Alloys Formed in Situ from Convertible Oxides

Alloys Formed In situ from Convertible Oxides 

A renewed interest in noncarbonaceous lithium alloy electrodes arose recently as the result of the announcement by Fuji Photo Film Co. of the development of a new generation of lithium batteries based upon the use of an amorphous tin-based composite oxide in the negative electrode [13]. It is claimed that these electrodes have a volumetric capacity of 3200 Ah L , which is four times that commonly achieved with carbonaceous negative electrodes, and a specific capacity of 800 mAh g , twice that generally found in carbon-containing negative electrodes. 

According to the public announcement, a new company, Fujifilm Celltec Co., has been formed to produce products based upon this approach. It was reported that some 200 patents have been applied for in this connection. Unfortunately, there is little yet available in the standard literature concerning these matters. To date, there are only references to some of the patents [14-17]. Flowever, what must be happening seems rather obvious. 

as an example, we make the assumption that the electrode initially has the composition SnO, if we introduce lithium into it there will be a displacement reaction in which Li20 will be formed at the expense of the SnO due to the difference in the values of their Gibbs free energies of formation (—562.1 kj mol for Ii20 and —256.8 kJ mol in the case of SnO). This is equivalent to a driving force of 1.58 V. The other product will be elemental Sn, and as additional Li is brought into the electrode this will react further to form the various Li-Sn alloys that are discussed in some detail later in this section. This simplified picture is consistent with what has been found in experiments of this general type [18]. 

Thermodynamic Basis for Electrode Potentials and Capacities under Conditions in which Complete Equilibrium can be Assumed 

4 Alloys Formed in Situ from Convertible Oxides 

The general thermodynamic treatment of binary systems which involve the incorporation of an electroactive species into a solid alloy electrode under the assumption of complete equilibrium was presented by Weppner and Huggins [19-21], Under these conditions the Gibbs Phase Rule specifies that the electrochemical potential varies with composition in the single-phase regions of a binary phase diagram, and is composition-independent in two-phase regions if the temperature and total pressure are kept constant. 

Thus the variations of the electrode potential during discharge and charge, as well as the phases present and the charge capacity of the electrode, directly reflect 

as an example, we make the assumption that the electrode initially has the composition SnO, if we introduce lithium into it there will he a displacement reaction in which LI2O will he formed at the expense of the SnO due to the difference in the values of their Gibbs free energies of formation (-562.1kJmor for LijO and -256.8 

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The recent development of the convertible oxide materials at Fuji Photo Film Co. will surely cause much more attention to be given to alternative lithium alloy negative electrode materials in the near future from both scientific and technological standpoints. This work has shown that it may pay not only to consider different known materials, but also to think about various strategies that might be used to form attractive materials in situ inside the electrochemical cell. 

The A1 and Cr in the Co-Cr-Al-Y are converted to oxides in situ but cannot form protective scales because of the rapid removal of cobalt from the alloy surface. 

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