- Email: [email protected]
Improved electrochemical performance of boron-doped SiO negative electrode materials in lithium-ion batteries
Journal of Power Sources 299 (2015) 25e31
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Improved electrochemical performance of boron-doped SiO negative electrode materials in lithium-ion batteries Jihoon Woo 1, Seong-Ho Baek 1, Jung-Soo Park, Young-Min Jeong, Jae Hyun Kim* Division of Nano Energy Convergence Research, Daegu-Gyeongbuk Institute of Science and Technology (DGIST), 711-873, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
One-step thermal doping and disproportionation reaction was introduced. Enhanced reversible capacity was attributed to the disproportionation of SiO. HBeSiO electrodes exhibit the best electrochemical performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 17 August 2015 Accepted 24 August 2015 Available online xxx
We introduce a one-step process that consists of thermal disproportionation and impurity doping to enhance the reversible capacity and electrical conductivity of silicon monoxide (SiO)-based negative electrode materials in Li-ion batteries. Transmission electron microscope (TEM) results reveal that thermally treated SiO at 900 C (HeSiO) consists of uniformly dispersed nano-crystalline Si (nc-Si) in an amorphous silicon oxide (SiOx) matrix. Compared to that of prinstine SiO, the electrochemical performance of HeSiO shows improved specific capacity, due mainly to the increased reversible capacity by ncSi and to the reduced volume expansion by thermally disproportionated SiOx matrix. Further electrochemical improvements can be obtained by boron-doping on SiO (HBeSiO) using solution dopant during thermal disproportionation. HBeSiO electrode without carbon coating exhibits significantly enhanced specific capacity superior to that of undoped HeSiO electrode, having 947 mAh g1 at 0.5C rate and excellent capacity retention of 93.3% over 100 cycles. Electrochemical impedance spectroscopy (EIS) measurement reveals that the internal resistance of the HBeSiO electrode is significantly reduced by boron doping. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lithium ion batteries Silicon monoxide Disproportionation reaction Boron doping Negative electrode materials
1. Introduction Lithium ion batteries (LIBs) have been widely used as mobile power sources for laptops, cameras, and smart phones. In recent
* Corresponding author. E-mail address: [email protected] (J.H. Kim). 1 These authors contributed equally to this paper. http://dx.doi.org/10.1016/j.jpowsour.2015.08.086 0378-7753/© 2015 Elsevier B.V. All rights reserved.
years, there has been an increasing demand for electric transportation using LIBs, which need much higher electrochemical performances including high energy density and long cycle life [1e3]. Many studies have been conducted to achieve high performance on negative electrode materials, such as conversion oxides [4], carbon [5], and silicon (Si) [6e12]. Among these materials, Si has been regarded as one of the most attractive candidates to replace carbon-based negative electrode materials in LIBs because
26
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
of its high theoretical specific capacity (4200 mAh g1) and small initial irreversible capacity compared to metal oxide based negative materials [6e10]. However, two main problems have been raised in Si-based negative electrodes: volume expansion and poor electrical conductivity [6,11,12]. A large volume change results in electrode pulverization, a loss of electrical contacts, and drastic capacity fading during alloying/de-alloying reactions [6,11]. Furthermore, Si materials, due to their low electrical conductivity, are known to suffer from sluggish lithiation at high current density as compared to intercalation reactions [12]. To overcome these issues, silicon monoxide (SiO) has been introduced as an alternative choice instead of Si negative electrode materials [13e26]. When SiO negative electrode undergoes lithiation processes, Li oxide (Li2O) and Li silicates (Li4SiO4) are mainly formed and that act as a more stable phase than the LieSi alloys during long cycles [13e15]. On the other hand, irreversible capacity and poor electrical conductivity are caused by inactive Li2O and Li4SiO4 phase, which consume a large amount of Li during the initial lithiation [16e19]. Many research groups have suggested the process of thermal disproportionation to improve the electrochemical performance of SiO negative electrodes [20e24]. Because SiO is thermodynamically unstable at all temperatures, it can be easily transformed into nano-crystalline Si (nc-Si) and SiOx during heat treatment, triggering a disproportionation reaction [25]. The nc-Si acts to increase the reversible capacity and the SiOx matrix plays an important role in accommodating the volume expansion of embedded nc-Si during alloying/de-alloying processes [23,26]. However, the electrical conductivity of heat-treated SiOx remains low because of this material's intrinsic semiconductor nature and inactive phases, which restrict the high rate capability during fast lithiation/delithiation processes. Carbon coating has been regarded as one of the most effective and typical ways to improve the electrochemical properties of SiOx. Dispersed carbon particles provide pathways for electron transfer and decrease the resistance of the electrode, thus resulting in improved conductivity and electrochemical properties of the SiO negative electrode [24,27e29]. However, Kim et al. suggested that local capacity fading caused by electrical loss between the active materials and the carbon coating is unavoidable during long cycle test [24]. Moreover, carbon coating requires additional processing with carbon sources. Herein, we have fabricated nc-Si embedded SiOx negative electrode materials using thermal disproportionation in order to enhance the reversible capacity for high performance LIBs. Furthermore, we have found that the electrochemical performance at high current rate, and the electrical conductivity are greatly improved by B-doping on SiO negative electrode materials using the facile spin-on dopants (SOD) technique. Until now, impuritydoped SiO negative electrode materials have been investigated by only a few groups [30e32]. Yi et al. investigated the effect of boron doping on the rate capability of SieC composites [30]. However, to the best of our knowledge, we here report for the first time results obtained using a thermal B-doping by SOD approach to improve the electrical properties and capacity retention of SiO-based electrode materials without boron powder (B2O3) and carbon coating. SOD methods have been widely used as doping processes for enhancement of electrical conductivity in Si-based semiconductor technology because of their uniform and consistent doping without displacement of toxic gases [33]. Moreover, this advanced process enables simultaneous boron diffusion in a gaseous phase and disproportionation reaction inside all SiO negative materials using only one heat treatment. Taking advantage of this unique process, disproportionated SiO negative electrode materials with B-dopant can be easily synthesized in one step, and the electrochemical performances of resulting materials without carbon coating has been widely discussed.
2. Experimental 2.1. Material synthesis Pure SiO powders (Aldrich, 325 mesh) and liquid spin-on dopants (SODs, Filmtronics) were used as starting materials. The HeSiO powders were synthesized by heat treatment at a temperature of 900 C for 3 h in a nitrogen (N2) gas ambient. The HBeSiO sample was prepared as follows. First, boron dopant solution was dropped (3 ml) on an Si wafer on a spin coater at a speed of 500 rpm and 3000 rpm for 5 s and 15 s, respectively, which results in a very uniform layer. The wafer was heated on a hot plate at 120 C for 10 min to remove the excess solvents. The next step of boron diffusion and the thermal disproportionation reaction was carried out in a furnace at a temperature of 900 C for 3 h in an N2 ambient. 2.2. Preparation of electrodes and cell assembly The electrodes were prepared by coating slurries that consisted of the active materials (68 wt.%), acetylene black (Super P, 20 wt.%) as a conducting agent, carboxymethyl cellulose (CMC, 9 wt.%) and styrene butadiene rubber (SBR, 3 wt.%) dissolved in DI water as a binder. The slurry was spread onto a current collector of copper (Cu) foil to the thickness of 20 mm and dried in a vacuum oven at 70 C for 12 h. CR2032-type coin cells were used to test the electrochemical performance of the samples. 1 M of LiPF6 in an ethylene carbonate (EC)/diethyl carbonate (DEC) with a volume ratio of 1:1 and 10% fluoro-ethylene carbonate (FEC) additive were used as electrolyte (Panaxetec. Inc). A Celgard 2400 microporous membrane was used as a separator in the coin cell. Li metal was used as a counter electrode. The cells were assembled in an Ar-filled glove box with concentrations of moisture and oxygen below 1 ppm. 2.3. Characterization The morphological properties of all the samples were characterized using field emission scanning electron microscopy (FE-SEM, Hitachi-S4800). X-ray diffraction (XRD, Panalytical Empyrean) was used to observe the crystallographic structures. Also, HeSiO and HBeSiO samples were identified using a field emission transmission electron microscope (FE-TEM, HF-3300). The elemental compositions of the SiO, HeSiO, and HBeSiO samples were determined using X-ray photoelectron spectroscopy (XPS) in an ultrahigh vacuum setup equipped with a monochromatic Al Ka X-ray source (1486.6 eV) and a high resolution analyzer. The binding energies were calibrated based on the graphite C1s peak. The optical properties of all samples were obtained by using Raman spectrometer with 532 nm excitation lasers (Thermo Nicolet ALMECA). Inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements were introduced to quantify the boron concentration (Thermo iCAP-7600). All samples were preserved in 1% nitric acid (trace metal grade) and calibration standards were determined using 1000 ppm of boron standard solution (Inorganic Ventures). Cycling tests were performed in galvanostatic mode between 0.01 and 1.5 V versus Li/Liþ with a TOSCAT 3100 battery tester (Toyo system). All the specific capacities of the samples were calculated on the basis of the weight of the active materials. The first discharge capacity of about 2400 mAh g1 at the rate of 1C was provided to examine the electrochemical reaction of SiO mixture model. The electrochemical impedance spectra were measured from 100 kHz to 0.01 Hz with an alternating current amplitude of 10 mV using a VersaSTAT3 potentiostat (Princeton Applied Research).
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
27
3. Results and discussion Fig. 1 provides the schematics of the sample preparation processes for SiO, HeSiO, and HBeSiO. The fabrication processes are described in detail in the Experimental section. Both samples (named as HeSiO and HBeSiO) were heat-treated in a crucible under identical conditions, except that the HBeSiO was placed under an SOD coated substrate wafer. Sohn et al. reported that a disproportionation temperature above 800 C led to the best electrochemical performance in terms of initial coulombic efficiency and capacity retention because this temperature allowed the uniform dispersal of nc-Si within the amorphous SiOx matrix [22,23]. Also, it was possible that the electrochemical performance of disproportionated SiO was affected by the amounts and valence states of the various SiOx phases [16,22,23,26]. Thus, we confirm that nc-Si will be formed and surrounded by inactive SiOx under conditions of 900 C heating for 3 h. To examine the valence states of the SiO, HeSiO, and HBeSiO samples, XPS measurements were conducted with Ar sputtering. The Si 2p3/2 spectra, from the XPS results, were deconvoluted into five positions: 99.7 eV for Si0, 100.6 eV for Siþ, 101.6 eV for Si2þ, 102.7 eV for Si3þ, and 103.8 eV for Si4þ, as has been described by other groups [22,23,26]. Fig. 2 shows the XPS results of Si 2p3/2 spectra for the pristine SiO, HeSiO, and HBeSiO samples, and these results confirm that significant amounts of Si suboxides, such as Siþ, Si2þ, and Si3þ, are clearly resolved. These results are in good agreement with those of previous works [22,23,26]. The XPS results of the thermally treated samples (Fig. 2(b) and (c)) reveal that the number of Si0 and Si4þ states increases, while that of Siþ, Si2þ, and Si3þ states decreases due to the disproportionation reaction. According to these XPS results, we suggest that the volume fractions of crystalline Si and SiO2 should be increased during the thermal disproportionation of SiO. The amounts of Si oxidation states for each sample are summarized in Table 1. The crystal structures of the HeSiO and HBeSiO powders were analyzed by observing the XRD patterns as shown in Fig. 3. The XRD pattern of pure SiO powder is also provided for comparison. The pristine SiO powder showed an amorphous nature, and mainly consisted of amorphous Si surrounded by Si suboxide matrix [34]. After heat treatment of SiO, sharp peaks appear in the XRD patterns. As can be seen in Fig. 3, the newly observed peaks appearing at 2q ¼ 28.3 , 48.6 , and 56 were assigned to the (111), (220), and (311) planes of Si, respectively (JCPDS-27-1402). These peaks indicate that a small amount of crystalline Si was formed in the SiOx matrix due to thermal disproportionation. The average crystallite size of Si in both HeSiO and HBeSiO is approximately 4 ± 1 nm, which is in good agreement with the data shown in the FE-TEM images provided in Fig. 4(a) and (b). Crystallite size was calculated using the Scherrer formula:
D ¼ 0:94 l=ðB cosqÞ
(1)
where B is the full width at half maximum intensity (rad), l is the Xray wavelength (nm), D is the average of crystallite size (nm) and q is the diffraction angle. Compared to HeSiO sample, XRD spectrum of HBeSiO shows a very small amount of shift to higher angles which means a decrease of lattice constant by substituting smaller B atoms after B doping as shown in Fig. 3. FE-TEM images in Fig. 4(a) and (b) are obtained from the corresponding samples of HeSiO and HBeSiO, selected areas marked by red box in inset of Fig. 4, respectively. As can be seen in Fig. 4(a) and (b), FE-TEM images reveal that quantities of nc-Si with sizes of around 4 nm were formed in the SiOx matrix by disproportionation reaction for both heat-treated samples. These results match well with the XPS and XRD measurements. On the basis of XRD and FETEM analysis, it can be concluded that nc-Si was uniformly dispersed within an amorphous SiOx matrix after the heat treatment process. From the XRD and TEM results, however, we were not able to find sufficient evidence corresponding to the existence form of boron in the HBeSiO sample due to the small amount of boron dopants. To clarify the existence form of boron element, we measured Raman spectroscopy for the three types of samples as shown in Fig. 5. Raman spectroscopy is a useful technique to probe structural and electronic characteristics of materials, especially to examine the ordered and disordered structures. It should be noted that the Raman shift of Si peak from 508 cm1 for undoped SiO sample to 498 cm1 for B doped SiO is clearly observed. This shift is attributed to the distortion in the Si network due to the stress applied in the surrounding Si atomic structure after B doping [30]. Therefore, we suggest that the boron atoms exist in the form of doping in SiO negative electrode materials rather than in the form of boron particles. ICP-OES is known to be one of the most accurate techniques for quantitative analysis of elements. It has been applied to a wide range of samples to determine the concentration of doping elements [35,36]. Calibration with samples with known contents has to be carried out for such quantification, and this calibration has been successfully applied to determine the contents of trace elements in various powder samples [36]. To measure the concentration of boron element, thermally-treated samples (HeSiO, HBeSiO) were completely dissolved in perfluoroalkoxy (PFA) bottles with mixtures of 5 M nitric acid and 3.5 M hydrofluoric acid, respectively. Fig. S1 shows the ICP emission spectra for these two samples (see supporting information). The emission spectra for the two samples were acquired in the wavelength region of around l ¼ 249.77 nm, and the radiation intensity of boron elements was determined. From these data, we can conclude that the concentration of boron in
Fig. 1. Schematics of the fabrication processes for each sample.
28
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
Fig. 3. X-ray diffraction patterns of pure SiO, HeSiO, and HBeSiO powders.
conditions. It is ascribed to the out-diffusion of residual boron dopants in the furnace reactor during high temperature process. However, the B concentration in the HBeSiO sample is significantly
Fig. 2. XPS spectra of Si 2p for (a) SiO, (b) HeSiO, and (c) HBeSiO samples.
HBeSiO is higher than that in HeSiO, and the quantitative amounts of boron element in both samples were obtained as shown in Table 2. It is of importance to note that the small amounts of boron element were detected in the HeSiO sample under no dopant Table 1 Quantity of Si valence states from deconvolution of Si 2p spectra for disproportionate-SiO samples. Sample
Si0 (%)
Si1þ (%)
Si2þ (%)
Si3þ (%)
Si4þ (%)
SiO HeSiO HBeSiO
26.7 35.7 35.8
25.7 15.8 4.3
19.3 12 6.4
15.7 8.4 12.9
12.6 28.1 40.6
Fig. 4. FE-TEM images of (a) HeSiO and (b) HBeSiO powders. The inset shows lowmagnification TEM images of (a) HeSiO and (b) HBeSiO powders, respectively.
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
Fig. 5. Raman spectra of SiO, HeSiO, and HBeSiO powders.
greater than that in the HeSiO sample. From the Raman results, we observed a meaningful Raman shift caused by B doping only in HBeSiO. Therefore, we suggest that the effect of B doping on the electrochemical performance of HeSiO is negligible. To investigate the electrochemical performances of the three samples, CR2032-type coin cells with SiO, HeSiO, and HBeSiO as negative electrodes were assembled with Li metal as a counter electrode. Fig. 6(a) shows the initial chargeedischarge curves of the SiO, HeSiO, and HBeSiO electrodes at a 0.1C rate with cutoff voltages of 0.01 and 1.5 V versus Li/Liþ. The initial reversible capacities of SiO, HeSiO, and HBeSiO are 1017, 1543, and 1804 mAh g1, with corresponding coulombic efficiencies of 46.5, 54.3, and 65.1%, respectively. The low coulombic efficiency of the first cycle is attributed to the formation of irreversible Li2O and Li4SiO4 phases while lithiation proceeds. Compared to that of pristine SiO, however, the lower irreversible capacities of HeSiO and HBeSiO can be attributed to nc-Si (formation of Li15Si4) that uniformly dispersed within an amorphous SiOx matrix via thermal disproportionation. From the XPS results (Fig. 2(b) and (c)), we observed that the amounts of Si0 in HBeSiO are almost same as HeSiO, while the amounts of Si1þ and Si2þ are fewer than that of HeSiO. It is generally accepted that the electrochemical activity of Si at low valence state is higher than that of high valence state [16,19,32]. Despite its higher amount of valence states, HBeSiO showed a higher capacity than HeSiO, and this result will be discussed in Figs. 7 and 8 with electrochemical impedance spectroscopy (EIS). Fig. 6(b) shows the galvanostatic Li insertion/extraction of the HBeSiO electrode at a rate of 0.5C. During a period of 20e100 cycles, the HBeSiO negative electrode demonstrated a stable cycling performance. It is observable that there was a slight amount of capacity fading after 100 cycles. As can be seen in Fig. 6(c), HBeSiO and HeSiO maintain reversible capacity of 947 and 553 mAh g1, respectively, retaining above 90% of their initial capacity after 100 cycles at 0.5C rate. However, SiO shows drastic capacity fading with low capacity retention. It should be noted that thermally treated HeSiO and HBeSiO samples show no capacity fading after 100 cycles. We suggest that the increased Li2O and Li4SiO4 phases act as Table 2 ICP-OES results for HeSiO and HBeSiO samples. Sample
Boron concentration (ppm, mg/g)
HeSiO HBeSiO
1.79 33.82
29
buffer matrix preventing the volume expansion of nc-Si during the lithiation/delithiation processes. Also, it is noteworthy that the capacity of the HBeSiO electrode is 70% higher than that of the HeSiO electrode after 100 cycles at a 0.5C rate. This result supports our suggestion that the enhancement of capacity should be attributed to the suppression of irreversible capacity via the improvement of the electrical conductivity. The rate capabilities of the SiO, HeSiO, and HBeSiO electrodes were tested under various levels of current density (0.1e2C). As shown in Fig. 6(d), the specific capacity of HBeSiO is higher than that of HeSiO at all C rates. Even at high current rate (2C), HBeSiO retains a high capacity of over 700 mAh g1. These results confirm the importance of B-doping to maintain high capacities at both low and high C rates. And, after the C rate returned to 0.2C, HBeSiO recovered its initial capacity of over 1250 mAh g1, showing that the electrode did not fade during high current charge/discharge conditions. To elucidate the origin of the improvement of electrochemical performances, all samples were analyzed using the EIS measurements under delithiation state after the rate capability test. The depressed semicircle in the high-middle frequency region relates to the charge transfer resistances [37e39]. As can be seen in Fig. 7(a), the HBeSiO electrode shows a smaller semicircle than does the HeSiO, which suggests a much lower charge transfer resistance. For further investigation, the experimental impedance data was fitted to an equivalent circuit as plotted in Fig. 7(b). In this equivalent circuit, where semi-infinite diffusion is the rate determining step, W is the Warburg impedance of solid-phase diffusion, and Rs and Rct represent the electrolyte resistance and the charge transfer resistance related to the Li ion interfacial transfer, respectively [39]. The parameters of the equivalent circuit for three samples are reported in Table 3. According to Table 3, the Rs value of HBeSiO electrode is almost same as HeSiO electrode, but the Rct value of HBeSiO electrode is significantly lower than that of HeSiO. These results indicate that impurity doping provides effective electron conductive path on SiO negative electrodes, which helps reversible redox reaction during electrochemical processes [30e32]. Thus, we suggest that the excellent reversibility of HBeSiO due to the fast charge transfer probably contributes to the superior specific capacity and high coulombic efficiency compare to HeSiO. Furthermore, it should be considered that the oblique straight line in the low frequency region corresponds to the Li ion diffusion within the negative electrodes [37e39]. There is an observable change in Li ion diffusion behaviors after B doping. To investigate the behavior of Li ion in electrode materials, the relationship between the real impedance (Zre) and the reciprocal square root of the lower angular frequencies (u1/2) is plotted in Fig 8. The straight lines are attributed to the diffusion of the Li ions into the negative electrode materials, i.e., Warburg diffusion. From Fig. 8, Warburg impedance coefficient (su) can be derived from the slope of straight line according to the Eq. (2), and the diffusion coefficient values of the Li ions (DLi) in the bulk electrode materials are calculated using Eq. (3) [40,41]:
Zre ¼ Rs þ Rct þ su u1=2
DLi ¼
1 2
RT AF 2 su C
(2)
2 (3)
where the meanings of R is the gas constant, T the absolute temperature, F the Faraday's constant, A the area of the electrode surface, and C is the molar concentration of Liþ ions (mol/cm3). The calculated su and DLi is shown in Table 3. It is found that the value of su is 28.86 U cm2 s1/2 for HBeSiO, which is considerably lower than that of HeSiO. Also, the obtained DLi values for the samples
30
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
Fig. 6. Electrochemical performances of SiO, HeSiO, and HBeSiO electrodes: (a) Voltage profiles of SiO, HeSiO, and HBeSiO electrodes at 0.1C. (b) Charge/discharge curves of HBeSiO electrode after the 20, 40, 60, 80, and 100th cycle. (c) cycle performance and coulombic efficiency. (d) rate capability of SiO, HeSiO, and HBeSiO electrodes cycled at various rates from 0.1C to 2C.
Fig. 7. (a) Nyquist plots of the SiO, HeSiO, and HBeSiO electrodes. (b) equivalent circuit and fitting spectra of each sample.
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
31
Technology (DGIST) funded by the Ministry of Science, ICT and Future Planning of the Korean government (MSIP).
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.08.086.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] Fig. 8. Relationship between real impedance with the low frequencies of the SiO, HeSiO, and HBeSiO electrodes.
[10] [11] [12]
Table 3 The impedance parameters of SiO, HeSiO, and HBeSiO electrodes.
[13]
Sample
Rs (U)
Rct (U)
su (U cm2 s1/2)
D (cm2 s1)
SiO HeSiO HBeSiO
2.68 5.13 5.29
102.31 68.97 40.31
102.92 61.76 28.86
3.34 1012 9.28 1012 4.25 1011
explain the higher mobility for Liþ diffusion in HBeSiO electrode rather than the other samples. Therefore, the improved electrochemical performances of the HBeSiO electrode can be explained by its higher Liþ diffusivity and lower charge transfer resistance values. We confirm that B doping instead of carbon coating in SiO negative electrodes strongly influences the electrical and electrochemical behaviors in terms of specific capacity, rate capability during lithiation and delithiation processes. 4. Conclusions In summary, B-doped SiO as a negative electrode has been successfully prepared using a one-step SOD process that facilitates thermal disproportionation and impurity doping. Electrochemical results indicate that B-doped SiO, in contrast to HeSiO, greatly improves internal resistance of electrode, rate capability, and cycle performance. The excellent electrochemical performance is associated with the increased reversible capacity and electrical conductivity resulting from high Liþ diffusivity in negative electrode materials by B-doping. In particular, a high capacity of over 700 mAh g1 can be achieved by using HBeSiO at a 2C rate without any carbon coating, about 3 times higher than that of undoped HeSiO. Our approach provides a simple and effective route to prepare high-performance negative electrode materials in future LIB applications for electrical transportation. Acknowledgment This work was financially supported by a basic research program (15-EN-01) through the Daegu-Gyeongbuk Institute of Science and
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]
J.-M. Tarascon, M. Armand, Nature 414 (2001) 359e367. R.F. Service, Science 332 (2011) 1494e1496. B. Dunn, H. Kamath, J.-M. Tarascon, Science 334 (2011) 928e935. M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558e2563. S. Flandrois, B. Simon, Carbon 37 (1999) 165e180. J.O. Besenhard, J. Yang, M. Winter, J. Power Sources 68 (1997) 87e90. W.J. Weydanz, M. Wohlfahrt-Mehrens, R.A. Huggins, J. Power Sources 81 (1999) 237e242. T.D. Hatchard, J.R. Dahn, J. Electrochem. Soc. 151 (2004) A838eA842. X.W. Zhang, P.K. Patil, C. Wang, A.J. Appleby, F.E. Little, D.L. Cocke, J. Power Sources 125 (2004) 206e213. U. Kasavajjula, C.S. Wang, A.J. Appleby, J. Power Sources 163 (2007) 1003e1039. N. Ding, J. Xu, Y. Yao, G. Wegner, I. Lieberwirth, C. Chen, J. Power Sources 192 (2009) 644e651. R. Chandrasekaran, A. Magasinski, G. Yushin, T.F. Fuller, J. Electrochem. Soc. 157 (2010) A1139eA1151. H. Morimoto, M. Tatsumisago, T. Minami, Electrochem. Solid-State Lett. 4 (2001) A16eA18. A. Netz, R.A. Huggins, W. Weppner, J. Power Sources 119e121 (2003) 95e100. Y. Nagao, H. Sakaguchi, H. Honda, T. Fukunaga, T. Esaka, J. Electrochem. Soc. 151 (2004) A1572eA1575. M. Miyachi, H. Yamamoto, H. Kawai, T. Ohta, M. Shirakata, J. Electrochem. Soc. 152 (2005) A2089eA2091. J. Wang, H. Zhao, J. He, C. Wang, J. Wang, J. Power Sources 196 (2011) 4811e4815. G. Jeong, J.-H. Kim, Y.-U. Kim, Y.-J. Kim, J. Mater. Chem. 22 (2012) 7999e8004. C.C. Nguyen, H. Choi, S.-W. Song, J. Electrochem. Soc. 160 (2013) A906eA914. J.-H. Kim, H.-J. Sohn, H. Kim, G. Jeong, W. Choi, J. Power Sources 170 (2007) 456e459. C.-H. Doh, C.-W. Park, H.-M. Shin, D.-H. Kim, Y.-D. Chung, S.-I. Moon, B.-S. Jin, H.-S. Kim, A. Veluchamy, J. Power Sources 179 (2008) 367e370. C.-M. Park, W. Choi, Y. Hwa, J.-H. Kim, G. Jeong, H.-J. Sohn, J. Mater. Chem. 20 (2010) 4854e4860. Y. Hwa, C.-M. Park, H.-J. Sohn, J. Power Sources 222 (2013) 129e134. I. Choi, M.J. Lee, S.M. Oh, J.J. Kim, Electrochim. Acta 85 (2012) 369e376. M. Mamita, M. Kikuchi, H. Takei, J. Cryst. Growth 237 (2002) 1909e1914. M.N. Obrovac, L. Christensen, D.B. Le, J.R. Dahn, J. Electrochem. Soc. 154 (2007) A849eA855. X. Yuan, Y.-J. Chao, Z.-F. Ma, Electrochem. Commun. 9 (2007) 2591e2595. I.W. Seong, K.T. Kim, W.Y. Yoon, J. Power Sources 189 (2009) 511e514. D.J. Lee, M.-H. Ryou, J.N. Lee, B.G. Kim, Y.M. Lee, H.-W. Kim, B.-S. Kong, J.K. Park, J.W. Choi, Electrochem. Commun. 34 (2013) 98e101. R. Yi, J. Zai, F. Dai, M.L. Gordin, D. Wang, Electrochem. Commun. 36 (2013) 29e32. T. Tabuchi, H. Yasuda, M. Yamachi, J. Power Sources 146 (2005) 507e509. M. Miyachi, H. Yamamoto, H. Kawai, J. Electrochem. Soc. 154 (2007) A376eA380. Z.-T. Zhu, E. Menard, K. Hurley, R.G. Nuzzo, J.A. Rogers, Appl. Phys. Lett. 86 (2005) 133507. A. Hohl, T. Wieder, P.A. Aken, T.E. Weirich, G. Denninger, M. Vidal, S. Oswald, C. Deneke, J. Mayer, H. Fuess, J. Non Cryst. Solids 320 (2003) 255e280. D.A. Johnson, D.D. Siemer, W.F. Bauer, Anal. Chim. Acta 270 (1992) 223e230. M.V. Balarama Krishna, G. Venkateswarlu, S. Thangavel, D. Karunasagar, Anal. Methods 5 (2013) 1515e1523. E. Pollak, G. Salitra, V. Baranchugov, D. Aurbach, J. Phys. Chem. C 111 (2007) 11437e11444. R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, J. Phys. Chem. C 113 (2009) 11390e11398. D. Aurbach, J. Power. Sources 89 (2000) 206e218. A.J. Bard, L.R. Faulkner, Electrochemical Methods, second ed., John Wiley & Sons, New York, 2001, p. 380. N. Ding, J. Xu, Y.X. Yao, G. Wegner, X. Fang, C.H. Chen, I. Lieberwirth, Solid State Ionics 180 (2009) 222e225.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Improved electrochemical performance of boron-doped SiO negative electrode materials in lithium-ion batteries Jihoon Woo 1, Seong-Ho Baek 1, Jung-Soo Park, Young-Min Jeong, Jae Hyun Kim* Division of Nano Energy Convergence Research, Daegu-Gyeongbuk Institute of Science and Technology (DGIST), 711-873, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
One-step thermal doping and disproportionation reaction was introduced. Enhanced reversible capacity was attributed to the disproportionation of SiO. HBeSiO electrodes exhibit the best electrochemical performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 17 August 2015 Accepted 24 August 2015 Available online xxx
We introduce a one-step process that consists of thermal disproportionation and impurity doping to enhance the reversible capacity and electrical conductivity of silicon monoxide (SiO)-based negative electrode materials in Li-ion batteries. Transmission electron microscope (TEM) results reveal that thermally treated SiO at 900 C (HeSiO) consists of uniformly dispersed nano-crystalline Si (nc-Si) in an amorphous silicon oxide (SiOx) matrix. Compared to that of prinstine SiO, the electrochemical performance of HeSiO shows improved specific capacity, due mainly to the increased reversible capacity by ncSi and to the reduced volume expansion by thermally disproportionated SiOx matrix. Further electrochemical improvements can be obtained by boron-doping on SiO (HBeSiO) using solution dopant during thermal disproportionation. HBeSiO electrode without carbon coating exhibits significantly enhanced specific capacity superior to that of undoped HeSiO electrode, having 947 mAh g1 at 0.5C rate and excellent capacity retention of 93.3% over 100 cycles. Electrochemical impedance spectroscopy (EIS) measurement reveals that the internal resistance of the HBeSiO electrode is significantly reduced by boron doping. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lithium ion batteries Silicon monoxide Disproportionation reaction Boron doping Negative electrode materials
1. Introduction Lithium ion batteries (LIBs) have been widely used as mobile power sources for laptops, cameras, and smart phones. In recent
* Corresponding author. E-mail address: [email protected] (J.H. Kim). 1 These authors contributed equally to this paper. http://dx.doi.org/10.1016/j.jpowsour.2015.08.086 0378-7753/© 2015 Elsevier B.V. All rights reserved.
years, there has been an increasing demand for electric transportation using LIBs, which need much higher electrochemical performances including high energy density and long cycle life [1e3]. Many studies have been conducted to achieve high performance on negative electrode materials, such as conversion oxides [4], carbon [5], and silicon (Si) [6e12]. Among these materials, Si has been regarded as one of the most attractive candidates to replace carbon-based negative electrode materials in LIBs because
26
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
of its high theoretical specific capacity (4200 mAh g1) and small initial irreversible capacity compared to metal oxide based negative materials [6e10]. However, two main problems have been raised in Si-based negative electrodes: volume expansion and poor electrical conductivity [6,11,12]. A large volume change results in electrode pulverization, a loss of electrical contacts, and drastic capacity fading during alloying/de-alloying reactions [6,11]. Furthermore, Si materials, due to their low electrical conductivity, are known to suffer from sluggish lithiation at high current density as compared to intercalation reactions [12]. To overcome these issues, silicon monoxide (SiO) has been introduced as an alternative choice instead of Si negative electrode materials [13e26]. When SiO negative electrode undergoes lithiation processes, Li oxide (Li2O) and Li silicates (Li4SiO4) are mainly formed and that act as a more stable phase than the LieSi alloys during long cycles [13e15]. On the other hand, irreversible capacity and poor electrical conductivity are caused by inactive Li2O and Li4SiO4 phase, which consume a large amount of Li during the initial lithiation [16e19]. Many research groups have suggested the process of thermal disproportionation to improve the electrochemical performance of SiO negative electrodes [20e24]. Because SiO is thermodynamically unstable at all temperatures, it can be easily transformed into nano-crystalline Si (nc-Si) and SiOx during heat treatment, triggering a disproportionation reaction [25]. The nc-Si acts to increase the reversible capacity and the SiOx matrix plays an important role in accommodating the volume expansion of embedded nc-Si during alloying/de-alloying processes [23,26]. However, the electrical conductivity of heat-treated SiOx remains low because of this material's intrinsic semiconductor nature and inactive phases, which restrict the high rate capability during fast lithiation/delithiation processes. Carbon coating has been regarded as one of the most effective and typical ways to improve the electrochemical properties of SiOx. Dispersed carbon particles provide pathways for electron transfer and decrease the resistance of the electrode, thus resulting in improved conductivity and electrochemical properties of the SiO negative electrode [24,27e29]. However, Kim et al. suggested that local capacity fading caused by electrical loss between the active materials and the carbon coating is unavoidable during long cycle test [24]. Moreover, carbon coating requires additional processing with carbon sources. Herein, we have fabricated nc-Si embedded SiOx negative electrode materials using thermal disproportionation in order to enhance the reversible capacity for high performance LIBs. Furthermore, we have found that the electrochemical performance at high current rate, and the electrical conductivity are greatly improved by B-doping on SiO negative electrode materials using the facile spin-on dopants (SOD) technique. Until now, impuritydoped SiO negative electrode materials have been investigated by only a few groups [30e32]. Yi et al. investigated the effect of boron doping on the rate capability of SieC composites [30]. However, to the best of our knowledge, we here report for the first time results obtained using a thermal B-doping by SOD approach to improve the electrical properties and capacity retention of SiO-based electrode materials without boron powder (B2O3) and carbon coating. SOD methods have been widely used as doping processes for enhancement of electrical conductivity in Si-based semiconductor technology because of their uniform and consistent doping without displacement of toxic gases [33]. Moreover, this advanced process enables simultaneous boron diffusion in a gaseous phase and disproportionation reaction inside all SiO negative materials using only one heat treatment. Taking advantage of this unique process, disproportionated SiO negative electrode materials with B-dopant can be easily synthesized in one step, and the electrochemical performances of resulting materials without carbon coating has been widely discussed.
2. Experimental 2.1. Material synthesis Pure SiO powders (Aldrich, 325 mesh) and liquid spin-on dopants (SODs, Filmtronics) were used as starting materials. The HeSiO powders were synthesized by heat treatment at a temperature of 900 C for 3 h in a nitrogen (N2) gas ambient. The HBeSiO sample was prepared as follows. First, boron dopant solution was dropped (3 ml) on an Si wafer on a spin coater at a speed of 500 rpm and 3000 rpm for 5 s and 15 s, respectively, which results in a very uniform layer. The wafer was heated on a hot plate at 120 C for 10 min to remove the excess solvents. The next step of boron diffusion and the thermal disproportionation reaction was carried out in a furnace at a temperature of 900 C for 3 h in an N2 ambient. 2.2. Preparation of electrodes and cell assembly The electrodes were prepared by coating slurries that consisted of the active materials (68 wt.%), acetylene black (Super P, 20 wt.%) as a conducting agent, carboxymethyl cellulose (CMC, 9 wt.%) and styrene butadiene rubber (SBR, 3 wt.%) dissolved in DI water as a binder. The slurry was spread onto a current collector of copper (Cu) foil to the thickness of 20 mm and dried in a vacuum oven at 70 C for 12 h. CR2032-type coin cells were used to test the electrochemical performance of the samples. 1 M of LiPF6 in an ethylene carbonate (EC)/diethyl carbonate (DEC) with a volume ratio of 1:1 and 10% fluoro-ethylene carbonate (FEC) additive were used as electrolyte (Panaxetec. Inc). A Celgard 2400 microporous membrane was used as a separator in the coin cell. Li metal was used as a counter electrode. The cells were assembled in an Ar-filled glove box with concentrations of moisture and oxygen below 1 ppm. 2.3. Characterization The morphological properties of all the samples were characterized using field emission scanning electron microscopy (FE-SEM, Hitachi-S4800). X-ray diffraction (XRD, Panalytical Empyrean) was used to observe the crystallographic structures. Also, HeSiO and HBeSiO samples were identified using a field emission transmission electron microscope (FE-TEM, HF-3300). The elemental compositions of the SiO, HeSiO, and HBeSiO samples were determined using X-ray photoelectron spectroscopy (XPS) in an ultrahigh vacuum setup equipped with a monochromatic Al Ka X-ray source (1486.6 eV) and a high resolution analyzer. The binding energies were calibrated based on the graphite C1s peak. The optical properties of all samples were obtained by using Raman spectrometer with 532 nm excitation lasers (Thermo Nicolet ALMECA). Inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements were introduced to quantify the boron concentration (Thermo iCAP-7600). All samples were preserved in 1% nitric acid (trace metal grade) and calibration standards were determined using 1000 ppm of boron standard solution (Inorganic Ventures). Cycling tests were performed in galvanostatic mode between 0.01 and 1.5 V versus Li/Liþ with a TOSCAT 3100 battery tester (Toyo system). All the specific capacities of the samples were calculated on the basis of the weight of the active materials. The first discharge capacity of about 2400 mAh g1 at the rate of 1C was provided to examine the electrochemical reaction of SiO mixture model. The electrochemical impedance spectra were measured from 100 kHz to 0.01 Hz with an alternating current amplitude of 10 mV using a VersaSTAT3 potentiostat (Princeton Applied Research).
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
27
3. Results and discussion Fig. 1 provides the schematics of the sample preparation processes for SiO, HeSiO, and HBeSiO. The fabrication processes are described in detail in the Experimental section. Both samples (named as HeSiO and HBeSiO) were heat-treated in a crucible under identical conditions, except that the HBeSiO was placed under an SOD coated substrate wafer. Sohn et al. reported that a disproportionation temperature above 800 C led to the best electrochemical performance in terms of initial coulombic efficiency and capacity retention because this temperature allowed the uniform dispersal of nc-Si within the amorphous SiOx matrix [22,23]. Also, it was possible that the electrochemical performance of disproportionated SiO was affected by the amounts and valence states of the various SiOx phases [16,22,23,26]. Thus, we confirm that nc-Si will be formed and surrounded by inactive SiOx under conditions of 900 C heating for 3 h. To examine the valence states of the SiO, HeSiO, and HBeSiO samples, XPS measurements were conducted with Ar sputtering. The Si 2p3/2 spectra, from the XPS results, were deconvoluted into five positions: 99.7 eV for Si0, 100.6 eV for Siþ, 101.6 eV for Si2þ, 102.7 eV for Si3þ, and 103.8 eV for Si4þ, as has been described by other groups [22,23,26]. Fig. 2 shows the XPS results of Si 2p3/2 spectra for the pristine SiO, HeSiO, and HBeSiO samples, and these results confirm that significant amounts of Si suboxides, such as Siþ, Si2þ, and Si3þ, are clearly resolved. These results are in good agreement with those of previous works [22,23,26]. The XPS results of the thermally treated samples (Fig. 2(b) and (c)) reveal that the number of Si0 and Si4þ states increases, while that of Siþ, Si2þ, and Si3þ states decreases due to the disproportionation reaction. According to these XPS results, we suggest that the volume fractions of crystalline Si and SiO2 should be increased during the thermal disproportionation of SiO. The amounts of Si oxidation states for each sample are summarized in Table 1. The crystal structures of the HeSiO and HBeSiO powders were analyzed by observing the XRD patterns as shown in Fig. 3. The XRD pattern of pure SiO powder is also provided for comparison. The pristine SiO powder showed an amorphous nature, and mainly consisted of amorphous Si surrounded by Si suboxide matrix [34]. After heat treatment of SiO, sharp peaks appear in the XRD patterns. As can be seen in Fig. 3, the newly observed peaks appearing at 2q ¼ 28.3 , 48.6 , and 56 were assigned to the (111), (220), and (311) planes of Si, respectively (JCPDS-27-1402). These peaks indicate that a small amount of crystalline Si was formed in the SiOx matrix due to thermal disproportionation. The average crystallite size of Si in both HeSiO and HBeSiO is approximately 4 ± 1 nm, which is in good agreement with the data shown in the FE-TEM images provided in Fig. 4(a) and (b). Crystallite size was calculated using the Scherrer formula:
D ¼ 0:94 l=ðB cosqÞ
(1)
where B is the full width at half maximum intensity (rad), l is the Xray wavelength (nm), D is the average of crystallite size (nm) and q is the diffraction angle. Compared to HeSiO sample, XRD spectrum of HBeSiO shows a very small amount of shift to higher angles which means a decrease of lattice constant by substituting smaller B atoms after B doping as shown in Fig. 3. FE-TEM images in Fig. 4(a) and (b) are obtained from the corresponding samples of HeSiO and HBeSiO, selected areas marked by red box in inset of Fig. 4, respectively. As can be seen in Fig. 4(a) and (b), FE-TEM images reveal that quantities of nc-Si with sizes of around 4 nm were formed in the SiOx matrix by disproportionation reaction for both heat-treated samples. These results match well with the XPS and XRD measurements. On the basis of XRD and FETEM analysis, it can be concluded that nc-Si was uniformly dispersed within an amorphous SiOx matrix after the heat treatment process. From the XRD and TEM results, however, we were not able to find sufficient evidence corresponding to the existence form of boron in the HBeSiO sample due to the small amount of boron dopants. To clarify the existence form of boron element, we measured Raman spectroscopy for the three types of samples as shown in Fig. 5. Raman spectroscopy is a useful technique to probe structural and electronic characteristics of materials, especially to examine the ordered and disordered structures. It should be noted that the Raman shift of Si peak from 508 cm1 for undoped SiO sample to 498 cm1 for B doped SiO is clearly observed. This shift is attributed to the distortion in the Si network due to the stress applied in the surrounding Si atomic structure after B doping [30]. Therefore, we suggest that the boron atoms exist in the form of doping in SiO negative electrode materials rather than in the form of boron particles. ICP-OES is known to be one of the most accurate techniques for quantitative analysis of elements. It has been applied to a wide range of samples to determine the concentration of doping elements [35,36]. Calibration with samples with known contents has to be carried out for such quantification, and this calibration has been successfully applied to determine the contents of trace elements in various powder samples [36]. To measure the concentration of boron element, thermally-treated samples (HeSiO, HBeSiO) were completely dissolved in perfluoroalkoxy (PFA) bottles with mixtures of 5 M nitric acid and 3.5 M hydrofluoric acid, respectively. Fig. S1 shows the ICP emission spectra for these two samples (see supporting information). The emission spectra for the two samples were acquired in the wavelength region of around l ¼ 249.77 nm, and the radiation intensity of boron elements was determined. From these data, we can conclude that the concentration of boron in
Fig. 1. Schematics of the fabrication processes for each sample.
28
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
Fig. 3. X-ray diffraction patterns of pure SiO, HeSiO, and HBeSiO powders.
conditions. It is ascribed to the out-diffusion of residual boron dopants in the furnace reactor during high temperature process. However, the B concentration in the HBeSiO sample is significantly
Fig. 2. XPS spectra of Si 2p for (a) SiO, (b) HeSiO, and (c) HBeSiO samples.
HBeSiO is higher than that in HeSiO, and the quantitative amounts of boron element in both samples were obtained as shown in Table 2. It is of importance to note that the small amounts of boron element were detected in the HeSiO sample under no dopant Table 1 Quantity of Si valence states from deconvolution of Si 2p spectra for disproportionate-SiO samples. Sample
Si0 (%)
Si1þ (%)
Si2þ (%)
Si3þ (%)
Si4þ (%)
SiO HeSiO HBeSiO
26.7 35.7 35.8
25.7 15.8 4.3
19.3 12 6.4
15.7 8.4 12.9
12.6 28.1 40.6
Fig. 4. FE-TEM images of (a) HeSiO and (b) HBeSiO powders. The inset shows lowmagnification TEM images of (a) HeSiO and (b) HBeSiO powders, respectively.
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
Fig. 5. Raman spectra of SiO, HeSiO, and HBeSiO powders.
greater than that in the HeSiO sample. From the Raman results, we observed a meaningful Raman shift caused by B doping only in HBeSiO. Therefore, we suggest that the effect of B doping on the electrochemical performance of HeSiO is negligible. To investigate the electrochemical performances of the three samples, CR2032-type coin cells with SiO, HeSiO, and HBeSiO as negative electrodes were assembled with Li metal as a counter electrode. Fig. 6(a) shows the initial chargeedischarge curves of the SiO, HeSiO, and HBeSiO electrodes at a 0.1C rate with cutoff voltages of 0.01 and 1.5 V versus Li/Liþ. The initial reversible capacities of SiO, HeSiO, and HBeSiO are 1017, 1543, and 1804 mAh g1, with corresponding coulombic efficiencies of 46.5, 54.3, and 65.1%, respectively. The low coulombic efficiency of the first cycle is attributed to the formation of irreversible Li2O and Li4SiO4 phases while lithiation proceeds. Compared to that of pristine SiO, however, the lower irreversible capacities of HeSiO and HBeSiO can be attributed to nc-Si (formation of Li15Si4) that uniformly dispersed within an amorphous SiOx matrix via thermal disproportionation. From the XPS results (Fig. 2(b) and (c)), we observed that the amounts of Si0 in HBeSiO are almost same as HeSiO, while the amounts of Si1þ and Si2þ are fewer than that of HeSiO. It is generally accepted that the electrochemical activity of Si at low valence state is higher than that of high valence state [16,19,32]. Despite its higher amount of valence states, HBeSiO showed a higher capacity than HeSiO, and this result will be discussed in Figs. 7 and 8 with electrochemical impedance spectroscopy (EIS). Fig. 6(b) shows the galvanostatic Li insertion/extraction of the HBeSiO electrode at a rate of 0.5C. During a period of 20e100 cycles, the HBeSiO negative electrode demonstrated a stable cycling performance. It is observable that there was a slight amount of capacity fading after 100 cycles. As can be seen in Fig. 6(c), HBeSiO and HeSiO maintain reversible capacity of 947 and 553 mAh g1, respectively, retaining above 90% of their initial capacity after 100 cycles at 0.5C rate. However, SiO shows drastic capacity fading with low capacity retention. It should be noted that thermally treated HeSiO and HBeSiO samples show no capacity fading after 100 cycles. We suggest that the increased Li2O and Li4SiO4 phases act as Table 2 ICP-OES results for HeSiO and HBeSiO samples. Sample
Boron concentration (ppm, mg/g)
HeSiO HBeSiO
1.79 33.82
29
buffer matrix preventing the volume expansion of nc-Si during the lithiation/delithiation processes. Also, it is noteworthy that the capacity of the HBeSiO electrode is 70% higher than that of the HeSiO electrode after 100 cycles at a 0.5C rate. This result supports our suggestion that the enhancement of capacity should be attributed to the suppression of irreversible capacity via the improvement of the electrical conductivity. The rate capabilities of the SiO, HeSiO, and HBeSiO electrodes were tested under various levels of current density (0.1e2C). As shown in Fig. 6(d), the specific capacity of HBeSiO is higher than that of HeSiO at all C rates. Even at high current rate (2C), HBeSiO retains a high capacity of over 700 mAh g1. These results confirm the importance of B-doping to maintain high capacities at both low and high C rates. And, after the C rate returned to 0.2C, HBeSiO recovered its initial capacity of over 1250 mAh g1, showing that the electrode did not fade during high current charge/discharge conditions. To elucidate the origin of the improvement of electrochemical performances, all samples were analyzed using the EIS measurements under delithiation state after the rate capability test. The depressed semicircle in the high-middle frequency region relates to the charge transfer resistances [37e39]. As can be seen in Fig. 7(a), the HBeSiO electrode shows a smaller semicircle than does the HeSiO, which suggests a much lower charge transfer resistance. For further investigation, the experimental impedance data was fitted to an equivalent circuit as plotted in Fig. 7(b). In this equivalent circuit, where semi-infinite diffusion is the rate determining step, W is the Warburg impedance of solid-phase diffusion, and Rs and Rct represent the electrolyte resistance and the charge transfer resistance related to the Li ion interfacial transfer, respectively [39]. The parameters of the equivalent circuit for three samples are reported in Table 3. According to Table 3, the Rs value of HBeSiO electrode is almost same as HeSiO electrode, but the Rct value of HBeSiO electrode is significantly lower than that of HeSiO. These results indicate that impurity doping provides effective electron conductive path on SiO negative electrodes, which helps reversible redox reaction during electrochemical processes [30e32]. Thus, we suggest that the excellent reversibility of HBeSiO due to the fast charge transfer probably contributes to the superior specific capacity and high coulombic efficiency compare to HeSiO. Furthermore, it should be considered that the oblique straight line in the low frequency region corresponds to the Li ion diffusion within the negative electrodes [37e39]. There is an observable change in Li ion diffusion behaviors after B doping. To investigate the behavior of Li ion in electrode materials, the relationship between the real impedance (Zre) and the reciprocal square root of the lower angular frequencies (u1/2) is plotted in Fig 8. The straight lines are attributed to the diffusion of the Li ions into the negative electrode materials, i.e., Warburg diffusion. From Fig. 8, Warburg impedance coefficient (su) can be derived from the slope of straight line according to the Eq. (2), and the diffusion coefficient values of the Li ions (DLi) in the bulk electrode materials are calculated using Eq. (3) [40,41]:
Zre ¼ Rs þ Rct þ su u1=2
DLi ¼
1 2
RT AF 2 su C
(2)
2 (3)
where the meanings of R is the gas constant, T the absolute temperature, F the Faraday's constant, A the area of the electrode surface, and C is the molar concentration of Liþ ions (mol/cm3). The calculated su and DLi is shown in Table 3. It is found that the value of su is 28.86 U cm2 s1/2 for HBeSiO, which is considerably lower than that of HeSiO. Also, the obtained DLi values for the samples
30
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
Fig. 6. Electrochemical performances of SiO, HeSiO, and HBeSiO electrodes: (a) Voltage profiles of SiO, HeSiO, and HBeSiO electrodes at 0.1C. (b) Charge/discharge curves of HBeSiO electrode after the 20, 40, 60, 80, and 100th cycle. (c) cycle performance and coulombic efficiency. (d) rate capability of SiO, HeSiO, and HBeSiO electrodes cycled at various rates from 0.1C to 2C.
Fig. 7. (a) Nyquist plots of the SiO, HeSiO, and HBeSiO electrodes. (b) equivalent circuit and fitting spectra of each sample.
J. Woo et al. / Journal of Power Sources 299 (2015) 25e31
31
Technology (DGIST) funded by the Ministry of Science, ICT and Future Planning of the Korean government (MSIP).
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.08.086.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] Fig. 8. Relationship between real impedance with the low frequencies of the SiO, HeSiO, and HBeSiO electrodes.
[10] [11] [12]
Table 3 The impedance parameters of SiO, HeSiO, and HBeSiO electrodes.
[13]
Sample
Rs (U)
Rct (U)
su (U cm2 s1/2)
D (cm2 s1)
SiO HeSiO HBeSiO
2.68 5.13 5.29
102.31 68.97 40.31
102.92 61.76 28.86
3.34 1012 9.28 1012 4.25 1011
explain the higher mobility for Liþ diffusion in HBeSiO electrode rather than the other samples. Therefore, the improved electrochemical performances of the HBeSiO electrode can be explained by its higher Liþ diffusivity and lower charge transfer resistance values. We confirm that B doping instead of carbon coating in SiO negative electrodes strongly influences the electrical and electrochemical behaviors in terms of specific capacity, rate capability during lithiation and delithiation processes. 4. Conclusions In summary, B-doped SiO as a negative electrode has been successfully prepared using a one-step SOD process that facilitates thermal disproportionation and impurity doping. Electrochemical results indicate that B-doped SiO, in contrast to HeSiO, greatly improves internal resistance of electrode, rate capability, and cycle performance. The excellent electrochemical performance is associated with the increased reversible capacity and electrical conductivity resulting from high Liþ diffusivity in negative electrode materials by B-doping. In particular, a high capacity of over 700 mAh g1 can be achieved by using HBeSiO at a 2C rate without any carbon coating, about 3 times higher than that of undoped HeSiO. Our approach provides a simple and effective route to prepare high-performance negative electrode materials in future LIB applications for electrical transportation. Acknowledgment This work was financially supported by a basic research program (15-EN-01) through the Daegu-Gyeongbuk Institute of Science and
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]
J.-M. Tarascon, M. Armand, Nature 414 (2001) 359e367. R.F. Service, Science 332 (2011) 1494e1496. B. Dunn, H. Kamath, J.-M. Tarascon, Science 334 (2011) 928e935. M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558e2563. S. Flandrois, B. Simon, Carbon 37 (1999) 165e180. J.O. Besenhard, J. Yang, M. Winter, J. Power Sources 68 (1997) 87e90. W.J. Weydanz, M. Wohlfahrt-Mehrens, R.A. Huggins, J. Power Sources 81 (1999) 237e242. T.D. Hatchard, J.R. Dahn, J. Electrochem. Soc. 151 (2004) A838eA842. X.W. Zhang, P.K. Patil, C. Wang, A.J. Appleby, F.E. Little, D.L. Cocke, J. Power Sources 125 (2004) 206e213. U. Kasavajjula, C.S. Wang, A.J. Appleby, J. Power Sources 163 (2007) 1003e1039. N. Ding, J. Xu, Y. Yao, G. Wegner, I. Lieberwirth, C. Chen, J. Power Sources 192 (2009) 644e651. R. Chandrasekaran, A. Magasinski, G. Yushin, T.F. Fuller, J. Electrochem. Soc. 157 (2010) A1139eA1151. H. Morimoto, M. Tatsumisago, T. Minami, Electrochem. Solid-State Lett. 4 (2001) A16eA18. A. Netz, R.A. Huggins, W. Weppner, J. Power Sources 119e121 (2003) 95e100. Y. Nagao, H. Sakaguchi, H. Honda, T. Fukunaga, T. Esaka, J. Electrochem. Soc. 151 (2004) A1572eA1575. M. Miyachi, H. Yamamoto, H. Kawai, T. Ohta, M. Shirakata, J. Electrochem. Soc. 152 (2005) A2089eA2091. J. Wang, H. Zhao, J. He, C. Wang, J. Wang, J. Power Sources 196 (2011) 4811e4815. G. Jeong, J.-H. Kim, Y.-U. Kim, Y.-J. Kim, J. Mater. Chem. 22 (2012) 7999e8004. C.C. Nguyen, H. Choi, S.-W. Song, J. Electrochem. Soc. 160 (2013) A906eA914. J.-H. Kim, H.-J. Sohn, H. Kim, G. Jeong, W. Choi, J. Power Sources 170 (2007) 456e459. C.-H. Doh, C.-W. Park, H.-M. Shin, D.-H. Kim, Y.-D. Chung, S.-I. Moon, B.-S. Jin, H.-S. Kim, A. Veluchamy, J. Power Sources 179 (2008) 367e370. C.-M. Park, W. Choi, Y. Hwa, J.-H. Kim, G. Jeong, H.-J. Sohn, J. Mater. Chem. 20 (2010) 4854e4860. Y. Hwa, C.-M. Park, H.-J. Sohn, J. Power Sources 222 (2013) 129e134. I. Choi, M.J. Lee, S.M. Oh, J.J. Kim, Electrochim. Acta 85 (2012) 369e376. M. Mamita, M. Kikuchi, H. Takei, J. Cryst. Growth 237 (2002) 1909e1914. M.N. Obrovac, L. Christensen, D.B. Le, J.R. Dahn, J. Electrochem. Soc. 154 (2007) A849eA855. X. Yuan, Y.-J. Chao, Z.-F. Ma, Electrochem. Commun. 9 (2007) 2591e2595. I.W. Seong, K.T. Kim, W.Y. Yoon, J. Power Sources 189 (2009) 511e514. D.J. Lee, M.-H. Ryou, J.N. Lee, B.G. Kim, Y.M. Lee, H.-W. Kim, B.-S. Kong, J.K. Park, J.W. Choi, Electrochem. Commun. 34 (2013) 98e101. R. Yi, J. Zai, F. Dai, M.L. Gordin, D. Wang, Electrochem. Commun. 36 (2013) 29e32. T. Tabuchi, H. Yasuda, M. Yamachi, J. Power Sources 146 (2005) 507e509. M. Miyachi, H. Yamamoto, H. Kawai, J. Electrochem. Soc. 154 (2007) A376eA380. Z.-T. Zhu, E. Menard, K. Hurley, R.G. Nuzzo, J.A. Rogers, Appl. Phys. Lett. 86 (2005) 133507. A. Hohl, T. Wieder, P.A. Aken, T.E. Weirich, G. Denninger, M. Vidal, S. Oswald, C. Deneke, J. Mayer, H. Fuess, J. Non Cryst. Solids 320 (2003) 255e280. D.A. Johnson, D.D. Siemer, W.F. Bauer, Anal. Chim. Acta 270 (1992) 223e230. M.V. Balarama Krishna, G. Venkateswarlu, S. Thangavel, D. Karunasagar, Anal. Methods 5 (2013) 1515e1523. E. Pollak, G. Salitra, V. Baranchugov, D. Aurbach, J. Phys. Chem. C 111 (2007) 11437e11444. R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, J. Phys. Chem. C 113 (2009) 11390e11398. D. Aurbach, J. Power. Sources 89 (2000) 206e218. A.J. Bard, L.R. Faulkner, Electrochemical Methods, second ed., John Wiley & Sons, New York, 2001, p. 380. N. Ding, J. Xu, Y.X. Yao, G. Wegner, X. Fang, C.H. Chen, I. Lieberwirth, Solid State Ionics 180 (2009) 222e225.