Nikolas
Oehl
*a,
Guido
Schmuelling
*b,
Martin
Knipper
a,
Richard
Kloepsch
b,
Tobias
Placke
b,
Joanna
Kolny-Olesiak
a,
Thorsten
Plaggenborg
a,
Martin
Winter
b and
Juergen
Parisi
a
aUniversity of Oldenburg, Energy and Semiconductor Research Laboratory, Institute of Physics, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany. E-mail: nikolas.oehl@uni-oldenburg.de
bUniversity of Muenster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, 48149 Muenster, Germany
First published on 8th October 2015
In situ X-ray diffraction (XRD) was performed to study the formation of the α-Sn structure in nanocrystalline Sn-based electrodes during electrochemical lithium insertion and extraction at room temperature. Therefore, pure β-Sn nanoparticles were synthesised and further processed into electrodes. The lithiation and de-lithiation process of the β-Sn nanoparticles follows the formation of discrete lithium–tin phases which perfectly fits the voltage plateaus in the charge/discharge diagram. However, unlike bulk electrodes, where no α-Sn is formed, we observed the formation of the semiconducting α-modification at 870 mV vs. Li within the first de-lithiation process. This observation explains earlier reports of an increasing internal resistance of such an electrode. Additionally, our study supports earlier suggestions that predominantly small tin crystallites are transformed from the β-Sn phase into the α-Sn phase, while larger crystallites retain their metallic β-Sn structure.
In situ X-ray diffraction (XRD) measurements by Rhodes et al.10 on bulk Sn electrodes showed that the lithiation and de-lithiation process follows the formation of discrete lithium–tin phases. The presence of four phases including β-Sn, Li2Sn5, β-LiSn and Li22Sn5, which were formed during lithiation and de-lithiation, was identified and could be correlated to the voltage plateaus in the charge/discharge profile. In their work, no α-Sn phase formation was observed.
In this study, we investigated the first-cycle lithiation and de-lithiation process of Sn/SnOx core/shell nanoparticles using an in situ XRD technique in order to get an insight into the mechanism of the α-Sn formation. The in situ XRD measurements show that the lithiation/de-lithiation process follows the formation of lithium–tin phases similar to those in Sn bulk electrodes. However, in contrast to the bulk material, the α-Sn phase is formed at the end of the first de-lithiation process at 870 mV vs. Li. Furthermore, the measurements show that the α-Sn phase gets lithiated contemporaneous with the β-Sn phase, forming the same Li–Sn phases.
XRD investigations were performed using a Bruker D8 Advance X-ray diffractometer (Bruker AXS GmbH) equipped with a copper target X-ray tube (λ = 1.54 Å). In situ XRD analysis of the Sn active material upon galvanostatic cycling was carried out using a self-designed in situ cell, which has been described previously.12 An electrode paste consisting of 80 wt% Sn nanoparticles, a 12 wt% conductive carbon agent Super C65 (Imerys) and 8 wt% sodium carboxymethylcellulose (Na-CMC) as a binder (Walocel CRT 2000 PPA 12, Dow Wolff Cellulosics) was applied onto a beryllium (Be) window serving as both the current collector and the X-ray transparent window. Metallic lithium foil (Rockwood Lithium) served as a counter electrode, Whatman glass fiber (grade GF/D) as a separator and 1 M LiPF6 in EC/DEC 3:
7 (by weight) with 2 wt% VC (UBE) as an electrolyte. After resting, the cell was galvanostatically cycled at a current rate of C/15 (based on the tin active weight m = 6.52 mg, theoretical specific capacity = 950 mAh g−1) between 0.025 V and 1.5 V with a constant voltage step at the lower cut-off voltage (I ≤ C/20). Simultaneously, XRD measurements were performed in the 2θ angular range of 21–81° with a step size of 0.015 degrees and a step time of 0.7 s, resulting in 50 minutes per XRD scan at an accelerating voltage of 40 kV and a current of 40 mA. The Rietveld refinement of the diffraction pattern was applied with the Maud software 2.4.9 in the 2θ angular range of 21–47°. The crystal structure parameters for the β-Sn (03-065-7657) and the α-Sn (01-087-0794) phases were taken from the ICDD database. A polynomial of the fourth degree was used to refine the background; the incident intensity factor, lattice constants and crystallite size and strain were refined in the second and third steps.
The size distribution was determined by TEM measurements and reveals a mean diameter of 9.8 nm and a volume-weighted mean diameter of 24 nm. The volume-weighted mean diameter was calculated according to eqn (1):
![]() | (1) |
This size is large enough to get strong reflections for the in situ X-ray diffraction measurements and it is small enough to observe the desired β/α-phase transformation.
Rietveld refinement with the Maud software version 2.3.3 was applied in order to analyse the diffraction pattern of the as-prepared nanoparticles. The crystal structures of Sn and SnO were considered to calculate the diffraction pattern. Fig. 2 shows the result of the refinement.
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Fig. 2 XRD measurement of the prepared Sn/SnOx nanoparticles and the result of the Rietveld refinement. The weighted R factor is 2.1%. |
All strong reflections in this pattern could be assigned to the β-Sn phase. A broad peak at 2θ between 30° and 35° can be explained with the SnOx phase. This strong broadening results from the small crystalline domains of that phase. Therefore we conclude that the nanoparticles consist of a crystalline β-Sn core and a SnOx shell with only very small crystalline domains. This conclusion is supported by HR-TEM measurements (Fig. 1b) which show a highly crystalline core and an amorphous shell. The results from the refinement of the diffraction pattern of the as-prepared nanoparticles are in good agreement with the TEM measurements. Fig. 3 shows the volume-weighted size distribution of the as-prepared Sn nanoparticles. According to earlier observations, at a critical size of 17(4) nm, 33(7) wt% of the particles should be transformed into the α-modification after electrochemical lithium insertion and extraction. The X-ray diffraction patterns of the in situ XRD measurement and the corresponding voltage profile of the first charge/discharge cycle are displayed in Fig. 4. The results show that the lithiation process follows the formation of discrete lithium–tin phases, which is perfectly correlated to the voltage plateaus from the charge/discharge profile. The lithiation process follows the formation of the same lithium–tin phases which are formed in bulk Sn-based electrodes, i.e. Li2Sn5, LiSn and Li22Sn5.10 At the beginning, the assembled in situ cell has an initial open circuit voltage of 2.78 V and the diffraction pattern only shows the reflections from β-Sn (Fig. 4a) and the (100) reflection originating from the Be window at 2θ = 46.1°. The transformation of the SnOx shell into the Li2O phase is not observed in the X-ray diffraction measurements because of the small crystalline domain of the shell and the small scattering cross-section between X-rays and light elements. Nevertheless, a small voltage plateau at 0.9 V indicates such a transformation. When the cell voltage reaches the first plateau at 0.56 V, the reflections from the β-Sn phase vanish and a crystalline Li2Sn5 (Fig. 4b) phase is formed. Shortly after the voltage falls below 0.42 V, the reflections from Li2Sn5 disappear and the signals of the LiSn (Fig. 4c) phase appear. Once the voltage decreases below 0.31 V, the characteristic reflections13,14 (a broad reflection at 22° and a sharp one at 38°) of the highest lithiated phase Li22Sn5 (Fig. 4d) occur. In some literature reports, Li17Sn4 was identified as the highest lithiated phase.15 Nevertheless, the Li17Sn4 crystal structure does not fit the XRD pattern from the highest electrochemically lithiated Sn phase in our experiment.
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Fig. 3 Volume-weighted size histogram of the prepared tin nanoparticles determined by TEM measurements. |
During de-lithiation (Fig. 4d–g), the same lithium–tin phases appear and vanish in reverse order and with more overlap between diffraction peaks representing the various phases. However, an uprising phase can still be correlated to a new plateau in the voltage profile. When the voltage exceeds 0.87 V, the reflections from β-Sn return, and additionally, the reflections from α-Sn (Fig. 4g) appear as well. In the following cycles (Fig. 4h–j), the signals from the α-Sn phase appear and vanish contemporaneous with the β-Sn phase, indicating that the α-Sn phase gets lithiated and de-lithiated in a similar way as the β-Sn phase. Fig. 5 shows the XRD patterns and the result of the structural Rietveld refinement for the measurement before and after the first charge/discharge cycle. Fig. 5a shows the XRD pattern before the lithiation process starts. All detectable reflections can be assigned to the β-Sn phase and the Be window. After one complete cycle (see Fig. 5b), three additional reflections appear. These reflections can be explained by the formation of the α-Sn phase. The results of the refinement are summarized in Table 1.
![]() | ||
Fig. 5 XRD measurement and refinement of the nano-Sn-based electrode before (a) and after (b) the first charge/discharge cycle. |
β-Sn | α-Sn | w α/wβa | R wp (%) | ||||
---|---|---|---|---|---|---|---|
a (Å) | c (Å) | L (nm)c | a (Å) | L (nm) | |||
a w α/wβ denotes the weight ratio of the α-phase/β-phase. b R wp denotes the weighted R factor. c L denotes the crystallite size. d In the de-lithiated state. | |||||||
As prepared | 5.8388(1) | 3.186(1) | 24.6(9) | — | — | — | 3.5 |
After the 1st cycled | 5.8393(1) | 3.188(1) | 26(2) | 6.467(1) | 7.1(4) | 0.23 | 3.4 |
The results of the refinement show that the crystallites of the α-Sn phase (L = 7.1(4) nm) are significantly smaller than the crystallites of the β-Sn phase (L = 26(2) nm). The mean crystallite size of the β-Sn nanoparticles increases. Therefore we believe that the original size distribution is split into α- and β-phase distributions. This observation is in conformity with earlier reports that the β-/α-phase equilibrium depends on the size of the crystallites.7 From the ratio of the peak intensities, we calculated the weight ratio. Therefore, after one complete cycle, the crystalline part of the Sn nanoparticles consists of 19(1) wt% α-Sn and 81(1) wt% β-Sn. On the assumption that only the smallest particles in this distribution are transformed into the α-phase, one could calculate a critical size of 11(3) nm for the transformation. Our measurements show for the first time that the α-Sn phase is already formed during the first charge/discharge cycle at 870 mV and simultaneously with the β-phase.
Although the size-dependent phase transformation is very common for nanoparticle crystal structures, the phase transformation for Sn nanocrystals is poorly studied so far. Im et al.5 used ab initio calculations to study the emergence of the α-phase. They concluded that once the α-phase is formed, it preserves its crystal structure, while the β-phase is easily transformed into an amorphous phase during lithiation. Nevertheless, the formation of the α-phase is still supposed to be a size-related effect, which is not fully understood, so far. Besides the contribution of the surface energy, the formation and the stability of α-Sn depends on other factors as well. Gallerneault et al.16 showed that small amounts (0.6 wt%) of Si can enhance the α-phase stability. Furthermore, a matrix is able to stabilize the α-Sn phase up to a melting point of 200 °C.17 However, there are no reports about the α-phase in Sn nanoparticles without the participation of lithium. Therefore, a lithium contribution to the α-phase formation should be considered as well. How the size of the crystallites, the incorporation of lithium and the matrix stabilization might affect the α-phase formation needs to be studied in future work.
Furthermore, the influence of the α-phase on the overall electrochemical performance of Sn-based anode materials needs to be examined more carefully. For example, Im et al.5 reported that crystalline α-Sn can lead to improved rate performance. However, α-Sn is a semiconductor and may lead to an increased internal resistance compared to metallic β-Sn. Studies by Kaghazchi18–20 showed that the insertion of Li in α-Sn is kinetically more favorable than that in β-Sn. Nevertheless, our measurements show that the α-phase gets lithiated contemporaneous with the β-Sn phase. Therefore the kinetic influence of Li insertion into the α-Sn phase seems to be negligible. Therefore, the insertion of lithium into the α-Sn phase and the effect of an increased electrical resistance need to be investigated more carefully and will be the topic of our future work.
Footnote |
† Electronic supplementary information (ESI) available: Cif data of β-Sn and α-Sn nanoparticle crystal structures. See DOI: 10.1039/c5ce01841b |
This journal is © The Royal Society of Chemistry 2015 |