JP2007213953A - Negative electrode material for battery and secondary battery using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode material for lithium secondary battery having thermal resistance capable of enduring a reflow soldering, with little capacity deterioration at the time of repeated charge and discharge, and capable of using stably. <P>SOLUTION: The negative electrode material for lithium secondary battery contains a component which is bonded with lithium and has a melting temperature higher that of lithium (Li) in 1-30% in total quantity in atomic conversion in Li system matrix. As an example, the negative electrode material has the component which is at least one kind out of an element group made of B, Al, C, Si, P, S, Mg, Ca, Sr, N, O. The lithium secondary battery using this material for the negative electrode maintains a high capacity even after mounting by reflow soldering and can suppress for a long period to the minimum deterioration of capacity and adhesion with electrolyte interface even if charge and discharge are repeated. Thereby, it can be provided for practical use stably. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a negative electrode material for a battery and a lithium secondary battery using the same.

  The negative electrode material of the lithium secondary battery using an organic electrolyte includes a cylindrical carbon type, a small coin type, a metal Li type, a Li-Al alloy type, a Li-Si alloy type, Examples include Li-Sn alloy-based ones. Among these, a metal Li-based material has the largest discharge capacity per unit volume, and is therefore the ultimate material. However, pure metal Li reacts with the electrolytic solution during repeated charging and discharging, and acicular crystals are deposited, which may break through the separator and reach the anode to cause a short circuit. For this reason, contrivances have been made to suppress this, such as composite materials in which carbon and Li metal foil are laminated, and composites with wood metal. For example, Non-Patent Documents 1 to 3 and Patent Documents 1 and 2 introduce an outline of development of a negative electrode material for a lithium secondary battery using such an organic electrolyte. According to the descriptions in these non-patent documents, lithium-aluminum-based materials have been mainly used as negative electrode materials.

On the other hand, Patent Documents 4 and 5 propose an all-solid-state battery, that is, a solid-electrolyte-type secondary battery that is composed of only a solid electrolyte without using an organic electrolyte solution. These documents introduce as a negative electrode material a composite material in which lithium metal or metallic Li particles are embedded in a carbon-based material.
; Battery manual issued by Maruzen Co., Ltd., 1990, pp. 328 to 329 The latest battery handbook published by Asakura Shoten in 1996, pages 651 to 656 ; Electrochimica Acta, 1999, pp. 31-50 Japanese Patent Application Laid-Open No. 2005-079077 ; JP-A-2005-0505585 ; JP-A-2005-071683 ; JP-A-2004-179158 Japanese Patent Application Laid-Open No. 2004-127743

  The coin-type lithium secondary battery is usually mounted on the electric circuit by reflow soldering. In this case, the battery is placed at a temperature close to 300 ° C. For this reason, the improvement in heat resistance of the organic electrolyte (the above organic electrolyte solution) and the improvement in heat resistance when the above-described Li metal or Li alloy system whose main component is a low melting point (180 ° C.) are used for the negative electrode material are as follows. This is an important issue for the practical application of secondary batteries. In order to solve this problem, for example, in the case of using an organic electrolytic solution, as described in Patent Document 3, a solution in which a specific component such as sulfolane or tetraglyme is added to the electrolytic solution has been proposed. As a result, thermal decomposition of the organic electrolyte during mounting is alleviated. However, the problem of deterioration due to added components during use remains. In addition, when the negative electrode material is a lithium-aluminum alloy, the melting point is high, so that it has heat resistance. However, when the discharge capacity is increased, the material itself becomes fine powder and deterioration is accelerated.

  On the other hand, as described above, there is a solid electrolyte type battery that does not use an organic electrolyte but uses a solid electrolyte (solid electrolyte) having the same degree of lithium ion conductivity. For example, a sulfide-based solid electrolyte is sufficiently stable over a wide temperature range from −20 ° C. to 300 ° C. Therefore, the electrolyte itself can sufficiently withstand reflow soldering. However, when a lithium-based metal is used as the negative electrode material, as described above, it may be melted during reflow soldering. Even if a lithium-aluminum alloy is used, as described above, the powder itself tends to be pulverized during charging and discharging. Moreover, interface peeling from the solid electrolyte is likely to occur due to its own three-dimensional expansion and contraction, and there is a possibility that the electrical connection between the two is broken.

  The present invention has been made to improve the heat resistance and stability in practical use of the negative electrode used in the lithium secondary battery as described above. The material of the present invention can also be used effectively in a secondary battery using an organic electrolyte, but especially when used in a secondary battery using a solid electrolyte, the electrolyte itself is superior in heat resistance. It can be used more advantageously without being affected by deterioration of the properties. That is, in the present invention, a component that binds to lithium and has a melting temperature higher than that of lithium (Li) is converted into an atom in a Li-based matrix (a matrix containing Li and a Li compound containing Li as a main component). Is a negative electrode material for a lithium secondary battery containing 1 to 30% in total. In the present invention, as an example, the secondary component is a lithium secondary in which at least one element group consisting of B, Al, C, Si, P, S, Mg, Ca, Sr, N, and O is specified. Battery negative electrode materials are included. The present invention also includes a lithium secondary battery using the above materials.

  The material of the present invention has sufficient heat resistance. For this reason, the lithium secondary battery using the material of the present invention for the negative electrode maintains a high capacity even after reflow solder mounting, and even after repeated charging and discharging, the decrease in the capacity and adhesion with the electrolyte interface is prolonged. It can be kept small. For this reason, it can be stably put to practical use.

  The present invention relates to a negative electrode for a lithium secondary battery that is bonded to lithium and has a melting temperature higher than that of lithium (Li) in a Li-based matrix in an amount of 1 to 30% in terms of atoms. Material. The reason why the total amount of the above components is 1 to 30% in terms of atoms is that if the temperature is lower than the lower limit, the shape cannot be maintained because it melts when the temperature rises near 300 ° C., for example, during reflow soldering. If the upper limit is exceeded, pulverization at the time of charge / discharge is likely to proceed, and the initial capacity reduction is accelerated. The total amount is preferably 5 to 20 atomic%, more preferably 7 to 15 atomic%. Since the negative electrode material of the present invention is mainly composed of lithium and is uniformly distributed in the above total amount range in a state where a component having a higher melting temperature and lithium are combined with each other, it is 300 ° C. by reflow soldering. Even if the temperature rises nearby, it does not melt. In addition, by controlling the total amount of components having a high melting temperature within the above range, expansion / contraction during charging / discharging is suppressed in the plane direction, occurs only in the two-dimensional plane in the thickness direction, and expands in the three-dimensional direction. No contraction occurs. For this reason, when the electrolyte is in a solid state, stable charge and discharge can be repeated without disconnecting electrical contact therewith. Furthermore, an interface with a low ion conductivity barrier with the electrolyte can be formed, and the current density during charging and discharging can be stably increased.

  In the present invention, as an example, the secondary component is a lithium secondary material specified as at least one element group consisting of B, Al, C, Si, P, S, Mg, Ca, Sr, N, and O. A negative electrode material for a battery is included. In that case, a plurality of types of elements in the group may be included in a form combined with Li. In addition, a plurality of kinds of elements may form a compound by themselves. Examples of the compound in this case include oxides, nitrides, and oxynitrides of Al, B, Si, and Mg.

  Hereinafter, an example of the form of the present invention will be described with reference to examples, but the present invention is not limited thereto.

In order to confirm the heat resistance in reflow solder mounting of the negative electrode material of the present invention, a sample of a secondary battery in which the negative electrode material is changed is prepared, and repeated when heated to 300 ° C. comparable to the reflow solder mounting temperature and when not heated The capacity reduction rate after charge / discharge was compared. First, a laminate of a positive electrode and a solid electrolyte was prepared. The laminate is a positive electrode in which a LiCoO ₂ -based positive electrode active material layer having a thickness of 5 μm is formed on a pure aluminum (Al) or pure copper (Cu) foil having a diameter of 16 mm and a thickness of 20 μm by a KrF excimer laser ablation method. And a Li ₂ S—P ₂ S ₅ based solid electrolyte layer having a thickness of 10 μm is formed thereon.

The specific preparation procedure of the laminated body is as follows. First, a support body on which a base material made of pure aluminum or pure copper foil masked with a stainless steel thin plate having a hole having a diameter of 15 mm is fixed. Prepare. The substrate temperature was controlled at 700 ° C., and a positive electrode target (commercially available powdered LiCoO ₂ and Li ₂ O were prepared in a container with an oxygen pressure of 30 Pa. Is mixed by pulverization at a ratio of 10 mol%, and this mixture is pressure-molded by a mold). While rotating this at 5 rpm, a 10 nm pulsed excimer laser is irradiated to form a positive electrode layer. To do. In this example, the positive electrode film obtained at this point was analyzed by X-ray diffraction, and it was confirmed that it was a LiCoO ₂ single phase.

Next, commercially available powdery Li ₂ S and P ₂ S ₅ were prepared for each, and a mixture of 80 mol% Li ₂ S and 20 mol% P ₂ S ₅ chemical composition was prepared in the same procedure as described above. Make it. These mixtures are molded into a diameter of 20 mm and a thickness of 5 mm to obtain a solid electrolyte target. On the other hand, a substrate made of a pure aluminum or pure copper foil on which a positive electrode layer having a diameter of 15 mm prepared in the above procedure is formed is fixed to a support. Next, the solid electrolyte target and the support on which the substrate is placed are both put into a sealed container, sent to the laser film forming chamber, placed at a predetermined position, and an excimer laser in an argon (Ar) gas atmosphere of 10 ⁻² Pa. To form a solid electrolyte layer on the positive electrode layer of the substrate. The thickness of the film is monitored by a quartz oscillation type film thickness meter. The film formation sample is stored in a sealed container, sent to a glow box with dew point control, masked and stored.

Thereafter, a negative electrode material having the chemical composition shown in Table 1 was vacuum-deposited on the solid electrolyte layer of the substrate according to the following procedure. First, a perforated mask having a diameter of 14 mm is attached to the base material and fixed to the substrate. This is put in a sealed container and sent into a vacuum apparatus, and fixed to a support base with a cooling mechanism. Next, after depositing a vapor deposition source component including a raw material lithium metal and a component that forms a melting point substance higher than lithium by combining with the raw material in an electron beam heating vessel, the inside of the vapor deposition apparatus is evacuated to 10 ⁻⁴ Pa. In this state, each raw material is evaporated by heating with an electron beam, and is deposited on a substrate. As a result, a layer having a thickness of 5 μm in which a component bonded to lithium is dispersed in the lithium matrix on the solid electrolyte of the base material is formed.

  At the time of vapor deposition, for example, an element that can be supplied from a gas phase such as N or O is used as an atmospheric gas. As a result, oxidation or nitridation of the component bonded to lithium can be performed at the same time. Of course, if the gas pressure is controlled at this time, it is possible to control the intake amount from the atmospheric gas such as oxygen (O) or nitrogen (N) in the material. If the component to be combined with lithium is a metal, the chemical composition of the negative electrode layer can be controlled by the input power to the heating container. Depending on the sample, some of the plural types of elements are not bonded to the lithium metal, but are dispersed in the lithium matrix in a state of being bonded only by them (sample 34 to sample 1 in Table 1). 39). The film thickness is monitored by a quartz oscillation type film thickness meter as described above. The film formation sample is stored in an airtight container and sent to a glow box with dew point control.

  Through the above procedure, a negative electrode material layer containing components having various compositions shown in Table 1 was prepared. The chemical composition was prepared by using an inductively coupled plasma atomic emission spectrometry (ICP) metal element using a monitor specimen separately deposited in a vapor deposition layer and depositing a vapor deposition source component on a polyethylene substrate. And nonmetallic elements such as oxygen and nitrogen were quantified by X-ray electron spectroscopy (XPS).

  Using each battery element in which these negative electrode materials were changed, a coin-type battery was produced by the following procedure. The all-solid-state thin film battery was placed in a stainless steel container of a coin-type battery with a heat-resistant gasket with the base material surface made of Al or copper foil facing down, and dried in a vacuum with an outer diameter of 17 mm and an inner diameter of 12 mm. Cover with a donut-shaped polyimide film. Further, a stainless steel disk having a diameter of 10 mm and a thickness of 0.2 mm is placed at the center, and a coin-type battery cover is placed on the center, and caulking is performed using a caulking machine.

Using these battery samples, prepare those with and without heating at 300 ° C for about 10 seconds at the time of reflow solder mounting, and compare the reduction rate of the battery capacity after repeated charging and discharging loads did. Specifically, the procedure was as follows. The heated and non-heated ones were divided into 30 pieces, and 500 cycles of charge / discharge were repeated under the conditions of an upper limit potential of 4.2 V, a lower limit potential of 3.0 V, and a current value of 0.1 mA. The discharge capacity (A ₁ ) and the discharge capacity at the 500th cycle (A ₅₀₀ ) are confirmed. The capacity decrease rate (A ₁ -A ₅₀₀ ) / A ₁ was calculated from these data. The results are shown in Table 1.

The display of Table 1 is as follows. “Al” and “Cu” in the base material column of the positive electrode are respectively pure aluminum and pure copper, and “LCO-LO” in the active material column is a composition of 90 mol% LiCoO ₂ and 10 mol% Li ₂ O, respectively. “LS-PS” in the column of the solid electrolyte layer represents the chemical composition of 80 mol% Li ₂ S and 20 mol% P ₂ S ₅ , respectively. The column of the secondary component element of the Li negative electrode (here, the secondary component element refers to an element that combines with Li and constitutes a component whose melting temperature is higher than that of Li) is the type of the secondary component element of each sample. (Species) and the atomic% (content in terms of atoms) in the negative electrode material comprising the Li-based matrix are shown. The column of the battery capacity decrease rate (%) after the charge / discharge cycle indicates the decrease rate of the charge / discharge cycle battery capacity evaluated under the above conditions. The “unheated” and “after heating” categories are the rate of decrease of the sample before and after heating at 300 ° C., respectively. Furthermore, the remarks column shows the confirmation items of the sample group subdivided in the table.

  In addition, the structure of the secondary component element contained in Li negative electrode of each sample, and those presence forms are as follows. Most of the secondary component elements of Samples 1 to 23 and 40 to 42 are combined with Li in the Li matrix to form an intermetallic compound, an alloy, etc., and raise the melting temperature of the matrix itself. In the samples 22 and 23, the former contains Si 8 atom% and C7 atom%, and the latter contains Al 8 atom% and Mg 7 atom%, respectively. The secondary component elements of Samples 24 to 31 exist in the form of Li oxide, Sample 32 in the form of Li nitride, and Sample 33 in the form of Li oxynitride.

In the samples 34 to 39, most of the subordinate component elements are combined with Li to form an intermetallic compound, an alloy, etc., but a small amount of compound as described in the “Components outside Li compound” column is formed. There is also a part. These compounds are mainly contained in the form of nitrides and oxides at the grain boundaries in the Li-based matrix. A small amount of compounds contained in Samples 34 to 39 are aluminum nitride (AlN), spinel (MgAl ₂ O ₄ ), and boron oxide (B ₂ O ₃ ), respectively, as described in the “Components outside Li compound” column. Or lithium borate (Li ₂ B ₄ O ₇ or the like), boron nitride (BN), calcium oxide (CaO), phosphorus sulfide (P ₂ O ₅ ), or the like. For example, the negative electrode material of Sample 34 contains 7 atomic% of Al and 7 atomic% of N, and a total of 14 atomic%, and 1 atomic% or less of Al and N are less than 1 μm in particle size. The AlN particles are uniformly distributed in the Li-based matrix. The other samples are almost similar forms of tissue. Since these compounds all have a melting temperature higher than that of Li, the melting temperature of the Li-based matrix is increased.

  Separately, battery samples 1 'and 6' of the same type using the same positive electrode and negative electrode as samples 1 and 6 and using an organic electrolyte having a solute of lithium bisimide and an electrolyte of polyethylene glycol dialkyl ether were prepared. A battery sample heated at 270 ° C. and 300 ° C. for 10 seconds and an unheated battery sample were prepared and subjected to charge / discharge cycles under the same conditions as described above, and the capacity reduction rate was confirmed. As a result, both the samples 1 'and 6' were almost the same as the samples 1 and 6 in the unheated sample and the 270 ° C heated sample, respectively. In the sample heated at 300 ° C., a capacity drop of 45% was confirmed for sample 1 ′ and 7% for sample 6 ′.

From the above results, a negative electrode material excellent in heat resistance can be obtained by including, in the Li-based matrix, a component having a melting temperature higher than that of Li and bonded to Li in a range of 1 to 30 atomic%. By incorporating the secondary battery element using this negative electrode material into the secondary battery, the shape can be maintained without melting even at a high temperature of about 300 ° C., which is the same as when reflow soldering is mounted, and the capacity remains unchanged even after repeated charge and discharge. It is possible to provide an excellent secondary battery that has not been lowered and is not found in the past.

Claims (3)

  A negative electrode material for a lithium secondary battery, wherein a lithium-based matrix contains 1 to 30% in terms of atoms of a component that is bonded to lithium and whose melting temperature is higher than that of lithium (Li).   2. The lithium according to claim 1, wherein the component is at least one selected from an element group consisting of B, Al, C, Si, P, S, Mg, Ca, Sr, N, and O. 3. Negative electrode material for secondary batteries. A lithium secondary battery using the material according to claim 1.