JP5298659B2 - Active material for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyanionic active material that excels in high rate discharge performance, and to provide a lithium secondary battery using the same. <P>SOLUTION: The active material for a lithium secondary battery is represented by general formula: Li<SB>a</SB>M<SB>b</SB>(PO<SB>4</SB>)<SB>1-x</SB>(BO<SB>3</SB>)<SB>x</SB>, where M is one or more transition metal elements selected from the group consisting of Ti, V, Cr, Mn, Fe, Co and Ni; 0&lt;a, 0&lt;b, 0.5&lt;a+b&le;2; 0&lt;x&lt;1; a and b being specified such that the general formula remains electroneutral. In particular, the active material for a lithium secondary battery is represented by the general formula: Li<SB>3</SB>V<SB>2</SB>(PO<SB>4</SB>)<SB>3-x</SB>(BO<SB>3</SB>)<SB>x</SB>, where 0&lt;x&le;1/4. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an active material for a polyanionic lithium secondary battery and a lithium secondary battery using the same.

  In recent years, non-aqueous electrolyte secondary batteries represented by lithium secondary batteries with high energy density and low self-discharge and good cycle performance have been used as power sources for mobile devices such as mobile phones and laptop computers, and electric vehicles. Attention has been paid.

The current mainstream of lithium secondary batteries is for consumer use, mainly for mobile phones of 2 Ah or less. Many positive electrode active materials for lithium secondary batteries have been proposed. The most commonly known positive electrode active materials are lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide whose operating voltage is around 4V. object is a (LiNiO _2), or lithium manganese oxide having a spinel structure lithium-containing transition metal oxide to basic configuration (LiMn ₂ O ₄₎ or the like. Among these, lithium cobalt oxide is widely adopted as a positive electrode active material for small-capacity lithium secondary batteries up to a battery capacity of 2 Ah because of its excellent charge / discharge characteristics and energy density.

  When considering the development of non-aqueous electrolyte batteries for automotive applications such as HEV and EV, which are expected to have a large demand in the future, and other industrial applications, the batteries will be used in high-temperature environments that will not be used in small consumer applications. It must be assumed that it will be used. In such a high temperature environment, not only conventional non-aqueous electrolyte secondary batteries, but also nickel-cadmium batteries and lead batteries have a very short life, and there is no conventional battery that satisfies user requirements. Further, although the capacitor can only be used in this temperature range, it has a low energy density and does not satisfy the user's requirements in this respect, and a battery having a high temperature and a long life and a high energy density is required. At the same time, since safety is very important, lithium manganese oxide is sometimes used instead of lithium cobalt oxide, which is highly safe and stable in terms of thermal stability in the charged state. However, even with lithium manganese oxide, at a high temperature of 300 ° C. or higher, oxygen is released into the battery as it decomposes, so that sufficient safety cannot be ensured when the battery generates heat in the event of an internal short circuit or the like. .

  Recently, polyanionic active materials having excellent thermal stability have attracted attention. This polyanion-based active material is immobilized by covalently bonding oxygen to elements other than transition metals, so it does not release oxygen even at high temperatures and can be used as an electrode active material for battery safety. It is thought that can be dramatically improved.

As such a polyanion positive electrode active material, lithium iron phosphate (LiFePO ₄ ) is currently widely studied. Lithium iron phosphate has a theoretical capacity of 170 mAh · g ^−1, which is larger than that of lithium manganate, but Li insertion / desorption is 3.4 Vvs. Since it is carried out at a low potential of Li / Li ⁺ , its energy density per weight is only the same as that of lithium manganate.

Therefore, a phosphoric acid-based polyanion positive electrode active material capable of inserting and removing Li at a higher potential than lithium iron phosphate was investigated, and phosphorous with manganese or vanadium as the central metal instead of iron. Lithium manganese oxide (LiMnPO ₄ ) and lithium vanadium phosphate (Li ₃ V ₂ (PO ₄ ) ₃ ) have been found.

Lithium vanadium phosphate has a high theoretical capacity of 197 mAh / g because the insertion / extraction potential of lithium is close to that of LiCoO ₂ and a maximum of 1.5 Li is operated for one V. The energy density per weight far exceeds that of lithium manganate, and can be said to be a positive electrode active material having both high safety and high energy density.

  In the medium and large industrial batteries, the high energy density characteristics are not as important as those for consumer equipment, but the output characteristics, that is, the battery capacity that can be taken out when performing high rate discharge is low. The ratio with respect to the capacity | capacitance which can be taken out when rate discharge is performed is calculated | required. Thus, the polyanion positive electrode material excellent in safety is required to have good high-rate discharge characteristics.

The Examples of Patent Document 1, it was Electrochemical evaluation of vanadium phosphate lithium _{Li 3 V 2 (PO 4)} 3 is described.

In an example of Patent Document 2, a material in which a portion of M represented by Li ₃ M ₂ (PO ₄ ) ₃ is composed of an equal amount of two kinds of transition metals (for example, Li ₃ AlV (PO ₄ ) ₃ , Li ₃ BV (PO ₄ ) ₃ , Li ₃ MgV (PO ₄ ) ₃ etc.) are described. In Patent Document 2, there is no description or suggestion of what the performance of these materials will be, but all of Al, B, and Mg have a valence associated with the electrochemical redox of the materials. Since it is an element that does not fluctuate, the battery characteristics including the discharge capacity are expected to be halved.

Patent Document 3 suggests that (PO ₄ ), which is a polyanion site, is partially substituted with (SiO ₄ ), but there is no specific description as an example, and what performance There is no description or suggestion about whether or not the effect can be expected.

Table 1 of Patent Document 4 describes LiFeP _0.95 B _0.05 O _3.95 (Example 2), and the conductivity of LiFePO ₄ (Comparative Example 1) is 1 × 10 ⁻¹² S / cm or less. It was shown that it was 8 × 10 ⁻⁴ S / cm. It is also shown that the discharge capacity has increased. In Patent Document 4, the ionic radius of tetracoordinate P ⁵⁺ is 0.017 nm, the ionic radius of tetracoordinate B ³⁺ is 0.012 nm, and B can be substituted at the P position. It is described. However, there is no description about substitution of tricoordinate BO ₃ . In addition, although there is a description about conductivity, since polyethylene glycol is added in the examples and sintered in an Ar atmosphere, the conductivity of carbon remaining at the time of sintering is also measured. The effect of substitution or the effect of remaining carbon is not clear.
Special Table 2001-2001655 gazette Japanese translation of PCT publication No. 2002-530835 Special table 2002-519836 gazette JP 2004-178835 A Q.CHen et al., Electrochimica Acta, 2007, vol.52, page 5251-5257.

  An object of the present invention is to provide a polyanionic active material excellent in high rate discharge performance and a lithium secondary battery using the same.

In Patent Document 4, the problem is that the electronic conductivity of LiFePO ₄ is as low as 1 × 10 ⁻¹² S / cm or less, but within the scope of the present inventors, it will be described later. As shown in the examples, the present invention has an electron conductivity of 1 × 10 ⁻³ S / cm or more regardless of the value of x, and the premise of the present invention is that in terms of electron conductivity There are no issues.

  The configuration and effects of the present invention are as follows. However, the action mechanism described in this specification includes estimation, and its correctness does not limit the present invention.

In the present invention, the general formula Li _{a Mb} (PO ₄ ) _1-x (BO ₃ ) _x (where M is one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, or Two or more transition metal elements, 0 <a, 0 <b, 0.5 <a + b ≦ 2, 0 <x <1, and a and b are selected so that the general formula is electrically neutral. ) Is an active material for a lithium secondary battery.

Further, the present invention is a general formula _{Li 3 V 2 (PO 4)} 3-x (BO 3) x active material for a lithium secondary battery represented by (0 <x ≦ 1/4 ).

  Here, it does not prevent other elements from dissolving in part of V.

  Moreover, the present invention is characterized in that the active material has an electron-conducting carbonaceous material deposited on a surface thereof.

  Moreover, this invention is a lithium ion secondary battery provided with the positive electrode containing the said active material, the negative electrode containing the negative electrode active material which occludes / releases lithium ion, and a nonaqueous electrolyte.

The active material according to the present invention is selected from the group consisting of the general formula Li _{a Mb} (PO ₄ ) _1-x (BO ₃ ) _x (where M is Ti, V, Cr, Mn, Fe, Co, Ni). One or more transition metal elements, 0 ≦ a <3, 0 <b ≦ 1, 0.5 <a + b ≦ 2, 0 <x <1, and a and b are of the general formula The values of transition metals M and a and b are LiFe ⁺² (PO ₄ ) _1-x (BO ₃ ) _x , LiMn ⁺² (PO ₄ ) _{1 − x} (BO ₃ ) _x , LiCo ⁺² (PO ₄ ) _1-x (BO ₃ ) _x , LiNi ⁺² (PO ₄ ) _1-x (BO ₃ ) _x , LiV ⁺² (PO ₄ ) _1-x (BO ₃ ) _x , LiV ⁺³ _2/3 (PO ₄ ) _1-x (BO ₃ ) _x , LiFe ⁺³ _2/3 (PO ₄ ) _1-x (BO ₃₎ ) _X , LiTi ⁺³ _2/3 (PO ₄ ) _1-x (BO ₃ ) _x , LiCr ⁺³ _2/3 (PO ₄ ) _1-x (BO ₃ ) _x , Fe ⁺² (PO ₄ ) _1-x (BO) ₃ ) _x , Mn ⁺² (PO ₄ ) _1-x (BO ₃ ) _x , Co ⁺² (PO ₄ ) _1-x (BO ₃ ) _x , Ni ⁺² (PO ₄ ) _1-x (BO ₃ ) _x , etc. Is mentioned.

  The method for producing an active material according to the present invention is not limited, but basically, a raw material containing a metal element constituting the active material, a raw material that becomes a phosphoric acid source, and a raw material that becomes a boric acid source. The raw material contained according to the composition of the active material to be prepared can be prepared and fired. At this time, the composition of the compound actually obtained may slightly vary compared to the composition calculated from the raw material composition ratio. The present invention can be carried out without departing from the technical idea or main features thereof, and the scope of the present invention is only that the composition of the product obtained as a result of the production does not exactly match the above composition formula. Needless to say, it should not be construed as not belonging to. In particular, it is known that a part of the lithium source easily volatilizes during firing. For this reason, it is a common practice to charge a lithium source as a raw material before firing more than the intended composition.

As a raw material for synthesizing Li _a M _b (PO ₄ ) _1-x (BO ₃ ) _x , the general formula is not limited at all. For example, as a Li source, LiOH, LiOH.H ₂ O , LiNO ₃ , Li ₂ CO ₃ , CH ₃ COOLi · 2H ₂ O, Li ₂ SO ₄ · H ₂ O, Li ₂ C ₂ O ₄ , M sources include Ti, TiO, TiO ₂ , Ti ₂ O ₃ , Ti (OCH ₃ ) ₄ , V, V ₂ O ₅ , V ₂ O ₃ , V ₂ O ₄ , NH ₄ VO ₃ , VOSO ₄ , 2H ₂ O, VOCl ₃ , V (C ₅ H ₇ O ₂ ) ₃ , _{V (C 5 H 7 O 2} ) 3, (OC 2 H 5) 2 VO, Cr, CrO 3, Cr 2 O 3, Cr (CH 3 COO) 3 · xH 2 O, Cr (NO 3) 3 · 9H _{2 O, Cr (OH) 3} , Mn, MnO, MnO 2, _{n 2 O 3, Mn 3 O} 4, MnC 2 O 4, Mn (CH 3 COO) 2 · 4H 2 O, Mn (NO 3) 2 · 6H 2 O, MnSO 4 · 5H 2 O, Fe, FeO, Fe ₂ O ₃ , Fe ₃ O ₄ , Fe (NO ₃ ) ₃ .9H ₂ O, FeSO ₄ .7H ₂ O, FeSO ₄ , FeC ₂ O ₄ .2H ₂ O, Fe (C ₃ H ₅ O ₃ ) _2. 3H ₂ O, Co, CoO, CoO (OH), Co ₃ O ₄ , CoC ₂ O ₄ , Co (CH ₃ COO) ₂ .4H ₂ O, Co (NO ₃ ) ₂ .6H ₂ O, CoSO ₄ .7H _{As a} source of ₂ O, Ni, NiO, Ni (NO ₃ ) ₂ .6H ₂ O, NiSO ₄ .6H ₂ O, NiC ₂ O ₄ .2H ₂ O, PO ₄ , NH ₄ H ₂ PO ₄ , (NH ₄ ) _{2 HPO 4, H 3 PO 4} , P 2 O 5, BO 3 _{As, H 3 BO 3, B 2} O 3, (NH 4) 2 O · 5B 2 O 3, (NH 4) 2 B 4 O 7 · 4H 2 O, Li 3 PO 4 as Li and PO ₄ source, LiH ₂ PO _4, M and P sources as _{Fe 3 (PO 4) 2 ·} 8H 2 O, FePO 4 · 4H 2 O, Co 3 (PO 4) 2 · 8H 2 O, Li 2 B 4 as Li and BO ₃ sources O ₇ , Li ₂ B ₄ O ₇ .3H ₂ O, LiBO ₂ .2H ₂ O, or the like can be used.

  The method for synthesizing the polyanionic active material according to the present invention is not particularly limited. Specific examples include a solid phase method, a liquid phase method, a sol-gel method, and a hydrothermal method. Here, the polyanionic active material can be finally obtained by firing. If the firing temperature is too low or too high, the desired polyanionic active material cannot be obtained, and is preferably 500 ° C to 1000 ° C, more preferably 650 ° C to 850 ° C.

  For the purpose of supplementing the electron conductivity, it is preferable that carbon is adhered and coated on the surface of the active material particles mechanically or by thermal decomposition of organic matter.

In the present invention, the polyanionic active material is preferably used for a positive electrode for a lithium secondary battery as a powder having an average particle size of 100 μm or less of secondary particles. In particular, it is preferable that the particle size is small, the average particle size of the secondary particles is more preferably 0.5 to 20 μm, and the particle size of the primary particles constituting the secondary particles is preferably 1 to 500 nm. The specific surface area of the powder particles is preferably large in order to improve the high rate discharge characteristics of the positive electrode, and is preferably 1 to 100 m ² / g. More preferably, it is 5-100 m < ² > / g. In order to obtain the powder in a predetermined shape, a pulverizer or a classifier can be used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill, a sieve, or the like can be used. At the time of pulverization, wet pulverization in which an organic solvent such as water or alcohol or hexane coexists may be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like can be used dry or wet as necessary.

  As the conductive agent and the binder, well-known ones can be used in a well-known prescription.

  The amount of water contained in the positive electrode containing the active material of the present invention is preferably as small as possible, specifically less than 500 ppm.

  Moreover, the thickness of the electrode mixture layer to which the present invention is applied is preferably 20 to 500 μm in view of the balance with the energy density of the battery.

The negative electrode of the battery of the present invention is not limited in any way, but lithium metal, lithium alloy (lithium metal such as lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloy) Alloys), alloys capable of inserting and extracting lithium, carbon materials (eg, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.), metal oxides, lithium metal oxides (Li ₄ Ti ₅ O ₁₂₎ Etc.), polyanionic compounds and the like. You may use the active material for lithium secondary batteries which concerns on this invention. Among these, graphite is preferable as a negative electrode material because it has an operating potential very close to that of metallic lithium and can realize charge and discharge at a high operating voltage. For example, artificial graphite and natural graphite are preferable. In particular, graphite in which the surface of the negative electrode active material particles is modified with amorphous carbon or the like is desirable because it generates less gas during charging.

  In general, the form of a non-aqueous electrolyte battery is composed of a positive electrode, a negative electrode, and a non-aqueous electrolyte containing an electrolyte salt in a non-aqueous solvent. Is provided.

  Non-aqueous solvents include cyclic carbonates such as propylene carbonate and ethylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate Chain esters such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyl jig Examples include ethers such as lime; nitriles such as acetonitrile and benzonitrile; dioxolane or a derivative thereof; ethylene sulfide, sulfolane, sultone or a derivative thereof alone or a mixture of two or more thereof. Is limited to There is no.

Examples of the electrolyte salt include ionic compounds such as LiBF ₄ and LiPF ₆ , and these ionic compounds can be used alone or in admixture of two or more. The concentration of the electrolyte salt in the nonaqueous electrolyte is preferably 0.5 mol / l to 5 mol / l, and more preferably 1 mol / l to 2.mol in order to reliably obtain a lithium ion secondary battery having high battery characteristics. 5 mol / l.

  In the present specification, a non-aqueous electrolyte battery will be described in detail, particularly among lithium secondary batteries. However, the active material according to the present invention may be used for a positive electrode of an aqueous lithium secondary battery. The effects of the invention are effectively exhibited.

  Hereinafter, the present invention will be described in more detail by way of examples. However, the present invention is not limited to the following embodiments.

  The present inventors use a sol-gel method using citric acid. By using citric acid as a complex-forming agent, it becomes possible to mix uniformly and primary particles can be made smaller. Moreover, citric acid is arrange | positioned on the primary particle surface as a carbon source by baking.

Example 1
(Synthesis of Li ₃ V ₂ (PO ₄ ) _11/2 (BO ₃ ) _1/4 corresponding to x = _{1/4 in} the general formula Li ₃ V ₂ (PO ₄ ) _3-x (BO ₃ ) _x )
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 .5: 11/4: Weighed to 1/4. These were added to purified water in the order described and stirred, and it was confirmed that each time each raw material was added, it was dissolved. Next, the solvent was removed on a hot plate at 80 ° C. to obtain a precursor. This was pulverized well in an automatic mortar. This precursor was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and the atmosphere was replaced with a nitrogen gas (flow velocity) using an atmosphere substitution type firing furnace (a table vacuum gas substitution furnace KDF-75 manufactured by Denken). Firing was performed at 1.0 l / min). The calcining temperature for pre-firing was 350 ° C., the calcining time (the time for maintaining the calcining temperature) was 3 hours, the main calcining temperature was 850 ° C., and the calcining time was 6 hours. The rate of temperature increase was 5 ° C./minute, and the temperature was naturally cooled. Next, it grind | pulverized for 1 hour with the automatic mortar, and the secondary particle diameter was 50 micrometers or less. In this state, a carbonaceous material derived from citric acid is disposed on the surface of the primary particles of Li ₃ V ₂ (PO ₄ ) ₃ . This also applies to the following examples and comparative examples. This is designated as active material a1 of the present invention.

(Example 2)
(Synthesis of Li ₃ V ₂ (PO ₄ ) _23/8 (BO ₃ ) _1/8 corresponding to x = _{1/8 in} the general formula Li ₃ V ₂ (PO ₄ ) _3-x (BO ₃ ) _x
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 .5: 23/8: An active material for a lithium secondary battery was prepared in the same manner as in Example 1 except that the weight was adjusted to be 1/8. This is designated as active material a2 of the present invention.

(Example 3)
(Synthesis of Li ₃ V ₂ (PO ₄ ) _47/16 (BO ₃ ) _1/16 corresponding to x = _{1/16 in} the general formula Li ₃ V ₂ (PO ₄ ) _{3 -x} (BO ₃ ) _x
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 5: 47/16: An active material for a lithium secondary battery was prepared in the same manner as in Example 1 except that the weight was adjusted to be 1/16. This is designated as active material a3 of the present invention.

Example 4
(Synthesis of Li ₃ V ₂ (PO ₄ ) _95/32 (BO ₃ ) _1/32 corresponding to x = _{1/32 in} the general formula Li ₃ V ₂ (PO ₄ ) _3−x (BO ₃ ) _x
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 .5: 95/32: An active material for a lithium secondary battery was prepared in the same manner as in Example 1 except that the weight was adjusted to be 95/32: 1/32. This is designated as an active material a4 of the present invention.

(Example 5)
(Synthesis of Li ₃ V ₂ (PO ₄ ) _191/64 (BO ₃ ) _1/64 corresponding to x = _{1/64 in} the general formula Li ₃ V ₂ (PO ₄ ) _3−x (BO ₃ ) _x
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 .5: 191/64: An active material for a lithium secondary battery was prepared in the same manner as in Example 1 except that the weight was adjusted to be 1/64. This is designated as active material a5 of the present invention.

(Comparative Example 1)
(Synthesis of Li ₃ V ₂ (PO ₄ ) _5/2 (BO ₃ ) _1/2 corresponding to x = _{1/2 in} the general formula Li ₃ V ₂ (PO ₄ ) _3−x (BO ₃ ) _x )
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 A lithium secondary battery active material was prepared in the same manner as in Example 1 except that the weight was adjusted to 5: 5/2: 1/2. This is referred to as a comparative active material b1.

(Comparative Example 2)
(Synthesis of Li ₃ V ₂ (PO ₄ ) ₂ (BO ₃ ) ₁ corresponding to x = 1 in the general formula Li ₃ V ₂ (PO ₄ ) _{3 -x} (BO ₃ ) _x )
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 A lithium secondary battery active material was prepared in the same manner as in Example 1 except that the weight was adjusted to 5: 2: 1. This is referred to as a comparative active material b2.

(Comparative Example 3)
(Synthesis of Li ₃ V ₂ (PO ₄ ) ₃ corresponding to x = 0 in the general formula Li ₃ V ₂ (PO ₄ ) _{3 -x} (BO ₃ ) _x )
Lithium hydroxide monohydrate (LiOH.H ₂ O), ammonium vanadate (NH ₄ VO ₃ ), citric acid monohydrate, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Boric acid (H ₃ BO ₃ ) in molar ratio with LiOH.H ₂ O: NH ₄ VO ₃ : citric acid monohydrate: NH ₄ H ₂ PO ₄ : H ₃ BO ₃ = 3.03: 2: 1 A lithium secondary battery active material was prepared in the same manner as in Example 1 except that the weight was adjusted to 5: 3: 0. This is referred to as a comparative active material b3.

Powder X-ray diffraction measurement (XRD) using CuKα rays was performed on the active materials for lithium secondary batteries prepared in all Examples and Comparative Examples. The XRD pattern is shown in FIG. From FIG. 1, the larger the value of x, the lower the crystallinity. In particular, in x = 1/2 (comparative active material b1) and 1 (comparative active material b2), it is not attributed to Li ₃ V ₂ (PO ₄ ) _3. Peaks were confirmed at 2θ = 16.8, 22.2, and 53.8.

  Next, the powder resistance was measured with respect to the active material for lithium secondary batteries produced in all Examples and Comparative Examples. Table 1 shows the results of electron conductivity together with the raw material composition ratios.

All of the active materials shown in this table have an electron conductivity of 1 × 10 ⁻³ S / cm or more regardless of the value of x indicating the substitution amount of BO ₃ .

(Preparation of positive electrode)
A positive electrode containing the active material, acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 82: 10: 8 and using N-methyl-2-pyrrolidone (NMP) as a solvent. The paste was adjusted. The positive electrode paste is applied on both sides of an aluminum mesh current collector with an aluminum terminal attached, NMP is removed at 80 ° C., the applied part is folded in half, and then pressed to a thickness of 400 μm. A positive electrode was obtained. The application area of the active material is 2,25 cm ² and the application weight is 0.071 g. The positive electrode was vacuum-dried at 150 ° C. for 5 hours or longer to remove moisture from the electrode plate.

(Preparation of negative electrode)
A negative electrode was prepared by attaching a lithium metal foil having a thickness of 300 μm to both surfaces of a stainless steel mesh current collector to which a stainless steel terminal was attached and pressing it.

(Production of reference electrode)
A reference electrode was prepared by attaching a lithium metal foil having a thickness of 300 μm to a current collector rod made of stainless steel.

(Preparation of electrolyte)
In a mixed solvent in which ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate are mixed at a volume ratio of 1: 1: 1, a fluorine-containing electrolyte salt, LiPF ₆ , is dissolved at a concentration of 1.0 mol / l, and a non-aqueous electrolyte is obtained. Was made. The amount of water in the non-aqueous electrolyte was less than 50 ppm.

(Battery assembly)
A glass lithium ion secondary battery was assembled in an Ar box having a dew point of −40 ° C. or lower. Each of the positive electrode, the negative electrode, and the reference electrode was sandwiched between gold-plated clips whose conductors were previously fixed to the lid portion of the container, and then fixed so that the positive and negative electrodes were opposed to each other. The reference electrode was fixed at a position on the back side of the positive electrode when viewed from the negative electrode. Next, a polypropylene cup containing a certain amount of electrolyte was placed in a glass container, and a battery was assembled by covering the positive electrode, the negative electrode, and the reference electrode so as to be immersed therein.

  Using the active materials of Examples 1 to 5 and Comparative Examples 1 to 3, lithium secondary batteries were assembled according to the above procedure.

(Charge / discharge test)
The lithium secondary battery was subjected to a charge / discharge process in which charge / discharge of two cycles was performed at a temperature of 25 ° C. The charging conditions were a constant current / constant voltage charging with a current of 0.9 mA and a voltage of 4.5 V for 15 hours, and the discharging conditions were a constant current discharge with a current of 0.9 mA and a final voltage of 2.7 V. The discharge capacity and initial discharge capacity obtained in the first cycle are shown in Table 2.

(High rate discharge test)
After charging at a temperature of 25 ° C. under the same conditions as in the initial charge / discharge step, constant current discharge with a discharge current of 45 mA and a discharge end voltage of 2.7 V was performed. The discharge capacity at this time was defined as a high rate discharge capacity, and the percentage of the capacity with respect to the discharge capacity at the first cycle of the charge / discharge process was determined to obtain a “high rate discharge characteristic value (%)”. The results are also shown in Table 2.

Some of the results in each column of Table 2 were plotted on a graph and shown as FIGS. 2 to 4, the horizontal axis represents the boric acid substitution rate (%) of the polyanion portion. The value on the horizontal axis is uniquely derived from the value of _x in Li ₃ V ₂ (PO ₄ ) _3−x (BO ₃ ) _x and is also shown in Table 2.

From Table 2, some BO ₃ was replaced was _{Li 3 V 2 (PO 4)} 3-x (BO 3) x of PO ₄ are both deteriorated in the initial discharge capacity.

However, for high rate discharge characteristic values, in the range of x = 1/64 to 1/4, surprisingly, Li ₃ V ₂ (PO ₄ ) ₃ (x = 0) not substituted with BO ₃ It can be seen that it has improved. In particular, it can be seen that the high rate discharge capacity itself is improved in the range of x = 1/64 to 1/8. In addition, at x = 1/2 and 1 where a large amount of BO ₃ is substituted, the high rate discharge characteristic value decreases.

This coincides with the result of XRD. When x = 1/2 or more, impurities that cannot be attributed to Li ₃ V ₂ (PO ₄ ) ₃ increase, so that not only high rate discharge but also low rate discharge performance is achieved. It is predicted that the decline of

Since the electronic conductivity shows an equivalent value from Table 1, the effect of improving the high rate discharge performance by substituting a small amount with BO ₃ is that P as described in Patent Document 4 is changed to B. It can be seen that the substitution is not solely due to the improvement in electron conductivity.

Incidentally, a part of the polyanion portion (PO ₄₎ is replaced by (BO ₃₎ can be confirmed by Raman spectroscopic analysis, infrared absorption spectrum measurement.

It is a powder X-ray diffraction diagram of the active material for lithium secondary batteries which concerns on an Example and a comparative example. It is the figure which compared the initial stage discharge capacity of the lithium secondary battery which concerns on an Example and a comparative example. It is the figure which compared the high rate discharge characteristic value of the lithium secondary battery which concerns on an Example and a comparative example. It is the figure which compared the high rate discharge capacity of the lithium secondary battery which concerns on an Example and a comparative example.

Claims (3)

Formula _{Li 3 V 2 (PO 4)} 3-x (BO 3) x cathode active material for a lithium secondary battery represented by (0 <x ≦ 1/4 ). The active material, the positive electrode active material for lithium secondary battery according to claim 1, electron conductive carbonaceous material is deposited on the surface. A lithium secondary battery comprising: a positive electrode including the active material according to claim 1; a negative electrode including a negative electrode active material that absorbs and releases lithium ions; and a nonaqueous electrolyte.