Effect of Phosphorus-Doping on Electrochemical Performance of

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Effect of Phosphorus-Doping on Electrochemical Performance of Silicon Negative Electrodes in Lithium-Ion Batteries Yasuhiro Domi,†,‡ Hiroyuki Usui,†,‡ Masahiro Shimizu,†,‡ Yuta Kakimoto,†,‡ and Hiroki Sakaguchi*,†,‡ †

Department of Chemistry and Biotechnology, Graduate School of Engineering, and ‡Center for Research on Green Sustainable Chemistry, Tottori University, 4-101 minami, Koyama-cho, Tottori 680-8552, Japan S Supporting Information *

ABSTRACT: The effect of phosphorus (P)-doping on the electrochemical performance of Si negative electrodes in lithiumion batteries was investigated. Field-emission scanning electron microscopy was used to observe changes in surface morphology. Surface crystallinity and the phase transition of Si negative electrodes before and after a charge−discharge cycle were investigated by Raman spectroscopy and X-ray diffraction. Li insertion energy into Si was also calculated based on computational chemistry. The results showed that a low P concentration of 124 ppm has a meaningful influence on the electrochemical properties of a Si negative electrode; the cycle performance is improved by P-doping of Si. P-doping suppresses the changes in the surface morphology of a Si negative electrode and the phase transition during a charge−discharge cycle. Li insertion energy increases with an increase in the P concentration; Li insertion into P-doped Si is energetically unfavorable, which indicates that the crystal lattice of Si shrinks as a result of the replacement of some Si atoms with smaller P atoms, and therefore, it is more difficult to insert Li into P-doped Si. These results reveal that suppression of the phase transition reduces the large change in the volume of Si and prevents a Si negative electrode from disintegrating, which helps to improve the otherwise poor cycle performance of a Si electrode. KEYWORDS: lithium-ion battery, negative electrode, silicon, phosphorus doping, morphology, crystallinity, and phase transition

1. INTRODUCTION If lithium-ion batteries (LIBs) are to be used as a power source in electric vehicles and stationary power supply systems, an additional increase in their energy density is required. Silicon (Si) is a promising active material for use as a negative electrode in next-generation LIBs due to its high theoretical capacity of 3580 mA h g−1 (Li15Si4) compared to that of graphite (372 mA h g−1), which is used currently.1,2 It has been reported that crystalline Si (c-Si) forms an amorphous alloy phase that consists of Si and Li (a-LixSi) in the first charge process according to eq 1 and that the resulting a-LixSi subsequently alloys with Li to form a c-Li3.75Si (Li15Si4) phase according to eq 2:1−7 c ‐Si + x Li+ + x e− → a‐LixSi

(1)

a‐LixSi + y Li+ + ye− → c ‐Li3.75Si

(2)

(3)

a‐LixSi → a‐Si + x Li+ + xe−

(4)

a‐Si + x Li+ + x e− → a‐LixSi

(5)

Importantly, Si undergoes significant volume expansion and contraction during the alloying (charge) and dealloying (discharge) reactions with Li, respectively. The volumetric change ratio per Si atom from Si to Li3.75Si corresponds to 380%, which generates high stresses and large strains in the active materials.5 The strains that accumulate under repeated charge−discharge cycling cause disintegration of the Si negative electrode, leading to a rapid decrease in capacity. In addition, Si has disadvantages of a low electrical conductivity (∼1 × 105 Ω cm) and a low diffusion coefficient of Li+ within it (DLi+: 1 × 10−14 to 1 × 10−12 cm2 s−1).11,12 These limitations hinder the practical application of Si-active material as a negative electrode. To address these issues, researchers have proposed various approaches, including the preparation of composite electrodes to improve mechanical properties,13,14 coating of Si with conductive materials to reduce electrical resistivity,15,16 and the

In the dealloying (discharge) process, the c-Li3.75Si phase reverses to the a-LixSi phase (eq 3), which continues to dealloy to amorphous Si (a-Si) instead of c-Si according to eq 4.8−10 aSi formed in the first discharge process alloys with Li to form aLixSi after the second charge process (eq 5). After that, the reactions in eqs 2−5 proceed repeatedly. Therefore, the charge−discharge reaction of a Si negative electrode proceeds with a phase transition. © XXXX American Chemical Society

c ‐Li3.75Si → a − LixSi + y Li+ + ye−

Received: January 12, 2016 Accepted: March 3, 2016

A

DOI: 10.1021/acsami.6b00386 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

guide tube, and sprayed from the nozzle onto the Cu substrate in the chamber with a base pressure of several tens of Pa. Electrochemical measurements were carried out with a laboratorymade beaker-type three-electrode cell. The working electrode was the fabricated GD thick-film electrode. Both the counter and reference electrodes consisted of Li metal sheets (Rare Metallic, 99.90%, thickness; 1 mm). The electrolyte solution used was 1 M lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) in propylene carbonate (PC; C4H6O3, Kishida Chemical Co., Ltd.). The cell was assembled and disassembled in an Ar-filled glovebox (Miwa MFG, DBO2.5LNKP-TS) with a dew point below −100 °C and an oxygen content below 1 ppm. A charge−discharge test was carried out using an electrochemical measurement system (HJ-1001SD8 Hokuto Denko Co., Ltd. or BS2506, KEISOKUKI) in the potential range between 0.005 and 2.000 V vs. Li+/Li at 303 K under a constant current density of 1.0 A g−1 (0.28 C). The electrical resistance of P-doped and undoped Si powders was measured by a laboratory-made system. After a powder-filled cylinder was subjected to a pressure of up to 60 MPa to create a pelletlike sample, axial electric resistance was measured with the four-terminal method. 2.2. Structural and Morphological Characterization. The phase transition and surface crystallinity of Si thick-film electrodes were investigated using XRD (Ultima IV, Rigaku) and Raman spectroscopy (NanofinderFLEX, Tokyo Instruments, Inc.), respectively. XRD patterns were obtained at a voltage of 40 kV and a current of 40 mA with Cu−Kα radiation (λ = 1.5406 Å). After the charge− discharge test, the electrochemical cell was disassembled in an Ar-filled glovebox to prevent exposure to the atmosphere, and the electrode was washed with dimethyl carbonate (DMC, Kishida Chemical Co., Ltd.) to remove residual electrolytes and covered with kapton film. XRD patterns were identified based on a comparison with patterns in the Inorganic Crystal Structure Database (ICSD). The electrode was irradiated with the 532 nm line of a Nd:YAG laser beam at room temperature through a 100-powered objective lens to obtain Raman spectra. The scattered light was collected in a backscattering geometry. The surface morphology of the electrode was observed before and after the charge−discharge test by FE-SEM (JSM-6701F, JEOL Co., Ltd.). The surface of the thick-film electrode was coated with gold to prevent a charge up. The cross-sectional surface of the electrode was observed using FE-SEM (JSM-7800F, JEOL Co., Ltd.) A focused ionbeam (FIB, JIB-4501, JEOL Co., Ltd.) was used to fabricate the crosssectional surface. The surface of the thick-film electrode was coated with carbon to protect it against damage by the Ga+ beam of FIB. 2.3. Calculation of Li Insertion Energy. Einsertion was calculated using the plane wave code of the Vienna Ab initio Simulation Package (VASP),35 with the supplied Projector Augmented Wave (PAW).36 A Generalized Gradient Approximation was used as an exchangecorrelation functional. We used a kinetic energy cutoff of 350 eV and 8 × 8 × 8 k points. Einsertion was calculated according to eq 6.22 When this Einsertion is negative, Li is likely to be inserted into Si.

construction of nanostructured Si materials to buffer volume expansion.17−19 Doping of Si with impurities, such as phosphorus (P),20−24 boron (B),22,25−27 copper (Cu),28 arsenic (As),27 and aluminum (Al),29 has also been attempted to increase the electrical conductivity of Si. Kong et al. found that a P-doped Si/graphite composite electrode exhibited lower resistivity than an undoped Si/graphite electrode.20 They also reported that the former shows better capacity retention than the latter. Rousselot et al. investigated the electrical resistivity of B-doped Si with various B concentrations, and showed that the resistivity of B-doped Si is less than that of undoped Si by 3 orders of magnitude.25 However, they concluded that the increase in the conductivity of Si under B-doping has no significant effect on the electrochemical properties of a composite electrode consisting of Si as an active material, carbon black as a conductive additive, and carboxymethyl cellulose as a binder. On the other hand, a computational chemistry approach has been used to elucidate the change in Li insertion energy (Einsertion) under doping with impurities. Long et al. reported that Einsertion for B-doped Si is negative, whereas that for P-doped Si is positive; thus, whether Einsertion is positive or negative can depend on the dopant.22 Positive Einsertion (Pdoping) indicates that Li insertion into Si is energetically unfavorable, whereas a negative value (B-doping) means that it is easy to insert Li into Si. As seen from the above, the effect of doping with an impurity on the electrochemical performance of a Si negative electrode in LIBs has not yet been clarified. It is very important to elucidate the relationships between the electrochemical performance of the doped Si and the reaction behavior, or between the electrochemical performance of doped Si and Einsertion. To understand these relationships, it should be enlightening to use doped Si itself as an active material for a negative electrode without any binder or conductive agent, and the gas-deposition (GD) method should be very useful for preparing a thick-film electrode consisting of only doped Si.14,30 P-doped Si is an attractive candidate as a negative electrode in next-generation LIBs because it has already been used as an ntype semiconductor material in Si doped with impurities.31,32 In the present study, the effect of P-doping on the electrochemical performance of a Si negative electrode in LIBs was investigated by X-ray diffraction (XRD), Raman spectroscopy, fieldemission scanning electron microscopy (FE-SEM), and so on. We also discuss the above effect with respect to the size of the dopant, surface morphology, surface crystallinity, and Einsertion.

E insertion = E(Li + Si + dopants) − E(Si + dopants)

2. EXPERIMENTAL SECTION

− E(Li metal)

2.1. Electrode Preparation and Electrochemical Measurements. P-doped Si (P concentration; 0, 50, 124, 633, and 1000 ppm) powders were supplied by Elkem (Silgrain e-Si). The average particle size of 0, 50, and 1000 ppm P-doped Si powders was more than 100 μm, which was not adequate for the formation of a thick-film by the GD method. Therefore, the powders were mechanically milled using a container and ball consisting of Y2O3-stabilized ZrO2 with a rotary speed of 380 rpm. The average particle diameter of the milled powders was 7−10 μm, which was confirmed by a particle size distribution analyzer (SALD-2300, Shimazu). In contrast, the 124 and 633 ppm Pdoped Si powders were used as received, and the average particle size of these powders was also 7−10 μm. For GD, a current collector consisting of a 20 μm thick Cu foil substrate was set at a distance of 10 mm from a nozzle in a vacuum chamber.33,34 The nozzle, with a diameter of 0.8 mm, was connected to the end of a guide tube. An aerosol consisting of Ar gas (differential pressure of 7.0 × 105 Pa) and active material powder of undoped or P-doped Si was generated in the

(6)

3. RESULTS AND DISCUSSION 3.1. Characterization of P-Doped Si Negative Electrode. Figure 1A, B show XRD patterns of P-doped Si powders with various P concentrations and the expansion of part A between 28 and 29°, respectively. As shown in Figure 1A, peaks of Si (111), Si (220), and Si (311) were observed between 20 and 60°. In Figure 1B, the XRD peak of Si (111) slightly shifts toward a higher angle with an increase in the P concentration, which indicates a decrease in the lattice spacing. To confirm this observation, we estimated the real lattice constant. Figure 2 shows the correlation between the real lattice constant of Si and the P concentration. The real lattice constant was estimated by B


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The electrical resistivity of P-doped Si (124 ppm) and undoped Si powders was estimated to be 50 and 8700 Ω cm, respectively; the electrical resistivity of Si decreased with Pdoping. The increase in electrical conductivity under P-doping indicates the formation of an n-type semiconductor of Si. A similar increase in electrical conductivity under doping with an impurity has been reported.39,40 3.2. Effects of P-Doping on the Electrochemical Performance of Si Negative Electrodes. Figure 3 shows

Figure 3. First charge−discharge curves of P-doped Si thick-film electrodes with each P concentration in 1 M LiTFSA/PC at a constant current density of 1.0 A g−1 (0.28 C).

the first charge−discharge (Li insertion-extraction) curves of undoped and P-doped Si thick-film negative electrodes in 1 M LiTFSA/PC. In every case, potential plateaus were observed at ca. 0.1 and 0.4 V vs. Li+/Li on charge and discharge curves, respectively. These plateaus are attributed to the alloying and dealloying reactions of Si with Li (eqs 1−4).41,42 Although the charge−discharge capacity of 50 ppm P-doped Si was almost the same as that of undoped Si, it decreased with an increase in the P concentration above 124 ppm. If we take into account the decrease in electrical resistivity under 124 ppm P-doping, the increase in the electrical conductivity would not always correspond to an increase in the initial charge−discharge capacity. The potential plateaus of 124, 633, and 1000 ppm Pdoped Si electrodes were observed at about 0.2 V on last charge curves, while they remained at around 0.4 V on last discharge curves. This is because alloying reaction of a-Si with Li after second cycle (eq 5) proceeds at around 0.2 V.43 On the other hand, no potential plateau and an overpotential for the discharge curves are confirmed for 0 and 50 ppm P-doped Si electrodes, which are caused by electrical isolation and deterioration of Si active material, respectively. Figure 4A shows the cycle performance of undoped and Pdoped Si thick-film negative electrodes in 1 M LiTFSA/PC. Figure 4B, C also show the dependence of the Coulombic efficiency and capacity retention of the thick-film electrodes on the cycle number, respectively. Rapid fading of the discharge capacity was gradually suppressed with an increase in the P concentration (Figure 4A), which indicates good cycle performance. Notably, a very low P concentration of 124 ppm significantly influenced the electrochemical performance of the Si negative electrode. Figure 4B shows that there was no drop in Coulombic efficiency in the initial cycle under 124− 1000 ppm P-doping, whereas the efficiency showed a significant decrease with 0 and 50 ppm P-doping, which later recovered after 10 and 100 cycles, respectively. It is widely accepted that a surface film forms through reductive decomposition of the

Figure 1. (A) XRD patterns of P-doped Si powders with various P concentrations. (B) Expansion of part A between 28 and 29°. Cu in part A is attributed to the substrate of the current collector.

Figure 2. Correlation between the lattice constant of Si and the P concentration.

a = a0 + a0Kcos θ

(7)

where a and a0 are the apparent and real lattice constants, respectively, and the other symbols have their usual meanings.37 a0 decreased with an increase in the P concentration in Si. The relationship between the lattice constant of Si and the concentration of a substitution element has been previously reported,38 and the result obtained here is consistent with that in the previous study. Because the atomic radius of P (0.106 nm) is smaller than that of Si (0.111 nm), but still within 15%, it is considered that some of the Si atoms may be replaced by P atoms and that P-doped Si can be a substitutional solid solution. C


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that P-doped Si electrodes in which the P concentration is more than 124 ppm maintain over 40% of the initial discharge capacity after 200 cycles, whereas pristine Si retains only 2% of the initial capacity. These results indicate that P-doping improves the electrochemical performance of Si negative electrodes in LIBs. If we take into account the results in Figures 1 and 2, it may be difficult to insert Li into the crystalline lattice of Si under a decrease in the lattice constant with P-doping, and the phase transition during the charge− discharge cycle is controlled. Such inhibition of the phase transition may limit the large volumetric change of Si during alloying and dealloying reactions with Li, which could improve the otherwise poor electrochemical properties of a Si negative electrode. 3.3. Changes in Surface Morphology. Figure 5 shows FE-SEM images of undoped and 124 ppm P-doped Si thickfilm electrodes before and after charge−discharge cycles. Before a charge−discharge cycle, the FE-SEM image of P-doped Si was almost the same as that of Si, as shown in Figure 5A, D. This result was expected from the fact that the two powders have a similar particle size distribution, as noted above. After the first charge−discharge cycle, the undoped Si thick-film electrode was full of cracks and the active material of Si was deteriorated (Figure 5B). On the other hand, as shown in Figure 5E, the surface morphology of P-doped Si was almost the same as that before the cycle, even though a slight crack was observed. If we take into account the greater charge−discharge capacities of an undoped Si electrode (Figure 3), we can consider that there is a large change in the volume of the active material of Si in the initial cycle. As shown in Figure 5C, F, cracks and exfoliation of the active material occur on a Si thick-film electrode because of a large volumetric change after the 40th cycle, whereas the surface morphology of P-doped Si remained uniform, which could be attributed to the small change in the volume of Si. Since a change in volume results in the generation of high stresses and large strains in the active materials and these accumulated strains cause a disintegration of the Si electrode, it is considered that P-doped Si shows superior electrochemical performance because of its smaller volumetric change compared to undoped Si (Figure 4). To estimate the three-dimensional root-mean-square roughness (Sq), the surface morphology of P-doped and undoped Si thick-film electrodes before and after charge−discharge cycles was observed using confocal laser scanning microscopy (CLSM,

Figure 4. Dependence of (A) discharge capacity, (B) Coulombic efficiency, and (C) capacity retention of P-doped Si thick-film electrodes with each P concentration on cycle number.

electrolyte solution during the initial cycle, and that the formation of this film is one of the reasons for a decrease in colombic efficiency. The resulting surface film should break up with the large volumetric change between 0 and 50 ppm Pdoped Si, and then a surface film should form again on the newly formed Si surface. Therefore, the efficiency decreases in the initial cycle. On the other hand, with 124−1000 ppm Pdoped Si, the volumetric change should be suppressed and the surface film should not be disrupted. Therefore, the efficiency with 124−1000 ppm P-doped Si was superior to that with 0 and 50 ppm P-doped Si. On the basis of Figure 4C, it is clear

Figure 5. FE-SEM images of (A−C) undoped Si and (D−F) 124 ppm P-doped Si GD thick-film electrodes (A, D) before and after the (B, E) first and (C, F) 40th charge−discharge cycles. D


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124 ppm P-doped Si electrode after the 40th lithiation was almost the same as that after the 40th delithiation. These results clarified that P-doping of Si suppresses the large change in the volume of Si during alloying and dealloying reactions with Li. Several subμm cracks were observed inside the Si active material layer after 40 cycles (Figure 6B, C, E, F and Figure S7C−F). On the other hand, cracks of several micrometers in size were confirmed at the undoped Si electrode regardless of the lithiation and delithiation state, but were not observed at the P-doped Si electrode; undoped Si should be damaged compared to P-doped Si. In addition, the large cracks observed in the undoped Si active material layer should contribute to its low Coulombic efficiency. Carbon and oxygen were found in 0 and 124 ppm P-doped Si active material layers after 40 cycles (Figures S3−S6), whereas they were scarcely detected before the charge−discharge cycle (Figures S1 and S2). These elements may arise from reductive decomposition products of the electrolyte solution which penetrates into the porous active material layers. Although an interspace was confirmed between the copper current collector and Si active material layer (Figure 6B, E), this interspace was not observed after lithiation (Figure 6C, F and Figure S7D, F). The disappearance of cracks is due to expansion of the Si active material through the formation of a Li−Si alloy. Therefore, electrical continuity is ensured for at least 40 cycles. 3.4. Change in Surface Crystallinity and the Phase Transition. Figure 7 shows the Raman spectra of P-doped Si thick-film electrodes with various P concentrations before and after 10 charge−discharge cycles. It has been reported that c-Si has a triply degenerated Raman active F2g-mode at around 520 cm−1.43,44 Before charge−discharge cycling, 0 and 50 ppm Pdoped Si electrodes exhibited a peak at 519 cm−1 which was assigned to F2g-mode, as shown in Figure 7A. The peak at around 520 cm−1 gradually shifted toward a lower wavenumber with an increase in the P concentration; 124, 633, and 1000 ppm P-doped Si electrodes showed peaks at 518, 514, and 513 cm−1, respectively. On the basis of a previous report, this shift was caused by P-doping.45 In addition, the full width at halfmaximum (fwhm) of the peak was almost the same for all of the electrodes; P-doping of Si and its concentration do not influence the surface crystallinity of Si. On the other hand, after 10 cycles, the peaks assigned to F2g-mode of 0, 50, 124, 633, and 1000 ppm P-doped Si electrodes shifted to 491, 493, 499, 506, and 512 cm−1, respectively (Figure 7B); the degree of the

VK-9700, Keyence). The surface morphology observed by CLSM was almost the same as that by FE-SEM. Although the Sq of undoped Si was almost the same as that of P-doped Si before cycling, as shown in Table 1, the percentage increase in Table 1. Root-Mean-Square Roughness of P-Doped Si (124 ppm) and Si before and after Charge−Discharge Cycles

P-doped Si (124 ppm) undoped Si

before cycle (μm)

after first cycle (μm)

after 40th cycle (μm)

0.40

0.63

3.5

0.38

1.0

5.4

Sq for undoped Si was higher than that for P-doped Si; Pdoping controlled changes in the surface morphology of the Si negative electrode during charge−discharge reactions. Figure 6 shows cross-sectional FE-SEM images of undoped and 124 ppm P-doped Si thick-film electrodes before and after the 40th charge−discharge cycle. Energy-dispersive X-ray spectroscopy (EDS) mappings of Figure 6A−F are also shown in Figures S1−S6, respectively. Before the charge− discharge cycle, the thicknesses of the thick-film were estimated to be 1.9 and 1.8 μm for 0 and 124 ppm P-doped Si thick-film electrodes, respectively (Figure 6A, D); the films have almost the same thickness. On the other hand, as shown in Figure 6B, E, the thicknesses of the thick film after the 40th delithiation were estimated to be 30.4 and 27.7 μm for 0 and 124 ppm Pdoped Si electrodes, respectively; after the 40th delithiation, both electrodes became obviously thicker compared to the pristine material. Because the active material layers became porous after 40 cycles, as shown in Figure S7A, B, the above increase in thickness is the result of a change in the structure with repeated charge−discharge cycles. In addition, because these cross-sectional FE-SEM images were taken in a delithiation state, the increase in thickness is not due to an increase in volume through the formation of a Si−Li alloy. In Figure 6C, F, the thicknesses of the thick film after the 40th lithiation were estimated to be 44.2 and 23.5 μm for 0 and 124 ppm P-doped Si electrodes, respectively. The thickness of undoped Si after the 40th lithiation was greater than that after the 40th delithiation. A possible explanation for why the volumetric change ratio did not correspond to the theoretical value of 380% is that a Li−Si alloy formed inside the porous Si active material layer. On the other hand, the thickness of the

Figure 6. Cross-sectional FE-SEM images of (A−C) undoped Si and (D−F) 124 ppm P-doped Si GD thick-film electrodes (A, D) before the first charge−discharge cycle, (B, E) after the 40th delithiation and (C, F) after the 40th lithiation. E


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Figure 9. Crystal structures of (A) Li0.125Si, (B) Li0.125P0.125Si0.875, and (C, D) Li0.125P0.25Si0.75.

summarized in Table 2. The Einsertion value increased with the P concentration, which indicates that it is difficult to insert Li

Figure 7. Raman spectra of P-doped Si thick-film electrodes with various P concentrations (A) before and (B) after 10 charge− discharge cycles.

Table 2. Li Insertion Energy of (A) Si, (B−D) P-Doped Si, (E) S-Doped Si, and (F) Al-Doped Si in Various Crystal Structuresa

peak shift increased with a decrease in the P concentration. Since the Raman peak of a-Si is observed at 490 cm−1,43,46,47 cSi transformed to a-Si during charge−discharge cycles according to eqs 1−5. Although the fwhm of the peak also increased after 10 cycles, it decreased with an increase in the P concentration. XRD was also used to elucidate the suppressive effect of Pdoping on the phase transition. Figure 8 shows XRD patterns

system

Einsertion (eV)

(A) Li0.125Si (B) Li0.125P0.125Si0.875 (C) Li0.125P0.25Si0.75 (D) Li0.125P0.25Si0.75 (E) Li0.125S0.125Si0.875 (F) Li0.125Al0.125Si0.875

0.5 0.7 0.8 0.6 0.8 −0.9

a

The formulas of A−D correspond to the structures in Figure 9, and the structures of E and F are the same as that in Figure 9B.

into bulk Si in the order C > B > A in Figure 9 and/or Table 2. For a Li0.125P0.25Si0.75 system, however, Einsertion differs depending on the Si site that is substituted by P. The difference in Einsertion can be attributed to the difference in distortion of the crystal lattice. The higher Einsertion value under P-doping, i.e., the poor insertion of Li into Si, was consistent with the observation of a lower first charge−discharge capacity (Figure 3) and suppression of the deterioration of crystallinity and the phase transition from c-Si to a-Si (Figures 7 and 8). To understand the relationship between Einsertion and the size of the dopant in detail, E insertion was calculated for Li0.125S0.125Si0.875 and Li0.125Al0.125Si0.875. P atoms in Figure 9B were substituted with S and Al atoms, and the Einsertion values of these structures are summarized in Table 2E, F). The atomic radius of Si (0.111 nm) is larger than that of S (0.102 nm), whereas it is smaller than that of Al (0.118 nm). Our results suggest that the insertion of Li into Si is energetically unfavorable in the order S-doped > P-doped > undoped > Al-doped Si. As expected, the value of Einsertion decreased in the above order, as shown in Table 2A, B, E, F. These results indicate that the crystal lattice of Si would shrink due to the displacement of some Si atoms with smaller S and P atoms, and therefore, it would be difficult to insert Li into Si. On the other hand, in the case of Al-doped Si, Li is likely to insert into Si because the crystal lattice of Si would be expected to expand when Si is replaced by a larger Al atom.

Figure 8. XRD patterns of P-doped Si negative electrodes after the first lithiation at 0.005 V vs. Li+/Li in 1 M LiTFSA/PC. Cu is assigned to the current collector.

of P-doped Si electrodes with various P concentrations after the first lithiation until 0.005 V vs. Li+/Li in 1 M LiTFSA/PC. Peaks assigned to Li15Si4 (332), Li15Si4 (422), Li15Si4 (521), Si (220), and Si (311) were observed. The peak intensity of Si increased and that of Li15Si4 decreased with an increase in the P concentration. Based on Raman spectra and XRD patterns, Pdoping of Si suppressed the formation of a c-Li15Si4 phase during the charge process and the c-Si phase remained with an increase in the P concentration. As a result, a deterioration in the crystallinity of Si, i.e., a phase transition from c-Si to a-Si, was also controlled by P-doping. 3.5. Li Insertion Energy into Si. The experimental results obtained here were verified by computational chemistry. Figure 9 shows various crystal structures of LixPySiz, and Einsertion values for these structures calculated according to eq 6 are F


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4. CONCLUSION The effect of P-doping on the electrochemical performance of a Si negative electrode in LIBs during alloying and dealloying with Li was investigated from the viewpoint of the atomic radius of the dopant, the surface morphology, the phase transition, and Einsertion. The electric resistivity and the first charge−discharge capacity of a Si electrode decreased under Pdoping; an increase in electrical conductivity does not always correspond to an increase in the initial charge−discharge capacity. On the other hand, P-doping with a very low concentration of 124 ppm helped to improve the electrochemical performance of Si negative electrodes. A change in the surface morphology of a Si negative electrode after charge− discharge cycles was suppressed by P-doping. The results of Raman spectroscopy and XRD revealed that P-doping controlled the formation of a c-Li15Si4 phase during the charge process and the phase transition from c-Si to a-Si. This suppression of the phase transition leads to a decrease in the large change in volume of Si and prevents the Si negative electrode from disintegrating. Dopants that are smaller than Si shrink the crystal lattice of Si and increase its Einsertion; under these conditions, it is difficult to insert Li into Si. On the other hand, with dopants that are larger than Si, it is easy to insert Li into Si. In addition, Einsertion increased with an increase in the P concentration. These effects of P-doping help to improve the otherwise poor cycle performance of a Si electrode.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00386. EDS mappings and high-magnification FE-SEM images of Figure 6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-857-31-5265. E-mail: [email protected]. ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant numbers 24350094 and 15K21166. The authors thank Prof. T. Kondo (Ochanomizu University) and Ms. N. Aoki (Ochanomizu University) for their assistance with cross-sectional FE-SEM observations.

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REFERENCES

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Effect of Phosphorus-Doping on Electrochemical Performance of

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