**1. Introduction**

The present study aims to understand the capacity degradation that occurs in SnS nanomaterials in terms of the morphological changes they undergo during Li-ion insertion. Sn has attracted significant attention for its use as an anode in Li-ion batteries, since it allows for a theoretical specific capacity that is over 990 mAh g−<sup>1</sup> [1]. This is approximately three times the capacity of commercially used graphitic anodes (372 mAh g<sup>−</sup>1) [2]. Despite its promising electrochemical performance, upon the formation of Li alloys, Sn suffers from severe volume expansions as high as 298%, which result in fracture of the Sn and delamination from the electrode. This leads to a loss of material that can host Li, and a rapid capacity decay occurs after several charge–discharge cycles. In order to alleviate high volume expansions, Sn is rarely used in its pure form, but is rather used as an oxide (SnO2) or as an alloy (such as SnSb and SnS) [1,3–5]. To further increase mechanical stability, the size of the Sn-based particles is reduced to the nanoscale [6,7] and they are embedded or attached on a carbon-based matrix, since it can buffer and constrain the volume expansion.

The maximum initial capacity of Sn or SnO2 cannot exceed 990 mAh g−1; however, that of SnS anodes is greater than 1000 mAh g−1. Despite this, the initial capacity of SnS anodes significantly decreases during cycling, as is the case for all Sn-based materials. For example, SnS nanoparticles (5–6 nm) embedded in a carbon matrix [5] experienced a capacity loss of ~500 mAh g−<sup>1</sup> after 40 cycles, while SnS single phase alloy anodes in the form of microflakes showed an initial capacity of 1085 mAh g−1, which dropped to ~550 mAh g−<sup>1</sup> after 45 cycles [8].

The capacity drop observed during the first cycle has been attributed to the conversion reaction that gives rise to the formation of Li2S [5,9], which forms irreversibly upon initial lithiation. The resulting microstructure is, therefore, Sn particles in a Li2S matrix, and with continuous lithiation/delithiation, LixSn alloys form reversibly. The mechanisms that give rise to the continuous capacity fade over long term cycling have not been explored.

A detailed examination of the microstructural changes that occur during the conversion and alloying reactions of SnS electrodes has not been performed. However, based on the severe fracture observed for Sn/C and SnO2/C nanocomposites during cycling [10], it is of interest to examine whether mechanical instability can be correlated to the capacity decay of SnS. Hence, the present article focuses on the long term cycling performance of SnS/C nanomaterials and captures the effect that conversion and alloying reactions have on the microstructure by performing transmission and scanning electron microscopy before and after cycling. The particular microstructure considered is that of a flower-like nanoscale SnS alloy attached to graphite. The motivation for using such structures is that their thin petal structure is not dense and could accommodate the volume expansion of Sn. Such flower-type active materials have not been cycled before, and although similar SnS microstructures have recently been fabricated [11], they were significantly larger and their "flower petals" were much denser.

In order to examine the effect of the carbon substrate in SnS/C, both artificial graphite and microcarbon microbeads were used as the matrix, while the SnS content was varied by allowing the precursor compounds in solution to be either 10% or 20% by weight. Although the nanoflower-like SnS materials used here have not been examined as electrodes before, the purpose of this article is not to propose a promising SnS/C electrode (since SnS cannot compare to the promising high-capacity Si-based anodes), but to provide insight as to why SnS materials cannot retain their initial high capacity of over 1000 mAh g<sup>−</sup>1, and interpret their capacity fade in terms of their microstructural changes.
