JP5262318B2 - 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, and a 2.9 V end discharge capacity after charging a battery using this as a positive electrode active material to 5 V was compared with LiFePO _4, and the Co substitution amount was 20%. In this case, although the discharge capacity was slightly increased compared to LiFePO ₄ , the discharge capacity when Co substitution amount was 50% and 80% was significantly lower than LiFePO ₄ (Fig. 2). ing.

In Non-Patent Document 2, LiFe _0.1 Co _0.9 PO ₄ corresponding to 10% as the amount of Co substitution was synthesized by a solid phase method, which was used as a positive electrode active material, and metallic lithium was used as a 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 as a positive electrode active material, metallic lithium was used for the negative electrode, non-aqueous electrolyte batteries were prepared, and the results of comparison of discharge capacities were described. Was reduced compared to LiFePO ₄ and the charge / discharge cycle performance was also reduced, and it was described that Co substitution did not have a good effect on the electrochemical properties of lithium iron phosphate.

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 high-temperature storage performance and charge / discharge cycle life 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 the examples described later, the positive electrode potential at the time of charging was set to 3.9V.

As described above, it has been reported in many literatures that the discharge capacity of a material obtained by substituting part of Fe of LiFePO ₄ with Co is actually lower than expected. Under the condition of charging to a high potential, even if there is a phenomenon that the capacity is increased by substituting part of Fe of LiFePO ₄ with Co, it is not understandable. 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.

By the way, if the efficiency at the time of first charge / discharge after the battery is assembled (hereinafter referred to as “initial efficiency”) is low, it is necessary to provide an extra active material in the battery corresponding to the efficiency. And so on. In this regard, in any of the above-mentioned patent documents and non-patent documents, a positive electrode active material in which a part of Fe of LiFePO ₄ is replaced with Co is used, and a carbon material capable of inserting and extracting lithium ions is used for the negative electrode. There is no description of the initial efficiency of the case. Also, regarding the initial efficiency when using a carbon material capable of occluding and releasing lithium ions for the negative electrode, from the above-mentioned patent documents and non-patent documents only describing batteries using metallic lithium for the negative electrode Is totally unpredictable. This is because the positive electrode active material according to the present invention has a room for selecting “discharge” as well as selecting “charge” with lithium elimination reaction as the first electrochemical operation after assembling the cell. Absent. Therefore, in this specification, “initial efficiency” indicating the ratio of the initial discharge capacity to the initial charge capacity means that a part of Li existing in the positive electrode active material reaches the carbon material of the negative electrode by charging, and subsequent discharge. This is because the ratio of Li that can sometimes return from the carbon material of the negative electrode is evaluated.
Japanese Patent No. 3523397 N. Penazzi et al. Journal of the European Ceramic Society, 2004, vol.24, page1381. D. Wang et al. Electrochimica Acta 2005, vol.50, page2955. D. Shanmukaraj et al. Materials Science and Engineering B, 2008, vol.149, page93. LFNazar 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 a polyanion-type positive electrode active material capable of improving the initial efficiency of a battery using a carbon material capable of occluding and releasing lithium ions as a negative electrode, and The object is to provide a non-aqueous electrolyte battery using the same.

  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 provides carbon to lithium iron cobalt phosphate represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0 <x ≦ 0.05, 0 ≦ y ≦ 1.2). A lithium secondary battery comprising a positive electrode including a positive electrode active material for a lithium secondary battery , a negative electrode including a carbon material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte .

In the present invention, the lithium cobalt iron phosphate represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0.01 ≦ x <0.02, 0 ≦ y ≦ 1.2) is carbonized. A lithium secondary battery provided with a positive electrode containing a positive electrode active material for a lithium secondary battery, a negative electrode containing a carbon material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte is preferable.

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 ≦ 0.05, 0 ≦ y ≦ 1.2). This does not exclude the case where a part of Fe or Li is substituted with a transition metal element other than Fe or Co, such as Mn or Ni, or a metal element other than Li, Fe or Co, such as Al. The polyanion part polyanion part (PO ₄ ) may be partly 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 ≦ 0.05, 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.

  ADVANTAGE OF THE INVENTION According to this invention, the polyanion type positive electrode active material which can make the initial efficiency of the battery which used the carbon material which can occlude / release lithium ion for the negative electrode excellent, and a nonaqueous electrolyte battery using the same are provided. can do.

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]
(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 LiFe _0.99 Co _0.01 PO ₄ at 2% by mass, it was calcined at 700 ° C. for 6 hours, and lithium cobalt phosphate A substituted with 1% of Co according to the present invention. (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 A (LiFe _0.99 Co _0.01 PO ₄ / C) was investigated using elemental analysis. In addition, the composition of LiFe _0.99 Co _0.01 PO ₄ was confirmed by ICP emission spectroscopic analysis.

(Production of positive electrode plate)
A mass ratio of lithium cobalt iron phosphate A (LiFe _0.99 Co _0.01 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 sides of a 10 μm thick copper foil current collector, dried, and then subjected to press working 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 2]
(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 iron cobalt phosphate B substituted with 2% of Co according to the present invention ( 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 B (LiFe _0.98 Co _0.02 PO ₄ / C) was investigated 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 B according to the present invention was assembled in the same manner as in Example 1 except that lithium cobalt iron phosphate B was used.

[Example 3]
(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 cobalt phosphate C with 5% substitution 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 C (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 C according to the present invention was assembled in the same manner as in Example 1 except that lithium cobalt iron phosphate C was used.

[Comparative Example 1]
(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 1 except that lithium iron phosphate D was used.

  Initial charging / discharging was performed with respect to the assembled lithium secondary batteries A to D. The initial charge was a constant current constant voltage charge of a constant current of 0.1 It (about 10 hours rate, 50 mA) up to 3.9 V and a constant voltage of 3.9 V for a total of 15 hours. The amount of electricity that flowed during the initial charge was measured with a coulomb meter and recorded as “initial charge capacity (mAh)”. The subsequent initial discharge was a constant current discharge of up to 2.0 V at a constant current of 0.1 It (about 10 hours, 50 mA), and the amount of discharge at this time was recorded as “initial discharge capacity (mAh)”. The percentage of the “initial discharge capacity (mAh)” with respect to the “initial charge capacity (mAh)” was calculated as “initial efficiency (%)”. The results are shown in Table 1.

  As can be seen from Table 1, the discharge capacity of the lithium secondary batteries A and B according to the present invention is larger than that of the lithium secondary battery D, and the initial efficiency is higher than that of the lithium secondary battery D. I understand that. In addition, the lithium secondary battery C according to the present invention has a smaller discharge capacity than the lithium secondary battery D, but the initial efficiency is higher than that of the lithium secondary battery D.

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 ≦ 0.05 as the value of x in LiFe _(1-x) Co _x PO ₄ The initial efficiency can be increased. Moreover, it turns out that the value of x is preferably 0.01 or more. It can also be seen that the value of x is preferably 0.02 or less.

As described above, in the configuration of the battery of the present invention, the “initial efficiency” measured in the examples indicates the ratio of the initial discharge capacity to the initial charge capacity, and a part of Li existing in the positive electrode active material is charged. Thus, the ratio of Li that reaches the carbon material of the negative electrode and can return from the carbon material of the negative electrode during the subsequent discharge is evaluated. In addition, the battery of the example is designed to have a negative electrode limitation, that is, at least during initial discharge, the discharge ends due to a sudden rise in the negative electrode side potential. From this, it is clear that the difference in Co content in the positive electrode active material has affected the charge / discharge efficiency on the negative electrode side. 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, by substituting a part of Fe of LiFePO ₄ with Co, some factors acted to optimize the formation of the carbon negative electrode coating and reduce the irreversible capacity. Although it may be considered, it has not yet been verified.

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 (2)

Lithium secondary formed by adding carbon to lithium cobalt iron phosphate represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0 <x ≦ 0.05, 0 ≦ y ≦ 1.2) A lithium secondary battery including a positive electrode including a positive electrode active material for a battery , a negative electrode including a carbon material capable of occluding and releasing lithium ions, and a nonaqueous electrolyte . Lithium obtained by adding carbon to lithium cobalt iron phosphate represented by the general formula Li _y Fe _(1-x) Co _x PO ₄ (0.01 ≦ x <0.02, 0 ≦ y ≦ 1.2) A lithium secondary battery including a positive electrode including a positive electrode active material for a secondary battery, a negative electrode including a carbon material capable of inserting and extracting lithium ions, and a nonaqueous electrolyte .