WO2016097516A1

WO2016097516A1 - Lithium accumulator and its preparing process

Info

Publication number: WO2016097516A1
Application number: PCT/FR2015/053246
Authority: WO
Inventors: Jean-Frédéric Martin; Hugo MAUCHAMP
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2014-12-16
Filing date: 2015-11-27
Publication date: 2016-06-23
Also published as: FR3030123A1

Abstract

The present invention relates to a process for preparing a lithium accumulator according to the following steps: preparing a positive-electrode ink comprising carbon black, a polymer binder and a lithium-containing positive-electrode active material; depositing this positive-electrode ink on a first current collector; preparing a negative-electrode ink comprising a negative-electrode active material and a polymer binder; depositing this negative-electrode ink on a second current collector; forming a stack comprising in succession the positive electrode, an electrode separator, and the negative electrode; introducing an electrolyte into the electrode separator; and obtaining the lithium accumulator; characterised in that the carbon black of the positive electrode is obtained beforehand by: treating carbon black with an acid; rinsing the acid-treated carbon black with water; and drying the acid-treated carbon black. The invention also relates to the lithium accumulator obtained with this process and its use at a potential higher than 4.5 V versus the potential of the Li/Li⁺ couple.

Description

LITHIUM ACCUMULATOR AND PROCESS FOR PREPARING SAME

FIELD OF THE INVENTION The present invention relates to a lithium battery whose positive electrode comprises carbon black having previously undergone acid treatment. This type of accumulator is particularly adapted to operate at a potential greater than 4.5 volts. PRIOR STATE OF THE TECHNIQUE

Lithium batteries typically include the following stack:

a positive electrode;

an electrode separator comprising an electrolyte;

a negative electrode.

These accumulators operate by insertion and de-insertion of lithium ions between the electrode materials. Lithium ions pass from one electrode to another via the electrolyte which, on the other hand, does not allow the conduction of electrons. The electrons are routed to an external circuit by means of current collectors associated with the electrodes.

In order to optimize the conduction of electrons, the electrodes also include conductive additives such as carbon black. However, these additives promote the degradation of the electrolyte when the battery is used at a high potential, for example above 4.5 volts. Li.

Under these conditions of use, Zhang et al (Journal of Power Sources, 2013, 227) highlighted:

the deterioration of the reversibility of the capacity resulting from the decomposition of the electrolyte (LiPF ₆ ) and the intercalation of PF ₆ ^" ions;

the degradation of the stability in cycling.

Moreover, Zhang et al (Journal of Power Sources, 2013, 227) have also observed that increasing the number of oxygen atoms on the carbon surface (in this case graphene) decreases its performance. In order to improve the performance of conducting materials regularly used in the electrodes, heat treatments under inert gas have been developed (US 2005/0063893). In any event, there is still a need to improve the properties and stability of the electronic conductors used in the accumulators, especially for a high operating potential.

The present invention solves this problem, thanks to the use of specific carbon black which drastically reduces high potential degradation phenomena.

SUMMARY OF THE INVENTION The Applicant has developed a lithium accumulator implementing a conductive additive allowing its use at high potential without reducing its life.

The properties of this conductive additive arise directly from a pretreatment in an acidic solution.

More specifically, the present invention relates to a method for preparing a lithium battery comprising the following steps:

preparing a positive electrode ink comprising carbon black, a polymeric binder and a lithiated positive electrode active material;

depositing this positive electrode ink on a first current collector; preparing a negative electrode ink comprising an active material, preferably lithiated, negative electrode and a polymeric binder;

depositing this negative electrode ink on a second current collector; forming a stack successively comprising the positive electrode, an electrode separator, and the negative electrode;

introducing an electrolyte into the electrode separator;

obtaining the lithium battery.

According to the invention, the carbon black of the positive electrode is obtained beforehand by:

optionally washing carbon black with water;

acid treatment of the optionally washed carbon black; rinsing the acid-treated carbon black with water, preferably de-ionized water;

drying acid treated carbon black. In this accumulator, the first and second current collectors are of course in contact with the face of the electrode opposite to the electrode separator.

The invention also relates to the lithium battery resulting from this process. Advantageously, it is a lithium ion battery. It is not a lithium-air accumulator.

In general, the positive electrode comprises at least:

75 to 98% by weight of a positive electrode active material, preferably 85 to 98% by weight, based on the weight of the electrode; and

at least 1 to 15% by weight of acid-treated carbon black, advantageously 1 to 5% by weight, relative to the weight of the electrode.

The carbon black of the positive electrode has properties resulting from the acid treatment.

This acid treatment modifies the surface of the carbon black, in particular by increasing the number of surface oxygen atoms and the acidification of these groups. The carbon black is advantageously treated by immersion in an aqueous solution comprising an acid chosen from the group comprising nitric acid HNO ₃ , sulfuric acid H ₂ SO ₄ , hydrochloric acid HCl, hydrobromic acid HBr, and hydrofluoric acid HF. Advantageously, the acid is nitric acid.

The pH of the acid solution is advantageously less than 4, more preferably less than 2. According to a particular embodiment, 1 to 200 grams of carbon black are immersed in the acid solution per liter of acid solution, more preferably 10 to 100 grams. During the treatment, the pH is maintained between the values indicated above.

The duration of the acid treatment of the carbon black is advantageously between 12 and 48 hours, more advantageously between 24 and 36 hours.

The temperature of the acid treatment of the carbon black is advantageously between 0 and 40 ° C., more advantageously between 20 and 30 ° C.

Once the carbon black has been treated with an acid solution, it is separated from the acid solution, advantageously by filtration. The acid-treated carbon black is then rinsed and dried.

As already indicated, the rinsing is advantageously carried out with deionized water. Drying can be carried out at a temperature between 80 and 200 ° C. It can especially be carried out under vacuum or in an oven.

According to a particular embodiment, the acid treatment of the carbon black may comprise the following steps:

- Prepare an acid solution as described above;

place carbon black in this acid solution, preferably a nitric acid solution;

maintaining the carbon black in the acid solution with mechanical stirring for a period of between 5 and 10 hours, and at a temperature advantageously between 0 and 40 ° C;

optionally, leaving the carbon black in the acid solution without mechanical agitation for a period of between 12 and 48 hours, and at a temperature advantageously between 0 and 40 ° C;

filter the carbon black;

- Rinse the carbon black, preferably with water that can be deionized;

to dry the carbon black. The carbon black, before or after the acid treatment, is advantageously in the form of a powder. Its specific surface area (BET, Brunauer-Emmett-Teller) is advantageously between 10 and 800 m ² / g, and even more advantageously between 20 and 100 m ² / g.

As already indicated, the positive electrode also comprises an active material. It is a lithiated material, that is to say a material comprising lithium. It can in particular be chosen from the group comprising LMNO

; L1COPO4; Li P0 ₄ ; Li ₃ V ₂ (PO ₄ ) 3; Li ₂ CoP ₂ O ₇ ; Li ₂ MnP ₂ O ₇ ; Li ₂ CoPO ₄ F; Li ₂ NiPO ₄ F; LiCoS0 ₄ F; LiNiSO ₄ F; LiCuS0 ₄ F; LiCoOSO ₄ ; Li OS0 ₄ ; Li ₂ NiSiO ₄ ; LiNi 0.5Mn 1.5 O 4; LiCr _y Mn ₂ _ _y 0 ₄ with y between 0.5 and 1 inclusive; LiCo _y Mn ₂ _ _y 0 ₄ with y between 0.5 and 1 inclusive; LiFe 0.5 Mn 1.5 O 4; LiCuo.5Mn1.5O4; and L1NIVO4.

The electrode separator makes it possible to ensure the reversible passage of the lithium ions between the positive electrode and the negative electrode.

This is usually an electrode separator soaked with electrolyte.

The separator may in particular be a material chosen from the group comprising polyolefins, in particular polyethylene or polypropylene; PVDF; and POE (poly (oxyethylene)). It may be a non-woven polypropylene for example.

The separator is electronic insulator but ionic conductor thanks to its electrolyte. The separator comprises an electrolyte which is generally a composition containing a lithium salt. The electrolyte may in particular be a composition comprising: a lithium salt which may be chosen from the group comprising in particular

LiPF ₆ ; L1CIO ₄ ; LiAsF ₆ ; LiBF ₄ ; LiTFSI; LiBETI; and LiBOB;

a solvent which may be selected from the group consisting of carbonates such as ethylene carbonate, methyl carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, and mixtures thereof; nitriles; esters, ethers; and sulfones.

The electrolyte may also include an additive such as VC (vinylenecarbonate) or FEC (fluoroethylenecarbonate) In general, the negative electrode comprises at least one active material. It is a preferably lithiated material, that is to say a material comprising lithium. It may especially be chosen from the group comprising LTO (Li.iTi ₅ 0 ₁₂ ); silicon; graphite carbon; and tin.

The negative electrode may also comprise at least one additive that may be selected from the group comprising: carbon black, carbon fiber, graphene, and NTC (carbon nanotubes). It can include carbon black. In this case, the carbon black is not necessarily treated with an acidic solution such as that of the positive electrode.

The positive and negative electrodes are made according to the conventional techniques forming part of the knowledge of those skilled in the art. They can in particular be made by depositing on a current collector by inkjet printing or coating, for example.

The electrode inks used generally comprise at least:

an electrode active material;

a binder;

a solvent;

an electronic conductive additive, in this case acid-treated carbon black for the positive electrode. The binder of the electrodes may in particular be chosen from the group comprising: PVDF (polyvinylidene fluoride), CMC (carboxymethylcellulose), Latex-SBR (styrene-butadiene SBR), and polyacrylic acid (styrene-butadiene rubber) . The solvent of the electrode inks may especially be chosen from the group comprising: water, NMP (N-methyl-2-pyrrolidone), acetone, DMSO (dimethylsulfoxide), MEK (methyl ethyl ketone).

The electronic conductive additive can be chosen especially from the group comprising carbon black, carbon fiber, graphene, and NTC (carbon nanotubes). In addition to the electrode active material and the acid-treated carbon black, the inks of the positive and negative electrodes do not necessarily include the same binders, solvents and conductive additives. The accumulator according to the invention differs from those of the prior art in that the positive electrode comprises a specific conductive additive. As already indicated, the properties of this additive, acid-treated carbon black, result directly from its acidic treatment process. Without making any hypothesis, the Applicant considers that this acid treatment makes it possible to modify the surface functions of the carbon black with the consequence of modifying:

the double layer capacitance, which could be an obstacle for the molecules likely to be oxidized;

- the interaction of carbon black and metal ions;

carbon black by removing impurities.

The accumulator according to the present invention is particularly adapted to operate at a potential greater than 4.5 V with respect to the potential of the Li / Li + pair, more advantageously between 4.8 and 5.5.

Thus, the invention also relates to the use (or method of operation) of this accumulator at such a potential. DESCRIPTION OF THE FIGURES

The invention and the advantages thereof will appear more clearly from the following figures and examples given to illustrate the invention and not in a limiting manner.

Figure 1 illustrates the thermal analysis curves of the carbon black according to the invention and according to a counterexample, showing the loss of CO.

FIG. 2 illustrates the thermal analysis curves of the carbon black according to the invention and according to a counterexample, showing the loss of CO ₂ .

FIG. 3 illustrates the cyclic voltammetry curves (tenth cycle) between 3.5 V and 5 V for carbon black according to the invention and according to a counterexample.

FIG. 4 illustrates the specific discharge capacity, as a function of the number of cycles, of a battery comprising an electrode based on LMNO and carbon black according to the invention and according to a counterexample. FIG. 5 illustrates the coulombic efficiency, as a function of the number of cycles, of a cell comprising an electrode based on LMNO and carbon black according to the invention and according to a counterexample. EXAMPLES OF CARRYING OUT THE INVENTION

Acid treatment of carbon black (C-LNV)

8 grams of carbon black (Super P) are washed with 3 liters of deionized water on a Buchner type filter. The carbon black is then immersed in an acid solution (2M, HN0 ₃ ) with a ratio of 10 mL of acid solution per gram of carbon black.

This mixture (carbon black / acid solution) is kept under mechanical stirring for 8 hours and then allowed to stand for 24 hours, all at room temperature.

At the end of this acid treatment, the carbon black is filtered on a Buchner-type filter and rinsed with the de-ionized water until the filtrate is stabilized.

The carbon black is then dried under vacuum at 110 ° C for at least 10 hours.

Characterization of acid-treated carbon black (C-INV) The acid treatment does not fundamentally modify the specific surface area of the carbon black (63.6 m ² / g against 61.3 m ² / g before treatment).

The PZC (zero energy point) calculated according to the conventional titration techniques is equal to 6 for the untreated carbon black while it is equal to 10 for the carbon black treated with the nitric acid solution. The increase in PZC after the acid treatment corresponds to a greater presence of acid sites on the surface of the carbon black.

PZC is obtained by conventional techniques, by mass titration.

Thermogravimetric analysis coupled with mass spectrometry (ATG-MS) shows a greater presence of carbonyl groups (CO) from 700 ° K for the acid-treated carbon black according to the invention (C-INV) relative to the untreated carbon black (C-CE) (Figure 1). In general, this corresponds to a relative increase in ether, carbonyl and / or quinone surface groups. Figure 2 shows the increase in C0 ₂ starting between 400 ° K and 700 ° K (right) related to the increased presence of carboxyl groups on the surface.

Analysis by EDX (energy dispersive analysis spectroscopy) shows the increase in oxygen content on carbon black, from 1.43% to 3.18% after acid treatment.

In summary, the acid-treated carbon black has a greater amount of oxygenated surface function and acidification of these groups, relative to the untreated carbon black. Button cell preparation - Impact of acid treatment on high potential electrochemistry

To appreciate the effect of acid treatment on carbon black, six button cells (Examples 1-6) were prepared.

Positive electrode Negative electrode

Examples Separator

(% by weight) (% by weight)

1-INV Super P-A (50) Super P-A (50)

S

(FIGS. 1 and 2) PVdF (50) PVdF (50)

2-CE Super P (50) Super P (50)

S

(FIGS. 1 and 2) PVdF (50) PVdF (50)

3-INV Super P-A (50)

S lithium metal (100) (Figure 3) PVdF (50)

4-CE Super P (50)

S lithium metal (100) (Figure 3) PVdF (50)

LTO (89)

LMNO (80)

5-INV Super P (5)

Super P-A (10) S

(Figures 4 and 5) CMC (2)

PVdF (10)

latex (4)

LTO (89)

LMNO (80)

6-CE Super P (5)

Super P (10) S

(Figures 4 and 5) CMC (2)

PVdF (10)

latex (4) Table 1: Composition of button cells according to Examples 1-6

INV = example according to the invention

CE = counterexample

S = two porous polypropylene membranes (Celgard 2400) impregnated with an electrolytic solution (1M LiPF ₆ in a mixture EC: EMC: DMC 1/1/1 vol).

The EC, EMC and DMC components of the electrolyte correspond respectively to ethylene carbonate, dimethyl carbonate, and ethylmethyl carbonate.

LMNO = LiNi 4Mni, ₆ 0 ₄

CMC = carboxymethylcellulose

Latex = emulsion in styrene-butadiene water SBR (styrene butadiene rubber)

PVdF = polyvinylidene fluoride

Super P-A = carbon black having undergone acid treatment according to the invention

Super P = conventional carbon black

The electrodes of the button cells according to Examples 1 and 2 were prepared according to the following steps:

preparing a mixture of acid-treated and non-acid-treated carbon black and PVdF in N-methyl-2-pyrrolidone (NMP). The carbon black (1 g) is first mixed with 8.33 g of a 12% by weight solution of PVdF in NMP. In order to obtain a satisfactory viscosity, 15 g of NMP are added to the ink;

coating (150μιη wet height) of this mixture on an aluminum foil;

drying the coating;

obtaining the electrode which is not calendered later. The batteries according to Examples 1 and 2 were mounted in order to achieve symmetrical impedances (Figures 1-2 and Table 2). The difference in mass of the electrodes between these two examples is less than 10% by weight.

As shown in Table 2, the acid treatment hardly modifies the transfer resistance R2 and the double layer capacity Cd1. On the other hand, it modifies the pseudo capacitance Q and the factor n. The reduction in n-factor after acid treatment corresponds to a decrease in surface homogeneity. In other words, the acid treatment does not change the surface of the carbon black continuously, which may be due to the natural asperities and hydrophobicity of the carbon black.

The increase in pseudo capacitance corresponds to a greater number of oxygen functions at the surface of the carbon black.

Table 2: R2 transfer resistance, pseudo capacitance Q, n factor and Cdl double layer capacity of carbon black treated according to the invention or not

The data in Table 2 are either measured or modeled variables from the electrochemical impedance results.

The double layer capacity Cdi is calculated from the formula Cdi = R2 ^{(1 "}

_γ Ά i _will _eur Cdi being with respect to the electrode surface.

In addition, the RI electrolyte resistance is not useful for this experiment. It is identical in both cases.

In order to test the potential strength of the acid-treated carbon black according to the invention, cyclic voltammetries were carried out on the positive electrodes of Examples 3 and 4 vs. Li metal between 3.5V and 5V at O.lmV / s.

Figure 3 shows that the oxidation current for Example 3 (INV) begins to drop from the reference to around 4.8V and the difference reaches 20% at 4.9V and about 15% at 5V. This proves that the acid treatment inhibited some of the oxidation reactions occurring at very high potentials (> 4.8V). Impact when using acid-treated carbon black as a conductive additive in a lithium battery

The battery electrodes according to Examples 5 and 6 were prepared by coating an aluminum foil with a mixture containing the components of Table 1 in NMP.

The potential of the LTO material platter is 1.55V. The N / P balancing (corresponding to the ratio of the negative electrode capacitances on the positive electrode) of the mounted cells has been set to 1.3 in order to ensure that, during charging, the positive electrode reaches the cutoff potential. Thus, during a charge at 3.45V vs. LTO, the positive electrode can go up to 5V. Fig. 4 corresponds to the specific discharge capacity for cycling of 15 cycles by gradually increasing the breaking potential every 5 cycles. We can observe that the passage of a cut from 3.25V to 3.45V vs. LTO saves just over 5% discharge capacity for the battery. The difference between a treated super P and an untreated is not significant.

On the other hand, the effect related to the acid treatment appears more clearly in FIG. 5 which corresponds to coulombic efficiency as a function of the number of cycles (for reasons of reproducibility, two series of tests were carried out for Examples 5 and 5). 6 of Figure 5).

The coulombic efficiency is almost identical for the carbon black according to the invention and for the untreated acid carbon black during the cycling up to 4.25V. It drops drastically in the case of untreated carbon black (Example 5) when changing to 4.35V then to 4.45V to stabilize below 98%.

For the acid-treated carbon black (Example 6), coulombic efficiency is little affected by the increase in potential and ends at 99%. These examples confirm that the acid treatment of the carbon black before its use in an electrode makes it possible to limit electrolyte degradations when the potential exceeds 4.8V.

Claims

A method of preparing a lithium battery comprising the steps of:

preparing a positive electrode ink comprising carbon black, a polymeric binder and a lithiated positive electrode active material; depositing this positive electrode ink on a first current collector;

preparing a negative electrode ink comprising a negative electrode active material and a polymeric binder;

depositing this negative electrode ink on a second current collector;

forming a stack successively comprising the positive electrode, an electrode separator, and the negative electrode;

introducing an electrolyte into the electrode separator;

obtaining the lithium battery;

characterized in that the carbon black of the positive electrode is previously obtained by:

acid treatment of carbon black;

rinsing acid-treated carbon black with water;

drying acid treated carbon black.

A process for preparing a lithium battery according to claim 1, characterized in that the acid treatment comprises immersing the carbon black in an aqueous solution comprising an acid selected from the group consisting of nitric acid, sulfuric acid, hydrochloric acid, hydrobromic acid, and hydrofluoric acid.

Process for the preparation of a lithium battery according to claim 1 or 2, characterized in that the acidic treatment is carried out for a period of between 12 and 48 hours.

4. Process for the preparation of a lithium battery according to one of claims 1 to 3, characterized in that the acid treatment is carried out at a temperature between 0 and 40 ° C. Process for the preparation of a lithium battery according to one of Claims 1 to 4, characterized in that the acidic treatment of the carbon black comprises the following steps:

place carbon black in a nitric acid solution;

filter the carbon black;

rinse the carbon black with deionized water;

to dry the carbon black.

A method of preparing a lithium battery according to one of claims 1 to 5, characterized in that the lithiated active material of the positive electrode is selected from the group consisting of LfNi0, 4Mni _i6 04; L1COPO4; LiNiPO ₄ ; Li ₃ V ₂ (PO ₄ ) 3; Li ₂ CoP ₂ O ₇ ; Li ₂ MnP ₂ O ₇ ; Li ₂ CoPO ₄ F; Li ₂ NiPO ₄ F; LiCoS0 ₄ F; LiNiSO ₄ F; LiCuS0 ₄ F; LiCoOSO ₄ ; LiNiOSO ₄ ; Li ₂ NiSiO ₄ ; LiNi 0.5Mn 1.5 O 4; LiCr _y Mn ₂ _ _y 04 with from 0.5 to 1 inclusive; LiCo _y Mn ₂ _ _y 04 with from 0.5 to 1 inclusive; LiFe 0.5 Mn 1.5 O 4; LiCuo.5Mn1.5O4; and LiNiV0 ₄ .

A method of preparing a lithium battery according to one of claims 1 to 6, characterized in that the electrode separator is made of a material selected from the group consisting of polyolefins; polyvinylidene fluoride; and poly (oxyethylene).

Process for the preparation of a lithium battery according to one of Claims 1 to 7, characterized in that the electrolyte comprises:

a lithium salt selected from the group consisting of LiPF ₆ ; L1CIO ₄ ;

LiAsFg; LiBF ₄ ; LiTFSI; LiBETI; and LiBOB;

a solvent selected from the group consisting of; ethylene carbonate, methyl carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, and mixtures thereof; nitriles; esters, ethers; and sulfones. 9. A method of preparing a lithium battery according to one of claims 1 to 8, characterized in that the lithiated active material of the negative electrode is selected from the group comprising Li.iTi ₅ 0 ₁₂ ; silicon; graphite carbon; and tin.

10. Lithium accumulator obtained by the method according to one of claims 1 to 9.

11. Use of the lithium battery according to claim 10 at a potential greater than 4.5 V relative to the potential of the Li / Li + pair, more preferably between 4.8 and 5.5.