FR3104150A1 - Positive electrode active material - Google Patents

Abstract

The present invention relates to a positive electrode active material of the following formula (I): NaxLiyMn1-yO2 (I), wherein x is a number ranging from 0.8 to 1; y is a number strictly greater than 0 and less than or equal to 1/3.

Description

Positive electrode active material

The invention relates to the general field of rechargeable batteries, sodium-ion (Na-ion) and lithium-ion (Li-ion) batteries, in particular Na-ion batteries.

The invention relates more specifically to the positive electrode active materials, and the positive electrodes comprising them.

Prior techniques

Na-ion batteries represent one of the most promising alternative solutions to lithium-ion batteries, sodium being more interesting than lithium from an economic point of view, in particular because of its abundance and its low cost.

Intensive research has been done on positive electrodes for Na-ion batteries. This work led to a classification of positive electrodes into two main categories.

The first category contains polyanionic compounds. Among these polyanionic compounds, the compound Na ₃ V ₂ (PO ₄ ) ₂ F ₃ has been identified as suitable for use in Na-ion batteries. Indeed, it is characterized in particular by ease of synthesis, stability when used in humid conditions, stability in air during storage, or even high specific energy, such as document WO 2014/009710 describes it.

However, the presence of vanadium within the electrode can pose a problem during the use of the Na-ion battery in the medium/long term, given its toxic nature. Furthermore, although the best results are obtained with this polyanionic compound, the specific capacity of the latter is limited due to its relatively high molecular weight.

The second category contains the lamellar oxides of sodium. These particular oxides have the general formula Na _b MO ₂ , where b is less than or equal to 1, and M designates at least one transition metal. These lamellar oxides seem more promising than the polyanionic compounds because they notably have a lower molecular mass. They are also interesting for their structural and compositional richness and their ease of synthesis. Furthermore, the gravimetric energy density of the lamellar oxides of sodium is higher than that of the compound Na ₃ V ₂ (PO ₄ ) ₂ F ₃ (approximately 4.5 g/cm ³ vs. approximately 3 g/cm ³ ).

Thus, many works on lamellar sodium oxides have been undertaken, with particular attention given to the nature of the metals. Certain transition metals are preferably to be avoided, in particular metals such as V, Co and Ni because of their toxic character.

In this context, interesting materials have been identified, in particular the material Na _d Fe _0.5 Mn _0.5 O ₂ , where d is less than or equal to 1, such as the document “P ₂ -type Nax[Fe _1/2 Mn _1/2 ]O ₂ made from earth-abundant elements for rechargeable Na batteries”, N. Yabuuchi et al., Nature Materials, 11, 512-517 (2012), or the material Na _f MnO ₂ , where f is less than or equal to 1, as described in the document “Electrochemical Properties of Monoclinic NaMnO ₂ ”, X. Ma, H. Chen, G. Ceder, J. Electrochem. Soc., 158, A1307 (2011), and the document “β-NaMnO ₂ : A High-Performance Cathode for Sodium-Ion Batteries”, J. Billaud et al., J. Am. Chem. Soc., 136, 17243-17248 (2014).

However, despite the promise that these materials offer due to their high theoretical capacity (240 mAh/g), the capacity of these materials is found to deteriorate during charging and discharging cycles of the Na-ion battery. . These lamellar oxides are also very sensitive to humidity, which complicates storage and processing.

Thus, there is a need to develop new active materials of positive electrode making it possible to overcome the drawbacks mentioned above, in particular that of the deterioration of the capacity, and of the stability to humidity.

It has been discovered that a particular positive electrode active material allows to obtain an improved cell capacity comprising said active material, which does not deteriorate with the repetition of the charging and discharging cycles, with an improved stability to the 'humidity.

A subject of the invention is therefore a positive electrode active material of formula (I) below:
Na _x Li _y Mn _1-y O ₂ (I),
in which :
- x is a number ranging from 0.8 to 1;
- y is a number strictly greater than 0 and less than or equal to 1/3.

Another object of the invention is a process for manufacturing the active material according to the invention.

The invention also relates to a positive electrode comprising at least one active material according to the invention.

Another object of the invention is an Na-ion battery cell, comprising the electrode according to the invention and also an Na-ion battery comprising at least one Na-ion battery cell.

The invention also relates to a Li-ion battery cell, comprising the electrode according to the invention and also a Li-ion battery comprising at least one Li-ion battery cell as defined above.

Other advantages and characteristics of the invention will appear more clearly on examination of the detailed description and the appended drawings in which:

is a graph representing the capacity of several Na-ion battery cells, as a function of the number of charge and discharge cycles;

is a graph representing the voltage of a Na-ion battery cell, as a function of the number of charge and discharge cycles;

is a representation of two diffractograms of two active materials;

is a representation of two diffractograms of two active materials;

is a representation of two diffractograms of two active materials;

is a graph representing the voltage of a Li-ion battery cell, as a function of capacity.

It is specified that the expression “from… to…” used in this description of the invention must be understood as including each of the limits mentioned.

The positive electrode active material according to the invention is of formula (I) as mentioned above.

Preferably, y varies from varies from 0.1 to 1/3, preferably from 0.2 to 1/3, more preferably y is equal to 1/3.

According to a particular embodiment of the invention, x varies from 0.9 to 1, preferably x is equal to 1.

According to a particularly preferred mode of the invention, the material of formula (I) as defined above is of formula NaLi _1/3 Mn _2/3 O ₂ .

The invention also relates to a process for manufacturing the active material according to the invention comprising the following steps:
(a) mixing at least one transition metal salt or transition metals with at least one lithium precursor and at least one sodium precursor;
(b) heating the mixture obtained at the end of step (a) to a temperature ranging from 500 to 900° C.;
(c) recovering said material.

Preferably, the transition metal salt is a manganese salt.

Advantageously, the lithium precursor is chosen from an oxide, a peroxide, a salt and mixtures thereof, more preferably chosen from lithium carbonate, lithium nitrate, lithium acetate, lithium sulphate, lithium hydroxide, lithium, Li ₂ O, Li ₂ O ₂ and mixtures thereof.

According to a particular embodiment, the sodium precursor is chosen from an oxide, a peroxide, a salt and mixtures thereof, more preferably chosen from sodium carbonate, sodium nitrate, sodium acetate, sodium sulphate , soda, Na ₂ O, Na ₂ O ₂ .

According to a preferred embodiment, the mixture obtained at the end of step (a) is heated to a temperature ranging from 600 to 900°C.

Preferably, step (b) takes place for a period ranging from 6 hours to 20 hours, preferably from 6 hours to 16 hours, more preferably from 6 hours to 12 hours, in a particularly preferred way, from 8 hours.

According to a particular embodiment, step (b) is carried out under an argon flow.

Advantageously, step (b) is followed by a cooling step.

For example, the mixture is heated to 700°C in an oven for 8 hours, then cooled to a temperature less than or equal to 300°C, then removed from the oven.

Another object of the invention is a positive electrode comprising at least one active material according to the invention.

Preferably, the positive electrode according to the invention further comprises at least one conductive compound.

According to a particular embodiment, the conductive compound is chosen from metal particles, carbon, and mixtures thereof, preferably carbon.

Said metallic particles can be particles of silver, copper or nickel.

The carbon can be in the form of graphite, carbon black, carbon fibers, carbon nanowires, carbon nanotubes, carbon nanospheres, preferably carbon black.

In particular, the positive electrode according to the invention advantageously comprises the carbon black SuperP® marketed by Timcal.

Preferably, the content of active material according to the invention varies from 50 to 90% by weight, preferably from 70 to 90% by weight, relative to the total weight of the positive electrode.

Advantageously, the conductive compound content varies from 10 to 50% by weight, preferably from 10 to 30% by weight, more preferably from 15 to 25% by weight, relative to the total weight of the positive electrode.

The present invention also relates to a Na-ion battery cell comprising a positive electrode comprising the active material according to the invention, a negative electrode, a separator and an electrolyte.

Preferably, the battery cell comprises a separator located between the electrodes and acting as an electrical insulator. Several materials can be used as separators. Separators are generally made of porous polymers, preferably polyethylene and/or polypropylene. They can also be made of glass microfibers.

Advantageously, the separator used is a CAT No. 1823-070® glass microfiber separator marketed by Whatman.

Preferably, said electrolyte is liquid.

This electrolyte may comprise one or more sodium salts and one or more solvents.

The sodium salt or salts can be chosen from NaPF ₆ , NaClO ₄ , NaBF ₄ , NaTFSI, NaFSI, and NaODFB.

The sodium salt or salts are preferably dissolved in one or more solvents chosen from aprotic polar solvents, for example ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl and ethyl carbonate.

Advantageously, the electrolyte comprises propylene carbonate mixed with the sodium salt NaPF ₆ at 1M.

The present invention also relates to an Na-ion battery comprising at least one cell as described above.

The present invention also relates to a Li-ion battery cell comprising a positive electrode comprising the active material according to the invention, a negative electrode, a separator and an electrolyte.

The present invention also relates to a Li-ion battery comprising at least one Li-ion battery cell as described above.

The present invention is illustrated in a non-limiting manner by the following examples.

Examples

Example 1

I. Preparation of Na-ion electrochemical half-cells

1. Synthesis of active materials
1.1 Synthesis of the active material Na _2/3 Fe _0.5 Mn _0.5 O ₂
777.30 mg of Fe ₂ O ₃ , 768.50 mg of Mn ₂ O ₃ and 687.90 mg of sodium carbonate are added. The temperature is brought to 900° C. at the rate of 3° C. per minute, then the assembly is calcined at 900° C. for 12 hours in an oven. The mixture is then taken out of the oven.
This material is synthesized according to the process described in the document “P2-type Nax[Fe _1/2 Mn _1/2 ]O ₂ made from earth-abundant elements for rechargeable Na batteries”, N. Yabuuchi et al., Nature Materials, 11, 512-517 (2012).
This comparative active material is called material A.
1.2 Synthesis of the active material NaMnO ₂
1436.10 mg of Mn ₂ O ₃ and 964.16 mg of sodium carbonate are added. The temperature is brought to 700° C. at the rate of 3° C. per minute, then the assembly is calcined at 700° C. for 10 hours in an oven. The mixture is then taken out of the oven.
This comparative active material is called material B.
1.3 Synthesis of the active material NaLi _1/3 Mn _2/3 O ₂
1078.40 mg of Mn ₂ O ₃ , 103.00 mg of Li ₂ O, and 809.50 mg of Na ₂ O ₂ are added. The temperature is brought to 700° C. at a rate of 2° C. per minute, then the assembly is calcined at 700° C. for 8 hours in an oven under an argon stream. The mixture is then cooled to 300° C. at the rate of 1° C. per minute.
This active material according to the invention is called material C.
1.4 Synthesis of the active material NaLi _0.2 Mn _0.8 O ₂
617.78 mg of Na ₂ O, 59.56 mg of Li ₂ O, 693.30 mg of MnO ₂ and 629.40 mg of Mn ₂ O ₃ are added. The temperature is brought to 600° C. at the rate of 3° C. per minute, then the assembly is calcined at 600° C. for 8 hours in an oven under an argon stream. The mixture is then cooled to 300° C. at the rate of 1° C. per minute.
This active material according to the invention is called material D.
1.5 Synthesis of the active material NaLi _0.1 Mn _0.9 O ₂
70.29 mg of Li ₂ CO ₃ , 1488.60 mg of MnO ₂ and 1008.21 mg of Na ₂ CO ₃ are added. The temperature is brought to 900° C. at the rate of 3° C. per minute, then the assembly is calcined at 900° C. for 12 hours under a flow of argon in an oven. The mixture is then cooled to 300° C. at the rate of 1° C. per minute.
This active material according to the invention is called material E.

2. Preparation of positive electrodes
From materials A, B and C, three positive electrodes were prepared, respectively named EN-A, EN-B, and EN-C. Positive electrodes EN-A and EN-B are comparative electrodes. The positive electrode EN-C is an electrode according to the invention.
The positive electrode EN-A is made by mixing 80% by weight of the active material A, which has been transferred directly to the glove box from the oven without exposure to air, and 20% by weight of the carbon black SuperP® , the mixture then being ground for 15 minutes using an SPEX 8000M mixer.
The other positive electrodes EN-B and EN-C are made by mixing 80% by weight of the active material, B and C respectively, and 20% by weight of the SuperP® carbon black, the mixtures then being ground in the same way as for the positive electrode EN-A. In the same way as for active material A, active materials B and C were directly transferred into the glove box from the oven without exposure to air.

3. Assembly of the electrochemical half-cells
Three electrochemical half-cells were then prepared comprising respectively the positive electrodes EN-A to EN-C. The half-cells are named CE-A, CE-B and CE-C respectively.
The assembly of the electrochemical half-cells is carried out in a glove box using a device consisting of a 12 mm diameter Swagelok® connector. Each of the half-cells includes a separator, a negative electrode and an electrolyte.
3.1 Assembling the CE-A half-cell
positive electrode
A mass of 6 mg of the EN-A electrode, in the form of a powder, is then spread over an aluminum piston placed in the CE-A half-cell.
Separator
Three layers of CAT No. 1823-070® glass microfiber separator are used to prevent any short circuit between the positive electrode and the negative electrode during charge and discharge cycles. These separators are cut to a diameter of 12 mm and a thickness of 675 micrometers.
negative electrode
Pellets 10 mm in diameter are cut from a sheet of sodium metal. The pellet obtained is then glued by pressure to a stainless steel current collector. This collector is then deposited on the separating membrane in the half-cell.
Electrolyte
The electrolyte used comprises a solution composed of 1M NaPF ₆ dissolved in propylene carbonate.
3.2 Assembling the CE-B and CE-C half-cells
Positive electrodes
A mass of 6 mg each of electrodes EN-B and EN-C, respectively, in the form of a powder, is then spread on an aluminum plunger placed in the half-cells CE-B and CE-C, respectively .
The separators, negative electrodes and electrolytes are identical to those used in the CE-A half-cell.

II. Electrochemical tests
1. Comparative CE-A half-cell
Galvanostatic cycling is performed using a BioLogic cycler at a cycling rate of C/8. The capacitance of the CE-A half-cell was measured as a function of the number of cycles, at voltages ranging from 4.3 to 1.5 V, as shown in Figure 1. The evolution of the capacitance is observed on curve A. The charge and discharge potential window of this material is that which is described in the document “P2-type Nax[Fe _1/2 Mn _1/2 ]O ₂ made from earth-abundant elements for rechargeable Na batteries”, N. Yabuuchi et al., Nature Materials, 11, 512-517 (2012).
Thus, capacity degradation can be observed with charging and discharging cycles. A capacity of around 145 mAh.g ^-1 is measured after 20 cycles and around 106 mAh.g ^-1 after 40 cycles.
2. Comparative CE-B half-cell
Galvanostatic cycling is performed using a BioLogic cycler at a cycling rate of C/8. The capacitance of the CE-B half-cell was measured as a function of the number of cycles, at voltages ranging from 3.8 to 2.0 V as shown in Figure 1. The evolution of the capacitance is observed on the curve B. The charge and discharge potential window of this material is that which is described in the document “Electrochemical Properties of Monoclinic NaMnO ₂ ”, X. Ma, H. Chen, G. Ceder, J. Electrochem. Soc., 158, A1307 (2011).
Thus, a very rapid degradation of capacity can be observed with charging and discharging cycles. Indeed, after only 20 charge and discharge cycles, the measured capacity is already lower than that of the CE-A half-cell, after the same number of cycles, i.e. approximately 120 mAh.g ^-1 after 20 cycles .
3. CE-C half-cell according to the invention
Galvanostatic cycling is performed using a BioLogic cycler at a cycling rate of C/8. The capacitance of the CE-C half-cell was measured as a function of the number of cycles, at voltages ranging from 4.5 to 1.5 V, as shown in Figure 1. The evolution of the capacitance is observed on the C curve.
Thus, a capacity of around 170 mAh.g ^-1 is measured after 20 cycles, and around 173 mAh.g ^-1 after 40 cycles.
Compared to the capacities of the CE-A and CE-B half-cells, the capacity of the CE-C half-cell according to the invention is higher and more stable over the charging and discharging cycles.
Thus, the capacity of the half-cell comprising the active material according to the invention is improved.

Furthermore, the average potential of the half-cell CE-C was measured as a function of the number of cycles, as shown in FIG. 2. The evolution of the average potential is observed on the curve C1.
It is clearly seen that the average potential of the CE-C half-cell does not deteriorate with the repetition of the charging and discharging cycles, and is therefore stable.

Example 2

I. Synthesis of active materials

The active materials A, B and C are synthesized according to the synthetic methods described above in example 1.

II. Evaluation of sensitivity to humidity

1. Comparative Material A

Material A, after synthesis, is characterized using a diffractogram, as indicated in FIG. 3 (curve A1).
Material A was also characterized using a diffractogram, after having been washed with water, as indicated in FIG. 3 (curve A2). Material A was washed in water with a ratio of 100 mg of powder in 10 mL of water, for 30 minutes, before being dried at 80° C. under vacuum overnight.
It is observed that this material A is very sensitive to humidity. Indeed, the two diffractograms do not overlap. One can in fact observe differences, materialized by the symbol # in FIG. 3, between the two diffractograms, thus denoting structural changes.

2. Comparative Material B

Material B, after synthesis, is characterized using a diffractogram, as indicated in FIG. 4 (curve B1).
Material B was also characterized using a diffractogram, after having undergone washing with water according to the same protocol as that described for material A, as indicated in FIG. 4 (curve B2).
It is also observed that this material B is very sensitive to humidity, much more sensitive than material A. Indeed, the two diffractograms absolutely do not overlap. Many differences can indeed be observed between the two diffractograms, thus indicating structural changes.

3. Material C according to the invention

Material C, after synthesis, is characterized using a diffractogram, as indicated in FIG. 5 (curve C2).
Material C was also characterized using a diffractogram, after having undergone washing with water according to the same protocol as that described for material A, as indicated in FIG. 5 (curve C3).
It is clearly observed that this material is not sensitive to humidity. Indeed, the two diffractograms are superimposed.

Thus, the material according to the invention has improved stability to humidity, compared to the materials of the prior art.

Example 3

I. Preparation of a Li-ion electrochemical half-cell

The material C according to the invention and the positive electrode were manufactured according to the methods described above in Example 1.
The assembly of the half-cell is carried out in a glove box using a device consisting of a 12 mm diameter Swagelok® coupling. The half-cell includes a separator, a negative electrode and an electrolyte.

positive electrode
A mass of 6 mg of the positive electrode, in the form of a powder, is then spread over an aluminum piston placed in the half-cell.

Separator
Two layers of CAT No. 1823-070® glass microfiber separator are used to prevent any short circuit between the positive electrode and the negative electrode during charge and discharge cycles. These separators are cut to a diameter of 12 mm and a thickness of 675 micrometers.

negative electrode
Pellets 10 mm in diameter are cut from a sheet of lithium metal. The pellet obtained is then glued by pressure to a stainless steel current collector. This collector is then deposited on the separating membrane in the half-cell.

Electrolyte
The electrolyte used comprises a solution composed of 1M of LiPF ₆ dissolved in a 50/50 mixture by volume of ethylene carbonate and dimethyl carbonate.

II. Electrochemical test

A cycling process comprising the application of a plurality of charge and discharge cycles at voltages ranging from 2 to 4.8 V, was carried out, at a cycling rate of C/10.
Half-cell voltage was measured as a function of capacitance, as shown in Figure 6 (curve C4).
Thus, it is observed that the capacity of the half-cell is stable with the repetition of charging and discharging cycles.
The capacity of the half-cell does not deteriorate with repeated charging and discharging cycles.
Consequently, the material C according to the invention can be an active material for a Li-ion battery, with good electrochemical behavior.

Claims (12)

Positive electrode active material of formula (I) below:
Na _x Li _y Mn _1-y O ₂ (I),
in which:
- x is a number ranging from 0.8 to 1;
- y is a number strictly greater than 0 and less than or equal to 1/3. Material. according to claim 1, characterized in that y varies from 0.1 to 1/3, preferably from 0.2 to 1/3, more preferably y is equal to 1/3. Material according to Claim 1 or 2, characterized in that x varies from 0.9 to 1, more preferably x is equal to 1. Process for manufacturing the active material as defined in any one of the preceding claims, comprising the following steps:
(a) mixing at least one transition metal or transition metal salt with at least one lithium precursor and at least one sodium precursor;
(b) Heating the mixture obtained at the end of step (a) to a temperature ranging from 500 to 900° C.;
(c) Recover said material. Positive electrode comprising at least one active material as defined in any one of claims 1 to 3. Positive electrode according to Claim 5, characterized in that it also comprises at least one conductive compound. Positive electrode according to Claim 6, characterized in that the conductive compound is chosen from metallic particles, carbon, and mixtures thereof, preferably carbon. Positive electrode according to the preceding claim, characterized in that the carbon is in the form of graphite, carbon black, carbon fibres, carbon nanowires, carbon nanotubes, carbon nanospheres, preferably black of carbon. A sodium-ion battery cell comprising a positive electrode as defined in any one of claims 5 to 8, a negative electrode, a separator and an electrolyte. Sodium-ion battery comprising at least one cell as defined in the preceding claim. A lithium-ion battery cell comprising a positive electrode as defined in any one of claims 5 to 8, a negative electrode, a separator and an electrolyte. Lithium-ion battery comprising at least one cell as defined in the preceding claim.