Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium

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Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium Ion Batteries: Comparative Phase Conversion of Germanium and Tin Hyung Soon Im,† Young Rok Lim,† Yong Jae Cho,† Jeunghee Park,*,† Eun Hee Cha,‡ and Hong Seok Kang§ †

Department of Chemistry, Korea University, Jochiwon, Sejong 339-700, Korea Department of Liberal Art and Literature, Hoseo University, Asan, Chungnam 336-795, Korea § Department of Nano and Advanced Materials, Jeonju University, Chonju, Chonbuk 560-709, Korea ‡

S Supporting Information *

ABSTRACT: Germanium and tin sulfide nanostructures are considered the most promising candidates for useful alternative materials in commercial Li−graphite anodes of lithium ion batteries. Selenides have received less attention, but the electrochemical reaction mechanism is still being debated. We report the novel synthesis of GeSex and SnSex (x = 1 and 2) nanocrystals by a gas-phase laser photolysis reaction and their excellent reversible capacity for lithium ion batteries. The capacity was 400−800 (mA h)/g after 70 cycles, which is close to the theoretical capacity (Li4.4Ge or Li4.4Sn). Remarkably, SnSex exhibited higher rate capabilities than GeSex. Ex situ X-ray diffraction and Raman spectroscopy revealed the cubic−tetragonal phase conversion of Ge and Sn upon lithiation/delithiation to support their distinctive lithium ion battery capacities. Firstprinciples calculations of the Li intercalation volume change indicate that the smallest volume expansion in the cubic Sn phase can guarantee the enhanced cycling capability of the Sn compounds.

1. INTRODUCTION Ge and Sn are elements of particular interest for lithium ion batteries (LIBs).1−4 Both these elements have high theoretical capacities, 1640 (Ge) and 990 (Sn) (mA h)/g, owing to Li alloy formation (i.e., Li4.4Ge and Li4.4Sn), fast Li+ ion diffusivity, and high electrical conductivity. Therefore, they are considered the most promising candidates for useful alternative materials in commercial Li−graphite anodes of LIB. However, large volume changes (up to 300%) during lithiation/delithiation lead to significant capacity fading, which is an issue yet to be solved. Recently, the use of oxide or sulfide nanostructures has been intensively investigated because they afford better cycling performance with the formation of the Li2O or Li2S matrix (via irreversible decomposition), alleviating catastrophic volume changes.5−24 In addition, the nanosize morphology can minimize volume changes and dissipate the mechanical stress because of increased surface:volume ratios. Selenides have received less attention, and the electrochemical reaction mechanism for the SnSe film with Li has been proposed to be either the same as or different from that in the case of SnO2 or SnS2.25−27 In the present work, we successfully synthesized GeSex and SnSex nanocrystals (NCs), where x = 1 and 2, by a gas-phase laser photolysis reaction. The GeSx and SnSx (x = 1 and 2) NCs were also synthesized for comparison. We investigate the cycling performance of the sulfide and selenide NCs in LIBs. All the selenide NCs had excellent LIB capacities comparable to those of their sulfide counterparts. The Sn coumpond (SnSex © XXXX American Chemical Society

and SnSx) NCs exhibited higher rate capabilities than did the Ge compound counterparts. Our group reported the production of metastable tetragonal-phase (ST12) Ge and cubic-phase Sn (α-Sn) upon lithiation/delithiation, respectively, for the GeS and SnS NCs.18,23 The phase conversion of selenide NCs was first investigated using ex situ X-ray diffraction (XRD) and Raman spectroscopy to compare it with that in the case of sulfide NCs. Ab initio quantum mechanical calculations on the Li-intercalated cubic- and tetragonal-phase Ge and Sn lattices were used to explain the experimental results.

2. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the high-purity and singlephase selenide NCs, orthorhombic GeSe (JCPDS no. 48-1226, Pnma, a = 10.84 Å, b = 3.834 Å, c = 4.390 Å), monoclinic GeSe2 (JCPDS no. 71-0117, a = 7.061 Å, b = 16.79 Å, c = 11.831 Å, β = 90.650°), orthorhombic SnSe (JCPDS no. 48-1224, Pnma, a = 11.49 Å, b = 4.15 Å, c = 4.440 Å), and hexagonal SnSe2 (JCPDS no. 23-0602, P3̅m1, a = 3.810 Å, c = 6.140 Å). The purity and phase of the GeS, GeS2, SnS, and SnS2 NCs were identified by XRD as shown in the Supporting Information, Figure S1. Received: July 22, 2014 Revised: August 30, 2014

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profiles over the selected area at the edge of the nanosheets. The composition of an individual NC was confirmed by energydispersive X-ray fluorescence spectroscopy (EDX) (Figure 2f). The EDX peak of Ge (K and L shells), Sn (L shell), and Se (K and L shells) elements with scanning TEM (STEM) images (insets) shows the Ge:Se and Sn:Se ratios are 1:1 or 1:2. The scanning electron microscopy (SEM) images of the NC powders are shown in the Supporting Information, Figure S2, with the EDX data that show the consistent composition. The electrochemical performance of LIB anode materials was examined for four selenide (GeSe, GeSe2, SnSe, and SnSe2) NCs with four sulfide (GeS, GeS2, SnS, and SnS2) NCs. We defined 1 C of Ge and Sn compounds as the theoretical capacity of Ge (1640 mA/g) and Sn (990 mA/g), respectively. Parts a−d of Figure 3 show the voltage profiles of coin-type

Figure 1. XRD patterns of GeSe, GeSe2, SnSe, and SnSe2 NCs.

Figure 2 shows the high-resolution transmission electron microscopy (HRTEM) images of GeSex and SnSx NCs. The

Figure 3. Discharge/charge voltage profiles of coin-type half-cells using (a) GeSe, (b) GeSe2, (c) SnSe, and (d) SnSe2 for 1, 2, 5, 10, 30, and 70 cycles tested between 0.01 and 1.5 V at a rate of 0.1 C.

half-cells prepared using GeSe, GeSe2, SnSe, and SnSe NCs, respectively, for 1, 2, 5, 10, 30, and 70 cycles at a discharge/ charge rate (current rate, C rate) of 0.1 C, tested between 0.01 and 1.5 V. The voltage profile data of sulfide NCs are shown in Figure S3 (Supporting Information). GeSe and GeSe2 show a plateau at ∼1 V in the first discharge process, which is ascribed to the irreversible decomposition into Ge and Li2Se. Another plateau in the first discharge process appears at a higher voltage (∼1.3 V) for GeSe2, probably because of irreversible decomposition into GeSe and Li2Se. The plateau region at ∼0.2 V appears in all the discharge curves, corresponding to the reversible process of lithium insertion/ deinsertion of Ge: Ge + xLi ↔ LixGe. SnSe and SnSe2 consistently show a plateau at ∼1.3 V in the first discharge process, which is ascribed to irreversible decomposition into Sn and Li2Se. SnSe2 has a plateau at ∼1.7 V, due to irreversible decomposition into SnSe and Li2Se. The plateau region at ∼0.2 V appears in the discharge curves and is caused by the lithium insertion process of Sn + xLi → LixSn. The first discharge/charge capacities have an initial Coulombic efficiency average of 60% because of the irreversible decomposition process into Ge or Sn: GeSex (or SnSex) + 2xLi+ + 2e− → Ge (or Sn) + xLi2Se. After the fifth cycle, all profiles show nearly perfect reversibility with an average Coulombic efficiency of 98%, because of the alloying/ dealloying process: Ge (or Sn) + xLi ↔ LixGe (or LixSn).

Figure 2. (a) HRTEM image showing the general morphology of GeSe, GeSe2, and SnSe NCs. Lattice-resolved and FFT images of (b) GeSe, (c) GeS2, and (d) SnSe NCs. (e) HRTEM and FFT images and corresponding intensity profile of SnSe2 nanosheets. (f) EDX data of GeSe, GeSe2, SnSe, and SnSe NCs with STEM images (insets).

GeSe, GeSe2, and SnSe NCs (average size 20 nm) usually agglomerate into a cluster with a random size up to a few hundred nanometers (Figure 2a). The lattice-resolved and fast Fourier transformed (FFT) images of the single-crystalline GeSe NC reveal that the d-spacing of the orthorhombic (101) planes is 2.9 Å, which is consistent with that of bulk GeSe (Figure 2b). The (010) planes of the monoclinic-phase GeSe2 NCs have a d-spacing of 16 Å (Figure 2c). The d-spacing of the (010) planes in the SnSe NCs is 4.0 Å, corresponding to that of the orthorhombic phase (Figure 2d). Figure 2e shows an HRTEM image of SnSe2 (size 20−50 nm) NCs consisting of 2−10 layered sheets (average thickness 5 nm) that are tangled to form a cluster. The d-spacing of the hexagonal (0001) planes of SnSe2 was clearly identified as 6.2 Å from the intensity line B

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The corresponding cyclic voltammetry curves are shown in Figure S3 (Supporting Information). Parts a and b of Figure 4 show the discharge/charge capacity for the Ge and Sn compound NCs, respectively, as a function of

Figure 5. XRD patterns of (a) GeSe and (b) SnSe electrodes after the 3rd and 20th cycles (discharged), where T and C denote tetragonaland cubic-phase Sn, respectively.

= 6.98 Å). The peaks were red-shifted by an average of 0.5°, probably due to the residual Li atoms. The peak at 23.1° matched the (220) plane of the cubic phase of Li15Ge4 (JCPDS no. 89-2584, a = 10.7860 Å).17,18 SnSe showed tetragonal (β) Sn (JCPDS no. 86-2265, a = 5.831 Å, c = 3.181 Å) and cubic (α) Sn (JCPDS no. 87-0794, a = 6.489 Å) peaks. The peaks at 25.7° and 29.9° were assigned to the (111) and (220) planes of Li2Se (JCPDS no. 77-2146, a = 6.489 Å), respectively. Li2Se would be formed by an irreversible conversion: SnSe + 2Li+ + 2e− → Sn + Li2Se. As the number of cycles increases, α-Sn becomes dominant. We concluded that the selenides undergo the same phase conversion as that of the sulfides. The lower intensity of Ge than that of Sn indicates considerable amorphization. Ex situ Raman spectra were measured for the Ge, GeS, and GeSe electrodes, as shown in parts a−c, respectively, of Figure 6. After the cells were first discharged, a peak at 290 cm−1

Figure 4. Discharge/charge capacity at 0.1 C versus cycle number for coin-type half-cells using (a) GeS, GeS2, GeSe, and GeSe2 and (b) SnS, SnS2, SnSe, and SnSe2 NCs cycled at a rate of 0.1 C. (c, d) Cycling performance as the rate is increased from 0.1 to 20 C.

the cycle number up to 70 cycles at a rate of 0.1 C. All samples exhibited high Coulombic efficiency after the first cycle. The capacities after 70 cycles were 1232, 1099, 867, and 647 (mA h)/g for GeS, GeS2, SnS, and SnS2, respectively. The capacities were 771, 574, 510, and 412 (mA h)/g for GeSe, GeSe2, SnSe, and SnSe2, respectively. This corresponds to a theoretical 4.4 mol of Li/mol of Ge (or Sn), which is the same as that of the sulfides (Supporting Information, Table S1). The rate capabilities are shown in Figure 4c,d. When the rate was increased to 20 C, the capacities of the Ge compounds averaged 50% retention of the initial capacities (fifth cycle at 0.1 C). In contrast, the Sn compounds exhibited a slight decrease to 75% of the initial capacities. When the rate was reverted to 0.1 C (after 80 cycles), the capacity was increased to 94% of the initial value. The capacities are summarized in Table S2 (Supporting Information). These results led us to conclude that the Sn compounds have higher rate capabilities than do the Ge compounds. The same behaviors in the sulfides and selenides indicate that the reversible lithiation/delithiation processes are exclusively led by Ge or Sn and that the Li2Se matrix is as effective as the Li2S matrix in increasing the capacities. It was reported that the diffusion rate of Li ions in bulk Sn ((2−6) × 10−7 cm2 s−1 at 25 °C) is much higher than in bulk Ge (3.1 × 10−9 cm2 s−1 at 150 °C).3,28 We can simply rationalize the higher rate capability of the Sn compounds on the basis of the higher diffusion rate of Li+ ions in Sn. However, our previous work revealed the cubic−tetragonal phase conversion of Ge and Sn. Therefore, we need to examine the phase underlying the lithiation/delithiation process for selenides. Parts a and b of Figure 5 show the ex situ XRD patterns of the GeSe and SnSe electrodes (discharged), respectively, after the 3rd and 20th cycling tests, where all the GeSe and SnSe had decomposed. For GeSe, the peaks were assigned to the tetragonal-phase (ST12) Ge (JCPDS no. 72-1425, a = 5.93 Å, c

Figure 6. Raman spectra of (a) Ge, (b) GeS, and (c) GeSe electrodes after the first and third cycles.

consistently emerged. The 10 cm−1 red shift from bulk cubic Ge is due to the amorphous phase upon lithiation. The peak was resolved by a Voigt function and yielded the C-phase peak (marked by blue shading) and a shoulder peak at 270 cm−1 (red shading). After the third discharge, the broader peak was resolved into two bands at 270 and 250 cm−1, which can be assigned to the ST12 Ge.29,30 As the number of cycles increased, the ST12 peaks red-shifted and disappeared, probably because of amorphization. Ge, GeS, and GeSe consistently show transformations from amorphous cubic into the ST12 phase after a few cycles, and this is followed by amorphization. Our previous ab initio calculation showed that the ST12 phase has stronger binding interactions of Li atoms than the cubic phase.18 C

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NCs can be considered as excellent high-capacity LIB anode materials.

We also monitored the Raman spectra of Sn compound electrodes but found that the intensity of the Sn peaks was too low to support the phase conversion. Nevertheless, XRD confirmed the production and persistence of the crystalline αSn, which is distinct from the amorphization of ST12 Ge. Since the electrical conductivity of the crystalline phase is usually higher than that of the amorphous phase, α-Sn promises enhanced cycling capability of the Sn compounds. First-principles calculations were performed on the intercalation of various numbers of Li atoms into the ST12- and cubic-phase Ge lattices and the tetragonal- and cubic-phase Sn lattices (see the details in the Supporting Information).18,23 Our calculations revealed that the binding energy of Li atoms per unit cell is higher (∼7%) for the tetragonal phase compared to that for the cubic phase, as shown in Figure 7a. The binding

4. MATERIALS AND METHODS Laser photolysis of tetramethylgermanium (Ge(CH3)4, TMG), tetramethyltin (Sn(CH3)4, TMT), hydrogen sulfide (H2S), and dimethylselenium (Se(CH3)2, DMS) mixtures was performed using a Nd:YAG pulsed laser (Coherent) operated at 1064 nm, with a repetition rate of 10 Hz and a pulse width of 10 ns. All the precursors were purchased from Sigma-Aldrich or Alfa Co. The precursors were degassed by several freeze (77 K)− pump−thaw cycles and used without further purification. In the typical process, the precursor vapors (