Negative electrode material for secondary battery and secondary battery using the same

Abstract

Alloy particles for negative electrode active material are proposed, which can impart anti-oxidation property to Si-containing alloy particles, and suppress oxidation of the negative electrode active material due to electrolyte at a considerably high level. A negative electrode material of secondary battery is achieved by a negative electrode material of secondary battery which is capable of intercalating and de-intercalating lithium and which consists of alloy particles including a silicon phase, a metal phase and bismuth, in which a crystallite size of the silicon phase is 10 nm or smaller, and the metal phase includes at least one kind of metal alloying with silicon but not with lithium, and the negative electrode material includes primary particles formed at least by the silicon and the metals.

Claims

1. A negative electrode material for a secondary battery capable of intercalating and de-intercalating lithium, wherein the negative electrode material comprises alloy particles, the alloy particles including a silicon phase, a metal phase, and bismuth, wherein the silicon phase has a crystallite size of 10 nm or smaller, the metal phase includes at least one kind of metal alloying with silicon but not with lithium.

2. The negative electrode material according to claim 1, further comprising granulated alloy particles, wherein the alloy particles have an average particle diameter of 0.01 m to 1 m and the granulated alloy particles are in a granule form granulated from the alloy particles, and have an average particle diameter of 0.1 m to 20 m and the granulated alloy particles have an aspect ratio of 5 or lower.

3. The negative electrode material according to claim 1, wherein the alloy particles include a portion in which the silicon is in an intermetallic compound with the at least one kind of metal, and a portion in which the silicon is present as a silicon single substance, and no peak is observed at (111) plane of the silicon obtained by X-ray diffraction measurement, and all the crystallite sizes of the alloy particles on the rest of the planes as calculated by diffraction spectra are 30 nm or smaller.

4. The negative electrode material of claim 1, wherein a content of the bismuth included in the alloy particles is 5 wt % or lower.

5. The negative electrode material of claim 1, wherein a content of the silicon included in the alloy particles is 40 wt % or higher.

6. The negative electrode material of claim 1, wherein the alloy particles are present in an amorphous or a microcrystalline state.

7. A negative electrode for a secondary battery comprising the negative electrode material as set forth in claim 1.

8. A secondary battery, comprising a positive electrode, a negative electrode, a nonaqueous electrolyte, and a separator, wherein the negative electrode is as set forth in claim 7.

9. The secondary battery of claim 8, wherein the secondary battery is a lithium secondary battery.

10. A fabricating method of the negative electrode material for a secondary battery as set forth in claim 1, the fabricating method comprising: preparing silicon, at least one kind of metal alloying with the silicon but not with lithium, and bismuth; forming at least the silicon and the at least one kind of metal into a master alloy, and forming alloy particles including a silicon phase, a metal phase, and bismuth, having a crystallite size of 10 nm or smaller, by subjecting the master alloy to mechanical alloying.

11. The fabricating method of claim 10, wherein the bismuth is added during forming of the master alloy and/or performing of the mechanical alloying.

Description

MODE FOR DISCLOSURE

Working Example 1

(1) (Preparation of Alloy Powders)

(2) The raw material powders were mixed at a ratio of Si:Cr:Ti:Bi=70:14:13:3 (wt %). After alloy particles were prepared with gas atomization, the particles were seived through so that the particle diameter was adjusted to below 45 m. The alloy powder was added with 1 wt % stearic acid, and placed into a receptacle of a vibrating mill along with 15 mm-diameter steel balls to fill 80% the vibrating mill receptacle. After substitution with nitrogen gas, mechanical alloying treatment was conducted at 1200 cpm vibrating frequency for 24 hr. The X-ray diffraction measurement on the obtained alloy powder revealed that the peaks were not observed from the (111) plane of silicon and that it was sufficiently amorphous.

(3) (Fabrication of Secondary Battery)

(4) <Fabrication of Negative Electrode>

(5) After passing the obtained alloy powder through an electromagnetic sieve to a diameter below 38 m, the alloying material and the graphite having average particle diameter of 15 m were mixed at a weight ratio of 25:75, so that a negative electrode active material was prepared. A mixture of 94 wt % of negative electrode active material, 2 wt % of carbon nanotubes as the conducting agent, and 4 wt % of polyvinylidene fluoride as the binder, was prepared and formed into slurry with N-methyl-2-pyrrolidone, which was applied onto 20 m-thick copper foil to a thickness of approximately 100 m. After vacuum-drying at 120 C. and pressing, a negative electrode of electrode density of 1.7 g/cc was prepared by punching out a 13 mm-diameter disc.

(6) <Fabrication of Positive Electrode>

(7) 0.3 mm-thick metal lithium was used for the positive electrode.

(8) <Preparation of Electrolyte>

(9) Ethylene carbonate and diethyl carbonate were mixed at a ratio of 3:7, and electrolyte solution containing 1 mole of LiPF.sub.6 were used.

(10) <Fabrication>

(11) 2016-type coin cell was fabricated with the constituent materials described above.

Working Example 2

(12) The coin cell was fabricated in the same manner as Example 1 except that the respective raw material powders were mixed at a ratio of Si:Cr:Ti=73:14:13 (wt %) and that Bi was not added.

Comparative Example 2

(13) Tin (Sn) having expandability as high as Bi was used in place of Bi. The coin cell was fabricated in the same manner as Example 1 except that the respective raw material powders were mixed at a ratio of Si:Cr:Ti:Sn=70:14:13:3 (wt %) and the alloy particles were prepared by gas atomization.

(14) The battery characteristics and analysis of Examples and Comparative Examples are listed in Table 1.

(15) <Evaluation Test 1: Charge-Discharge Cycle Test>

(16) The coin cells (secondary batteries) of the Examples and Comparative Examples were subjected to 50 repetitious charge-discharge cycles at 0.5 C current rate. After finishing 51st charge cycle state, the coin cells were disassembled and the thickness of the electrodes was measured.

(17) The thickness was divided by (discharge capacity at 50th cycleweight of active material containing conducting agent per unit area as measured before charging). As a result, volume of the mixed active material layer per capacity of the 51st charge cycle state was calculated. Table 1 below lists the result.

(18) TABLE-US-00001 TABLE 1 Capacity Electrode volume (%) per Initial retention capacity at 51st charge efficiency (%) after cycle (relative to Example (%) 50th cycle Example 1) Ex. 1 89.5 89.5 100 Ex. 2 88.8 83.4 108 Comp. Ex. 1 89.0 82.7 111 Comp. Ex. 2 87.6 82.0 113

(19) <Overall Evaluation>

(20) According to the present disclosure, by adding bismuth to silicon, and to at least one kind of metal alloying with silicon but not with lithium, and performing mechanical alloying, it is possible to suppress expansion experienced during charging and discharging, also enhance anti-oxidation property and thus enhance service life characteristic.