conceptual review - quiz1

Miscible system is how many phases?

1 phase

Example of Miscible System

Li_1-xTiS₂ where x is between 0-1

Voltage Profile of miscible/single-phase system

Single, sloping line

Advantages of miscible system

Easy to map state of charge (SoC)

Disadvantages of Miscible System

Low energy density

Mild Voltage Retention

Immiscible System has how many Phases?

2

Example of 2--phase/immiscible system?

LFP (LiFePO₄)

Example of 2-phase/immiscible system?

LTO (Lithium Titanium Oxide) - anode

Advantages of 2-Phase System?

Constant, high-stability system

Disadvantages of 2-phase system

Hard to track state of charge (SoC)

Can have mechanical straining (fixable w/ cracking material into nanoparticles)

Voltage Profile of Immiscible System

Plateaus and goes down

Does the Nernst Equation hold if the cathode is a phase change?

No, because at the interface between phases, the activity is the same in both phases

Making a voltage plot for a system with 2 materials, one being immiscible and the other being miscible

The resulting plot is a superposition of both materials, mixing doesn’t change the atomic environment

Voltage plot of system with 2 materials

E_cell =

E_cathode - E_anode

Positive (cathode) electrode should have a more positive or less positive redox potential than the anode (negative electrode)?

more positive (aka closer to 0 if negative, or larger)

How is the usable electrical energy calculated?

Using Gibbs Free Energy equation, you can’t find out without it

$$\Delta G = \Delta H - T\Delta S$$

Calculating Cell Energy from Cell Potential and Capacity

E = Q * V

V - cell potential

Q - capacity

Calculate Power Output

P = I * V

Most important things about battery metrics

1. Theoretical specific capacity

2. Specific Energy

3. Cell Voltage

Capacity units

Ah (Coulombs)

Ampere

Unit of electric current

Amp-hour means…

Describes total electric charge/capacity of a battery

1 amp hour means a battery can deliver

A steady current of 1 amp for 1 hour

Specific Capacity units

Ah/kg

What does 1 watt-hour literally represent?

Energy needed to power 1-watt bulb for 1 hour

Capacity density units

Ah/L

Energy units for Batteries

Wh

Energy in Watt Hours is

Wh = Capacity (Ah) * Potential (V)

Power = ?

W = Current * Voltage (integrated over time)

Specific Energy unit

Wh/kg

Energy density

Wh/L

Battery cycle life

Number of charge/discharge cycles a battery can go through before its capacity significantly decreases

Battery vs. Fuel Cell

Battery stores electrical energy inside itself

Fuel cell generates electricity by converting fuel like hydrogen

Why is the practical cell voltage lower than the theoretical cell voltage?

Due to overpotential loss and side reactions