Metal sulfides and their lithium storage properties
Date of Issue2017-03-16
School of Materials Science and Engineering
Lithium ion battery (LIB) is currently one of the most widely used forms of rechargeable energy storage system in portable electronics. To achieve LIB with enhanced lithium storage properties, better safety aspects and lower cost, extensive research has been conducted on the various components of a LIB for the past few decades. Although graphite is currently used as the anode material in most commercial LIB, its limited capacity and the safety issues related to its low electrochemical potential (vs. Li/Li+) have prompted scientists to search for alternative materials. Amongst the various conversion-typed materials, metal sulfides are particularly interesting as sulfur (S) has a rich and versatile chemistry, thus providing the possibility of altering the lithium storage properties of the materials by simply varying the stoichiometric ratio between the metal and sulfide ion in the compound. However, as with all sulfur and sulfide electrodes, polysulfides will be formed during their partial reduction, resulting in poor capacity retention and cycling performance in lithium and lithium ion batteries. In view of this, more study needs to be carried out to find approaches that can be undertaken to improve the lithium storage performance of metal sulfide electrodes. Hence, this thesis aims to gain an insight into how the amount of sulfide ion in the stoichiometry of a metal sulfide compound affects its lithium storage properties, with the focus placed on two metal sulfide systems, namely, iron sulfides (intercalation-conversion material) and tin sulfides (conversion-alloying material). To investigate the effect of how the amount of sulfide ion in the stoichiometry of an iron sulfide compound affects its lithium storage properties, pyrrhotite Fe1-xS and pyrite FeS2 with good purity have been successfully synthesized via a solution-based chemical synthesis method and their electrochemical properties were characterized. It was found that pyrite FeS2 exhibits better lithium storage capability than pyrrhotite Fe1-xS because of: (1) the lower polarization, better electron and Li+ ion transport at the interface between the active material and electrolyte at the pyrite FeS2 electrode and (2) the reversible lithiation and delithiation of iron sulfide (FeSy) during the galvanostatic cycling of the pyrite FeS2 electrode. SnS and SnS2 with good purity have also been successfully prepared via a solution-based chemical synthesis method to investigate the effect of how the amount of sulfide ion in the stoichiometry of a tin sulfide compound affects its lithium storage properties and the electrochemical properties of these compounds were characterized. It was found that SnS2 displayed a higher capacity and better cycling stability than SnS after prolonged cycling particularly at higher current densities. Since the SnS2 electrode was found to have a poorer electronic and ionic conductivity than the SnS electrode, its superior lithium storage performance is attributed to its chemical and structural properties. As evidenced by the higher discharge capacity attained from the intercalation and conversion reaction throughout the 100 cycles, more Li2S is formed during the lithiation of SnS2, thus providing a thicker layer to buffer the large volume change during the lithiation and delithiation of Sn. This can result in a reduction in the pulverization and better capacity retention of the electrode after prolonged cycling, as verified by the slower alloying capacity fading rate observed in the SnS2 electrode compared to the SnS electrode. It is found in this dissertation that for both iron and tin sulfides, the compound with a higher sulfide ion content in its stoichiometry i.e. FeS2 and SnS2 exhibits better lithium storage performance than its counterpart with lower sulfide ion content i.e. Fe1-xS and SnS when cycled in a voltage window of 0.001 – 3 V. For the pyrite FeS2 electrode, which undergoes intercalation and conversion reaction during cycling, the superior lithium storage performance is attributed to its better conductivity and reversibility of the lithiation and delithiation of FeSy. On the other hand, the SnS2 electrode, which undergoes conversion and alloying reaction during cycling, displayed a better lithium storage performance due to its ability to form a thicker Li2S layer which provides better buffering for the large volume change in the Sn particles during their alloying reaction, thus maintaining structural integrity of the electrode and result in slower capacity fading.