Article pubs.acs.org/JPCC
Improved Electrochemical Performance of Lanthanum Silicide/ Silicon Composite Electrode with Nickel Substitution for Lithium-Ion Batteries Yasuhiro Domi,†,‡ Hiroyuki Usui,†,‡ Yuya Takemoto,†,‡ Kazuki Yamaguchi,†,‡ and Hiroki Sakaguchi*,†,‡ †
Department of Chemistry and Biotechnology, Graduate School of Engineering, and ‡Center for Research on Green Sustainable Chemistry, Tottori University, Minami 4-101, Koyama-cho, Tottori 680-8552, Japan S Supporting Information *
ABSTRACT: The effect of nickel substitution on the electrochemical performance of a lanthanum silicide (LaSi2)/silicon (Si) composite electrode for lithium-ion batteries was studied. The results of X-ray diffraction analysis showed that LaSi2 forms a substitutional solid solution with Ni, and that only the Si site in LaSi2 is substituted by Ni, whereas elemental Si in the crystal structure is not substituted. Although the charge−discharge capacity of a LaNixSi2−x electrode (x = 0.06 and 0.12) was lower than that of a LaSi2 electrode, the LaNixSi2−x electrode exhibited a high-rate performance. A LaNi0.10Si1.90/Si (70:30 wt %) composite electrode showed a large initial discharge capacity and a superior long-term cycle performance compared to electrodes composed of Si alone and LaSi2/Si composite, and suppressed the decrease in the initial Coulombic efficiency of the Si electrode. The LaNi0.10Si1.90/Si electrode also exhibited an excellent high-rate performance with a reversible capacity of 2240 mA h g(Si)−1 at a rate of 10 C. The results of computational chemistry demonstrated that LaNi0.25Si1.75 favors Li migration in the pathway compared to LaSi2. These results indicate that Ni substitution in a LaSi2/Si composite negative electrode significantly improves its electrochemical performance.
1. INTRODUCTION The desired increase in the driving range of electric and hybrid electric vehicles has increased the demand for lithium-ion batteries (LIBs) with high energy density and a long lifetime. Silicon (Si) is a potential candidate as an active material for the negative electrode due to its high theoretical capacity of 3600 mA h g−1,1,2 compared to that of graphite electrode (372 mA h g−1), which is used currently. However, Si has disadvantages, including a low diffusion coefficient of Li+ (1 × 10−14−1 × 10−12 cm2 s−1) and a high electrical resistance (ca. 1 × 105 Ω cm).3,4 In addition, large volume expansion and contraction of Si occur during charge and discharge reactions, respectively.5 The expansion ratio per Si atom from Si to Li15Si4 reaches 380%, which generates high stresses and large strains in the active material.6 The accumulation of strains under repeated charge−discharge cycling result in cracking and pulverization of the active material, which causes a loss of electric contact with the current collector. Thus, a Si negative electrode shows a rapid decrease in capacity and a poor cyclability. To address these issues, many researchers have proposed various approaches, including the following: coating of Si with conductive material to increase the low electrical conductivity;7,8 preparation of nanostructured Si material to cushion volumetric change;9−11 doping of Si with impurities, such as phosphorus, boron, or copper, to reduce the electrical resistivity of Si and/or to change its properties including its phase transition, crystallinity, and morphology; 12−18 and the © XXXX American Chemical Society
application of an ionic liquid electrolyte to form a stable surface film.19−22 We have previously investigated various composite electrodes composed of elemental Si and a rareearth-metal silicide such as LaSi2/Si,23,24 mischmetal silicide (MmSi2)/Si (Mm is composed of four rare-earth elements (22% La, 60% Ce, 4% Pr, and 14% Nd in weight ratio)),25 and Gd−Si/Si,26 or of elemental Si and a base metal silicide such as Fe−Si/Si,25,27 NiSi2/Si,25 and VSi2/Si.25 We have also reported other materials for use as negative electrodes consisting of intermetallic compounds and elemental Si.28−31 These composite materials have been shown to require four main properties:32,33 (1) mechanical properties suitable for release of the stress due to Si, (2) low electronic resistivity, (3) moderate diffusing capability of Li+, and (4) high thermodynamic stability so that they will not decompose under repeated charge/ discharge cycles. The electrochemical performance differs depending on the kind of transition metal used in the silicide. Discharge capacities of various composite electrodes consisting of elemental Si and each transition metal at the initial and 1000th cycles are summarized in Table S1 in the Supporting Information.25 A LaSi2/Si composite electrode notably exhibits stable cycle performance; the electrode maintains ca. 43% of the initial capacity after the 1000th cycle. This is because LaSi2 Received: April 12, 2016 Revised: June 30, 2016
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Database (ICSD) was used to identify the obtained XRD patterns. A particle size distribution analyzer (SALD-2300, Shimadzu) was used to estimate the average particle size diameter of the obtained powders. 2.2. Electrode Preparation and Electrochemical Measurements. The working electrode was prepared by gas deposition (GD) method in a vacuum chamber equipped with a guide tube,30 and the detailed conditions of GD were described in our previous paper.12 A laboratory-made beakertype three-electrode cell was used for electrochemical measurements. The reference and counter electrodes consisted of Li sheets (Rare Metallic, 99.9%, thickness 1 mm). A 1 M lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) dissolved in propylene carbonate (PC, C4H6O3, Kishida Chemical Co., Ltd.) was used as the electrolyte solution. The cell assembly was performed in an Ar-filled glovebox (Miwa MFG, DBO2.5LNKP-TS) with a dew point below −100 °C. An electrochemical measurement system (HJ-1001SD8, Hokuto Denko Co., Ltd.) was used for a galvanostatic charge−discharge test. The test was carried out under a constant current rate of 2.8 C (current density 3.0 A g−1) unless otherwise stated. The potential range and temperature for the test were set between 0.005 and 2.000 V vs Li+/Li and at 303 K, respectively. In this study, 1 C is defined as 1.08 A g−1 because the weight ratio for LaNixSi2−x/Si composite is 70:30 and the theoretical capacity of Si is about 3.6 A h g−1. Since the capacity of LaNixSi2−x is very small, it was ignored here. 2.3. Calculation of the Charge Density and Li Migration Barrier. To investigate the behavior of Li+ migration in LaNixSi2−x (x = 0, 0.25), a first-principles calculation based on density functional theory (DFT) was performed using the projector augmented wave (PAW)34 method as implemented in the plane wave code of the Vienna Ab initio Simulation Package (VASP).35 The charge densities of some silicides were also evaluated based on a Bader analysis with a first-principles calculation.36−38 A generalized gradient approximation (GGA) was used as the term exchange correlation with a kinetic energy cutoff of 350 eV. Brillouin zone sampling was performed with a 8 × 8 × 8 k point mesh within a Gamma point centered mesh scheme. Eformation for Li insertion was calculated according to eq 1. The crystal structure and the lattice constant of LaSi2 for the calculation are shown in Figure S2.
remarkably reduces the electric resistivity of the Si negative electrode, releases the stress generated by the huge changes in volume of Si particles, and has high thermodynamic stability for a long charge−discharge cycle life.19−21,25 The improvement of the electrochemical performance of the LaSi2/Si composite electrode has been attempted using some ionic liquid electrolytes.24 As a result, the composite electrode exhibited better cyclability in a certain ionic liquid electrolyte than in an organic electrolyte. In this study, we consider that a ternary silicide that included another transition metal is effective for bringing out the potential of LaSi2/Si composite electrode to the maximum. We focus on the charge density of the transition metal to decide which third element should be included in the silicide. The charge density of Li0.25MSi2 (M = Ni, V, La, Gd, Sm, Dy) based on a first-principles calculation is shown in Table S2. While the charge density of Li in Li0.25MSi2 is positive independent of the kind of M, that of Si, except for Li0.25NiSi2, is negative. While most M’s have a positive charge density, Ni is only negative. These results indicate that Li has a high affinity for Si, except in Li0.25NiSi2, in which Li has a greater affinity for Ni than for Si, and hence, it is expected that nickel silicide (NiSi2) would have specific properties compared to other transition metal silicides. Figure S1 shows the first charge− discharge curve and the cycle performance of a NiSi2 electrode. Unlike in the case of other transition metal silicides such as VSi2, FeSi2, and MmSi2,25 clear potential plateaus were observed in the charge and discharge curves at about 0.1 and 0.4 V vs Li+/Li, respectively. The initial discharge capacity of a NiSi2 electrode was higher than that of other transition metal silicide electrodes.25 In addition, the electric resistivity of NiSi2 was 0.8 Ω cm, which is less than those of LaSi2 (2.2 Ω cm) and VSi2 (1.2 Ω cm). Based on these results, NiSi2 has specific properties of high reactivity with Li+ and high electronic conductivity. In the present study, elemental Ni was added to the silicide of LaSi2 as a third element to improve the electrochemical performance of a LaSi2/Si composite electrode; i.e., LaNixSi2−x/ Si powders were prepared. To evaluate the electrochemical performance of a LaNixSi2−x/Si composite electrode, the performance of a LaNixSi2−x electrode was also investigated. The charge density distribution of LaNixSi2−x was calculated based on computational chemistry to understand the affinity between each element. In addition, we discuss the migration barrier of Li in LaNixSi2−x on the basis of the formation energy (Eformation) for Li insertion.
Eformation = E(Li 0.25LaNixSi 2 − x) − [0.25E(Li) + E(LaNixSi 2 − x)]
(x = 0, 0.25) (1)
2. EXPERIMENTAL SECTION 2.1. Synthesis of LaNixSi2−x and LaNixSi2−x/Si Powders. Active material powders of LaSi2 alone and LaSi2/Si composite were synthesized by means of a mechanical alloying (MA) method.23−25 A mixture of elemental La chip and Si powder (stoichiometric ratio of 1:2 for LaSi2 alone and a weight ratio of 70:30 for LaSi2/Si composite) was placed in a ZrO2 pod along with balls. The weight ratio of the sample to the balls was about 1:15. Dry Ar gas was filled inside the pod. MA was conducted using a high-energy planetary ball mill (P-5, Fritsch) for 8 h with a rotary speed of 300 rpm at room temperature. Ni powder was added to LaSi2 alone and/or LaSi2/Si composite, and MA was continued for another 5 h. X-ray diffraction (XRD, Ultima IV, Rigaku) measurement was performed at a condition of 40 kV and 40 mA with Cu Kα radiation to verify the crystal structure of the powders. The Inorganic Crystal Structure
The Li migration barrier in LaNixSi2−x (x = 0, 0.25) was estimated on the basis of Eformation. If we assume that the Li site where Eformation is the smallest is a starting point, the Li migration barrier in LaNixSi2−x is defined as the change in Eformation when Li migrates to an adjacent stable site.
3. RESULTS AND DISCUSSION 3.1. Characterization of LaNixSi2−x and LaNixSi2−x/Si Powders. Figure 1A shows XRD patterns of LaNixSi2−x (x = 0−0.24) powders, and Figure 1B shows the expansion of Figure 1A between 41 and 44°. The resulting patterns were in good agreement with a LaSi2 tetragonal system (ICSD No. 01-0821924). While a single phase of LaNixSi2−x was confirmed in the range of x from 0 to 0.12, peaks assigned to a new crystal phase of the ternary compound (La3Ni3Si7) appeared when x had a B
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assigned to LaSi2 sharpened, peaks assigned to Ni did not appear. Consequently, it is considered that LaSi2 forms a solid solution with all of the added Ni; i.e., amorphous Ni should not remain after MA. Figure 2A shows XRD patterns obtained for LaNixSi2−x/Si (70:30 wt %, x = 0−0.10) powders with different amounts of
Figure 1. (A) XRD patterns of LaNixSi2−x (x = 0−0.24) powders and (B) magnified view of (A) between 41 and 44°. Cu powder was used as an internal standard.
maximum value of 0.24. Hereafter, we will focus on LaNixSi2−x (x = 0−0.12) except for the powder with x = 0.24, since it contained an impurity phase. It has been previously reported that LaSi2 forms a substitutional solid solution with Ni (LaNixSi2−x) until x is 0.12,39 which is consistent with the result obtained in this study. In addition, it was confirmed that there is no peak assigned to LaNi0.4Si1.6 (ICSD No. 01-0894014), which is similar to the composition of LaNixSi2−x (x = 0.06, 0.12, and 0.24). As shown in Figure 1B, the XRD peak of LaSi2(200) slightly shifted toward a higher angle with an increase in the degree of Ni substitution, which indicates a change in the lattice spacing. To verify this change, the lattice parameters of LaSi2 were calculated from the diffraction angles. The position of the diffraction peak was correctly determined by a calibration using an internal standard of copper. Figure S3 shows the relationship between the calculated lattice parameters and x, the amount of Ni substitution. The a and c axis lengths of the lattice constant decrease and increase, respectively, with an increase in x. The atomic radii of La and Si, which are the constituent elements of LaSi2, are 0.195 and 0.111 nm, respectively. It is considered that the partial Si atoms in LaSi2 can be replaced by Ni atoms and that LaNixSi2−x (x = 0−0.12) ought to be a substitutional solid solution, since the atomic radius of Ni (0.121 nm) is larger than that of Si, but still within 15%. On the other hand, La atoms in LaSi2 cannot be substituted by Ni atoms because the atomic radius of La is more than 60% larger than that of Ni. If we also consider the change in the a and c axis lengths of the lattice constant, the tetragonal system of LaSi2 ought to be strained. To confirm that LaSi2 forms a substitutional solid solution with all of the added Ni, the obtained powder of LaNi0.12Si1.88 was annealed under an Ar atmosphere at 773 K for 10 h. The temperature was increased and decreased at rates of 4.2 and −1.5 K min−1, respectively. The XRD patterns of the powder before and after annealing are shown in Figure S4. While the crystallinity of LaSi2 increased after annealing, since the peaks
Figure 2. (A) XRD patterns of LaNixSi2−x/Si (70/30 wt %, x = 0− 0.10) powders, and magnified views of (A) between (B) 41 and 43° and (C) 46.5 and 48.5°, respectively. Cu powder was used as an internal standard.
Ni (x) ranging from 0 to 0.10. Since x was within the range from 0 to 0.12, only peaks assigned to LaNixSi2−x and Si were confirmed; i.e., an impurity did not form. While the peak assigned to LaSi2(200) slightly shifted toward a higher angle with an increase in x, the peak position of Si(220) did not change (Figure 2B, C); only a Si site in LaSi2 was substituted by Ni, whereas elemental Si in the crystal structure was not substituted. In addition, the average particle size diameter of LaNixSi2−x/Si (70:30 wt %, x = 0 and 0.10) powders was 0.5− 0.7 μm, as shown in Figure S5; Ni substitution did not affect the particle size. 3.2. Electrochemical Performance of LaNixSi2−x Alone and LaNixSi2−x/Si Composite Electrodes. Figure 3A shows charge−discharge (lithiation−delithiation) curves of a LaNixSi2‑x (x = 0, 0.06, and 0.12) thick-film electrode during the first cycle in 1 M LiTFSA/PC. The charge−discharge capacities of LaNi0.06Si1.94 and LaNi0.12Si1.88 electrodes are expected to be greater than that of a LaSi2 electrode, reflecting the high reactivity of NiSi2 with Li+. However, in contrast to our expectations, the first capacity of the LaNixSi2−x (x = 0.06 and 0.12) electrode was less than that of a LaSi2 electrode. It is considered that the lattice of LaSi2 is strained because it forms a solid solution with Ni and that the number of sites for Li insertion in LaNixSi2‑x decreased. On the other hand, as shown in Figure 3B, a LaNixSi2−x (x = 0.06 and 0.12) electrode exhibited good high-rate performance. The capacity retention of a LaNixSi2−x (x = 0.06 and 0.12) electrode is higher than that C
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phase effectively promotes the lithiation and delithiation reactions of Si. Figure 5A shows the long-term cycle performance of a LaNixSi2−x/Si (70:30 wt %, x = 0, 0.05, and 0.10) composite
Figure 3. (A) First charge−discharge curves and (B) rate performance of LaNixSi2−x (x = 0−0.12) thick-film electrodes in 1 M LiTFSA/PC. Figure 5. Dependence of (A) discharge capacity and (B) Coulombic efficiency of LaNixSi2−x/Si (70/30 wt %, x = 0−0.10) thick-film electrodes on cycle number. The discharge capacity of LaNixSi2−x/Si electrodes was converted to capacity per unit mass of elemental Si.
of a LaSi2 electrode; when the current rate was 5.0 A g−1 (50fold greater that of the initial current density of 0.1 A g−1), the capacity retention of the LaNi0.06Si1.94 and LaNi0.12Si1.88 electrodes was ca. 57 and 60%, respectively, whereas that of the LaSi2 electrode was ca. 32%. The reason for the superior rate performance of LaNixSi2−x (x = 0.06 and 0.12) is discussed below based on computational chemistry. Figure 4 shows the charge−discharge curves of the first cycle for the LaNixSi2−x/Si (70:30 wt %, x = 0, 0.05, and 0.10)
thick-film electrode in 1 M LiTFSA/PC. To discuss the performance of elemental Si in the composite, the discharge capacity of a LaNixSi2−x/Si electrode was converted to capacity per unit mass of elemental Si. For comparison, the dependence of discharge capacity per mass of LaNixSi2−x/Si composite on the cycle number is shown in Figure S6. Figure 5B also shows the dependence of the Coulombic efficiency of the LaNixSi2−x/ Si electrode on the cycle number. Although the initial discharge capacities of the Si, LaSi2/Si, and LaNi0.05Si1.95/Si electrodes were almost the same, almost all of the discharge capacity of the Si electrode faded by the 100th cycle. On the other hand, the rapid decay of the discharge capacity was controlled little by little with an increment in the amount of added Ni. Notably, a LaNi0.10Si1.90/Si electrode showed the best cycle performance and the highest initial discharge capacity, which is almost the same as the theoretical capacity of a Si electrode. The discharge capacities of LaNi0.10Si1.90/Si, LaNi0.05Si1.95/Si, and LaSi2/Si electrodes at the 200th cycle were 2210, 1770, and 1410 mA h g(Si)−1, respectively. The capacity retention value was 61, 59, and 48% at 200 cycles for LaNi0.10Si1.90/Si, LaNi0.05Si1.95/Si, and LaSi2/Si electrodes, respectively. In addition, the LaNi0.10Si1.90/ Si electrode showed a discharge capacity of ca. 1000 mA h g(Si)−1 at the 500th cycle (data not shown). An initial drop in Coulombic efficiency was observed around the 40th cycle for the Si electrode in Figure 5B. Reductive decomposition of the electrolyte results in the formation of a surface film, which contributes to the decrement in Coulombic efficiency. The resulting surface film ought to collapse with the huge volumetric changes in Si during charge−discharge reactions, followed by a surface film should form again on the newly formed Si surface, and hence, the initial efficiency dropped. By contrast, the change in the volume of LaNixSi2−x/Si should be
Figure 4. First charge−discharge curves of LaNixSi2−x/Si (70/30 wt %, x = 0−0.10) thick-film electrodes in 1 M LiTFSA/PC at 2.8 C.
composite electrode. For all x, potential plateaus were observed at around 0.1 and 0.4 V vs Li+/Li on charge and discharge curves, respectively.12,40−42 Since the LaNixSi2−x electrode (x = 0, 0.06, and 0.12) did not show a discharge potential plateau (Figure 3A), the potential plateaus in Figure 4 are attributed to the lithiation and delithiation reactions of elemental Si in the LaNixSi2−x/Si composite. Unlike in the case of the LaSi2 electrode (Figure 3A), the first capacity of the LaSi2/Si composite electrode increased upon Ni substitution with Si in LaSi2. It is considered that the LaNi0.10Si1.90 which is a matrix D
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of LaSi2 and LaNi0.25Si1.75, respectively, which was illustrated by the Visualization for Electronic and STructural Analysis (VESTA) package.43 Figure 7C also shows the Li migration barrier in Li0.125LaNixSi2−x (x = 0, 0.25) when Li migrates between adjacent stable sites. While the Li migration barrier of LaSi2 is at a maximum of 0.27 eV, that of LaNi0.25Si1.75 is 0.14 eV; LaNi0.25Si1.75 exhibits a lower energy barrier for Li migration in the pathway than LaSi2. LaNi0.25Si1.75 is clearly more desirable for Li migration compared to LaSi2. The results of the calculation for smooth Li diffusion in Li0.125LaNixSi2−x corresponded with the high-rate performance of a LaNixSi2−x (x = 0.06 and 0.12) electrode, as shown in Figure 3B. To better understand the reason for the increase in the diffusing capability of Li in LaNixSi2−x (x = 0.06 and 0.12), the charge density of each silicide was investigated. Figure 8 and
suppressed because the LaNixSi2−x phase acts as an effective matrix for relaxation of the stress from Si, and because the proportion of Si in a LaNixSi2−x/Si electrode is lower than that in a Si electrode. Therefore, the surface film should not collapse and the LaNixSi2‑x/Si composite was more efficient than Si alone. Figure 6 shows the rate performance of a LaNixSi2−x/Si (70:30 wt %, x = 0 and 0.10) composite electrode in 1 M
Figure 6. Rate performance of thick-film electrodes consisting of LaNixSi2−x/Si (70/30 wt %, x = 0 and 0.10) or Si in 1 M LiTFSA/PC. The discharge capacity of composite electrodes was converted to capacity per unit weight of elemental Si.
LiTFSA/PC. The performance of a Si-alone electrode is also shown for comparison. The vertical axis represents the discharge capacity per mass of elemental Si. The LaSi 2 electrode showed better rate performance than the Si electrode. In addition, the discharge capacity of the LaNi0.10Si1.90/Si electrode was 2240 mA h g(Si)−1 even at a rate of 10 C, whereas a value of 2040 mA h g(Si)−1 was achieved for the LaSi2/Si electrode. When the rate was back to 0.4 C (60th cycle), the capacity retention of the Si electrode was only 42%. This poor rate performance originates from disintegration of the Si electrode because of the large change in volume during charge−discharge reactions. On the other hand, the capacity retentions of the LaNi0.10Si1.90/Si and LaSi2/Si electrodes were 88 and 87% at the 60th cycle, respectively; the LaNi0.10Si1.90/Si and LaSi2/Si electrodes did not disintegrate. 3.3. Li Migration Barrier and Charge Density Distribution Based on Computational Chemistry. To better understand the excellent electrochemical performance of LaNi0.10Si1.90/Si, the Li migration barrier and charge density distribution of LaSi2 with and without Ni substitution were investigated. Figure 7A, B shows a diffusion pathway between the most stable interstitial sites for Li in the tetragonal system
Figure 8. Charge density distributions of (A) Li0.25LaSi2, (B) Li0.25NiSi2, and (C) Li0.25LaNi0.25Si1.75.
Table 1 show the charge density distribution and its value for each transition metal silicide, respectively. Ni has a negative charge in Li0.25LaNi0.25Si1.75, as is the case with Li0.25NiSi2. In addition, the positive charges of Li and La in Li0.25LaNi0.25Si1.75 were slightly increased compared to those of Li0.25LaSi2, which indicates that Li and La show greater repulsion in
Figure 7. Diffusion pathway between the most stable interstitial sites for Li in crystal structures of (A) LaSi2 and (B) LaNi0.25Si1.75, and (C) change in formation energy of Li0.125LaNixSi2−x (x = 0, 0.25) calculated by GGA with PBE when Li migrates between adjacent stable sites. The formation energy at the most stable site was set to be 0. E
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Table 1. Charge Densities of Li0.25LaSi2, Li0.25NiSi2, and Li0.25LaNi0.25Si1.75 Li0.25LaSi2 Li0.25NiSi2 Li0.25LaNi0.25Si1.75
Li
La
Ni
Si
0.75 0.78 0.81
1.21 − 1.23
− −1.00 −0.76
−0.70 0.40 −0.71
Li 0.25 LaNi 0.25 Si 1.75 . Therefore, it is considered that Ni substitution introduces a great affinity of Ni for Li.
4. CONCLUSION The improvement of the electrochemical performance of a LaSi2/Si composite electrode for LIBs by Ni substitution was studied. The results of XRD analysis revealed that only the Si site in LaSi2 was substituted by Ni, while elemental Si in the crystal structure was not; LaSi2 forms a substitutional solid solution with Ni. While the charge−discharge capacity of a LaNixSi2−x electrode (x = 0.06 and 0.12) was lower than that of a LaSi2 electrode, the LaNixSi2−x electrode exhibited a high-rate performance. Ni substitution with a very low atomic concentration significantly improved the electrochemical performance of a LaSi2/Si (70:30 wt %) composite negative electrode; the LaNi0.10Si1.90/Si electrode exhibited superior long-term cycle and high-rate performances compared to Sialone and LaSi2/Si composite electrodes. Computational chemistry demonstrated that LaNi0.25Si1.75 is more favorable for Li migration in the pathway compared to LaSi2, and that Ni substitution introduces a great affinity of Ni for Li. Consequently, Ni substitution in a LaSi2/Si composite negative electrode helps to improve its electrochemical performance.
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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/acs.jpcc.6b03717. Electrochemical performance, lattice constants of LaSi2 and LaNixSi2−x (x = 0−0.12), XRD patterns before and after annealing, particle size distribution, comparison of discharge capacity, and charge density (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +81-857-31-5265. Fax: +81-857-31-5265. E-mail:
[email protected]. 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, Grants 24350094, 25620195, and 15K21166.
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REFERENCES
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