Abstract
The thermochemical reduction of silica to silicon using chemical reductants requires high temperature and has a high activation energy, which depends on the melting temperature of the reductant. The addition of bi-functional molten salts with a low melting temperature may reduce the required energy, and several examples using molten salts have been demonstrated. Here we study the mechanism of reduction of silica in the presence of aluminum metal reductant and aluminum chloride as bi-functional molten salts. An aluminum–aluminum chloride complex plays a key role in the reduction mechanism, reacting with the oxygen of the silica surfaces to lower the heat of reaction and subsequently survives a recycling step in the reaction. This experimentally and theoretically validated reaction mechanism may open a new pathway using bi-functional molten salts. Furthermore, the as-synthesized hollow porous silicon microsphere anodes show structural durability on cycling in both half/full cell tests, attributed to the high volume-accommodating ability.
Highlights
The thermochemical reduction of silica to silicon using chemical reductants requires high temperature and has a high activation energy, which depends on the melting temperature of the reductant
The conversion of Si from SiO2 through thermochemical process provides a general understanding on their mechanism under the given principle of Ellingham diagram, and as-reduced semiconducting Si materials have been utilized as anodes in lithium-ion batteries (LIBs)[13,14,15,16]
The reduction mechanism on disintegrated silica crumbs was investigated by the density function theory (DFT) calculation
Summary
The thermochemical reduction of silica to silicon using chemical reductants requires high temperature and has a high activation energy, which depends on the melting temperature of the reductant. As activated carbon or metal reductants undergo the explosive reactions of M (metal or carbon) + SiO2 (s) → MO2 + Si (s), etchable by-products are generated along with a large amount of exothermic heat This extensive and accumulated energy of entire system will greatly increase the risk of explosion, and appropriate mediation of system energy should be addressed. We establish the whole reaction pathway for the reduction of SiO2 with Al metal reductants and in a AlCl3 molten salt medium, named by low-temperature aluminothermic reduction reaction (LTARR), as follows: (i) complex formation between Al and AlCl3, (ii) SiO2 reduction with adsorbed Alcomplex, and (iii) recrystallization of Si seed to produce hollow porous Si sphere (HPSS) demonstrated by density function theory (DFT) calculations and consistent with experimental results. The large size of HPSS particles—over 3 μm—significantly increases the electrode density without further calendaring process, and high volumetric energy density is attained both in half/full cell demonstrations
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