JP5446036B2 - 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, particularly a polyanionic positive electrode active material, and a lithium secondary battery using the same.

  In recent years, lithium secondary batteries typified by lithium secondary batteries with high energy density and low self-discharge and good cycle performance have been attracting attention as power sources for mobile devices such as mobile phones and notebook computers, and electric vehicles. ing.

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 lithium 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.

  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 ) having an olivine structure has been actively studied. LiFePO ₄ has a theoretical capacity as large as 170 mAh / g, since the 3.4V (vs.Li/Li ⁺⁾ lithium insertion and desorption at high potential is conducted, the energy density is high in about comparable to LiCoO _2, LiCoO There is great expectation as a positive electrode active material that can replace ₂ .

  Several attempts have been reported to synthesize materials in which a part of iron in lithium iron phosphate is replaced by cobalt.

In Patent Document 1, LiCo _0.25 Fe _0.75 PO ₄ , LiCo _0.5 Fe _0.5 PO ₄ , LiCo _{0. 5} corresponding to 25%, 50%, 75% and 100% as Co substitution amounts _{. 75} Fe _0.25 PO ₄ and LiCoPO ₄ were synthesized by a solid phase method, and this was used as a positive electrode active material, and a non-aqueous electrolyte battery was fabricated using metallic lithium as a negative electrode, and the discharge capacity was charged to 5.3 V. The result of the evaluation was described, "The higher the cobalt content, the higher the discharge voltage, and the voltage at the flat portion of the discharge voltage exceeds 4.5 V" (paragraph 0022), the discharge capacity associated with the amount of Co Is described as increasing or decreasing according to the discharge end voltage (Table 1).

In Non-Patent Document 1, LiFe _0.8 Co _0.2 PO ₄ , LiFe _0.5 Co _0.5 PO _4, and LiFe _0.2 Co corresponding to Co substitution amounts of 20%, 50%, and 80% are disclosed. _0.8 PO ₄ was synthesized by a solid phase method, a single electrode evaluation cell was prepared using this as a positive electrode active material, a charge / discharge cycle test was conducted at a charging potential of 5 V, and LiFePO ₄ was used as a positive electrode active material. As a result, the results of conducting a charge / discharge cycle test at a charge potential of 4 V are described. When the Co substitution amount is 20%, the initial discharge capacity is slightly increased compared to LiFePO ₄ , but the capacity associated with the charge / discharge cycle is The retention rate deteriorated compared to LiFePO _4, and when the Co substitution amount was 50% and 80%, both the initial discharge capacity and the capacity retention rate associated with the charge / discharge cycle were LiF. extremely worse that compared to ePO ₄ (Fig.2) is described.

In Non-Patent Document 2, LiFe _0.9 Co _0.1 PO ₄ corresponding to 10% of the Co substitution amount was synthesized by a solid phase method, and this was used as a positive electrode active material, and metallic lithium was used for the negative electrode. Although a non-aqueous electrolyte battery was fabricated and the result of comparing the 2.0 V end discharge capacity after charging to 4.5 V with LiFePO ₄ was described, the high rate discharge characteristics at 5 C discharge were improved, but 0.2 C It is described that the discharge capacity in discharge decreased (see FIG. 3).

In Non-Patent Document 3, LiFe _1-x Co _x PO ₄ (x = 0.02, 0.04, 0.08,0) corresponding to Co substitution amount of 2%, 4%, 8% and 10%. .1) was synthesized, and this was used as a positive electrode active material, and a non-aqueous electrolyte battery was prepared using metallic lithium as a negative electrode. An initial discharge test and a charge / discharge cycle test were performed at a charge voltage of 5.1 V, and LiFePO ₄ The results of comparison with the case where the charging voltage was set to 4.25 V using the battery were described, and it was described in FIG. 7 that the discharge capacity was reduced compared to LiFePO ₄ in all the batteries replaced with Co. Abstract states that it did not have a positive effect on the electrochemical properties of lithium iron phosphate. It is also described that only a capacity of about 50% was obtained as a result of conducting a charge / discharge cycle test at a discharge rate of 1 C (corresponding to 1 It) (Fig. 8).

In Non-Patent Document 3, the various positive electrode active materials described above are prepared by dissolving LiOH.H ₂ O and FeC ₂ O ₄ .2H ₂ O in nitric acid, and dropping (NH ₄ ) H ₂ PO ₄ solution together with citric acid into this. The gel obtained by heating to 75 ° C. is dried at 110 ° C. to obtain a precursor, which is synthesized by baking at 850 ° C. for 2 hours in an Ar atmosphere followed by 750 ° C. for 10 hours. And it is described from the X-ray diffraction pattern that the presence of a small amount of Fe ₂ P impurity phase was confirmed in all the synthesized samples irrespective of the presence or absence of Co substitution. In this regard, in Non-Patent Document 4, the improvement in electron conductivity is not due to the doping of a small amount of different elements, but the phase-separated Fe formed by mixing and mixing such different elements. be phosphide phase such as _{2 P} is present, in combination with transmission that contribute to the improvement of electron conductivity electron microscope (TEM) electron energy loss spectroscopy (electron energy-loss spectroscopy, EELS ) measurements It is clear from the results.

By the way, the redox reaction accompanying electrochemical lithium insertion / extraction with respect to LiFePO ₄ proceeds at a relatively low potential in the vicinity of 3.4 V (vs. Li ^/ Li ⁺ ), whereas in LiCoPO ₄ , 4. It is known to proceed at a relatively high potential in the vicinity of 8 V (vs. Li ^/ Li ⁺ ). In general, the aim of substituting part of Fe of LiFePO ₄ with Co is to expect high energy density as a positive electrode active material by utilizing the high redox potential of LiCoPO ₄ . For this reason, in the above-described prior art documents, battery performance is evaluated by adopting a high potential sufficient to change the valence of Co by charging.

  However, in a lithium secondary battery that is actually intended for industrial use, it must be designed with the problem of the oxidation resistance potential of the non-aqueous electrolyte solution, so that the positive electrode potential exceeds 4.2V. Charging causes problems in terms of battery performance.

For this reason, as a precondition of the present invention intended for industrial use, taking advantage of the inherent feature of LiFePO ₄ that the insertion and elimination reaction of lithium proceeds at a relatively low potential, the positive electrode during charging Various problems must be solved under conditions where the ultimate potential does not exceed 4.2V. Here, it can be said that the present invention relates to a lithium secondary battery used in a region where the positive electrode potential does not exceed 4.2V. In Examples 5-7 and Comparative Examples 6-8, which will be described later, the positive electrode potential at the time of charging was 3.6V.

Even if the fact that the discharge capacity is reduced by substituting part of Fe of LiFePO ₄ with Co is reported in many of the above-mentioned literatures, the oxidation-reduction potential of LiCoPO ₄ is 4.8 V (vs. .Li / Li ⁺ ), for example, the energy density increases by substituting part of Fe in LiFePO ₄ with Co under the condition of charging to a sufficiently high potential such as 5 V. Even if there is such a phenomenon, it is not unclear. However, since the valence of Co cannot be changed under the condition that the potential to reach the positive electrode during charging does not exceed 4.2 V, the presence of Co in LiFePO ₄ is a battery such as a reversible discharge capacity. It will only lead to a prediction of performance degradation.

Patent Document 1 states that “as the negative electrode active material, lithium alloys and lithium compounds in addition to lithium, other conventionally known alkali metals such as sodium, potassium, and magnesium, alkaline earth metals, or alkali metals or alkaline earth metals. Substances capable of occluding and releasing ions, such as alloys of the above metals, carbon materials, etc., can be used ”(paragraph 0005), and carbon materials are also described as one line that can be used as the negative electrode active material. However, in any of the above-mentioned patent documents and non-patent documents, there is no specific description of a battery using a positive electrode active material in which a part of Fe of LiFePO ₄ is substituted with Co and a carbon material for the negative electrode, and LiFePO ₄ There is no description or suggestion of the charge / discharge cycle performance of a battery using a positive electrode active material in which a part of Fe is replaced with Co and using a carbon material for the negative electrode. Furthermore, regarding the charge / discharge cycle performance when using a carbon material capable of occluding / releasing lithium ions for the negative electrode, only the battery using metallic lithium for the negative electrode is specifically described above. It cannot be predicted at all from non-patent literature. This is because, when metal lithium is used for the negative electrode, the negative electrode capacity is generally excessive and the capacity balance becomes a positive electrode limited battery, whereas when a carbon material is used for the negative electrode, all lithium sources are from the positive electrode side. This is because the capacity balance is normally limited by the negative electrode since it is supplied, and therefore the charge / discharge cycle performance is largely influenced by the performance of the negative electrode.

Japanese Patent No. 3523397

N. Penazzi et al. Journal of the European Ceramic Society, 2004, vol.24, page1381. D. Wang et al. ElectrochimicaActa 2005, vol.50, page2955. D. Shanmukaraj et al. Materials Science and Engineering B, 2008, vol.149, page93. L.F.Nazar et al, Nature Materials, MARCH 2004, VOL.3, no.3, page.147-152.

  The present invention has been made in view of the above problems, and provides a polyanion-type positive electrode active material capable of improving the charge / discharge cycle performance of a lithium secondary battery and a lithium secondary battery using the same. It is intended to provide.

  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 relates to a lithium secondary battery comprising lithium cobalt iron phosphate represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0 <x <0.02, 0 ≦ y ≦ 1.2). Cathode active material. x is preferably 0.005 ≦ x ≦ 0.019.

  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. The negative electrode preferably contains a carbon material that can occlude and release lithium ions.

The positive electrode active material according to the present invention can be represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0 <x <0.02, 0 ≦ y ≦ 1.2). However, it is not excluded that a part of Fe or Li is substituted with a transition metal element other than Fe or Co, such as Mn or Ni, or Al or a metal element other than Li, Fe or Co. The polyanion part (PO ₄ ) may be partially dissolved (SiO ₄ ), and such a part is also included in the scope of the present invention.

  The method for producing a positive electrode active material according to the present invention is not limited, but basically, a raw material containing a metal element (Li, Fe, Co) constituting the active material and a raw material to be a phosphoric acid source are used. It can be obtained by adjusting the raw materials contained according to the composition of the active material and firing it. 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 usually performed that the lithium source is charged in a larger amount than equimolar with respect to Fe as a raw material before firing.

In order for the lithium cobalt iron phosphate represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0 <x <0.02, 0 ≦ y ≦ 1.2) to exhibit the effects of the present invention. Co is required to be dissolved in the LiFePO ₄ structure. When LiFePO ₄ and LiCoPO ₄ are simply mixed, the effect of the present invention is not expected. In the case of a simple mixture, the peak positions corresponding to the respective lattice constants in the X-ray diffraction diagram are different from each other, so that one peak is branched into two or three. The peak of the book does not branch. However, the positive electrode active material of the present invention may contain a Co compound not dissolved in the olivine structure as an impurity, and the positive electrode is mixed with another active material such as LiCoO ₂ in addition to the positive electrode active material of the present invention. And such an aspect is also included in the scope of the present invention.
In the examples described later, a positive electrode active material of LiFe _(1-x) Co _x PO ₄ with y = 1 has been proposed. However, as described above, the Li composition varies particularly during the synthesis of the active material. In addition to being easy, in the battery, the positive electrode material is released by charging and Li can reach 0, and Li is occluded by discharging and can reach 1.2, so 0 ≦ y ≦ 1 .2.

  ADVANTAGE OF THE INVENTION According to this invention, the polyanion type positive electrode active material which can make the charge / discharge cycle performance etc. of the lithium secondary battery excellent, and a lithium secondary battery using the same can be provided.

It is the figure which compared the charging / discharging cycling performance of the Example battery and the comparative example battery.

The method for synthesizing the polyanionic positive electrode 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 polyanion-type positive electrode active material can be finally obtained by firing. The composition after firing is preferably assumed that the impurity phases of Fe 2 _P is not observed. The present inventors have confirmed that when the positive electrode active material contains an Fe ₂ P phase, Fe ₂ P is eluted into the electrolytic solution, particularly affecting the negative electrode side and reducing various characteristics of the battery. Yes. In the present specification, when powder X-ray diffraction measurement using CuKα rays is performed on the obtained active material, in the X-ray diffraction diagram with the maximum peak at full scale, Fe ₂ P is about 2θ = 41 °. It is said that “the impurity phase of Fe ₂ P is not recognized” when the derived peak is not clearly observed to such an extent that it is visually recognized.

The production conditions for making the Fe ₂ P impurity phase not recognized in the composite after firing are not limited, but one is that the raw materials before firing are uniformly mixed. Very important. It is important that the firing atmosphere is not a reducing atmosphere. For example, firing in a nitrogen atmosphere containing 5% hydrogen is not preferable because formation of an Fe ₂ P phase is easily observed. Moreover, it is preferable that a calcination temperature is not too high, for example, 720 degrees C or less is preferable.

  For the purpose of supplementing the electron conductivity, it is preferable to adhere and cover carbon on the particle surface of the positive electrode active material mechanically or by thermal decomposition of organic matter.

In the present invention, the polyanion-type positive electrode active material is preferably used for a positive electrode for a lithium secondary battery as a powder having an average secondary particle size of 100 μm or less. 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 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 particularly limited as long as it is a carbon material capable of occluding and releasing lithium ions, and examples thereof include graphite, hard carbon, low-temperature fired carbon, and amorphous carbon. 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 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 non-aqueous electrolyte battery having high battery characteristics. / L.

  In the present specification, the non-aqueous electrolyte battery is particularly described in detail among lithium secondary batteries, and the positive electrode active material according to the present invention may be used for the positive electrode of an aqueous lithium secondary battery. The effect of the present invention is 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.

Example 1
(Synthesis of LiFe _0.995 Co _0.005 PO ₄ )
Iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate ( Li ₂ CO ₃ ) was weighed so that the molar ratio was 0.995: 0.005: 1: 0.51. Ethanol was added thereto 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 firing was performed at a rate of 0.0 l / min). The firing temperature was 700 ° C., and the firing time (the time for maintaining the firing temperature) was 2 hours. The rate of temperature increase was 5 ° C./minute, and the temperature was naturally cooled. The obtained product was confirmed to have a composition of LiFe _0.995 Co _0.005 PO ₄ by ICP emission spectroscopic analysis. Thus, the positive electrode active material for lithium secondary batteries was produced.

(Example 2)
(Synthesis of LiFe _0.99 Co _0.01 PO ₄ )
In preparation of the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in the same manner as in Example 1 except that the molar ratio was 0.99: 0.01: 1: 0.51. A positive electrode active material for a lithium secondary battery was produced. Note that the composition of LiFe _0.99 Co _0.01 PO ₄ was confirmed by ICP emission spectroscopic analysis.

(Example 3)
(Synthesis of LiFe _0.985 Co _0.015 PO ₄ )
In preparation of the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) as in Example 1, except that the molar ratio was 0.985: 0.015: 1: 0.51. A positive electrode active material for a lithium secondary battery was produced. In addition, the composition of LiFe _0.985 Co _0.015 PO ₄ was confirmed by ICP emission spectroscopic analysis.

Example 4
( _{Preparation of} LiFe _0.981 Co _0.019 PO ₄ )
In preparation of the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in the same manner as in Example 1 except that the molar ratio was 0.981: 0.019: 1: 0.51. A positive electrode active material for a lithium secondary battery was produced. ICP emission spectroscopic analysis confirmed the composition of LiFe _0.981 Co _0.019 PO ₄ .

(Comparative Example 1)
(Preparation of LiFePO ₄ )
In preparation of the positive electrode active material, 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 ) LiFePO ₄ was obtained in the same manner as in Example 1 except that the molar ratio was 1: 1: 0.51.

(Comparative Example 2)
(Production of LiFe _0.98 Co _0.02 PO ₄ )
In preparation of the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) as in Example 1, except that the molar ratio was 0.98: 0.02: 1: 0.51. A positive electrode active material for a lithium secondary battery was produced. The composition of LiFe _0.98 Co _0.02 PO ₄ was confirmed by ICP emission spectroscopic analysis.

(Comparative Example 3)
(Production of LiFe _0.97 Co _0.03 PO ₄ )
In preparation of the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in the same manner as in Example 1 except that the molar ratio was 0.97: 0.03: 1: 0.51. A positive electrode active material for a lithium secondary battery was produced. The composition of LiFe _0.97 Co _0.03 PO ₄ was confirmed by ICP emission spectroscopic analysis.

(Comparative Example 4)
(Production of LiFe _0.95 Co _0.05 PO ₄ )
In preparation of the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in the same manner as in Example 1 except that the molar ratio is 0.95: 0.05: 1: 0.51. A positive electrode active material for a lithium secondary battery was produced. The composition of LiFe _0.95 Co _0.05 PO ₄ was confirmed by ICP emission spectroscopic analysis.

(Comparative Example 5)
(Preparation of LiFe _0.90 Co _0.10 PO ₄ )
In preparation of the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in the same manner as in Example 1 except that the molar ratio was 0.90: 0.10: 1: 0.51. A positive electrode active material for a lithium secondary battery was produced. The composition of LiFe _0.90 Co _0.10 PO ₄ was confirmed by ICP emission spectroscopic analysis.

  When the positive electrode active materials for lithium secondary batteries prepared in all Examples and Comparative Examples were subjected to powder X-ray diffraction measurement (XRD) using CuKα rays, formation of a single phase was confirmed.

  The positive electrode active materials for lithium secondary batteries prepared in all the examples and comparative examples are prepared so that the mass ratio of the positive electrode active material and polyvinyl alcohol (degree of polymerization: about 1500) is 1: 1 before being used for the positive electrode. And then dry-mixing with a ball mill, placing the mixture in an alumina sagger, and firing at 700 ° C. for 1 hour in a nitrogen flow (1.0 liter / min) in an atmosphere substitution-type firing furnace. Carbon coating was performed.

(Preparation of positive electrode)
A positive electrode active material after the carbon coating, acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder are contained at a weight ratio of 80: 8: 12, and N-methyl-2-pyrrolidone (NMP) A positive electrode paste using as a solvent was prepared. The positive electrode paste is applied on both sides of an aluminum mesh current collector with an aluminum terminal attached, and after removing NMP at 80 ° C., the applied portions are doubled and folded so that the projected area of the applied portion is halved. Then, press working was performed so that the total thickness after bending was 400 μm, and a positive electrode was obtained. 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 300 μm-thick lithium metal foil to both surfaces of a SUS316 mesh current collector to which a SUS316 terminal was attached, and pressing it.
(Production of reference electrode)
A reference electrode was prepared by attaching a 300 μm thick lithium metal foil to a SUS316 current collector rod.

(Preparation of electrolyte)
A non-aqueous electrolyte was produced by dissolving LiPF ₆ as a fluorine-containing electrolyte salt at a concentration of 1.2 mol / l in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7. The amount of water in the non-aqueous electrolyte was less than 50 ppm.

(Battery assembly)
A glass non-aqueous electrolyte battery was assembled in an Ar box having a dew point of −40 ° C. or lower. A positive electrode, a negative electrode, and a reference electrode were sandwiched one by one on a gold-plated clip having a conducting wire portion fixed to the lid portion of the container in advance, and then fixed so that the positive and negative electrodes face 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 positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 5, 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 current of 0.1 ItmA (approximately 10 hour rate), a voltage of 4.9 V, and a constant current constant voltage charge of 15 hours. The discharging conditions were a current of 0.1 ItmA (approximately 10 hour rate) and a final voltage of 2.0 V. Constant current discharge. Table 1 shows the results of the discharge capacity obtained in the first cycle.

(High rate discharge test)
After charging at the temperature of 25 ° C. under the same conditions as in the initial charge / discharge step, a constant current discharge with a discharge current of 2.0 ItmA (about 0.5 hour rate) and a discharge end voltage of 2.0 V was performed. The percentage of the discharge capacity at this time with respect to the discharge capacity of 0.1 ItmA in the charge / discharge process was determined, and was defined as “high rate discharge characteristic value (%)”. The results are also shown in Table 1.

(Charge / discharge cycle test)
The battery that completed the high rate discharge test was subjected to a charge / discharge cycle test at a temperature of 25 ° C. The charging conditions were a current of 1.0 ItmA (about 1 hour rate), a voltage of 4.9 V, and a constant current constant voltage charging of 1.5 hours, and the discharging conditions were a current of 1.0 ItmA (about 1 hour rate) and a final voltage of 2. The constant current discharge was 0V. The percentage of the discharge capacity after 50 cycles with respect to the discharge capacity of the first cycle in this charge / discharge cycle test was determined and was defined as “charge / discharge cycle performance (%)”. The results are also shown in Table 1.

(Discharge capacity characteristics)
As can be seen from Table 1, the discharge capacity is reduced in Comparative Examples 2 to 5 where the Co substitution amount is 2%, 3%, 5%, and 10% compared to Comparative Example 1 in which Co substitution is not performed. did.

However, in Examples 1 to 4 in which the Co substitution amounts were 0.5%, 1%, 1.5%, and 1.9%, the discharge capacity was quite surprising compared to Comparative Example 1 (LiFePO ₄ ). increased.

That is, according to the present invention, in the general formula LiFe _(1-x) Co _x PO ₄ , the value of x is 0 <x <0.02, preferably the value of x is 0.005 or more and 0.019. It has been found that the following effect can be provided that a polyanion-type positive electrode active material having a larger discharge capacity than LiFePO ₄ can be provided, which cannot be predicted from the prior art. From Table 1, it can be seen that the value of x is more preferably 0.005 or more and 0.015 or less, and further preferably 0.010 or more and 0.015 or less.

Further, as can be seen from Table 1, the high rate discharge characteristics were obtained in Comparative Examples 2 to 4 in which the Co substitution amount was 2%, 3%, and 5%, except for Comparative Example 5 in which the Co substitution amount was 10%. In Examples 1 to 4 in which the Co substitution amounts were 0.5%, 1%, 1.5%, and 1.9%, respectively, compared to Comparative Example 1 in which Co substitution was not performed. Surprisingly, the result of not lower than the Comparative example 1 (LiFePO ₄₎ was observed.

That is, according to the present invention, in the general formula LiFe _(1-x) Co _x PO ₄ , the value of x is 0 <x <0.02, preferably the value of x is 0.005 or more and 0.019. By setting it as the following, not only can a polyanion type positive electrode active material having a large discharge capacity be provided, but also a positive electrode active material having excellent high rate discharge characteristics can be provided. From the results shown in Table 1, the high rate discharge characteristics are rather improved in the range where the value of x is 0.005 or more and 0.019 or less. In particular, the value of x is 0.010 or more and 0. It can be seen that the high rate discharge characteristic is particularly excellent when it is .015 or less.

In addition, as can be seen from Table 1, the charge / discharge cycle performance of Comparative Examples 2 to 5 in which the Co substitution amount was 2%, 3%, 5%, and 10% was higher than that of Comparative Example 1 (LiFePO ₄ ). On the other hand, in Examples 1 to 4 in which the Co substitution amounts were 0.5%, 1%, 1.5% and 1.9%, it was quite surprising that Comparative Example 1 (LiFePO ₄ ) As a result, no decrease was observed.

That is, according to the present invention, in the general formula LiFe _(1-x) Co _x PO ₄ , the value of x is 0 <x <0.02, preferably the value of x is 0.005 or more and 0.019. By setting it as follows, not only can a polyanion-type positive electrode active material having a large discharge capacity be provided, but also a positive electrode active material having excellent charge / discharge cycle performance can be provided. From the results shown in Table 1, the charge / discharge cycle performance is rather improved in the range where the value of x is 0.005 or more and 0.015 or less. In particular, the value of x is 0.010 or more. It can be seen that the charge / discharge cycle performance is particularly excellent when it is 0.015 or less.

A lithium secondary battery that can improve the discharge capacity as compared with LiFePO ₄ and has excellent high-rate discharge characteristics and charge / discharge cycle performance by setting the ratio of replacing part of Fe in LiFePO ₄ with Co to less than 2%. Although it is not necessarily clear about the mechanism of action that provides the remarkable effect that the positive electrode active material can be provided, the present inventors presume as follows. That is, when a part of Fe of LiFePO ₄ is replaced with Co, the crystal forms a solid solution. In this solid solution, the lattice constant of the crystal approaches the lattice constant of LiCoPO ₄ according to the amount of Co substitution. However, this phenomenon appears when the Co substitution amount is 2% or more. If the Co substitution amount is less than 2%, the lattice constant almost coincides with LiFePO ₄ . That is, the presence of Co having a small ionic radius but a little smaller than Fe while maintaining the same lattice volume has resulted in some effects such as the expansion of the Li ion path in the solid phase, resulting in Li ions. It is thought that this is because it becomes easier to move or because the structural stability is increased. On the other hand, when the amount of Co substitution is 2% or more, the lattice constant changes according to the amount of Co substitution. Therefore, it is considered that various electrochemical characteristics of the active material are deteriorated by losing the special effects as described above. It is done.

  In the following examples and comparative examples, the present invention will be described in more detail with respect to a battery in which a positive electrode including the positive electrode material of the present invention and a negative electrode including a carbon material (artificial graphite) are combined.

(Example 5)
(Production of LiFe _0.995 Co _0.005 PO ₄ / C)
First, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate tetrahydrate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio of 0.995: 0.005: 1.00: 0.51, Using an alcohol as a solvent, pulverization and mixing were performed for 2 hours in a ball mill to obtain a LiFe _0.995 Co _0.005 PO ₄ precursor. Next, after drying the LiFe _0.995 Co _0.005 PO ₄ precursor, the amount of carbon produced by thermal decomposition of methanol is reduced to a mixed gas of methanol and nitrogen that is put into a rotary kiln and vaporized. While supplying 2% by mass of LiFe _0.995 Co _0.005 PO ₄ , it was calcined at 700 ° C. for 6 hours, and cobalt iron phosphate substituted with 0.5% of Co according to the present invention. Lithium A (LiFe _0.995 Co _0.005 PO ₄ / C) was produced. The rotational speed of the kiln is 1 r. p. m. It was. Moreover, the mixed gas of vaporized methanol and nitrogen was manufactured by enclosing a methanol solution maintained at 45 ° C. in a sealed container and bubbling with nitrogen as a carrier gas.

The amount of carbon in the obtained lithium cobalt iron phosphate A (LiFe _0.995 Co _0.005 PO ₄ / C) was investigated using elemental analysis. Further, the composition of LiFe _0.995 Co _0.005 PO ₄ was confirmed by ICP emission spectroscopic analysis.

(Production of positive electrode plate)
The mass ratio of lithium cobalt iron phosphate A (LiFe _0.995 Co _0.005 PO ₄ / C) manufactured by the above method, acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder is 87. : N-methyl-2-pyrrolidone (NMP) was used as a solvent so as to have a ratio of 5: 8, and kneaded sufficiently to produce a positive electrode paste. This positive electrode paste was applied to both sides of an aluminum foil current collector with a thickness of 20 μm, dried, and then pressed to obtain a positive electrode plate.

(Production of negative electrode plate)
Artificial graphite as the negative electrode material (average particle size 6 μm, interplanar spacing (d002) 0.337 nm by X-ray diffraction analysis, c-axis direction crystal size (Lc) 55 nm) and PVdF as the binder in mass ratio A negative electrode paste was manufactured by sufficiently kneading using N-methyl-2-pyrrolidone (NMP) as a solvent so that the ratio was 94: 6. The negative electrode paste was applied to both surfaces of a 10 μm thick copper foil current collector, dried, and then pressed, to obtain a negative electrode plate. A negative electrode terminal was welded to the negative electrode plate by resistance welding.

After winding the positive electrode and the negative electrode so that a separator, which is a continuous porous body having a thickness of 25 μm and an air permeability of 90 seconds / 100 cc, is arranged between both electrodes, this is 48 mm high, 30 mm wide and 5 mm thick. Inserted into a 2 mm container. Furthermore, a lithium secondary battery A according to the present invention was assembled by injecting a non-aqueous liquid electrolyte (electrolytic solution) into the container and finally sealing the liquid injection port. The electrolyte solution used was a solution of 1 mol / l LiPF ₆ in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1: 1. It was. The design capacity was 500 mAh.

(Example 6)
(Production of LiFe _0.99 Co _0.01 PO ₄ / C)
First, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate tetrahydrate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio of 0.99: 0.01: 1.00: 0.51 Using an alcohol as a solvent, the mixture was pulverized and mixed with a ball mill for 2 hours to obtain a LiFe _0.99 Co _0.01 PO ₄ precursor. Next, after the LiFe _0.99 Co _0.01 PO ₄ precursor is dried, the amount of carbon produced by the thermal decomposition of methanol is reduced to a mixed gas of methanol and nitrogen that is put into a rotary kiln and vaporized. While supplying 2% by mass of LiFe _0.99 Co _0.01 PO ₄ , firing was performed at 700 ° C. for 6 hours, and lithium cobalt phosphate B substituted with 1% of Co according to the present invention B (LiFe _0.99 Co _0.01 PO ₄ / C) was produced. The rotational speed of the kiln is 1 r. p. m. It was. Moreover, the mixed gas of vaporized methanol and nitrogen was manufactured by enclosing a methanol solution maintained at 45 ° C. in a sealed container and bubbling with nitrogen as a carrier gas.

The amount of carbon in the obtained lithium cobalt iron phosphate B (LiFe _0.99 Co _0.01 PO ₄ / C) was examined using elemental analysis. In addition, the composition of LiFe _0.99 Co _0.01 PO ₄ was confirmed by ICP emission spectroscopic analysis.

  A lithium secondary battery B according to the present invention was assembled in the same manner as in Example 9 except that lithium cobalt iron phosphate B was used.

(Example 7)
(Production of LiFe _0.981 Co _0.019 PO ₄ / C)
First, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate tetrahydrate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio of 0.981: 0.019: 1.00: 0.51 Using alcohol as a solvent, the mixture was pulverized and mixed with a ball mill for 2 hours to obtain a LiFe _0.981 Co _0.019 PO ₄ precursor. Next, after the LiFe _0.981 Co _0.019 PO ₄ precursor is dried, the amount of carbon produced by thermal decomposition of methanol is reduced to a mixed gas of methanol and nitrogen that is put into a rotary kiln and vaporized. While supplying 2% by mass of LiFe _0.981 Co _0.019 PO ₄ , it was calcined at 700 ° C. for 6 hours, and cobalt iron phosphate substituted with 1.9% of Co according to the present invention. Lithium C (LiFe _0.981 Co _0.019 PO ₄ / C) was produced. The rotational speed of the kiln is 1 r. p. m. It was. Moreover, the mixed gas of vaporized methanol and nitrogen was manufactured by enclosing a methanol solution maintained at 45 ° C. in a sealed container and bubbling with nitrogen as a carrier gas.

The amount of carbon in the obtained lithium cobalt iron phosphate C (LiFe _0.981 Co _0.019 PO ₄ / C) was examined using elemental analysis. Further, the composition of LiFe _0.981 Co _0.019 PO ₄ was confirmed by ICP emission spectroscopic analysis.

  A lithium secondary battery C according to the present invention was assembled in the same manner as in Example 9 except that lithium cobalt iron phosphate C was used.

(Comparative Example 6)
(Production of LiFePO ₄ / C)
First, the 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 ) is 1. 00: 1.00: a After weighed to become 0.51, these using an alcohol as a solvent under a nitrogen atmosphere for 2 hours in a ball mill to obtain LiFePO ₄ precursor and milling and mixing. Next, after drying the LiFePO ₄ precursor, it is put into a rotary kiln, and the amount of carbon produced by thermal decomposition of methanol is 2% by mass of LiFePO ₄ by vaporizing the mixed gas of methanol and nitrogen. In this manner, the mixture was baked at 700 ° C. for 6 hours to produce lithium iron phosphate D (LiFePO ₄ / C) not substituted for Co. The rotational speed of the kiln is 1 r. p. m. It was. Moreover, the mixed gas of vaporized methanol and nitrogen was manufactured by enclosing a methanol solution maintained at 45 ° C. in a sealed container and bubbling with nitrogen as a carrier gas.

The amount of carbon in the obtained lithium iron phosphate D (LiFePO ₄ / C) was investigated using elemental analysis. Further, the composition of LiFePO ₄ was confirmed by ICP emission spectroscopic analysis.

  A lithium secondary battery D was assembled in the same manner as in Example 9 except that lithium iron phosphate D was used.

(Comparative Example 7)
(Production of LiFe _0.98 Co _0.02 PO ₄ / C)
First, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate tetrahydrate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio of 0.98: 0.02: 1.00: 0.51 Using an alcohol as a solvent, pulverization and mixing were performed for 2 hours with a ball mill to obtain a LiFe _0.98 Co _0.02 PO ₄ precursor. Next, after drying the LiFe _0.98 Co _0.02 PO ₄ precursor, the amount of carbon produced by thermal decomposition of methanol is reduced to a mixed gas of methanol and nitrogen that is put into a rotary kiln and vaporized. While supplying 2% by mass of LiFe _0.98 Co _0.02 PO ₄ , it was calcined at 700 ° C. for 6 hours, and lithium cobalt cobalt phosphate E (2%) substituted with Co of the present invention by 2%. LiFe _0.98 Co _0.02 PO ₄ / C) was produced. The rotational speed of the kiln is 1 r. p. m. It was. Moreover, the mixed gas of vaporized methanol and nitrogen was manufactured by enclosing a methanol solution maintained at 45 ° C. in a sealed container and bubbling with nitrogen as a carrier gas.

The amount of carbon in the obtained lithium cobalt iron phosphate E (LiFe _0.98 Co _0.02 PO ₄ / C) was examined using elemental analysis. Further, the composition of LiFe _0.98 Co _0.02 PO ₄ was confirmed by ICP emission spectroscopic analysis.

  A lithium secondary battery E according to the present invention was assembled in the same manner as in Example 9 except that lithium cobalt iron phosphate E was used.

(Comparative Example 8)
(Production of LiFe _0.95 Co _0.05 PO ₄ / C)
First, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), cobalt acetate tetrahydrate (Co (CH ₃ COO) ₂ .4H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H) ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio of 0.95: 0.05: 1.00: 0.51 Using alcohol as a solvent, the mixture was pulverized and mixed with a ball mill for 2 hours to obtain a LiFe _0.95 Co _0.05 PO ₄ precursor. Next, after drying the LiFe _0.95 Co _0.05 PO ₄ precursor, the amount of carbon produced by the thermal decomposition of methanol is reduced to a mixed gas of methanol and nitrogen that is put into a rotary kiln and vaporized. While supplying 2% by mass of LiFe _0.95 Co _0.05 PO ₄ , it was calcined under conditions of 700 ° C. for 6 hours, and lithium iron cobalt phosphate F substituted with 5% of Co according to the present invention ( LiFe _0.95 Co _0.05 PO ₄ / C) was produced. The rotational speed of the kiln is 1 r. p. m. It was. Moreover, the mixed gas of vaporized methanol and nitrogen was manufactured by enclosing a methanol solution maintained at 45 ° C. in a sealed container and bubbling with nitrogen as a carrier gas.

The amount of carbon in the obtained lithium cobalt iron phosphate F (LiFe _0.95 Co _0.05 PO ₄ / C) was investigated using elemental analysis. Further, the composition of LiFe _0.95 Co _0.05 PO ₄ was confirmed by ICP emission spectroscopic analysis.

  A lithium secondary battery F according to the present invention was assembled in the same manner as in Example 9 except that lithium cobalt iron phosphate F was used.

  An initial charge / discharge test was performed at 25 ° C. on the assembled lithium secondary batteries A to F. The initial charging was constant current constant voltage charging for 3 hours in total at a constant current of 1 It (about 1 hour rate, 500 mA) up to 3.6 V, and further at a constant voltage of 3.6 V. The subsequent initial discharge was a constant current discharge of up to 2.0 V at a constant current of 1 It (about 1 hour rate, 500 mA), and the amount of discharge at this time was recorded as “initial discharge capacity (mAh)”. Subsequently, the cycle test was performed at 45 ° C. for 50 cycles. The charging was performed at a constant current of 1 It (about 1 hour rate, 500 mA) up to 3.6 V, and further at a constant voltage of 3.6 V for a total current of 3 hours. The subsequent discharge was a constant current discharge up to 2.0 V at a constant current of 1 It (about 1 hour rate, 500 mA). Thereafter, the charge / discharge test was performed again at 25 ° C. The charging was performed at a constant current of 1 It (about 1 hour rate, 500 mA) up to 3.6 V, and further at a constant voltage of 3.6 V for a total current of 3 hours. The subsequent discharge was a constant current discharge of up to 2.0 V at a constant current of 1 It (about 1 hour rate, 500 mA), and the amount of electricity discharged at this time was recorded as “discharge capacity after 50 cycles (mAh)”. Further, the percentage of the “discharge capacity after 50 cycles (mAh)” with respect to the “initial discharge capacity (mAh)” was calculated as “charge / discharge cycle performance (%)”. The results are shown in Table 2 and FIG.

  As can be seen from Table 1 and FIG. 1, the initial discharge capacities of the lithium secondary batteries A, B and C according to the present invention are larger than those of the lithium secondary battery D, and the charge / discharge cycle performance is It can be seen that it is higher than the battery D. It can also be seen that the charge / discharge cycle performance of the battery C with x = 0.199 is higher than that of the batteries E and F with x ≧ 0.02.

From the above results, a battery using a carbon material capable of inserting and extracting lithium ions in the negative electrode by selecting 0 <x <0.02 as the value of x in LiFe _(1-x) Co _x PO ₄ The charge / discharge cycle performance can be improved. It can also be seen that the value of x is more preferably 0.005 or more.

As described above, there is no specific description of a battery using a positive electrode active material in which a part of Fe of LiFePO ₄ is substituted with Co and using a carbon material for the negative electrode in any known document, but it is assumed that LiFePO ₄ Even if the charge / discharge cycle performance of a battery using a positive electrode active material obtained by substituting a certain amount of Fe with Co for a specific amount and using a carbon material for the negative electrode is known, as shown in FIG. The present invention, which has been found to show a prominent tendency in terms of cycle performance, in the range where the value of x shown exceeds 0 and is less than 0.02, cannot be easily derived.

In addition, when the positive electrode active material for lithium secondary batteries produced in all Examples and Comparative Examples was subjected to powder X-ray diffraction measurement (XRD) using CuKα rays, formation of a single phase was confirmed, and Fe ₂ Since the peak corresponding to the P impurity phase was not observed near 41 °, the presence of the Fe ₂ P impurity phase as described in Non-Patent Document 3 was not confirmed.

The above test results by the present inventors are that, in Non-Patent Document 3, a part of LiFePO ₄ was replaced with Co, the initial discharge capacity was reduced, and the capacity during the charge / discharge cycle was low. Although it is not clear why the difference is significant, there is no Fe ₂ P impurity phase in the positive electrode active material, the positive battery reaching the charge is different, and the negative electrode is not a lithium cell or a single electrode cell, It is presumed that this is due to differences such as being a cell using a carbon material.

It is a matter that the inventor could not have predicted that such an action effect is achieved, and the action mechanism is not clear at present. According to one hypothesis of the present inventors, when LiFePO ₄ is used for the positive electrode, Fe elutes from the positive electrode side and has some adverse effect on the negative electrode, and a part of the Fe of LiFePO ₄ is replaced with Co. When this was used for the positive electrode, Co was eluted and reached the negative electrode, thereby mitigating the above-mentioned adverse effects on the negative electrode caused by Fe. As a result, an increase in the irreversible capacity of the negative electrode accompanying the cycle test was suppressed. Estimated.

  According to the present invention, a lithium secondary battery having excellent battery performance can be provided by using a thermally stable polyanionic positive electrode active material. The battery is particularly suitable for application in fields where high capacity and high output are required, and its industrial applicability is extremely large.

Claims (4)

Positive electrode active material for a lithium secondary battery containing lithium cobalt iron phosphate represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0 <x <0.02, 0 ≦ y ≦ 1.2) .   2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein x is 0.005 ≦ x ≦ 0.019.   The lithium secondary battery provided with the positive electrode containing the positive electrode active material of Claim 1 or 2, a negative electrode, and the nonaqueous electrolyte.   The lithium secondary battery according to claim 3, wherein the negative electrode includes a carbon material capable of inserting and extracting lithium ions.

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