JP7594754B2 - Electrode active material, its manufacturing method, and secondary battery - Google Patents

Description

The present invention relates to an electrode active material, a method for producing the same, and a secondary battery.

One method for improving the electrochemical properties of electrode active materials is surface modification. It is expected that surface modification can suppress side reactions such as oxidative and reductive decomposition of the electrolyte that occur at the active sites on the electrode surface, improving the coulombic efficiency, which is the ratio of discharge capacity to charge capacity.

It is known that the cycle characteristics and rate characteristics of the positive electrode active materials of lithium ion secondary batteries and all-solid-state lithium batteries can be improved and surface side reactions suppressed by coating them with various oxides (Non-Patent Documents 1 and 2). It has also been reported that the charge/discharge characteristics and cycle characteristics of positive electrode active materials that have a charge/discharge range of 4.5 V or higher relative to lithium metal can be improved by surface modification using a coupling agent containing a fluoroalkyl group (Patent Document 1).

International Publication No. 2012/081348

N. Ohta, K. Takada, I. Sakaguchi, L. Zhang, R. Ma, K. Fukuda, M. Osada, T. Sasaki, Electrochem. Commun. 9, 1486 (2007) Z. Chen, Y. Qin, K. Amine, Y.-K. Sun, J. Mater. Chem. 20, 7606 (2010); P. Guan, L. Zhou, Z. Yu, Y. Sun, Y. Liu, F. Wu, Y. Jiang, D. Chu, J. Energy. Chem. 43, 220 (2020) S. Yagi, Y. Ichikawa, I. Yamada, T. Doi, T. Ichitsubo, E. Matsubara, Jpn. J. Appl. Phys. 53, 119201 (2014) K. Ishii, S. Doi, R. Ise, T. Mandai, Y. Oaki, S. Yagi, H. Imai, J. Alloy Compound. 816, 152556 (2020)

Electrode active materials are the constituent materials of batteries, and many active materials are known that use the insertion and removal of mobile ions to charge and discharge. In the positive electrode active material of magnesium secondary batteries, insertion and removal of magnesium ions occurs. Since magnesium ions diffuse slowly within active materials, in order to improve their properties, it is necessary to nanoparticle the constituent materials of the electrode active material and shorten the diffusion distance of the magnesium ions within the particles. On the other hand, when the constituent materials of the electrode active material are nanoparticled, water adsorption on the surface becomes significant, which causes problems such as aggregation and separation of the slurry when preparing the slurry for making the coated electrode, making it difficult to apply uniformly. The same problem applies to lithium secondary batteries, and in order to make coating easier, efforts have been made to intentionally aggregate nanoparticles to form larger particles.

When materials that are highly reactive to the electrolyte, such as manganese or cobalt, are used as the transition metal elements that make up the electrode active material, side reactions such as oxidative and reductive decomposition of the electrolyte, which consists of solvents, supporting salts, additives, etc., become prominent during charging and discharging in addition to the insertion and removal of mobile ions, making it difficult to charge up to the theoretical capacity, and the cycle characteristics and coulombic efficiency are also low, which are problems.

The present invention was made in consideration of the above circumstances, and aims to provide an electrode active material that can prevent side reactions during charging and discharging and suppress deterioration of charge and discharge characteristics by surface modification of the electrode active material, and to provide a coated electrode that promotes mixing of a slurry containing the electrode active material and has excellent uniformity, smoothness, and adhesion to the electrode metal foil.

To solve the above problems, the present invention adopts the following measures.

(1) The electrode active material according to one embodiment of the present invention contains as its main component a surface-modified active material that is reactive to an electrolyte and has active material particles made of a compound of a transition metal element, and a first coating that covers the surfaces of the active material particles and is made of phosphoric acid or a phosphoric acid derivative.

(2) In the electrode active material described in (1) above, it is preferable that the first coating is made of a phosphate compound produced by reacting at least one of phosphoric acid, polyphosphoric acid, and derivatives thereof with a portion of the transition metal element.

(3) In the electrode active material described in either (1) or (2) above, it is preferable that the first coating film comprises a phosphoric acid compound produced by reacting at least one of phosphonic acid ^RPO (OH) ₂ containing an organic group R, phosphinic acid R1R2PO(OH ₎ containing organic groups _R1 and ^R2 , and their derivatives with a part of the transition metal element.

(4) In the electrode active material described in (3) above, it is preferable that at least one of the phosphonic acid RPO(OH) ₂ , the phosphinic acid R ¹ R ² PO(OH), and the derivatives thereof has a hydrocarbon group.

(5) In the electrode active material described in (4) above, it is preferable that at least one of the phosphonic acid RPO(OH) ₂ , the ^{phosphinic acid R1R2PO (} OH), or the derivatives thereof contains at least one of a phenyl group, a methyl group, an ethyl group, a propyl group, and a butyl group.

(6) In the electrode active material described in any one of (1) to (5) above, the surface-modified active material may further include a second coating that covers the surface of the first coating and is made of a compound that is non-reactive with the electrolyte.

(7) In the electrode active material described in any one of (1) to (6) above, the compound of the transition metal element may be capable of inserting and removing lithium or magnesium.

(8) In one embodiment of the secondary battery of the present invention, at least one of the positive electrode active material and the negative electrode active material is an electrode active material described in any one of (1) to (7) above.

(9) A method for producing an electrode active material according to one embodiment of the present invention is the method for producing an electrode active material described in (7) above, and includes a first step of surface-modifying a powder of a compound of a transition metal element in a solution of a salt of phosphoric acid, phosphonic acid, phosphinic acid, or a derivative thereof, and a second step of recovering the surface-modified powder after washing and drying it.

(10) The method for producing an electrode active material described in (9) above may further include a third step of heating the dried powder.

(11) In the method for producing an electrode active material described in either (9) or (10) above, the first step and the second step, or the first step, the second step, and the third step may be repeated in sequence.

The electrode active material of the present invention comprises a compound of a transition metal element reactive to an electrolyte as active material particles, the surface of which is covered with a first coating of phosphoric acid or a phosphoric acid derivative. Therefore, when the electrode active material of the present invention is used as a positive electrode active material or a negative electrode active material of a secondary electrode, it is possible to suppress side reactions during charging and discharging caused by contact of the transition metal element with the electrolyte, and to suppress deterioration of the charge and discharge characteristics. In addition, the electrode active material of the present invention controls the surface physical properties by coating the surface with phosphoric acid and its derivatives. The strong bonding force between phosphoric acid and transition metal ions and the slow rate of condensation polymerization between phosphoric acids allow for dense, thin, and uniform surface modification.

1 is a cyclic voltammogram of a composite electrode of surface-unmodified active material particles according to Comparative Example 1. 1 shows charge/discharge curves of a composite electrode of surface-unmodified active material particles according to Comparative Example 1. 1 shows Fourier transform infrared spectra of surface-unmodified active material particles according to Comparative Example 1 and surface-modified active material particles according to Examples 1 and 2. 4 is a cyclic voltammogram of a composite electrode of surface-modified active material particles according to Example 1. 13 is a cyclic voltammogram of a composite electrode of surface-modified active material particles according to Example 2. 13 is a cyclic voltammogram of a composite electrode of surface-modified active material particles according to Example 3. 13 is a cyclic voltammogram of a composite electrode of surface-modified active material particles according to Example 4. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 3. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 4. 13 is a charge/discharge curve of a composite electrode of surface-unmodified active material particles according to Comparative Example 2. 1A and 1B are Fourier transform infrared spectra of a powder of surface-unmodified active material particles according to Comparative Example 2 and a powder of surface-modified active material particles according to Examples 5 to 9. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 5. 13 is a charge/discharge curve of a composite electrode of surface-modified active material particles according to Example 7. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 8. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 9. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 10. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 11. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 12. 13 shows charge/discharge curves of a composite electrode of surface-modified active material particles according to Example 13.

The electrode active material, secondary battery, and manufacturing method thereof according to the embodiment of the present invention will be described in detail below with reference to the drawings. Note that the drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality. In addition, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them, and may be modified as appropriate within the scope of the present invention.

First Embodiment
The electrode active material according to the first embodiment of the present invention mainly contains, as a main component, a plurality of surface-modified active material particles each including an active material particle and a first coating covering the surface of the active material particle.

The active material particles are compounds that contain transition metal elements such as Sc, Ti, V, Cr, Mn, Co, Ni, and Cu, and that can insert and release mobile ions and are reactive to the electrolyte. Examples of mobile ions include lithium ions and magnesium ions. In the electrode active material, multiple active material particles are distributed in the form of aggregated porous secondary particles.

The first coating is made of a phosphoric acid compound produced by reacting at least one of phosphoric acid (H ₃ PO ₄ ), polyphosphoric acid, or a derivative thereof with a part of a transition metal element. Examples of phosphoric acid derivatives include phosphoric acid esters.

Phosphoric acid has the property of bonding strongly with transition metal elements, but polycondensation between phosphoric acids is slow. Therefore, when phosphoric acid or a phosphoric acid derivative is reacted with a transition metal element, the phosphoric acid quickly bonds to the exposed surface of the transition metal element, but bonds slowly to phosphoric acid that is already bonded to the transition metal element. As a result, a thin, dense, and uniform first coating made of phosphoric acid or a phosphoric acid derivative is formed around the active material particles in the secondary particle state.

The electrode active material can be mainly produced through the following steps: First, a powder of a transition metal element compound (e.g., MgCo2O4, MgMn2O4, ZnMn2O4, LiCoO2, _LiMn2O4 , _{etc. ) prepared} by a coprecipitation method or the like is added to a solution of a phosphate source (e.g., diammonium _{hydrogen phosphate ( NH4 ) 2HPO4 , triammonium} phosphate ₍ NH4 ₎ 3PO4, _etc. ) and stirred to perform surface modification (first step).

Next, the surface-modified powder is washed, collected, and dried to obtain a surface-modified active material in which the surfaces of the active material particles are modified with phosphoric acid or a phosphoric acid compound (second step). The first and second steps can be repeated in sequence, either once each or multiple times, to obtain an electrode active material.

If necessary, the dried sample may be heat-treated (third step). In this case, the first step, the second step, and the third step may be repeated in order, either once each or multiple times, to obtain an electrode active material.

The surface-modified active material may further include a second coating that covers the surface of the first coating and is made of a metal or a compound (oxide) thereof that is non-reactive with the electrolyte. The metal that constitutes the second coating strongly bonds with the phosphoric acid that constitutes the first coating, and can also reduce the gaps between the phosphoric acids that are bonded to the transition metal elements that constitute the active material particles, thereby improving the coverage of the active material particles. Examples of compounds that are non-reactive with the electrolyte include compounds of iron and aluminum. If necessary, any number of layers can be formed outside the second coating by alternating between layers of phosphoric acid or phosphoric acid compounds and compound layers.

As described above, the electrode active material according to this embodiment comprises a compound of a transition metal element that is reactive to the electrolyte as active material particles, the surface of which is covered with a thin, dense, and uniform first coating made of phosphoric acid or a phosphoric acid compound. Therefore, when the electrode active material according to this embodiment is used as the positive electrode active material or negative electrode active material of a secondary electrode, it is possible to prevent side reactions during charging and discharging caused by contact of the transition metal element with the electrolyte, and to suppress deterioration of the charging and discharging characteristics.

Second Embodiment
The electrode active material according to the second embodiment of the present invention is similar to the electrode active material according to the first embodiment in that it mainly comprises a plurality of surface-modified active material particles each having an active material particle and a first coating covering the surface of the active material particle, but differs in that the first coating is made of a phosphoric acid compound produced by reacting at least one of phosphonic ^acid RPO(OH) ₂ containing one organic group R, phosphinic acid ^R1R2PO (OH ₎ containing two organic groups _R1 and R2, and a part of a transition metal element with a part of a transition metal element. Examples of the organic groups R, _R1 , and _R2 include alkyl groups, aryl groups, cycloalkyl groups, haloalkyl groups, vinyl groups, allyl groups, and polyoxyethylene groups. The organic groups _R1 and _R2 may be the same or different from each other.

At least one of the phosphonic acid, phosphinic acid, and derivatives thereof constituting the first coating preferably has a hydrocarbon group. Examples of these include those containing at least one of a phenyl group, a methyl group, an ethyl group, a propyl group, and a butyl group, i.e., phenylphosphonic acid, diphenylphosphonic acid, methylphosphonic acid, ethylphosphonic acid, propylphosphonic acid, and butylphosphonic acid.

Even when phosphonic acid, phosphinic acid, or a derivative thereof is used as the phosphoric acid compound, a thin, dense, and uniform first coating can be formed. By covering the surface of the active material particles with this first coating, side reactions during charging can be suppressed and deterioration of the charge/discharge characteristics can be suppressed, as in the first embodiment.

Furthermore, according to this embodiment, the problem of water being adsorbed on the surface of the electrode active material (e.g., MgMn ₂ O ₄ , etc.) produced using the complex polymerization method and agglomerating in the slurry can be solved. That is, according to this embodiment, even if the active material particles have water adsorbed on the surface like MgMn ₂ O ₄ , the surface can be made hydrophobic by forming a first coating made of organic phosphonic acid, and the adsorption of water molecules can be suppressed. This is because the phenyl group, methyl group, ethyl group, propyl group, and butyl group contained in phosphonic acid and phosphinic acid are all hydrophobic. Therefore, according to this embodiment, a coated electrode excellent in uniformity, smoothness, and binding to the electrode metal foil can be provided.

The effects of the present invention will be made clearer by using the following comparative examples and examples. Note that the present invention is not limited to the following examples, and can be modified as appropriate without departing from the gist of the present invention.

(Comparative Example 1)
A composite electrode was prepared by a dry method using _MgCo2O4 powder prepared by coprecipitation according to the procedure of Non-Patent Document 3. 30 mg of active material powder, acetylene black, and polytetrafluoroethylene (PTFE) were weighed out in a weight ratio of _60:30:10 and kneaded in an agate mortar. The resulting mixture was pressed on a Pt mesh (80 mesh) in a state where about 2 mg was supported on the mesh to prepare a working electrode of the composite electrode.

Cyclic voltammetry and charge/discharge measurements were performed at 100°C using magnesium as the counter electrode, a silver wire immersed in 0.01M silver nitrate and 0.1M Mg(TFSA) ₂ solution of triglyme (G3) as the reference electrode, and tetraglyme (G4)-based solvated ionic liquid (0.3M [Mg(G4)(TFSA) ₂ ]/Pyr13TFSA) as the electrolyte. The sweep rate of cyclic voltammetry was 0.1mVs ^-1 , and the first sweep was started from the open circuit potential (OCV) toward the negative potential. Charge/discharge measurements were started from the discharge side in constant current mode, and the current value was 10.4mAg ^-1 .

Figures 1 and ₂ show the cyclic voltammogram and charge/discharge curve of unmodified _MgCo2O4 . In the cyclic voltammogram in Figure 1, an increase in current was observed at 3.5 V vs. Mg/Mg2 ^+, which is thought to be due to the oxidative decomposition of the electrolyte. In addition, the charge/discharge curve in Figure 2 shows that the ratio of the discharge capacity to the charge capacity (Coulomb efficiency) is 0.5, which is significantly smaller than 1. This is thought to be because part of the charge current was consumed by the decomposition of the electrolyte.

(Examples 1 and 2)
After dissolving 3.8 _mmol of ( _NH4 ) _2HPO4 in 192 mmol of water, 3.8 mmol of _MgCo2O4 powder prepared by _{coprecipitation} was added and stirred for 24 hours while heating to 60° C. The resulting suspension was centrifuged, and the precipitate was washed with water and centrifuged five times, after which the precipitate was suspended in water, recovered, and dried at 100° C. for 24 hours.

After drying, the sample was crushed in an agate mortar and placed in an alumina boat, and heated to 600°C at 200°C·h ^-1 in a tubular atmosphere furnace with simulated air (nitrogen 80 mL·min ^-1 , oxygen 20 mL·min ^-1 ) flowing, and then held for 1 hour to obtain a phosphoric acid-modified sample of Example 1 (r _P/MCO = 1, ×1). This procedure was also repeated to obtain a phosphoric acid-modified sample of Example 2 (r _P/MCO = 1, ×2). Here, r _P/MCO indicates the molar ratio of _P to _MgCO2O4 in the solution.

Figure 3 shows the Fourier transform infrared spectroscopy (ATR-FT-IR) spectra of unmodified MgCo ₂ O ₄ and phosphoric acid-modified MgCo ₂ O _4. In the phosphoric acid-modified sample, an absorption band attributable to the P-O stretching vibration (ν(P-O)) was observed at 850 to 1200 cm ^-1 . Furthermore, the intensity of the P-O stretching vibration increased with repeated phosphoric acid modification.

The results of cyclic voltammetry performed on the phosphoric acid-modified MgCo ₂ O ₄ of Examples 1 and 2 in the same manner as in Comparative Example 1 are shown in Figures 4 and 5. In both Examples 1 and 2, the increase in the current at 3.5 V vs. Mg/Mg ²⁺ was suppressed by phosphoric acid modification. Comparison between Examples 1 and 2 shows that the suppression effect is enhanced by repeating phosphoric acid modification twice.

(Examples 3 and 4)
After dissolving _2.4 mmol of ( _NH4 ) _2HPO4 in 555 mmol of water, 2.4 mmol of _MgCo2O4 powder prepared by _{coprecipitation} was added and stirred for 24 hours while heating to 60° C. The resulting suspension was centrifuged, and the precipitate was washed with water and centrifuged five times, after which it was suspended in water and collected.

Next, an aqueous solution of 2.4 mmol of iron nitrate nonahydrate was added to 555 mmol of water, and the mixture was stirred for 24 hours while being heated to 60°C. The resulting suspension was centrifuged, and the precipitate was washed with water and centrifuged five times, after which the precipitate was suspended in water, collected, and dried at 100°C for 24 hours. After drying, the precipitate was crushed in an agate mortar and placed in an alumina boat, and heated to 600°C at 200°C·h ^-1 in a tubular atmosphere furnace in which simulated air (nitrogen 80 mL·min ^-1 , oxygen 20 mL·min ^-1 ) was flowed, and then held for 1 hour to obtain the phosphate-iron composite modified sample ( _rP-Fe/MCO = 1) of Example 3. In addition, a phosphate-iron composite modified sample (r P-Fe/MCO = 2) of Example 4 was obtained by the same procedure, except that an aqueous solution in which 4.8 mmol of ( _{NH 4} ) ₂ HPO ₄ was dissolved in 555 mmol of water was added, and then 4.8 mmol of iron nitrate nonahydrate was dissolved in 555 mmol of water.

The results of cyclic voltammetry performed on the phosphate-iron composite-modified MgCo ₂ O 4 of Examples 3 and ₄ in the same manner as in Comparative Example 1 are shown in Figures 6 and 7. The results of charge/discharge measurements performed on the MgCo ₂ O ₄ of Examples 3 and 4 in the same manner as in Comparative Example 1 are shown in Figures 8 and 9. From these results, it can be seen that in both Examples 3 and 4, the phosphate-iron composite modification suppresses the increase in current at 3.5 V vs. Mg/Mg ²⁺ due to oxidative decomposition of the electrolyte during charging, improving the Coulombic efficiency.

(Comparative Example 2)
Charge and discharge measurements were performed on a composite electrode made of unmodified porous MgMn ₂ O ₄ powder by a dry method, which was prepared by a complex polymerization method according to the procedure in Non-Patent Document 4. The current value for charge and discharge measurements was 10 mA·g ^-1 , and charging was capacity-regulated at 135 mA·g ^-1 (13.5 hours), which is half of the theoretical capacity of MgMn ₂ O ₄ , 270 mA·g ^-1 . Other conditions were the same as those in Comparative Example 1.

Figure 10 shows the charge-discharge curves of unmodified MgMn ₂ O _4. During charging, a plateau was observed in the charge-discharge curve at 3.1-3.2 V vs. Mg/Mg ²⁺ , which is believed to be due to oxidative decomposition of the electrolyte. It can also be seen that the coulombic efficiency was significantly smaller than 1 (about 0.6) in the cycle where the plateau was observed.

A coated electrode was prepared using acetylene black as a conductive assistant and polyvinylidene fluoride (PVDF) as a binder in the following procedure. Acetylene black and N-methylpyrrolidone (NMP) were added to the porous MgMn ₂ O ₄ powder in that order, and each time they were added, they were kneaded for 10 minutes at 2000 rpm using a planetary mixer. A PVDF NMP solution (PVDF weight fraction 12%) was added to this and kneaded for 10 minutes in the same manner.

The amount of NMP added was 600 μL, the total weight of MgMn ₂ O ₄ , acetylene black, and PVDF was 500 mg, and the weight ratio was 80:10:10. The obtained slurry was applied to an aluminum foil with a thickness of 4 mil (101.6 μm). This was vacuum dried overnight at 80° C. using a vacuum dryer to obtain a coated electrode. The smoothness of the coated electrode was poor, and aggregates were observed here and there. In addition, the adhesion to the aluminum foil was weak, and it was easy to peel off.

(Examples 5 to 9)
Porous MgMn ₂ O ₄ whose surface was modified with phosphonic acid or phosphinic acid was prepared. In Examples 5 to 8, the modifying phosphonic acid was methylphosphonic acid (MePA), ethylphosphonic acid (EtPA), n-butylphosphonic acid (BuPA), or phenylphosphonic acid (PhPA), and in Example 9, the modifying phosphinic acid was diphenylphosphinic acid (Ph ₂ PA).

After dissolving 1 mmol of phosphonic or phosphinic acid in 1 g of water, 1.5 mmol ( _Ph2PA ) or 2.5 mmol (MePA, EtPA, BuPA, PhPA) of ammonia was added as 10% aqueous ammonia, and the resulting clear solution was dried at 80°C for 3 h to obtain diammonium phosphonate or ammonium phosphinate.

The ammonium salt was dissolved in 300 mmol of methanol, and then 2.5 mmol of porous _MgMn2O4 powder prepared by complex polymerization was added and stirred for 3 hours. The resulting suspension was centrifuged, and the precipitate was washed with methanol and centrifuged twice, and then suspended in ethanol and collected. The suspension was dried overnight at 60° _C to obtain a surface-modified active material powder.

11(a) and (b) show the ATR-FT-IR spectra of unmodified MgMn ₂ O ₄ and MgMn ₂ O ₄ modified with phosphonic acid, respectively. In the phosphonic acid modified sample, the absorption band assigned to the P-O stretching vibration (ν(P-O)) was observed at 950-1150 cm ^-1 , which indicates that a phosphonic acid layer was formed on the sample surface by the surface modification treatment. The absorption intensity of the P-O stretching vibration in the diphenylphosphonic acid modified sample was smaller than that of the other samples. In addition, the broad peak assigned to the O-H stretching vibration (ν(O-H)) of adsorbed water, observed at 3000-3500 cm ^-1 , was reduced by the phosphonic acid modification treatment. At the same time, a peak due to surface isolated OH groups appeared at about 3500 cm ^-1 , except for the diphenylphosphonic acid modified sample. These results show that the surface of the porous MgMn ₂ O ₄ was hydrophobized by the organic phosphonic acid modification, and the amount of adsorbed water was reduced.

12 to 15 show the results of charge/discharge measurements performed according to the procedure of Comparative Example 2 for the cases where methylphosphonic acid (MePA), n-butylphosphonic acid (BuPA), phenylphosphonic acid (PhPA), and diphenylphosphinic acid (Ph ₂ PA) were modified onto porous MgMn ₂ O ₄ powder, respectively.

The plateau of 3.1-3.2 V vs. Mg/Mg ²⁺ , which is thought to be due to oxidative decomposition of the electrolyte, was shorter than that of the unmodified sample and almost disappeared in the butylphosphonic acid-modified and phenylphosphonic acid-modified samples. In addition, the Coulombic efficiency of the butylphosphonic acid-modified and phenylphosphonic acid-modified samples was almost 1.

When a coated electrode was prepared for the phenylphosphonic acid modified sample according to the procedure of Comparative Example 2, the surface was smooth, no aggregates were observed, and the adhesion to the aluminum foil was good.

(Examples 10 to 12)
In addition, a coated electrode was prepared by the following procedure using carbon nanotubes (CNT) as a conductive assistant and polyvinylidene fluoride (PVDF) as a binder. NMP and a CNT NMP dispersion (CNT weight fraction 4%) were added to the phenylphosphonic acid-modified porous MgMn ₂ O ₄ powder in order, and each time they were added, they were kneaded for 10 minutes at 2000 rpm using a rotation and revolution mixer. A PVDF NMP solution (PVDF weight fraction 12%) was added to this and kneaded for 10 minutes in the same manner.

The amount of NMP added was 300 μL, the total weight of MgMn ₂ O ₄ , CNT, and PVDF was 150 mg, and the weight ratio was 98-w _CNT :w _CNT :2 (w _CNT =0.5, 1.0, 2.0). The cases where w _CNT was 0.5, 1.0, and 2.0 were designated as Examples 10 to 12, respectively. All of the coated electrodes were smooth, no aggregates were observed, and the adhesion to the aluminum foil was good.

About 2 mg of the obtained slurry was supported on a Pt mesh (80 mesh), and after drying in a vacuum at 80°C overnight, it was pressed to prepare a composite electrode. The results of charge/discharge measurements performed according to the procedure of Comparative Example 2 using this composite electrode as the working electrode are shown in Figures 16 to 18. The composite electrode with _{w CNT} = 2.0% had a charge/discharge capacity comparable to that of the acetylene black composite electrode (Figure 13). On the other hand, the capacity of the composite electrode with w CNT ≦ 1% was small, but this is thought to be due to the low CNT content and insufficient current collection. From these results, it can be seen that CNT can be used as a conductive assistant in the surface-modified MgMn ₂ O ₄ , and the weight fraction of the positive electrode active material in the positive electrode can be increased to 96%.

Example 13
A coated electrode of phenylphosphonic acid-modified porous MgMn ₂ O ₄ powder was prepared in the same manner as in Examples 10 to 12. The amount of NMP added was 800 μL, the total weight of CNT and PVDF was 400 mg, and the weight ratio was 92:4:4. The obtained coated electrode was smooth, no aggregates were observed, and the adhesion to the aluminum foil was good.

A 2032-type coin cell was fabricated consisting of a coated electrode positive electrode, an AZ31 alloy negative electrode, a glass fiber separator, and a 0.3 M Mg[B(HFIP) ₄ ] ₂ /G3 electrolyte, and a constant current charge-discharge test was performed at 25° C. with a current value of 10 mAhg ⁻¹ . The charge-discharge curve is shown in Figure 19. The initial discharge capacity was 70 mAhg ⁻¹ , and no significant capacity deterioration was observed even after four charge-discharge cycles.

(Comparative Example 3)
190 mg (2 mmol) of anhydrous magnesium chloride was dissolved in a mixed solvent of 25 mL of methanol and 25 mL of ethylene glycol dimethyl ether, and 723 mg (2 mmol) of tetrabutylammonium permanganate was added. After stirring for 30 minutes, water was added, and the mixture was centrifuged, washed, and dried to synthesize MgMn ₂ O ₄ powder. The molar ratio of Mg:Mn in MgMn ₂ O ₄ obtained by SEM-EDX analysis was 28.0:72.0.

30 mg of the obtained MgMn ₂ O ₄ powder was mixed with acetylene black and polytetrafluoroethylene (PTFE) in a weight ratio of 60:30:10 to obtain a composite electrode of 4 mg, which was then pressed onto an Al mesh (10 mmφ, 100 mesh) to prepare a positive electrode. The electrode was vacuum dried overnight at 120°C and then placed in a glove box. A 2032-type coin cell was prepared consisting of an AZ31 alloy negative electrode, a glass fiber separator, and a 0.3M Mg[B(HFIP) ₄ ] ₂ /G3 electrolyte, and a constant current discharge test was performed at 25°C with a current value of 10 mAg ^-1 and a discharge end potential of 0.1 V. The initial discharge capacity was 25 mAhg ^-1 .

(Example 14)
To 7 g (10 mmol) of tetrabutylammonium hydroxide (37% methanol solution), 790 mg (5 mmol) of phenylphosphonic acid was added and stirred for 1 hour, and then the solvent was removed by evaporation to obtain 3 g of ditetrabutylammonium phenylphosphonate.

190 mg (2 mmol) of anhydrous magnesium chloride was dissolved in a mixed solvent of 25 mL of methanol and 25 mL of ethylene glycol dimethyl ether, and 723 mg (2 mmol) of tetrabutylammonium permanganate was added. After stirring for 30 minutes, 135 mg (0.2 mmol) of ditetrabutylammonium phenylphosphonate was added and stirred for another 30 minutes. Water was added to the reaction solution, which was then centrifuged, washed, and dried to synthesize phenylphosphonic acid modified MgMn ₂ O ₄ powder.

The molar ratio of Mg:Mn:P of the phenylphosphonic acid modified MgMn ₂ O ₄ obtained by SEM-EDX analysis was 28.7:64.9:6.3. The charge-discharge measurement of the phenylphosphonic acid modified MgMn ₂ O ₄ was carried out in the same manner as in Comparative Example 3. The initial discharge capacity was 29 mAhg ^-1 .

Claims (9)

active material particles having a reactivity with an electrolyte and made of a compound of a transition metal element;
The active material includes, as a main component, a surface-modified active material having a first coating film made of phosphoric acid or a phosphoric acid derivative, the first coating film covering the surfaces of the active material particles,
The electrode active material is characterized in that the first coating comprises a phosphoric acid compound produced by reacting at least one of a phosphonic acid RPO(OH)2 containing an organic group R ^, _a phosphinic ^{acid R1R2PO} (OH) containing organic groups _R1 and R2 _, and a derivative thereof with a portion of the transition metal element . 2. The electrode active material of claim 1 , wherein at least one of the phosphonic acid RPO(OH) ₂ , the ^{phosphinic acid R1R2PO (} OH), and the derivatives thereof has a hydrocarbon group. 3. The electrode active material of claim 2, wherein at least one of the phosphonic acid RPO(OH) ₂ , the ^{phosphinic acid R1R2PO (} OH ) , and the derivatives thereof contains at least one of a phenyl group, a methyl group, an ethyl group, a propyl group, and a butyl group. The electrode active material according to any one of claims 1 to 3 , characterized in that the surface-modified active material further comprises a second coating film covering a surface of the first coating film and made of a compound having non-reactivity with an electrolyte. 5. The electrode active material according to claim 1, wherein the compound of the transition metal element is capable of inserting and desorbing lithium or magnesium. A secondary battery, characterized in that at least one of a positive electrode active material and a negative electrode active material is the electrode active material according to any one of claims 1 to 5 . A method for producing an electrode active material according to claim 5 ,
A first step of surface-modifying a powder of a compound of a transition metal element in a solution of a salt of phosphoric acid, phosphonic acid, phosphinic acid, or a derivative thereof;
and a second step of washing the surface-modified powder, recovering it, and drying it. 8. The method for producing an electrode active material according to claim 7 , further comprising a third step of heating the dried powder. The method for producing an electrode active material according to claim 8 , characterized in that the first step and the second step, or the first step, the second step, and the third step are sequentially repeated.

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