Ion conduction Flashcards
Is ionic or atom diffusion faster or slower in solids than other phases
- Much slower than other phases- liquids and gas
Give examples of when diffusion in solids is still important in solid state technology
- Synthesis of solids (usually at high temp)
2. Ionic conductors or solid electrolytes in batteries, gas sensors and fuel cells
What are diffusing ions linked to
- Mobile point defects
2. Generally the lowest-energy pathway involves defect sites
Give 3 examples of a mobile point defect
- Vacancy mechanism
- Direct interstitial mechanism
- Interstitialy or knock-on mechanism
Describe a vacancy mechanism
- Ion migrates into adjacent vacant sites (continues through the lattice)
Describe a direct interstitial mechanism
- Migration of interstitial ion directly into an adjacent unoccupied site
Describe a knock-on mechanism
- Interstitial ion moved into adjacent lattice site, displacing the ion into a new interstitial site
Give an example of a knock-on mechanism and uses
- AgCl or AgBr
2. For Ag+ conduction (from NMR, radio-tracer experiment and computer modelling)
What is ionic conductivity dependent on and give an equation to show this
- Ionic conductivity (sigma) is temperature dependent and shows ‘Arrhenius behaviour’
- Sigma = Aexp(-Ea/RT)
- A= pre-exponential factor
- R= gas constant
How can the equation linking ionic conductivity to temperature be manipulated to be used on a graph
- ln(sigma)= -Ea/RT + ln(A)
- y= mx + c
- Draw graph of ln(sigma) over 1/T
- Slope= -Ea/RT
What does the gradient of the slope tell us
- The lower the gradient of the slope the lower the activation energy
- This leads to high ionic conductivity
What compound has a high ionic conductivity
- Na(Beta2prime)-Al2O3
- Electrolyte use in sodium batteries
- Has low activation energy- smallest gradient
Name the structure and migrating defect for Na(Beta2prime)-Al2O3
- ?
2. Na(i).
Name the structure and migrating defect for Y3+ doped ZrO2
- Fluorite
2. V(O)..
Name the structure and migrating defect for NaCl
- Rock-salt
2. V(Na)I
Name the structure and migrating defect for AgCl
- Rock-salt
2. Ag(i).
Name the structure and migrating defect for Sr2+ doped LaGaO3
- Perovskite
2. ?
What are the three types of conductors
- Ionic conductor (solid electrolyte)- medium- 0.1-10^2 sm-1
- Electronic conductor ( metals) - fastest 10^3-10^7sm-1
- Electronic insulator (diamond)- slowest 10^-7sm-1
What is intercalation
- The incorporation (insertion) of atoms, ions or molecules into a structure with minimal structural change
- It involves a redox process (oxidation or reduction) and often they are reversible reaction
Give an example of intercalation
- Lithium intercalation in rechargeable Li-ion batteries
2. Led to the portable revolution and now important for electric vehicles
Describe what a Li-ion battery cell is composed of
- Battery cell consists of +ve electrode LiCoO2 and -ve electrode (graphite) separated by a Li+ conductivity electrolyte
- Both electrodes are intercalation host structures for small and light lithium
- Charging/discharging involves lithium intercalation (and extraction) and Li-ion conduction
Describe what occurs at the LiCoO2 electrode
- Li(x)CoO2 x= 1.0 - Co3+ or x=0.0 Co4+
- Cobalt gets oxidised/reduced with x ranging from 1.0–>0.2
- Structure of LiCoO2 is layered with edge-sharing [CoO6] octahedra and [LiO6] octahedra
- Co, Li coordination number= 6
Describe what occurs at the graphite electrode
- Has layered structure of hexagonal carbon rings, strongly bonded within the 2D layers (separated by 3.5 A)
- Weak interlayer interactions (Van der Waals) which allow for atom/ion intercalation between the layers.
How does lithium intercalation occur
- In both LiCoO2 and graphite there is fast Li+ ion diffusion within the 2D layers
What is the overall Li-ion battery cell reaction
- LiCoO2 + 6 graphite LiC6 + CoO2
- Forward is charge
- Backward discharge
Describe the current research involving Li-ion
- LiCoO2 - replace Co which is toxic and expensive with Fe, Ni, Mn
- NMC electrodes Li(Ni,Mn,Co)O2
- NMC-622 Li(Ni(0.6),Mn(0.2), Co(0.2))O2- used in Nissan-leaf, BMW-I3
What is another example of a compound that can be used to produce intercalation compounds
- Buckminster fullerene
What is buckminster fullerene
- C60
- Football shaped with 6-membered rings and 5-membered rings of carbon
- Reacts with alkaline metal ions (M+) to produce intercalation compounds
How is buckminster fullerene used
- K3C(primer)60
- The ion insertion leads to a superconductor at Tc around 18K
- 0 resistance to electronic conduction at Tc
- The other important property of super conductor is powerful magnetism (Meissner effect) used in NMR, MRI, and Maglev trains
What does a graph of electrical resistance over Tc look like for a superconductor
- At specific Tc goes straight up and then diagonal line which could be semi-conductor
