JP2010257736A - Manufacturing method of lithium iron nitride, anode active material for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method enabling direct-solid phase method synthesizing of a stratified lithium iron nitride Li<SB>3-x</SB>Fe<SB>x</SB>N of an Li3N structure simply at low temperature and enabling composition control, discarding a conventional method using calcination at 900°C or more in an N<SB>2</SB>atmosphere or complicated procedures through an antifluorite structure. <P>SOLUTION: A temperature profile and atmospheric control at baking are interlocked, and a method of directly synthesizing Li<SB>3-x</SB>Fe<SB>x</SB>N of a wide composition area at 600°C by a solid phase method is established. More concretely, (1) restraint of decomposition of a raw material at a low-temperature range by augmenting velocity at temperature raising, (2) restraint of decomposition of a raw material at a low-temperature range under a nitrogen atmosphere only at temperature raising and, after raising of temperature, stabilization of the Li<SB>3-x</SB>Fe<SB>x</SB>N under an inert atmosphere, (3) adoption of inert gas for controlling a reduction property of a baking atmosphere, (4) ensued stabilization of iron with a low formal charge, and further, stabilization of the Li<SB>3-x</SB>Fe<SB>x</SB>N having a stratified structure are to be essential features. The Li<SB>3-x</SB>Fe<SB>x</SB>N thus made shows excellent lithium ion secondary battery anode characteristics. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  An object of this invention is to provide the manufacturing method of the layered lithium iron nitride excellent in the charge / discharge characteristic which is a negative electrode active material of a lithium ion secondary battery.

  Moreover, an object of this invention is to provide the negative electrode active material for lithium secondary batteries excellent in the charge / discharge characteristic which consists of the layered lithium iron nitride manufactured using the said manufacturing method.

  Another object of the present invention is to provide a lithium ion secondary battery using an excellent negative electrode active material for a lithium ion secondary battery comprising a layered lithium iron nitride obtained by the production method of the present invention. To do.

In general, the crystal structure of the Li—M—N ternary system (M = 3d transition metal) is classified into an inverted fluorite type for M = Ti to Fe and a Li ₃ N type layered structure for M = Fe to Cu. .

In the case of M = Fe, which is positioned at the boundary of phase stability between the two crystal structures, Li ₃ FeN ₂ having an inverted fluorite structure is more stable, and Li _3−x Fe _x N having a Li ₃ N type structure is more stable. In order to obtain it, it was necessary to perform firing at 850 ° C. or higher using a sealed container in N ₂ and complicated synthesis via an inverted fluorite structure, and it was difficult to control the composition.

Regarding lithium iron nitride as a negative electrode material for lithium ion batteries, see J.A. L. C. Rowsell et al. Report that the electrochemically reversible capacity (550 mA · h / g) of a compound of Li _2.7 Fe _0.3 N is as good as Li _2.6 Co _0.4 N. (Non-patent document 1). The compound formation reaction is carried out by enclosing a pressure-molded mass of Li ₃ N in a high-purity iron container under a nitrogen pressure of 300 kPa, heating in a temperature range of 850-1050 ° C. for 12 hours, and then rapidly cooling.

Non-Patent Document 2 discloses that a single crystal of Li ₂ [(Li _1−x Fe ^I _x ) N] (x = 0.63) is 1: 1 of Li ₃ [FeN ₂ ] and an alkali metal (Li, Na). It is reported that it can be obtained by heat-treating a mixture with a molar ratio of 1 at Ar under an atmosphere of 1 atm. In the heat treatment, the mixture was first heated to 523 K (250 ° C.) at 1.3 K / min for 12 hours, then heated to 1173 K (900 ° C.) at 3 K / min to reach the maximum temperature, The reaction product is cooled to room temperature at 3 K / min.

Jesse L. C. Rowsell, Vale´rie Pralong, and LindaF. Nazar J. Am. Chem. Soc. 123, 8598-8599 (2001). J. Klatyk and R. Kniep, Z. Kristallogr. NCS 214, 447-448 (1999).

The direct synthesis of layered lithium iron nitride Li _3-x Fe _x N having a Li ₃ N-type structure at a simple and low temperature is currently most demanded, but has not yet been obtained.

  That is, the technique described in Non-Patent Document 1 is not industrial in production, and the obtained layered lithium iron nitride is still not sufficient as a negative electrode active material for a lithium ion secondary battery. .

  In Non-Patent Document 2, a lithium iron nitride having a reverse fluorite structure is used as a raw material to obtain lithium iron nitride, which is a manufacturing method not suitable for industrial application. The composition of the ratio is also different. It is also unclear whether it can be used as a negative electrode active material for lithium ion secondary batteries.

  Moreover, it is difficult to control the amount of solid solution in any of the methods of the non-patent literature.

Thus, the present invention provides a production method that enables direct solid phase synthesis of Li ₃ N-type layered lithium iron nitride Li _3-x Fe _x N at a low temperature and composition control. For the purpose.

  Another object of the present invention is to provide an excellent negative electrode active material for a lithium ion secondary battery comprising a layered lithium iron nitride obtained by the production method of the present invention.

  Another object of the present invention is to provide a lithium ion secondary battery using an excellent negative electrode active material for a lithium ion secondary battery comprising a layered lithium iron nitride obtained by the production method of the present invention. To do.

  The technical problem can be achieved by the present invention as follows.

The present invention relates to a method for producing a layered lithium iron nitride having a composition formula of Li _3-x Fe _x N (0 <x <0.4), in which lithium nitride and iron nitride are mixed, nitrogen This is a method for producing a layered lithium iron nitride characterized by heating to 400 to 700 ° C. in an atmosphere and then switching to an Ar atmosphere and heating and firing (Invention 1).

The present invention relates to a method for producing a layered lithium iron nitride having a composition formula of Li _3-x Fe _x N (0 <x <0.4), in which lithium nitride and iron nitride are mixed, nitrogen Alternatively, it is a method for producing a layered lithium iron nitride characterized by heating to 400 to 700 ° C. at a temperature rising rate of 10 ° C./min or more in an Ar atmosphere, and then switching to an Ar atmosphere and baking. 2).

  The present invention is the method for producing a layered lithium iron nitride according to the present invention 1 or 2, wherein the mixing ratio of lithium nitride and iron nitride is 5 or more as the molar ratio of Li / Fe. There is (Invention 3).

Moreover, this invention is a negative electrode active material for lithium secondary batteries which consists of layered lithium iron nitride manufactured with the manufacturing method in any one of this invention 1 thru | or 3 (this invention 4).

Moreover, this invention is a lithium ion secondary battery using the negative electrode active material for lithium secondary batteries of this invention 4 (this invention 5).

The method for producing lithium iron nitride according to the present invention can synthesize Li ₃ N-type layered lithium iron nitride Li _3-x Fe _x N easily and directly at low temperature. It is suitable as a manufacturing method.

2 is an X-ray diffraction pattern of lithium iron nitride obtained in Example 2 and Example 3. FIG. This is a comparison of Fe raw materials. (Example 2 and Example 3 from above) 4 is an X-ray diffraction pattern of lithium iron nitride obtained in Example 4. This is a production example in which the temperature was raised and fired in an Ar atmosphere (Example 4). The lower X-ray diffraction pattern Li _2.7 Fe _0.3 N model simulation is based on ICSD (inorganic crystal structure database). 2 is an X-ray diffraction pattern of lithium iron nitride obtained in Comparative Example 1. FIG. This is a production example in which the temperature is raised and fired in an N ₂ atmosphere (Comparative Example 1). The lower X-ray diffraction pattern is an X-ray diffraction pattern of the inverted fluorite crystal structure of Li ₃ FeN ₂ based on ICSD (inorganic crystal structure database). 2 is an X-ray diffraction pattern of lithium iron nitride obtained in Examples 1 and 5. FIG. The temperature rising atmospheres are compared (Examples 5 and 1 from the top). 2 is an X-ray diffraction pattern of lithium iron nitride obtained in Examples 1 and 7 to 9 and Comparative Example 3. FIG. The firing temperatures were compared (Examples 7, 1, 8, 9 and Comparative Example 3 from the top). It is a charging / discharging characteristic of the lithium iron nitride obtained in Examples 1, 10, 11 and Comparative Example 4 (Comparative Example 4, Examples 10, 11 and 1 from the top).

  A method for producing a layered lithium iron nitride according to the present invention will be described.

Conventionally, the synthesis method of Li _3-x Fe _x N has only been reported as a method using a complicated procedure through calcination at 850 ° C. or higher in an N ₂ atmosphere or an inverted fluorite structure. In the present invention, Li _{3 -x} Fe _x N having a wide composition range is directly applied at a low temperature of 400 to 700 ° C. by a solid phase method by interlocking temperature raising conditions (temperature raising rate and atmosphere), temperature, and atmosphere control during firing. A method of synthesis was established.

  In the present invention, after mixing lithium nitride and iron nitride, the mixed powder is heated to a heating and firing temperature in a nitrogen atmosphere, and is switched to an Ar atmosphere and fired.

Specifically, (1) Suppressing decomposition of raw materials in a low temperature region by increasing the rate of temperature increase. (2) Li _3-x Fe _x N is stabilized by suppressing the decomposition of the raw material in a low temperature region as a nitrogen atmosphere only at the time of temperature rise, and by setting the inert atmosphere after temperature rise. (3) Use of an inert gas to control the reducibility of the firing atmosphere. (4) Stabilize iron with a low formal charge, and thereby stabilize Li _{3 -x} Fe _x N having a layered structure. The produced Li _3-x Fe _x N exhibited excellent lithium ion secondary battery negative electrode characteristics.

The present inventors paid attention to the decomposition of the raw material in a low temperature region at the time of temperature increase. For this reason, the present inventors have considered that the generation of impurity metallic iron can be substantially suppressed by increasing the rate of temperature rise so as to be in the range of 10 ° C./min to 35 ° C./min.

In addition, iron takes a lower formal charge in the Li ₃ N-type layered structure (Li _3−x Fe ^I _x N) than in the reverse fluorite structure (Li ₃ Fe ^III N ₂ ), so that it is more reducible than N ₂ . by firing in strong Ar atmosphere, iron is stabilized in the lower valence state, lithium iron nitride having a layered structure Li _{3-x Fe x N} is that it would be stably obtained inventors Thought.

In the present invention, Li ₃ N can be used as the lithium nitride raw material. The iron nitride raw material is Fe ₄ N, Fe ₃ N, or Fe _x N (3 <x <4), and more preferably Fe ₄ N.

  The mixing ratio of lithium nitride and iron nitride is preferably 5 or more in terms of the molar ratio of Li to Fe (Li / Fe). More preferably, the ratio of the mixing ratio is 6.5 or more.

  Mixing of lithium nitride and iron nitride may be performed according to a conventional method. Preference is given to uniform mixing with a planetary ball mill or the like.

  The atmosphere at the time of temperature increase in heating and firing is a nitrogen atmosphere or an Ar atmosphere. Preferably, the temperature is raised in a nitrogen atmosphere.

  The heating rate during heating is preferably 10 ° C./min or more. More preferably, it is 20 ° C./min or more. More preferably, it is 30 ° C./min or more.

  The atmosphere for heating and baking is an Ar atmosphere.

  The temperature for heating and baking is 400 to 700 ° C. When the heating and baking temperature is less than 400 ° C., there is no reaction. When the heating and baking temperature exceeds 700 ° C., dissolution occurs.

The layered lithium iron nitride obtained by the production method of the present invention has a composition formula of Li _3-x Fe _x N (0 <x <0.4). The range of x is preferably 0 <x ≦ 0.32. More preferably, 0.1 ≦ x ≦ 0.32. More preferably, 0.16 ≦ x ≦ 0.32.

  The c-axis lattice constant of the layered lithium iron nitride obtained by the production method of the present invention is preferably 3.79 to 3.87%.

  The layered lithium iron nitride obtained by the production method of the present invention has a diffraction maximum count number ratio between the main peak (100 plane) and the main peak of metal Fe (110 plane) in the X-ray diffraction of the layered lithium iron nitride. Is preferably 1.8 or more. The intensity ratio is preferably 9 or more. More preferably, it is 30 or more. More preferably, the main peak of metallic Fe is not observed.

  Next, a negative electrode using a negative electrode active material made of layered lithium iron nitride according to the present invention will be described.

  When producing a negative electrode using the negative electrode active material in the present invention, a conductive agent and a binder are added and mixed according to a conventional method. As the conductive agent, ketjen black (manufactured by Ketjenblack International Corporation), acetylene black, carbon black, graphite and the like are preferable, and as the binder, polytetrafluoroethylene, polyvinylidene fluoride and the like are preferable.

  The secondary battery manufactured using the negative electrode active material in this invention is comprised from the said negative electrode, a positive electrode, and electrolyte.

  As the positive electrode active material, conventionally known positive electrode active materials such as lithium cobaltate, lithium nickelate, lithium manganate, olivine compounds, and those substituted with a different element can be used.

  In addition to the combination of ethylene carbonate and diethyl carbonate, an organic solvent containing at least one of carbonates such as propylene carbonate and dimethyl carbonate and ethers such as dimethoxyethane can be used as the solvent for the electrolytic solution.

  Further, as the electrolyte, in addition to lithium hexafluorophosphate, at least one lithium salt such as lithium perchlorate and lithium tetrafluoroborate can be dissolved in the above solvent and used.

<Action>
The present inventor considers the reason why the method for producing the layered lithium iron nitride according to the present invention is simple and can be carried out at a low temperature as follows.

The reducing atmosphere during firing was controlled using the difference in reducing properties between Ar and N ₂ . Calcination is performed using weakly reducing N ₂ in order to suppress decomposition of the raw material at the time of temperature rise, and highly reducing Ar in order to stabilize Li _3-x Fe _x N after reaching the reaction temperature. went. In consideration of the fact that the decomposition temperature of the raw material is lower than the reaction temperature, the temperature was raised at a fast rate of 10 to 35 ° C./min to prevent iron precipitation due to the decomposition of the raw material as much as possible. In principle, there is no need to consider the upper limit of the heating rate, but it has been confirmed experimentally up to 50 ° C./min.

Conventionally, Li _3−x Fe _x N could not be synthesized unless the firing conditions were 850 ° C. or higher, but synthesis at 600 ° C. became possible by using an Ar atmosphere having a strong reducing property during firing. In the present invention, the procedure is simplified by using solid phase synthesis without using a complicated procedure.

The production of metallic iron derived from the raw material was suppressed by increasing the temperature rise rate or using the N ₂ atmosphere at the time of temperature rise, and succeeded in obtaining almost single-phase Li _3-x Fe _x N under optimum conditions.

  A typical embodiment of the present invention is as follows.

The crystal structure of the product was identified by powder X-ray diffraction measurement using CoKα as a radiation source, and the c-axis lattice constant was determined. In addition, the X-ray diffraction maximum count ratio between the main peak (100 plane) and the main peak (110 plane) of metal Fe (α-Fe) in the X-ray diffraction of the layered lithium iron nitride Li _3-x Fe _x N Asked. The X-ray diffraction maximum count is obtained by subtracting the baseline count.

  The solid solution amount x of the product Fe was determined as follows. The composition ratio of Li and Fe was determined by atomic absorption analysis, and the solid solution amount x was determined from the value.

  A typical embodiment of the present invention is as follows.

Example 1 <Preparation of layered lithium iron nitride>
The raw materials were mixed and fired under an inert atmosphere (dew point <−65 ° C.). The starting materials were Li ₃ N (High Purity Chemical Laboratory, 99% up) and Fe ₄ N (High Purity Chemical Laboratory, 99.9%). 1.8303 g and 1.1697 g of the starting materials were put in a planetary ball mill (ITO MFG. Co., Ltd .; LP.4), and pulverized and mixed at 60 rpm for 30 minutes, 120 rpm for 30 minutes, and 240 rpm for 5 hours. Thereafter, the mixture was pelletized, heated to 600 ° C. at 35 ° C./min in an N ₂ atmosphere, switched to the Ar atmosphere after reaching the firing temperature, held for 12 hours, and then allowed to cool to room temperature.

<Creation and charge / discharge test of battery cell using layered lithium iron nitride>
A mixture of the obtained layered lithium iron nitride, acetylene black, and polytetrafluoroethylene in a composition of 9: 1: 1 was used as a working electrode, a metal lithium foil as a counter electrode, and 1M-LiPF ₆ / A battery cell was prepared using ethylene carbonate-diethyl carbonate (3: 7).

  The charge / discharge test is performed in a low current (C / 20) -low voltage (<C / 200) mode (assuming 1C = 550 mAh / g) in a potential range of 0.05 to 1.3 V (vs. Li / Li +). I went there.

Examples 2-12, Comparative Examples 1-6
Samples were prepared in the same manner as in Example 1 except for the conditions described in Table 1. The properties and test results of the obtained samples are summarized in Table 2 together with the results of Example 1.

The lithium iron nitride obtained in Comparative Examples 1 and 2 had an inverted fluorite structure. In Comparative Example 3, the reaction did not proceed and only a small amount of lithium iron nitride having an inverted fluorite structure was obtained. Comparative Example 4 is an example in which the Fe solid solution amount x = 0. The c-axis lattice constant of Comparative Example 4 is that of Li ₃ N having the same crystal structure as the layered lithium iron nitride.

Since the layered lithium iron nitride obtained by the production method according to the present invention has a small amount of metallic Fe produced as an impurity, it can be expected to have a high initial oxidation capacity / reduction capacity in a charge / discharge test.

  The layered lithium iron nitride obtained by the method for producing a layered lithium iron nitride of the present invention is suitable as a negative electrode active material for a lithium ion secondary battery.

Claims (5)

A method for producing a layered lithium iron nitride represented by a composition formula of Li _3-x Fe _x N (0 <x <0.4), in which lithium nitride and iron nitride are mixed, A method for producing a layered lithium iron nitride, wherein the temperature is raised to 700 ° C., followed by switching to an Ar atmosphere and heating and firing. A method for producing a layered lithium iron nitride represented by a composition formula of Li _3-x Fe _x N (0 <x <0.4), wherein lithium nitride and iron nitride are mixed, A method for producing a layered lithium iron nitride, wherein the temperature is raised to 400 to 700 ° C. at a rate of temperature increase of 10 ° C./min or more, and then switched to an Ar atmosphere and fired.   The method for producing a layered lithium iron nitride according to claim 1 or 2, wherein a mixing ratio of lithium nitride and iron nitride is 5 or more as a molar ratio of Li / Fe. The negative electrode active material for lithium secondary batteries which consists of layered lithium iron nitride manufactured with the manufacturing method in any one of Claims 1 thru | or 3. The lithium ion secondary battery using the negative electrode active material for lithium secondary batteries of Claim 4.