Hydrothermal Synthesis of Submicron SnO Crystallites
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SnO crystallites having a size ranging 0.1–0.2 m were synthesized by a hydrothermal process, which consisted of prolonged solution aging under pH ⳱ 1.0, followed by hydrothermal treatment under pH ⳱ 9 to 10 at 75–95 °C. Oxidation of Sn+2 to Sn+4 during the hydrothermal stage was effectively inhibited by increasing the solution-aging time. This was attributed to the formation, upon aging, of polymeric hydrous SnO colloids, which are more oxidation resistant than aquo-hydroxo tin complex monomers. For solutions with a sufficiently long (艌240 h) aging time, the SnO yield increased with increasing pH and temperature during the hydrothermal treatment.
SnO has been recognized as a potential alternative to graphite as an anode material of the lithium-ion secondary battery with a higher specific capacity (mA h/g). For graphite anodes, Li ions intercalate between basal planes during charging to a maximum content of LiC6, which corresponds to a theoretical capacity of 372 mA h/g carbon. When SnO is used,1–3 on the other hand, Li ions react with SnO during the first charging cycle to form alloys with a maximum theoretical Li content up to Li4.4Sn: 6.4 e− + 6.4 Li+ + SnO → Li2O + Li4.4Sn
. (1)
The reversible anode half-cell reaction during subsequent discharging–charging cycles is expressed as Li4.4Sn ↔ 4.4 e− + 4.4 Li+ + Sn
.
(2)
Whereas Reaction (2) predicts a reversible theoretical capacity of 993 mA h/g Sn, the practical capacity has been found to depend on SnO powder composition (i.e., SnO2 impurity) and morphology.3–6 Reaction (2) involves an almost threefold change in specific volume. The irreversible by-product Li2O formed via Reaction (1) has been said to help to stabilize the anode structure by establishing a stable matrix.3 The advantage, however, would be offset by unnecessary excessive Li ion loss when SnO2, rather than SnO, is used.6 In this report, the formation of SnO submicron crystallites by hydrothermal synthesis is described. Although the low-temperature characteristics of this technique are particularly attractive for producing metal suboxide
a)
Address all correspondence to this author. J. Mater. Res., Vol. 15, No. 7, Jul 2000
http://journals.cambridge.org
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materials, literature on the synthesis of SnO is scarce.7 The effects of different processing variables, including solution-aging time, solution acidity, and hydrothermal temperature on competitive formation between SnO and SnO2 were investigated. A rather unexpected profound effect due to solution aging prior to the hydrothermal stage was observed. The entire hydrothermal process involves the following procedures. A solution containing 0.06 M Sn2+ with pH ∼ 1.0 was prepared by dissolving SnCl2 ⭈ 2H2O(s) into aqueous HCl solution. After aging for a selected period of time, ammonia was introduced into the solution, producing white precipitate, until the desired pH was reached. The solution-aging time ranges from a few minutes to over 720 h, whereas the solution pH for condensation varies between 3 and 10. The entire so
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