METHOD FOR DELITHIATING AT LEAST ONE LITHIUM AND TRANSITION-METAL NITRIDE

Abstract

A method for delithiating a lithium and transition metal nitride. The method involves mixing an oxidising agent with the lithium and transition metal nitride and recovering the material obtained. The transition metal may be Mn, Fe, Co, Ni, Cu, or a mixture thereof. The material obtained by the method may be used as a negative electrode material for a lithium-ion battery.

Claims

1. A method for delithiating a lithium and transition metal nitride, the method comprising: a) mixing an oxidising agent with the lithium and transition metal nitride; and b) recovering a material obtained at the end of a).

2. The method of claim 1, wherein the transition metal is at least one selected from the group consisting of Mn, Fe, Co, Ni, Cu, and mixtures thereof.

3. The method of claim 1, wherein the lithium and transition metal nitride is present as at least one material having a formula selected from the group consisting of Li.sub.7MnN.sub.4, Li.sub.3FeN.sub.2, Li.sub.2.6Co.sub.0.4N, Li.sub.2.0Ni.sub.0.67N, and Li.sub.2.57Cu.sub.0.43N.

4. The method of claim 1, wherein the lithium and transition metal nitride is of formula Li.sub.7MnN.sub.4 or of formula Li.sub.3FeN.sub.2.

5. The method of claim 1, wherein the oxidising agent comprises a cobaltocenium salt.

6. The method of claim 1, wherein the molar ratio between the oxidising agent and the lithium and transition metal nitride is 0.5 to 3.

7. The method of claim 1, wherein a) is carried out in the presence of a solvent.

8. The method of claim 7, wherein the solvent is an aprotic organic solvent.

9. The method of claim 7, wherein the solvent is at least one selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl and methyl carbonate, and mixtures thereof.

10. A negative electrode active material for a lithium-ion battery, comprising the material obtained by the method of claim 1.

11. The method of claim 5, wherein the oxidising agent comprises at least one selected from the group consisting of cobaltocenium hexafluorophosphate, cobaltocenium tetrafluoroborate, bis(pentamethylcyclopentadienyl)cobalt hexafluorophosphate, bis(pentamethylcyclopentadienyl)cobalt tetrafluoroborate hexafluorophosphate, and mixtures thereof.

12. The method of claim 8, wherein the solvent comprises at least one selected from the group consisting of acetonitrile, tetrahydrofuran, dimethylformamide, dichloromethane, ethyl acetate, and mixtures thereof.

Description

[0020] Other advantages and features of the invention will become clearer on examination of the detailed description and the accompanying drawings in which:

[0021] FIG. 1 shows diffractogram of the Li.sub.7MnN.sub.4 material and the material obtained at the end of the method according to the invention;

[0022] FIG. 2A is a scanning electron microscope image of the Li.sub.7MnN.sub.4 material;

[0023] FIG. 2B is a scanning electron microscope image of a material obtained by the method according to the invention;

[0024] FIG. 3 is a graph representing the galvanostatic curve of a material obtained by the method according to the invention against Li metal.

[0025] It should be noted that the expression from . . . to . . . used in this description of the invention should be understood to include each of the limits mentioned.

[0026] The expression at least one means one or more.

[0027] As indicated above, according to step a) of the method according to the invention, at least one oxidising agent is mixed with the lithium and transition metal nitride.

[0028] Advantageously, the transition metal or metals are selected from Mn, Fe, Co, Ni, Cu and mixtures thereof.

[0029] Preferably, said lithium and transition metal nitride is selected from materials of formula Li.sub.7MnN.sub.4, Li.sub.3FeN.sub.2, Li.sub.2.6Co.sub.0.4N, Li.sub.2.0Ni.sub.0.67N and Li.sub.2.57Cu.sub.0.43N.

[0030] More preferably, said lithium and transition metal nitride is of formula Li.sub.7MnN.sub.4 or of formula Li.sub.3FeN.sub.2.

[0031] Preferably, said oxidising agent is selected from those belonging to the family of metallocenes in oxidised form.

[0032] Advantageously, said oxidising agent is selected from cobaltocenium salts, preferably from cobaltocenium hexafluorophosphate, cobaltocenium tetrafluoroborate, bis(pentamethylcyclopentadienyl)cobalt hexafluorophosphate, bis(pentamethylcyclopentadienyl)cobalt tetrafluoroborate hexafluorophosphate and mixtures thereof.

[0033] More preferably, said oxidising agent is selected from cobaltocenium hexafluorophosphate.

[0034] According to a preferred embodiment, the molar ratio between said oxidising agent and said lithium and transition metal nitride ranges from 0.5 to 3, preferably from 1 to 2.

[0035] In a preferred manner, step a) is performed in the presence of at least one solvent.

[0036] Advantageously, the solvent is selected from aprotic organic solvents, preferably from acetonitrile, tetrahydrofuran, dimethylformamide, dichloromethane, ethyl acetate and mixtures thereof, preferably from acetonitrile.

[0037] According to another embodiment, any solvent that can be used in a Li-ion battery electrolyte can also be used, preferably the solvent is selected from ethylene carbonate (denoted EC), propylene carbonate (denoted PC), dimethyl carbonate (denoted DMC), diethyl carbonate (denoted DEC) and ethyl and methyl carbonate (denoted EMC) and mixtures thereof.

[0038] According to another embodiment, the solvent selected from aprotic organic solvents, such as those mentioned above, may be in a mixture with the solvent that may be used in an Li-ion battery electrolyte, such as those mentioned above.

[0039] In a preferred manner, the solvent is acetonitrile.

[0040] As indicated above, according to step b) of the method according to the invention, the material obtained at the end of step a) is recovered.

[0041] Advantageously, the material can be recovered by centrifugation or by filtration.

[0042] The material can then be rinsed with solvent, preferably selected from the solvents mentioned above, possibly several times.

[0043] Then, the material can be dried in a vacuum.

[0044] The materials Li.sub.7-xMnN.sub.4 (0<x2) and Li.sub.3-yFeN.sub.2 (0<y1.2) can be obtained in this way.

[0045] The subject-matter of the invention is also the use of the material obtained by the method according to the invention, as a negative electrode active material for lithium-ion batteries.

[0046] As indicated above, the method according to the invention is a method for delithiating at least one lithium and transition metal nitride.

[0047] A protocol for delithiating at least one lithium and transition metal nitride can be described according to an embodiment below.

[0048] Advantageously, an oxidising agent, such as those mentioned above, can first be added to a solvent, such as those mentioned above, to obtain a solution comprising said oxidising agent.

[0049] Then, lithium and transition metal nitride, such as for example Li.sub.7MnN.sub.4 or Li.sub.3FeN.sub.2, can be added to said solution. Said lithium and transition metal nitride can be in the form of powder.

[0050] The amount of lithium and transition metal nitride can be adjusted such that the molar ratio between said oxidising agent and said lithium and transition metal nitride can range from 0.5 to 3, preferably from 1 to 2.

[0051] The lithium and transition metal nitride can then be mixed in the said solution comprising said oxidising agent.

[0052] The temperature can then be adjusted to a temperature ranging from 5 C. to 50 C.

[0053] All of these steps can be carried out in a controlled environment, such as in a glove box.

[0054] Advantageously, the oxidising agent used can be a cobaltocenium salt. The colour of the solvent may change during the reaction. This is a signal that the reaction is in progress.

[0055] At the end of the reaction, the material obtained can be recovered.

[0056] Advantageously, the material can be separated from the solution by centrifugation or filtration.

[0057] The material can then be rinsed with solvent several times to remove any undesirable products. Then, the material can be dried in a vacuum.

[0058] The present invention will now be described more specifically with reference to examples, which are by no means limiting to the scope of the invention. However, the examples provide support for specific features, variants, and preferred embodiments of the invention.

EXAMPLES

Example 1: Method According to the Invention

[0059] The lithium and transition metal nitride of formula Li.sub.7MnN.sub.4 is used.

[0060] A diffractogram is made of the material in its initial state, as shown in FIG. 1 (curve A, material A). The characteristic peaks of Li.sub.7MnN.sub.4 can be identified on this diffractogram.

[0061] A scanning electron microscope image of the material in its initial state is taken, as shown in FIG. 2A (material A).

[0062] The delithiation takes place in a glove box at a temperature of 20 C. The molar ratio between said oxidising agent and said lithium and transition metal nitride is 1.5.

[0063] Approximately 320 mg (0.002 mol) Li.sub.7MnN.sub.4 is placed in an Erlenmeyer flask, then approximately 7.5 mL acetonitrile and a bar magnet are introduced into the same Erlenmeyer flask. Approximately 1 g cobaltocenium hexafluorophosphate (0.003 mol) is dissolved in 7.5 ml acetonitrile to prepare the oxidising solution. The oxidising solution was added dropwise using an additional ampoule. A total of 15 mL acetonitrile was used and the oxidising solution had a concentration of 0.5 mol/L. Vigorous mixing was maintained after the addition overnight for the reaction between the oxidant and the nitride.

[0064] The mixture was then decanted and transferred to a centrifuge tube. Centrifugation was carried out at a speed of 5000 rpm for 5 minutes using a centrifuge. After centrifugation, the powder and solution were separated.

[0065] Approximately 5 mL acetonitrile was introduced a second time to the centrifuge tube for rinsing, then a second centrifugation was carried out.

[0066] This rinsing-centrifugation step was repeated several times (3 times). At the end, the powder was placed in an oven (Buchi type) for drying at 90 C. under vacuum for one hour.

[0067] The delithiated material was obtained after drying.

[0068] A material is thus obtained at the end of the method according to the invention.

[0069] A diffractogram of the material obtained is then produced, as shown in FIG. 1 (curve B, material B).

[0070] A scanning electron microscope image of the material obtained by the method of the invention was also taken, as shown in FIG. 2B (material B).

[0071] It is clear that the material of formula Li.sub.7MnN.sub.4 has been modified.

[0072] More precisely, the material of formula Li.sub.5,3MnN.sub.4 was obtained. The presence of the material of formula Li.sub.5,3MnN.sub.4 was confirmed by the lattice parameter calculated from this diffractogram, which is 9.35 A.

[0073] A delithiation of the material of formula Li.sub.7MnN.sub.4 was thus achieved using the method according to the invention.

Example 2: Use of the Material Obtained in Example 1 in a Half-Cell

[0074] The active material obtained in Example 1 was prepared in the form of a composite electrode and tested with a piece of lithium metal in a CR2032 button cell. The composite electrode was prepared by mixing 70% by weight of the active material obtained in example 1 with 22% by weight acetylene black and 8% by weight polytetrafluoroethylene (PTFE).

[0075] The separator used was a glass microfibre separator CAT No. 1823-070@marketed by Whatman.

[0076] The electrolyte used was 1 mol/L lithium hexafluorophosphate dissolved in a mixture of carbonate solvents with a 1:1:1 volume ratio of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC).

[0077] Then a half-cell was assembled. The half-cell was assembled in a glove box.

Electrochemical Test

[0078] Galvanostatic cycling was performed using a BioLogic VMP3 potentiostat with a cycling regime of C/20, as shown in FIG. 3. The potential window is between 1.6 V and 0.9 V.

[0079] In FIG. 3, the starting point x=0 corresponds to the active material obtained at the end of example 1. The potential at this point is E=1.7 V.

[0080] As x increases, the material reduces electrochemically and the potential of the material decreases. It is known that the initial form of Li.sub.7MnN.sub.4 is obtained when the potential has dropped to E=0.9 V. It can then be seen in FIG. 3 that x is approximately equal to 1.7 when E=0.9 V. In other words, when E=0.9 V, approximately 1.7 lithium cells have been electrochemically re-intercalated into the half cell. Thus, when x=0, the formula can be determined as being Li.sub.5,3MNn.sub.4.

[0081] Thus, the material obtained by way of the method according to the invention is the material of formula Li.sub.7-xMnN.sub.4 (with x=1.7), a material which has therefore been delithiated.

[0082] This material can be used as an active material for the negative electrode of an Li-ion battery.