Intercalation Compounds

The Kinetics and Mechanism of Intercalation ... (Problems with the more reactive alkali metals ... benzophenone solutions...

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Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali

Intercalation Compounds

Prof. Dr. Antonella AntonellaGlisenti Glisenti--Dip. Dip.Scienze ScienzeChimiche Chimiche--Università Universitàdegli degliStudi StudididiPadova Padova

Bibliography 1. M.S. Whittingham, A.J. Jacobson Intercalation Chemistry – Academic Press 2. D. O’Hare Inorganic Intercalation Compounds in Inorganic Materials Wiley

Summary Introduction The Kinetics and Mechanism of Intercalation Lamellar Host Lattices and their Intercalates Chain Structure Host Lattice and their Intercalates Framework Hosts and their Intercalates Molecular Hosts and their Intercalates

History The phenomenon of intercalation appears to have been discovered by the Chinese around 600-700 A.D. In 1840 C. Schafhäutl reported his observations on attempting to dissolve graphite in sulfuric acid In 1926 Karl Fredenhagen and Gustav Cadenbach described the uptake of potassium vapour by graphite. 1960s: Intercalation compounds as reversible electrodes for high energy density batteries, superconductivity and catalysis. In 1983 Thomas and 1984 Schöllhorn suggested the possibility to intercalate not layered compounds.

Chemistry of Intercalation Intercalation: Solid state reaction involving reversible insertion of guest species, G, into a host structure, [Hs]. The host provides an interconnected system of accessible unoccupied sites, □.

xG + □x[Hs]

Gx[Hs]

The reaction is topotactic because the host matrix retains its integrity (structure and composition) during intercalation and disintercalation.

The guest and host may experience a spectrum of perturbations in their geometrical, chemical, and electronic environment depending on the individual characteristics of either the chosen host or guest. These properties may be subtly controlled and tailored to meed specific requirements, such as catalytic activity, electrochromic displays, battery technology or as lubricants.

Li in TiS2: an intercalation model Li reduces the layers of TiS2 and Li+ cations are inserted into the vacant interlayer sites to compensate for the negative layer charge caused by the electron transfer; The final product, LiTiS2, has a structure which is slightly expanded in the direction perpendicular to the layers.

Li Li+ + eLi+ + e- + TiS2 Li+[TiS2]Ability to adapt to the geometry of the inserted guest species by adjustment of the interlayer separation.

HOSTS Strong intralayer bonding and weaker interlayer interactions Neutral layers: Interlayer bonding: van der Waals; Interlayer space: a connected network of empty lattice sites Charged layers: Interlayer bonding: electrostatic forces; Interlayer sites: partially or completely filled by ions or ions/solvent molecules

GUESTS

STAGING The reaction of a potential guest molecule with a inorganic host lattice is a heterogeneous reaction involving bonds breaking in the host and the formation of new interactions. The interlayer bond breaking penality can be minimisided by the phenomenon of STAGING. LixTiS2 (0≤x ≤1)

{An+x/n[MS2]x-(H2O)y} (An+ = alkali, alkaline earth, TM; M = Ti, Nb, Ta) Li in graphite

The size of the guest may affect staging: Li+ and Cu+ do not induce stage formation, even at low concentration. > interlayer bonding strength > intercalation difficulty: S < Se < Te

Kinetics and mechanisms of intercalation The reaction seems to be initiated at defects on the host surface (different batch = different reactivity) LixTiS2 (0≤x ≤1) Step reaction leading to a coexistence of three homogeneous phases differing in lithium content At the biginning the kinetics is dominated by nucleation processes. Other factors: Ò > stiffness = > activation energy Ò layer thickness

Kinetics and mechanisms of intercalation Na+ ions into crystals of 2H-NbS2TiS2 1. Initial stages: a. host lattice b. hydrated Na+ phase with bilayers of water c. hydrated phase with monolayers of water d. 3° and 2° stage compound

2. All the 2H-NbS2 has been reduced b. hydrated Na+ phase with bilayers of water c. hydrated phase with monolayers of water

3. Diffusion of water into the crystal b. hydrated Na+ phase with bilayers of water

Synthetic Methods Intercalation compounds are usually insoluble: ) clean reactions ) synthetic strategy adapted to obtain products free of impurity phases

Direct reaction Ion Exchange Electrointercalation methods

Direct reaction: the simplest method m G + MXx G in NH3

LiBun + MXx

( Good general method for less GgMXx + (m-g) G reactive post transition metals ( Problems with the more reactive alkali metals (thermal instability of the product, over reaction, reaction with the vessel) ( Frequently: co-intercalation of ammonia GgMXx ( Heating in vacuo to remove NH3 = undesidered side reactions

LiyMXx + (y/2) C8H18

Metal aluminohydrides and borohydrides (M = Li, Na) Sodium and potassium naphthalide or benzophenone solutions.

( LiBun dissolved in hexane into which is suspended the host lattice ( When reaction is complete (time depending on host) the reaction product is isolated by filtration, washed (pure solvent) to remove any excess LiBun and dried in vacuum

Intercalation reagent and ease of reduction of the host Relative reduction potentials for some common host lattices relative to some common lithium intercalation reagents

Reactivity of the intercalation reagent

X LiI + V2O5 LiBun + V2O5

LixV2O5 + (x/2) I2 no intercalation compounds

Reaction conditions Examples of intercalation compounds formed by direct reaction of the host lattice with the appropriate guest molecules Guest reagent liquids or low melting solids = used as net reactants organic and organometallic solids = dissolved in polar organic solvents. Solvent: an experimental choice z toluene, acetonitrile, dimethoxyethane, dimethylformamide, water have proved to be successful for a wide range of host/guest systems. z highly polar solvents often accelerate the intercalation process but they can complicate the product distribution by cointercalating with the guest molecules

Reducing power and ability to intercalate The electron transfer mechanism involving oxidation of the guest and electron transfer to the conduction band of the host lattice accounts for the correlation which exists between the reducing power of the organometallic guest and its ability to intercalate specific host lattices. The first row transition metal metallocenes: an example First ionization potential: Cobaltocene = 5.5 eV Ferrocene = 6.88 eV Host lattice oxidising power: FeOCl > TaS2

[Co(η-C5H5)2] + TaS2 [Co(η-C5H5)2] + FeOCl

IC IC

[Fe(η-C5H5)2] + TaS2 [Fe(η-C5H5)2] + FeOCl

The reducing power is a necessary byt not sufficient condition:

[Co(η-C5Me5)2] (1° IP = 4.7 ev) + MS2 (M = Nb, Ta, Zr, Sn) [Cr(η5-C9Me7)2] (1° IP = 4.6 ev) + MS2 (M = Nb, Ta, Zr, Sn)

IC

Ion exchange: a useful strategy ) Once an intercalation compound has been formed the intercalated guest ions can often be replaced by immersing the material in a concentrated solution containing another potential guest ion ) Pre-intercalation provides a useful strategy for intercalating large guest cations that do not intercalate by direct reaction NaxTiS2 + xs LiPF6 KCl aqueous solution MnPS3

LiyTiS2 + x NaPF6 Tris(2,2’-bipyridyl) ruthenium dications

Mn0.8PS3K0.4(H2O)y Ru containing IC

) Intercalation reactions involving the zeolites, pyrochlores, layered perovskites, silicates, and clay hosts all generally proceed by ionexchange processes

Examples of intercalation compounds prepared by ionexchange methods

Electrointercalation: easy and fast ) In electrochemical intercalation the host lattice serves as the cathode of an electrochemical cell ) Stoichiometry control, fast reaction at RT, thermodynamic measurements, ease of studying staging CuxTiS2 + δx Cu

Cu(x+δx)TiS2

Experimental set-up for the cathodic reduction of TiS2

Cathode

Anode

Aprotic electrolyte

Electrointercalation: easy and fast Potential/charge transfer diagram for the cathodic reduction of TiS2

0.0 ≤ x ≤ 0.5 0.7 ≤ x ≤ 0.9

Not ideal for: bulk samples, insulating hosts or neutral guest species