Chapter 9 - Characterisation

Question 1

What are soft x-ray absorption spectroscopy (sXAS) used for?

Answer 1

Tool to probe the electronic states through excitation of core-level electrons to unoccupied states above the Fermi level.

3d-states of TM and p-electrons of anions can be studied.

Question 2

What are some properties of soft X-ray absorption spectroscopy (sXAS)?

Answer 2

• Extremely surface sensitive
• Different modes with different probing strength (1 - 500 nm, Auger < Partial electron yield < Total electron yield < Total fluorescense yield)

Question 3

What are some limitations with soft X-ray absorption spectroscopy (sXAS)?

Answer 3

• Requires UHV -> limits operando capabilities

Question 4

What is x-ray diffraction (XRD) used for?

Answer 4

To probe the average crystal structure:

- lattice parameter, phase, strain, atomic positions, site occupancy, texture, stacking faults

Question 5

What is pair distribution function (PDF)?

Answer 5

A total scattering technique. Looks at Bragg peaks and diffuse scattering.

Yields information about the local structure. Interatomic distances, coordination numbers.

Covers mid-range structure between XRD and EXAFS.

Question 6

What is hard x-ray absorption spectroscopy (XAS) used for?

Answer 6

Looking at valence state changes during electrochemical reactions (reveals charge compensation)

• Looking at coordination environment
• Looking at local structural changes (bond length and degree of disorder).

Question 7

What can scanning/transmission x-ray microscopy (STXM/TXM) be used for?

Answer 7

• Evaluation of morphology and micro-structure evaluation in micro/meso/macro scale (microcracking, particle fracture, tortuosity)
• Chemical information (elemental / chemical mapping, revealing concentration gradient, oxidation states)

Question 8

What are some factors that complicate operando neutron experiments?

Answer 8

• High incoherent scattering cross section of elements in the battery (especially hydrogen).
• Lithium, while having good contrast in neutron experiments, also is a good neutron absorber which reduces signal.
• Amount of material needed for experiments

Question 9

What are some great advantages of neutrons?

Answer 9

• High sensitivity to lithium
• High contrast between neighbouring elements in the periodic table
• High penetrating power allowing whole battery to be imaged

Question 10

How can we study the motion of atoms with neutron scattering?

Answer 10

By looking at the inelastic scattering. This is where neutrons start or cancel oscilliations in the atoms, creating phonons or magnons and thus loosing energy.

Wavelength and energy are comparable to typical atomic spacing and vibrational energy.

Question 11

How are neutrons at a neutron facility generated?

Answer 11

Through a fission process of e.g. 235 U

Question 12

What are some fundamental properties of neutrons?

Answer 12

• Is neutral
• Has magnetic moment
• Has spin
• Has energies similar to vibrational modes
• Has wavelengths similar to atomic spacings

Question 13

How does the scattering cross section of x-rays vary with Z?

Answer 13

Increases with increasing Z (scatters of electron cloud)

Question 14

How does the scattering cross section of neutrons vary with Z?

Answer 14

Randomly - dependent on isotopes for same element as well.

Question 15

What does the neutron scattering cross section depend on?

Answer 15

Atomic element, isotope and nuclear spin state.

Question 16

What is the scattering length, b, a measure of?

Answer 16

The strength of the neutron-nucleus interaction.

Question 17

How is the scattering length, b, determined for elements and its isotopes?

Answer 17

Experimentally.

Question 18

What can we see in the low Q regime of scattering experiments?

Answer 18

Large distances. Superstructures.

Question 19

What are some tips in designing a cell for operando x-ray studies?

Answer 19

• Should be easy to assemble / disassemble at the facility
• Should be easy to incorporate in existing beamline setup
• Should allow x-ray to reach the sample region and detector
• Window material should not react with cell components
• Window material should be impermeable to oxygen and moisture
• Window material should be stiff to apply uniform pressure to the battery
• Should measure background signal from an empty cell without active material