Facile Synthesis of hollow flower-like SnO and Applications in Lithium Ion Batteries

: Novel hollow flower-like SnO structures have been successfully synthesized by using a facile solvothermal method approach, which injecting LiN(SiMe 3 ) 2 to the solution of SnCl 2 and oleamine. The as-synthesized hollow flower-like SnO structures were aggregated by ultrathin SnO nanosheets with large surface area and exhibit narrow size distribution of ~2.0 µm. The electrochemical performance of hollow flower-like SnO structures were examined as an anode material in coin cells for lithium-ion batteries. It demonstrates excellent lithium ion batteries performance as characterized by the cycling stability, specific capacity, cyclic voltammetry and rate performance. The measured discharge capacity is 361.4 mAh g -1 after 50 charge/discharge cycles with 46% capacity retention, which indicated a improving in cyclic stability. This result demonstrated that the specific hollow structure has great potential as the electrodes for lithium ion batteries. This work may provide an attractive road to synthesize other hollow-flower structures of transition metal oxides.


Introduction
In recent years, lithium ion battery (LIBs) is one of the most advanced energy storage systems which have been widely studied for cell phones, handheld computers portable, electronic vehicles and biomedicine, due to their characteristics of eco-friendly, high energy density and long cycle life. Many attempts have been conducted to develop potential materials (theoretical capacity higher than traditional graphite) as candidates to replace the currently commercial graphite anode materials, such as metal nitrides, metal oxides, metal sulfide and alloys. Recently, Sn-based anode materials have been widely used in LIBs due to their high theoretical capacity, low cost and low potential for lithium alloying. SnO is p-type semiconductor (direct band gap of 2.5-3.4 eV [1,2] ) which has been utilized in catalyst [3][4][5][6] , sensor [7,8] and energy storage [9,10] . It is noteworthy that the theoretical capacity of SnO is 875 mAh g -1 , which is higher than SnO2 (783 mAh g -1 ). This suggests that SnO is a potential cathode material for lithium ion batteries. As known, the properties of materials have been strong influenced on their sizes and morphologies. In recent research reports, SnO with various morphologies morphologies have been reported, such as diskettes, platelets, sheets, belts and flowers [3,6,9] . Due to the higher active specific areas and short path lengths for electronic transport, nanostructured SnO is supposed to be more suitable for long-lived anode materials.
In this work, we successfully prepared hollow flower-like SnO through the reaction of SnCl2 and LiN(SiMe3)2 in oleamine. The hollow flower-like structures provides vast 3 surface states and defects with high surface area and accelerates the transport of conductive ions. On the other hand, the volume changes associated with Li + insertion and extraction can be well buffered due to the hierarchical structure. Benefiting from the structural features, the hollow flower-like SnO delivers an initial discharge capacity of 1235.8 mAh g -1 , and remains 361.4 mAh g -1 after 50 cycles. The results demonstrate a significantly improving in cyclic stability. In this sense, the hierarchical structure holds promise for anode material for rechargeable lithium ion batteries.

Results and discussion
The morphology of as-synthesized SnO was characterized by SEM, which shown in  (1) ) and 212 cm -1 (A 1g ), showing the c-axis polarization [11] .
The EDS spectrum (Figure 2a) indicated that only Sn and O elements are detected. 5 The atom ration is 49.43:50.57, close to 1:1, exhibiting the stoichiometric phase SnO.
Moreover, the EDS line-scan intensity profile across the single SnO sample was shown in Figure 2b. The element of O and Sn exhibit weak but nonzero intensity in core region of the structure, which indicated the hollow structure of aggregated SnO nanosheets. TEM and HRTEM were carried out to investigate the structure and crystallography of as-synthesized SnO nanostructure, which shown in Figure 2c Meanwhile, the corresponding SAED pattern can be indexed as the [001] zone axis of tetragonal SnO. Due to the grains have the slowest growth velocity along the direction of < 001 >, the formation of SnO crystals mainly exhibits a (001) face [12] .
A probable growth mechanisms of the as-synthesized hollow-flower SnO structures is proposed, which shown in Figure 3. Initially, SnCl2 reacted with Li[N(SiMe3)2] to form Sn[N(SiMe3)2]2 which will form SnO nuclei through reduction reaction. Driven by the minimization of the total energy of the system, the SnO nuclei then form 'petals' nanosheets in a very short time. The petals aggregated in oleyamine to assemble into hollow flower-like SnO nanostructure. The role of oleyamine in the synthesis is not only a solvent, but also a surfactant [13] .
To explore the potential applications in lithium batteries, the as-synthesized SnO were fabricated into CR-2430 coin cells as anode. The galvanostatic charge/discharge voltage profiles of first three cycles are exhibited in Figure 4a. In the first static lithiation process, the initial capacity were 1235.8 mAh g -1 . The high initial discharge capacity is attributed to the formation of solid electrolyte interface (SEI) and the 6 volume expansion when SnO was transformed to LixSn (0≤x≤4.4) alloy. Figure 4b shows the cycling stability of as-synthesized SnO, which the discharge capacity remained 361.4 mAh g -1 after 50 charge/discharge cycles. The coulombic efficiencies of SnO is 59.3% in the initial and increased to 97% after several cycles, which revealing a good reversibility of the SnO samples. In addition, the SnO electrode has a rate performance with reversible capacities of 537.3, 287.9, 221.6 and 122.5 mAh g -1 at 0.2, 0.4, 0.8, and 1.6 C (1 C=500 mA g -1 ), respectively (Figure 4c). When returns to 0.2 C, the coin cell displayed a stable discharge capacity of 436.1 mAh g -1 , indicated an excellent rate capacity. The Li + insertion-extraction behavior of SnO electrode was investigated by cyclic voltammetry (CV). According to Figure 4d, the reduction peaks at 1.02 V and 0.61 V can be observed in the first cathodic scan, which could attributed to the SnO reduced to Sn (Eq. 1) and the irreversible formation of solid electrolyte interface. The Sn is then reacts with Li to form LixSn alloys. The Li matrix can promote the alloying and de-alloying reactions resulting in better cyclic performance [14] . The peaks at 0.06 and 0.59 V can be ascribed to the reaction process in Eq. 2.
Furthermore, compared to the second and third cycles, the peak intensities of the first cycle are lower. This might be due to the unique hierarchical structures [9] . Compared with SnO of other morphologies, the SnO sample demonstrated excellent cyclic stability of lithium ion batteries, which is shown in Table. 1. The excellent electrochemical performance of hollow-flower SnO can be ascribed to vast surface states and defects with high surface area on hollow-flower SnO accelerating the transport of conductive ions.