JP5939623B2 - Highly crystalline aLi2MnO3- (1-a) Li (Nix, Coy, Mnz) O2-based nanostructure electrode material and method for producing the same by electrospinning - Google Patents

Description

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

The development of clean energy devices has been actively carried out in order to realize a sustainable society where global environmental issues are actively taken up. Among them, in the development of storage batteries for electric vehicles and plug-in hybrid vehicles, research on Li-ion batteries has attracted attention, and the development of large-capacity electrode materials has attracted particular attention. In addition, the present situation is that improvement of high output characteristics suitable for in-vehicle use is also desired.
In recent years, there have been many reports showing that high-output characteristics can be obtained by synthesizing nanoelectrode materials for increasing the output of Li-ion batteries and using them in batteries (Non-patent Document 1). High output can be obtained by using nanomaterials for the electrodes. By making the particle size of the solid active material constituting the electrodes nano, the electrodes pass through the gap where Li ions exist between multiple nanoparticles. This is because the inside of the electrode can be easily diffused in a short time because the distance within the solid active material particles can be easily moved and the distance for diffusing in the solid active material particles is greatly reduced.
In addition, in the active material in which the volume change of the electrode is large due to the insertion and removal of Li into the solid active material, this was a factor causing cycle deterioration of the battery characteristics, but by the size of the solid active material becoming nano, It is expected that the volume change of the electrode is alleviated and the cycle characteristics of the battery are improved.
In the developing large _{material, aLi 2 MnO 3 - (1} -a) Li (Ni x, Co y, Mn z) of O ₂ solid solution, as an indication of large positive characteristics, recently been noticed (Non-Patent Document 2).

JP2008-285372

Nano active materials for lithium-ionbatteries, Y. G. Wang et al., NANOSCALE 2, 1294-1305, 2010 Detailed Studies of a High-Capacity Electrode Material for Rechargeable Batteries, Li2MnO3-LiCo1 / 3Ni1 / 3Mn1 / 3O2, N. Yabuuchi et al., J. Am. Chem. Soc. 2011, 133, 4404-4419

As described above, nanomaterials are attracting attention as electrode materials for high-power Li-ion batteries that are used in vehicles.
However, many nanoparticles tend to aggregate and hardly exhibit properties as a nanomaterial, and are difficult to handle.
There is also a concern that many nanoparticles have low crystallinity. When the crystallinity is low, it is difficult to show a stable charge / discharge potential when used as an electrode material. In order to obtain a highly crystalline material, heat treatment at a high temperature is usually performed. Generally, when a nanoparticle material is heat-treated at a high temperature, the crystallinity is improved, but a large grain growth occurs, and the nanoparticle As a characteristic is lost.
The present inventors have studied the use of single crystal nanowires as electrode materials as nanomaterials having high crystallinity (Patent Document 1), but the use of single crystal nanowires as electrode materials has been realized. Therefore, it is necessary to develop a process for easily synthesizing single crystal nanowires in large quantities.
In addition, the above-described solid solution of aLi ₂ MnO _3- (1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ has a large capacity, and therefore, a large amount of Li can be inserted and removed, Although it is expected to be used, on the other hand, in addition to concerns about deterioration due to volume change due to the deinsertion of a large amount of Li, a reaction including degassing occurs in the initial charging process, and activation by this reaction has a large capacity It is considered to be important for the development of characteristics, and the volume change during this activation process is likely to lead to cycle deterioration.
Therefore, aLi ₂ MnO ₃ - in order to use (1-a) Li (Ni x, Co y, Mn z) a solid solution of O ₂ as the electrode material, by nano the size of the solid solution, the charge and discharge It is conceivable to reduce the volume change during the process and improve the cycle characteristics. However, it is difficult to produce solid solution nanowires using the hydrothermal method used to produce conventional single crystal nanowires, and appropriate techniques for controlling the nanostructure of such solid solution system materials are known. Not.

Therefore, the present invention finds a method for controlling the nanostructure of a solid solution of aLi ₂ MnO _3- (1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ , and the obtained aLi ₂ MnO ₃ -(1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ solid solution nanoparticles are used as an electrode material to form a large-capacity lithium ion battery with little cycle deterioration. To do.

By using the electrospinning method, the inventors have
We found that a nanoparticle with high crystallinity of a solid solution of aLi ₂ MnO _3- (1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ can be easily produced, and using this as an electrode material The present inventors have found that a large-capacity lithium ion battery with little cycle deterioration can be constructed.
The electrospinning method is a technique in which a polymer is dissolved in a sol-gel solution or MOD solution of a metal compound, and a fiber-like precursor is obtained by applying a voltage. By firing this, a fiber-like metal oxide is obtained. Etc. can be obtained.
Specifically, the solid solution used for the electrode material for Li-ion batteries of the present invention is:
aLi ₂ MnO _3- (1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ (where, in the above chemical formula, 0 <a <1, 0 ≦ x, y, z ≦ 1, and X + y + z = 1).
A polymer component is added to a solution of a metal salt corresponding to the composition of the solid solution to prepare a solution of the solid solution precursor, and an electrospinning method is applied to the solution to produce a solid solution precursor fiber.
Next, the solid solution precursor fiber obtained by the electrospinning method is fired at a high temperature, so that the solid solution is composed of nanoparticles of the solid solution having high crystallinity and has a wall thickness of about several tens to several hundreds of nanometers. A material having a tube-shaped structure is prepared.
By using this as a positive electrode material, a Li-ion battery exhibiting high capacity electrode characteristics can be produced.
Since the solid solution fiber obtained by the above method has a hollow shape with a thin wall, the supply of a substance becomes two-dimensional during heat treatment at a high temperature as compared with the case of a bulk. It is thought that large grain growth was suppressed. In addition, the solid solution fiber is formed in a hollow tube form so that the nanoparticles are two-dimensionally connected to each other, thereby improving the conductivity between the particles when the Li-ion battery electrode is formed. Therefore, it is suitable as an electrode material for a high-power battery. Even if the tube-like shape is broken in the electrode production process or the like, it is considered that the connection of the two-dimensional nanoparticles remains as a fragmented sheet.

The present invention has been completed based on these findings, and specifically provides the following inventions.
<1> ALi ₂ MnO _3- (1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ solid solution nanoparticles with high crystallinity, wall thickness of about tens to hundreds of nm Highly crystalline aLi ₂ MnO _3- (1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ Li-ion battery electrode comprising a one-dimensional structure having a hollow tube-like structure material.
(Here, in the above chemical formula, 0 <a <1, 0 ≦ x, y, z ≦ 1, and x + y + z = 1.)
<2> A solid solution precursor fiber is produced from a precursor solution containing a metal salt corresponding to the composition of the solid solution and a polymer component by electrospinning, and the obtained precursor fiber is fired at a high temperature. The manufacturing method of the electrode material as described in <1>.
<3> An electrode for a Li ion battery containing the electrode material according to <1> as an electrode active material.
<4> A lithium ion secondary battery using the electrode according to <3> as an electrode.

According to the present invention, a solid solution based nanostructured electrode material of aLi ₂ MnO _3- (1-a) Li (Ni _x , Co _y , Mn _z ) O ₂ having high crystallinity easily by using an electrospinning method Can be produced. In addition, by using the solid solution nanostructure electrode material thus obtained as a positive electrode material, a large-capacity lithium ion battery with little cycle deterioration can be configured.

SEM image of precursor fiber obtained by electrospinning. XRD of solid solution material obtained by heat treatment of precursor fiber. SEM image of solid solution material obtained by heat treatment of precursor fiber. TEM image of solid solution material obtained by heat treatment of precursor fiber. Charging / discharging curve and cycle characteristics of Li-ion battery using solid solution material at current density of 10mA / g. Charge / discharge curves and cycle characteristics of Li-ion batteries using solid solution materials at a current density of 30 mA / g.

Below, the form for implementing this invention is explained in full detail.
ALI ₂ MnO ₃ used as an electrode material of the present invention - (1-a) Li as the _{(Ni x, Co y, Mn} z) a solid solution of O _2, is not particularly limited as long as it corresponds to the composition For example,
Examples include 0.5Li ₂ MnO ₃ —0.5LiNi _1/3 Co _1/3 Mn _1/3 O ₂ solid solution, 0.5Li ₂ MnO ₃ —0.5LiNi _0.5 Mn _0.5 O ₂ solid solution, and the like.
In the present invention, the polymer component added to the precursor solution of the solid solution is not particularly limited as long as it is a polymer that is made into a fiber by an electrospinning method. For example, polyvinyl alcohol, polyacrylonitrile, polyacrylic acid Polyvinylidene fluoride, polyethylene oxide, polyester and the like are used.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention in detail, this invention is not restrict | limited at all to description of these Examples.

Example 1.
Lithium acetate (0.400M), nickel acetate tetrahydrate (0.044M), cobalt acetate tetrahydrate (0.044M), manganese acetate tetrahydrate (0.178M), polyvinyl alcohol (1.5g), polyoxyethylene (10) Put octylphenyl ether (0.05 g) in ion-exchanged water (15 ml), ethanol (4 ml), acetic acid (1 ml), age at 90 ° C. for 1 hour, and stir at room temperature to obtain a precursor solution. .
Using the obtained precursor solution, a voltage of 20 kV is applied by a nanofiber electrospinning unit manufactured by Kato Tech Co., Ltd., the solution is extruded at a syringe speed of 0.2 mm / min, and the fiber is collected in an aluminum foil collector. After vacuum drying at 100 ° C for 1 hour, the fiber is peeled off the aluminum foil collector. An SEM image of the peeled fiber is shown in FIG.
The peeled fiber is heat-treated at 800 ° C for 3 hours using an electric furnace.
A 0.5Li ₂ MnO ₃ -0.5LiNi _1/3 Co _1/3 Mn _1/3 O ₂ solid solution positive electrode material was obtained. FIG. 2 shows an XRD pattern of the obtained material. The XRD in FIG. 2 shows a pattern due to the solid solution, and it can be seen from the sharp peak that the crystallinity is high.
Moreover, the SEM image and TEM image of the obtained material are shown in FIGS. As can be seen from FIG. 3, the obtained solid solution has a hollow tube shape as a whole, and its wall thickness is about several tens to several hundreds of nm as seen from the upper and lower images of FIG. It turns out that it is. Moreover, it can be seen from the central image in FIG. 4 that the wall is composed of particles two-dimensionally connected to each other, and each particle is a nanoparticle having a diameter of several tens to 100 nm.

Example 2
The solid solution positive electrode material (75%) obtained in Example 1 was mixed with acetylene black (20 wt%) as a conductive auxiliary agent and Teflon (registered trademark) (5 wt%) as a binder to form a paste. . This paste was pressed onto an Al mesh current collector, which was used as an electrode. Using a metal Li as the counter electrode, a polypropylene film as the separator, and an organic electrolyte containing 1M LiPF ₆ dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate, a 2032 type coin cell is made in a glove box filled with argon, an inert gas. did. The charge / discharge characteristics were evaluated at current densities of 10 mA / g and 30 mA / g.
FIGS. 5 and 6 show charge / discharge curves and cycle characteristics when charge / discharge is performed at each current density. 5, (a) and (b) are charge / discharge curves and cycle characteristics when charging / discharging is performed at a current density of 10 mA / g. In FIG. 6, (c) and (d) are current densities. It is a charging / discharging curve and cycle characteristic when charging / discharging at 30 mA / g.
At any current density, a charging curve having a plateau near 4.5 V, which is characteristic of the first charging of the solid solution system, is shown in the first charging ((a), (c)). Further, during discharge, a shoulder is present in the vicinity of 3V ((a), (c)), and a stable discharge capacity exceeding 200 mAhg ⁻¹ is shown ((b), (d)). On the other hand, it is known that the discharge capacity of a conventional Li-ion battery using LiCoO ₂ or LiMn ₂ O ₄ as an electrode material is about 150 mAhg ^−1. It is shown that a Li-ion battery with a capacity was obtained.

  The large-capacity positive electrode material of the present invention is used as a positive electrode material for batteries used in various applications such as mobile devices, stationary power sources, and automobile batteries.

Claims (3)

High crystallinity having aLi2MnO3- (1-a) Li consists (Nix, Coy, Mnz) O2 solid solution nanoparticles has the structure of hollow tube forms hundreds n m wall thickness of several tens nm A highly crystalline aLi2MnO3- (1-a) Li (Nix, Coy, Mnz) O2 type positive electrode material for Li-ion battery comprising a structural body (where, in the above chemical formula, 0 <a <1, 0 ≦ x, y, z ≦ 1, and x + y + z = 1.) is mixed with a conductive additive and a binder, pasted, and pressure-bonded to a current collector. By-elect Russia spinning, by to produce a fiber of a solid solution precursor from a precursor solution containing a metal salt and a polymer component corresponding to the composition of the solid solution, calcining the resulting precursor fibers at high temperatures, the positive electrode material produced, mixed with which the conductive additive and the binder, a paste, wherein the pressure-bonding to the current collector, the production method of a positive electrode for a lithium ion battery according to claim 1. A lithium ion secondary battery using the positive electrode according to claim 1 as a positive electrode.