Lithium-ion secondary battery and method of manufacturing the same

Abstract

A lithium-ion secondary battery that includes a positive electrode including a sulfur-based positive active material containing at least sulfur and a negative electrode including a silicon-based negative active material containing at least silicon or a tin-based negative active material containing tin, in which lithium ions are easily implanted and moved. A positive electrode includes a positive current collector and a sulfur-based positive active material containing at least sulfur (S). A negative electrode includes a negative current collector and a silicon-based negative active material containing at least silicon (Si) or a tin-based negative active material containing tin (Sn). The positive current collector is made of an aluminum foil having a plurality of through holes. The negative current collector is made of a copper foil having a plurality of through holes. The positive electrode and the negative electrode are stacked via a separator to form an electrode group.

Claims

1. A lithium-ion secondary battery comprising: a positive electrode including a positive current collector and a sulfur-based positive active material containing at least sulfur (S); a negative electrode including a negative current collector and a silicon-based negative active material containing at least silicon (Si) or a tin-based negative active material containing tin (Sn); and a separator, wherein: the positive current collector is made of an aluminum perforated foil having a plurality of through holes formed to pierce the foil from a front surface to a back surface thereof; the negative current collector is made of a copper foil having a plurality of through holes; and the positive electrode and the negative electrode are stacked via the separator; the through holes formed in the aluminum perforated foil and the through holes formed in the copper foil each have a density of 110.sup.4 holes/m.sup.2 or more and a hole opening rate of 3 to 50%; and when the respective average inside diameters of the through holes formed in the aluminum perforated foil and the through holes formed in the copper foil are defined as R (m), the respective hole opening rates of the aluminum perforated foil and the copper foil are indicated by the following expression: $\begin{array}{cc}\mathrm{hole}\mathrm{opening}\mathrm{rate}\left(\%\right)=\frac{R^2}{4}{10}^{-12}\mathrm{density}\left(\mathrm{holes}/m^2\right)100.& \left[\mathrm{Expression}1\right]\end{array}$

2. The lithium-ion secondary battery according to claim 1, wherein: the aluminum perforated foil has a foil thickness of 50 m or less; the through holes formed in the aluminum perforated foil have an average inside diameter of 2 m to 500 m; the copper foil has a foil thickness of 50 m or less; and the through holes formed in the copper foil have an average inside diameter of 30 m to 500 m.

3. The lithium-ion secondary battery according to claim 2, wherein the aluminum perforated foil has a hole opening rate of 5 to 30%.

4. The lithium-ion secondary battery according to claim 2, wherein the through holes formed in the copper foil have an average inside diameter of 50 m to 300 m.

5. The lithium-ion secondary battery according to claim 3, wherein the through holes formed in the copper foil have an average inside diameter of 50 m to 300 m.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1A illustrates a schematic configuration of an example lithium-ion secondary battery according to an embodiment of the present invention; FIG. 1B illustrates a positive electrode; FIG. 1C illustrates a negative electrode; FIG. 1D illustrates a separator; and FIG. 1E illustrates a laminate film.

(2) FIG. 2 schematically illustrates the internal configuration of the embodiment.

DESCRIPTION OF EMBODIMENTS

(3) An example lithium-ion secondary battery according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1A illustrates a schematic configuration of an example lithium-ion secondary battery 1 according to an embodiment of the present invention. FIG. 1B illustrates a positive electrode 3. FIG. 1C illustrates a negative electrode 7. FIG. 1D illustrates a separator 11. FIG. 1E illustrates a laminate film 12. FIG. 2 schematically illustrates the internal configuration of the embodiment.

(4) The positive electrode 3 includes a positive current collector 4 and a sulfur-based positive active material 5 containing at least sulfur (S). The negative electrode 7 includes a negative current collector 8 and a silicon-based negative active material 9 containing at least silicon (Si) or a tin-based negative active material 9 containing tin (Sn). Reference numerals 6 and 10 denote a positive electrode terminal lead and a negative electrode terminal lead, respectively.

(5) The positive current collector 4 is made of an aluminum foil having a plurality of through holes. The aluminum foil is an aluminum perforated foil having a plurality of through holes formed to pierce the foil from a front surface to a back surface thereof. The aluminum perforated foil has a foil thickness of 50 m or less. The density of the through holes is 110.sup.4 holes/m.sup.2 or more. The average inside diameter of the through holes is 2 to 500 m.

(6) The sulfur-based positive active material 5 is formed, for example, by mixing sulfur powder and carbon source compound powder with a binder to prepare a mixed paste raw material, applying the mixed paste raw material to the positive current collector, and thereafter heating (performing a heat treatment on) the positive current collector. Examples of the carbon source compound include pitch, polyisoprene, polycyclic aromatic hydrocarbons obtained by condensing three or more six-membered rings, and vegetable carbon materials such as coffee beans and seaweeds, etc.

(7) The negative current collector 8 is made of a copper foil having a plurality of through holes. The copper foil has a foil thickness of 50 m or less. The density of the through holes is 110.sup.4 holes/m.sup.2 or more. The average inside diameter of the through holes is 30 to 500 m.

(8) The negative active material layer contains a negative active material, a binder, and a conductive assistance, and is applied to both surfaces of the current collector. The negative active material contains silicon (Si) or tin (Sn).

(9) The positive electrode 3 and the negative electrode 7 are stacked via the separator 11. The separator 11 is a rectangular sheet made of a polypropylene resin. The laminate film 12 has a structure in which a resin and an aluminum foil are stacked. In the embodiment, an electrode group 13 is sandwiched between a set of two laminate films 12, three sides of the laminate films 12 are sealed into a bag shape, and a predetermined non-aqueous electrolyte is poured into a space between the laminate films 12. After that, the remaining side of the laminate films 12 is sealed to form a laminate cell with the four sides tightly sealed and in which the electrode group 13 and the non-aqueous electrolyte are tightly enclosed.

(10) In the embodiment, in order to dope the negative active material with lithium ions, a stack of two copper foils 8A and 8B each having a plurality of through holes and a lithium metal sheet 14 disposed therebetween, is disposed on one side of the electrode group 13. During doping, one or both of the copper foils 8A and 8B each having a plurality of through holes and the negative current collector 7 are electrically connected to each other. Through the electrical connection, lithium ions are eluted from the lithium metal sheet 14, and pass through the plurality of through holes formed in the positive current collector 4 and the negative current collector 8 so that the silicon-based negative active material 9 or tin-based negative active material 9 is doped with the lithium ions. When the lithium-ion secondary battery is completely charged, the lithium metal sheet 14 has been dissolved and does not exist any longer.

(11) In the embodiment, the negative active material is doped with lithium ions through the through holes in the electrode foil. This significantly facilitates doping even in a structure in which the positive electrode and the negative electrode are stacked via the separator. Moreover, the negative active material can be uniformly doped with the lithium ions.

EXAMPLES

(12) Example lithium-ion secondary batteries according to the present invention will be described below.

Example 1-1

(13) A lithium-ion secondary battery was manufactured using a positive active material containing sulfur (S) and a negative active material containing silicon (Si).

(14) (Positive Electrode)

(15) An aluminum foil (thickness: 20 m) for use in ordinary lithium-ion secondary batteries was subjected to a boring process by electrolytic etching to obtain a positive current collector having through holes formed thereon at a density of 110.sup.9 holes/m.sup.2 and with an average inside diameter of 10 m. The hole opening rate was calculated by illuminating the foil with light from below, capturing an image of transmitted light and a non-transparent portion using a microscope, and binarizing the image. The hole opening rate was 22.5%.

(16) A mixed paste raw material prepared by mixing 4.8 g of sulfur (Wako Pure Chemical Industries, Ltd.; product number 195-04625; purity 98%) as a positive active material, 2.4 g of acetylene black (Denki Kagaku Kogyo; product number HS-100) as a conductive assistance, and 6.7 g of PVDF (Kureha Corporation; product number L#1120) as a binder was applied to both surfaces of the positive current collector. The positive current collector was heated to prepare a positive electrode. The positive active material mixture after being heated had 4.0 mg/cm.sup.2, and was shaped into 7 cm7.5 cm by shaving unnecessary portions.

(17) (Negative Electrode)

(18) A copper foil (thickness: 15 m) for use in ordinary lithium-ion secondary batteries was subjected to pattern printing and thereafter etching to form through holes to obtain a negative current collector with through holes having a density of 110.sup.11 holes/m.sup.2 and an average inside diameter of 1 m. The measured weight of a square piece of 10 cm10 cm was 1.080 g, which was 7.9% (hole opening rate) of the theoretical weight (1.347 g).

(19) A mixed paste raw material prepared by mixing 6.4 g of silicon monoxide (SiO) (Wako Pure Chemical Industries, Ltd.; product number 198-05612; purity 99.9%) as a negative active material, 0.4 g of acetylene black (Denki Kagaku Kogyo; product number HS-100) as a conductive assistance, and 10 g of PVDF (Kureha Corporation; product number L#1120) as a binder was applied to both surfaces of the negative current collector. The negative current collector was heated to prepare a negative electrode. The negative active material mixture after being heated had 4.0 mg/cm.sup.2, and was shaped into 7 cm7.5 cm by shaving unnecessary portions.

(20) (Assembly of Battery)

(21) The separator was a rectangular sheet made of a polypropylene resin. The laminate film was formed mainly from nylon, aluminum, and a polypropylene resin. The separator and the laminate film were for use in ordinary lithium-ion secondary batteries. As illustrated in FIG. 2, the positive electrode and the negative electrode were stacked via the separator in two layers for the positive electrode and in three layers for the negative electrode to form an electrode group. The laminate film of 20 cm11 cm was folded into half along the long side. Two sides of the laminate film were sealed into a bag shape. After that, the electrode group was inserted and a non-aqueous electrolyte was poured into the laminate film. The non-aqueous electrolyte included ethylene carbonate, dimethyl carbonate, and methylethyl carbonate, and was for use in ordinary lithium-ion secondary batteries. After that, the four sides of the laminate film were tightly sealed.

(22) In the cell, as illustrated in FIG. 2, a stack of two copper foils each having a plurality of through holes and a lithium metal sheet disposed therebetween, was disposed on one side of the electrode group. The copper foils and the negative current collector were electrically connected to each other by ultrasonic welding. The cell was left to stand for two weeks at 50 C., which allowed lithium ions to be eluted from the lithium metal sheet and to be doped into the negative active material containing silicon.

(23) When the lithium metal sheet was dissolved, charge was completed, thereby obtaining a lithium-ion secondary battery according to Example 1-1.

Examples 1-2 to 1-7

(24) Lithium-ion secondary batteries according to Examples 1-2 to 1-7 were obtained under the same conditions as Example 1-1 except that the average inside diameter and the number of the through holes in the negative current collector were 2 m and 110.sup.11 holes/m.sup.2, 10 m and 110.sup.9 holes/m.sup.2, 50 m and 110.sup.8 holes/m.sup.2, 300 m and 110.sup.6 holes/m.sup.2, 500 m and 510.sup.8 holes/m.sup.2 and 700 m and 2.510.sup.6 holes/m.sup.2, respectively.

Comparative Example 1-1

(25) A lithium-ion secondary battery according to Comparative Example 1-1 was obtained under the same conditions as Example 1-1 except that the negative current collector had no through holes.

(26) TABLE-US-00001 TABLE 1 Average inside Number Hole diameter (m) of (holes/m.sup.2) opening Performance through holes of holes rate (%) Capacity Evaluation Comp. Ex. 1-1 No through holes 0 0 39 mAh x: Doping did not progress Ex. 1-1 1 .sup.1 10.sup.11 7.9 71 mAh : Electrolyte did not sufficiently permeate Ex. 1-2 2 .sup.1 10.sup.11 31.4 195 mAh Ex. 1-3 10 1 10.sup.9 7.9 202 mAh Ex. 1-4 50 1 10.sup.8 19.6 212 mAh Ex. 1-5 300 1 10.sup.6 7.1 206 mAh Ex. 1-6 500 5 10.sup.8 9.8 197 mAh Ex. 1-7 700 2.5 10.sup.6 9.6 140 mAh : Material fell off through holes in foil during application

(27) The discharge capacity was measured in an environment at 25 C. First, the battery was subjected to a constant current charge with a current value of 0.5 CA to an upper limit voltage of 2.2 V. When the upper limit voltage was reached, the battery was subjected to a constant voltage charge at 2.2 V for one hour. The battery was subjected to a constant current discharge with a current value of 0.5 CA to a lower limit voltage of 1V to calculate the capacity.

(28) While doping did not progress when an aluminum foil with no through holes was used, doping progressed when an aluminum foil having through holes was used. The lithium-ion batteries, which used the aluminum foil having through holes with an average inside diameter of 2 m to 500 m, exhibited good performance. However, the non-aqueous electrolyte did not sufficiently permeate the through holes in the lithium-ion battery that used the aluminum foil having through holes with an average inside diameter of 1 m, and the material fell off through the holes when applied to the positive current collector for the lithium-ion battery that used the aluminum foil having through holes with an average inside diameter of 700 m. This result indicates lowered performance compared to lithium-ion batteries that used the aluminum foil having through holes with an average inside diameter of 2 m to 500 m.

Example 2-1

(29) A lithium-ion secondary battery was manufactured using a positive active material containing sulfur (S) and a negative active material containing silicon (Si). A lithium-ion secondary battery according to Example 2-1 was prepared under the same conditions as Example 1-1 except that an aluminum foil having through holes with an average inside diameter of 50 m and with the number of the through holes being 110.sup.8 holes/m.sup.2 was used as the positive current collector and a copper foil having through holes with an average inside diameter of 10 m and with the number of the through holes being 110.sup.9 holes/m.sup.2 was used as the negative current collector.

Examples 2-2 to 2-6

(30) Lithium-ion secondary batteries according to Examples 2-2 to 2-6 were obtained under the same conditions as Example 2-1 except that the average inside diameter and the number of the through holes in the positive current collector were 30 m and 510.sup.8 holes/m.sup.2, 50 m and 110.sup.8 holes/m.sup.2, 300 m and 110.sup.6 holes/m.sup.2, 500 m and 510.sup.5 holes/m.sup.2, and 700 m and 2.510.sup.5 holes/m.sup.2, respectively. Example 2-1 was the same as Example 1-3.

Comparative Example 2-1

(31) A lithium-ion secondary battery according to Comparative Example 2-1 was obtained under the same conditions as Example 2-1 except that the positive current collector had no through holes.

(32) TABLE-US-00002 TABLE 2 Average inside Number Hole diameter (m) of (holes/m.sup.2) opening Performance through holes of holes rate (%) Capacity Evaluation Comp. Ex. 2-1 No through holes 0 0 2 mAh x: Doping did not progress Ex. 2-1 10 1 10.sup.9 7.9 155 mAh : Doping was not uniform Ex. 2-2 30 5 10.sup.8 35.3 215 mAh Ex. 2-3 50 1 10.sup.8 19.6 210 mAh Ex. 2-4 300 1 10.sup.6 7.1 199 mAh Ex. 2-5 500 5 10.sup.5 9.8 204 mAh Ex. 2-6 700 2.5 10.sup.5 9.6 177 mAh : Material fell off through holes in foil during application

(33) While doping did not progress when a copper foil with no through holes was used, doping progressed when a copper foil having through holes was used. The lithium-ion batteries, which used the aluminum foil having through holes with an average inside diameter of 30 m to 500 m, exhibited good performance. However, doping was not uniform in the lithium-ion battery that used the aluminum foil having through holes with an average inside diameter of 10 m, and the material fell off through the holes when applied to the positive current collector for the lithium-ion battery that used the aluminum foil having through holes with an average inside diameter of 700 m. This result indicates lowered performance compared to lithium-ion batteries that used the aluminum foil having through holes with an average inside diameter of 30 m to 500 m.

Examples 3 and 4

(34) A lithium-ion secondary battery was manufactured using a positive active material containing sulfur (S) and a negative active material containing tin (Sn).

(35) A mixed paste raw material prepared by mixing 6.8 g of tin oxide (SnO.sub.2) (Wako Pure Chemical Industries, Ltd.; product number 329-94293; purity 99.7%) as a negative active material, 0.5 g of acetylene black (Denki Kagaku Kogyo; product number HS-100) as a conductive assistance, and 5.8 g of PVDF (Kureha Corporation; product number L#1120) as a binder was applied to both surfaces of the negative current collector. The negative current collector was heated to prepare a negative electrode. The negative active material mixture after being heated had 4.0 mg/cm.sup.2, and was shaped into 7 cm7.5 cm by shaving unnecessary portions. The mixed paste raw material was applied to the negative current collector to prepare a negative electrode.

(36) Lithium-ion secondary batteries according to Examples 3-1 to 3-7, Comparative Example 3-1, Examples 4-1 to 4-6, and Comparative Example 4-1 were prepared through the same processes as Example 1-1 etc. described above except for preparation of the negative electrode. The discharge capacities of the lithium-ion secondary batteries were measured and evaluated. The results are indicated in Tables 3 and 4.

(37) TABLE-US-00003 TABLE 3 Average inside Number Hole diameter (m) of (holes/m.sup.2) opening Performance through holes of holes rate (%) Capacity Evaluation Comp. Ex. 3-1 No through holes 0 0 30 mAh x: Doping did not progress Ex. 3-1 1 .sup.1 10.sup.11 7.9 78 mAh : Electrolyte did not sufficiently permeate Ex. 3-2 2 .sup.1 10.sup.11 31.4 208 mAh Ex. 3-3 10 1 10.sup.9 7.9 198 mAh Ex. 3-4 50 1 10.sup.8 19.6 195 mAh Ex. 3-5 300 1 10.sup.6 7.1 212 mAh Ex. 3-6 500 5 10.sup.8 9.8 203 mAh Ex. 3-7 700 2.5 10.sup.6 9.6 165 mAh : Material fell off through holes in foil during application

(38) TABLE-US-00004 TABLE 4 Average inside Number Hole diameter (m) of (holes/m.sup.2) opening Performance through holes of holes rate (%) Capacity Evaluation Comp. Ex. 4-1 No through holes 0 0 3 mAh x: Doping did not progress Ex. 4-1 10 1 10.sup.9 7.9 172 mAh : Doping was not uniform Ex. 4-2 30 5 10.sup.8 35.3 208 mAh Ex. 4-3 50 1 10.sup.8 19.6 192 mAh Ex. 4-4 300 1 10.sup.6 7.1 197 mAh Ex. 4-5 500 5 10.sup.5 9.8 203 mAh Ex. 4-6 700 2.5 10.sup.5 9.6 156 mAh : Material fell off through holes in foil during application

(39) In this way, it was found that lithium-ion secondary batteries having performance equivalent to that of the lithium-ion secondary batteries in which a silicon-based negative active material was used according to Example 1-1 etc. were obtained even if a tin-based negative active material containing tin (Sn) was used.

INDUSTRIAL APPLICABILITY

(40) According to the present invention, an aluminum foil having a plurality of through holes is used as a positive current collector and a copper foil having a plurality of through holes is used as a negative current collector in a lithium-ion battery in which a positive electrode includes the positive current collector and a sulfur-based positive active material containing at least sulfur (S) and a negative electrode includes a negative current collector and a silicon-based negative active material containing at least silicon (Si) or a tin-based negative active material containing tin (Sn). Lithium ions are moved through the through holes formed in the current collectors. Therefore, the negative active material can be easily doped uniformly with the lithium ions, thereby facilitating manufacture of a lithium-ion battery in which a positive electrode and a negative electrode are stacked via a separator.

DESCRIPTION OF REFERENCE NUMERALS

(41) 1 lithium-ion secondary battery 3 positive electrode 4 positive current collector 5 sulfur-based positive active material 6 positive electrode terminal lead 7 negative electrode 8 negative current collector 8A, 8B copper foil having a plurality of through holes 9 silicon-based negative active material or tin-based negative active material 10 negative electrode terminal lead 11 separator 12 laminate film 13 electrode group 14 lithium metal sheet