HARD CARBON BEADS, THEIR PREPARATION, AND ENERGY STORAGE DEVICE COMPRISING THE SAME

Abstract

Provided are hard carbon beads, their preparation method, and an energy storage device comprising the same. Microwave heating is used to synthesize cross-linked phenolic formaldehyde for reducing energy consumption and controlling the crosslinking density of cured phenolic formaldehyde. The problems caused by high temperature heating and hydrothermal process for curing resin can be solved by the instant disclosure, which can increase the economic values of electrode and energy storage device comprising the hard carbon beads.

Claims

1. A method of preparing hard carbon beads comprising: step (a): dissolving phenol-formaldehyde resin, a cross-linking reagent and a protection reagent into a solvent to form a mixture and heating the mixture by microwave for cross-linking reaction to obtain a suspension containing phenol-formaldehyde beads, wherein the phenol-formaldehyde resin is set to be 100 parts by weight, the cross-linking reagent is greater than or equal to 5 parts by weight and less than or equal to 70 parts by weight, the protection reagent is greater than or equal to 1 part by weight and less than or equal to 10 parts by weight, a heating temperature of microwave is greater than or equal to 100° C. and less than or equal to 180° C., the cross-linking reagent is selected from the group consisting of: hexamethylenetetramine, formaldehyde acetal, furfural, furfural alcohol and trimethylol phosphine oxide, and the protection reagent is selected from the group consisting of: polyvinyl alcohol, methyl cellulose and polyoxyethylene polyoxypropylene; step (b): drying the suspension containing phenol-formaldehyde beads to obtain phenol-formaldehyde beads; and step (c): subjecting the phenol-formaldehyde beads for carbonization under inert gas to obtain the hard carbon beads, wherein a carbonization temperature is greater than or equal to 500° C. and less than or equal to 1500° C.

2. The method as claimed in claim 1, wherein the phenol-formaldehyde resin is set to be 100 parts by weight in step (a), and the cross-linking reagent is greater than or equal to 5 parts by weight and less than or equal to 50 parts by weight.

3. The method as claimed in claim 1, wherein the phenol-formaldehyde resin is set to be 100 parts by weight in step (a), and the cross-linking reagent is greater than or equal to 5 parts by weight and less than or equal to 30 parts by weight.

4. The method as claimed in claim 1, wherein the carbonization temperature in step (c) is greater than or equal to 600° C. and less than or equal to 1500° C.

5. The method as claimed in claim 1, wherein the carbonization temperature in step (c) is greater than or equal to 700° C. and less than or equal to 1200° C.

6. The method as claimed in claim 1, wherein the phenol-formaldehyde resin has a number average molecular weight greater than or equal to 200 and less than or equal to 10000.

7. The method as claimed in claim 1, wherein the phenol-formaldehyde resin is nitrogen-doped phenol-formaldehyde resin or phenol-formaldehyde resin without nitrogen doping.

8. The method as claimed in claim 1, wherein the solvent comprises 0 vol % to 100 vol % water and 0 vol % to 100 vol % alcohol, and the alcohol is methanol, ethanol or the combination thereof.

9. The method as claimed in claim 1, wherein the phenol-formaldehyde resin has a number average molecular weight greater than or equal to 200 and less than or equal to 3000 and the solvent is water, methanol or ethanol.

10. The method as claimed in claim 1, wherein the phenol-formaldehyde resin has a number average molecular weight greater than or equal to 2000 and less than or equal to 4000 and the solvent comprises 20 vol % to 80 vol % water and 20 vol % to 80 vol % methanol or ethanol.

11. A hard carbon bead having a maximum particle size and a minimum particle size, wherein a ratio of the maximum particle size to the minimum particle size is greater than or equal to 1 and less than or equal to 1.1.

12. The hard carbon bead as claimed in claim 11, wherein the ratio of maximum particle size to minimum particle size is greater than or equal to 1.021 and less than or equal to 1.098.

13. The hard carbon bead as claimed in claim 11, wherein the hard carbon bead has an average group particle size greater than or equal to 3.5 μm and less than or equal to 4.8 μm.

14. The hard carbon bead as claimed in claim 11, wherein the hard carbon bead has an average group particle size greater than or equal to 3.8 μm and less than or equal to 4.6 μm.

15. The hard carbon bead as claimed in claim 13, wherein the hard carbon bead has a standard deviation of particle size greater than or equal to 1.2 μm and less than or equal to 2.8 μm.

16. The hard carbon bead as claimed in claim 11, wherein the hard carbon bead has a graphitic length L.sub.a micro structure greater than or equal to 2.9 nm and less than or equal to 3.5 nm.

17. The hard carbon bead as claimed in claim 11, wherein the hard carbon bead has a graphitic length L.sub.a micro structure greater than or equal to 3.0 nm and less than or equal to 3.4 nm.

18. The hard carbon bead as claimed in claim 11, wherein the Raman spectrum of the hard carbon bead has D1 band and G band, and the ratio of the intensity of D1 band to the intensity of G band is greater than or equal to 2.0 and less than or equal to 2.5.

19. An energy storage device comprising a negative electrode and a lithium foil as a counter electrode, wherein the negative electrode comprises the hard carbon beads as claimed in claim 11, the negative electrode coupling with the counter electrode of lithium foil has a Galvanostatic charge-discharge curve, and the Galvanostatic charge-discharge curve comprises a plateau region ranging from 0.003 V to 0.12 V and a sloping area ranging from 0.12 V to 1.5 V, a specific capacity of the plateau region is greater than or equal to 90 mAh/g and less than or equal to 220 mAh/g, and a specific capacity of the sloping area is greater than or equal to 120 mAh/g and less than or equal to 320 mAh/g.

20. The energy storage device as claimed in claim 19, wherein the energy storage device has a total specific capacity greater than or equal to 280 mAh/g and less than or equal to 500 mAh/g.

Description

BRIEF DESCRIPTION OF THE DRAWING(S)

[0039] FIG. 1 is a flow chart of a method of preparing hard carbon beads.

[0040] FIG. 2A to FIG. 2L are field-emission scanning electron microscope (FE-SEM) images of Examples 1 to 12 in order.

[0041] FIG. 3A is a particle size distribution graph of Examples 1 to 3.

[0042] FIG. 3B is a particle size distribution graph of Examples 1, 4 and 7.

[0043] FIG. 4A is a high resolution transmission electron microscopy (HR-TEM) image of Example 1.

[0044] FIG. 4B is an image of partial enlargement of FIG. 4A.

[0045] FIG. 5A is an X-ray diffraction (XRD) graph of Examples 1 to 3.

[0046] FIG. 5B is an X-ray diffraction (XRD) graph of Examples 1, 4 and 7.

[0047] FIG. 6 is a Raman spectrum of Example 1.

[0048] FIG. 7A is a cyclic voltammetry graph of Examples 1B to 3B.

[0049] FIG. 7B is a cyclic voltammetry graph of Examples 1B, 4B and 7B.

[0050] FIG. 8A is a Galvanostatic charge/discharge cycle graph of Examples 1B to 3B.

[0051] FIG. 8B is a Galvanostatic charge/discharge cycle graph of Examples 1B, 4B and 7B.

[0052] FIG. 8C is a Galvanostatic charge/discharge cycle graph of Examples 2B, 5B and 8B.

[0053] FIG. 8D is a Galvanostatic charge/discharge cycle graph of Examples 3B, 6B and 9B.

[0054] FIG. 8E is a Galvanostatic charge/discharge cycle graph of Examples 11B and 12B.

[0055] FIG. 8F is a Galvanostatic charge/discharge cycle graph of Examples 10C.

[0056] FIG. 9A is a rate capability graph of Examples 1B to 3B.

[0057] FIG. 9B is a rate capability graph of Examples 1B, 4B and 7B.

[0058] FIG. 9C is a rate capability graph of Example 4B and Comparative Examples 1B and 2B.

[0059] FIG. 10 is a capacity retention graph of Example 4B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] Hereinafter, multiple examples are provided to illustrate the implementation of the method of preparing hard carbon beads of the instant disclosure, while multiple comparative examples are provided as comparison. A person having ordinary skill in the art can easily realize the advantages and effects of the instant disclosure from the following examples and comparative examples. The descriptions proposed herein are just preferable embodiments for the purpose of illustrations only, not intended to limit the scope of the instant disclosure. Various modifications and variations could be made in order to practice or apply the instant disclosure without departing from the spirit and scope of the instant disclosure.

[0061] Hard Carbon Bead

EXAMPLES 1 TO 10

[0062] Phenolic-formaldehyde resin was used as starting material of Examples 1 to 10. By microwave-assisted hydrothermal method, fast cross-linking reaction was carried out and phenolic-formaldehyde beads in ball shape were obtained. Then after carbonization at appropriate temperature, Examples of hard carbon beads were obtained. With reference to FIG. 1, the manufacturing process of Examples 1 to 10 was described as follows.

[0063] First, 846 g phenol (9 mol) and 1600 g of 30 wt % formaldehyde solution (16 mol) were mixed at 20° C. 38 wt % concentrated sodium hydroxide solution (about 110 ml) was added to aforesaid mixture with constant stirring and the pH value of the mixture was adjusted to be 9.5 to 10. Then the mixture was heated at 60° C. for 4 hours. After the reaction ended, the mixture was analyzed by titration and only 1 wt % formaldehyde was detected. Then aforesaid mixture was cooled down, about 5 wt % hydrochloric acid was added with constant stirring and the pH value of the mixture became almost neutral. Aforesaid mixture should not be acidic. Specifically, the pH value of the mixture should be about 7.2 to 7.5. Afterwards, water and excess phenol were evaporated under vacuum and the inner temperature reached about 60° C. Finally, 1360 g of viscous resin known as phenol-formaldehyde resin without nitrogen doping was obtained. Herein, the phenol-formaldehyde resin without nitrogen doping had a number average molecular weight around 200 and the viscosity around 35 cps.

[0064] Afterwards, 0.8 g phenol-formaldehyde resin without nitrogen doping was dissolved in 0.2 g methanol to obtain 1 g of 80 wt % phenol-formaldehyde resin solution without nitrogen doping. Phenol-formaldehyde resin without nitrogen doping was set to be 100 parts by weight. 5 parts by weight polyvinyl alcohol and 10 parts by weight to 50 parts by weight HMTA (vendor: SHOWA) were added to 1 g of 80 wt % phenol-formaldehyde resin solution without nitrogen doping. 19 ml deionized water was added to aforesaid mixture and the mixture was transferred to a reaction tube. The reaction tube was sealed with a lid and placed in a focused microwave reactor (model: CEM discover). The temperature of the focused microwave reactor was about 130° C., the power of the focused microwave reactor was about 60 watt, and the time of heating and constant stirring was 20 minutes. By microwave-assisted hydrothermal method, fast cross-linking reaction was carried out and a suspension containing phenol-formaldehyde beads without nitrogen doping was obtained.

[0065] Afterward, aforesaid suspension containing phenol-formaldehyde beads without nitrogen doping was cooled down to room temperature. Yellow phenol-formaldehyde beads without nitrogen doping were significantly suspended within aforesaid suspension. The suspension containing phenol-formaldehyde beads without nitrogen doping was rinsed by acetone two to three times and then aforesaid suspension was filtered. The solid product was obtained and placed in an 80° C. oven. After drying for 24 hours, phenol-formaldehyde beads without nitrogen doping were obtained.

[0066] Finally, aforesaid phenol-formaldehyde beads without nitrogen doping were put in a high-temperature furnace. The temperature of the high-temperature furnace was set to be about 600° C. to 1000° C. The temperature of the high-temperature furnace was raised at the speed of 1° C. to 50° C. per minute to the target temperature in nitrogen atmosphere. Carbonization was carried out at the target temperature for 6 hours and each Example of hard carbon beads was obtained.

[0067] The differences between Examples 1 to 10 were recruitment of HMTA and carbonization temperature. The parameters were shown in Table 1.

TABLE-US-00001 TABLE 1 recruitment of phenol-for maldehyde resin and cross-linking reagent and caronization temperature of Examples 1 to 10 (E1 to E10) Amount of phenol- Amount of cross- formaldehyde resin linking reagent Carbonization (part by weight) (part by weight) temperature E1 100 10  800° C. E2 100 25  800° C. E3 100 50  800° C. E4 100 10  900° C. E5 100 25  900° C. E6 100 50  900° C. E7 100 10 1000° C. E8 100 25 1000° C. E9 100 50 1000° C. E10 100 10  600° C.

EXAMPLES 11 TO12

[0068] Examples 11 to 12 of hard carbon beads were rapidly produced by a similar microwave-assisted hydrothermal method and carbonization as stated in Examples 1 to 10. The differences were described as follows. (1) Different phenol-formaldehyde resin was used as starting material. (2) Polyvinyl alcohol was not added. The manufacturing process of Examples 11 to 12 was described as follows.

[0069] First, 0.8 g nitrogen-doped phenol formaldehyde resin (vendor: DIC Corporation, model: PHENOLITE LA-1356) was dissolved in methanol to obtain 1 g of 80 wt % nitrogen-doped phenol-formaldehyde resin solution. Nitrogen-doped phenol-formaldehyde resin was set to be 100 parts by weight. 70 parts by weight HMTA (vendor: SHOWA) was added to 1 g of 80 wt % nitrogen-doped phenol formaldehyde resin solution. 9.5 ml ethanol and 9.5 ml deionized water (total 19 ml liquid) were added to aforesaid mixture and the mixture was transferred to a reaction tube. The reaction tube was sealed with a lid and placed in a focused microwave reactor (model: CEM discover). The temperature of the focused microwave reactor was about 130° C., and the time of heating and constant stirring was 20 minutes. By microwave-assisted hydrothermal method, fast cross-linking reaction was carried out and a suspension containing nitrogen-doped phenol-formaldehyde beads was obtained.

[0070] Afterward, aforesaid suspension containing nitrogen-doped phenol-formaldehyde beads was cooled down to room temperature. Yellow nitrogen-doped phenol-formaldehyde beads were significantly suspended within aforesaid suspension. The suspension containing nitrogen-doped phenol-formaldehyde beads was rinsed by acetone two to three times and then aforesaid suspension was filtered. The solid product was obtained and placed in an 80° C. oven. After drying for 24 hours, nitrogen-doped phenol-formaldehyde beads were obtained.

[0071] Finally, aforesaid nitrogen-doped phenol-formaldehyde beads were put in a high-temperature furnace. The temperature of the high-temperature furnace was set to be about 900° C. to 1000° C. The temperature of the high-temperature furnace was raised at the speed of 1° C. to 50° C. per minute to the target temperature in nitrogen atmosphere. Carbonization was carried out at the target temperature for 6 hours and Examples 11 and 12 of hard carbon beads were obtained.

[0072] The difference between Examples 11 and 12 was the carbonization temperature. The carbonization temperature was 900° C. in Example 11 and the carbonization temperature was 1000° C. in Example 12.

TEST EXAMPLE 1

Analysis of Particle Size

[0073] Field-emission scanning electron microscope (vendor: Hitachi, model: SU8010) was used to observe Examples 1 to 12 of hard carbon beads in the test example. The results were shown as FIG. 2A to FIG. 2L in order. As shown in FIG. 2A to FIG. 2L, Examples 1 to 12 of hard carbon beads obtained by aforesaid manufacturing process were classic ball type hard carbon materials.

TEST EXAMPLE 1-1

Average Particle Size of Single Particle and Standard Deviation of Particle Size of Single Particle

[0074] IC Measure was used to analyze FIG. 2A to FIG. 2L. Three hard carbon beads were chosen as samples in each FIG. 100-time measurements were conducted from different angles to render the particle size of each hard carbon bead. Three hard carbon beads were a big hard carbon bead (with particle size ranging from 6 μm to 8 μm), a medium hard carbon bead (with particle size ranging from 5 μm to 6 μm), and a small hard carbon bead (with particle size ranging from 4 μm to 5 μm) respectively. Average particle size of single particle, standard deviation of particle size of single particle and ratio of maximum particle size to minimum particle size were rendered by 100 measurements of the big hard carbon bead, 100 measurements of the medium hard carbon bead, and 100 measurements of the small hard carbon bead. The results were shown as Table 2.

TABLE-US-00002 TABLE 2 average particle size of single particle, standard deviation of particle size of single particle and ratio of maximum particle size to minimum particle size of three kinds of particle sizes in FIG. 2A to FIG. 2L E1 Sample Big Medium Small Average particle size 7.5145 5.6196 4.6109 of single particle(μm) Standard deviation of particle 0.0959 0.0844 0.0872 size of single particle(μm) (Max particle size)/ 1.061 1.075 1.082 (Min particle size) E2 Sample Big Medium Small Average particle size 7.4525 5.6892 4.2131 of single particle(μm) Standard deviation of particle 0.0841 0.0826 0.0941 size of single particle(μm) (Max particle size)/ 1.021 1.054 1.046 (Min particle size) E3 Sample Big Medium Small Average particle size 7.1244 5.4892 4.1563 of single particle(μm) Standard deviation of particle 0.0845 0.0726 0.0941 size of single particle(μm) (Max particle size)/ 1.035 1.071 1.056 (Min particle size) E4 Sample Big Medium Small Average particle size 7.4122 5.2416 4.2394 of single particle(μm) Standard deviation of particle 0.0852 0.0946 0.0975 size of single particle(μm) (Max particle size)/ 1.033 1.059 1.045 (Min particle size) E5 Sample Big Medium Small Average particle size 7.3243 5.2466 4.6289 of single particle(μm) Standard deviation of particle 0.0963 0.0945 0.0961 size of single particle(μm) (Max particle size)/ 1.084 1.065 1.074 (Min particle size) E6 Sample Big Medium Small Average particle size 7.1023 5.3642 4.1256 of single particle(μm) Standard deviation of particle 0.0841 0.0785 0.0772 size of single particle(μm) (Max particle size)/ 1.023 1.056 1.044 (Min particle size) E7 Sample Big Medium Small Average particle size 7.5124 5.0211 4.6987 of single particle(μm) Standard deviation of particle 0.0855 0.0841 0.0942 size of single particle(μm) (Max particle size)/ 1.054 1.069 1.085 (Min particle size) E8 Sample Big Medium Small Average particle size 7.5142 5.2314 4.5866 of single particle(μm) Standard deviation of particle 0.0845 0.0956 0.0825 size of single particle(μm) (Max particle size)/ 1.045 1.066 1.098 (Min particle size) E9 Sample Big Medium Small Average particle size 7.3654 5.3255 4.6236 of single particle(μm) Standard deviation of particle 0.0586 0.0428 0.0652 size of single particle(μm) (Max particle size)/ 1.032 1.087 1.062 (Min particle size) E10 Sample Big Medium Small Average particle size 7.5823 5.0244 4.3289 of single particle(μm) Standard deviation of particle 0.0521 0.0854 0.0751 size of single particle(μm) (Max particle size)/ 1.095 1.068 1.072 (Min particle size) E11 Sample Big Medium Small Average particle size 6.3255 5.4122 4.6858 of single particle(μm) Standard deviation of particle 0.0856 0.0821 0.0925 size of single particle(μm) (Max particle size)/ 1.085 1.073 1.062 (Min particle size) E12 Sample Big Medium Small Average particle size 6.5546 5.3245 4.3328 of single particle(μm) Standard deviation of particle 0.0868 0.0841 0.0855 size of single particle(μm) (Max particle size)/ 1.091 1.083 1.088 (Min particle size)

[0075] As shown in Table 2, standard deviation of particle size of single particle of Examples 1 to 12 in the three kinds of particle sizes was less than 0.1 μm, which indicated that the values of particle size were centralized. The ratio of maximum particle size to minimum particle size of Examples 1 to 12 in the three kinds of particle sizes was greater than 1 and less than 1.1, which confirmed that Examples 1 to 12 of hard carbon beads were close to ideal ball type and different from conventional flake carbon material or lump carbon material.

TEST EXAMPLE 1-2

Average Group Particle Size and Standard Deviation of Particle Size

[0076] Afterwards, field-emission scanning electron microscope and IC Measure were used to analyze the polydispersity index (PDI) of FIG. 2A to FIG. 2L. The particle size distribution of Examples 1 to 12 of hard carbon beads was rendered. Average group particle size and standard deviation of particle size of Examples 1 to 12 were rendered by the particle size distribution. Aforesaid standard deviation was rendered by 100 samples of each Example. The results were shown in Table 3. To investigate the effect of recruitment of cross-linking reagent to particle size distribution of hard carbon beads, particle size distribution of Examples 1 to 3 was shown together in FIG. 3A. Besides, to investigate the effect of carbonization temperature to particle size distribution of hard carbon beads, particle size distribution of Examples 1, 4 and 7 was shown together in FIG. 3B.

TABLE-US-00003 TABLE 3 average group particle size and standard deviation of particle size of Examples to 12 (E1 to E12) Average group Standard deviation particle size (μm) of particle size (μm) E1 4.16 1.49 E2 4.56 1.36 E3 3.89 2.51 E4 4.33 1.22 E5 4.21 1.42 E6 4.14 2.34 E7 4.51 2.01 E8 4.35 2.14 E9 4.02 2.65 E10 4.22 1.23 E11 4.52 1.24 E12 4.35 1.42

[0077] The results in Table 3 and FIG. 3A were further analyzed. The analysis result of Examples 1 to 9 indicated that as the recruitment of HMTA grew up, the hard carbon beads had slightly less average group particle size and greater standard deviation of particle size. The results in Table 3 and FIG. 3B were further analyzed. The analysis result indicated that as the carbonization temperature increased, the hard carbon beads had greater average group particle size. The results of standard deviation of particle size of Examples 1, 4 and 7 indicated that Example 7 had the highest standard deviation of particle size among the three due to the highest carbonization temperature.

TEST EXAMPLE 2

Closed Micropore

[0078] The number of closed micropores in hard carbon beads and storage capacity of lithium ion and sodium ion were highly related. Transmission electron microscope (vendor: JEOL, model: JEM 2100F) was used to observe Example 1 of hard carbon beads in the test example. The HR-TEM and the partial enlargement thereof were shown as FIG. 4A and FIG. 4B. As shown in FIG. 4A and FIG. 4B, Example 1 of hard carbon bead had classic non-graphitizable carbon structure. That is, Example 1 had both amorphous areas and partially graphitized areas. There were micro-graphitic structures surrounded by nm-scale closed micropores shown as circle marks in FIG. 4B.

TEST EXAMPLE 3

Crystal Structure

[0079] X-ray diffractometer (vendor: Bruker, model: D8 Advance) was used to identify the crystal structures of Examples 1 to 4, and 7. A Cu Kα radiation source (λ=1.5405 Å) was used to measure aforesaid crystal structures. The results were shown as FIG. 5A and FIG. 5B.

[0080] The 2θ diffraction position of facet (002) and facet (100) ranged from 23° to 25° and 43° to 45° respectively. As shown in FIG. 5A and FIG. 5B, the characteristic peaks of facet (002) and facet (100) of Examples 1 to 4 and 7 were weak and broad compared to the ones of conventional graphitic material (MCMB), which indicated that aforesaid Examples had lots of amorphous areas, few graphene sheets and hollow closed micropores.

[0081] To investigate the influence of recruitment of cross-linking reagent and carbonization temperature to micro-graphitic structure of hard carbon bead, X-ray diffraction curves of Examples 1 to 4 and 7 were combined into FIG. 5A and FIG. 5B according to different categories. As shown in FIG. 5A, the 2θ diffraction position of facet (002) did not shift as cross-linking density of phenol-formaldehyde resin changed, which indicated nm-scale structure of hard carbon bead was basically not influenced by cross-linking density. As shown in FIG. 5B, the 2θ diffraction position of facet (002) shifted to right side (toward theoretical 2θ diffraction position of facet (002) which is 26.7°) as carbonization temperature increased. The results indicated that as carbonization temperature increased, the obtained nm-scale structure of hard carbon beads tended to form a perfect micro-graphitic structure.

[0082] Further analysis of XRD of Examples 1 to 9 was conducted to obtain 2θ.sub.002 of facet (002) and 2θ.sub.100 of facet (100). Interlayer d-spacing (d.sub.002) was rendered by Bragg's equation: nλ=2d.sub.002 sin θ.sub.002. L.sub.c (graphitic length along c-axis) was rendered by Scherrer's equation: L.sub.c=0.89λ/β.sub.002 cos θ.sub.002. β.sub.002 was the full width at half maximum of facet (002) characteristic peak. L.sub.a (graphitic length along a-axis) was rendered by Scherrer's equation: L.sub.a=1.84λ/β.sub.100 cos θ.sub.100. β.sub.100 was the full width at half maximum of facet (100) 5 characteristic peak. The results were shown in Table 4.

[0083] Besides, Raman spectra (model: JY HR 800) of Examples 1 to 9 were used to analyze the disordering degree of carbon structure. Origin (fitting mode: Gaussian) was used to carry out fitting analysis. As shown in FIG. 6, D4 band (1210 cm.sup.−1) was contributed by sp.sup.3 bonds of C-C stretching, D1 band (1350 cm.sup.−1) was contributed by defect of carbon structure, D3 band (1520 cm.sup.−1) was contributed by sp.sup.2 bonds of C-C stretching, and G band (1600 cm.sup.−1) was contributed by micro-graphitic structure.

[0084] The ratio of the intensity of D1 band to the intensity of G band (I.sub.D1/I.sub.G) in Raman spectrum of each Example was used to evaluate the disordering degree of carbon structure. Aforesaid intensity of D1 band and intensity of G band were calculated by the signal areas of aforesaid two bands. The results were shown in Table 4.

TABLE-US-00004 TABLE 4 2θ.sub.002   d.sub.002   L.sub.c   2θ.sub.100   L.sub.a and I.sub.D1/I.sub.G of Examples 1 to 9 (E1 to E9) 2θ.sub.002(°) d.sub.002(nm) L.sub.c(nm) 2θ.sub.100(°) L.sub.a(nm) I.sub.D1/I.sub.G E1 22.442 0.39614 0.9597 43.878 3.0112 2.447 E2 22.451 0.39599 0.9479 43.965 3.0082 2.367 E3 22.475 0.39557 0.9836 44.049 2.9311 2.283 E4 22.608 0.39328 0.9772 43.976 3.2704 2.434 E5 22.631 0.39289 0.9459 43.922 3.2584 2.312 E6 22.658 0.39242 0.9852 43.996 3.1501 2.241 E7 22.935 0.38775 0.9371 43.982 3.3543 2.308 E8 22.937 0.38780 0.9629 44.041 3.3234 2.198 E9 22.938 0.38782 0.9500 44.005 3.3021 2.145

[0085] As shown in Table 4, as cross-linking density of phenol-formaldehyde resin grew up, I.sub.D1/I.sub.G of hard carbon beads after carbonization was decreased. Aforesaid phenomenon indicated that the disordering degree was decreased. As shown in Table 4, each Raman spectrum of Examples 1 to 9 had D1 band and G band and the ratio of the intensity of D1 band to the intensity of G band was greater than or equal to 2.0 and less than or equal to 2.5.

[0086] As shown in Table 4, as cross-linking density of phenol-formaldehyde resin grew up, the graphitic length L.sub.a micro structure was decreased due to the influence of closed micropore of micro-graphitic structure. As shown in Table 4, the graphitic length L.sub.a micro structure of Examples 1 to 9 was greater than or equal to 2.9 nm and less than or equal to 3.5 nm.

[0087] Negative Electrode

EXAMPLES 1A TO 12A

[0088] Hard carbon beads of Examples 1 to 12 were used as negative electrode active material for preparing negative electrodes of Examples 1A to 12A. Each negative electrode of lithium ion batteries was obtained through following manufacturing process. The manufacturing process of Examples 1A to 12A was described as follows.

[0089] First, aforesaid hard carbon beads, carbon black (model: XC-72) and polyvinylidene fluoride (PVDF) were mixed at weight ratio of 85:5:10. Aforesaid mixture was dissolved in N-methyl-2-pyrrolidone (NMP) and the weight ratio of solid to liquid was 1:8. Then the negative electrode slurry was obtained.

[0090] Afterwards, the negative electrode slurry was coated on a copper foil by doctor blade method. The amount of hard carbon beads on the copper foil was around 2 mg per square centimeter. Then the copper foil coated with negative electrode slurry was placed in 80° C. vacuum environment for 24-hour drying to obtain a negative electrode containing hard carbon beads.

COMPARATIVE EXAMPLE 1A AND 2A

[0091] Negative electrodes of Comparative Examples 1A and 2A and Negative electrodes of Examples 1A to 12A were almost the same. The differences were that commercial mesocarbon microbeads (MCMB, vendor: China Steel Chemical Corporation, model: MG11) was used as negative electrode active material in Comparative Example 1A and that commercial soft carbon (vendor: China Steel Chemical Corporation, model: MSC-2) was used as negative electrode active material in Comparative Example 2A.

[0092] Lithium Ion Battery

EXAMPLES 1B TO 9B, 11B TO 12B

[0093] Negative electrodes of Examples 1A to 9A and 11A to 12A with same kind of positive electrode and electrolyte were used for preparing lithium ion batteries of Examples 1B to 9B and 11B to 12B respectively. The manufacturing process of Examples 1B to 9B and 11B to 12B were described as follows.

[0094] Coin cell (model: CR2032) was used and aforesaid negative electrode and lithium foil (counter electrode) were divided by a glass fiber (grade: GF/A). Lithium hexafluorophosphate (LiPF.sub.6) was dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (volume ratio: 1:1:1) to obtain 1M LiPF.sub.6 solution as electrolyte. 1 wt % vinylene carbonate (VC) was added in aforesaid electrolyte. 80 μl aforesaid electrolyte was added to the coin cell in argon atmosphere (contents of moisture and oxygen less than 0.1 ppm) to obtain lithium ion batteries of Examples 1B to 9B and 11B to 12B. The content of water in aforesaid electrolyte was less than 10 ppm by measurement of Karl Fisher titration.

COMPARATIVE EXAMPLES 1B TO 2B

[0095] Lithium ion batteries of Comparative Examples 1B to 2B and lithium ion batteries of Examples 1B to 9B and 11B to 12B were almost the same. The differences were that negative electrode of Comparative Example 1A was used in Comparative Example 1B and that negative electrode of Comparative Example 2A was used in Comparative Example 2B.

[0096] Sodium Ion Battery

EXAMPLE 10C

[0097] Sodium ion battery of Example 10C was almost the same as lithium ion battery of Example 1B. The differences were that negative electrode of Example 10A was used in sodium ion battery of Example 10C and that the electrolyte in Example 10C was 1M sodium hexafluorophosphate (NaPF.sub.6) solution. Aforesaid 1M NaPF.sub.6 solution was obtained by NaPF.sub.6 dissolving in the mixture of EC, EMC and DMC (volume ratio: 1:1:1).

TEST EXAMPLE 5

Cyclic Coltammetry

[0098] Lithium ion batteries of Examples 1B to 4B and 7B were used as samples in this test example. The cyclic voltammetry was conducted between 0.003 V and 1.5V (vs. Li.sup.+/Li) at scanning rate of 1 mV/s and atmosphere of 1 atm. The results were shown in FIGS. 7A and 7B.

[0099] As shown in FIGS. 7A and 7B, the cyclic voltammetry curves of Examples 1B to 4B and 7B were similar, which indicated that mechanisms of lithium ion storage of aforesaid Examples were similar and lithium ion storage started around 0.8 V. As shown in FIG. 7B, Example 4B has the best efficiency of lithium ion storage.

TEST EXAMPLE 6

Galvanostatic Charge/Discharge Cycle, GCD Cycle)

[0100] Lithium ion batteries of Examples 1B to 9B and 11B to 12B and sodium ion battery of Example 10C were used as samples in this test example. GCD cycle was conducted at a current density of 50 mA/g (0.2C), voltage ranging between 0.003 V and 1.5 V (vs. Li.sup.+/Li), and atmosphere of 1 atm. The results were shown in FIG. 8A to FIG. 8F. As shown in FIG. 8A to FIG. 8F, the GCD curves comprised plateau region (about 0.003 V to 0.12 V) and sloping area (about 0.12 V to 1.5 V). The specific capacities of plateau region and sloping area were shown in Table 5 respectively.

[0101] FIG. 8A and FIG. 8B were taken as examples. As shown in FIG. 8A, Examples 1B to 3B had the similar slope of sloping area in GCD curves, which indicated that the greater specific capacity of plateau region contributed to higher total specific capacity of lithium ion battery. As shown in FIG. 8B, the slope of sloping area in Examples 1B and 4B was less than the one in Example 7B, which indicated that the specific capacity of sloping area in Examples 1B and 4B was greater than the one in Example 7B. The GCD curve of Example 4B had the most significant plateau region among the three, which indicated that the total specific capacity of Example 4B was the highest among the three.

TABLE-US-00005 TABLE 5 specific capacity of plateau region, specific capacity of sloping area, total specific capacity and initial Faraday efficiency (FE) of GCD curves of Examples 1B to 9B and 11B to 12B and Example 10C (current density: 0.2C, unit of specific capacity: mAh/g) Specific Specific capacity of capacity of plateau region sloping area Total specific Initial FE (mAh/g) (mAh/g) capacity (mAh/g) (%) E1B 171 290 461 64 E2B 137 277 414 60 E3B 97 276 373 56 E4B 200 280 480 65 E5B 180 248 428 61 E6B 142 223 365 58 E7B 109 216 325 60 E8B 106 205 311 61 E9B 100 197 297 59 E11B 110 240 350 45 E12B 140 198 338 44 E10C 100 130 230 73

[0102] As shown in Table 5, cross-linking density of phenol-formaldehyde resin had great influence on formation of closed micropore. Hard carbon bead carbonized by phenol-formaldehyde resin with low cross-linking density had more micro-graphitic structures and/or closed micropores, which was beneficial to lithium ion storage.

[0103] As shown in Table 5, cross-linking density of phenol-formaldehyde resin had decisive influence on specific capacity of plateau region. The specific capacity of plateau region of Example 4B was 200 mAh/g, and the total specific capacity of Example 4B was 480 mAh/g.

[0104] As shown in Table 5, GCD curves of Examples 1B to 9B and 11B to 12B had significant plateau region and sloping area. The specific capacity of aforesaid plateau region was greater than or equal to 90 mAh/g and less than or equal to 220 mAh/g, the specific capacity of aforesaid sloping area was greater than or equal to 180 mAh/g and less than or equal to 320 mAh/g, and the total specific capacity was greater than or equal to 280 mAh/g and less than or equal to 500 mAh/g.

[0105] As shown in FIG. 8F, the hard carbon bead of the instant disclosure served as negative electrode could apply to the sodium ion battery. The GCD curve of aforesaid sodium ion battery had significant plateau region, which indicated the hard carbon bead of the instant disclosure had the ability to store sodium ion. The sodium ion battery of Example 10C had total specific capacity of 230 mAh/g.

[0106] In one of the embodiments, the initial Faraday efficiency of lithium ion batteries of Examples 1B to 9B and 11B to 12B may be raised by pre-lithiation of negative electrode.

TEST EXAMPLE 7

Rate Capability

[0107] Lithium ion batteries of Examples 1B to 4B and 7B and Comparative Examples 1B to 2B were used as samples in this test example. The test was conducted at current density ranging from 50 mA/g to 2500 mA/g (0.2C to 10C) and atmosphere of 1 atm. The results were shown in FIG. 9A to FIG. 9C. As shown in FIG. 9A to FIG. 9B, Examples 1B to 4B and 7B had good rate capability at current density ranging from 2C to 10C.

[0108] As shown in FIG. 8C, at current density of 0.2C, the total specific capacity of Example 4B was greater than the total specific capacity of Comparative Example 1B and the total specific capacity of Comparative Example 2B. At current density of 10C, the total specific capacity of Comparative Example 1B was close to 0 mAh/g, but the total specific capacity of Example 4B was still about 100 mAh/g, which indicated that Example 4B had abilities of lithium ion storage and lithium ion transmittance at high speed charge-discharge process.

TEST EXAMPLE 8

Capacity Retention

[0109] Lithium ion battery of Example 4B was used as sample in this test example. Initial 3 charge-discharge cycles were measured at current density of 50 mA/g (0.2C) and atmosphere of 1 atm and subsequent 50 charge-discharge cycles were measured at current density of 250 mA/g (1C) and atmosphere of 1 atm. The result was shown as FIG. 10.

[0110] As shown in FIG. 10, as current density increased from 0.2C to 1C, the total specific capacity of Example 4B was decreased from 480 mAh/g to 300 mAh/g. Although subsequent capacity retention test was carried out at current density of 1C, the total specific capacity of Example 4B still had 90% initial total specific capacity after 50 charge-discharge cycles.

[0111] In summary, the method of preparing hard carbon beads involves a cross-linking reaction of phenol-formaldehyde resin heated by microwave. The method can reduce energy consumption, control the curing extent, and solve the problems of side reaction and high energy-consumption occurring at high temperature cross-linking reaction. Therefore, economic values of the negative electrode, lithium ion battery, sodium ion battery, lithium ion capacitor, and sodium ion capacitor comprising the hard carbon beads can be increased.