JP2008016329A - Negative electrode material for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode material for a lithium secondary battery capable of maintaining properly high rate characteristics, even if chargings and dischargings are repeated in a secondary battery. <P>SOLUTION: The negative electrode material for the lithium secondary battery is provided with porous support bodies 3, 13 having a porous structure, and a negative electrode active material 5 which is carried by the porous support bodies 3, 13 and which contains lithium. According to this constitution, a negative electrode surface area is increased, a battery internal resistance is reduced, and high rate discharging characteristics are improved. Moreover, optimization of the material and the constitution of the porous support bodies, improvement in the cycle discharging characteristics, and thermal resistance or the like can be realized. Both a liquid electrolyte and a solid electrolyte can be applied as the electrolyte. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a negative electrode material for a lithium secondary battery.

In a lithium battery, when the negative electrode material is a flat surface lithium metal or lithium alloy, the surface area used for lithium ion desorption is very limited. Therefore, in order to increase the contact area between the negative electrode material and the electrolyte, a proposal has been made to use porous lithium of 25% or more and less than 75% of the bulk density of solid lithium as a negative electrode material by electrodeposition or the like (Patent Document). 1). By using such porous lithium as the negative electrode material, the contact area between the electrolyte and the negative electrode material can be increased, the internal resistance of the battery can be reduced, and heat generation and voltage loss at a large current can be reduced. Charging / discharging capacity at a large current can be increased. In other words, high rate characteristics can be realized.
JP-T-4-507171

  When the porous lithium is used in a primary battery, the above effect can be obtained, but the same effect cannot be expected as a negative electrode material for a secondary battery. This is because, in a disposable primary battery, it has an optimal porous structure at the beginning, and at the time of the first discharge, the porous lithium material of the negative electrode can achieve a high density and a large amount of lithium elution as intended. When used in a secondary battery, when lithium is re-deposited on the negative electrode during charging of the secondary battery, a phenomenon such as closing a porous hole occurs. As a result, there is a problem in that good characteristics similar to those of the first discharge are not reproduced in the second discharge. Moreover, it is difficult to control the porous structure such as the pore diameter and the porosity over a wide range only by the electrodeposition conditions of the plating treatment.

  An object of this invention is to provide the negative electrode material for lithium secondary batteries which can maintain a favorable high rate characteristic even if charging / discharging is repeated in a secondary battery.

  The lithium secondary battery negative electrode material of the present invention is characterized by comprising a porous support having a porous structure and a negative electrode active material containing lithium carried on the porous support.

  With the above configuration, the surface area of the negative electrode is increased, the internal resistance of the battery is reduced, the heat generation and the voltage loss at the time of a large current are reduced, and therefore the charge / discharge capacity can be expanded. That is, a high rate characteristic can be obtained. In addition, since the negative electrode active material containing lithium and the porous support are used as separate members, the outer shape of the porous support is stably maintained even when charging and discharging are repeated, and is supported on the porous support. Lithium simply elutes and precipitates repeatedly. For this reason, the negative electrode material of the secondary battery which can maintain a high rate characteristic over many cycles of discharge / charge can be provided. In addition, since the negative electrode active material and the porous support are separate members, the porosity is very large compared to the lithium porous support by electrodeposition, and the average pore diameter is set to 90% by volume or more. Can be bigger. For this reason, the porous support body of a more preferable form as a secondary battery can be obtained. The negative electrode material for a lithium secondary battery may be targeted for a liquid phase lithium secondary battery, or a solid phase lithium secondary battery for an electrolyte.

  The porous support can be formed of a conductor. In this configuration, there is no need for a current collecting layer for current collection, and as a result, for example, it is not necessary to consider securing the contact area between the current collecting layer and the electrode plate, and the configuration of the negative electrode material can be simplified. it can.

  Moreover, it is preferable that the porous support is formed of a metal having an oxidation potential equivalent to or higher than that of copper. With this configuration, the corrosion resistance of the porous support can be maintained when incorporated in a battery.

  The porous support may be formed of a material containing 50% or more of elemental. With this configuration, a porous support having both corrosion resistance and conductivity can be obtained.

  In addition, the porous support described above is formed of an insulator, and the porous support can also be configured to carry a current collecting layer electrically connected to the negative electrode active material for current collection. With this configuration, the choice of material for forming the porous support can be expanded to the insulator, and, for example, diversity can be ensured with respect to the shape of the pores of the porous support.

  The porous support is preferably formed of a material having a glass transition temperature of 100 ° C. or higher. With this configuration, it is possible to obtain thermal characteristics required when, for example, a negative electrode active material containing lithium is formed on a porous support.

  Moreover, the oxidation potential of the current collecting layer can be a metal equivalent to or higher than copper. With this configuration, the corrosion resistance of the current collecting layer incorporated in the battery can be maintained.

  In the above porous support, the average pore diameter (μm)> 2 × {(current collector layer thickness (μm)) + (negative electrode active material layer thickness (μm))} may be satisfied (this inequality is represented by And inequality (1). Here, when the current collecting layer is not used (when the porous support is formed of a conductor), the current collecting layer thickness is zero. The negative electrode active material layer thickness (μm) is the layer thickness before the start of use. With this configuration, the pores of the porous support can be prevented from being blocked or filled with an active material or the like.

  Moreover, a lithium ion conductive solid electrolyte can be arrange | positioned in contact with said negative electrode active material. With this structure, dendritic growth of lithium metal that causes a short circuit failure when the negative electrode active material is repeatedly eluted and precipitated can be suppressed. As a matter of course, the negative electrode material in which the lithium ion conductive solid electrolyte is arranged as described above is intended for a solid electrolyte lithium secondary battery.

  The lithium ion conductive solid electrolyte preferably contains Li, P, S, and O elements. With this configuration, it is possible to obtain a secondary battery with high rate characteristics with little performance deterioration due to use over time.

(Embodiment 1)
Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a negative electrode material 10 for a lithium secondary battery according to Embodiment 1 of the present invention. In FIG. 1, a porous support 3 made of a conductor carries a negative electrode active material 5 containing lithium. More specifically, the surface of the porous support 3 inside and outside the pores is covered with the lithium-containing negative electrode active material 5. The porous support 3 has a larger surface area when the porosity is higher, and when the Li-containing negative electrode active material 5 is supported on the surface thereof, more Li-containing negative electrode active material 5 can be supported on the surface. The porous support 3 preferably has continuous pores, but does not necessarily have to be continuous pores. In FIG. 1, the Li-containing negative electrode active material 5 is formed in a layer shape so as to cover the surface of the porous support 3, but it does not necessarily have a layer shape, and there is a portion that is only formed in an island shape depending on the region. In such a case, island-like regions and layer-like regions may be mixed. In the negative electrode material 10 for a lithium secondary battery shown in FIG. 1, since the porous support 3 is formed of a conductor, a current collecting layer for collecting current is not particularly necessary. When the porous support 3 is formed of a conductive material, the bottom side of the porous support is preferably fixed in a state where it is electrically connected to an electrode plate (not shown).

  FIG. 2 is a cross-sectional view showing negative electrode material 10 for a lithium secondary battery according to Embodiment 1 of the present invention when the porous support is formed of an insulator. Since the porous support 13 is formed of an insulator, the current collection layer 7 is formed so as to cover the surface of the porous support 13 for current collection, and lithium is formed on the current collection layer 7. The negative electrode active material layer 5 containing is formed. The current collecting layer 7 is preferably formed on the surface of the porous support 13 in a layered manner, but the lithium-containing negative electrode active material 5 formed thereon is preferably formed in a layered shape, but its area If it is large enough, it may be layered or island-shaped. When the current collector layer 7 and the Li-containing active material 5 are carried on the insulating porous support 13, the porous support 13 is attached to the electrode plate 1 as shown in FIG. The current collecting layer 7 is preferably extended to a position covering the electrode plate 1 to maintain electrical contact and secure a path through which electrons can move from the negative electrode to the external circuit.

  When the negative electrode material 10 shown in FIG. 1 to FIG. 3 is used for a secondary battery of a liquid phase electrolyte, the liquid phase electrolyte enters the pores of the porous support 3 and is supported on the surface. The state in contact with the contained active material 5 is maintained. The case of a solid electrolyte will be described in Embodiment 2 of the present invention.

  The average pore diameter (μm) of the porous supports 3 and 13 is larger than 2 × {(current collector layer thickness (μm)) + (Li-containing active material layer thickness (μm))} (the inequality (1) is To meet). When the average pore diameter of the porous support satisfies the above-mentioned conditions, the porous support 3, even when the cycle of elution and precipitation of the active material is repeated according to charge and discharge when applied to a secondary battery. The 13 pores are hardly filled with the Li-containing active material. That is, it is easy to realize the initial surface area of the active material layer before the start of charge / discharge. Obviously, if the porous support is a conductor (no current collecting layer) or the porous support is an insulator (necessary current collecting layer), which is easier to meet this condition, it is clearly porous The case of FIG. 1 in which the support is formed of a conductor and does not use a current collecting layer is easier to fill. When the porous support in FIG. 2 is an insulator, the average pore diameter of the porous support is substantially reduced by twice the thickness of the current collecting layer, and is easily blocked by the Li-containing active material 5. Because it becomes. Therefore, when the porous support is formed of a conductor, the pores are less likely to be blocked by the Li-containing active material due to repeated charge and discharge. However, even when the porous support is formed of an insulator, there is no problem as long as the average pore diameter can be formed sufficiently large. The average pore diameter can be imaged as the maximum value of the diameter of a sphere that can be pushed into the pores. The pore shape of FIG. 1 or FIG. 2 is a shape in which a continuous path is formed, but is not continuous as described above, and may not be similar to the pore shape shown in FIG. 1 or FIG. .

  The material of the porous support may be formed of any material as long as a sufficiently large average pore diameter and porosity can be secured. For example, an insulator such as a polymer resin or inorganic ceramics, or a conductor such as metal or carbon may be used. Specifically, polymer resins such as polypropylene, polyethylene, polyethersulfone, polyimide, polyetheramide, and polytetrafluoroethylene, and inorganic materials such as porous alumina, porous silica, porous zirconia, and porous silicon nitride Ceramics and conductors such as carbon (carbon), copper, aluminum, nickel, and stainless steel can be used. The porous manufacturing method may be any method, for example, a liquid phase process such as electrodeposition or electrodeposition, or a gas phase process such as various vapor depositions.

  The porosity of the porous supports 3 and 13 is preferably 10% by volume to 99% by volume. Among these, 50 volume%-95 volume% are especially preferable. This is because a sufficiently large surface area for supporting the Li-containing negative electrode active material can be secured. The average pore diameter of the porous supports 3 and 13 is preferably 2 μm to 150 μm.

  The current collecting layer 7 is made of a metal or alloy such as copper or nickel, and its thickness is preferably 0.1 μm to 10 μm in consideration of current collection. A more preferable thickness range is 0.5 μm to 5 μm. Moreover, it is preferable that the thickness of the Li containing active material layer 5 is 1 micrometer-50 micrometers. Considering the range of the average pore diameter of the porous support described above and the thickness range of the current collecting layer 7 and the Li-containing active material 5, it is convinced that the above inequality (1) is sufficiently satisfied. The

When the negative electrode material for a lithium secondary battery shown in FIGS. 1 to 3 is prepared so that the porosity and the average pore diameter satisfy the above-described conditions, and the Li layer is used as the negative electrode active material 5, for example, 10 Li deposition can be performed by resistance heating in a vacuum of ⁻⁶ torr or less. For Li deposition, any method other than the resistance heating method, for example, CVD (Chemical Vapor Deposition) can be used. Further, any method such as an electrodeposition method or an electrodeposition method may be used. When the porous support is formed of an insulator, the current collecting layer 7 can be formed by any vapor phase growth method such as direct current magnetron sputtering or CVD. Moreover, you may form the current collection layer 7 by the formation methods from liquid phases, such as an electrodeposition method and an electrodeposition method.

(Embodiment 2)
In the first embodiment described above, the negative electrode material 10 is mainly intended for a secondary battery of a liquid phase electrolyte. In Embodiment 2 of the present invention, an example in which a lithium ion conductive solid electrolyte (hereinafter referred to as a solid electrolyte) is used as an electrolyte will be described. FIG. 4 is a cross-sectional view showing a negative electrode material 10 for a lithium secondary battery according to Embodiment 2 of the present invention. In FIG. 4, the pores of the porous support 3 are filled with the solid electrolyte 9 and are in contact with the lithium-containing negative electrode active material 5. The solid electrolyte 9 can prevent surface roughness of the Li-containing active material 5 during charging and discharging. As a result, the internal resistance of the lithium secondary battery is reduced, and heat generation and voltage loss during charge / discharge of a large current can be reduced. And the capacity | capacitance at the time of charging / discharging of a large current can be enlarged.

Next, the outstanding characteristic of the negative electrode material for lithium secondary batteries of this invention is clarified by an Example. The lithium secondary battery to which the negative electrode material 10 of the present invention is applied is a coin-type secondary battery having an electrode area of about 2 cm ^{2 as} shown in FIG. The positive electrode material 20 used was an aluminum foil coated with LiCoO ₂ powder as an active material and acetylene black as a current collector in a thickness of 100 μm together with a binder. The separator 35 was made of polypropylene and had a thickness of 25 μm. As an electrolytic solution, an equal volume mixture of EC (Ethyl Carbonate) and DEC (Diethyl Carbonate) was used, and LiPF ₆ was dissolved at a concentration of 1M as a supporting salt. LiClO ₄ may be used instead of LiPF _{6 as} the supporting salt.

  Examples 1 to 4 of the present invention and Comparative Example 1 shown in Table 1 were used for the negative electrode material 10. Carbon (carbon material) and copper conductors and polypropylene insulators were used for the porous support of the present invention example. The porosity is 95% by volume of carbon, 96% by volume of copper, 90% by volume of polypropylene, and 90% by volume or more, and the average pore diameter is 20 μm. Considering that the conventional porous lithium has a porosity of 75% by volume at the maximum (Patent Document 1), the very large advantage of the present invention in which the porous support is formed of a member different from lithium is sufficient. Can be recognized. On the other hand, in Comparative Example 1, a solid copper layer was used. Further, in Example 3 of the present invention, which is an insulating porous support, a Ni current collecting layer was formed. Each negative electrode active material was a Li layer having a thickness of 5 μm.

  The above-described lithium secondary battery was subjected to a charge / discharge test between a current density of 30 mA and 4.2 V to 3 V. The results obtained are shown in Table 2. According to Table 2, in Examples 1 to 3 of the present invention, even after 10 cycles, the capacity was deteriorated only within the error range in Example 2 of the present invention, and the capacity of 100% was maintained. On the other hand, in Comparative Example 1, the capacity is reduced to 68% of the capacity of the first cycle when 10 cycles have elapsed. From the results of this Example, it was confirmed that when the negative electrode material of the present invention was used, a large discharge charge capacity was obtained and a lithium secondary battery excellent in high rate characteristics was obtained.

  Although the embodiments and examples of the present invention have been described above, the embodiments and examples of the present invention disclosed above are merely examples, and the scope of the present invention is the implementation of these inventions. It is not limited to the form. The scope of the present invention is indicated by the description of the scope of claims, and further includes meanings equivalent to the description of the scope of claims and all modifications within the scope.

  By using the negative electrode material for a lithium secondary battery of the present invention, the internal resistance of the lithium secondary battery can be reduced, and a high rate characteristic with little loss during charge / discharge of a large current is maintained even after repeated use. be able to.

It is sectional drawing which shows the negative electrode material for lithium secondary batteries in Embodiment 1 of this invention. It is sectional drawing which shows another negative electrode material for lithium secondary batteries in Embodiment 1 of this invention. It is a figure which shows the negative electrode material for lithium secondary batteries of FIG. 2 in detail. It is sectional drawing which shows the negative electrode material for lithium secondary batteries in Embodiment 2 of this invention. It is sectional drawing which shows the lithium secondary battery used for the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode plate, 3 Porous support body (conductor), 5 Negative electrode active material, 7 Current collection layer, 9 Solid electrolyte, 10 Negative electrode material for lithium secondary batteries, 13 Porous support body, 20 Positive electrode material, 35 Separator.

Claims (10)

A porous support having a porous structure;
A negative electrode material for a lithium secondary battery, comprising: a negative electrode active material containing lithium supported on the porous support.   The lithium secondary battery negative electrode material according to claim 1, wherein the porous support is made of a metal.   The lithium secondary battery negative electrode material according to claim 2, wherein the porous support is formed of a metal having an oxidation potential equal to or higher than that of copper.   The lithium secondary battery negative electrode material according to claim 2 or 3, wherein the porous support is made of a material containing 50% or more of carbon.   The porous support is formed of an insulator, and the porous support also supports a current collecting layer electrically connected to the negative electrode active material for current collection. The negative electrode material of a lithium secondary battery as described in 1.   The lithium secondary battery negative electrode material according to claim 5, wherein the porous support is formed of a material having a glass transition temperature of 100 ° C or higher.   7. The negative electrode material for a lithium secondary battery according to claim 5, wherein an oxidation potential of the current collecting layer is a metal equivalent to or higher than copper. 8.   In the porous support, the average pore diameter (μm)> 2 × {(current collecting layer thickness (μm)) + (negative electrode active material layer thickness (μm))}. The lithium secondary battery negative electrode material according to any one of 7.   The lithium secondary battery negative electrode material according to claim 1, wherein a lithium ion solid electrolyte is disposed in contact with the negative electrode active material. The lithium secondary battery negative electrode material according to claim 9, wherein the solid electrolyte contains Li, P, S, and O.

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