JP3331691B2 - Positive electrode for lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode for a lithium secondary battery, and more particularly to a positive electrode for a lithium secondary battery having a high discharge energy density during large current discharge.

[0002]

2. Description of the Related Art Lithium metal, which has the lowest potential and the highest energy density per unit weight and unit volume, has attracted attention as a negative electrode active material for secondary batteries aiming at higher energy density. As a positive electrode active material for the negative electrode, LiMn ₂ giving a high energy density is used.
Attention has been paid to metal oxides containing lithium such as O ₄ , especially spinel compounds. Such a lithium secondary battery is reported, for example, in JP-A-2-139860.

[0003]

As the LiMn ₂ O ₄ constituting the positive electrode of the lithium secondary battery, LiMn ₂ O ₄ powder obtained by mixing MnO ₂ and LiOH and firing the mixture is used. Then, a binder and a conductive agent are mixed with the LiMn ₂ O ₄ powder and formed into a plate to form a positive electrode. The positive electrode thus prepared has a crystal defect in the LiMn ₂ O ₄ powder itself, a large number of crystal grain boundaries, and an amorphous material comprising a binder and a conductive agent mixed in the LiMn ₂ O ₄ powder. The region inhibits lithium migration. Therefore, the apparent mobility of lithium ions at the positive electrode is low. In particular,
Even if a high-current discharge is attempted, crystal defects, crystal grain boundaries, and amorphous regions are large and the rate of movement of lithium ions is limited, and high-current discharge cannot be performed. Only parts can be taken out. The present inventor has observed a phenomenon that the discharge current density decreases as the discharge current density increases, using the above-described conventional positive electrode.

[0004] Such a problem occurs not only at the time of discharging but also at the time of charging. For example, there is a problem that quick charging cannot be performed. The present invention has I to solve such problems Utosuru increases the mobility of the apparent lithium ions in the positive electrode, and an object thereof is to provide a positive electrode for a high lithium secondary battery discharge energy density at the time of large current discharge .

[0005]

Means for increasing the apparent mobility of lithium ions at the positive electrode include increasing the apparent positive electrode area, reducing or eliminating amorphous regions, and eliminating crystal defects and crystal grain boundaries. That is. The present invention embodies such means. That is,
The positive electrode for a lithium secondary battery of the first invention includes a conductive substrate and a metal oxide containing lithium protruding on the surface of the substrate.
A positive electrode active material comprising:
The angle between the plate surface normal direction and the normal direction should be 0 ≦ θ <90 °
It is characterized by being projected and growing in a columnar shape .

The positive electrode for a lithium secondary battery according to the second aspect of the present invention comprises a conductive substrate and a positive electrode active material composed of LiMn ₂ O ₄ crystal composed of one single crystal in the thickness direction of the substrate surface. It is characterized by having. In both the first invention and the second invention, a metal plate of Ni, Al, Cu or the like or a plate material formed of other conductive material can be used as the substrate of the positive electrode. The size and shape of the substrate can be arbitrary depending on the application.

The positive electrode according to the first aspect of the present invention comprises the substrate
Positive electrode made of metal oxide containing lithium protruding on the surface
And the positive electrode active material is formed by projecting so that an angle θ with respect to the normal direction of the substrate surface satisfies 0 ≦ θ <90 ° and growing in a columnar shape . Examples of lithium-containing metal oxides include LiMn ₂ O ₄ , Li _X WO ₃ , LiCoO ₂ , L
iNi _0.4 Co _0.6 O ₂ , Li _0.5 MnO ₂ , LiV ₂
O _{5 and the} like can be mentioned.

[0008] and that these metal oxides substrate surface normal direction and the angle theta has grown into columnar is 0 ≦ θ <90 °
As means for performing this, oblique incidence evaporation in vacuum evaporation film formation can be employed. As shown schematically in FIG. 1, this method uses the substrate 2 and the target 2 so that the angle θ between the normal line of the substrate 2 and the incident direction of the vapor deposition particles in the vacuum chamber 1 satisfies 0 ≦ θ <90 °. The target 3 is irradiated with energy such as an ion beam, and the constituent element particles of the target 3 are beaten out and adhered to the substrate 2. By this oblique incidence evaporation, columnar projections having a diameter of about several nanometers are formed on the substrate. The direction in which the protrusion extends coincides with the incident direction. Therefore, as the angle θ between the normal and the incident direction of the vapor deposition particles increases, the columnar projections formed become more inclined with respect to the substrate surface. Further, as θ increases, the interval between adjacent columnar projections tends to increase. However, if θ is too large, it becomes difficult to perform uniform deposition on the substrate surface. Therefore, the preferable angle of θ is 5
About 80 degrees.

As a target, a metal oxide constituting the positive electrode active material can be used as it is. Alternatively, only a metal oxide except lithium may be vacuum-deposited to form a projection on the substrate, and then lithium may be absorbed. In order to widen the interval between the columnar projections, it is also preferable to perform etching with an appropriate etching agent to widen the interval between the columnar projections.

As a specific example, Li_xWO_ThreeThe positive electrode active material
A method for manufacturing a positive electrode will be described. Conductive plate as substrate
Was used. The target is WO_ThreeWith
Was. Put this substrate and target in a vacuum chamber,
Between the normal of the target and the direction of incidence of the particle incident from the target
The degree θ was 75 °. Then, 6.66 × 10
^-6mbar, and the temperature of the substrate is kept at 26 ° C.
The target is irradiated with a beam and sputtering is performed.
WO at a deposition rate of 10ｓｅ / sec_ThreeWas deposited. this
WO on the substrate by_ThreeThe columnar projections are densely forested
The metal oxide thin film shown in FIG. Height of this columnar projection
The length was about 0.3 μm and the diameter was 0.04 μm. This pillar
WO_ThreeAnode active material by adsorbing lithium on
Quality Li_xWO _ThreeIs a book protruding in a columnar shape standing on the substrate surface
The positive electrode for a lithium secondary battery of the first invention is obtained.

The positive electrode for a lithium secondary battery according to the first aspect of the present invention has a positive electrode active material composed of protrusions protruding from the substrate surface. Naturally, a space is formed between the protrusions. The electrolyte that carries lithium ions penetrates into this space. In other words, the electrolytic solution penetrates into a deep part away from the surface of the positive electrode. As a result, the contact area between the apparent positive electrode surface and the electrolytic solution is dramatically increased. Therefore, the transfer of lithium ions between the positive electrode active material and the electrolyte becomes extremely easy.

Further, no binder or conductive agent is used for the columnar positive electrode active material. Therefore, the transfer of lithium ions is not hindered by the binder and the conductive agent. The problem is that since the positive electrode active material is formed by vacuum evaporation, the crystals of the lithium-containing metal oxide are relatively small, and many of them contain crystal grain boundaries. The grain boundaries hinder the movement of lithium ions. This problem can be overcome by making the columnar projections single crystallized.

The positive electrode for a lithium secondary battery according to the second aspect of the present invention comprises:
LiMn ₂ formed on the substrate surface as the positive electrode active material
O ₄ crystals are used. This crystal is preferably a single crystal, but may be composed of one crystal in the thickness direction of the thin film constituting the positive electrode active material, and may be composed of a plurality of crystals in the direction of spreading on the surface of the substrate. . As shown in FIG. 3, the single crystal of LiMn ₂ O ₄ is composed of oxygen atoms 11 shown by white spheres, manganese atoms 12 shown by black spheres, and lithium atoms 13 shown by dashed spheres, as shown in FIG. Various octahedrons are connected to form an octahedral frame shown in FIG. Oxygen atoms occupy each vertex of the octahedron, and manganese atoms exist at the center of the octahedron. Lithium atoms exist in a space extending in the three-dimensional direction of the octahedral frame. The arrows shown in FIG. 3 indicate these spaces. Lithium atoms move through these three-dimensionally extending tunnel-like spaces. Then, at the time of charging, lithium atoms in the electrolyte enter the tunnel-like space of the crystal and advance into the crystal. vice versa,
At the time of discharge, lithium atoms are released into the electrolyte from these spaces.

In this specification, the term “LiMn ₂ O ₄ single crystal” is used, but lithium atoms enter and exit the crystal by charging and discharging the battery. Therefore, the expression of a single crystal of LiMn ₂ O ₄ is not strictly accurate. Here, one in which the octahedral frame shown in FIG. 5 extends three-dimensionally is defined as one single crystal and one LiMn ₂ O ₄ single crystal.

Since the tunnel-like space existing in one LiMn ₂ O ₄ single crystal extends continuously three-dimensionally, lithium atoms can easily reach and be held deep in the crystal, and Conversely, it can move to the crystal surface more easily from the deep part of the crystal. Since the thin-film active material of the positive electrode for a lithium secondary battery of the second invention has only one single crystal in the thickness direction of the thin film, it is held on the surface of the thin film in contact with the electrolyte. Lithium atoms can easily travel through the tunnel-like space of the single crystal to a deep portion of the thin film near the substrate on the opposite side of the thin film.

[0016] Incidentally, LiM a Mn of LiMn ₂ O ₄ or partially substituted by another element, during lamination structure of LiMn ₂ O ₄
By introducing a layer of a substance having a different lattice constant from n ₂ O ₄ , the crystal of LiMn ₂ O ₄ is intentionally strained,
It is also conceivable to increase the mobility of lithium atoms by expanding the tunnel-like space. Further, it is conceivable to improve the fatigue fracture strength of the thin-film crystal by forming an inclined structure in which only the outermost surface layer in contact with the electrolyte of the thin-film crystal is made amorphous.

There are the following methods for producing the thin film active material for the positive electrode for a lithium secondary battery of the second invention. This method is based on the idea of atomic manipulation,
As shown in FIG. 2, Mn atoms 12 are arranged on the surface of the substrate 2 so as to match the lattice constant of the Mn layer of LiMn ₂ O ₄ .
Next, on these Mn atoms 12, Li atoms 13 and O atoms 1
1 are independently laminated. Thereafter, a heat treatment is performed to form a seed crystal in which a single crystal of LiMn ₂ O ₄ is uniformly formed on the entire surface of the substrate. Then, based on this seed crystal, L
iMn ₂ O ₄ is grown in the thickness direction of the substrate to form a thin film crystal.

One method for growing LiMn ₂ O ₄ in the thickness direction of the substrate is solid phase epitaxy employing the concept of single crystal growth by a zone melting method. In this method, each atomic layer of lithium, manganese, and oxygen is laminated on the single crystal of the substrate on which the single crystal has been formed to a thickness of a practical positive electrode. Thereafter, the substrate is heated in a belt shape from one end side of the substrate, and each of the laminated atomic layers is heat-treated in a belt shape. Then, the heater is slowly moved to the other end side, and the heated portion is moved to the other end side in a belt shape in accordance with the heater. Thus, the crystal is grown from one end to the other end. The defects existing in each of the stacked atomic layers are collected at the other end by heating by the heater. By removing the other end, a LiMn ₂ O ₄ single crystal can be grown on the entire surface of the substrate.

Another method is immersion pulling of the raw material melt.
Method, a single layer of LiM
n_TwoO_FourSeveral atomic layers of Li are formed in the same manner as when a single crystal is made.
Mn _TwoO_FourMake a seed crystal consisting of a single crystal. This seed crystal
LiMn_TwoO_FourImmersion in the raw material melt, then relax
LiMn_TwoO_FourGrown single crystal
And a single crystal having a thickness of the positive electrode.

[0020]

In the positive electrode for a lithium secondary battery according to the first aspect of the present invention, a metal oxide containing lithium constituting a positive electrode active material is protruding in a columnar shape on the surface of a substrate forming the positive electrode. Therefore, a space is formed between the positive electrode active materials protruding in a columnar shape. This space is filled with the electrolyte. Therefore, the contact area of the positive electrode in contact with the electrolytic solution is extremely large. Therefore, even if there is an electric double layer between the electrolytic solution and the positive electrode active material that inhibits the transfer of lithium ions, the electric double layer can be compensated for by the large contact area. Further, the columnar projection has an extremely small cross section. Therefore, the lithium atoms held on the surface of the columnar projection can relatively easily move to the center of the columnar projection.

By these effects, the positive electrode for a lithium secondary battery according to the first aspect of the present invention has a high apparent lithium ion mobility and can easily discharge and charge a large current. In addition, even when a large current flows, the energy density does not decrease much. The positive electrode for a lithium secondary battery according to the second aspect of the present invention comprises a thin film LiMn ₂ O ₄ crystal having a positive electrode active material formed on the surface of a substrate. This thin film LiMn ₂ O ₄ crystal is composed of one single crystal in the thickness direction of the thin film. That is, a plurality of single crystals are not stacked in the thickness direction. For this reason,
Lithium ions held on the surface of the positive electrode can easily move in the crystal through the tunnel-like space of the crystal. Therefore, the apparent mobility of lithium ions is high.

The high mobility of lithium ions makes it easy to discharge and charge a large current. In addition, even when a large current flows, the energy density does not decrease much. Further, since the thin film active material is a single crystal in the thickness direction, there are no crystal grain boundaries or defects in the grains in the thickness direction. In addition, there is naturally no substance such as a binder that hinders the movement of lithium ions. These also act to increase the mobility of lithium ions. Also, if there is a defect in the LiMn ₂ O ₄ particles, strain energy is accumulated in the crystal defect due to the expansion and contraction of the crystal due to the inflow and out of lithium ions accompanying the charging and discharging of the battery, and the crystal structure is destroyed and finely divided. Occur. As a result, the life of the positive electrode of the battery is shortened. Since the thin film crystal of the present invention is a single crystal, there is no such a defect. Therefore, destruction of the crystal structure hardly occurs. Also, pulverization is unlikely to occur. Therefore, the positive electrode of the second invention can be expected to have a charge / discharge life of about 500 times.

[Brief description of the drawings]

FIG. 1 is a schematic view of a vapor deposition apparatus illustrating oblique incidence vacuum vapor deposition for forming columnar projections of a positive electrode for a lithium secondary battery according to the first invention.

FIG. 2 is an electron micrograph showing the columnar crystal structure of WO ₃ obtained by oblique incidence vacuum deposition.

FIG. 3 is a view showing an arrangement of atoms in a LiMn ₂ O ₄ single crystal of the second invention.

FIG. 4 is a perspective view of an octahedron illustrating an arrangement of oxygen and manganese in a LiMn ₂ O ₄ single crystal.

FIG. 5 is a perspective view showing an octahedral arrangement of a LiMn ₂ O ₄ single crystal.

FIG. 6 is a conceptual diagram illustrating a method for forming a single layer of LiMn ₂ O ₄ single crystal on a substrate.

[Explanation of symbols]

1. Vacuum chamber 2. Substrate 3.
Target 11: oxygen atom 12: manganese atom 13
... Lithium atom

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-328245 (JP, A) JP-A-4-328244 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02 H01M 10/40

Claims (2)

(57) [Claims] 1. A conductive substrate and a projecting rib on the surface of the substrate.
A positive electrode active material comprising a metal oxide containing titanium.
And, positive electrode active material is the substrate surface normal direction and the angle is 0 ≦ theta <
It is noted that it is projected at 90 ° and grown in a columnar shape.
The positive electrode for lithium secondary batteries. 2. A lithium secondary battery comprising a conductive substrate and a positive electrode active material comprising a LiMn ₂ O ₄ crystal composed of one single crystal in a thickness direction of the substrate surface. Positive electrode.