ENERGY MATERIALS Flashcards
Describe the components of a Li-ion battery with a graphite based anode. Describe how lithium enters into the anode on charging. What is understood with “staging”?
Graphite anode separated from the cathode part by a electrolyte/separator.
On charging, lithium ions will travel from the cathode, through the electrolyte to the anode. It will take place in the graphite staging, so the staging height increases.
Why is the liquid electrolyte not always stable at the interface towards the anode and cathode materials?
Very reducing potential at the anode side
Very oxidative potential at the cathode side
The organic electrolyte has a certain electrochemical stability window. The very reducing conditions at the Li/graphite interface and the oxidizing conditions at the cathode side is often outside that window. Decomposition reaction are expected –kinetics can be slow.
Which mechanism can possibly make a Li-ion battery unsafe, and trigger thermal runaway?
Oxygen evolution at the cathode side –if overcharged –alternatively formation of reactive peroxide.These can oxidize the conducting carbon in the electrode, or the organic electrolyte.The oxidation is highly exothermic. That might accelerate the reactions, give more heat formation. Gas pressure may evolve; rapturing or explosions may take place.
Li(s) may deposit on the anode, and one may have dendritic growth (more problematic for Li-metal batteries then for Li-ion batteries), and dendrites may come in contact with the cathode and make short-circuiting.
What is meant by the “SEI layer”? Describe mechanism that may take place at the interface between the electrode materials and the electrolyte?
A passivation layer called the solid electrolyte interphase (SEI) is formed on electrode surfaces from decomposition products of electrolytes. The SEI allows Li+ transport and blocks electrons in order to prevent further electrolyte decomposition and ensure continued electrochemical reactions.
Which chemical parameters are essential for providing a high energy Li-ion battery cell? Consider redox aspects of the cathode and anode materials.
Mobility of the ions and electrons, proper electrolyte, optimized SEI layer. Half-reactions at the anode and cathode should not generate unwanted species.
What could be positive role of an ultrathin coating layer with an inert material on the fine particles of the cathode material?
It may prevent dissolution of cations into the electrolyte. It may stabilize the particle from breaking up –at conditions where lithiation/delithiation results in major volume work owing to differences in molar volume between the involved compounds.
Explain what is understood with an ionic liquid, and provide one example.
Ionic liquid is defined as a salt with melting point below the boiling point of water.
Ex. dicyanide.
Why can a solid electrolyte be beneficial as replacement of the liquid electrolyte? What is the main challenge for having a well working solid electrolyte that may facilitate reasonable charge/discharge times? What is understood with a charge rate of 1C?
BENEFITS
-avoid organic flammable liquid
avoid formation of SEI; loss of electroactive material
-can have higher potential for the cathode material
-reduced risk for thermal runaways/short circuiting
CHALLENGES
-sufficiently high ionic conductivity
1C = full charge/discharge of the entire battery (cell/electrode) in 1 h
Why is it important to add conductive carbons to the powder mix that is used to fabricate the electrodes? What is typically the electronic properties of the redox active materials of the cathode?
Design of nanoporous conducting network.
Are such ion-batteries limited to lithium? Suggest other possible ions as “rocking ions”, shuffling the charge between anode and cathode. What could be the challenges/open questions?
Na and K from group I;
Mg and Ca from group II
ISSUES:
Potential? Not that much changed… slightly less for M = Na
Anode materials (intercalation; graphite not OK for Na)
Diffusion divalent cations -too slow
Lack of structural stability by removing 2+ cations…
What is the performance problem with alloying anodes, e.g. silicon? What can be done to improve their cycling stability?
Main problem: huge volume work. Breaking up of particles. Loss of connectivity to the conducting network of carbons.
