Negative electrode material comprising silicon flakes and preparing method of silicon flakes

Abstract

The present disclosure relates to a negative electrode material including, as an active material, silicon flakes with a hyperporous structure, represented by the following chemical formula 1:
xSi.(1−x)A  (1) where 0.5≤x≤1.0, and A is an impurity, and includes at least one compound selected from the group consisting of Al.sub.2O.sub.3, MgO, SiO.sub.2, GeO.sub.2, Fe.sub.2O.sub.3, CaO, TiO.sub.2, Na.sub.2O K.sub.2O, CuO, ZnO, NiO, Zr.sub.2O.sub.3, Cr.sub.2O.sub.3 and BaO, and a preparing method of the silicon flakes.

Claims

1. A negative electrode material comprising, as an active material, silicon flakes with a hyperporous structure, represented by the following chemical formula 1:
xSi.(1−x)A  (1) where 0.5≤x≤1.0, and A is an impurity, and includes at least one compound selected from the group consisting of Al.sub.2O.sub.3, MgO, SiO.sub.2, GeO.sub.2, Fe.sub.2O.sub.3, CaO, TiO.sub.2, Na.sub.2O, K.sub.2O, CuO, ZnO, NiO, Zr.sub.2O.sub.3, Cr.sub.2O.sub.3 and BaO, wherein the silicon flakes have a BET surface area of 120 m.sup.2/g to 250 m.sup.2/g, wherein the silicon flakes have a hyperporous structure including macropores having a pore size in the range of greater than 50 nm to 500 nm, mesopores having a pore size of greater than 2 nm to 50 nm, and micropores having a pore size of 0.5 nm to 2 nm, wherein mesopores and micropores are formed on a frame surface of the silicon flakes, and macropores are through holes created inside of the silicon flakes frame.

2. The negative electrode material according to claim 1, wherein the silicon flakes have an average pore diameter of 100 nm to 150 nm.

3. The negative electrode material according to claim 1, wherein the silicon flakes have a porosity of 100 to 5000 based on a total volume.

4. The negative electrode material according to claim 1, wherein the silicon flakes have a thickness of 20 to 100 nm.

5. The negative electrode material according to claim 1, wherein the silicon flakes have a size of 200 nm to 50 μm.

6. The negative electrode material according to claim 1, wherein the silicon flakes further include carbon coating.

7. The negative electrode material according to claim 6, wherein the carbon coating has a thickness of 1 to 100 nm.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a scanning electron microscopy (SEM) image of clay (talc) used in example 1.

(2) FIG. 2 is an SEM image of silicon flakes prepared in example 1.

(3) FIG. 3 is a transmission electron microscopy (TEM) image of silicon flakes prepared in example 1.

(4) FIG. 4 is a high resolution TEM image of silicon flakes prepared in example 1.

(5) FIG. 5 is an X-ray diffraction (XRD) graph of silicon flakes prepared in example 1.

(6) FIG. 6 is a TEM image showing a pore formation process of silicon flakes prepared in example 1 as a function of reaction time.

(7) FIG. 7 is an SEM image of carbon-coated silicon flakes prepared in example 2.

(8) FIG. 8 is an XRD graph of carbon-coated silicon flakes prepared in example 2.

(9) FIG. 9 is an SEM image of clay (nanoclay) used in comparative example 1.

(10) FIG. 10 is an SEM image of porous silicon prepared in comparative example 1.

(11) FIG. 11 is an SEM image of silicon flakes prepared with an addition of carbon compound in comparative example 2.

(12) FIG. 12 is an XRD graph of silicon flakes prepared with an addition of carbon compound in comparative example 2.

(13) FIG. 13 is an SEM image of carbon-coated porous bulk silicon prepared in comparative example 3.

(14) FIG. 14 is an SEM image of silicon flakes with no hyperporous structure prepared in comparative example 4.

(15) FIG. 15 is an XRD graph of silicon flakes with no hyperporous structure prepared in comparative example 4.

(16) FIG. 16 is an SEM image of silicon flakes prepared in comparative example 5.

(17) FIG. 17 is an SEM image of silicon flakes prepared in comparative example 6.

(18) FIG. 18 is an initial charge/discharge graph of coin cell fabricated in experimental example 4 using silicon flakes of example 1 and example 2 as an active material.

(19) FIG. 19 is an initial charge/discharge graph of coin cell fabricated in experimental example 4 using bulk silicon of comparative example 1 as an active material.

(20) FIG. 20 is an initial charge/discharge graph of coin cell fabricated in experimental example 4 using silicon flakes with added carbon compound of comparative example 2 as an active material.

(21) FIG. 21 is an initial charge/discharge graph of coin cell fabricated in experimental example 4 using carbon-coated porous bulk silicon of comparative example 3 as an active material.

(22) FIG. 22 is an initial charge/discharge graph of coin cell fabricated in experimental example 4 using silicon flakes with no hyperporous structure of comparative example 4 as an active material.

(23) FIG. 23 is a graph showing charge/discharge life characteristics of coin cell fabricated in experimental example 5 using silicon flakes of example 1 and example 2 as an active material.

(24) FIG. 24 is a graph showing charge/discharge life characteristics of coin cell fabricated in experimental example 5 using silicon flakes coated with carbon at different thicknesses of examples 3 and 4 as an active material.

(25) FIG. 25 is a graph showing charge/discharge life characteristics of coin cell fabricated in experimental example 5 using bulk silicon of comparative example 1 as an active material.

(26) FIG. 26 is a graph showing charge/discharge life characteristics of coin cell fabricated in experimental example 5 using silicon flakes with added carbon compound of comparative example 2 as an active material.

(27) FIG. 27 is a graph showing charge/discharge life characteristics of coin cell fabricated in experimental example 5 using carbon-coated porous bulk silicon of comparative example 3 as an active material.

(28) FIG. 28 is a graph showing charge/discharge life characteristics of coin cell fabricated in experimental example 5 using silicon flakes with no hyperporous structure of comparative example 4 as an active material.

(29) FIG. 29 is a graph showing charge/discharge characteristics as a function of C-rate for coin cell fabricated in experimental example 6 using silicon flakes of example 1 and example 2 as an active material.

(30) FIG. 30 is an SEM image of diatomite used in comparative example 7.

(31) FIG. 31 is an SEM image of silicon flakes finally obtained in comparative example 7.

(32) FIG. 32 is an SEM image of slag used in comparative example 8.

(33) FIG. 33 is an SEM image of silicon flakes finally obtained in comparative example 8.

(34) FIG. 34 is an initial charge/discharge graph of coin cell fabricated in experimental example 4 using each silicon material of example 1, example 2, comparative example 7, and comparative example 8 as an active material.

(35) FIG. 35 is a graph showing charge/discharge life characteristics of coin cell fabricated in experimental example 5 using silicon material of example 1, example 2, comparative example 7, and comparative example 8 as an active material.

(36) FIG. 36 is a graph showing charge/discharge characteristics as a function of C-rate for coin cell fabricated in experimental example 6 using silicon material of example 1, example 2, comparative example 7, and comparative example 8 as an active material.

(37) FIG. 37 is a TEM image of silicon flakes having undergone thermal treatment reaction of 650° C. for 6 hours.

(38) FIG. 38 is a diagram of silicon flakes with hyperporous structure according to an embodiment of the present disclosure.

MODE FOR CARRYING OUT THE INVENTION

(39) Hereinafter, the embodiments/examples are described to explain the present disclosure in more detail, but the following embodiments/examples are provided for illustration only, and the scope of the present disclosure is not limited thereto.

Example 1

(40) 1 g of clay (talc) was mixed with 0.7 g of magnesium while milling uniformly using a mortar. The uniformly mixed mixture was put into a reaction container, and heated in argon atmosphere at 650° C. for 3 hours to drive reduction reaction. The silicon flakes having undergone reaction were added to 200 mL of 0.5M HCL aqueous solution and mixed at 35° C. for 3 hours to remove magnesium oxide and other impurities. Removal of silica remaining after reaction was carried out by mixing silicon flakes with 0.1 to 5% of hydrofluoric acid for 5 to 30 minutes. Finally, silicon flakes with hyperporous structure were synthesized through a vacuum filter.

Example 2

(41) Acetylene gas was introduced to silicon flakes synthesized in example 1 in 900° C. argon atmosphere for 3 minutes to drive reaction. After naturally cooling to room temperature, finally, silicon flakes coated with carbon 9 nm in thickness were synthesized.

Example 3

(42) Silicon flakes were synthesized in the same way as example 2 except that the thickness of carbon was 5 nm.

Example 4

(43) Silicon flakes were synthesized in the same way as example 2 except that the thickness of carbon coated was 15 nm.

Comparative Example 1

(44) The same process was performed except the type of clay used in synthesis example of silicon flakes. That is, clay used in comparative example 1 was nanoclay.

Comparative Example 2

(45) Silicon composite including graphene oxide was obtained in the same way as example 1 except that reaction took place with an addition of 0.05 g of carbon compound to 1 g of clay.

Comparative Example 3

(46) 50 g of bulk silicon was added to 100 mL of mixed solution of 40 mM of CuSO.sub.4 with 5M HF and stirred at 50° C. for 12 hours. After reaction, porous silicon including macropores was filtered out through a filter, and remaining Cu metal was removed by stirring in 50 mL of nitric acid solution at 50° C. for 3 hours. Acetylene gas was introduced to the porous silicon in 900° C. argon atmosphere for 28 minutes to drive reaction. After naturally cooling to room temperature, finally, porous silicon coated with 15 wt % of carbon was synthesized.

Comparative Example 4

(47) Silicon flakes were synthesized in the same way as example 1 except that the type of clay used was illite.

Comparative Example 5

(48) Silicon flakes were synthesized in the same way as example 1 except that the reduction reaction temperature was 400° C.

Comparative Example 6

(49) Silicon flakes were synthesized in the same way as example 1 except that the reduction reaction time was 10 minutes.

Comparative Example 7

(50) Silicon flakes were synthesized by the same method as example 1 except that diatomite was used instead of clay (talc).

Comparative Example 8

(51) Silicon flakes were synthesized by the same method as example 1 except that slag was used instead of clay (talc). In this instance, the used slag is also called iron slag, and its composition was CaO (43.3 wt %), SiO.sub.2 (34.5 wt %), Al.sub.2O.sub.3 (13.3 wt %), MgO (3.6 wt %), TiO.sub.2 (1.7 wt %), and Fe.sub.2O.sub.3 (1.1 wt %).

Experimental Example 1

Experimental Example 1-1

(52) For evaluation of appearance, SEM image and/or TEM image of clay used in examples 1 and 2 and comparative examples 1 to 6 and silicon flakes or bulk silicon prepared using the same are shown in FIGS. 1 to 4, 7, 9 to 11, 13, 14, 16 and 17.

(53) Specifically, FIG. 1 is a scanning electron microscopy (SEM) image of clay (talc) used in example 1, and FIG. 2 is an SEM image of silicon flakes finally obtained in example 1.

(54) Referring to FIG. 1, a layered structure of raw material, i.e., clay (talc) is found to include flakes stacked in 2 to 4 layers through covalent bond of silica and metal oxide, and when the stacked flakes are a unit structure, the thickness is 50 to 200 nm.

(55) Referring to FIG. 2, it can be seen that silicon flakes synthesized in example 1 has a hyperporous structure.

(56) Furthermore, to understand the structure of silicon flakes finally obtained in example 1 better, a TEM image was taken and is shown in FIG. 3.

(57) Referring to FIG. 3, it can be seen that macropores having 100 to 150 nm size are uniformly formed on the surface of silicon flakes, and silicon flakes are stacked in 2 to 4 layers.

(58) A smaller drawing in FIG. 4 is a high resolution image of FIG. 4. Referring to FIG. 4, it can be seen that mesopores and micropores are formed on the surface of silicon flakes.

(59) That is, analyzing FIGS. 3 and 4 together, it can be seen that silicon flakes according to example 1 have a hyperporous structure.

(60) Furthermore, FIG. 7 is an SEM image of carbon-coated silicon flakes prepared in example 2, and a smaller drawing in FIG. 7 is a high resolution image of FIG. 7.

(61) Referring to FIG. 7, it can be seen that silicon flakes maintain a hyperporous structure well even after they are coated with a carbon layer.

(62) Meanwhile, FIG. 9 is an SEM image of clay (nanoclay) used in comparative example 1, and FIG. 10 is an SEM image of bulk silicon finally obtained in comparative example 1.

(63) Referring to FIG. 9, a layered structure of raw material, i.e., clay (nanoclay) is found to include nanosheets stacked through covalent bond of silica and metal oxide, and when the stacked nanosheets are a unit structure, the thickness is about 10 to 50 nm.

(64) In contrast, referring to FIG. 10, it can be seen that silicon synthesized in comparative example 1 does not have an existing layered structure, and shows a structure of 3-dimensional (3D) bulk silicon primarily having 100 to 300 nm macropores.

(65) In the case of nanoclay, because metal oxide (i.e., aluminum oxide) inside cannot act as a negative catalyst (heat scavenger), an excessive amount of heat generated during reduction reaction is fully transmitted to the structure, and nanoclays of small size are agglomerated. Furthermore, 100 to 300 nm macropores formed by removing MgO produced as a result of reduction are all that is formed, and accordingly, there is no hyperporous structure. For this reason, obviously, an existing layered structure or a layered structure such as silicon flakes cannot be maintained.

(66) FIG. 11 is an SEM image of silicon flakes with added carbon compound prepared in comparative example 2, and a smaller drawing in FIG. 11 is a high resolution image of FIG. 11.

(67) Referring to FIG. 11, in the case of silicon flakes synthesized with an addition of carbon compound, the structure of flakes is maintained on predetermined level, but during reduction reaction, the carbon compound hinders the reduction reaction, and as a result, the pore structure is not definite, and a non-uniform structure is formed.

(68) FIG. 13 is an SEM image of carbon-coated porous bulk silicon prepared in comparative example 3.

(69) Referring to FIG. 13, a porous structure composed only of macropores is formed in bulk silicon with 3D structure, and a hyperporous structure is not found.

(70) Further, to understand the structure of silicon flakes finally obtained in comparative example 4 better, an SEM image was taken and is shown in FIG. 14.

(71) Referring to FIG. 14, silicon prepared from raw material, i.e., clay (illite) shows a flake structure, but a hyperporous structure on the surface is absent.

(72) FIG. 15 is an X-ray diffraction (XRD) graph of silicon flakes with no hyperporous structure prepared in comparative example 4.

(73) Furthermore, FIG. 16 is an SEM image of silicon flakes obtained in comparative example 5, and an inside figure in FIG. 16 is a high resolution image of FIG. 16.

(74) Referring to FIG. 16, silicon flakes synthesized under temperature condition of 400° C. have a structure of flakes of the used clay (talc), but the pore structure is irregular and does not achieve a hyperporous structure.

(75) A general magnesium reduction reaction is accomplished when magnesium melts around the melting point of magnesium and reacts with silica, and magnesium does not fully participate in reaction at 400° C., failing to achieve full reduction of silicon flakes, and as a consequence, a hyperporous structure is not formed.

(76) FIG. 17 is an SEM image of silicon flakes obtained in comparative example 6.

(77) A smaller drawing in FIG. 17 is a high resolution image of FIG. 17. Referring to FIG. 17, it can be seen that macropores are not formed on the surface of silicon flakes under reaction conditions of 650° C. and 10 minutes, and a layer composed of mesopores and micropores covers the surface. When the reaction time is 10 minutes, reaction is not fully made, and silicon flakes do not perfectly form a hyperporous structure.

(78) That is, analyzing FIGS. 16 and 17 together, the synthesis conditions including reaction temperature and reaction time applied to example 1 are found reasonable.

Experimental Example 1-2

(79) For evaluation of appearance, SEM images of diatomite and slag used in comparative examples 7 and 8, and silicon flakes prepared using the same are shown in FIGS. 30 to 33.

(80) Specifically, FIG. 30 is an SEM image of diatomite used in comparative example 7, and FIG. 31 is an SEM image of silicon flakes finally obtained in comparative example 7.

(81) Furthermore, FIG. 32 is an SEM image of slag used in comparative example 8, and FIG. 33 is an SEM image of silicon flakes finally obtained in comparative example 8.

(82) For silicon flakes prepared using talc for clay, compared to FIG. 2 in which porous silicon of 2D structure was synthesized, in comparative example 7, porous silicon of 3D structure was synthesized, and in comparative example 8, mesoporous silicon particles having 500 nm size were synthesized and a hyperporous structure was not formed.

Experimental Example 2

(83) To understand the pore structure formation process of silicon flakes finally obtained in example 1 as a function of reaction time better, a TEM image was taken and is shown in FIG. 6.

(84) Referring to FIG. 6, when the reaction time is 10 minutes, macropores structure of the silicon flakes was hardly observed, and the entire surface was composed of mesopores. It can be also seen that the thickness is more than the level of clay. Additionally, it can be seen that crystallinity of silicon reduces.

(85) When the reaction time is 30 minutes, macropores structure starts to form, and due to reaction heat, a layer of silicon flakes delaminated from the stack structure of clay is formed.

(86) Furthermore, it can be seen that 50 to 70% of the surface of silicon flakes is composed of mesopores, and crystallinity increases.

(87) When the reaction time is 1 hour, macropores have a more uniform size, and judging from the observation of overlapping macropores, it can be seen that delaminated silicon flakes start to be stacked in 2 to 3 layers.

(88) Furthermore, it can be seen that 10 to 30% of the surface of silicon flakes is composed of mesopores, and crystallinity increases.

(89) When the reaction time is 3 hours, macropores have slightly smaller size, and a stack of 3 to 4 layers is observed.

(90) Furthermore, the mesoporous structure on the surface of silicon flakes disappears, and a frame of dense structure is observed.

(91) In conclusion, analyzing the TEM image of FIG. 6 together, an excessive amount of mesopores are formed on the surface by a metal reducing agent at the initial time of reaction, and as the time passes, the reaction proceeds into the clay, making delamination and macropores structure more clear, and when the reaction time is longer than 30 minutes, stacks are observed again, and the macropore size reduces. Accordingly, the reaction time of at least 30 minutes is required to obtain the silicon flakes according to the present disclosure, and specifically, the most preferred reaction time is 1 hour to 3 hours.

(92) FIG. 37 is a TEM image of silicon flakes having undergone thermal treatment reaction at 650° C. for 6 hours. Referring to FIG. 37, it can be seen that as the thermal treatment time increases, all the particles increase in size. This is thought to be aggregation typically appearing when subjected to reaction at high temperature for a long time, and one more point to note is a size reduction of macropores to very small size and consequential indefinite boundary as described previously. It may be caused by agglomeration of many layers of silicon flakes.

Experimental Example 3

(93) To understand the structure of silicon flakes finally obtained in examples 1 and 2 and comparative examples 2 and 4 better, XRD graphs were plotted and are shown in FIGS. 5, 8, 12, and 15.

(94) The XRD equipment (D8 Advance, Bruker) was used to measure at 3 kW X-ray power, 20 kV measurement voltage, 50 mA measurement current, and the measurement range between 10 and 90 degrees.

(95) Referring to FIGS. 5, 8 and 12, it can be seen that pure silicon is synthesized without impurities.

(96) Referring to FIG. 8, a thin carbon layer is coated, but it is too small to be observed through XRD analysis, and referring to FIG. 12, it can be seen that silicon and graphene oxide exist together.

Experimental Example 4

Experimental Example 4-1

(97) Various silicon prepared in example 1 and comparative examples 1 to 4 was used as a negative electrode active material, polyacrylic acid (PAA)/CMC was used as a binder and carbon black was used as a conductive material. The negative electrode active material:binder:conductive material were mixed well in water at a weight ratio of 8:1:1, applied to a 18 μm thick Cu foil and dried at 150° C. to manufacture a negative electrode. For a positive electrode, a lithium foil was used, and a half coin cell was manufactured using an electrolyte solution containing 1M LiPF.sub.6 and 10 wt % of fluoroethylene carbonate (FEC) in a solvent of ethylene carbonate (EC):diethyl carbonate (DEC)=3:7.

(98) For the manufactured half coin cell, charge/discharge capacity was measured at 25° C. with 0.05 C current and voltage ranging from 0.01 to 1.2 V, and discharge capacity and charge/discharge efficiency results are shown in FIGS. 18 to 22.

(99) Referring to FIG. 18, the half coin cells manufactured using silicon flakes and carbon-coated silicon flakes have the specific charge/discharge capacity of 2209/2383 and 2984/3216, and the initial charge/discharge efficiency of 92.67% and 92.77% respectively.

(100) In contrast, referring to FIG. 19, the half coin cell manufactured using bulk silicon prepared from nanoclay has the specific charge/discharge capacity of 1772/2497, and the initial charge/discharge efficiency of 70.96%.

(101) Referring to FIG. 20, the half coin cell manufactured using silicon flakes with added carbon compound had the specific charge/discharge capacity of 2027/2475, and the initial charge/discharge efficiency of 81.9%.

(102) Referring to FIG. 21, the half coin cell manufactured using carbon-coated porous silicon prepared through a metal-assisted chemical etching method has the specific charge/discharge capacity of 2565/2886, and the initial charge/discharge efficiency of 88.88%.

(103) Referring to FIG. 22, the half coin cell manufactured using silicon flakes with no hyperporous structure has the specific charge/discharge capacity of 1243/1381, and the initial charge/discharge efficiency of 90.05%.

Experimental Example 4-2

(104) Various silicon prepared in example 1, example 2, comparative example 7 and comparative example 8 was used as a negative electrode active material, polyacrylic acid (PAA)/CMC was used as a binder, and carbon black was used as a conductive material. The negative electrode active material:binder:conductive material were mixed well in water at a weight ratio of 8:1:1, applied to a 18 μm thick Cu foil and dried at 150° C. to manufacture a negative electrode. For a positive electrode, a lithium foil was used, and a half coin cell was manufactured using an electrolyte solution containing 1M LiPF.sub.6 and 10 wt % of fluoroethylene carbonate (FEC) in a solvent of ethylene carbonate (EC):diethyl carbonate (DEC)=3:7.

(105) For the manufactured half coin cell, charge/discharge capacity was measured at 25° C. with 0.05 C current and voltage ranging from 0.01 to 1.2 V, and discharge capacity and charge/discharge efficiency results are shown in FIG. 34.

(106) Referring to FIG. 34, the half coin cells manufactured using silicon flakes (example 1) and carbon-coated silicon flakes (example 2) have the specific charge/discharge capacity of 2209/2383 and 2984/3216, and the initial charge/discharge efficiency of 92.67% and 92.77%.

(107) In contrast, in the case of silicon synthesized from diatomite and slag (comparative example 7 and comparative example 8), the charge/discharge specific capacity was 2742/3483 and 2320/2674, and the initial charge/discharge efficiency was 78.77% and 86.76, respectively.

(108) In conclusion, when silicon flakes according to the present disclosure are used as an active material, the initial charge/discharge efficiency is considerably high compared to the case using bulk silicon and silicon flakes with no hyperporous structure, and further, it can be seen that they have higher initial charge/discharge efficiency than silicon flakes including carbon compound and carbon-coated porous silicon.

Experimental Example 5

Experimental Example 5-1

(109) The carbon-coated silicon prepared in examples 3 and 4 was used as a negative electrode active material, PAA/CMC was used as a binder, and carbon black was used as a conductive material. The negative electrode active material:binder:conductive material were mixed well in water at a weight ratio of 8:1:1, applied to a 18 μm thick Cu foil, and dried at 150° C. to manufacture a negative electrode. For a positive electrode, a lithium foil was used, and a half coin cell was manufactured using an electrolyte solution containing 1M LiPF.sub.6 and 10 wt % of FEC in a solvent of EC:DEC=3:7.

(110) Using the half coin cells manufactured as above and half coin cells manufactured in experimental example 4, life characteristics were evaluated at 0.2 C (1 C=3 A/g) current in 100 charge/discharge cycles, and the results are shown in FIGS. 23 to 28.

(111) Referring to FIG. 23, the half coin cells manufactured using silicon flakes (example 1) and carbon-coated silicon flakes (example 2) as an active material have the cycle life of 89.0% and 94.6% in 100 charge/discharge cycles at the rate of 0.2 C respectively.

(112) Referring to FIG. 24, life characteristics of the half coin cells manufactured using silicon flakes of examples 3 and 4 as an active material were evaluated in 100 charge/discharge cycles at the rate of 0.2 C. They have sufficiently good life characteristics compared to comparative examples.

(113) Referring to FIG. 25, the life of the half coin cell using bulk silicon of comparative example 1 as an active material expired in 100 charge/discharge cycles at the rate of 0.2 C.

(114) Referring to FIG. 26, the half coin cell manufactured using silicon flakes with added carbon compound has a rapid reduction in charge/discharge cycles at the rate of 0.2 C, and has the cycle life of only 18.3% in 70 charge/discharge cycles at the rate of 0.5 C.

(115) Referring to FIG. 27, the life of the half coin cell manufactured using carbon-coated porous silicon expired in 100 charge/discharge cycles at the rate of 0.2 C.

(116) Referring to FIG. 28, the half coin cell manufactured using silicon flakes with no hyperporous structure has the cycle life of 71.74% in 90 charge/discharge cycles at the rate of 0.2 C.

Experimental Example 5-2

(117) Various silicon prepared in example 1, example 2, comparative example 7, and comparative example 8 was used as a negative electrode active material, PAA/CMC was used as a binder, and carbon black was used as a conductive material. The negative electrode active material:binder:conductive material were mixed well in water at a weight ratio of 8:1:1, applied to a 18 μm thick Cu foil, and dried at 150° C. to manufacture a negative electrode. For a positive electrode a lithium foil was used, and a half coin cell was used using an electrolyte solution containing 1M LiPF.sub.6 and 10 wt % of FEC in a solvent of EC:DEC=3:7.

(118) Using the manufactured half coin cells, life characteristics were evaluated at 0.2 C (1 C=3 A/g) current in 50 charge/discharge cycles, and the results are shown in FIG. 35.

(119) Referring to FIG. 35, the half coin cells manufactured using silicon flakes (example 1) and carbon-coated silicon flakes (example 2) show cycle characteristics of 100% and 95.97% in 50 charge/discharge cycles at 0.2 C rate, respectively. In contrast, silicon synthesized from diatomite and Ferro-Si (comparative example 7 and comparative example 9) showed life characteristics of 56.47% and 71.36% respectively.

(120) In conclusion, when the silicon flakes according to the present disclosure are used as an active material, the cycle life was longer than the case using bulk silicon and silicon flakes with no hyperporous structure, and further, in the case of using carbon-coated silicon flakes, there is an effect on significantly improving the life characteristics compared to silicon flakes with added carbon compound.

Experimental Example 6

(121) According to experimental example 4, with half coin cells using silicon flakes prepared in example 1 and example 2 as a negative electrode active material, rate characteristics were evaluated under the charge/discharge condition with the current of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C and its results are shown in FIG. 29.

(122) Referring to FIG. 29, the half coin cells manufactured using silicon flakes (example 1) and carbon-coated silicon flakes (example 2) have the specific capacity of about 25 and 585 at 10 C rate.

(123) Furthermore, according to experimental example 4, with half coin cells using silicon flakes prepared in example 1, example 2, comparative example 7 and comparative example 8 as a negative electrode active material, rate characteristics were evaluated under the charge/discharge condition with the current of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and its results are shown in FIG. 36.

(124) Referring to FIG. 36, when silicon flakes of example 1 and carbon-coated silicon flakes of example 2 are used as a negative electrode active material, each specific capacity at 5 C is 134 and 958, and when each silicon synthesized from diatomite of comparative example 7 and Ferro-Si of comparative example 8 is used, each specific capacity is 28 and 87 (unit of specific capacity: mAhg.sup.−1).

(125) In conclusion, when the silicon flakes according to the present disclosure are used as an active material, the performance is also good in terms of rate characteristics, but when carbon-coated silicon flakes are used, it is much better in terms of rate characteristics.

(126) Through the electrochemical analysis, it can be seen that silicon flakes and carbon-coated silicon flakes according to an embodiment of the present disclosure have better initial efficiency, life characteristics, and rate characteristics than other silicon materials synthesized through Mg reduction.

(127) The reason for the performance improvement can be explained as below.

(128) First, the silicon flakes according to an embodiment of the present disclosure can have greatly improved electrolyte solution impregnation characteristics through the hyperporous structure. Most of the porous silicon structures have pores on the surface only, while the silicon flakes according to an embodiment of the present disclosure have large macropores over the entire structure, bringing an electrolyte solution and an electrode material into uniform contact from the initial cycle, which helps improve the diffusion of lithium ions.

(129) Second, the silicon flakes according to an embodiment of the present disclosure have various types of pores including macropores as well as mesopores and micropores, which is advantageous in terms of accommodating the volume expansion. According to earlier studies, volume expansion was reduced through porous structure, and 3D porous structure was proposed. Completely different from the earlier technology, the silicon flakes according to an embodiment of the present disclosure are applied as a silicon negative electrode active material with 2D porous structure.

(130) In addition, the silicon flakes according to an embodiment of the present disclosure exhibits superior reversible capacity at a high C rate despite a large particle size, which greatly improves electrolyte solution impregnation characteristics through the hyperporous structure, helping improve the diffusion of lithium ions, and thus it is advantageous in terms of accommodating the volume expansion. The reason why a material with 2D structure is more advantageous in volume expansion than 3D structure is that even under the same volume expansion, a material with 2D structure can maintain the structure better and a material with 3D structure changes the structure to 2D structure in many cycles. 2D structure expands in surface direction and vertical direction, and even though the same volume expansion is taken into account, an expansion rate in a real electrode is much smaller, so it is superior to the material of 3D structure expanding in all directions.

(131) FIG. 38 is a diagram of silicon flakes with hyperporous structure according to an embodiment of the present disclosure. Referring to FIG. 38, the largest and round part is macropore, and pores disposed on the surface thereof are mesopore and micropore. That is, mesopores and micropores are formed on the frame surface of silicon flakes, and macropores are holes created inside of the silicon flakes frame, that is, in the frame itself.

(132) While the present disclosure has been hereinabove described with regard to a limited number of embodiments and drawings, the present disclosure is not limited thereto and it is obvious that various modification and alteration may be made thereto by person having ordinary skill in the technical field pertaining to the present disclosure within the technical aspect of the present disclosure and the equivalent scope to which the appended claims are entitled.