JP2009277381A - Lithium battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium battery in which a negative electrode layer is less apt to be peeled off from a solid electrolyte layer, even if chargings and dischargings are repeated. <P>SOLUTION: The battery is equipped with a positive electrode layer 13, a negative electrode layer 14, and a solid electrolyte layer (SE layer 15) to mediate the conduction of lithium ions between these both layers, and furthermore, equipped with an interface layer 16 between the negative electrode layer 14 and the SE layer 15. The negative electrode layer 14 in this lithium battery 1 contains at least Li, and the interface layer 16 contains at least fourteenth group element in the Periodic Table. Then, when the thickness of the negative electrode layer 14 is set to Tn, the thickness of the interface layer will be Tb, and the relation 0.005<Tb/Tn<0.5 is satisfied. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lithium battery in which a negative electrode layer is difficult to peel off from a solid electrolyte layer even when charging and discharging are repeated while having a solid electrolyte layer.

  A lithium battery has a negative electrode layer formed on a negative electrode current collector, a positive electrode layer, and an electrolyte layer interposed between both electrode layers. Among such lithium batteries, a lithium secondary battery that can be repeatedly charged and discharged (hereinafter simply referred to as a lithium battery) has attracted attention as a main power source for portable communication terminals and portable electronic devices.

  In recent years, as this lithium battery, an all-solid-state battery that does not use an organic electrolyte for lithium conduction between the positive and negative electrodes has been proposed (see, for example, Patent Document 1). All-solid-state batteries use a solid electrolyte (SE) as an electrolyte. Problems associated with using an organic electrolyte, such as a safety problem due to leakage of the electrolyte, and the boiling point of the organic electrolyte at high temperatures The problem of heat resistance due to volatilization exceeding, the problem that the ionic conductivity of the organic electrolytic solution is greatly reduced at a low temperature and the battery reaction is lowered, or the electrolytic solution is frozen can be solved.

  Here, Patent Document 1 discloses the use of a carbon material, Si, or Si oxide that forms an alloy with Li as the negative electrode layer (first electrode). In the same document, it is disclosed that Li metal itself or Li alloy is used as the negative electrode layer.

JP 2004-335455 A

  However, the lithium battery as disclosed in Patent Document 1 has a problem that the capacity of the battery decreases due to insufficient bonding between the negative electrode layer and the solid electrolyte layer as the battery is charged and discharged.

  For example, when the negative electrode layer is formed of an element such as Si that forms an alloy with Li, the negative electrode layer easily peels off from the solid electrolyte layer because the volume change of the negative electrode layer accompanying charging / discharging of the battery is large.

  On the other hand, when the negative electrode layer is formed of Li itself or a Li alloy, Li precipitation occurs at the time of charging the battery and Li dissolves at the time of discharging at the interface between the negative electrode layer and the solid electrolyte layer. It becomes difficult to maintain the bonding with the solid electrolyte layer. As described above, when the bonding between the negative electrode layer and the solid electrolyte layer is interrupted, the effective area of the battery is reduced and the capacity of the battery is reduced.

  The present invention has been made in view of the above circumstances, and one of its purposes is to maintain the junction between the negative electrode layer and the solid electrolyte layer even when the battery is repeatedly charged and discharged, and the discharge capacity can be maintained. An object of the present invention is to provide a lithium battery that does not easily decrease.

  The present invention achieves the above-mentioned object by forming an interface layer that closely adheres both the negative electrode layer and the solid electrolyte layer, and by defining a thickness ratio between the interface layer and the negative electrode layer.

  The present invention relates to a lithium battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer that mediates conduction of lithium ions between the two layers. The lithium battery of the present invention includes an interface layer between the negative electrode layer and the solid electrolyte layer, the negative electrode layer contains at least Li, and the interface layer contains at least a group 14 element of the periodic table. The lithium battery of the present invention is characterized by satisfying 0.005 <Tb / Tn <0.5, where Tn is the thickness of the negative electrode layer and Tb is the thickness of the interface layer.

  In the lithium battery of the present invention having the interface layer, the interface layer promotes diffusion of Li, which moves from the positive electrode layer to the negative electrode layer during charging, into the negative electrode layer. In particular, the interface layer containing a Group 14 element in the periodic table has a high ability (diffusion ability) to occlude and diffuse Li that has moved from the positive electrode layer through the solid electrolyte layer during charging of the lithium battery. . Therefore, according to the configuration of the present invention, Li does not easily precipitate at the negative electrode layer / solid electrolyte layer interface during charging, and the interface between the two layers is not expanded. As a result, it is possible to obtain a lithium battery in which the bonding between the negative electrode layer and the solid electrolyte layer is favorably maintained even when charging and discharging are repeated, and the discharge capacity is hardly reduced.

  Further, setting the ratio of the thickness of the interface layer to the thickness of the negative electrode layer (Tb / Tn) in an appropriate range also plays an important role in maintaining the bonding between the negative electrode layer and the solid electrolyte layer. Specifically, if Tb / Tn is too large, the interfacial layer itself has better Li storage than the Li diffusion capacity of the interfacial layer into the negative electrode layer, and the volume change of the interfacial layer becomes larger, resulting in a negative electrode layer / solid electrolyte. The bond at the layer interface is broken. If Tb / Tn is too small, the effect of providing the interface layer is low, and the junction at the negative electrode layer / solid electrolyte layer interface is also destroyed. A more preferable range of Tb / Tn is 0.01 to 0.4.

  Hereinafter, preferred embodiments of the present invention will be described.

  As one form of the lithium battery of the present invention, the group 14 element of the periodic table contained in the interface layer is preferably Si.

  By constituting the interface layer with Si, a lithium battery having further excellent cycle characteristics can be obtained.

  As one form of the lithium battery of the present invention, the negative electrode layer is preferably formed by a vapor phase method.

  By forming the negative electrode layer by a vapor phase method, the lithium battery can be thinned.

As one form of the lithium battery of the present invention, the solid electrolyte layer preferably contains at least Li ₂ S and P ₂ S ₅ .

  If it is sulfide as described above, a solid electrolyte layer having high lithium ion conductivity can be obtained. As a result, the internal resistance of the lithium battery can be reduced.

By the way, when a solid electrolyte layer is formed of sulfide, a bias of lithium ions occurs between the solid electrolyte layer and the positive electrode layer, and the internal resistance of the battery tends to increase. Therefore, as one embodiment of the lithium battery of the present invention, it is preferable to provide a buffer layer between the positive electrode layer and the solid electrolyte layer to buffer the deviation of lithium ions in the vicinity of the interface between these two layers. As a material of the buffer layer, a lithium conductive oxide, for example, Li _X La _{(2-X) / 3} TiO ₃ (X = 0.1 to 0.5), Li ₄ Ti ₅ O ₁₂ , Li _{3. 6} Si _0.6 P _0.4 O ₄ , Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _1.8 Cr _0.8 Ti _1.2 (PO ₄ ) ₃ , Li _{1 And} at least one of _0.4 In _0.4 Ti _1.6 (PO ₄ ) ₃ , LiTaO _3, and LiNbO ₃ .

  As one embodiment of the lithium battery of the present invention, the positive electrode layer is preferably an oxide of lithium and at least one element selected from Mn, Fe, Co, and Ni.

  By using an oxide containing the above element as the positive electrode layer, a lithium battery having a large discharge capacity can be obtained.

  According to the lithium battery of the present invention, peeling of the negative electrode layer from the solid electrolyte layer accompanying charging / discharging of the battery can be prevented. As a result, it is possible to obtain a lithium battery whose capacity does not easily decrease even when used repeatedly.

  Hereinafter, preferred embodiments of the present invention will be described in detail.

≪Overall structure≫
FIG. 1 is a longitudinal sectional view of a lithium battery according to the present embodiment. The lithium battery 1 has a laminated structure in which a positive electrode layer 13, a solid electrolyte layer (SE layer) 15, an interface layer 16, a negative electrode layer 14, and a negative electrode current collector layer 12 are laminated on a positive electrode current collector layer 11 in this order. have. In the lithium battery 1 of the present invention, the range of Tb / Tn is defined when the thickness of the negative electrode layer 14 is Tn and the thickness of the interface layer 16 is Tb. Hereinafter, each configuration will be described in detail, and Tn, Tb and Tb / Tn will also be described.

≪Each component≫
(Positive electrode current collector layer)
The positive electrode current collector layer 11 shown in FIG. 1 is a metal thin plate having a predetermined thickness, and also serves as a substrate that supports each layer described later. As the positive electrode current collector layer 11, one selected from aluminum (Al), nickel (Ni), alloys thereof, and stainless steel can be suitably used. In addition, a positive electrode current collector layer may be formed by forming a metal film over an insulating substrate. The current collector layer 11 made of a metal film can be formed by a PVD method (physical vapor deposition method) or a CVD method (chemical vapor deposition method). Examples of the PVD method include a vacuum deposition method, a sputtering method, an ion plating method, and a laser ablation method. Examples of the CVD method include a thermal CVD method and a plasma CVD method.

(Positive electrode layer)
The positive electrode layer 13 is a layer containing an active material that occludes and releases lithium ions. As the positive electrode active material, an oxide of lithium and at least one element selected from Mn, Fe, Co, and Ni can be suitably used. For example, there may be mentioned, such as LiCoO ₂ and _{LiNiO 2, LiMnO 2, LiNi 0.5} Mn 0.5 O 2, LiCo 0.5 Fe 0.5 O 2.

  The positive electrode layer 13 may further contain a conductive additive. As a conductive support agent, what consists of metal fibers, such as carbon black, such as acetylene black, natural graphite, thermal expansion graphite, carbon fiber, ruthenium oxide, titanium oxide, aluminum, and nickel, can be utilized, for example. In particular, carbon black is preferable because it can secure high conductivity in a small amount.

  As a method for forming the positive electrode layer 13 described above, a dry method such as a PVD method or a CVD method, or a wet method such as a coating method or a screen printing method can be used. If a wet method is used, a binder is included in the positive electrode layer so that the active materials are bound to each other, or the active material and a material other than the active material (electrolyte particles and conductive aid). Also good. As such a binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or the like can be used.

(Solid electrolyte layer)
The solid electrolyte layer (SE layer) 15 preferably has a lithium ion conductivity (20 ° C.) of 10 ⁻⁵ S / cm or more and a lithium ion transport number of 0.999 or more. In particular, the lithium ion conductivity may be 10 ⁻⁴ S / cm or more and the lithium ion transport number may be 0.99999 or more. The SE layer 15 preferably has an electronic conductivity of 10 ⁻⁸ S / cm or less. Examples of the material of the SE layer 15 include oxides (for example, Li—P—O—N), sulfides (for example, Li—PS—O and Li—PS), amorphous films, and polycrystalline films. It is preferable to comprise. In particular, Li—PS, which consists of Li ₂ S and P ₂ S ₅ , has excellent lithium ion conductivity, so that when it is employed in the SE layer, the performance of the battery can be improved.

  As a method for forming the SE layer 15, a solid phase method or a vapor deposition method can be used. Examples of the solid phase method include forming a raw material powder using a mechanical milling method and sintering the raw material powder. On the other hand, examples of the vapor deposition method include a PVD method and a CVD method. When the SE layer 15 is formed by the vapor deposition method, the thickness of the SE layer 15 can be made thinner than when the SE layer is formed by the solid phase method.

(Interface layer)
The interface layer 16 is a layer that plays a role of ensuring the bonding between the SE layer 15 and a negative electrode layer 14 described later. As the material of the interface layer 16, a group 14 element (particularly, Si) of the periodic table can be used. The content of Group 14 element in the periodic table in the interface layer is preferably 50% by mass or more, and more preferably 80% by mass or more. In particular, when the interface layer 16 is substantially made of Si, a lithium battery having excellent discharge characteristics can be obtained.

  The thickness Tb of the interface layer 16 is preferably in the range of 0.01 μm to 0.4 μm. By having such Tb, the function of the interface layer for maintaining the bonding between the negative electrode layer and the solid electrolyte layer can be sufficiently exhibited. A more preferable range of Tb is 0.02 μm to 0.2 μm.

(Negative electrode layer)
The negative electrode layer 14 is composed of a layer containing an active material that occludes and releases lithium ions. Specifically, Li metal or Li alloy (for example, Li—Al, Li—Mn—Al, etc.) is used as the negative electrode layer 14. Since such a negative electrode layer 14 can provide the negative electrode layer 14 itself with a function as a current collector, the negative electrode current collector 12 described later can be omitted.

  The thickness Tn of the negative electrode layer 14 is preferably in the range of 0.1 μm to 10 μm. By having such negative electrode layer thickness Tn, it can be set as a lithium battery provided with the discharge capacity which can be used for various uses. A more preferable range of the thickness Tn of the negative electrode layer 14 is 0.5 μm to 2 μm.

  Moreover, the formation method of the negative electrode layer 14 utilizes vapor phase vapor deposition methods, such as PVD method and CVD method. By using the vapor deposition method, the thickness of the negative electrode layer 14 can be reduced and contact with the SE layer 15 can be sufficiently ensured as compared with the case where the negative electrode layer 14 is formed with a metal foil.

(Negative electrode current collector layer)
The negative electrode current collector layer 12 is a metal film for supplying power to the negative electrode layer 14. As the negative electrode current collector layer 12, one selected from copper (Cu), nickel (Ni), iron (Fe), chromium (Cr), and alloys thereof can be suitably used. Similarly to the case of the positive electrode current collector layer 11, the negative electrode current collector layer 12 can also be formed by a PVD method or a CVD method. As already described, the negative electrode current collector layer 12 can be omitted.

(Tb / Tn specification)
In the lithium battery 1 of the present invention, Tb / Tn, which is the ratio between the thickness Tn of the negative electrode layer 14 and the thickness Tb of the interface layer 16, is defined as follows.
0.005 <Tb / Tn <0.5

  By setting Tb / Tn, which is the ratio of the thickness of the interface layer 16 and the negative electrode layer 14, to 0.005 to 0.5, a sufficient bond between the SE layer 15 and the negative electrode layer 14 can be secured. In particular, Tb / Tn is preferably 0.01 to 0.4. When Tb / Tn is 0.005 or less, the interface layer thickness Tb is relatively thin, and it is difficult to maintain the bonding between the negative electrode layer and the solid electrolyte layer. On the other hand, when Tb / Tn is 0.5 or more, the negative electrode layer thickness Tb is relatively thick. Therefore, the volume change of the interface layer accompanying the charging / discharging of the battery is large, and the negative electrode layer, the solid electrolyte layer, It becomes difficult to maintain the bonding.

(Other)
Although not shown in FIG. 1, a buffer layer may be provided between the positive electrode layer 13 and the SE layer 15 so as to buffer the deviation of lithium ions in the vicinity of the interface between these two layers. When the uneven distribution of lithium ions occurs, the electrical resistance at that position increases and the discharge capacity of the lithium battery decreases. As the buffer layer, for example, a lithium ion conductive oxide, specifically, Li _x La _{(2-x) / 3} TiO ₃ (x = 0.1 to 0.5), Li ₄ Ti ₅ O ₁₂ , Li _3.6 Si _0.6 P _0.4 O ₄ , Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _1.8 Cr _0.8 Ti _1.2 (PO ₄ ) ₃ , LiNbO ₃ , LiTaO ₃ , Li _1.4 In _0.4 Ti _1.6 (PO ₄ ) _{3 and the} like.

  Hereinafter, a coin cell type lithium battery (samples 1 to 7) having the configuration described in the embodiment was actually manufactured, and charge / discharge cycle characteristics were examined. In the lithium battery of this example, the negative electrode current collector is omitted.

  In this example, by changing the thickness of the interface layer 16, seven samples having different ratios Tb / Tn between the thickness Tb of the interface layer 16 and the thickness Tn of the negative electrode layer 14 were produced. The configuration other than the interface layer 16 is common to all samples.

<Preparation of sample>
A thin plate made of SUS316L having a thickness of 100 μm was prepared as the positive electrode current collector layer 11. This thin plate also serves as a base material for supporting each layer of the battery 1.

A positive electrode layer 13 made of LiCoO ₂ was formed on the positive electrode current collector layer 11 by electron beam evaporation. The film thickness of the positive electrode layer 13 was 1 μm.

A buffer layer was formed by depositing LiNbO ₃ on the positive electrode layer 13 by excimer laser ablation. The thickness of the buffer layer was 0.02 μm (20 nm).

An SE layer 15 having a Li—PS composition was formed on the buffer layer by excimer laser ablation. When the SE layer 15 was formed, lithium sulfide (Li ₂ S) and phosphorus pentasulfide (P ₂ S ₅ ) were used as raw materials, and the molar ratio of Li / P in the SE layer 15 was adjusted to 2.0. . The thickness of the SE layer 15 was 5 μm.

  An interface layer 16 made of Si was formed on the SE layer 15 by resistance heating vapor deposition. When the interface layer 16 was formed, seven samples having different interface layer thicknesses were prepared by adjusting the amount of Si deposited. The thickness Tb of the interface layer 16 of each sample 1 to 7 was 0.005 μm, 0.01 μm, 0.02 μm, 0.1 μm, 0.2 μm, 0.4 μm, and 0.5 μm in the order of the sample numbers.

  The negative electrode layer 14 was formed by vapor-depositing Li on the interface layer 16 by a resistance heating vapor deposition method. The thickness Tn of the negative electrode layer 14 was 1 μm.

<Measurement of internal resistance value>
A charge / discharge cycle test was performed using the lithium batteries of Samples 1 to 7 described above. Specifically, the charge / discharge current density was set to 0.05 mA / cm ² , the battery was charged to 4.2 V, and then the charge / discharge cycle test was performed 100 cycles with the work of discharging to 3.0 V as one cycle. The capacity maintenance rate was obtained. The capacity maintenance rate is obtained by the following equation.
Capacity maintenance rate (%) = (discharge capacity at each cycle / maximum discharge capacity) × 100
The results of the charge / discharge cycle test are shown in Table 1.

  As is clear from the results in Table 1, in Samples 2 to 6 where Tb / Tn was more than 0.005 and less than 0.5, the capacity retention rate after 100 cycles exceeded 90%. On the other hand, Sample 1 with Tb / Tn of 0.005 and Sample 7 with the same ratio of 0.5 had a capacity retention rate of 80%.

  In response to the above results, the lithium batteries of Samples 1 to 7 were disassembled and examined. In the batteries of Samples 2 to 6, the interface layer was not peeled, whereas the batteries of Samples 1 and 7 were used. Then, peeling of the interface layer was observed.

  From the above, it has been clarified that by defining the negative electrode layer thickness Tn, the interface layer thickness Tb and the ratio Tb / Tn in the lithium battery, a lithium battery having excellent cycle characteristics can be obtained. .

  The embodiments of the present invention are not limited to those described above, and can be appropriately changed without departing from the gist of the present invention. For example, C, Ge, Sn, and Pb, which are Group 14 elements of the periodic table other than Si, can also be used as constituent elements of the interface layer. These Group 14 elements of the periodic table have the common property that they can occlude Li that has migrated from the positive electrode layer during battery charging and can be diffused into the negative electrode layer, so that it is similar to the interface layer made of Si. It can be expected to demonstrate the functions of.

  The lithium battery of the present invention can be suitably used as a power source for portable devices.

It is a schematic block diagram of the lithium battery shown to embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lithium battery 11 Positive electrode collector layer 12 Negative electrode collector layer 13 Positive electrode layer 14 Negative electrode layer 15 SE layer 16 Interface layer

Claims (6)

A lithium battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer that mediates conduction of lithium ions between the two layers,
An interface layer is provided between the negative electrode layer and the solid electrolyte layer,
The negative electrode layer contains at least Li,
The interface layer contains at least a group 14 element of the periodic table,
When the thickness of the negative electrode layer is Tn and the thickness of the interface layer is Tb,
A lithium battery characterized in that the ratio of Tn and Tb satisfies the following formula.
0.005 <Tb / Tn <0.5   The lithium battery according to claim 1, wherein the group 14 element of the periodic table contained in the interface layer is Si.   The lithium battery according to claim 1, wherein the negative electrode layer is formed by a vapor phase method. The lithium battery according to claim 1, wherein the solid electrolyte layer contains at least Li ₂ S and P ₂ S ₅ . Furthermore, between the positive electrode layer and the solid electrolyte layer, provided with a buffer layer for buffering the bias of lithium ions in the vicinity of the interface between these two layers,
The buffer _{layer, Li X La (2-X} ) / 3 TiO 3 (X = 0.1~0.5), Li 4 Ti 5 O 12, Li 3.6 Si 0.6 P 0.4 O 4 , Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _1.8 Cr _0.8 Ti _1.2 (PO ₄ ) ₃ , Li _1.4 In _0.4 Ti _1.6 ( The lithium battery according to claim 4, wherein the lithium battery is at least one of PO ₄ ) ₃ , LiTaO _3, and LiNbO ₃ .   The lithium battery according to claim 1, wherein the positive electrode layer is an oxide of lithium and at least one element selected from Mn, Fe, Co, and Ni.

Images