CHE30043 Materials Chemistry Catalysis Solid State Chemistry lecture

CHE-30043 Materials Chemistry & Catalysis : Solid State Chemistry lecture 4 Rob Jackson LJ 1. 16, 01782 733042 r. a. jackson@keele. ac. uk www. facebook. com/robjteaching @robajackson

Plan of lecture • • Defects in crystalline materials Classification of point defects Intrinsic and extrinsic defects Ionic conductivity in crystalline materials Solid state electrolytes Fast ion conductors Lithium ion batteries che-30043 lecture 4 2

Defects in crystalline materials • Crystalline materials are not perfect above 0 K, since at finite temperatures atoms can move from their lattice positions. • Also, impurities are nearly always introduced during crystal growth (intentionally or not!) • We will be concentrating on point defects, which affect isolated atom sites, and have a significant effect on the chemistry of the crystal. che-30043 lecture 4 3

Classification of point defects • First, distinguish between intrinsic and extrinsic defects. • Intrinsic defects do not affect the chemical composition of the crystal, and include: vacancies – atoms missing from the lattice interstitials – atoms at non-lattice positions che-30043 lecture 4 4

Classification of intrinsic defects • Frenkel defects occur when an atom moves from a lattice site to an interstitial site. (this will be illustrated) • Schottky defects occur when a formula unit of vacancies is created. Note that this is still a neutral defect since both cation and anion vacancies are created (e. g. Na+ and Clvacancies in Na. Cl). (this will be illustrated) che-30043 lecture 4 5

Which intrinsic defects form? - 1 • Frenkel defects occur on one or other sublattice in the crystal (i. e. in MX, either the M+ or X- ions are involved). • A Frenkel defect is called a ‘Frenkel pair’ – it is the combination of a vacancy and an interstitial. • Frenkel pair formation depends on the availability of space in the lattice for interstitial formation. che-30043 lecture 4 6

Which intrinsic defects form? - 2 • Schottky defects will normally form when there is insufficient space in the lattice for interstitials. e. g. the rock salt structure – very little space, so Schottky defects predominate. • However, in the fluorite structure there alternate unoccupied cube centres and Frenkel defects can form. che-30043 lecture 4 7

Extrinsic defects • Impurity atoms present in crystals are called extrinsic defects. • They may be there deliberately, or as a consequence of the preparation process. – Examples are K+ ions in Na. Cl • Charge must always be balanced – e. g. if Ca 2+ substitutes for Na+ in Na. Cl, a Na+ vacancy or Cl- interstitial must be formed. che-30043 lecture 4 8

Ionic Conduction Mechanisms • The presence of point defects makes it possible for ions to move through a structure. • There are two common mechanisms that are adopted, the vacancy mechanism and the interstitial mechanism. • These mechanisms will be illustrated – but see the diagrams in Smart and Moore for an alternative view! che-30043 lecture 4 9

How is ionic conductivity defined? • Ionic conductivity ( ) is defined as follows: = n Ze where n is the number of charge carriers (defects per unit volume), Ze is their charge (e is the electron charge), and is the mobility of the ions – a measure of the speed with which they move through the lattice. • So depends on (i) the number of available defects (ii) the charge of the defect species, and (iii) how easily the ion can move through the lattice. che-30043 lecture 4 10

Some conductivity values conduction type material conductivity (ohm -1) ionic conductors ionic crystals solid electrolytes liquid electrolytes electronic conductors metals semiconductors insulators che-30043 lecture 4 10 -16 – 10 -3 10 -1 – 103 103 – 107 10 -3 – 104 < 10 -10 11

Solid Electrolytes • Solid electrolytes are ionic crystals with a sufficient volume of defects to be able to act like a liquid electrolyte. • Example – Li. I (rock salt structure, Schottky defects) has sufficient vacancies in its crystal structure to enable Li+ ions to move freely. • It was one of the first solid electrolytes to be used in the design of lithium batteries. che-30043 lecture 4 12

Lithium batteries: how they work - 1 • Lithium batteries are made up of: – a lithium anode, ‘iodine’ cathode (iodine embedded in a polymer, poly-2 -vinyl-pyridine). – a lithium iodide electrolyte. • Li+ ions can pass through the electrolyte by the vacancy mechanism, and the valence electrons go round the external circuit. • A diagram will be drawn in the lecture. che-30043 lecture 4 13

Lithium batteries: how they work - 2 • The following cell reaction occurs: anode: 2 Li(s) 2 Li+ + 2 ecathode: I 2(s) + 2 e- 2 Ioverall: 2 Li(s) + I 2(s) 2 Li. I(s) • Note – the same principal applies to different Li salts (e. g. Li 2 CO 3). • Lithium batteries have been replaced by lithium ion batteries, considered later. che-30043 lecture 4 14

Fast ion conductors • Fast ion conductors are particular examples of solid electrolytes. • We will consider some examples and show they work. • There are two mechanisms: (i) structures which have unoccupied lattice sites without defects being present. (ii) structures whose conductivity occurs because of defects. che-30043 lecture 4 15

Example 1 - -Ag. I - 1 • The structure of -Ag. I is such that there are more possible sites for the Ag+ ions than needed (see Smart and Moore for diagrams). Structure is BCC with I- anions at the BCC sites. • Conductivity occurs because the Ag+ ions can easily move through the lattice via unoccupied lattice sites. • Conductivity is 131 ohm-1 (high). che-30043 lecture 4 16

Example 1 - -Ag. I - 2 • We can relate the high conductivity to properties of the structure: – Low charge on the Ag+ ions – Low coordination of the Ag+ ions – Many vacant Ag+ ion sites • However the -phase of Ag. I only exists above 146 C, limiting its use. che-30043 lecture 4 17

More examples of fast ion conductors • -Ag. I can be modified to lower the temperature at which fast ion conduction occurs. – Partial replacement of Ag by Rb results in Rb. Ag 4 I 5, which has an ionic conductivity of 25 ohm-1 at room temperature. • Materials with the fluorite structure should have good potential as fast ion conductors, since there are many interstitial sites. che-30043 lecture 4 18

Zirconia and stabilised zirconia • Zirconia, Zr. O 2 exists naturally as the mineral baddeleyite, with a monoclinic structure. • On heating, the structure transforms first to a tetragonal phase, and then, at 1600 C, to a cubic fluorite phase, where it would be expected to have good ionic conduction properties. • It is possible to stabilise the structure in the cubic phase at room temperature by adding, e. g. 15 mol % Ca. O. che-30043 lecture 4 19

Stabilised zirconia • Addition of Ca. O (or other oxides, including Y 2 O 3), produces a cubic fluorite phase. • The structure is a good ion conductor because of the interstitial sites, but addition of the stabiliser ions improves this further. • If Ca 2+ is substituted for Zr 4+, the charge has to be compensated. • This can be achieved by creating O 2 vacancies – so oxygen ion conduction can occur. Stabilised zirconia is widely used in fuel cells. che-30043 lecture 4 20

Oxygen ion conduction in Ca. O stabilised zirconia - summary • The Ca. O can be added up to 28 mol%. • Addition of the Ca. O stabilises the structure in the cubic phase at room temperature. • The interstitial sites mean oxygen ion conduction will be possible via the interstitial mechanism. • Further oxygen vacancy sites are created to compensate for the charge imbalance caused by Ca 2+ substituting for Zr 4+. This enhances conduction further. che-30043 lecture 4 21

Lithium ion batteries • Lithium batteries were not always ideal to work with, mainly because of the reactivity of the lithium metal! • Lithium ion batteries were developed in the 1990 s, mainly by Sony, to provide reliable rechargeable batteries. They are now used routinely in laptop computers, mobile phones, mp 3 players etc. che-30043 lecture 4 22

How lithium ion batteries work – (i) • Their design is based on intercalation compounds (compounds formed by the reversible addition of ‘guest’ ions to a host lattice). • The electrolyte is a conducting polymer such as polyacetylene: n (H-C C-H) che-30043 lecture 4 23

How lithium ion batteries work – (ii) • The anode is composed of Li embedded in graphitic carbon, forming Lix. C 6. • The cathode is made from Li combined in an intercalation compound with a transition metal oxide like Co. O 2, forming Lix. Co. O 2. • The electrolyte is a conducting polymer. che-30043 lecture 4 24

How lithium ion batteries work – (iii) • The Li+ ions move between intercalation compounds. the two – At the anode, Lix. C 6 = x. Li+ + 6 C + xe– At the cathode: x. Li+ + Co. O 2(S) + xe- = Lix. Co. O 2 • The Li+ ions move through the electrolyte while the electrons go around the external circuit. • The process is reversed on charging. • See diagram on the next slide. che-30043 lecture 4 25

Lithium ion battery diagram che-30043 lecture 4 26

Improvements to the lithium ion battery • There is much current research on improving the performance of lithium batteries. • These have focussed on using nanostructured materials for the cathode and anode. – The rationale is that the ‘hopping distance’ for the Li+ ions is reduced. che-30043 lecture 4 27

Improvements to anode materials Journal of New Materials for Electrochemical Systems, 15(4) 2012, 233 -236 che-30043 lecture 4 28

Nanoporous cathode materials che-30043 lecture 4 29

Summary of lecture • Defects in solids have been introduced and classified • Ion migration mechanism have been discussed. • Applications to fast ion conductors and battery materials have been described. che-30043 lecture 4 30