Energy Storage Summary Questions Flashcards
Energy stored in capacitors
E = ½ CV^2
Why supercapacitors are “super”
Due to the very small Hemholtz layer the distance is very small. The formula for charge in a capacitor is C = e_0e_rA/d and a very small d gives a large C. The much larger C in a supercapacitor compared to conventional capacitors is the reason for the “super”
Why supercapacitors are also called double-layer
Due to the alignment of ions in the electrolyte that create a double layer on either side of the capacitor.
The solid electrode plate and the lined up ions (forming a liquid electrode wall) together create a capacitor with a potential difference across them. Hence the term “double-layer”
What is the Hemholtz layer?
Hemholtz layer is the name of the layer of ions that builds up to form a wall next to the solid electrode plate in a super capacitor.
Why are supercapacitor electrodes porous?
The porous electrodes increase surface allowing max area to store electric charge.
What is a common anode material?
Graphite
What is a common cathode material?
Lithium Cobalt Oxide
What are the advantages/disadvantages of silicon anode?
Silicon Nanowires
Advantages:
1. Ten times current capacity
Disadvantages:
1. Silicon films expand drastically when absorbing lithium ions during charging, and shrink when releasing ions during discharging leading to fracture/fatigue and short battery life.
How do we calculate stored energy in a battery?
Capacity is the term used to characterize battery performance which is equal to current (A) x Time (h)
What plot or diagram could we refer to for selecting a suitable electrode material?
Voltage vs. Capacity
Ragone plot
What is the driving energy for a chemical reaction?
Gibbs free energy
Standard chemical reaction: A + B = AB
Specifically the difference in the values of the standard Gibbs free energy of the
Products - Reactants
What is the difference between a chemical and an electrochemical reaction?
Key differences in a chemical reaction and an electrochemical reaction is the presence of an electrolyte and electrical path in the electrochemical reaction (battery). These do not exist in a chemical reaction.
A chemical reaction is the combination of A + B = AB
The electrochemical reaction is A < electrolyte > B with the connection made between A and B by an electrical path (wire).
What is Gibbs free energy as a function of temperature?
G = U + pV - TS
G = H - TS
How do you derive change of voltage in a battery and what is their reaction formula?
Derivation of change of voltage with respect to Temperature (dE/dT)
Delta G = Delta H - T*Delta S
Differentiating with respect to temperature: d(Delta G)/dT = -Delta S
Using the relation of voltage to Gibbs energy: E = -DeltaG/zF
We obtain: dE/dT = Delta S/zF
Describe 5 important parameters in a battery
Specific Energy: Energy per unit weight
Energy Density: Energy per unit volume
Operating Voltage: Voltage at which the battery operates during discharge - when supplying electrical energy and power - and charging
Energy quality: Ability of the form of energy to be converted to useful work
Charge Capacity
Stored Charge
Others
Why is the practical capacity lower than maximum theoretical capacity of an electrode material in a battery?
Presence of passive components
Effective utilization of active components is less than optimal
Active-turned-passive materials add to the weight and volume, but dont contribute
What is cyclic capacity and coulombic efficiency?
Cyclic capacity is when the capacity of the battery is reduced over time due to cycling (charging and discharging).
Coulombic efficiency is the fraction of prior charge capacity that is available during the following discharge.
What is self-discharge in a battery?
The decrease in capacity with time, even without use.
Classification of electrolytes in LIB’s
A. Hard Solid Electrolytes (inorganic)
1. Ceramic/glass
B. Liquid Electrolytes
1. Organic liquids
2. Ionic liquids
C. Polymer electrolytes (organic)
1. Solid Polymer Electrolytes (SPE)
i. Polymer/salt “molecular composite”
ii. Solid Phase
2. Gel Polymer Electrolyte (GPE)
i. Cross-linked polymer matrix
ii. Swollen with plasticizer/salt solution
iii. Two main phases: liquid salt/plasticizer solution and solid polymer matrix
Typical liquid electrolytes
PC: Propylene Carbonate
EC: Ethylene Carbonate
DMC: Di-Methyl Carbonate
DE: Diethyl Carbonate
Advantages of polymer electrolytes
- Act as both electrolyte and separator
- Can eliminate dendrite growth
- Can conform to the varying volume of electrodes during recycling
- Less reactive and thermodynamically more stable towards lithium compared to liquid electrolyte
- Safer
- Flexible
Methods of increasing Li ion conductivity in polymer electrolytes
Addition of nanofillers and/or plasticizer
Ion conductivity mechanisms in polymer electrolytes
Two main approaches:
- Continuum-based models
- Discrete models
Relation between free volume, Tg, diffusion and ion conductivity
Lithium ion conductivity is associated with polymer chain segmental mobility, free volume, and glass temperature (Tg)
The volume of the polymeric material consists of occupied volume and free volume
Occupied volume is sum of the space occupied by actual molecules and the volume due to thermal vibrations of the molecules
Free volume is the volume due to holes or voids caused by polymer packing irregularities
At Tg, the free volume reaches a minimum limit, where it is too small for any chain confirmation movement, therefore, the free volume effectiely “freezes” at Tg
What is the theoretical role of fillers in polymer electrolytes?
Can be added to polymer electrolytes to increase ion conductivity
Specific Energy
Energy per unit weight
Energy Density
Energy per unit volume
Operating Voltage
Voltage at which the battery operates during discharge - when it supplies electrical energy and power - and charging
Energy quality
Ability of the form of energy to be converted to useful work
What is intensive quantity? What is an example of an intensive quantity?
Independent of the amount of material present
Key example is voltage
What is extensive quantity? What is an example of an extensive quantity?
The amount of charge depends on the amount of material that can be stored in the battery
Key example is capacity
What is a primary cell/battery?
A battery that can NOT be recharged
What is a secondary cell/battery?
A battery that CAN be recharged
Free volume Theory
Volume of the polymeric material consists of “occupied volume” and “free volume”
Occupied volume is the sum of the space occupied by the actual molecules and the volume due to thermal vibrations of the molecules
Free volume is the volume due to holes or voids caused by polymer packing irregularities
What is the key driving force for flow of ions and electrons?
Potential difference between anode and cathode
What does free volume and vibrations (increase temperature) apply to with regard to polymer chains?
Increased ion conductivity
What is a formation reaction?
A + B = AB
What is a displacement reaction?
A + Bx = Ax + B
Define the type of reaction for the following Gibbs free energy G
G < 0
G = 0
G > 0
G < 0 means the reaction is forward and spontaneous
G = 0 means the reaction is at equilibrium
G > 0 means the reaction is non-spontaneous
Problems with rechargeability of Li Metal as Anode
-Deposition at unwanted locations
-Shape Change
-Dendrites
-Filamentary Growth
-Thermal Runaway
Examples of Solid Polymer Electrolytes
PEO: PolyEthylene Oxide
PVDF: PolyVinyliDene
Examples of Li Salts
LiClO4, LiPF6
Charge Capacity
Amount of charge stored in a battery
Maximum Theoretical Specific Energy (MTSE)
Amount of energy stored per unit mass.
“Insertion Reaction”
Voltage formula for LIB
E = -G/zF
