HIGH PERFORMANCE SILICON-BASED MATERIALS FOR LITHIUM ION BATTERY ANODES

Abstract

Amorphous silicon carbon composite particles contain, as components of the particles, 85 to 99.63 wt.-% silicon content, 0.3 to 15 wt.-% carbon content, and at least 0.07 wt.-% hydrogen content, where the components sum up to 100 wt.-%. The carbon content in the area beneath the surface of the particles, starting from the surface and reaching up to at least 30 nm from the surface in a direction to the centre of the particles, is at least 3 wt.-% higher than in the area of the centre of the particles. The area of the centre is the remaining part of the particles and is directly joined to the area beneath the surface.

Claims

1: Silicon carbon composite particles, comprising the components; 85 to 99.63 wt.-% of silicon content, 0.3 to 15 wt.-% of carbon content, at least 0.04 wt.-% of hydrogen content, and optionally, 0 to 1 wt.-% of oxygen content, wherein the components sum up to 100 wt.-%, and wherein the carbon content in an area beneath a surface of the particles, starting with the surface and reaching up to at least 30 nm from the surface in a direction to a centre of the particles, is at least 3 wt.-% higher than in an area of the centre of the particles, wherein the area of the centre of the particles is a remaining part of the particles and is directly joined to the area beneath the surface of the particles.

2: The silicon carbon composite particles according to claim 1, wherein the particles have a chlorine content below 0.7 ppm-wt.

3: The silicon carbon composite particles according to claim 1, wherein an average particle size of the particles is less than 300 nm.

4: The silicon carbon composite particles according to claim 1, wherein the particles have a content of amorphous silicon comprising hydrogen, a SiH-species, and/or (poly-[SiH.sub.2]—).

5: The silicon carbon composite particles according to claim 1, wherein the silicon content of the particles is at least 0.5 wt.-% to 30 wt.-% higher in the area of the centre of the particles than in the area beneath the surface of the particles.

6: The silicon carbon composite particles according to claim 5, wherein the silicon content of the particles is at least 2.0 to 15 wt.-% higher in the area of the centre of the particles than in the area beneath the surface of the particles.

7: The silicon carbon composite particles according to claim 1, wherein the carbon content of the particles is at least 3 wt.-% to 30 wt.-% higher in the area beneath the surface of the particles than in the area of the centre of the particles.

8: The silicon carbon composite particles according to claim 1, wherein the carbon content of the particles is from 2 wt.-% to 15 wt.-%.

9: The silicon carbon composite particles according to claim 1, wherein the particles have a content of at least one aliphatic carbon-hydrogen compound.

10: The silicon carbon composite particles according to claim 1, wherein the particles possess differential capacity (dQ/dV) versus voltage curves with a peak corresponding to crystalline Li.sub.15Si.sub.4 (c-Li.sub.15Si.sub.4) formation, whereby a ratio of an area of the peak to the area of the differential capacity (dQ/dV) in a range between 0.38 and 0.8 V is in a range of 0 to 0.1 during at least the first cycle.

11: A process for the production of silicon carbon composite particles according to claim 1, the process comprising: reacting a) a gaseous stream comprising at least one precursor silane selected from the group consisting of monosilane, disilane, trisilane, tetrasilane, pentasilane, hexasilane, heptasilane, octasilane, iso-tetrasilane, iso-pentasilane, neo-pentasilane, cyclopentasilane, cyclohexasilane a fully hydrogenated organosilane comprising 1 to 8 silicon atoms and 1 to 10 carbon atoms, and a mixture of at least two of the aforementioned silanes, and b) a gaseous stream comprising a precursor hydrocarbon selected from the group consisting of an olefinic hydrocarbon, an alkine, and a mixture of at least two of the aforementioned hydrocarbons, within a tubular reactor at a reaction temperature of 400 to 700° C., wherein a reaction mainly takes place within a heated tubular inert gas stream during a time frame of 500 milliseconds to 20 seconds, and optionally, quenching a reaction mixture with an inert gas possessing a lower temperature than the reaction mixture, and optionally collecting the silicon carbon composite particles.

12: The process according to claim 11, wherein in (b) the precursor hydrocarbon is at least one precursor olefinic hydrocarbon selected from the group consisting of an alkene comprising 1 to 10 carbon atoms, a cycloalkene comprising 1 to 10 carbon atoms, and a mixture of at least two of the aforementioned olefinic hydrocarbons.

13: The process according to claim 11, wherein the reaction takes place within a reaction zone that is located within the heated tubular inert gas stream during a time frame of 500 milliseconds to 20 seconds, and optionally, wherein the heated tubular inert gas stream possesses an outer circumference and an inner circumference, wherein the inner circumference is joined with the reaction zone which is within the heated tubular inert gas stream, and wherein a radial distance from the inner circumference to the outer circumference of the heated tubular inert gas stream has a ratio of 2:1 to 1:10, in relation to a radius of the reaction zone within the heated tubular inert gas stream.

14: The process according to claim 11, wherein the reaction takes place within a reaction zone that is located within the heated tubular inert gas stream during a time frame of 500 milliseconds to 20 seconds, and wherein the heated tubular inert gas stream possesses a temperature of about 400 to 520° C. when the precursor silane, the precursor olefinic hydrocarbon, or a mixture thereof is injected into the reaction zone of the tubular reactor.

15: The process according to claim 11, wherein the inert gas is selected from the group consisting of argon, helium, neon, and nitrogen.

16: The process according to claim 11, wherein the heated tubular inert gas stream has a volume flow of 1 to 10 Nm.sup.3/h.

17: An anode, comprising the silicon carbon composite particles according to claim 1 and optionally comprising a binder.

18: A battery, comprising at least one anode according to claim 17.

19: The silicon carbon composite particles according to claim 3, wherein the average particle size of the particles is less than 250 nm.

20: The battery according to claim 18, wherein the battery is a secondary lithium ion battery.

Description

[0100] FIG. 1a: Amorphous silicon carbon composite particles (Si/C 22) produced at 640° C. via gas-phase reaction in the hot-wall reactor.

[0101] FIG. 1b: Amorphous silicon carbon composite particles (Si/C 2) produced at 690° C. via gas-phase reaction in the hot-wall reactor.

[0102] FIG. 2a: .sup.13C-CP/MAS spectrum 9.091 pp/cm, 914.7 Hz/cm. No aromatic bonding is observed, the broad signal is purely due to the instrument background.

[0103] FIG. 2b: .sup.29Si-CP/MAS spectrum 9.091 ppm/cm, 727.7 Hz/cm.

[0104] FIG. 3a: Potential versus capacity in the first cycle measured at a rate of 0.2 C using the cutoff potentials of 0.01-2.5 V vs. Li/Li.sup.+ for pure crystalline Si (from Alfa Aesar, SSA about 28 m.sup.2/g),

[0105] FIG. 3b: Potential versus capacity in the first cycle measured at a rate of 0.2 C using the cutoff potentials of 0.01-2.5 V vs. Li/Li.sup.+ for amorphous Si produced at 640° C. (Si-am) in a gas-phase reactor,

[0106] FIG. 3c: Potential versus capacity in the first cycle measured at a rate of 0.2 C using the cutoff potentials of 0.01-2.5 V vs. Li/Li.sup.+ for amorphous silicon carbon composite particles (Si/C 22) produced at 640° C. in a gas-phase reactor,

[0107] FIG. 3d: Differential capacity curves dQ/dV obtained during dilithiation at a rate of 0.2 C using the cutoff potentials of 0.01-2.5 V vs. Li/Li.sup.+ for pure crystalline Si (from Alfa Aesar, SSA about 28 m.sup.2/g),

[0108] FIG. 3e: Differential capacity curves dQ/dV obtained during dilithiation at a rate of 0.2 C using the cutoff potentials of 0.01-2.5 V vs. Li/Li.sup.+ for amorphous Si produced at 640° C. (Si-am) in a gas-phase reactor,

[0109] FIG. 3f: Differential capacity curves dQ/dV obtained during dilithiation at a rate of 0.2 C using the cutoff potentials of 0.01-2.5 V vs. Li/Li.sup.+ for amorphous silicon carbon composite particles (Si/C 22) produced at 640° C. in a gas-phase reactor.

[0110] FIG. 3g: Differential capacity curves dQ/dV obtained during dilithiation at the first and second cycle at a rate of 0.2 C using the cutoff potentials of 0.005-1 V vs. Li/Li.sup.+ for amorphous silicon carbon composite particles (Si/C 15) produced at 665° C. in a gas-phase reactor.

[0111] FIG. 4a: X-ray diffraction patterns of silicon and amorphous silicon carbon composite particles with different carbon content synthesized at 640° C. with a residence time of 5 s.

[0112] FIG. 4b: X-ray diffraction patterns of pure amorphous silicon (Si-am) and amorphous silicon carbon composite powders (Si/C 14: 6.3 wt.-% C, Si/C 17:14.2 wt.-%) with different carbon contents synthesized at 640° C. with a residence time of 5 s and subsequently annealed at 690° C. for 1 h.

[0113] FIG. 4c: XPS carbon concentration depth profiles recorded from pure amorphous silicon (Si-am) and amorphous silicon carbon composite particles with different carbon content.

[0114] FIG. 4d: DSC signals collected from amorphous silicon (Si-am) and amorphous silicon carbon composite powders comprising 3.5 wt.-% carbon (Si/C 22), 6.3 wt.-% carbon (Si/C 14) and 14.2 wt.-% carbon (Si/C 17).

[0115] FIG. 5: The ratio of the area of the peak in the differential capacity corresponding to the formation of the crystalline c-Li.sub.15Si.sub.4, A[c-Li.sub.15Si.sub.4], to the area of the differential capacity (dQ/dV) in the range between 0.38 and 0.8 V, A[c-Li.sub.15Si.sub.4+a-Si″ ], for pure crystalline Si (from Alfa Aesar), amorphous silicon (Si-am) and amorphous silicon carbon composite particles.

[0116] FIG. 6a to d: Transmission electron microscopy (STEM) images and energy dispersive spectra (EDS) of silicon obtained from (a) agglomerated amorphous silicon nanoparticles after 270 full (de)lithiation cycles and (b) agglomerated amorphous silicon carbon composite particles after 310 full (de)lithiation cycles using the cutoff potentials of 0.05-1 V vs. Li/Li.sup.+. The nanoparticles are scrapped from electrodes in the delithiated state. The electrodes contain 60 wt.-% Si or amorphous silicon carbon composite particles, respectively, 15 wt. % carbon black, 10 wt.-% graphite and 15 wt.-% polyacrylic acid binder (PAA). Electrodes are assembled in a coin cell with a pure lithium counter electrode. As electrolyte solution, a mixture of 1 M LiPF.sub.6 in ethylene carbonate:ethyl methyl carbonate (3:7) with 2 wt.-% vinylene carbonate and 10 wt.-% fluoroethylene carbonate (FEC) is used. First cycle dilithiation capacity of the anodes is about 2900 mAh/g with the first cycle Coulombic efficiency of about 90%; a mass loading is 1.6-1.7 mg.sub.anode/cm.sup.−2.

[0117] FIG. 7: Coulombic efficiency vs number of cycles and specific capacity vs number of cycles of amorphous silicon carbon composites particles (amorphous Si/C, 100% amorphous) and crystalline silicon (100% crystalline). Dilithiation capacity (full symbols) and Coulombic efficiency (open symbols) of the fully crystalline silicon (circle symbols) and amorphous silicon carbon composite particles (square symbols).

[0118] The chemical and physical properties of the silicon carbon particles as well as their electrochemical properties as material in anodes of the FIGS. 1 to 7 are described below.

[0119] The silicon carbon particles were analysed by Rietveld analysis of XRPD, the silicon, carbon, hydrogen and oxygen contents were determined by elemental analysis (Table 2). XRPD confirmed that the particles were produced as amorphous particles, with the amount of the amorphous phase from 99 wt.-% to 100 wt.-%.

[0120] FIGS. 2a and 2b show solid state NMR spectra obtained from silicon carbon composite particles. No aromatic bonding is observed by .sup.13C-CP/MAS spectrum 9.091 pp/cm, 914.7 Hz/cm (FIG. 2a) and by .sup.29Si-CP/MAS spectrum 9.091 ppm/cm, 727.7 Hz/cm (FIG. 2b); the broad signal in the .sup.13C-CP/MAS spectrum is purely due to the instrument background. .sup.29Si-CP/MAS (CP: cross polarisation) possesses a very broad signal over circa 130 ppm that can be attributed according to literature (Lit: H. Brequel et al., J. of. Material Synthesis and Processing, 8, 5/6, 2000, 369-375) to the following fragments as proposed in table 3.

TABLE-US-00003 TABLE 3 .sup.29Si-CP/MAS signals and their correlation Chem. Shift SiC +: (attributed to fragment) .sup.29Si [ppm] Correlation (+): may be attributed to fragment −20 SiC.sub.4 + −80 SiSi.sub.4 (+)

[0121] With .sup.29Si (High performance decoupling) the observed signal is also broad but less intensive and shifted to a higher field (neg. chem. Shift). In .sup.13C-NMR spectra (CP/MAS, CP: cross polarisation) the signal occurs at +14 ppm with a Gaussian profile verifying pure aliphatic carbon signals. The broad signals are also typical for amorphous systems. Therefore, the .sup.29Si-NMR and .sup.13C-NMR spectra both prove that the amorphous silicon carbon composite particles are free from crystalline phases and free from polymeric SiC. Crystalline SiC possesses three very sharp signals at −16 ppm, −20 ppm and 27 ppm in .sup.29Si-NMR spectra as well as in .sup.13C-NMR spectra a set of three sharp signals at 27 ppm, 20 ppm and 15 ppm.

[0122] FIG. 3a-3g: The potential curves were measured on electrodes containing 60 wt.-% Si or amorphous silicon carbon composite particles, respectively, 20 wt. % acetylene black and 20 wt.-% polyimide binder (PI); a mass loading is 1.7-1.9 mg.sub.anode/cm.sup.−2. Electrodes are assembled in a coin cell with a pure lithium counter electrode. As electrolyte solution, a mixture of 1 M LiPF.sub.6 in ethylene carbonate:ethyl methyl carbonate (3:7) with 3 wt.-% vinylene carbonate is used. The numbers 1, 3, 10 and 20 correspond to the 1.sup.st, 3.sup.rd, 10.sup.th and 20.sup.th cycle. a-Si′ and a-Si″ mark dQ/dV peaks due to lithiated amorphous phases of various composition; c-Li.sub.15Si.sub.4 mark the position of the dQ/dV peak due to the crystalline c-Li.sub.15Si.sub.4 phase.

[0123] The electrochemical feature of the c-Li.sub.15Si.sub.4 formation is a characteristic voltage plateau near 0.42 V in the voltage versus capacity curves or a corresponding peak in the differential capacity curves dQ/dV at about 0.42 V during dilithiation (FIG. 3a, 3b, 3d, 3e). Both features are indicative of the first-order phase transformation and consequently large volume changes during the transformation and result in lower stability of the silicon-based particles.

[0124] Both crystalline and amorphous silicon show this plateau/peak at around 0.42 V indicating the formation of c-Li.sub.15Si.sub.4 (FIG. 3a, 3b, 3d, 3e).

[0125] In electrodes containing the silicon carbon composite particles (synonym to amorphous silicon carbon composite particles), the formation of c-Li.sub.15Si.sub.4 is effectively suppressed, as confirmed by the absence of the characteristic voltage plateau in the voltage versus capacity curves and absent or only a weak peak in the differential capacity dQ/dV curves around 0.42 V (FIG. 3c, 3f, 3g). On the contrary, a distinct peak in the differential capacity curves is observed near 0.42 V in the amorphous Si (Si-am) with the peak area being about four times that of amorphous silicon carbon composite particles (FIG. 3e); the peak area corresponds to the c-Li.sub.15Si.sub.4 amount formed during cycling.

[0126] The ratio of the area of the peak in the differential capacity corresponding to the formation of the crystalline c-Li.sub.15Si.sub.4, A[c-Li.sub.15Si.sub.4], to the area of the differential capacity (dQ/dV) in the range between 0.38 and 0.8 V, A[c-Li.sub.15Si.sub.4+a-Si″ ], for pure crystalline Si (from Alfa Aesar), amorphous silicon (Si-am) and amorphous silicon carbon composite particles is shown in FIG. 5 and Table 4 in dependence on cycle number. The ratio A[c-Li.sub.15Si.sub.4]/A[c-Li.sub.15Si.sub.4+a-Si″ ] is below 0.1 for the amorphous silicon carbon composite particles and remains essentially unchanged during cycling, whereas this ratio is between 0.1 and 0.37 in pure crystalline Si (from Alfa Aesar) and amorphous Si (Si-am). The ratio A[c-Li.sub.15Si.sub.4]/A[c-Li.sub.15Si.sub.4+a-Si″ ] for amorphous silicon carbon particles produced at 665° C. (Si/C 15) is zero, i.e. no formation of the crystalline c-Li.sub.15Si phase is observed (FIG. 3g).

TABLE-US-00004 TABLE 4 Differential capacity (dQ/dV) as ratio A[c-Li.sub.15Si.sub.4]/A[c-Li.sub.15Si.sub.4 + a-Si″] Ratio A[c-Li.sub.15Si.sub.4]/A[c-Li.sub.15Si.sub.4 + a-Si″] Cycle Pure crystalline Si number (from Alfa Aesar)* Si-am** Si/C 22 Si/C 15 1 0.17 0.22 0.07 0 3 0.23 0.32 0.08 0 10 0.18 0.30 0.06 Not measured 20 0.10 0.37 0.06 Not measured *100% crystalline, **100% amorphous

[0127] The first cycle Coulombic efficiency (CE) is retained at a high level of 89.2% in the amorphous silicon carbon composite powders; for comparison, the first cycle CE of Si-am is 90.2% and that of pure crystalline Si (from Alfa Aesar) is 77.2% (the corresponding lithiation/dilithiation curves are shown in FIG. 3).

[0128] Remarkably, the suppression of the c-Li.sub.15Si.sub.4 phase formation is observed in silicon carbon composite particles at particle sizes around 200 nm and without the addition of FEC (FIGS. 3c and 3f). These powders of silicon carbon composite particles have a low specific surface area (SSA) of about 6 m/g; a small specific surface (SSA) of electrode materials is a promising strategy for implementation in LIB. The addition of carbon in the silicon carbon particles of the invention disturbs the local atomic environment in the amorphous phase and thus suppresses the transformation to the crystalline c-Li.sub.15Si.sub.4 modification.

[0129] Thus, the design of silicon carbon composite particles represents the new concept for suppressing the c-Li.sub.15Si.sub.4 formation.

[0130] The reaction between silane and ethylene at 640° for 5 s leads to fully (100%) amorphous silicon carbon composite particles with a characteristic amorphous halo in the x-ray diffraction patterns independent of the silane and ethylene concentration in the gas mixture (FIG. 4a). Specifically, the intensity of the halo is decreased and its position shifts to higher diffraction angles with increasing carbon concentration indicating the formation of the silicon and carbon comprising amorphous phase.

[0131] To study the distribution of carbon within the silicon carbon composite particles (Si/C nanoparticles), XPS spectra of carbon were collected from the amorphous Si (Si-am) and amorphous silicon carbon composite particles and depth profiles were measured using sputtering. Amorphous Si (Si-am) shows a low but constant background carbon concentration, which is commonly observed for samples that have been handled under environmental conditions. In contrast, a clear carbon concentration gradient is observed in the silicon carbon composite particles. The higher the carbon content in the silicon carbon composite particles determined by elemental analysis, the steeper is the depth-dependent carbon concentration gradient. The structure of the silicon carbon composite particles can thus be described as C-lean in the particle center and C-rich closer to the particle surface. This particular Si/C structure in silicon carbon composite particles of the invention is distinctly different from the conventional Si+C composites, where Si is coated by/embedded in a carbon layer.

[0132] DSC curves of silicon and silicon carbon composite powders show several distinct features (FIG. 4d). The feature around 400° C. to 450° C. can be attributed to hydrogen desorption. An exothermic peak at 680° C. appears most prominent in samples without carbon or with a lower content of carbon. This peak indicates the crystallization of the amorphous silicon, as confirmed by x-ray diffraction analysis of the materials annealed at 690° C. for 1 h (FIG. 4b). Remarkably, the silicon carbon composite particles show a broader and less intense exothermic silicon crystallization signal compared to the amorphous Si (FIG. 4d). In addition, to amorphous Si, the silicon carbon composite particles exhibit an additional exothermic peak between 880 and 920° C. which can be attributed to the in-situ formation of the silicon carbide. The Rietveld analysis of the x-ray data shown in FIG. 4b reveals that the pure amorphous Si is completely crystallized after the annealing at 690° C. for 1 h. The amount of the crystalline phase in the silicon carbon composite powders after annealing is significantly lower than that in Si and decreases from 40 wt.-% to 20 wt.-% with increasing carbon concentration from 6.3 to 14.2 wt.-%, respectively. From the DSC measurements it can be concluded that the carbon is advantageous in suppressing the crystallization of the amorphous silicon-based materials, which is of particular importance for implementation of silicon carbon composite particles as anode material.

[0133] The presence of carbon in the amorphous silicon carbon composite particles modifies the dealloying reaction and has a stabilization effect against silicon particle degradation, as qualitatively shown by scanning TEM (STEM) analysis (FIG. 6a to 6d): annular dark-field (ADF) scanning transmission electron microscopy images and energy dispersive spectra (EDS) of the amorphous silicon carbon composite particles show a lower degree of porosity compared to the pure amorphous Si after 310 and 270 full (de)lithiation cycles with the cutoff potentials of 0.005-1 V vs. Li/Li.sup.+, respectively (electrodes were cycled to a similar degree of degradation in terms of specific capacity values). This example shows that in the amorphous silicon carbon composite particles the degree of degradation via dealloying reaction can be reduced and thus a better control or attenuation of the evolution of the surface area in the amorphous silicon carbon composite particle containing materials can be achieved.

[0134] The electrochemical capacity and the Coulombic efficiency of the amorphous silicon carbon composite particles were compared to those of the crystalline Si materials (Si-cr) (FIG. 7). The preparation of the electrodes was as follows: The electrochemical performance was evaluated in half cells using lithium metal as counter electrode. Silicon or silicon carbon composite particles were mixed with carbon (TIMCAL, Carbon black C45) and binder (poly acrylic acid) in a ratio of 80:5:15 weight percent. All ingredients were mixed together with an ethanol/water mixture for about 7 minutes in a centrifugal mixer to get a homogenous slurry. The slurry was the then doctor-bladed on copper sheets and dried at 60° C. for 16 h. The sheets were cut into round pieces with a diameter of 13 mm. The mass loading was in the range of 1 mg/cm. The electrodes were mounted in a glovebox in Swagelok T-cells. Lithium foil was used as the counter and reference electrode. The electrolyte was a mixture of 1 M LiPF6, ethylene carbonate (EC) and dimethyl carbonate (DMC) and fluoroethylene carbonate (FEC). Glass fiber material was used as a separator. All electrochemical measurements were performed at 25° C. and with a specific current corresponding to 0.05 C, 0.1 C and 0.5 C using a 4000 Battery Tester (Maccor).

[0135] The initial capacity is around 3500 mAh/g for pure crystalline silicon and about 3070 mAh/g in case of the amorphous silicon carbon composite particles. In both materials the capacity drops to about 2200 mAh/g after the second cycle. However, the Coulombic efficiency of amorphous silicon carbon composite particles is high in contrast to the pure crystalline silicon and reaches 85 and 92% for the first and second formation cycle, respectively, and levels at >99.5% in the subsequent cycles. We attribute the high Coulomb efficiency to reduced formation of a solid electrolyte interphase because of the carbon content, especially at the particle surface. The high initial capacity is due to the high overall content of silicon and indicates that the silicon is electrically well connected. The amorphous silicon carbon composite particles show a good performance during the first 120 cycles together with high specific capacity. After 120 cycles the capacity is at 1700 mA/g, whereas that for pure crystalline silicon is around 415 mAh/g.

[0136] Further modification of the synthesis of the silicon carbon composite particles at lower temperature in a well-defined reaction zone within a tubular inert gas stream, that surrounds the precursor silane and precursor olefinic hydrocarbon at a lower temperature of about 490° C. to 640° C. within a short time frame of 500 milliseconds to 20 seconds, preferred to 10 seconds results in silicon carbon particles of very uniform, spherical particles of a particle size of less than 250 nm, preferred 90 to 230 nm. The tubular inert gas stream is in particular a laminar inert gas stream. Due to the very specific reaction conditions, forming a reaction zone within the tubular inert gas stream of a temperature from 400° C. to 670° C., preferred from 430° C. to 500° C. where the gaseous stream comprising precursor silane and precursor olefinic hydrocarbon is injected into the reaction zone 100% amorphous silicon carbon particles comprising a carbon content decreasing from the area beneath the surface to the area of the centre of the particles can be obtained, which possess a high capacity of suppression of crystalline Li.sub.15Si.sub.4 phase formation.