JP2007265849A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of improving discharge capacity density, and of providing excellent cycle characteristic and load characteristic. <P>SOLUTION: For a positive electrode active material of a positive electrode 1, Li<SB>x</SB>Ti<SB>2</SB>(PO<SB>4</SB>)<SB>3</SB>(0≤x≤5) being a Nasicon type oxyacid lithium compound and having a crystalline system belonging to a space group R-3c is used. For a negative electrode 2, a material capable of storing and releasing lithium ions is used. In this nonaqueous electrolyte secondary battery, plateaus are respectively present, by lithium reference potential, in a region of 2.6 to 3 V, a region of 2.2 to 2.6 V, a region of 2.1 to 2.5, and a region not higher than 2V in discharging and charging. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte.

  Currently, non-aqueous electrolyte secondary batteries that use a non-aqueous electrolyte and charge and discharge by moving lithium ions between a positive electrode and a negative electrode are used as secondary batteries with high energy density.

In such a non-aqueous electrolyte secondary battery, a lithium transition metal composite oxide such as lithium cobaltate (LiCoO ₂ ) is generally used as the positive electrode, and a carbon material capable of occluding and releasing lithium as the negative electrode, lithium metal, lithium An alloy or the like is used. In addition, a non-aqueous electrolyte in which a lithium salt such as lithium tetrafluoroborate (LiBF ₄ ) or lithium hexafluorophosphate (LiPF ₆ ) is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate is used. ing.

In the non-aqueous electrolyte secondary battery as described above, when lithium cobaltate (LiCoO ₂ ) is used as the positive electrode, cobalt (Co) has limited reserves and is a scarce resource, which increases production costs. . Moreover, when the temperature becomes high at the time of charging which cannot be considered in a normal use state, oxygen in the positive electrode is released, and the reaction with the electrolyte is increased, resulting in a problem that the thermal stability is lowered.

For this reason, utilization of lithium manganate (LiMn ₂ O ₄ ), lithium nickelate (LiNiO ₂ ), etc. as a positive electrode material replacing lithium cobaltate (LiCoO ₂ ) has been studied.

However, when lithium manganate (LiMn ₂ O ₄ ) is used as the positive electrode material, a sufficient discharge capacity cannot be expected, and problems such as dissolution of manganese (Mn) occur when the battery becomes hot. On the other hand, when lithium nickelate (LiNiO ₂ ) is used as the positive electrode material, problems such as a low discharge voltage arise.

Therefore, in recent years, a nasicon-type lithium oxyacid compound and an olivine-type lithium oxyacid compound have attracted attention as positive electrode materials that replace lithium cobaltate (LiCoO ₂ ).

In particular, the NASICON lithium oxyacid compound has a high theoretical capacity density. For example, the theoretical capacity density of Li ₃ V ₂ (PO ₄ ) ₃ is 190 mAh / g. Therefore, when a NASICON type lithium oxyacid compound is used, it is theoretically possible to obtain a high discharge capacity density. In addition, it is believed that the NASICON type lithium oxyacid compound can suppress a decrease in battery capacity density even if rapid charging and discharging are performed due to its structure.

For example, Patent Document 1 discloses a lithium battery using Li ₃ V ₂ (PO ₄ ) ₃ which is a NASICON type lithium oxyacid compound as a positive electrode active material.

According to Patent Document 1, Li ₃ V ₂ (PO ₄ ) ₃ can reversibly insert and desorb lithium ions, and at the time of charging, Li ₃ V ^III ₂ (PO ₄ ) ₃ with lithium ion desorption. Sequentially changes to Li ₂ V ^IV V ^III (PO ₄ ) ₃ , LiV ^IV ₂ (PO ₄ ) ₃ , and V ^V V ^IV (PO ₄ ) ₃ , and everything in Li ₃ V ₂ (PO ₄ ) ₃ Of lithium ions can be deintercalated. The superscripts III to V above indicate the valence of vanadium (V).

Thus, in the charging of Li ₃ V ₂ (PO ₄ ) ₃ , the reaction proceeds so that the valence of vanadium increases with the elimination of lithium.

On the other hand, in Patent Document 2, Li _n M ₂ (XO ₄ ) ₃ (0 ≦ n ≦ 3, M: one or more metal elements selected from Fe, Ti, Ni, F, Nb and Mn, X: A lithium ion secondary battery using a positive electrode active material containing a NASICON compound represented by P, S, M, Mo, W or As) is disclosed.

In this Patent Document 2, LiTi ₂ (PO ₄ ) ₃ is synthesized by mixing and heating titanium oxide, ammonium phosphate and lithium carbonate, and using the LiTi ₂ (PO ₄ ) ₃ as a positive electrode active material. It is described that when a lithium ion battery was manufactured, the operating voltage of the obtained battery was 2.5V.
Special Table 2001-2001655 gazette JP 2002-56848 A

  As described above, in the lithium ion battery of Patent Document 2, the potential during discharge is constant at 2.5 V, and it is assumed that an ideal charge / discharge reaction is not performed.

  Therefore, in the lithium ion battery of Patent Document 2, it is considered that the theoretical characteristics of the NASICON lithium oxyacid compound cannot be obtained, and the discharge capacity density and the cycle characteristics cannot be sufficiently improved.

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery capable of improving discharge capacity density and obtaining good cycle characteristics and load characteristics.

(1) A nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode including a positive electrode active material, a negative electrode, and a nonaqueous electrolyte, and the positive electrode active material is Li _x Ti ₂ belonging to the space group R-3c. It includes a NASICON lithium oxyacid compound having (PO ₄ ) ₃ as a basic skeleton, and x is 0 or more and 5 or less.

In the non-aqueous electrolyte secondary battery according to the present invention, the positive electrode active material contains a Nasicon-type lithium oxyacid compound having Li _x Ti ₂ (PO ₄ ) ₃ belonging to the space group R-3c as a basic skeleton, It is considered that reactions of the following formulas (1) to (4) occur during discharging, and reverse reactions of the following formulas (1) to (4) occur during charging.

LiTi ^IV ₂ (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₂ Ti ^IV Ti ^III (PO ₄ ) ₃
・ ・ ・ ・ (1)
Li ₂ Ti ^IV Ti ^III (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₃ Ti ^III ₂ (PO ₄ ) ₃
(2)
Li ₃ Ti ^III ₂ (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₄ Ti ^III Ti ^II (PO ₄ ) ₃
.... (3)
Li ₄ Ti ^III Ti ^II (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₅ Ti ^II ₂ (PO ₄ ) ₃
.... (4)
In this case, it is possible to obtain characteristics close to the theoretical characteristics as a NASICON type lithium oxyacid compound. Accordingly, it is possible to improve the discharge capacity density and obtain good cycle characteristics and load characteristics.

  (2) First, second, and third plateaus are formed during discharging, and fourth, fifth, and sixth plateaus are formed during charging, and the first and fourth plateaus are at the lithium reference potential. The second and fifth plateaus are in the region of 2.2 V to 2.6 V at the lithium reference potential, and the third and sixth plateaus are in the lithium reference potential. The third plateau is formed at a lower potential than the second plateau, the second plateau is formed at a lower potential than the first plateau, and the sixth plateau The plateau may be formed at a lower potential than the fifth plateau, and the fifth plateau may be formed at a lower potential than the fourth plateau.

  In this case, the reaction of the above formula (1) and the reverse reaction thereof occur in the region of 2.6 V to 3 V at the lithium reference potential, and the reaction of the formula (2) and the reverse reaction thereof are 2.2 V at the lithium reference potential. It is considered that the reaction occurs in the region of 2.6 V or less and the reaction of the formula (3) and the reverse reaction occur in the region of 2.1 V or more and 2.5 V or less at the lithium reference potential. In that case, it is considered that the reaction of the positive electrode active material proceeds according to the theoretical reaction formula of the NASICON type lithium oxyacid compound. Therefore, it is possible to obtain characteristics closer to the theoretical characteristics as a NASICON type lithium oxyacid compound.

  (3) When the discharge current and the charging current are 0.1 It, the potential difference between the first plateau and the fourth plateau, the potential difference between the second plateau and the fifth plateau, and the third plateau and the first plateau The potential difference from the plateau 6 may be within 0.1V.

  In this case, the plateau potential at the time of discharging and the potential of the plateau at the time of charging are substantially equal, so that favorable cycle characteristics can be obtained.

(4) The positive electrode includes a positive electrode mixture including a positive electrode active material, a conductive agent, and a binder, and a current collector, and the positive electrode mixture is applied onto the current collector and applied onto the current collector. The packing density of the positive electrode mixture may be 1.4 g / cm ³ or more.

In this case, when the filling density of the positive electrode mixture applied on the current collector is 1.4 g / cm ³ or more, the electron conductivity in the positive electrode mixture is improved. Therefore, the electron conductivity of the positive electrode is sufficiently ensured. As a result, the load characteristics of the nonaqueous electrolyte secondary battery are improved.

  According to the present invention, it is possible to improve the discharge capacity density and obtain good cycle characteristics and load characteristics.

  Hereinafter, the nonaqueous electrolyte secondary battery according to the present embodiment will be described. The nonaqueous electrolyte secondary battery according to the present embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte.

  The various materials described below and the thicknesses and concentrations of the materials are not limited to those described below, and can be set as appropriate.

(1) Production of positive electrode The positive electrode is composed of a positive electrode mixture and a current collector, and the positive electrode mixture includes a positive electrode active material, a binder, and a conductive agent.

As the positive electrode active material, Li _x Ti ₂ (PO ₄ ) ₃ (0 ≦ x ≦ 5) which is a NASICON type lithium oxyacid compound and has a crystal system belonging to the space group R-3c is used. Hereinafter, the compound represented by Li _x Ti ₂ (PO ₄ ) ₃ is referred to as lithium titanium phosphate.

  When the above-described lithium titanium phosphate is used as the positive electrode active material, a region of 2.6 V to 3 V, a region of 2.2 V to 2.6 V, 2.1 V to 2 at the lithium reference potential during discharging and charging. There are plateaus in the region below 5V and the region below 2V.

  Hereinafter, during discharge, a plateau existing in the region of 2.6 V to 3 V at the lithium reference potential is referred to as a first plateau, and a plateau existing in the region of 2.2 V to 2.6 V is referred to as the second plateau. A plateau existing in the region of 2.1V to 2.5V is called a third plateau. During charging, a plateau existing in the region of 2.6 V to 3 V at the lithium reference potential is called a fourth plateau, and a plateau existing in the region of 2.2 V to 2.6 V is called the fifth plateau. A plateau existing in the region of 2.1V to 2.5V is called a sixth plateau. Note that the second plateau exists at a lower potential than the first plateau, and the third plateau exists at a lower potential than the second plateau. In addition, the fifth plateau exists at a lower potential than the fourth plateau, and the sixth plateau exists at a lower potential than the fifth plateau.

Here, when lithium titanium phosphate (Li _x Ti ₂ (PO ₄ ) ₃ ) is used as the positive electrode active material, theoretically, reactions of the following formulas (1) to (4) occur during discharging, and during charging: It is considered that the reverse reaction of the following formulas (1) to (4) occurs in In the following formulas (1) to (4), the valence of titanium (Ti) is indicated by superscripts II, III and IV. The first to third plateaus correspond to the reactions of the following formulas (1) to (4), and the fourth to sixth plateaus correspond to the reverse reactions of the following formulas (1) to (4). It is believed that there is.

LiTi ^IV ₂ (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₂ Ti ^IV Ti ^III (PO ₄ ) ₃
・ ・ ・ ・ (1)
Li ₂ Ti ^IV Ti ^III (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₃ Ti ^III ₂ (PO ₄ ) ₃
(2)
Li ₃ Ti ^III ₂ (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₄ Ti ^III Ti ^II (PO ₄ ) ₃
.... (3)
Li ₄ Ti ^III Ti ^II (PO ₄ ) ₃ + Li ⁺ + e ⁻ → Li ₅ Ti ^II ₂ (PO ₄ ) ₃
.... (4)
That is, the reaction of formula (1) and the reverse reaction thereof occur in the region of 2.6 V to 3 V at the lithium reference potential, and the reaction of formula (2) and the reverse reaction thereof are 2.2 V to 2 at the lithium reference potential. It is considered that the reaction of Formula (3) and the reverse reaction occur in the region of 2.1 V to 2.5 V at the lithium reference potential.

  The reaction of the formula (4) and the reverse reaction are considered to occur in the region of 2 V or less at the lithium reference potential. Note that the reaction of the formula (4) is not used except when the nonaqueous electrolyte secondary battery is used in a potential region of 2 V or less.

  Thus, in this embodiment, by using lithium titanium phosphate having a crystal system belonging to the space group R-3c as the positive electrode active material, the above formula (1 ) To (3) occur, and the reverse reactions of the above formulas (1) to (3) are considered to occur during charging. These reactions are presumed to occur at different potentials step by step. In this case, it is considered that characteristics closer to the theoretical characteristics of the NASICON type lithium oxyacid compound can be sufficiently obtained.

For example, the reactions of the above formulas (1) to (3) occur during discharge, and LiTi ₂ (PO ₄ ) ₃ changes to Li ₄ Ti ₂ (PO ₄ ) ₃ , so that the theoretical capacity is as high as 197 mAh / g. Density is obtained. Therefore, the discharge capacity density can be improved also in the nonaqueous electrolyte secondary battery according to the present embodiment.

As the particle diameter of the positive electrode active material, the median diameter (R _median ) and mode diameter (R _mode ) when measured with a laser diffraction particle size distribution analyzer are both preferably 10 μm or less, and more preferably 5 μm or less. preferable.

  Lithium titanium phosphate having a NASICON structure has a slow lithium insertion / release reaction during charge / discharge. For this reason, when the particle diameter is larger than 10 μm, resistance associated with lithium insertion / extraction increases, so that the central part of the particle cannot be used as an active material. On the other hand, when the particle diameter is 10 μm or less, the diffusion distance of lithium in the particle is shortened, so that the resistance associated with lithium insertion / extraction during charge / discharge can be reduced. Therefore, the utilization factor of the positive electrode active material can be improved by setting the particle diameter of lithium titanium phosphate to 10 μm or less, and the utilization factor can be further improved by setting the particle diameter to 5 μm or less. As a result, charge / discharge characteristics can be improved.

The specific surface area of the positive electrode active material calculated from the Brunauer Emmettteller (BET) equation is preferably 10 m ² / g or more. In this case, the contact area between the positive electrode active material and the conductive agent and the contact area between the positive electrode active material and the current collector are increased, and the electron conductivity of the positive electrode can be improved.

  Further, as the positive electrode active material, a mixture of a lithium-containing compound having a NASICON structure and another positive electrode material may be used.

  As the conductive agent, conductive powder is used. For example, a conductive carbon material, a metal oxide or the like is used, and preferably a conductive carbon powder is used. By mixing a conductive agent with the positive electrode active material, a conductive network formed of the conductive agent is formed around the particles of the positive electrode active material. Thereby, the electronic conductivity in a positive electrode mixture can be improved. In addition, when the addition amount of a conductive agent increases too much, the ratio of the positive electrode active material in the positive electrode mixture decreases and a high capacity cannot be obtained. Therefore, it is preferable that the addition amount of a electrically conductive agent is 10 weight% or less of the whole positive mix.

  In the present embodiment, the positive electrode is preferably formed by rolling a sufficiently dried positive electrode mixture on a current collector. A rolling roller, a press machine, etc. can be used for rolling. Thus, the density of the positive electrode active material can be improved by rolling the positive electrode mixture. Thereby, the volume energy density of a positive electrode active material can be improved. Further, the density of the positive electrode mixture is improved by rolling, and the contact area between the positive electrode active material and the conductive agent is increased. Thereby, the electronic conductivity in the positive electrode mixture is improved, and the load characteristics can be improved.

In addition, in order to ensure sufficient electron conductivity of the positive electrode, the positive electrode mixture is preferably formed at a density equal to or higher than a predetermined value. However, since the electron conductivity of the positive electrode depends on the electron conductivity between the positive electrode mixture and the current collector in addition to the electronic conductivity in the positive electrode mixture, the preferred range of the density of the positive electrode mixture is It depends on the electron conductivity between the positive electrode mixture and the current collector. For example, when the roughened current collector is used to improve the adhesion between the positive electrode mixture and the current collector and sufficient electron conductivity is ensured, the density of the positive electrode mixture is 1.4 g / It is preferable that it is cm ³ or more.

  As the binder to be added to the electrode, one or more selected from polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber and carboxymethylcellulose are used. be able to.

  When the amount of the binder added to the electrode is large, the ratio of the active material contained in the positive electrode becomes small, so that a high energy density cannot be obtained. Therefore, the amount of the binder is in the range of 0 to 30% by weight, preferably 0 to 20% by weight, more preferably 0 to 10% by weight.

(2) Structure of negative electrode As the negative electrode, a material capable of inserting and extracting lithium ions is used. Examples of this material include one or more materials selected from the group consisting of lithium, silicon, carbon, tin, germanium, aluminum, lead, indium, gallium, lithium-containing alloys, and compounds containing lithium and carbon. Can be used. An example of the above compound containing lithium and carbon is a graphite intercalation compound in which lithium is inserted between graphite layers.

(3) Preparation of nonaqueous electrolyte As a nonaqueous electrolyte, what dissolved electrolyte salt in the nonaqueous solvent can be used.

  Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, and the like, and combinations thereof.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate and the like, and those in which some or all of these hydrogen groups are fluorinated can be used. For example, trifluoropropylene carbonate, fluoro Examples include ethyl carbonate.

  Examples of chain carbonic acid esters include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, and the like. Some of these hydrogen groups are fluorinated. It is possible to use.

  Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5. -Trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether and the like.

  Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl. Ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1 -Dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethy Glycol dimethyl and the like.

  Nitriles include acetonitrile and the like, and amides include dimethylformamide and the like.

  Among the above non-aqueous solvents, particularly from the viewpoint of voltage stability, it is preferable to use cyclic carbonates such as ethylene carbonate and propylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate and dipropyl carbonate. .

As electrolyte salts, LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (C _l F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (l and m are integers of 1 or more) _{, LiC (C p F 2p +} 1 SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) (p, q, r is an integer of 1 or more), by the following structural formula The difluoro (oxalato) lithium borate etc. which are shown are mentioned.

  In addition, you may use 1 type in the said electrolyte salt, and may use it in combination of 2 or more type.

  The electrolyte salt is dissolved in the nonaqueous solvent at a concentration of 0.1 to 1.5 mol / l, preferably dissolved at a concentration of 0.5 to 1.5 mol / l.

(4) Effects in the present embodiment In this embodiment, lithium titanate (Li _x Ti ₂ (Li _x Ti ₂ () having a crystal system belonging to the space group R-3c and being a Nasicon type lithium oxyacid compound as the positive electrode active material). By using PO ₄ ) ₃ (0 ≦ x ≦ 5), the reaction of the above formulas (1) to (3) occurs at the time of discharging in the region of 2V or more actually used, and the above formula ( It is thought that the reverse reaction of 1) to (3) occurs. In this case, it is possible to obtain characteristics closer to the theoretical characteristics as a NASICON type lithium oxyacid compound. Thereby, the discharge capacity density can be improved, and good cycle characteristics and load characteristics can be obtained.

  Further, since the plateau potential at the time of discharging and the potential of the plateau at the time of charging become substantially equal, good cycle characteristics can be obtained.

(Preparation of positive electrode)
Lithium carbonate (Li ₂ CO ₃ ), titanium oxide (TiO ₂ ), and ammonium hydrogen phosphate were used as starting materials for the positive electrode active material. Note that ammonium hydrogen phosphate includes diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ) and ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ).

  These raw materials were weighed so as to have a ratio according to the stoichiometry, carbon powder (PRINTEX XE2-B manufactured by Degussa) and acetone were added, and mixed for 3 hours at a rotation speed of 150 rotations / minute by a planetary ball mill.

  The carbon powder was used to take out titanium by reducing titanium oxide having strong bonds. Excess carbon powder acts as a conductive agent. Acetone was used for wet mixing of the ball mill. These carbon powder and acetone do not affect the synthesis of the positive electrode active material.

  Subsequently, acetone was evaporated from the slurry obtained by ball mill mixing to obtain a powdered positive electrode active material. Next, this powdery positive electrode active material was formed into pellets (small particles) and pre-baked in an argon (Ar) atmosphere at 300 ° C. for 8 hours.

  Next, the calcined positive electrode active material was pulverized from a pellet to a powder, and again molded into a pellet. Thereafter, the pellet-shaped positive electrode active material was calcined in an argon (Ar) atmosphere at 850 ° C. for 8 hours.

  Then, the positive electrode active material after the main firing was pulverized again into a powder and measured by XRD (X-ray diffractometer). Note that copper (Cu) was used as the X-ray source, the working voltage and working current were 40 kV and 40 mA, respectively, and a range of 2θ (10 to 80 degrees) was measured at a scanning speed of 1.0 degree / minute.

FIG. 1 shows the XRD measurement results for the positive electrode active material after the main firing. In the lower part of FIG. 1, for comparison, XCP data of JCPDS (Joint Committee on Powder Diffraction Standards), which contains X-ray diffraction data of about 6000 kinds of inorganic compounds and organic compounds, is a space group R- The X-ray diffraction data of LiTi ₂ (PO ₄ ) ₃ with card number 350754 belonging to 3c is shown.

As shown in FIG. 1, the XRD pattern of the positive electrode active material after the main firing substantially matches the pattern of JCPDS card number 350754. Therefore, it was found that the positive electrode active material obtained by the main firing had a crystal system belonging to the space group R-3c similar to LiTi ₂ (PO ₄ ) ₃ .

  Further, the powdered positive electrode active material after the main firing was observed with a scanning electron microscope (SEM). FIG. 2 is an enlarged photograph of the positive electrode active material after the main firing by SEM.

  From FIG. 2, it was found that the particle diameter of the positive electrode active material was approximately 5 μm or less.

  Next, the positive electrode active material after the main firing and the conductive agent acetylene black (Electrochemical Black, manufactured by Denki Kagaku Kogyo) were mixed so as to be 80% by weight and 10% by weight, respectively, of the whole positive electrode. Thereafter, polyacrylonitrile (PAN) as a binder is added to the mixture of the positive electrode active material and the conductive agent so that the total amount of the positive electrode is 10% by weight, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) is added. Mixed. This produced the slurry as a positive electrode mixture.

This slurry was applied as a positive electrode mixture onto a roughened aluminum foil as a current collector by a doctor blade method, and dried at 80 ° C. using a hot plate. Thereafter, the current collector coated with the positive electrode mixture was cut into a 2 cm square and rolled using a roller. At this time, the density of the positive electrode mixture was 1.4 g / cm ³ . Then, it dried at 100 degreeC under vacuum. In this way, a positive electrode was produced.

(Preparation of negative electrode and reference electrode)
Lithium metal was used for each of the negative electrode and the reference electrode.

(Nonaqueous electrolyte adjustment)
As a non-aqueous electrolyte, lithium hexafluorophosphate (LiPF ₆ ) was added to a non-aqueous solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 30:70 so as to have a concentration of 1 mol / l. A thing was used.

(Production of reference electrode)
As the reference electrode, lithium metal cut to a predetermined size was used.

(Production of test cell)
FIG. 3 is a schematic explanatory view of a test cell produced in the example. As shown in FIG. 3, the lead was attached to the positive electrode (working electrode) 1 and the lead was attached to the negative electrode (counter electrode) 2 in an inert atmosphere. The separator 4 was inserted between the positive electrode 1 and the negative electrode 2, and the positive electrode 1, the negative electrode 2, and the reference electrode 3 were disposed in the test cell container 10. The non-aqueous electrolyte 5 was injected into the test cell container 10 to produce the test cell of the example.

(Charge / discharge test)
Using the produced test cell, a charge / discharge test was performed under the following conditions, and the discharge capacity per unit mass of the positive electrode active material was measured. Note that the rated current is 1.0 It under the following conditions. Here, the rated current means a current value when the rated discharge capacity is completely discharged in one hour, and the rated discharge capacity is a virtual discharge assumed from the weight of the positive electrode active material and the area of the positive electrode mixture. It is capacity.

  First, charging was performed until the potential of the positive electrode 1 with reference to the reference electrode 3 reached 4.5 V at a current value of 0.1 It reached 4.5 V, and then discharging was performed until the potential reached 2 V for 30 cycles.

  FIG. 4 shows charge / discharge curves in the first cycle and the second cycle.

  As can be seen from FIG. 4, in the discharge curves of the first and second cycles, a region of 2.6 V to 3.0 V, a region of 2.2 V to 2.6 V, and a voltage of 2.1 V to 2.5 V A plateau could be confirmed in each area.

  Also, check the plateau in the region of 2.6V to 3.0V, the region of 2.2V to 2.6V, and the region of 2.1V to 2.5V in the second cycle charging curve. I was able to.

  FIG. 5 shows the cycle characteristics of the nonaqueous electrolyte secondary battery of this example. In FIG. 5, the horizontal axis indicates cycle characteristics, and the vertical axis indicates discharge capacity density.

  As shown in FIG. 5, from the first cycle to the 30th cycle, the discharge capacity density changed within the range of 85 to 90 mAh / g, and the discharge capacity density of the 30th cycle with respect to the discharge capacity density of the 1st cycle. The ratio (capacity maintenance ratio) was 99.5%.

  Table 1 shows the load characteristics at room temperature of the nonaqueous electrolyte secondary battery of this example. The discharge current was 0.1 It, 0.2 It, 0.5 It, 1 It and 2 It, the discharge start voltage was 4.5 V, and the discharge end voltage was 2 V.

  As shown in Table 1, a discharge capacity density of about 80 to 85 mAh / g was obtained when the discharge current was in the range of 0.1 to 2 It. Further, the ratio of the discharge capacity density in the case of 2 It to the discharge capacity density in the case of the discharge current of 0.1 It was 95%.

(Evaluation)
By using lithium titanium phosphate (LiTi ₂ (PO ₄ ) ₃ ) belonging to space group R-3c as the positive electrode active material, a region of 2.6 V to 3.0 V, 2.2 V to 2.6 V It was found that plateaus were formed in the discharge curve and the charge curve in the region of 2.1 and in the region of 2.1 V to 2.5 V.

  Further, it was found that the difference between the potential at which the plateau can be confirmed in the discharge curve and the potential at which the plateau can be confirmed in the charging curve is 0.1 V or less.

  Furthermore, it was found that good cycle characteristics and load characteristics can be obtained by using the above lithium titanium phosphate as the positive electrode active material.

It was also found that good cycle characteristics and load characteristics can be obtained when the particle diameter of the positive electrode active material is approximately 5 μm or less and the density of the positive electrode mixture is 1.4 g / cm ³ or more.

  In this example, a high discharge capacity density could not be obtained, but this is considered to be because impurities exist in the positive electrode active material.

  From the results of the examples, when lithium titanium phosphate belonging to the space group R-3c is used as the positive electrode active material, it is considered that the reaction proceeds according to the theoretical reaction formula. Therefore, when lithium titanium phosphate belonging to space group R-3c is used as the positive electrode active material, it is considered that a high discharge capacity density can be obtained by reducing impurities.

  The nonaqueous electrolyte secondary battery according to the present invention can be used as various power sources such as a portable power source and an automotive power source.

It is a figure which shows the result of the XRD measurement of the positive electrode active material in an Example. It is an enlarged photograph by SEM of the positive electrode active material in an Example. It is a schematic explanatory drawing of the test cell produced in the Example. It is the graph which showed the charging / discharging curve of the nonaqueous electrolyte secondary battery in an Example. It is the graph which showed the discharge capacity density obtained in each cycle of the nonaqueous electrolyte secondary battery in an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Reference electrode 4 Separator 5 Nonaqueous electrolyte 10 Cell container

Claims (4)

A positive electrode containing a positive electrode active material, a negative electrode, and a non-aqueous electrolyte,
The positive electrode active material includes a Nasicon type lithium oxyacid compound having Li _x Ti ₂ (PO ₄ ) ₃ belonging to the space group R-3c as a basic skeleton,
Said x is 0 or more and 5 or less, The nonaqueous electrolyte secondary battery characterized by the above-mentioned. During discharge, first, second and third plateaus are formed,
During charging, the fourth, fifth and sixth plateaus are formed,
The first and fourth plateaus are in a region of 2.6 V or more and 3 V or less at a lithium reference potential,
The second and fifth plateaus are in the region of 2.2 V to 2.6 V at the lithium reference potential,
The third and sixth plateaus are in the region of 2.1 V to 2.5 V at the lithium reference potential,
The third plateau is formed at a lower potential than the second plateau, the second plateau is formed at a lower potential than the first plateau,
The non-aqueous solution according to claim 1, wherein the sixth plateau is formed at a lower potential than the fifth plateau, and the fifth plateau is formed at a lower potential than the fourth plateau. Electrolyte secondary battery. When the discharge current and the charging current are 0.1 It, the potential difference between the first plateau and the fourth plateau, the potential difference between the second plateau and the fifth plateau, and the third plateau The non-aqueous electrolyte secondary battery according to claim 2, wherein a potential difference between the first plateau and the sixth plateau is within 0.1V. The positive electrode includes a positive electrode mixture containing the positive electrode active material, a conductive agent and a binder, and a current collector,
The positive electrode mixture is applied onto the current collector,
The nonaqueous electrolyte secondary battery according to claim 1, wherein a packing density of the positive electrode mixture applied on the current collector is 1.4 g / cm ³ or more.

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