Lithium-silicon alloys

Attention has been given for some time to the use of lithium alloys as an alternative to elemental lithium. Groups working on batteries with molten salt electrolytes that operate at temperatures of 400-450 °C, well above the melting point of lithium, were especially interested in this possibility. Two major directions evolved. One involved the use of lithium-aluminium alloys [5, 6], whereas another was concerned with lithium-silicon alloys [7-9]. 

The cell operated at 300°C, at which temperature lithium metal is liquid, so that the lithium anode was replaced by a lithium-silicon alloy which is solid and which exhibits a reversible uptake of lithium, as discussed in Chapter 8. 

One of the battery prototypes for electric vehicles had a volume of 3201 and mass of 820 kg. The positive electrode is manufactured from FeS with the addition of C0S2. A few layers of the active material alternating with graphitized fabric are placed into a basket of molybdenum mesh welded to the central molybdenum current collector. The positive electrode is wrapped into a two-layer separator. The inner layer consists of Zr02 fabric and the outer layer of BN fabric. The negative electrode consists of a lithium-silicon alloy in the porous nickel matrix. The container and the cover are manufactured from stainless steel and electrically connected to the negative electrode. The prototype was drained with current up to 50 A, and the specific power was as high as 53 W/kg (Martino FJ et al, 1978). 

A semiquantitative analysis carried out by integrating the silicon-related peak intensities revealed that at the beginning of the first lithiation Li preferentially saturated the graphite phase (intercalation), and later, silicon became involved in the reaction (alloying). However, due to the milder alloying conditions, in the presence of the carbonaceous phase, the formation of lithium-silicon alloys with high lithium content on the surface of the alloying silicon particles has been suppressed. Therefore, the carbonaceous media led to a more uniform distribution of Li within the bulk of the active silicon particles. 

Thermal treatment of the mixture resulted in formation of a nanocomposite material made of tiny grains with a diameter of ca. 20 run. Hydrogen was considered to be the key in forming the nanocomposite material in this reaction. The hydrogen atoms were successively absorbed and desorbed from the lithium-silicon alloy many... 

There is strong evidence that failure of the silicon anode is caused by massive volume variations during lithiation and deUthiation of the lithium-silicon alloys because the intermetallics formed have greater molar volume than the parent pure silicon phase. For example, Li22Si5 has a 300% volume expansion over pure silicon, which causes substantial internal stress during lithiation or alloying. The fully lithiated phase of Lii5Si4 exhibits a capacity of 3,579 mAh/g and has a density of 1.179 g/cm calculated from XRD data, which represents... 

Limthongkul P, Jang Y-I, Dudney NJ, Chiang Y-M (2003) Electrochemically-driven solid-state amorphization in lithium-silicon alloys and implications for lithium storage. Acta Mater 51 1103-1113... 

The cells can be made of either an Li-Al or lithium-silicon alloy as the anode and a GVD thin film of titanium disulfide as the cathode. 

This battery produced by P.R. Mallory (now Duracell) operates at 300°C and uses a lithium-silicon alloy in cell construction, as lithium melts below its operating temperature. The cell has a voltage of 2.4 V at 300°C and a practical energy density of 280Wh/kg". The basic cell reaction is ... 

More work has been done on lithium-aluminium alloys than lithium-silicon alloys. However, Gould, Inc. (US) in their cell development have combined lithium-silicon alloy with lithium-aluminium alloy in their negative electrode. This increases the specific and volumetric capacity and avoids corrosion of cell components. 

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