JP5277929B2 - Positive electrode active material for lithium secondary battery and lithium secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery and a lithium secondary battery.

In recent years, non-aqueous electrolyte secondary batteries represented by lithium secondary batteries with high energy density, low self-discharge and good cycle characteristics as power sources for portable devices such as mobile phones and notebook computers and electric vehicles Is attracting attention. 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.

  However, when considering the development of non-aqueous electrolyte batteries for future medium-sized and large-sized, especially industrial applications where large demand is expected, safety is very important, so the specifications for current small batteries are not always sufficient. It cannot be said. One of the factors is the thermal instability of the positive electrode active material, and various countermeasures have been taken, but it is still not sufficient. In industrial applications, it is necessary to assume that the battery is used in a high-temperature environment that is not used in small consumer products. 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.

Therefore, recently, lithium iron phosphate (LiFePO ₄ ), which is a polyanion-type positive electrode active material excellent in thermal stability, has attracted attention. Since the polyanion portion of this LiFePO ₄ is covalently bonded to phosphorus and oxygen, it does not release oxygen even at high temperatures, and exhibits high safety even when all Li is extracted from the Li site. By using it as an active material, the safety of the battery can be dramatically improved. However, since LiFePO ₄ has a small theoretical capacity of 170 mAh / g, it has a lower energy density than the 4V positive electrode active material.

On the other hand, it is a polyanion positive electrode active material that employs Mn that provides a higher working potential than when the transition metal portion is Fe and replaces the polyanion portion with boric acid (BO ₃ ), which is lighter than phosphoric acid (PO ₄ ). Certain lithium manganese borate (LiMnBO ₃ ) is as safe as LiFePO ₄ and has a theoretical capacity of 220 mAh / g, which is larger than that of LiFePO ₄ .

Non-Patent Document 1 reports the synthesis of LiMnBO ₃ and its electrochemical properties together with LiFeBO ₃ and LiCoBO ₃ .

However, LiMnBO ₃ is shown in FIG. As it is described in 6 (a) that the reversible capacity in low current charge / discharge at a rate of about 60 hours is far less than 0.1 mol with respect to Li in the molecule, it can hardly be used as an active material. There was a problem.

Patent Document 1 has the following description.
"Formula _{A a D d M m Z z} O O N n F f
(Wherein A is an alkali metal,
D is selected from alkaline earth metals and group 3 elements of the periodic table (excluding B);
M is a transition metal or a mixture of transition metals,
Z is a non-metal selected from S, Se, P, As, Si, Ge, Sn and B;
O is oxygen, N is nitrogen, and F is fluorine;
a, d, m, z, o, n and f are real numbers greater than or equal to 0 and are selected to ensure electrical neutrality)
An electrode active compound of
A method for preparing a composite material comprising a conductive compound such as carbon,
Shortly mixed precursors containing all the elements A, D, M, Z, O, N and F forming the electrode active compound and one or more organic compounds and / or organometallic compounds A method of obtaining the composite material by pyrolysis over time. (Claim 1),
“A method according to claim 1 or 2, wherein D is selected from Mg, Al and Ga, and mixtures thereof.” (Claim 4),
“The method according to claim 1, wherein M is selected from Fe, Ni, Co, Mn, V, Mo, Nb, W and Ti, and mixtures thereof.” ),
“A is Li or Na and the electrode active compound is a lithium insertion compound or a sodium insertion compound such as LiFePO ₄ , LiFeBO ₃ or NaFeBO ₃ . Method "(Claim 6),

However, there is no specific description about lithium manganese borate in Patent Document 1, and LiFeBO ₃ is selected from the compounds described in Claim 6 of Patent Document 1, and claims instead of this Fe. In the case where Mn is selected from the elements M listed in 5, there is no description as to what effect can be achieved by selecting Mg from the elements D listed in claim 4. In addition, there is no description on how the value of d is selected in that case.
Legagneur V. et al., Solid State Ionics, 2001, vol.139, page 37-46. Special table 2007-520038 gazette

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a lithium secondary battery that uses a lithium manganese borate positive electrode active material and is excellent in high rate discharge performance.

  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.

The present invention is a positive electrode active material for a lithium secondary battery including a compound in which a part of Mn of lithium manganese borate (LiMnBO ₃ ) is substituted with Mg.

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

There are a plurality of types of crystal structures represented by LiMnBO ₃ , but the crystal structure of the lithium manganese borate compound-based positive electrode active material according to the present invention belongs to the space group P6-. Here, “P6-” (P6 bar) is supposed to be expressed by adding a horizontal bar (bar) on “6”, but it is expressed as “P6-” for convenience in this specification.

The positive electrode active material according to the present invention includes a crystal that can be represented by the general formula Li _x Mn _(1-y) Mg _y BO ₃ (0.5 ≦ x ≦ 1.5, 0 <y ≦ 0.5). The crystal does not prevent some transition metal elements other than Mn and Mg from being dissolved in part. Also, PO ₄ or SiO ₄ does not prevent that the solid solution as part polyanionic structure.

If the amount of Mg y in the general formula Li _x Mn _(1-y) Mg _y BO ₃ is too small, it is difficult to obtain the effects of the present invention. Moreover, since LiMgBO ₃ does not occlude / release Li, if the amount of Mg substitution is too large, the theoretical capacity of the active material decreases, which is not preferable. Therefore, the preferable Mg substitution amount y is 0.001 ≦ y ≦ 0.5, and more preferably 0.01 ≦ y ≦ 0.2.

The claim 4 of Patent Document 1, wherein _{A a D d M m Z z} O O N n F f of claim 1, wherein the same document (wherein, A is an alkali metal, D is an alkaline earth Selected from metals and Group 3 elements of the Periodic Table of Elements (excluding B), M is a transition metal or a mixture of transition metals, Z is S, Se, P, As, Si, Ge, Sn and B A non-metal selected from: O is oxygen; N is nitrogen; and F is fluorine; a, d, m, z, o, n and f are real numbers greater than or equal to 0; And Mg, Al and Ga are listed in the same column without distinction, substituting a part of Mn of the lithium manganese borate compound, Mg is selected as a suitable element for improving the electrochemical performance. The reason is that Mn is a divalent valence element in the lithium manganese borate compound, and among the elements listed in claim 4 of the same document, Al is a trivalent valence element. Therefore, it is unsuitable as an element that substitutes a part of Mn. Ga is also an element that takes mainly a trivalent valence, and is therefore unsuitable as an element that substitutes a part of Mn.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material which can be used as the lithium secondary battery excellent in high rate discharge performance can be provided using a lithium manganese borate compound type positive electrode active material. Moreover, according to this invention, the lithium secondary battery excellent in high-rate discharge performance can be provided using a lithium manganese borate compound-type positive electrode active material.

The synthesis process of the positive electrode active material according to the present invention is not particularly limited as long as a single phase crystal of LiMnBO ₃ can be synthesized. Specific examples include a solid phase method, a liquid phase method, a sol-gel method, and a hydrothermal method. Moreover, it is preferable to adhere and coat carbon on the surface of Li _x Mn _(1-y) Mg _y BO ₃ particles mechanically or by thermal decomposition of organic matter for the purpose of supplementing electron conductivity.

The positive electrode active material according to the present invention 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. In particular, a smaller particle size is preferable, the average particle size of the secondary particles is 0.5 to 20 μm, and the particle size of the primary particles is more preferably 1 to 500 nm. The specific surface area of the powder particles is preferably large in order to improve the high rate performance of the positive electrode, and is preferably 1 to 100 m ² / g. More preferably, it is 5-100 m < ² > / g. For the purpose of obtaining 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.

Li _x Mn _(1-y) Mg _y BO ₃ may inevitably coexist with impurities for the purpose of improving the performance as an active material. The effect is never lost.

  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 positive electrode 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.), polyphosphoric acid compounds and the like. 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 the lithium secondary battery is composed of a positive electrode, a negative electrode, and a non-aqueous electrolyte in which an electrolyte salt is contained in a non-aqueous solvent. Generally, a separator and these are interposed between the positive electrode and the negative electrode. 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 non-aqueous electrolyte is preferably 0.5 mol / l to 5 mol / l, more preferably 1 mol / l to 2.5 mol in order to reliably obtain a lithium secondary battery having high battery characteristics. / L.

  Hereinafter, the method for producing a lithium secondary battery of the present invention will be exemplified, but the present invention is not limited to the following embodiment.

Example 1
(Production of LiMn _0.95 Mg _0.05 BO ₃ )
Manganese acetate tetrahydrate (Mn (CH ₃ CO ₂ ) ₂ .4H ₂ O), magnesium acetate tetrahydrate (Mg (CH ₃ CO ₂ ) ₂ .4H ₂ O), boric acid (H ₃ BO ₃ ) , Lithium acetate dihydrate (LiCH ₃ CO ₂ .4H ₂ O) and citric acid were weighed so that the molar ratio was 0.95: 0.05: 1: 1.02: 0.5. Dissolved in purified water. This solution was evaporated to dryness while stirring on a 120 ° C. hot stirrer to obtain a precursor.

The precursor was pulverized for 30 minutes using an automatic mortar, then placed in an alumina mortar (outside dimensions 90 × 90 × 50 mm), and an atmosphere substitution type firing furnace (a tabletop vacuum gas substitution furnace KDF-75 made by Denken). ) Under a nitrogen gas flow (flow rate 1.0 liter / min). The firing temperature was 600 ° C., and the firing time (time for maintaining the firing temperature) was 2 hours. In this procedure, a positive electrode active material was synthesized. Here, carbon adheres to the surface of the synthesized material. This is formed by carbonizing citric acid. Moreover, when the powder X-ray-diffraction measurement of the synthesize | combined material was performed, the single phase X-ray-diffraction figure which belongs to the space group P6- was obtained. Here, all peak positions were slightly shifted to the higher angle side as compared with LiMnBO ₃ . From this, it was found that Mg was substituted at least in part of the Mn site of the crystal structure of LiMnBO ₃ .

(Preparation of positive electrode)
The positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder are contained at a mass ratio of 80: 8: 12, and N-methyl-2-pyrrolidone (NMP) is a solvent. A positive electrode paste was prepared. The positive electrode paste was applied to one side of an aluminum foil current collector having a thickness of 20 μm, dried, and then pressed to obtain a positive electrode. An aluminum positive electrode terminal was connected to the positive electrode by ultrasonic welding.

(Preparation of negative electrode)
A negative electrode was prepared by pasting a lithium metal foil having a thickness of 100 μm onto a nickel foil current collector having a thickness of 10 μm. A negative electrode terminal made of nickel was connected to the negative electrode by resistance welding.

(Preparation of electrolyte)
A non-aqueous electrolyte is prepared by dissolving LiPF ₆ as a fluorine-containing electrolyte salt at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate are mixed at a volume ratio of 6: 7: 7. did. The amount of water in the non-aqueous electrolyte was less than 50 ppm.

(Battery assembly)
A lithium secondary battery was assembled in a dry air atmosphere with a dew point of −40 ° C. or lower. Each of the positive electrode and the negative electrode is opposed to each other via a polypropylene separator having a thickness of 20 μm, and as an exterior body, a polyethylene terephthalate (15 μm) / aluminum foil (50 μm) / metal adhesive polypropylene film (50 μm The electrode group was hermetically sealed except for the portion serving as a liquid injection hole so that the open ends of the positive electrode terminal and the negative electrode terminal were exposed to the outside. After injecting a certain amount of non-aqueous electrolyte from the injection hole, the injection hole part was heat sealed in a reduced pressure state, and a battery was assembled.

(Comparative Example 1)
(Preparation of LiMnBO ₃ )
In the preparation of the precursor, as raw materials to be dissolved in purified water, manganese acetate tetrahydrate (Mn (CH ₃ CO ₂ ) ₂ .4H ₂ O), magnesium acetate tetrahydrate (Mg (CH ₃ CO ₂ ) ₂ 4H ₂ O), boric acid (H ₃ BO ₃ ), lithium acetate dihydrate (LiCH ₃ CO ₂ .4H ₂ O) and citric acid in a molar ratio of 0.95: 0.05: 1: 1. Except for weighing up to 02: 0.5, LiMnBO ₃ was synthesized in the same manner as in Example 1, and a lithium secondary battery was assembled using this as a positive electrode active material.

(High rate charge / discharge test)
The lithium secondary batteries of Example 1 and Comparative Example 1 were charged and discharged at a high rate of 5 cycles at a temperature of 20 ° C. Here, the charging conditions are a current of 0.1 ItmA (about 10 hours rate), a voltage of 4.5 V, and a constant current and constant voltage charging of 15 hours, and the discharging conditions are a current of 0.1 ItmA (about 10 hours rate) and an end voltage. The constant current discharge was 1.5V.

  Here, the high rate discharge capacity at the fifth cycle of the battery of Example 1 was 49 mAh, and the high rate discharge capacity at the fifth cycle of the battery of Comparative Example 1 was 33 mAh. FIG. 1 shows the discharge curve of the fifth cycle of the battery of Example 1, and FIG. 2 shows the discharge curve of the fifth cycle of the battery of Comparative Example 1.

(Low rate discharge test)
Subsequently, after performing constant current and constant voltage charging at a current of 0.01 ItmA (approximately 100 hour rate), a voltage of 4.5 V, and 150 hours, a constant current of 0.01 ItmA (approximately 100 hour rate) and a final voltage of 1.5 V was established. A current discharge was performed.

  As a result, the low rate discharge capacity of the battery of Example 1 was 78 mAh, and the low rate discharge capacity of the battery of Comparative Example 1 was 50 mAh.

  Table 1 shows the performance ratio of the battery of Example 1 when the performance of the battery of Comparative Example 1 is set to 1 for each test result.

From the above results, by using Mg as a substitution element for LiMnBO ₃ , the battery using this as a positive electrode active material has improved discharge performance compared to the battery using LiMnBO ₃ as the positive electrode active material. I understood it.

(Comparative Example 2)
(Preparation of LiFePO ₄ )
Molar ratio of iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) to 1: 1: 0.51 Then, 2-propanol was added to make a paste, and wet mixing was performed for 2 hours using a ball mill (Fritsch planetary mill, ball diameter: 1 cm).

The mixture was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and the atmosphere was replaced with a nitrogen gas (flow rate 1) using an atmosphere substitution type firing furnace (a tabletop vacuum gas substitution furnace KDF-75 manufactured by Denken). And fired at 0.0 liter / min). The firing temperature was 700 ° C., and the firing time (the time for maintaining the firing temperature) was 2 hours. In addition, a constant temperature period of 350 ° C. was provided for 2 hours during the temperature increase. The rate of temperature rise was 5 ° C./min, and the temperature was naturally cooled. The obtained fired powder was weighed with polyvinyl alcohol (degree of polymerization of about 1500) so that the mass ratio was 1: 1, and then dry-mixed with a ball mill, and the mixture was placed in an alumina slag and fired in an atmosphere substitution type. LiFePO ₄ coated with carbon was synthesized by firing for 1 hour at 700 ° C. under nitrogen flow (1.0 liter / min) in a furnace.

(Comparative Example 3)
(Production of LiFe _0.99 Mg _0.01 PO ₄ )
Iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), magnesium oxalate (MgC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ And LiFe _0.99 Mg _0.01 PO ₄ in the same manner as in Comparative Example 2 except that the molar ratio is 0.99: 0.01: 1: 0.51. Synthesized.

(Comparative Example 4)
(Production of LiFe _0.95 Mg _0.05 PO ₄ )
Iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), magnesium oxalate (MgC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ LiFe _0.95 Mg _0.05 PO ₄ in the same manner as in Comparative Example 2 except that the molar ratio is 0.95: 0.05: 1: 0.51. Synthesized.

(Comparative Example 5)
(Production of LiFe _0.9 Mg _0.1 PO ₄ )
Iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), magnesium oxalate (MgC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ And LiFe _0.9 Mg _0.1 PO ₄ in the same manner as in Comparative Example 2 except that the molar ratio is 0.9: 0.1: 1: 0.51. Synthesized.

  Lithium secondary batteries of Comparative Examples 2 to 5 were assembled in the same manner as in Example 1 except that these positive electrode active materials were used.

(Charge / discharge test)
The batteries of Comparative Examples 2 to 5 were charged and discharged for 5 cycles at a temperature of 20 ° C. Here, the charging conditions are a current of 0.1 ItmA (about 10 hours rate), a voltage of 3.8 V, and a constant current constant voltage charging of 15 hours, and the discharging conditions are a current of 0.1 ItmA (about 10 hours rate) and a final voltage. The constant current discharge was 2.0V.

(High rate discharge characteristics test)
Subsequently, after performing constant current and constant voltage charging with a current of 0.1 ItmA (about 10 hours), a voltage of 3.8 V, and 15 hours, a current of 5.0 ItmA (about 0.2 hours) and a final voltage of 2.0 V The constant current discharge was performed.

  For each test result, the performance ratio of each battery is shown in Table 2 based on the performance of the battery of Comparative Example 2.

As is apparent from Table 2, when Mg is applied as a substitution element to LiFePO ₄ , the battery using this as the positive electrode active material is discharged more than the battery using LiFePO ₄ as the positive electrode active material. It was found that the performance did not improve, but on the contrary deteriorated or comparable.

In general, the transition metal compound used in the positive electrode active material for a lithium secondary battery is attempted to replace a part of the transition metal site with another element, as an example in other active materials such as LiMn ₂ O ₄ having a tetragonal spinel structure. Many have been studied, not to mention. However, the effects of substitution of different elements vary depending on the active material, and in this technical field, it is quite difficult to predict whether the effects manifested in different materials will be manifested in other materials as well. I don't wait for that.

As shown in Comparative Examples 2-5, even looking at the discharge performance was or worse not at all improved when a portion of Fe with respect to LiFePO ₄ was replaced by Mg, the Mn against LiMnBO ₃ It cannot be predicted at all that the high-rate discharge performance is excellent by selecting Mg as an element to replace a part. Moreover, since Mg is not an element that changes in valence with electrochemical redox, as shown in Comparative Examples 2 to 5, even if it can be said that the prediction is possible if the discharge performance is reduced. However, it is impossible to predict that the discharge capacity will be improved.

Thus, by applying the Mg as substituent element relative LiMnBO _3, compared to LiMnBO _3, which will also act mechanism discharging performance of the battery can be improved using as a positive electrode active material, it is not necessarily clear.

However, the present inventors can make some inferences about the mechanism of action of the present invention in the face of the surprising and unexpected results described above. That is, in the lithium manganese borate compound, Mn is an element that takes a divalent valence, and Mg also takes a divalent valence and has an ionic radius close to that leads to an improvement in discharge performance. I guess that is the cause. In addition, the powder X-ray diffraction peak of LiMn _0.95 Mg _0.05 BO ₃ synthesized in Example 1 was slightly shifted to the higher angle side, suggesting that the lattice constant was reduced by Mg substitution. It was done. From this, the present inventors speculate that the minute change of this crystal may have the effect of reducing the resistance associated with the insertion and release of Li ions in LiMnBO ₃ .

  According to the present invention, it is possible to provide a positive electrode active material for a lithium secondary battery composed of a lithium manganese borate compound excellent in energy density and a lithium secondary battery using the same, and therefore, electric vehicles and the like that are expected to be developed in the future The battery is suitable for application in a field that requires a particularly high capacity in industrial batteries, and its industrial applicability is extremely large.

It is a figure which shows the charge / discharge behavior of the lithium secondary battery which concerns on an Example. It is a figure which shows the charge / discharge behavior of the lithium secondary battery which concerns on a comparative example.

Claims (2)

A positive electrode active material for a lithium secondary battery, comprising a compound in which a part of Mn of lithium manganese borate (LiMnBO ₃ ) is substituted with Mg. The lithium secondary battery provided with the positive electrode containing the positive electrode active material of Claim 1, a negative electrode, and the nonaqueous electrolyte.