Cathode active material and secondary battery including same

Abstract

The present invention provides a cathode active material comprising primary particles which include at least one transition metal and further comprise an iron phosphide compound, and thus exhibiting excellent charge / discharge capacity and rate performance on the basis of excellent electronic conductivity.

Description

positive electrode active material and secondary battery containing the same

[0001] The present invention relates to a positive electrode active material and a secondary battery containing the same, and more specifically, to a positive electrode active material that exhibits excellent electronic conductivity by including an iron phosphide compound in the primary particle and a secondary battery containing such a positive electrode active material.

[0002] While lithium iron phosphate (LFP) possesses advantages such as low cost, eco-friendliness, long lifespan, cell stability, and high thermal stability, it is not utilized in many applications. This is because it exhibits degraded electrochemical properties due to low electronic conductivity and ion diffusion coefficients.

[0003] Recently, in order to increase low electronic and ionic conductivity, there have been ongoing attempts to solve the problem by micronizing LFP particles to nanoscale to shorten the diffusion path of lithium ions or by coating the particle surface with carbon to enhance conductivity.

[0004] Meanwhile, when LFP in a semi-finished state is calcined during the manufacturing process, it undergoes a calcination process at a specific atmosphere and temperature for particle crystallization and carbonization of the carbon coating source. During this process, various calcination byproducts may be generated, and these calcination byproducts act as resistance to electrochemical properties, thereby having a negative effect.

[0005] While some calcination byproducts contribute to electronic conductivity, there is still a lack of research regarding their utilization.

[0006] Therefore, there is a high need in the industry for new technologies that can appropriately utilize the calcination by-products inevitably generated during the manufacturing process of LFP and, if possible, contribute to the improvement of electronic conductivity. Furthermore, as the application fields of secondary batteries have recently diversified and the demand for high-speed charging and discharging is high, technologies capable of improving rate characteristics are also required.

[0007] The present invention aims to solve the problems of the prior art described above and technical challenges that have been requested over time.

[0008] After conducting in-depth research and various experiments, the inventors of the present application developed a new type of cathode active material in which an iron phosphide compound is included in the primary particles, and discovered that excellent charge / discharge capacity and rate characteristics can be achieved through by-products generated during the manufacturing process that contribute to the improvement of electronic conductivity, and thus completed the present invention.

[0009] Accordingly, the positive electrode active material according to the present invention comprises primary particles comprising one or more transition metals, and the primary particles further comprise an iron phosphide compound.

[0010] As confirmed by the inventors of the present application, iron phosphide compounds can contribute to the electronic conductivity of the positive electrode active material, and consequently, secondary batteries using such positive electrode active materials have increased charge / discharge capacity and, in particular, significantly improved rate characteristics.

[0011] The primary particles above may preferably include at least Fe as a transition metal, and the positive electrode active material of the present invention may consist solely of primary particles, but may also consist of secondary particles in the form of aggregated primary particles, or may consist of a mixture of primary and secondary particles. Accordingly, according to the present invention, an iron phosphide compound is ultimately incorporated in the positive electrode active material composed of primary particles and / or secondary particles.

[0012] The above iron phosphide compound may be, for example, FeP, Fe2P, Fe3P, etc., and preferably may be Fe2P, which has excellent electronic conductivity.

[0013]

[0014] Since such a positive electrode active material can be preferably applied to lithium iron phosphate with low electronic conductivity, in one preferred example, the positive electrode active material may have a composition represented by the following chemical formula 1.

[0015] Li x MP a O b (1)

[0016] In the above formula,

[0017] 0 <x≤2, 0≤a≤2, 0<b≤4;

[0018] M includes Fe and optionally may further include one or more of the following: transition metals of groups 3 to 12 excluding Fe, post-transition metals and metalloids of groups 13 to 15, alkaline earth metals, nonmetals of groups 14 to 16, lanthanides, and actinides.

[0019] In the above,

[0020] "Group 3 to 12 transition metals excluding Fe" may be, for example, Ni, Co, Mn, Sc, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, etc.

[0021] "Post-transition metals and metalloids in groups 13 to 15" may be, for example, Al, Ga, In, Sn, Tl, Pb, Bi, Po, B, Si, Ge, As, Sb, Te, At, etc., and

[0022] "Alkali earth metals" can be, for example, Be, Mg, Ca, Sr, Ba, Ra, etc., and

[0023] "Nonmetallic elements in groups 14 to 16" may be, for example, C, P, S, Se, etc.

[0024] A preferred cathode active material may be in the form of LiFePO4 or LiFePO4 doped with one or more of transition metals, post-transition metals, metalloids, alkaline earth metals, nonmetals of groups 14 to 16, lanthanides, and actinides, and additionally, may be in the form coated with carbon, etc. as needed.

[0025]

[0026] In one specific example, the primary particle comprises a core and a surface portion, and the content of the iron phosphide compound in the surface portion may be equal to or greater than the content of the iron phosphide compound in the core portion, and preferably, the content of the iron phosphide compound in the surface portion may be relatively greater than the content of the iron phosphide compound in the core portion.

[0027] In the process of manufacturing the positive electrode active material according to the present invention, the formation of iron phosphide compounds proceeds from the outer surface of the particles under the conditions of the material introduced during the manufacturing process or the calcination atmosphere, so iron phosphide compounds tend to be present more on the surface of the primary particles.

[0028] Furthermore, if the iron phosphide compound is present in too large an amount in the center of the primary particles, it may actually cause a problem of capacity degradation; therefore, it may be desirable for the content in the center to be less than the content in the surface. Accordingly, when primary particles aggregate to form secondary particles, the contribution to electronic conductivity may be higher when the iron phosphide compound is present in a high amount at the interface of the primary particles.

[0029] In the above, the surface portion may be a portion extending from the outer surface to a predetermined width based on the radius (r) of the primary particle, for example, a portion extending from 0.6r to r or from 0.9r to r from the outer surface, and the remaining portion may be distinguished as the center.

[0030]

[0031] In another specific example, the positive active material of the present invention may further include a junction connecting adjacent primary particles.

[0032] Looking at the FE-SEM image of the positive electrode active material of Example 1 disclosed in Fig. 1, amorphous junctions can be observed between the primary particles. These junctions tend to be more clearly observed when conditions are set to increase the amount of iron phosphide compound produced during the manufacturing process of the positive electrode active material, and they have the effect of connecting the primary particles to each other to improve mutual electron conductivity. However, if these junctions are excessively formed, a problem of capacity degradation may occur proportional to the volume occupied by the junctions; therefore, it may be desirable to form only the desired amount of junctions by controlling appropriate firing conditions.

[0033] Under the premise that the scope of the rights of the present invention is not subject to a restrictive interpretation, it is presumed that such a joint is composed mainly of carbon or in a form in which carbon and iron phosphide compounds are mixed.

[0034] Accordingly, the above-mentioned joint may be in a form that includes carbon and optionally further includes an iron phosphide compound.

[0035]

[0036] In the cathode active material of the present invention, the primary particles may, for example, have a D50 of 0.9 to 1.5 μm. This is a relatively large size compared to the primary particles of general cathode active materials, and it is thought that this is because conditions that induce the formation of iron phosphide compounds during the manufacturing process of the cathode active material contribute to the growth of the particle size of the primary particles.

[0037]

[0038] In one specific example, the primary particles in the cathode active material of the present invention include LiFePO4, and the content ratio of Fe2P to LiFePO4 may be in the range of 1.2 to 3% by weight.

[0039] When preparing LiFePO4 as a positive electrode active material or as one component thereof, the formation of the iron phosphide compound Fe2P may depend on the input amount of a carbon source, which is a raw material for carbon coating, and the content of H2 gas for forming a reducing atmosphere as a calcination atmosphere.

[0040] For example, as shown in the reaction equations below, Fe2P can be produced as a portion of LiFePO4 is decomposed by carbon (C) or H2 gas.

[0041] 6LiFePO4+ 8C(or 16C) → 2Fe2P + 2Li3PO4+ 2FeP + 8CO2↑ (or 16CO↑)

[0042] → 3Fe2P + 2Li3PO4+ P↑

[0043] 6LiFePO4+ 16H2→ 2Fe2P + 2Li3PO4+ 2FeP + 16H2O↑

[0044] → 3Fe2P + 2Li3PO4+ P↑

[0045] Therefore, it is necessary to appropriately control the amount of LiFePO4 so that it does not decompose excessively and reduce capacity. On the other hand, if the amount of Fe2P produced is excessively low, the electronic conductivity decreases, and the capacity of the active material particles cannot be fully realized, which may lead to a problem of reduced capacity. In this case, the rate characteristics may also be significantly degraded. A more desirable content ratio of Fe2P to LiFePO4 may be 2.5 to 2.8%.

[0046]

[0047] In another specific example, the primary particles may further contain Li3PO4, in which case the content ratio of Li3PO4 to LiFePO4 may be in the range of 0.4 to 1.2% by weight.

[0048] As can be seen from the above reaction equations, when a portion of LiFePO4 is decomposed by carbon (C) or H2 gas to produce Fe2P, Li3PO4 also tends to be produced. Unlike Fe2P, Li3PO4 does not contribute to the improvement of electronic conductivity and does not exhibit electrochemical properties. However, since Li3PO4 has a complementary relationship with the Fe2P content, it can be defined as an indicator of an appropriate Fe2P content, and more preferably, it can be in the range of 0.8 to 1.0%.

[0049]

[0050] In this relationship between Fe2P and Li3PO4, the Fe2P / Li3PO4 content ratio on a weight basis may preferably be in the range of 2 to 3.3. While an Fe2P content that is at least twice as high as the Li3PO4 content ensures improved electronic conductivity, an excessively high Fe2P content may lead to a decrease in capacity as previously explained; therefore, an upper limit of the ratio of 3.3 times can be used as a standard for an appropriate Fe2P content.

[0051]

[0052] As can be confirmed from the experimental details described below, the positive electrode active material according to the present invention has the following compositional characteristics, which are itself differentiated from conventional positive electrode active materials.

[0053] That is, the primary particles may contain Li3PO4 in an amount of 0.4 to 1.1 at% and / or Fe2P in an amount of 1 to 9 at%.

[0054]

[0055] The present invention also relates to a secondary battery characterized by comprising the above-mentioned positive active material.

[0056] Since the composition and manufacturing method of secondary batteries are known in the art, a detailed description thereof is omitted in this specification.

[0057] As explained above, the positive electrode active material according to the present invention exhibits excellent electronic conductivity by including specific calcination by-products generated during the manufacturing process, and accordingly, can exhibit high charge / discharge capacity and particularly excellent rate characteristics.

[0058] Figure 1 is an FE-SEM image of the positive electrode active material of Example 1;

[0059] Figure 2 is an FE-SEM image of the positive electrode active material of Comparative Example 2;

[0060] FIG. 3 is an FE-SEM image of the positive electrode active material of Example 4;

[0061] Figure 4 shows the EDS measurement results of the positive electrode active material of Example 2;

[0062] Figure 5 shows the EDS measurement results of the positive electrode active material of Comparative Example 2.

[0063] The present invention will be described further below with reference to embodiments thereof, but the scope of the invention is not limited by them.

[0064]

[0065] [Comparative Example 1]

[0066] A lithium raw material Li2CO3 and an FePO4 precursor were mixed in a Li / Me ratio of 1.02, glucose was added as a carbon source, and the mixture was added to distilled water as a solvent so that the solid content was approximately 40%, and then stirred for 1 hour to prepare a mixed solution. The amount of the carbon source added is 5 wt% based on the total amount of the lithium raw material, the FePO4 precursor, and the carbon source.

[0067] The above mixed solution was coarsely ground using an attrition mill. Zirconium beads with a size of Φ2.0 mm were used inside the coarse grinding equipment. After setting the cooling water temperature used in the equipment to 5~10℃, the milling zone rpm was set to 1000 rpm and the pump rpm for solution transfer was set to 200~300 rpm, respectively, to prepare a wet solution ('coarse grinding solution') with a size of D50 ≤ 2.0 µm.

[0068] Fine grinding was performed on the above coarse grinding solution using a fine mill (Daehwa Tech) machine. Zirconium beads with a size of Φ0.3 mm were used inside the fine grinding machine. The rpm of the machine's milling zone was set to 2000 to 2300 rpm, and the pump rpm for solution transfer was set to 200 to 300 rpm, respectively, to prepare a wet solution ('fine grinding solution') with a size of D50 ≤ 0.5 μm.

[0069] The above finely ground solution was powdered to produce a semi-finished product using a spray drying device (Ein System). The internal chamber temperature of the spray drying device was set to 250 to 300°C and the outlet temperature to 100 to 115°C, respectively, and the mixed solution was sprayed.

[0070] The semi-finished product obtained by the above spraying was filled to about 80% capacity into a carbon crucible (Shinhan Science TI) and then placed into a tube furnace. N2 gas was filled into the tube at a rate of about 20 L / min, and firing was started after 20 minutes. Firing was carried out by setting the step temperature to 440–460℃ for 2 hours and the final firing temperature to 770–800℃ for 8 hours.

[0071] The calcined product obtained by the above calcination was ground using a Pin mill (Retsch) machine. During Pin mill grinding, the milling rpm was set to 15,000 to 18,000 and the grinding was repeated 3 to 5 times, and a LiFePO4 cathode active material powder was obtained using a mesh of 0.08 to 0.15 mm.

[0072]

[0073] [Example 1]

[0074] LiFePO4 cathode active material powder was prepared using the same method as Comparative Example 1, except that the amount of carbon source input was changed to 15 wt%.

[0075]

[0076] [Comparative Example 2]

[0077] LiFePO4 cathode active material powder was prepared in the same manner as Comparative Example 1, except that calcination was performed with N2+H2 mixed gas (H2 content: 5 mol%) filled in a tube furnace instead of N2 gas.

[0078]

[0079] [Example 2]

[0080] LiFePO4 cathode active material powder was prepared in the same manner as Comparative Example 1, except that the amount of carbon source input was changed to 7 wt% and calcination was performed with N2+H2 mixed gas (H2 content: 5 mol%) filled in the tube furnace instead of N2 gas.

[0081]

[0082] [Example 3]

[0083] LiFePO4 cathode active material powder was prepared in the same manner as in Example 2, except that the amount of carbon source input was changed to 10 wt%.

[0084]

[0085] [Example 4]

[0086] LiFePO4 cathode active material powder was prepared in the same manner as in Example 2, except that the amount of carbon source input was changed to 12 wt%.

[0087]

[0088] [Comparative Example 3]

[0089] LiFePO4 cathode active material powder was prepared in the same manner as in Example 2, except that the amount of carbon source input was changed to 15 wt%.

[0090]

[0091] [Comparative Example 4]

[0092] LiFePO4 cathode active material powder was prepared in the same manner as Comparative Example 1, except that the amount of carbon source input was changed to 1 wt% and calcination was performed with N2+H2 mixed gas (H2 content: 10 mol%) filled in the tube furnace instead of N2 gas.

[0093]

[0094] [Example 5]

[0095] LiFePO4 cathode active material powder was prepared in the same manner as Comparative Example 4, except that the amount of carbon source input was changed to 5 wt%.

[0096]

[0097] [Comparative Example 5]

[0098] LiFePO4 cathode active material powder was prepared in the same manner as Comparative Example 4, except that the amount of carbon source input was changed to 10 wt%.

[0099]

[0100] [Comparative Example 6]

[0101] LiFePO4 cathode active material powder was prepared in the same manner as Comparative Example 1, except that the amount of carbon source input was changed to 1 wt% and calcination was performed with N2+H2 mixed gas (H2 content: 15 mol%) filled in the tube furnace instead of N2 gas.

[0102]

[0103] [Example 6]

[0104] LiFePO4 cathode active material powder was prepared in the same manner as Comparative Example 6, except that the amount of carbon source input was changed to 5 wt%.

[0105]

[0106] [Example 7]

[0107] Except for adding a predetermined amount of Ti to the mixture of lithium raw material and FePO4 precursor and changing the input amount of the carbon source to 5 wt%, LiFe was prepared in the same manner as in Comparative Example 6. 1-x Ti x PO4 (x = 0.001 ~ 0.005) cathode active material powder was prepared.

[0108]

[0109] [Example 8]

[0110] LiFe was prepared in the same manner as Comparative Example 6, except that a predetermined amount of Mn was added to the mixture of lithium raw material and FePO4 precursor and the amount of carbon source added was changed to 5 wt%. 1-x Mn x PO4 (x = 0.001 ~ 0.005) cathode active material powder was prepared.

[0111]

[0112] [Comparative Example 7]

[0113] LiFePO4 cathode active material powder was prepared in the same manner as in Example 7, except that the amount of carbon source input was changed to 10 wt%.

[0114]

[0115] [Comparative Example 8]

[0116] Except for changing the amount of carbon source added to 10 wt%, LiFe is used in the same manner as in Example 8. 1-x Mn x PO4 (x = 0.001 ~ 0.005) cathode active material powder was prepared.

[0117]

[0118] [Comparative Example 9]

[0119] Except for changing the amount of carbon source added to 10 wt%, LiFe is used in the same manner as in Example 9. 1-x Mn x PO4 (x = 0.001 ~ 0.005) cathode active material powder was prepared.

[0120]

[0121] [Experimental Example 1]

[0122] Measurements were performed on the cathode active material powders prepared in Comparative Examples 1 to 9 and Examples 1 to 9, respectively, by the following methods, and the results are shown together in Table 1.

[0123]

[0124] Measurement method of PSD (Particle Size Distribution)

[0125] Mastersizer 3000 equipment (Malvern Panalytical) was used, and the measurement conditions are as follows.

[0126] - Sample amount: 0.45 g

[0127] - Sample Addition Dispersant: 10% Sodium Hexamethaphosphate 1ml

[0128] - Sample input solvent: 40 ml distilled water

[0129] - Sample ultrasonic dispersion: 40 kHz, 3 min

[0130] - Refraction Index: 1.692

[0131]

[0132] FE-SEM measurement method

[0133] JSM-IT800 (Jeol) was used as the SEM measurement equipment. A sample was sampled onto carbon tape, and after Pt coating was performed in a vacuum atmosphere, it was placed into the FE-SEM equipment for measurement. The acceleration voltage was set to a range of 10 to 30 kV, and the magnification during measurement was changed to ×5000, ×10000, and ×20000, respectively.

[0134]

[0135] EDS measurement method

[0136] The Aztec program was executed to operate the EDS equipment, and images were captured using the FE-SEM equipment and transmitted to the Aztec program. Analysis points were selected by choosing square, line, and dot types for the points to be measured, and elements contained within the chemical substance were selected on the periodic table to confirm the presence and content of the constituent elements through EDAX and mapping.

[0137] For EDS calculation, the measured atomic % was verified using the Aztec program, and based on this, the amount of byproducts was determined using the following calculation method.

[0138]

[0139]

[0140] XRD measurement method

[0141] The D8 ENDEAVO instrument (Bruker) was used, and the measurement conditions are as follows.

[0142] - Sample preparation: Flatten approximately 5 g of the sample in the sample holder.

[0143] - Measurement range: 10 ~ 80°

[0144] - Scan rate: 2.0° / min

[0145] Phase fractions were calculated by performing 'Rietveld Refinement' using the 'Topas' program.

[0146]

[0147] From the results of Table 1 above, the following items can be confirmed.

[0148]

[0149] (1) It can be seen that the Fe2P content in the cathode active material depends on the amount of carbon source input and the amount of H2 gas in the calcination atmosphere during the manufacturing process of the cathode active material. That is, if the amount of carbon source input increases or the amount of H2 gas in the calcination atmosphere increases, the Fe2P content increases.

[0150]

[0151] (2) As in Example 1, even if there is no H2 gas in the firing atmosphere, if the amount of carbon source added is large, it can be seen that the Fe2P content increases significantly compared to Comparative Example 1.

[0152]

[0153] (3) When referring to Comparative Examples 3, 5, and 7 to 9, if the amount of carbon source added is excessively large while the calcination atmosphere contains a predetermined amount of H2 gas, the Fe2P content increases significantly compared to the examples corresponding to the above comparative examples, and this results in undesirable electrochemical characteristics, as confirmed in Experimental Example 2.

[0154]

[0155] (4) When the overall amount of carbon source input and H2 gas content is small (see Comparative Example 2) or when the amount of carbon source input is excessively small (Comparative Examples 4 and 6), it can be seen that the Fe2P content does not reach a level that contributes to the improvement of electronic conductivity.

[0156]

[0157] (5) When the overall amount of carbon source input and H2 gas content is high, size growth due to particle bonding occurs, which can be confirmed by the fact that the size of D50 in Example 4 and Comparative Example 9 is very large in the form of secondary particles.

[0158]

[0159] (6) It can be seen that the cathode active materials of the embodiments according to the present invention have a Fe2P content of 1 to 9 at%, more specifically 1.22 to 2.89 at% in EDS analysis, a Fe2P content of 1 to 9 at%, more specifically 1.21 to 2.77 at% in XRD analysis, a Li3PO4 content of 0.4 to 1.1 at%, more specifically 0.42 to 1.07 at%, and in terms of the mutual ratio of the materials, Fe2P / LFP is 1.2 to 3%, Li3PO4 / LFP is 0.4 to 1.2%, and the Fe2P / Li3PO4 ratio is 2 to 3.3.

[0160]

[0161] (7) FIG. 1 shows an FE-SEM image of the positive active material of Example 1 and FIG. 2 shows an FE-SEM image of the positive active material of Comparative Example 2. When compared with FIG. 2, the positive active material of FIG. 1 shows a bonding structure connecting particles around the particles, which is predicted to contribute to particle bonding and electron conductivity as a byproduct of calcination. In FIG. 1, multiple primary particles aggregate to form secondary particles, and these secondary particles can be seen in the FE-SEM image of the positive active material of Example 4 shown in FIG. 3.

[0162]

[0163] (8) Figure 4 shows the EDS measurement results of the positive electrode active material of Example 2 and Figure 5 shows the EDS measurement results of the positive electrode active material of Comparative Example 2. When compared with Figure 5, it can be seen that the amount of oxygen in the positive electrode active material of Figure 4 has decreased, and instead, the amount of P and Fe has increased. This is because calcination byproducts such as Fe2P are generated as part of the LFP decomposes.

[0164]

[0165] (9) As shown in Table 2 below, which shows the particle cross-section Point EDS results for Comparative Example 1 and Example 3, the material distribution in the surface and center of the primary particles is such that in the case of the positive active material of Comparative Example 1, a small amount of Fe2P is present only in the surface, whereas in the case of the positive active material of Example 3, a small amount of Fe2P and Li3PO4 is present in the center as well, and Fe2P and Li3PO4 are present in a larger amount in the surface, which contributes significantly to the improvement of electronic conductivity.

[0166]

[0167]

[0168] [Experimental Example 2]

[0169] The cathode active material powders prepared in Comparative Examples 1 to 9 and Examples 1 to 9, respectively, were mixed with PVdF (KF1100) as a binder and Super-P as a conductive material in a weight ratio of 95:2.5:2.5, and added to NMP (N-Methyl-2-pyrrolidone) solvent to prepare a cathode active material slurry. The slurry was coated onto an aluminum foil (Al foil, thickness: 20 μm) serving as a cathode current collector, dried at 120°C, and then rolled to produce a cathode electrode plate. The loading level of the rolled cathode was 12 mg / cm². 2 The rolled density is 2.40 g / cm³ 3 The above positive electrode plate was stamped to 13.8Φ, and a 2032 coin-type half cell was fabricated using a 15Φ lithium metal negative electrode and an electrolyte (1M LiPF6 in EC / DMC=1:1).

[0170] After aging the above coin-type half cell at room temperature for 10 hours, charge-discharge tests were performed, and the results are shown in Table 3 below. Capacity evaluation was based on a standard of 150 mAh / g at a 0.1C rate, and charge-discharge conditions were executed under constant current (CC) / constant voltage (CV) within a voltage range of 3.7 to 2.5.

[0171]

[0172] As shown in Table 3 above, it can be seen that the secondary batteries of the examples have superior overall charge and discharge capacities compared to the comparative examples, and consequently, relatively high efficiency. In particular, regarding rate characteristics, it can be seen that the secondary batteries of the examples are significantly superior, which can be understood as being largely influenced by the improvement in electronic conductivity.

[0173] When referring to Tables 1 and 3, which compare the characteristics of the preceding examples and comparative examples, it appears that the battery characteristics of the examples are superior to those of Comparative Examples 1, 2, 4, and 6, which generally have low Fe2P content. This appears to be because Fe2P played a role in compensating for the low electron conductivity of the LFP active material. However, as seen in Comparative Examples 3, 5, 7 to 9, when Fe2P is excessively high, the LiFePO4 decomposes to that extent, resulting in a significant decrease in capacity; thus, it is evident that appropriate control of calcination conditions is necessary. Of course, as previously mentioned, the content of Fe2P can also be expressed in relation to LiFePO4 or Li3PO4.

[0174]

[0175] The present invention is not limited to the above embodiments and can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without changing the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1.

1. A positive electrode active material characterized by comprising primary particles containing one or more types of transition metals, wherein the primary particles further comprise an iron phosphide compound.

2. The positive electrode active material according to claim 1, characterized in that the iron phosphide compound comprises Fe2P.

3. The positive active material according to claim 1, characterized in that the positive active material has a composition represented by the following chemical formula 1: There x MP a OR b (1) In the above formula, 0 <x≤2, 0≤a≤2, 0<b≤4; M includes Fe and may optionally further include one or more of the following: transition metals of groups 3 to 12 excluding Fe, post-transition metals and metalloids of groups 13 to 15, alkaline earth metals, nonmetals of groups 14 to 16, lanthanides, and actinides.

4. The positive electrode active material according to claim 1, wherein the primary particle comprises a core and a surface portion, and the content of the iron phosphide compound in the surface portion is equal to or greater than the content of the iron phosphide compound in the core.

5. The positive active material according to claim 1, further comprising a junction connecting adjacent primary particles.

6. An anode active material according to claim 5, wherein the junction comprises carbon and optionally further comprises an iron phosphide compound.

7. The positive active material according to claim 1, characterized in that the D50 of the primary particle is 0.9 to 1.5 μm.

8. A positive electrode active material according to claim 1, characterized in that the primary particles comprise LiFePO4, and the content ratio of Fe2P to LiFePO4 is 1.2 to 3%.

9. A positive electrode active material according to claim 8, characterized in that, during the manufacture of the above LiFePO4, the formation of Fe2P, an iron phosphide compound, depends on the amount of carbon source input, which is a raw material for carbon coating, and the content of H2 gas for forming a reducing atmosphere as a calcination atmosphere.

10. The positive active material according to claim 8, characterized in that the primary particles further comprise Li3PO4.

11. A positive electrode active material according to claim 10, wherein the primary particles comprise LiFePO4, and the content ratio of Li3PO4 to LiFePO4 is 0.4 to 1.2%.

12. A positive electrode active material according to claim 10, characterized in that the Fe2P / Li3PO4 content ratio is 2 to 3.

3.

13. The positive active material according to claim 10, characterized in that the primary particles contain 0.4 to 1.1 at% of Li3PO4.

14. The positive active material according to claim 2, characterized in that the primary particles contain 1 to 9 at% of Fe2P.

15. A secondary battery characterized by including a positive electrode active material according to claim 1.

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