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Electrochemical properties of polydopamine coated Ti-Si alloy anodes for Li-ion batteries
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Electrochemical properties of polydopamine coated Ti-Si alloy anodes for Li-ion batteries Ji-Sun Lee, Min-Seon Shin, Sung-Man Lee* Department of Nano Applied Engineering, Kangwon National University, Chuncheon, Gangwon-Do 200-70, Republic of Korea
A R T I C L E I N F O
Article history: Received 30 August 2016 Received in revised form 16 November 2016 Accepted 16 November 2016 Available online xxx Keywords: Li-ion battery Ti-Si alloy anode SEI film Polydopamine coating
A B S T R A C T
We report a strategy to enhance the cycling performance of a Ti-Si alloy anode by coating it with Polydopamine (PDA) as an adhesive and elastic polymer layer. The PDA was coated on a Ti-Si alloy prepared using the mechanical alloying method. The PDA coating is confirmed by X-ray photoelectron spectroscopy (XPS) and cross sectional transmission electron microscopy (TEM). The PDA coated Ti-Si alloy shows improved cycling performance with enhanced coulombic efficiency during cycling. Because the PDA coating suppresses direct contact between the Ti-Si alloy and the electrolyte during chargedischarge cycling and, hence, the initially formed SEI film remains in a stable state during cycling. After 50 cycles, the SEI layer formed on the alloy particles is much thicker in the bare alloy electrode than in the PDA coated alloy electrode. This result is supported by electrochemical impedance spectroscopy (EIS) measurement and cross-sectional scanning electron microscopy (SEM) images before cycling and after 50 cycles. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Graphite has been widely used as anode material in lithium ion batteries (LIBs). However, its low specific capacity (372 mAh g1) limits the applications of LIBs. Silicon is a promising anode material due to its large gravimetric and volumetric capacity. It has been well known that the main drawback to using Si-based anode materials is their huge volume expansion/contraction during Li insertion/extraction. Much effort has been made to solve this problem. Many reviews have summarized recent progress in Sibased anode materials for LIBs [1–9]. Based on a number of studies, the capacity fading mechanism of Si electrode has been classified into three types: active material pulverization, morphology and volume change of the whole electrode, and continuous solidelectrolyte interphase (SEI) growth [6]. It has also been determined that these challenging behaviors can be significantly mitigated by using nanoscale Si particles and nanostructured Si electrodes [5,10–12]. Nevertheless, the large volume change during lithiation/ delithiation makes the SEI film unstable and, thus, a fresh Si surface is re-exposed to the electrolyte, resulting in continuous SEI formation upon charge/discharge cycling, which leads to severe capacity fading during cycling [13]. Mitigation of this unstable SEI
* Corresponding author. Tel.: +82 33 250 6266, fax: +82 33 259 5545. E-mail address: [email protected] (S.-M. Lee).
formation problem appears to be critical to achieve good cycling stability of Si and Si based anodes in LIBs. On the other hand, the electrochemical performances of lithium alloying anodes, such as Si anodes, can be considerably improved by alloying them with other metal elements M to form Si-M alloys [14–20]. Si-M alloys (M: a transition metal) have been extensively studied as anode material for LIBs. The Si-M alloys were in general prepared by sputtering or ball-milling method, resulting in Si and Si-M silicides. Especially, when Si-M alloys are synthesized by the process of mechanical alloying, a nano-sized Si phase is uniformly distributed in the silicide matrix. The Si-M silicides in Si-M alloys may buffer the volume changes of Si during charge-discharge cycling and enhance the electronic conduction between Si phases [21–24]. Moreover, these nanostructured microparticles are promising as electrode materials because they are easy to prepare and their use leads to higher volumetric energy density compared to that of nanoscale materials. Considering that the capacity fading of a Si-based anode during charge/discharge is mainly caused by the unstable SEI formation resulting from severe volume change, it is important to find a method to overcome the unstable SEI problem in Si electrodes. If Si and Si-based anode materials are coated with an adhesive and elastic polymer layer, we can expect that the growth of SEI film during charge/discharge cycling may be restricted by limiting the direct contact between the anode materials and electrolyte.
http://dx.doi.org/10.1016/j.electacta.2016.11.094 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J.-S. Lee, et al., Electrochemical properties of polydopamine coated Ti-Si alloy anodes for Li-ion batteries, Electrochim. Acta (2016), http://dx.doi.org/10.1016/j.electacta.2016.11.094
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Recently, polydopamine (PDA), originally inspired by the adhesive properties displayed by mussels, has been shown to be a promising material for adhesive surface coating on a wide range of substrates [25–30]. Reduced grapheme oxide (RGO)/SnO2 composite-based anode for LIBs was coated by PDA layer which functioned as a buffer [31]. PDA coating has been also used to make carbon-coated anode materials for LIBs [32,33]. PDA coating can be carried out by dip-coating objects in a weak alkaline aqueous solution of dopamine, in which the dopamine can self-polymerize and forms PDA on the substrates [26]. Moreover, the thickness of a PDA coating can be conveniently controlled in a range from nearly one monolayer to tens of nanometers [32,34]. As the aim of this work is to investigate the effect of coating of PDA on electrochemical performances of Si alloys, Ti-Si alloy was chosen as an example of Si alloy anodes. In this work, we show that coating of PDA on a Ti-Si alloy enhances the electrochemical performance, especially the cycling stability, of Ti-Si alloy anodes for LIBs. To the best of our knowledge, the surface-modification of Si-transition metal alloy anodes by coating with adhesive and elastic PDA has never been reported. 2. Experimental 24Ti-76Si alloy (at%) was prepared by mechanically milling a mixture containing Si (325 mesh. 98%) and titanium hydride (TiH2)
in Ar atmosphere for 3 h. The ball to powder weight ratio was 10:1. Details of the ball- milling apparatus are described in a previous study [35]. A brittle TiH2 instead of Ti has been used as a starting material to avoid an agglomeration [36]. For PDA coated Ti-Si alloy, the mechanically alloyed Ti-Si alloy powders were dispersed in a Tris-HCl solution (pH = 8.5, 10 mM) and then DopamineHCl (1 mg/ ml) was added to the solution, which was polymerized for 5 h at room temperature. The coating thickness was controlled by changing the weight ratios of the Ti-Si alloy to DopamineHCl, in which samples prepared with weight ratios of 32/1 and 2/1 are denoted as P1 and P2. The PDA coated Ti-Si alloy was collected by centrifugation and washed with water 3 times for further use. The PDA content in the PDA coated Ti-Si alloy was determined by thermogravimetric analysis (TGA, TA lnstruments SDT Q600) at a heating rate of 5 C/min in air. The structural evolution of the bare sample and PDA coated Ti-Si alloys was characterized using an Xray diffractometer (XRD, PANanalytical X-PERT PRO) with Cu Ka radiation. The particle size, morphology and structure were studied via Particle size analyzer (PSA, Malvern Mastersize200), field emission scanning electron microscopy (FESEM, HITACHI S4300), and cross-sectional transmission electron microscopy (TEM, JEOL JEM-2100F), respectively. The chemical composition of the coating layer was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K Alpha+). XPS and TEM experiments were performed before the electrochemical test. For evaluation of the
Fig. 1. XRD patterns and card numbers of bare Ti-Si alloy, P1, and P2.
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electrochemical characteristics, electrodes were fabricated by pasting an aqueous slurry containing 80 wt% active material, 10 wt% conductive carbon (Super-P, Timcal, Switzerland), and 10 wt% poly(acrylic acid) (PAA, molecular weight: 345,000, Lubrizol Co.) on a 20 mm thick copper foil. In electrode design, electrode thickness is an important parameter affecting the electrochemical performances of a cell. However, with increasing electrode thickness the mass transport limitations of lithium ions within electrodes may become dominating [37,38]. In this work, we used a thin electrode of 20 mm to minimize those effects. The electrodes were then dried under vacuum at 180 C for 12 h and subsequently pressed. But, when we used the polydopamine coated Ti-Si alloy as active materials, to induce cross-liking bonding between PDA and PAA [31], we dried the electrodes under vacuum at 150 C for 2 h and then pressed them. The pressing process was carried out using a roll pressing machine for all electrodes. Coin cells with working electrodes and Li foil counter electrodes were assembled in an Ar-filled glove-box using 1 M LiPF6 in a mixture of ethylene carbonate/diethyl carbonate (1:1 by volume) as the electrolyte. The cells were charged (lithiated) and discharged (delithiated) between 0.02 and 1.5 V (vs. Li/Li+) at 30 C. Cells were charged in constant current–constant voltage (CC CV) mode, consisting of a constant current of 100 mA/g followed by a constant voltage of 0.02 V (vs Li/Li + ) until the current tapered down to 10 mAg1; cells were discharged at a constant current of 100 mA/g for first the two cycles, and thereafter cycled in CC mode at 100 mA/g. Electrochemical impedance spectroscopy (EIS, IM6 Zahner Elektrik) of bare and PDA coated Ti-Si alloy electrodes after the 10th and 50th cycles was also performed in discharged state in a frequency range of 1 MHz to 10 mHz with AC amplitude of 5 mV. The cross-sectional morphologies of the electrodes before and after cycling were observed. The sample preparation procedure for cross-sectional SEM observation is as follows. To obtain cycled electrodes, cells in discharged state were disassembled in an argon-filled glove box. The delithiated electrodes were then washed with diethyl carbonate (DEC) and vacuum dried. For
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samples before cycling, electrodes were soaked for 12 h in electrolyte solution, washed with DEC, and vacuum dried. The electrodes (both before and after cycling) subjected to crosssectional observation were cut using a cross-section polisher (CP, JEOL). 3. Results and discussion XRD analysis (Fig. 1) demonstrates that the mechanically alloyed (MA) Ti-Si alloy consists of two phases of Si and the metastable silicide (U-phase), and the phase and crystallinity of the MA alloy are not affected by PDA coating treatment. SEM images of the bare Ti-Si alloy and the PDA coated Ti-Si alloys (P1 and P2) are given in Fig. 2, in which SEM images of Si and TiH2 used as starting materials are also presented. It seems to be hard to compare the thickness of PDA coating layer in the SEM images. Fig. 3 displays the cross sectional TEM image of bare, P1 and P2 samples. The oxidized layer of about 2 nm is observed on the bare particle. XPS was used to obtain the information of elements in the oxide layer. The wide scan spectra of XPS (Fig. 4 a) shows that the dominant signals are from Si, Ti, and O. Narrow scan spectra of Si 2p, Ti 2p and O 1s are shown in Fig. 4b, c and d, respectively. The peaks of observed chemical states were indexed using several reference data [39–41]. In the spectra of Si 2p, the binding energy at 102 eV exhibits the presence of the Si4+ state (SiO2) and the peak at 99 eV corresponds to Sio (Si Si bonds). For Ti 2p, multiple valence states corresponding to Tio, Ti2+, Ti3+ and Ti4+ with a main presence of Ti4+ at 458.9 eV. Therefore, it can be concluded that the native oxide layer on Ti-Si alloy is mainly composed of TiO2 and SiO2. On the other hand, O 1s spectra comprise two extra peaks 532 eV (oxygen state in OH) and 533.5 eV (adsorbed H2O) with a peak of the O2 state at 530.5 eV. After PDA coating, the thickness of the amorphous layer increased with increasing dopamine concentration, 12 nm and 38 nm for P1 and P2, respectively. However, it should be noted here that TEM analysis reveals that there are particle dependent deviations in the coating thickness.
Fig. 2. FE-SEM images of Ti-Si alloy samples: (a) TiH2, (b) Si, (c) Bare Ti-Si alloy (d) P1 and (e) P2.
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Fig. 3. Cross sectional TEM image of (a) bare Ti-Si alloy, (b) P1, and (c) P2. (inset of (c): electron diffraction pattern of coating layer and core regions).
2p and Si 2s decreased. This result indicates that the PDA has been successfully coated on the surface of the Ti-Si alloy. Highresolution spectra of the C 1s and N 1s regions obtained on P1 and P2 samples are shown in Fig. 5(b)–(e). The functional group of CHx/C-NH2, C-O/C-N and C¼O for C 1s can be observed at 284.4,
XPS analysis provides information about the elemental compositions of the PDA coating layer. Fig. 5(a) shows the XPS spectra of the bare, P1 and P2 samples. XPS signals for bare sample were suppressed after PDA coating. After the PDA coating, two new peaks of N 1s and C 1s appeared and the intensity of the peaks of Ti
(a)
600
Intensity(a.u.)
Intensity (a.u.)
C 1s
Ti 2p
800
Si 2s Si 2p
400
200
0
SiO2
(b)
108
106
104
102
100
98
96
94
Binding energy(eV)
Binding energy(eV)
(c)
(d)
Ti 2p
Ti-O or Ti-Si TiO2
470 468 466 464 462 460 458 456 454 452 450 448
Binding energy(eV)
O 2s
initial data OH O2
Intensity(a.u.)
Initial data Ti Ti-O or Ti-Si TiO2
Intensity(a.u.)
Si 2p
Initial data Si-Ti Si SiOx
O 1s
H2O
536
534
532
530
528
Binding energy(eV)
Fig. 4. (a) XPS spectra of bare Ti-Si alloy, (b) Si 2P spectrum, (c) Ti 2P spectrum and (d) O 2s spectrum of bare Ti-Si alloy.
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Fig. 5. (a) XPS spectra of bare Ti-Si alloy, P1 and P2, (b) (c) C 1s spectrum of P1 and P2, and (d) (e) N 1s spectrum of P1 and P2.
285.6 and 287.7 eV, respectively (Fig. 5(b) and (c)). N 1s spectrum is fit with three peaks at 402, 400 and 398.6 eV assigned to R-NH2, RNH-R and ¼N-R (Fig. 5(d) and (e)). The percent contribution for each functional group in N 1s and C 1s spectra are given in Table 1. The percent contribution for each functional group in N 1s and C 1s spectra are given in Table 1. The relative portions of each CHx/C– NH2, C–O/C–N and C¼O species in the C 1s region are similar for both P1 and P2. In the N 1s region, on the other hand, among three peaks assigned to R–NH2, R–NH–R, and ¼N–R amine functionalities, the fraction of R–NH2 species increased in P2 compared to P1, while the relative amount of R–NH–R appears to be lower in P2 than in P1. On the basis of the chemical structures of dopamine and PDA, the primary (R–NH2) amine is associated with dopamine, the secondary (R–NH–R) amine is associated with PDA and the tertiary (¼NR) amine is associated with tautomeric species of the intermediate species(5,6-dihydroxyindole and 5,6-indolequinone) [42–44]. Therefore, a higher fraction of dopamine is present in P2 than in P1, indicating that there is more dopamine that has not been converted to PDA during polymerization reaction for PDA coating in P2 than in P1. TG analysis was used to determine the content of the PDA coating, with results shown in Fig. 6, in which the TG curves of the
Table 1 XPS Functional Group Percentages of P1 and P2. Sample
P1 P2
C 1s
N 1s
C-Hx, C-NH2
C-O, C-N
C¼O
¼N-R
R-NH-R
R-NH2
66 68
24 23
10 9
12 21
78 42
10 37
bare sample and the PDA are also added for comparison. The weight loss of the PDA occurs between 200 and 600 C, while for the bare Ti-Si alloy the weight gain due to oxidation begins from around 500 C. As estimated from the TG curves, the content of PDA in P1 and P2 was 2.0 and 3.64 wt. %, respectively. Note that the difference between the PDA content between P1 and P2 seems very small considering the huge difference in dopamine content added to the solution for PDA coating. Moreover, the amounts of PDA coating estimated from TGA results appear to be less than those of the dopamine in the solution. This is because the conversion of dopamine to PDA is critically dependent on dopamine concentration and polymerization time [26,34]. At low dopamine concentration (e.g., 0.1 mg/ml), almost all dopamine molecules can be converted into the PDA coating, whereas at a much higher concentration (e.g., 1.5 mg/ml), only certain fractions are converted. In the latter case, the PDA coating thickness is a function of the immersion time for polymerization, in which once the surface is almost covered by a monolayer of PDA, the surface polymerization slow down [26,34]. It seems therefore that after polymerization for 5 h used in this work, only a small fraction of dopamine were converted to PDA. A series of electrochemical tests of the bare Ti-Si alloy and the PDA coated samples (P1 and P2) were performed. The chargedischarge profiles of those electrodes are shown in Fig. 7a–c. The first charge profiles of the bare Ti-Si and P1 electrodes appear to be similar, while in case of the P2 electrode a plateau at around 0.02 V during the initial charge reaction is observed, which is attributed to the polarization resistance caused by the thick PDA coating layer, as can be inferred from the cross sectional TEM images shown in Fig. 3. The thickness of the PDA coating affects the polarization of electrodes during charge-discharge cycling. The polarization, as measured by the voltage drop at the cut-off voltage for the
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125
120
Bare P1 P2 Polydopamine
120
100
80
60
110
40 105
Weight / %
Weight / %
115
20 100 0 95 100
200
300
400
500
600
700
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Temperature / Fig. 6. TG curves of polydopamine, bare Ti-Si alloy, P1, and P2.
(a)
(b)
1.0
0.5
0.0
(c) 1st 3rd 5th 7th 10th 30th 50th
1.5
1.0
0.5
200
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600
Capacity(mAh/g)
800
1000
1.0
0.5
0.0
0.0 0
1st 3rd 5th 7th 10th
1.5
Voltage(V)
Voltage(V)
1.5
Voltage(V)
1st 3rd 5th 7th 10th 30th 50th
0
200
400
600
800
0
1000
200
400
600
800
1000
Capacity(mAh/g)
Capacity(mAh/g) Fig. 7. Charge-discharge profiles of (a) Bare Ti-Si alloy, (b) P1, and (c) P2 electrodes.
capacity curves of P1 and P2 electrodes were compared with that of 600 nm thick Si thin film electrode prepared using sputtering method in Fig. 9. The differential capacity curves were derived from the the 3rd charge – discharge curves. Two broad peaks (A and B) during charging (lithiation) and two corresponding peaks (A’ and B’) during discharge (delithiation) can be observed for the three samples. These broad peaks are typical of amorphous Si. The
Si TiSi2 Cu(foil)
Intensity(a.u)
discharge reaction, increases with cycling as shown in Fig. 7. The polarization increase after 5 cycles is more prominent in the bare sample than in P1 electrode, probably due to the increase of the SEI film resistance. On the other hand, in the case of the P2 electrode, the polarization increases significantly over the initial cycles, resulting in fast capacity fading. It appears that the thickness of the PDA coating has a crucial role in improving electrochemical performances of Si based alloys. Si-M silicides in Si-M alloys are known as an inactive phase because the alloy capacities are well predicted based on the assumption that the silicon alloys consist of active phase (Si) and inactive phase (Si-M silicides) [45,46]. On the other hand, it has been reported that nanocrystalline or nanostructured silicides have activity toward lithium, although those electrochemical behaviors may be dependent on Si-M systems [47–52]. TiSi2 nanonets synthesized by chemical vapor deposition have a significant reversible capacity of 943 mAh g1 [52]. In the present work, the measured capacity appears to be similar to the value calculated based on the assumption that the silicide is inactive. It is also worth noting that in the XRD results performed before and after cycling, the silicide peak remains unchanged even after 10 cycles as shown in Fig. 8, indicating that the Ti-Si silicide is inactive phase during cycling. It is also meaningful to compare the lithiation and delithiation voltages of Ti-Si alloy and pure Si electrodes. The differential
10th delithiation
3rd delithiation
As prepared
10
20
30
40
50
60
70
80
Diffration angle(2θ) Fig. 8. XRD results performed before and after cycling of bare Ti-Si alloy.
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B'
2000
A'
(a) Bare
0 -2000 -4000
B
A
dQ/dV
2000
(b) P1
0 -2000 -4000 500
(c) Si thin film 0 -500
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Voltage (V) Fig. 9. Differential capacity curves of (a) Bare Ti-Si alloy, (b) P1, and (c) Si thin film electrodes.
peak A of P1 and P2 electrodes shifts a little bit to lower voltage relative to that of Si thin film electrode, while the peak B and the delithiation voltages (A’ and B’) remain unchanged. The initial discharge capacity/coulombic efficiency values of the bare Ti-Si, P1, and P2 electrodes were 814/87, 813/86.6, and 728/ 80.5 mAh g1/%, respectively. The cycling performance and coulombic efficiency are compared in Fig. 10. The P1 electrode shows a more enhanced cyclic performance than that of the bare sample, while the capacity of P2 can be seem to decay rapidly. This indicates that that too thick PDA coating in P2 deteriorates the performance of the alloy. Particularly noteworthy is the fact that the coulombic efficiency of P1 remains as high as about 99%, but
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that of bare the Ti-Si alloy is below 97%. It has been reported that the irreversible capacity of Si-based electrodes is mainly due to the loss of electrical contact and the continuous SEI growth with cycling [6]. The SEI film formed on Si becomes unstable and can be broken because a significant volume change occurs during chargedischarge cycling, re-exposing the fresh Si surface to the electrolyte and thus forming the SEI again. This continuous SEI growth with cycling limits Li ion diffusion through the thick SEI layer and results in continuous consumption of the Li-ion electrolyte, which leads to fading of the electrode capacity [13]. To investigate the change in resistance components with cycling, AC impedance spectrum measurements were conducted Nyquist plots for the bare alloy and the P1 samples, obtained for the fully lithiated state after the 10th and 30th cycles, are shown in Fig. 11a and b. The spectra consist of a depressed semicircle in the high frequency region and an oblique straight line in the low frequency region, both of which are related to Li-ion diffusion within particles. The diameter of the semicircle indicates the charge-transfer resistance and might contain the SEI film resistance and the inter-particle contact resistance [53,54]. It seems like it is hard to separate the effect of PDA layer and SEI because the PDA exhibits a good conductivity of lithium ions [28]. As can be seen in Fig. 11a and b, the diameter of the semicircle in the bare alloy sample significantly increased after 30 cycles, whereas in the case of P1 the size of the semicircle after the 30th cycles is almost the same as that after the 10th cycles. Therefore, in the case of the bare alloy electrode, the SEI film becomes thicker and thicker upon charge/discharge cycling, and thus the interparticle contact resistance increases. On the other hand, the SEI film in the P1 electrode is stable during cycling. This result has been confirmed by considering the cross-sectional SEM images before cycling and after the 50th cycles (Fig. 12a–d). After the 50th cycles, the SEI layer formed on the alloy particles was much thicker in the bare alloy electrode (Fig. 12b) than in the P1 electrode (Fig. 12d). So, it is considered that the PDA coating suppresses direct contact between the Ti-Si alloy and the electrolyte during chargedischarge cycling and, hence, the initially formed SEI film remained in a stable state during cycling. It should also be noted that the dimensional change of the electrode is larger in the bare sample than in P1. The increase in the electrode thickness was 35% in the
Fig. 10. Cycling performance and coulombic efficiency of bare Ti-Si alloy and P1 electrodes and P2. Note that the coulombic efficiency of P2 is not included.
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Fig. 11. Nyquist plots of two types of electrodes: (a) Bare Ti-Si alloy, (b) P1.
bare electrode, but 7% in the P1 electrode; both of these values were obtained or the delithiated state after the 50th cycles. In order to investigate the effect of the PDA coating layer on the rate performance, the rate capability values of the bare Ti-Si and P1 electrodes were compared. On the other hand, P2 electrode seems to be not suitable for investigation of the rate capability because it
shows a rapid capacity fading during cycling as illustrated in Fig. 10. In the rate performance test, both electrodes were found to have similar discharge capacities at current densities up to 10C (Fig. 13). Considering that the PDA exhibits a good conductivity for lithium ions [28], the similar behavior in rate capability of P1 and bare alloy is probably attributed to the PDA coating thickness. Therefore, if
Fig. 12. SEM cross-sectional images of Bare Ti-Si alloy (a) before cycling and (b) after 50 cycles and P1 Ti-Si alloy (c) before cycling and (d) after 50 cycles.
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Fig. 13. Rate capabilities of Bare Ti-Si alloy and P1 electrodes.
the coating layer is thin enough, the rate capability will be able to be more enhanced. These results indicate that PDA coating is an effective approach to improve the electrochemical performance. It appears that the thickness of the PDA coating has a crucial role in improving electrochemical performances of Si based alloys. Further investigation is on going to optimize the performance through controlling the thickness of the PDA coating. Hopefully this will further improve the electrochemical properties of the PDA coated Si alloys as anode for LIBs. It should be also noted that the cycle performance of Si-based alloy anodes is significantly affected by the microstructural characteristics, and thus if a Si alloy with a properly designed microstructure is used as core alloy then the PDA coated anode material will present better performance. 4. Conclusions Polydopamine as an adhesive and elastic polymer layer was coated on Ti-Si alloy and its electrochemical performance as an anode material for LIBs was investigated. The PDA coating suppresses direct contact between the Ti-Si alloy and the electrolyte during charge-discharge cycling and, hence, the initially formed SEI film remains in a stable state during cycling. The cycling performance and coulombic efficiency of the Ti-Si alloy anode were improved by PDA coating with a thin layer. The coating of PDA does not negatively affect the rate capability of the Ti-Si alloy anode. Acknowledgements This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government(MEST)" (NRF2011-C1AAA001-0030538). This study was also supported by a 2015 Research Grant from Kangwon National University (No. 120140163). References [1] X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, Silicon-Based Nanomaterials for Lithium-Ion Batteries: A Review, Adv. Energy Mater. 4 (2014) 1.
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Electrochemical properties of polydopamine coated Ti-Si alloy anodes for Li-ion batteries Ji-Sun Lee, Min-Seon Shin, Sung-Man Lee* Department of Nano Applied Engineering, Kangwon National University, Chuncheon, Gangwon-Do 200-70, Republic of Korea
A R T I C L E I N F O
Article history: Received 30 August 2016 Received in revised form 16 November 2016 Accepted 16 November 2016 Available online xxx Keywords: Li-ion battery Ti-Si alloy anode SEI film Polydopamine coating
A B S T R A C T
We report a strategy to enhance the cycling performance of a Ti-Si alloy anode by coating it with Polydopamine (PDA) as an adhesive and elastic polymer layer. The PDA was coated on a Ti-Si alloy prepared using the mechanical alloying method. The PDA coating is confirmed by X-ray photoelectron spectroscopy (XPS) and cross sectional transmission electron microscopy (TEM). The PDA coated Ti-Si alloy shows improved cycling performance with enhanced coulombic efficiency during cycling. Because the PDA coating suppresses direct contact between the Ti-Si alloy and the electrolyte during chargedischarge cycling and, hence, the initially formed SEI film remains in a stable state during cycling. After 50 cycles, the SEI layer formed on the alloy particles is much thicker in the bare alloy electrode than in the PDA coated alloy electrode. This result is supported by electrochemical impedance spectroscopy (EIS) measurement and cross-sectional scanning electron microscopy (SEM) images before cycling and after 50 cycles. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Graphite has been widely used as anode material in lithium ion batteries (LIBs). However, its low specific capacity (372 mAh g1) limits the applications of LIBs. Silicon is a promising anode material due to its large gravimetric and volumetric capacity. It has been well known that the main drawback to using Si-based anode materials is their huge volume expansion/contraction during Li insertion/extraction. Much effort has been made to solve this problem. Many reviews have summarized recent progress in Sibased anode materials for LIBs [1–9]. Based on a number of studies, the capacity fading mechanism of Si electrode has been classified into three types: active material pulverization, morphology and volume change of the whole electrode, and continuous solidelectrolyte interphase (SEI) growth [6]. It has also been determined that these challenging behaviors can be significantly mitigated by using nanoscale Si particles and nanostructured Si electrodes [5,10–12]. Nevertheless, the large volume change during lithiation/ delithiation makes the SEI film unstable and, thus, a fresh Si surface is re-exposed to the electrolyte, resulting in continuous SEI formation upon charge/discharge cycling, which leads to severe capacity fading during cycling [13]. Mitigation of this unstable SEI
* Corresponding author. Tel.: +82 33 250 6266, fax: +82 33 259 5545. E-mail address: [email protected] (S.-M. Lee).
formation problem appears to be critical to achieve good cycling stability of Si and Si based anodes in LIBs. On the other hand, the electrochemical performances of lithium alloying anodes, such as Si anodes, can be considerably improved by alloying them with other metal elements M to form Si-M alloys [14–20]. Si-M alloys (M: a transition metal) have been extensively studied as anode material for LIBs. The Si-M alloys were in general prepared by sputtering or ball-milling method, resulting in Si and Si-M silicides. Especially, when Si-M alloys are synthesized by the process of mechanical alloying, a nano-sized Si phase is uniformly distributed in the silicide matrix. The Si-M silicides in Si-M alloys may buffer the volume changes of Si during charge-discharge cycling and enhance the electronic conduction between Si phases [21–24]. Moreover, these nanostructured microparticles are promising as electrode materials because they are easy to prepare and their use leads to higher volumetric energy density compared to that of nanoscale materials. Considering that the capacity fading of a Si-based anode during charge/discharge is mainly caused by the unstable SEI formation resulting from severe volume change, it is important to find a method to overcome the unstable SEI problem in Si electrodes. If Si and Si-based anode materials are coated with an adhesive and elastic polymer layer, we can expect that the growth of SEI film during charge/discharge cycling may be restricted by limiting the direct contact between the anode materials and electrolyte.
http://dx.doi.org/10.1016/j.electacta.2016.11.094 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J.-S. Lee, et al., Electrochemical properties of polydopamine coated Ti-Si alloy anodes for Li-ion batteries, Electrochim. Acta (2016), http://dx.doi.org/10.1016/j.electacta.2016.11.094
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Recently, polydopamine (PDA), originally inspired by the adhesive properties displayed by mussels, has been shown to be a promising material for adhesive surface coating on a wide range of substrates [25–30]. Reduced grapheme oxide (RGO)/SnO2 composite-based anode for LIBs was coated by PDA layer which functioned as a buffer [31]. PDA coating has been also used to make carbon-coated anode materials for LIBs [32,33]. PDA coating can be carried out by dip-coating objects in a weak alkaline aqueous solution of dopamine, in which the dopamine can self-polymerize and forms PDA on the substrates [26]. Moreover, the thickness of a PDA coating can be conveniently controlled in a range from nearly one monolayer to tens of nanometers [32,34]. As the aim of this work is to investigate the effect of coating of PDA on electrochemical performances of Si alloys, Ti-Si alloy was chosen as an example of Si alloy anodes. In this work, we show that coating of PDA on a Ti-Si alloy enhances the electrochemical performance, especially the cycling stability, of Ti-Si alloy anodes for LIBs. To the best of our knowledge, the surface-modification of Si-transition metal alloy anodes by coating with adhesive and elastic PDA has never been reported. 2. Experimental 24Ti-76Si alloy (at%) was prepared by mechanically milling a mixture containing Si (325 mesh. 98%) and titanium hydride (TiH2)
in Ar atmosphere for 3 h. The ball to powder weight ratio was 10:1. Details of the ball- milling apparatus are described in a previous study [35]. A brittle TiH2 instead of Ti has been used as a starting material to avoid an agglomeration [36]. For PDA coated Ti-Si alloy, the mechanically alloyed Ti-Si alloy powders were dispersed in a Tris-HCl solution (pH = 8.5, 10 mM) and then DopamineHCl (1 mg/ ml) was added to the solution, which was polymerized for 5 h at room temperature. The coating thickness was controlled by changing the weight ratios of the Ti-Si alloy to DopamineHCl, in which samples prepared with weight ratios of 32/1 and 2/1 are denoted as P1 and P2. The PDA coated Ti-Si alloy was collected by centrifugation and washed with water 3 times for further use. The PDA content in the PDA coated Ti-Si alloy was determined by thermogravimetric analysis (TGA, TA lnstruments SDT Q600) at a heating rate of 5 C/min in air. The structural evolution of the bare sample and PDA coated Ti-Si alloys was characterized using an Xray diffractometer (XRD, PANanalytical X-PERT PRO) with Cu Ka radiation. The particle size, morphology and structure were studied via Particle size analyzer (PSA, Malvern Mastersize200), field emission scanning electron microscopy (FESEM, HITACHI S4300), and cross-sectional transmission electron microscopy (TEM, JEOL JEM-2100F), respectively. The chemical composition of the coating layer was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K Alpha+). XPS and TEM experiments were performed before the electrochemical test. For evaluation of the
Fig. 1. XRD patterns and card numbers of bare Ti-Si alloy, P1, and P2.
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electrochemical characteristics, electrodes were fabricated by pasting an aqueous slurry containing 80 wt% active material, 10 wt% conductive carbon (Super-P, Timcal, Switzerland), and 10 wt% poly(acrylic acid) (PAA, molecular weight: 345,000, Lubrizol Co.) on a 20 mm thick copper foil. In electrode design, electrode thickness is an important parameter affecting the electrochemical performances of a cell. However, with increasing electrode thickness the mass transport limitations of lithium ions within electrodes may become dominating [37,38]. In this work, we used a thin electrode of 20 mm to minimize those effects. The electrodes were then dried under vacuum at 180 C for 12 h and subsequently pressed. But, when we used the polydopamine coated Ti-Si alloy as active materials, to induce cross-liking bonding between PDA and PAA [31], we dried the electrodes under vacuum at 150 C for 2 h and then pressed them. The pressing process was carried out using a roll pressing machine for all electrodes. Coin cells with working electrodes and Li foil counter electrodes were assembled in an Ar-filled glove-box using 1 M LiPF6 in a mixture of ethylene carbonate/diethyl carbonate (1:1 by volume) as the electrolyte. The cells were charged (lithiated) and discharged (delithiated) between 0.02 and 1.5 V (vs. Li/Li+) at 30 C. Cells were charged in constant current–constant voltage (CC CV) mode, consisting of a constant current of 100 mA/g followed by a constant voltage of 0.02 V (vs Li/Li + ) until the current tapered down to 10 mAg1; cells were discharged at a constant current of 100 mA/g for first the two cycles, and thereafter cycled in CC mode at 100 mA/g. Electrochemical impedance spectroscopy (EIS, IM6 Zahner Elektrik) of bare and PDA coated Ti-Si alloy electrodes after the 10th and 50th cycles was also performed in discharged state in a frequency range of 1 MHz to 10 mHz with AC amplitude of 5 mV. The cross-sectional morphologies of the electrodes before and after cycling were observed. The sample preparation procedure for cross-sectional SEM observation is as follows. To obtain cycled electrodes, cells in discharged state were disassembled in an argon-filled glove box. The delithiated electrodes were then washed with diethyl carbonate (DEC) and vacuum dried. For
3
samples before cycling, electrodes were soaked for 12 h in electrolyte solution, washed with DEC, and vacuum dried. The electrodes (both before and after cycling) subjected to crosssectional observation were cut using a cross-section polisher (CP, JEOL). 3. Results and discussion XRD analysis (Fig. 1) demonstrates that the mechanically alloyed (MA) Ti-Si alloy consists of two phases of Si and the metastable silicide (U-phase), and the phase and crystallinity of the MA alloy are not affected by PDA coating treatment. SEM images of the bare Ti-Si alloy and the PDA coated Ti-Si alloys (P1 and P2) are given in Fig. 2, in which SEM images of Si and TiH2 used as starting materials are also presented. It seems to be hard to compare the thickness of PDA coating layer in the SEM images. Fig. 3 displays the cross sectional TEM image of bare, P1 and P2 samples. The oxidized layer of about 2 nm is observed on the bare particle. XPS was used to obtain the information of elements in the oxide layer. The wide scan spectra of XPS (Fig. 4 a) shows that the dominant signals are from Si, Ti, and O. Narrow scan spectra of Si 2p, Ti 2p and O 1s are shown in Fig. 4b, c and d, respectively. The peaks of observed chemical states were indexed using several reference data [39–41]. In the spectra of Si 2p, the binding energy at 102 eV exhibits the presence of the Si4+ state (SiO2) and the peak at 99 eV corresponds to Sio (Si Si bonds). For Ti 2p, multiple valence states corresponding to Tio, Ti2+, Ti3+ and Ti4+ with a main presence of Ti4+ at 458.9 eV. Therefore, it can be concluded that the native oxide layer on Ti-Si alloy is mainly composed of TiO2 and SiO2. On the other hand, O 1s spectra comprise two extra peaks 532 eV (oxygen state in OH) and 533.5 eV (adsorbed H2O) with a peak of the O2 state at 530.5 eV. After PDA coating, the thickness of the amorphous layer increased with increasing dopamine concentration, 12 nm and 38 nm for P1 and P2, respectively. However, it should be noted here that TEM analysis reveals that there are particle dependent deviations in the coating thickness.
Fig. 2. FE-SEM images of Ti-Si alloy samples: (a) TiH2, (b) Si, (c) Bare Ti-Si alloy (d) P1 and (e) P2.
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Fig. 3. Cross sectional TEM image of (a) bare Ti-Si alloy, (b) P1, and (c) P2. (inset of (c): electron diffraction pattern of coating layer and core regions).
2p and Si 2s decreased. This result indicates that the PDA has been successfully coated on the surface of the Ti-Si alloy. Highresolution spectra of the C 1s and N 1s regions obtained on P1 and P2 samples are shown in Fig. 5(b)–(e). The functional group of CHx/C-NH2, C-O/C-N and C¼O for C 1s can be observed at 284.4,
XPS analysis provides information about the elemental compositions of the PDA coating layer. Fig. 5(a) shows the XPS spectra of the bare, P1 and P2 samples. XPS signals for bare sample were suppressed after PDA coating. After the PDA coating, two new peaks of N 1s and C 1s appeared and the intensity of the peaks of Ti
(a)
600
Intensity(a.u.)
Intensity (a.u.)
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Ti 2p
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Si 2s Si 2p
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0
SiO2
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100
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initial data OH O2
Intensity(a.u.)
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Intensity(a.u.)
Si 2p
Initial data Si-Ti Si SiOx
O 1s
H2O
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532
530
528
Binding energy(eV)
Fig. 4. (a) XPS spectra of bare Ti-Si alloy, (b) Si 2P spectrum, (c) Ti 2P spectrum and (d) O 2s spectrum of bare Ti-Si alloy.
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Fig. 5. (a) XPS spectra of bare Ti-Si alloy, P1 and P2, (b) (c) C 1s spectrum of P1 and P2, and (d) (e) N 1s spectrum of P1 and P2.
285.6 and 287.7 eV, respectively (Fig. 5(b) and (c)). N 1s spectrum is fit with three peaks at 402, 400 and 398.6 eV assigned to R-NH2, RNH-R and ¼N-R (Fig. 5(d) and (e)). The percent contribution for each functional group in N 1s and C 1s spectra are given in Table 1. The percent contribution for each functional group in N 1s and C 1s spectra are given in Table 1. The relative portions of each CHx/C– NH2, C–O/C–N and C¼O species in the C 1s region are similar for both P1 and P2. In the N 1s region, on the other hand, among three peaks assigned to R–NH2, R–NH–R, and ¼N–R amine functionalities, the fraction of R–NH2 species increased in P2 compared to P1, while the relative amount of R–NH–R appears to be lower in P2 than in P1. On the basis of the chemical structures of dopamine and PDA, the primary (R–NH2) amine is associated with dopamine, the secondary (R–NH–R) amine is associated with PDA and the tertiary (¼NR) amine is associated with tautomeric species of the intermediate species(5,6-dihydroxyindole and 5,6-indolequinone) [42–44]. Therefore, a higher fraction of dopamine is present in P2 than in P1, indicating that there is more dopamine that has not been converted to PDA during polymerization reaction for PDA coating in P2 than in P1. TG analysis was used to determine the content of the PDA coating, with results shown in Fig. 6, in which the TG curves of the
Table 1 XPS Functional Group Percentages of P1 and P2. Sample
P1 P2
C 1s
N 1s
C-Hx, C-NH2
C-O, C-N
C¼O
¼N-R
R-NH-R
R-NH2
66 68
24 23
10 9
12 21
78 42
10 37
bare sample and the PDA are also added for comparison. The weight loss of the PDA occurs between 200 and 600 C, while for the bare Ti-Si alloy the weight gain due to oxidation begins from around 500 C. As estimated from the TG curves, the content of PDA in P1 and P2 was 2.0 and 3.64 wt. %, respectively. Note that the difference between the PDA content between P1 and P2 seems very small considering the huge difference in dopamine content added to the solution for PDA coating. Moreover, the amounts of PDA coating estimated from TGA results appear to be less than those of the dopamine in the solution. This is because the conversion of dopamine to PDA is critically dependent on dopamine concentration and polymerization time [26,34]. At low dopamine concentration (e.g., 0.1 mg/ml), almost all dopamine molecules can be converted into the PDA coating, whereas at a much higher concentration (e.g., 1.5 mg/ml), only certain fractions are converted. In the latter case, the PDA coating thickness is a function of the immersion time for polymerization, in which once the surface is almost covered by a monolayer of PDA, the surface polymerization slow down [26,34]. It seems therefore that after polymerization for 5 h used in this work, only a small fraction of dopamine were converted to PDA. A series of electrochemical tests of the bare Ti-Si alloy and the PDA coated samples (P1 and P2) were performed. The chargedischarge profiles of those electrodes are shown in Fig. 7a–c. The first charge profiles of the bare Ti-Si and P1 electrodes appear to be similar, while in case of the P2 electrode a plateau at around 0.02 V during the initial charge reaction is observed, which is attributed to the polarization resistance caused by the thick PDA coating layer, as can be inferred from the cross sectional TEM images shown in Fig. 3. The thickness of the PDA coating affects the polarization of electrodes during charge-discharge cycling. The polarization, as measured by the voltage drop at the cut-off voltage for the
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125
120
Bare P1 P2 Polydopamine
120
100
80
60
110
40 105
Weight / %
Weight / %
115
20 100 0 95 100
200
300
400
500
600
700
800
Temperature / Fig. 6. TG curves of polydopamine, bare Ti-Si alloy, P1, and P2.
(a)
(b)
1.0
0.5
0.0
(c) 1st 3rd 5th 7th 10th 30th 50th
1.5
1.0
0.5
200
400
600
Capacity(mAh/g)
800
1000
1.0
0.5
0.0
0.0 0
1st 3rd 5th 7th 10th
1.5
Voltage(V)
Voltage(V)
1.5
Voltage(V)
1st 3rd 5th 7th 10th 30th 50th
0
200
400
600
800
0
1000
200
400
600
800
1000
Capacity(mAh/g)
Capacity(mAh/g) Fig. 7. Charge-discharge profiles of (a) Bare Ti-Si alloy, (b) P1, and (c) P2 electrodes.
capacity curves of P1 and P2 electrodes were compared with that of 600 nm thick Si thin film electrode prepared using sputtering method in Fig. 9. The differential capacity curves were derived from the the 3rd charge – discharge curves. Two broad peaks (A and B) during charging (lithiation) and two corresponding peaks (A’ and B’) during discharge (delithiation) can be observed for the three samples. These broad peaks are typical of amorphous Si. The
Si TiSi2 Cu(foil)
Intensity(a.u)
discharge reaction, increases with cycling as shown in Fig. 7. The polarization increase after 5 cycles is more prominent in the bare sample than in P1 electrode, probably due to the increase of the SEI film resistance. On the other hand, in the case of the P2 electrode, the polarization increases significantly over the initial cycles, resulting in fast capacity fading. It appears that the thickness of the PDA coating has a crucial role in improving electrochemical performances of Si based alloys. Si-M silicides in Si-M alloys are known as an inactive phase because the alloy capacities are well predicted based on the assumption that the silicon alloys consist of active phase (Si) and inactive phase (Si-M silicides) [45,46]. On the other hand, it has been reported that nanocrystalline or nanostructured silicides have activity toward lithium, although those electrochemical behaviors may be dependent on Si-M systems [47–52]. TiSi2 nanonets synthesized by chemical vapor deposition have a significant reversible capacity of 943 mAh g1 [52]. In the present work, the measured capacity appears to be similar to the value calculated based on the assumption that the silicide is inactive. It is also worth noting that in the XRD results performed before and after cycling, the silicide peak remains unchanged even after 10 cycles as shown in Fig. 8, indicating that the Ti-Si silicide is inactive phase during cycling. It is also meaningful to compare the lithiation and delithiation voltages of Ti-Si alloy and pure Si electrodes. The differential
10th delithiation
3rd delithiation
As prepared
10
20
30
40
50
60
70
80
Diffration angle(2θ) Fig. 8. XRD results performed before and after cycling of bare Ti-Si alloy.
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B'
2000
A'
(a) Bare
0 -2000 -4000
B
A
dQ/dV
2000
(b) P1
0 -2000 -4000 500
(c) Si thin film 0 -500
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Voltage (V) Fig. 9. Differential capacity curves of (a) Bare Ti-Si alloy, (b) P1, and (c) Si thin film electrodes.
peak A of P1 and P2 electrodes shifts a little bit to lower voltage relative to that of Si thin film electrode, while the peak B and the delithiation voltages (A’ and B’) remain unchanged. The initial discharge capacity/coulombic efficiency values of the bare Ti-Si, P1, and P2 electrodes were 814/87, 813/86.6, and 728/ 80.5 mAh g1/%, respectively. The cycling performance and coulombic efficiency are compared in Fig. 10. The P1 electrode shows a more enhanced cyclic performance than that of the bare sample, while the capacity of P2 can be seem to decay rapidly. This indicates that that too thick PDA coating in P2 deteriorates the performance of the alloy. Particularly noteworthy is the fact that the coulombic efficiency of P1 remains as high as about 99%, but
7
that of bare the Ti-Si alloy is below 97%. It has been reported that the irreversible capacity of Si-based electrodes is mainly due to the loss of electrical contact and the continuous SEI growth with cycling [6]. The SEI film formed on Si becomes unstable and can be broken because a significant volume change occurs during chargedischarge cycling, re-exposing the fresh Si surface to the electrolyte and thus forming the SEI again. This continuous SEI growth with cycling limits Li ion diffusion through the thick SEI layer and results in continuous consumption of the Li-ion electrolyte, which leads to fading of the electrode capacity [13]. To investigate the change in resistance components with cycling, AC impedance spectrum measurements were conducted Nyquist plots for the bare alloy and the P1 samples, obtained for the fully lithiated state after the 10th and 30th cycles, are shown in Fig. 11a and b. The spectra consist of a depressed semicircle in the high frequency region and an oblique straight line in the low frequency region, both of which are related to Li-ion diffusion within particles. The diameter of the semicircle indicates the charge-transfer resistance and might contain the SEI film resistance and the inter-particle contact resistance [53,54]. It seems like it is hard to separate the effect of PDA layer and SEI because the PDA exhibits a good conductivity of lithium ions [28]. As can be seen in Fig. 11a and b, the diameter of the semicircle in the bare alloy sample significantly increased after 30 cycles, whereas in the case of P1 the size of the semicircle after the 30th cycles is almost the same as that after the 10th cycles. Therefore, in the case of the bare alloy electrode, the SEI film becomes thicker and thicker upon charge/discharge cycling, and thus the interparticle contact resistance increases. On the other hand, the SEI film in the P1 electrode is stable during cycling. This result has been confirmed by considering the cross-sectional SEM images before cycling and after the 50th cycles (Fig. 12a–d). After the 50th cycles, the SEI layer formed on the alloy particles was much thicker in the bare alloy electrode (Fig. 12b) than in the P1 electrode (Fig. 12d). So, it is considered that the PDA coating suppresses direct contact between the Ti-Si alloy and the electrolyte during chargedischarge cycling and, hence, the initially formed SEI film remained in a stable state during cycling. It should also be noted that the dimensional change of the electrode is larger in the bare sample than in P1. The increase in the electrode thickness was 35% in the
Fig. 10. Cycling performance and coulombic efficiency of bare Ti-Si alloy and P1 electrodes and P2. Note that the coulombic efficiency of P2 is not included.
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Fig. 11. Nyquist plots of two types of electrodes: (a) Bare Ti-Si alloy, (b) P1.
bare electrode, but 7% in the P1 electrode; both of these values were obtained or the delithiated state after the 50th cycles. In order to investigate the effect of the PDA coating layer on the rate performance, the rate capability values of the bare Ti-Si and P1 electrodes were compared. On the other hand, P2 electrode seems to be not suitable for investigation of the rate capability because it
shows a rapid capacity fading during cycling as illustrated in Fig. 10. In the rate performance test, both electrodes were found to have similar discharge capacities at current densities up to 10C (Fig. 13). Considering that the PDA exhibits a good conductivity for lithium ions [28], the similar behavior in rate capability of P1 and bare alloy is probably attributed to the PDA coating thickness. Therefore, if
Fig. 12. SEM cross-sectional images of Bare Ti-Si alloy (a) before cycling and (b) after 50 cycles and P1 Ti-Si alloy (c) before cycling and (d) after 50 cycles.
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Fig. 13. Rate capabilities of Bare Ti-Si alloy and P1 electrodes.
the coating layer is thin enough, the rate capability will be able to be more enhanced. These results indicate that PDA coating is an effective approach to improve the electrochemical performance. It appears that the thickness of the PDA coating has a crucial role in improving electrochemical performances of Si based alloys. Further investigation is on going to optimize the performance through controlling the thickness of the PDA coating. Hopefully this will further improve the electrochemical properties of the PDA coated Si alloys as anode for LIBs. It should be also noted that the cycle performance of Si-based alloy anodes is significantly affected by the microstructural characteristics, and thus if a Si alloy with a properly designed microstructure is used as core alloy then the PDA coated anode material will present better performance. 4. Conclusions Polydopamine as an adhesive and elastic polymer layer was coated on Ti-Si alloy and its electrochemical performance as an anode material for LIBs was investigated. The PDA coating suppresses direct contact between the Ti-Si alloy and the electrolyte during charge-discharge cycling and, hence, the initially formed SEI film remains in a stable state during cycling. The cycling performance and coulombic efficiency of the Ti-Si alloy anode were improved by PDA coating with a thin layer. The coating of PDA does not negatively affect the rate capability of the Ti-Si alloy anode. Acknowledgements This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government(MEST)" (NRF2011-C1AAA001-0030538). This study was also supported by a 2015 Research Grant from Kangwon National University (No. 120140163). References [1] X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, Silicon-Based Nanomaterials for Lithium-Ion Batteries: A Review, Adv. Energy Mater. 4 (2014) 1.
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Please cite this article in press as: J.-S. Lee, et al., Electrochemical properties of polydopamine coated Ti-Si alloy anodes for Li-ion batteries, Electrochim. Acta (2016), http://dx.doi.org/10.1016/j.electacta.2016.11.094