Principio de universalidad

Most intercalation materials are prepared directly by solid-state reaction or sol-gel process, but some of them are not, for instance, cubic Ti$2 can only be prepared in two

steps by: first synthesising CuTiS^ using a solid state reaction and then removing Cu by deintercalation [20].

2-2-1 Solid-state reaction and sol-gel reaction 2-2-1-1 Solid-state reactions

Solids do not usually react together at room temperature over normal time scales and it is necessary to heat them to much higher temperatures in order for reaction to occur at an appreciable rate. Thus a solid-state reaction is often characterised by a high- temperature. Fig 2-2-1 illustrates how a solid-state reaction proceeds. The reaction starts at the interface between MgO and Al^Og particles (Fig 2-2-1(a)), and then the produced phase, MgAl2 0^ in this case, grows at each interface associating with the AP+ and Mg2+ diffusion in the opposite direction to each other (Fig 2-l-l(b)) [2], It can be seen from this process that the rate of a solid state reaction relies on the particles size of the reactants and contact area between the grains. Heating promotes ion diffusion. It is also always good practised to regrind a solid mixture during the reaction so as to maximise the surface areas of contact between the reactants. Sometimes solid reactants aie pelletised to increase further the contact between grains.

(a) MgO AI2 Og (b) MgO j ^ original I y/ interface 31--- AI2O3 •MgAlproduct20/, layer new reactant - product interface

Fig 2-2-1 Schematic reaction, by interdiffusion of cations, of single crystals of MgO and AI2O3 to give MgAl20^ (quoted from [21]).

2-2-1-2. Sol-gel reactions

The sol-gel method offers an approach to the synthesis of oxide materials via wet chemical reactions [22-26]. The theory and practice of sol-gel reactions are summarised in the reference 23. In general, preparation of a powdered oxide using the sol-gel technique is divided into two steps, (1) a solution reaction involving hydroxylation and condensation of molecular precursors, and (2) sintering of the gel at a suitable temperature [22]. Compared with the conventional "powder" route (solid state reaction), the sol-gel approach undoubtedly offers a high level of chemical homogeneity in multi-component materials and so sintering temperatures are usually lower. It has been widely acknowledged that the sol-gel technique is a low temperature route to the preparation of high performance intercalation materials [27]. The preparation of amorphous LiVgOg via a sol-gel reaction is a typical example [28].

2-2-2 Chemical and electrochemical lithiation 2-2-2-1 Chemical lithiation

Chemical lithiation operates mostly in solution, and is classified as soft chemistry or chimie douce [22]. These terms refer to the synthesis of solids by mild, low temperature routes where control of the resulting structure is possible. In general, a soluble lithiation agent reacts with a solid host (MXj^) which does not dissolve in the solution, and meanwhile this agent also acts as a source of lithium ions. For example, lithiation by n-buthyl lithium (n-BuLi) in hexane, heptane, or acetonitrile solution [9],

xn-BuLi + MXj-j —> Lij^ MX^ + x/2CgHjg.

The ability of a reagent to intercalate lithium depends on its reduction potential. From thermodynamics, a reagent can reduce (i.e. lithiate) a host (MX^) if its redox potential

is lower than that of the host. Fig 2-2-2 presents redox potentials of various reagents. It is obvious that n-BuLi is a very strong reducing agent, and so is often used to prepare fully lithiated compounds. According to the Nernst equation

[Ox]

^OxIRed = %/Red + _[RedY

it is possible to adjust the reducing power of a reagent by controlling the reaction temperature and concentration when the reagent is a reversible couple. For example, Tarascon and Guyomard used Lil to lithiate LiMu204 to Lii+xMn204 (0< x < 1) at an elevated temperature (83 °C) and by controlling the amount of Lil [29].

REAGENTS ELECTRODE fMTER I AL S DDO ( R ) - -3.0 l ~ _{™ — (R)} H2 ( HgO ) —— ( N R ) H b z p h — — ( R) B H f —— ( N R ) n - B u L i — — ( N R ) b z p h “ —— ( R ) n a ph — ( R ) L i ( N H 3 ) — — ( R ) - -2.0 V2O5 Mn O2 VsOis V S 2 TiSz M 0 O 3 NiPSs M0O2 - -1.0 WO2 I - L i

Fig 2-2-2 A comparison of the redox potentials of a variety of intercalation reagents and hosts, the scale is an estimate based on observations from Li intercalation. (R) denotes reagents for which the redox couple is reversible and (NR) denotes irreversibility (quoted from [9]).

For deintercalation the reagents must be oxidising, such as I2 and B12 [3, 9]. Fig 2-2-2 shows that water itself can act as a reagent; a famous example is Hunter's method for

the preparation of spinel-related manganese oxide (À-Mn0 2) by leaching lithium from LiMu204 in acidic aqueous solution [30]. A more powerful oxidising agent is nitryl hexafluorouphosphate (NO2PF6) (^*1 ^ vs. NHE) [31] It has been reported that it can extract lithium from LiNi02 and LiCo02 [31, 32].

2“2-2-2 Electrochemical lithiation

Electrochemical lithiation has been described in section 2-1-2, where lithium ions are inserted into TiS2. The lithium content x in LijjTiS2 can be controlled by controlling the charge passed and by controlling the current the rate of intercalation may be fixed. The electrochemical method therefore offers advantages over chemical methods [9, 27]. However, electrochemical reactions are not ideal for preparing large quantities of samples for ex situ studies [9].