Göran Lindbergh | SHC
Viscosity and Ionic Conductivity
Ionic conductivity (σ) Viscosity (η) Walden product (σ*η)
Göran Lindbergh | SHC
Raman spectroscopy Lithium-ion solvation/de-solvation
à LiPF6 coordinates to TPP rather than to EC:DEC
Göran Lindbergh | SHC
Diffusion Resistivity
Diffusion resisitivity Ohmic resistivity
Göran Lindbergh | SHC
XPS Surface changes
Göran Lindbergh | SHC
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
Göran Lindbergh | SHC
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.
Göran Lindbergh | SHC 21