MATERIALS

The invention relates to the use of a metal carbodiimide or a metal cyanamide as a new active material for a negative electrode, a negative electrode comprising said metal carbodiimide or metal cyanamide, its preparation method, a battery comprising said negative electrode, and a method for the preparation of a composite material.

Lithium battery technology has become increasingly important in recent years and has proved to be the most efficient energy storage strategy for a wide range of portable devices like cellular phones, laptops and digital electronics. These batteries are also of interest because of their possible application in hybrid electric vehicles (HEV), plug in hybrid electric vehicles (PHEV) and pure electric vehicles (PEV). However, their use in said vehicles requires from two to five times more energy density than the present lithium batteries technology can offer (150 Wh/kg).

The commercialized Li-ion batteries with high power generally comprise a carbon derivative as the negative electrode material because of its low potential with respect to Li ⁺ /Li and its high energy density with the formation of LiC ₆ . The carbon derivative may be a "hard carbon", containing primarily sp ² carbon atoms, a "soft carbon" containing primarily sp ³ carbon atoms, or an intermediate variety of carbon in which variable proportions of sp ² carbon atoms and sp ³ carbon atoms coexist. The carbon derivative may also be natural graphite or artificial graphite, optionally covered with ungraphitized carbon which protects against exfoliation during electrochemical operation. The major drawback of these negative electrode materials is the consumption of a part of the current, and hence of lithium ions originating from the positive electrode during the first charge, the result of this being the formation on the negative electrode of a protective layer, called passivating layer (or SEI layer), which prevents subsequent reaction of the electrolyte on the negative electrode into which the lithium is inserted. This phenomenon gives rise to a decrease in the energy density of the battery, since the lithium rendered unusable is withdrawn from the positive electrode material, which then has a lower specific capacity (90-210 mAh.g ^"1 ). In practice, between 5% and 25% of the initial capacity is lost in this way.

As lithium resources continue to decline worldwide, studies have also been carried on other potential candidates to replace lithium in particular with sodium since it is cheaper, non-toxic, more evenly spread worldwide and more abundant than lithium. In a sodium battery, a carbon derivative can also be used as a negative electrode material and leads to reversible Na ⁺ insertions of the order of 200 mAh.g ^"1 . However, here as well, the formation of a passivating layer represents a loss of 15% to 25% of the initial capacity in the first cycle. In addition, sodium is employed for use in place of lithium in applications where the stored energy density is less critical than for portable electronics or automotive transport.

Therefore, efforts have been made in lithium batteries as well as in sodium batteries to replace graphite or carbon derivatives with other negative electrode materials such as intercalation/de-intercalation materials (e.g. Ti0 ₂ , Li ₄ Ti ₅ 0i ₂ ), alloy/de-alloy materials (e.g. Si, Ge, Sn, Al, Bi, Sn0 ₂ ) and conversion materials (e.g. transition metal oxides such as Mn _x O _y , NiO, Fe _x O _y , CuO, Cu ₂ 0, Mo0 ₂ , metal sulfides, metal phosphides and metal nitrides). Indeed, most of them possess a specific capacity higher than 500 mAh.g ^"1 . However, in order to address both energy and power demands for future energy storage applications, the development and selection of other suitable negative electrode materials which can provide high reversible capacities, increased power capability, long cycling stability and free from safety concerns are still required. The objective of the present invention is to overcome in part or in full the aforementioned disadvantages of the prior art, and more particularly provide the use of a new active material for a negative electrode, capable of inserting alkali metal or alkaline earth metal ions (e.g. lithium or sodium ions) reversibly, while guaranteeing good electrochemical properties in terms of capacities and cycling stability. This objective is also to provide a new negative electrode which comprises said new active material, a method for producing the negative electrode, and a battery which comprises said negative electrode, said electrode and said battery having good electrochemical properties in terms of capacities and cycling stability.

Indeed, the inventors have found that, surprisingly, the use of metal carbodiimides or metal cyanamides as negative electrode materials leads to good electrochemical properties in a primary or secondary battery, and more particularly in a lithium or sodium primary or secondary battery. The first object of the present invention is thus the use of a metal carbodiimide or a metal cyanamide responding to the following formula (I):

in which :

* M is a metal selected from the group consisting of Cr, Fe, Co, Ni, Cu, Zn, Cd, Pb, Ag, Si, In, Ti and Sn,

* M' is a metal different from M and selected from the group consisting of Mn, Cr, Fe, Co, Ni, Cu, Zn, Cd, Pb, Ag, Si, In, Ti and Sn,

* l≤ x + y≤ 2,

* 1≤ w≤ 3, * 0 < x≤ 2, and preferably 1≤ x≤ 2,

* 0≤ y≤ 2, preferably 0≤ y < 2, and more preferably 0≤ y≤ 1,

* each of x, y and w is an integer or a non-integer, and

* the values of x, y and w are chosen such that the metal carbodiimide or the metal cyanamide (I) is electronically neutral in view of the oxidation state(s) of the element or elements represented by M and M', as an active material for a negative electrode. In one embodiment, y = 0. Examples of such metal carbodiimides or metal cyanamides (I) can be FeNCN, ZnNCN, PbNCN or Cr ₂ (NCN) ₃ .

In another embodiment, y≠ 0.

Examples of such metal carbodiimides or metal cyanamides (I) can be Fei- _a CO _a NCN, Fei _-a Ni _a NCN or Fei _-a Mn _a NCN, in which 0 < a < 1, such as Fe ₀ . ₈ Co ₀ . ₂ NCN.

Different methods have been reported to prepare metal carbodiimides or metal cyanamides responding to formula (I) in which y = 0.

As an example, Liu et al. [Z. Naturforsch. B, 2005, 60, 593] have described a slow oxidation approach to prepare CuNCN from Cu ₄ (NCN) ₂ NH ₃ at room temperature. The same compound was prepared by Koziej et al. [J. Mater. Chem., 2009, 19, 5122-5124] following a non-aqueous soft chemistry approach via the reaction of copper chloride (CuCI ₂ ) with cyanamide and benzylamine. This is one of the most promising route leading to metal carbodiimides or metal cyanamides, nevertheless other routes are also possible, such as the metathesis reaction involving the heating of a mixture comprising an appropriate metal carbodiimide or metal cyanamide (e.g. zinc cyanamide) and a chloride of another appropriate metal (e.g. manganese (II) chloride or chromium (III) chloride), or the decomposition under argon of a compound of formula M ¹ (NCNH) ₂ (M ¹ being for example Ni, Co or Fe).

The inventors have found that metal carbodiimides or metal cyanamides of formula (I) display higher capacities than graphite which is the most commonly used negative active material in commercialized batteries. In addition, they are able to be used under high currents such as under very high charge/discharge rates (e.g. under 32 C ~ 9000 mA.g ^"1 ).

In one preferred embodiment, the metal carbodiimide or the metal cyanamide responding to the following formula (I) as described in the present invention is used as an active material for a negative electrode in a primary or secondary battery, and more particularly in a lithium or sodium primary or secondary battery. A second object of the present invention is a negative electrode comprising a negative electrode material including an active material, optionally a binder and optionally a conductive agent, wherein the active material is a metal carbodiimide or a metal cyanamide (I) as defined in the first object of the present invention.

The negative electrode material preferably includes:

- at least about 50% by weight of a metal carbodiimide or a metal cyanamide (I) as defined in the first object of the present invention; - from about 0% to 40% by weight of a conductive agent; and

- from about 0% to 40% by weight of a binder.

In one preferred embodiment, the negative electrode material includes:

- from about 50% to 80% by weight of a metal carbodiimide or a metal cyanamide (I) as defined in the first object of the present invention;

- from about 10% to 40% by weight of a conductive agent; and

- from about 10% to 40% by weight of a binder.

In a more preferred embodiment, the negative electrode material includes:

- from about 50% to 65% by weight of a metal carbodiimide or a metal cyanamide (I) as defined in the first object of the present invention;

- from about 15% to 35% by weight of a conductive agent; and - from about 10% to 25% by weight of a binder.

The conductive agent may be carbon black, Super P carbon black, acetylene black, ketjen black, channel black, natural or synthetic graphite, carbon fibers, carbon nanotubes, vapor grown carbon fibers or a mixture thereof.

In an advantageous embodiment, the conductive agent is a mixture of a conventional carbon conductive agent such as carbon black, Super P carbon black, acetylene black, ketjen black, channel black, natural or synthetic graphite and of conductive fibers such as carbon fibers, carbon nanotubes or vapor grown carbon fibers. A mixture of carbon black and vapor grown carbon fibers is preferred.

The binder is preferably a polymer which has a high modulus of elasticity (e.g. of the order of several hundred MPa), and which is stable under the temperature and voltage conditions in which the negative electrode is intended to operate. The binder maintains adhesion of the negative electrode material to the current collector, maintain ionic contact, and facilitate the formation of a stable interface with the electrolyte. The binder may be selected from fluoropolymers such as poly(vinylidene fluoride) or poly(tetrafluoroethylene), cellulose fibers, cellulose derivatives such as starch, carboxymethyl cellulose (CMC), diacetyl cellulose hydroxyethyl cellulose or hydroxypropyl cellulose, styrene butadiene rubber (SBR) and a mixture thereof. Cellulose derivatives, more particularly carboxymethyl cellulose (CMC), are preferred.

The negative electrode material (i.e. which includes an active material, optionally a binder and optionally a conductive agent) preferably has a thickness ranging from about 10 to 25 prn, and more preferably from about 15 to 20 pm.

If the thickness of the negative electrode material is greater than 25 pm, the specific capacity decreases due to a lower ionic and electronic percolation.

The loading of active material (i.e. metal carbodiimide or metal cyanamide (I)) on the negative electrode may range from about 1 to 5 mg/cm ² , and preferably from about 1 to 3 mg/cm ² . A negative electrode according to the invention can further comprise a current collector which carries the aforementioned negative electrode material.

The negative electrode material can then be in the form of a film carried by a current collector or in the form of a compacted powder layer carried by a current collector.

The negative electrode material is preferably in the form of a film carried by the current collector, so as to improve the electrochemical properties (e.g. the specific capacity).

The current collector is preferably composed of a conductive material, more particularly of a metallic material which may be selected from copper, aluminum, nickel, steel and iron.

The metallic material may be in the form of a metallic foil, a metallic grid or a metallic foam, and preferably in the form of a metallic foil.

The thickness of the current collector can range from about 10 to 25 prn.

Metal carbodiimides or metal cyanamides based-negative electrodes of the present invention are operating in voltage window significantly higher than 0 V, which prevents Li-plating and renders said electrodes safer than graphite.

In addition, they have also better cycling stability than published negative electrodes used in sodium batteries and excellent stability for operating at high rates.

It is particularly advantageous to prepare the negative electrode according to the following steps:

- preparing a slurry of a mixture comprising a metal carbodiimide or a metal cyanamide (I) as defined in the first object of the present invention, a binder and a conductive agent as defined in the second object of the present invention,

- grinding said slurry, preferably by ball-milling, - applying the resulting slurry on a support, preferably by tape casting, and

- drying the resulting slurry applied on a support.

Thanks to this method, improved electrochemical performances are obtained.

The slurry is preferably an aqueous slurry. However, a volatile organic solvent can also be used alone or in combination with water, so as to form the slurry. The volatile solvent is generally added to improve the solubility properties of the binder in the slurry. The volatile organic solvent may be selected from acetone, tetrahydrofuran, diethyl ether, hexane, and N-methylpyrrolidone.

The components of the negative electrode material, namely the metal carbodiimide or the metal cyanamide (I), the binder and the conductive agent, may represent from about 10 to 50% by weight of the total weight of the slurry, and preferably from about 10 to 30% by weight of the total weight of the slurry.

The grinding step is preferably performed by planetary ball-milling, more particularly during about 30 minutes to one hour.

The drying step is preferably carried out during at least about 12 hours, advantageously at a temperature ranging from about room temperature (i.e. about 20-25°C) to about 120°C.

The drying step aims at removing the volatile organic solvent or the water or the mixture thereof.

The support may be the current collector as defined in the second object of the present invention.

In another embodiment, the support may be a glass support so as to prepare a film of negative electrode material on said support. Thus, after the drying step, the method can further comprise a step of removing the film formed after drying from the support, and applying said film to a current collector. A third object of the present invention is a battery comprising :

- at least one positive electrode,

- at least one negative electrode,

- said positive and negative electrodes being separated by an electrolyte,

- said battery operating by circulation of cations A ⁿ⁺ between said positive electrode and said negative electrode, with n = 1 or 2 and said cation A ⁿ⁺ being selected from alkali metal ions and alkaline earth metal ions, preferably from Li, Na, K, Ca, and Mg, and more preferably from Li and Na, - the electrolyte comprising mobile cations A ⁿ⁺ , and wherein the negative electrode is as defined in the second object of the present invention.

The positive electrode of a battery according to the invention comprises a positive electrode material including an active material capable of reversible insertion of cations A ⁿ⁺ or reaction with cations A ⁿ⁺ at a potential greater than the operating potential of the negative electrode, optionally a conductive agent and optionally a binder.

The positive electrode can further comprise a current collector which carries the aforementioned positive electrode material. The current collector may be composed of a conductive material, more particularly of a metallic material which may be selected from aluminium, steel and iron.

The binder and the conductive agent may be defined as in the second object of the present invention. A ⁿ⁺ is preferably a sodium ion Na ⁺ or a lithium ion Li ⁺ .

Examples of active materials for the positive electrode of a lithium battery and thus, capable of reversibly inserting lithium ions may be: - transition metal chalcogenides, more particularly oxides Li _v T ^a 0 ₂ , in which 0≤ v≤ 1 and T ^a represents at least one element selected from Co, Ni, and Mn, a part of which may be replaced by Mg or Al;

- phosphates of olivine structure Li _v ^b P0 ₄ , in which 0≤ v'≤ 1 and T ^b represents at least one element selected from Fe, Mn and a mixture of Fe and

Mn, a part of which may be replaced by Co, Ni or Mg;

- silicates Li _2-s T ^c Si0 ₄ and fluorophosphates Li _r T ^c P0 ₄ F, in which 0≤ s < 2, 1≤ r≤ 2 and T ^c represents at least one element selected from Fe, Mn, Co, Ni, and Ti, a part of which may be replaced by Mg or Al; or - fluorosulfates Li _t T ^d S0 ₄ F, in which 0≤ t≤ 1 and T ^d represents at least one element selected from Fe, Mn, Co, and Ni, a part of which may be replaced by Mg and a part of the sulfate groups S0 ₄ ^2" of which may be replaced by the isosteric and iso-charge group P0 ₃ F ^2" .

Examples of active material for the positive electrode of a sodium battery and thus capable of reversibly inserting sodium ions, may be:

- lamellar fluorophosphates Na ₂ T' ^a P0 ₄ F, in which T' ^a represents a divalent element selected from Fe, Mn, Co, and Ni, which may be replaced partially by Mg or Zn;

- fluorosulfates NaT' ^b S0 ₄ F, in which T' ^b represents at least one element selected from Fe, Mn, Co, Ni and mixture thereof, a part of which is optionally replaced by Mg, and a part of the sulfate groups S0 ₄ ^2" of which is optionally replaced by the isosteric and iso-charge group P0 ₃ F ^2" ;

- Na _V "T'O ₂ , in which 0.4 ≤ v"≤ 1, and T' ^c represents Mn or Co (for example Nao. ₄₄ Mn0 ₂ or Na ₀ . ₇ MnO ₂ ); or - dithiocarbamates Na[CS ₂ NR'R"], in which each of the groups R' and

R" represents a methyl, ethyl, or propyl radical, or else, or R' and R" form a ring (for example, pyrrolidine or morpholine).

The positive electrode material can be in the form of a film carried by a current collector or in the form of a compacted powder layer carried by a current collector. The positive electrode material is preferably in the form of a film carried by a current collector.

The thickness of the current collector can range from about 10 to

25 prn. The positive electrode material preferably has a thickness ranging from about 10 to 25 prn.

The electrolyte of the battery according to the invention is generally a solution comprising a salt of said alkali metal or alkaline earth metal A, preferably a salt of Li, Na, K, Ca or Mg, and more preferably a salt of Li or Na, and a solvent.

The solvent may be a liquid solvent, optionally gelled by a polymer, or a polar polymer solvent which is optionally plasticized by a liquid.

The proportion of liquid solvent in the solvent may vary from about 2% by volume (corresponding to a plasticized solvent) to about 98% by volume (corresponding to a gelled solvent).

In a sodium battery, the alkali metal salt is a sodium salt, preferably selected from the group consisting of NaCI0 ₄ , NaBF ₄ , NaPF ₆ , Na ₂ S0 ₄ , NaN0 ₃ , Na ₃ P0 ₄ , Na ₂ C0 ₃ , sodium salts having a perfluoroalkanesulfonate anion, sodium bis(perfluoroalkanesulfonyl)methane, sodium tris(perfluoroalkanesulfonyl)methane and sodium bis(perfluoroalkanesulfonyl)imide (e.g. NaTFSI, also known under the chemical name sodium bis(trifluoromethanesulfonyl)imide).

In a lithium battery, the alkali metal salt is a lithium salt, preferably selected from the group consisting of LiCI0 ₄ , LiBF ₄ , LiPF ₆ , Li ₂ S0 ₄ , LiN0 ₃ , Li ₃ P0 ₄ , Li ₂ C0 ₃ , lithium salts having a perfluoroalkanesulfonate anion, lithium bis(perfluoroalkanesulfonyl)methane, lithium tris(perfluoroalkanesulfonyl)methane and lithium bis(perfluoroalkanesulfonyl)imide (e.g. LiTFSI, also known under the chemical name lithium bis(trifluoromethanesulfonyl)imide). The liquid solvent may be composed of at least one polar aprotic solvent selected from cyclic and linear carbonates (e.g. ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, vinylene carbonate, fluoroethylene carbonate), esters, nitriles (e.g. acetonitrile, benzonitrile), nitro derivatives (nitromethane, nitrobenzene), amides (e.g. dimethylformamide, diethylformamide, N-methylpyrrolidone), sulfones (e.g. dimethylsulfone, tetramethylene sulfone), sulfolanes, alkylsulfamides (tetraalkylsulfonamides having from 5 to 10 carbon atoms), partially hydrogenated hydrocarbons, cyclic and linear ethers (e.g. diethyl ether, tetrahydrofuran, dimethoxyethane, dioxane, glyme, dimethyltetrahydrofuran, methyl), polyethylene glycol ethers (e.g. RO(CH ₂ CH ₂ 0)n ' in which R and R' are CH ₃ or C ₂ H ₅ and 1≤ n ≤ 12), tetraalkyl sulfamides (e.g. RR'NS0 ₂ NR"R"' in which R, R', R", and R'" are CH ₃ or C ₂ H ₅ ), 3-methyl-l,3-oxazolidin-2-one, and cyclic esters (e.g. γ-butyrolactone).

Said liquid solvent may optionally be gelled by addition of a polymer obtained, for example, from one or more monomers selected from ethylene oxide, propylene oxide, methyl methacrylate, methyl acrylate, acrylonitrile, methacrylonitrile, N-vinylpyrrolidone and vinylidene fluoride, said polymer having a linear, comb, random, alternating, or block structure, and being crosslinked or not.

The polar polymer solvent may be selected from crosslinked or non-crosslinked solvating polymers which do or do not carry grafted ionic groups. A solvating polymer is a polymer which comprises solvating units containing at least one heteroatom selected from sulphur, oxygen, nitrogen and fluorine. Examples of solvating polymers include linear, comb or block polyethers which do or do not form a network, based on poly(ethylene oxide), or copolymers containing the ethylene oxide or propylene oxide or allyl glycidyl ether unit or at least 50% of repeating units -CH ₂ CH ₂ 0- and having a linear, comb, random, alternating or block structure, polyphosphazenes, crosslinked networks based on polyethylene glycol crosslinked by isocyanates, or the networks obtained by polycondensation and carrying moieties which allow the incorporation of crosslinkable moieties. Also included are block copolymers in which some blocks carry functions which have redox properties. Of course, the list above is not limitative, and all polymers having solvating properties may be used. Said polar polymer solvent may optionally be plasticized by addition of a liquid, more particularly a polar aprotic solvent as defined in the present invention.

In one particular embodiment, each of the electrodes of a battery according to the invention is composed of a thin film. By "thin film" is meant a film having a thickness of the order of 10 to

40 pm.

When the electrolyte is a solid polymer electrolyte, it is also in the form of a thin film.

When the electrolyte is a liquid electrolyte, said liquid impregnates a separator in the form of a thin film.

The separator may be a conventional polymer-based separator such as a Celgard ^® separator or a Whatman ^® borosilicate glass fiber separator.

The use of a solution of an alkali metal salt or an alkaline earth metal in a liquid solvent as the electrolyte for the battery of the invention is particularly preferred, so as to obtain a liquid electrolyte.

In a preferred embodiment, the electrolyte is a solution comprising a salt of said alkali metal or alkaline earth metal A and one or more of the carbonates selected from ethylene carbonate, propylene carbonate, dimethylcarbonate and vinylene carbonate. In a more preferred embodiment, the solution (of the electrolyte) further comprises from about 1 to 5% in volume of fluoroethylene carbonate.

A battery according to the invention may be composed of a single electrochemical cell comprising two electrodes (i.e. one positive electrode and one negative electrode) separated by an electrolyte; of a plurality of cells assembled in series; of a plurality of cells assembled in parallel; or of a combination of the two assembly types.

A fourth object of the present invention is a method for the preparation of a composite material, wherein it comprises at least one step of submitting a battery as defined in the third object of the present invention to a charge, so as to transform the metal carbodiimide or the metal cyanamide responding to the formula (I) into said composite material.

The composite material obtained during the method may comprise:

- at least one phase P ¹ consisting of a metal at the oxidation state (0) selected from the group consisting of M, M' and a mixture thereof, M and M' being as defined in the first object of the present invention,

- at least one phase P ² consisting of a metal carbodiimide or a metal cyanamide responding to the following formula (II):

AzNCN (II) in which :

* A is an alkali or alkaline earth metal, preferably an alkali or alkaline earth metal selected from the group consisting of Na, Li, K, Ca, Mg, Sr and Ba, and more preferably selected from Li and Na, and

* z = 1 or 2, and - optionally a phase P ³ consisting of a metal carbodiimide or a metal cyanamide responding to the formula (I) as defined in the first object of the present invention.

The metal M (respectively the metal M') of the phase P ¹ is the same as the metal M (respectively the metal M') of the phase P ³ . The alkali or alkaline earth metal A is the same as the one used in the form of a salt in the electrolyte of the battery.

When the battery according to the third object of the present invention is submitted to a charge (e.g. during cycling), the metal carbodiimide or the metal cyanamide responding to the formula (I) used as an active material in a negative electrode is transformed into said composite material.

Thus, during the cycling of the battery, the metal carbodiimide or the metal cyanamide responding to the formula (I) is capable of reversible insertion or reaction with said cations A ⁿ⁺ so as to form said composite material.

The composite material of the present invention may represent an intimate mixture of phases P ¹ and P ² or an intimate mixture of phases P ¹ , P ² and P ³ . In a preferred embodiment, z = 2 and A is Li or Na.

In a more preferred embodiment, the composite material is selected from the following composite materials:

* (1-u) FeNCN / u Li ₂ NCN / u Fe,

* (1-u) FeNCN / u Na ₂ NCN / u Fe, * (1-u) ZnNCN / u Li ₂ NCN / u Zn,

* (1-u) ZnNCN / u Na ₂ NCN / u Zn,

* [(2-u')/2] Cr ₂ NCN ₃ / (3u'/2) Li ₂ NCN / u' Cr, and

* [(2-u')/2] Cr ₂ NCN ₃ / (3u'/2) Na ₂ NCN / u' Cr, in which 2u and 3u' represent the molar proportion of Li or Na in the composite materials, 0 < u≤ 1 and 0 < u'≤ 2.

The present invention is illustrated in more detail in the examples below, but it is not limited to said examples.

EXAMPLES

The starting materials used in the examples which follow, are listed below:

Carboxymethyl cellulose (CMC) : Aldrich, DS = 0.7, Mw = 250 000 g/mol; - Vapor grown carbon fiber (VGCF-H) : Showa Denko, diameter of about 100-200 nm and length of about 10-20 prn; and

- Carbon black: Sn2A, Y50A, specific surface of about 70 m^g ^"1 , and size of particles of about 30-60 nm. Unless noted otherwise, these starting materials were used as received from the manufacturers, without additional purification.

Example 1

Use of FeNCN as an active material in a negative electrode of a sodium battery according to the invention FeNCN was prepared according to the method described in Liu et al.

[Chem. Eur. J., 2009, 15, 1558] by reaction of [Fe(NH ₃ ) ₆ ] ²⁺ with H ₂ NCN in water so as to form Fe(NCNH) ₂ and thermal decomposition of Fe(NCNH) ₂ into FeNCN at 400 °C.

0.579 g of FeNCN was mixed with 0.173 g of carboxymethyl cellulose (CMC), 0.088 g of vapor grown carbon fibers and 0.160 g of carbon black in 6 ml of water to form an aqueous slurry containing 8.3% by weight of FeNCN, 2.5% by weight of CMC and 3.5% by weight of conductive agent.

The components of the negative electrode material, namely the metal carbodiimide or the metal cyanamide (I), the binder and the conductive agent, represent about 14.3% by weight of the total weight of the aqueous slurry.

The aqueous slurry was homogeneously mixed and ground for about 40 min in a planetary ball-mill commercialized under the brand name Pulverisette 7 by the firm Fritsch. It was then tape casted on a 18 prn thick copper foil, dried at room temperature for about 12h and finally at about 120°C under vacuum for another 12h.

The negative electrode material included 57.9% by weight of FeNCN, 17.3% by weight of CMC and 24.8% by weight of conductive agent.

The final mass loading of FeNCN on the negative electrode was about 1.5 mg/cm ² and the thickness of the negative electrode material was about 17 pm. The resulting negative electrode was mounted in an electrochemical cell in which the counter-electrode was composed of sodium metal, the electrolyte was a 1 M solution of NaPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (1 : 1 in volume), said mixture further including about 5% in volume of fluoroethylene carbonate, and the separator was a Whatman GF/D borosilicate glass fiber disk. The cell was sealed and then a reducing current corresponding to a rate of C/5 (which corresponds to 0.2 Na ⁺ per hour) was applied to it.

Figure 1 shows the variation in potential E (in volts (V) vs. Na ⁺ /I\la) as a function of the equivalent of the number x of Na ⁺ ions inserted [Eq(x)] during the first seven charging/discharging cycles and figure 2 shows the specific capacity (in mAh.g ^"1 ) (curve with solid triangles during the charge and curve with empty triangles during the discharge) and the coulombic efficiency (in %) (curve with empty stars) as a function of the number n of cycles. Figure 1 demonstrates an irreversible phenomenon only during the 1st cycle. Then, during the following galvanostatic cycles, the insertion of Na ⁺ into the iron carbodiimide FeNCN is perfectly reversible. In addition, it is shown that it is possible to reversibly insert about 1.4 sodium ions at most into the metal cyanamide FeNCN. In addition, the electrode displays a low average potential (1.25 V) which makes it particularly useful as a negative electrode in a sodium battery.

Figure 2 shows long cycling stability both on charge and on discharge (i.e. over more than 220 cycles), a high specific capacity of about 350-400 mAh.g ^"1 and an excellent coulombic efficiency (almost 100% after 20 cycles). Example 2

Use of FeNCN as an active material in a negative electrode of a lithium battery according to the invention

FeNCN was prepared according to the method described in example 1.

0.579 g of FeNCN were mixed with 0.173 g of carboxymethyl cellulose (CMC), 0.088 g of vapor grown carbon fibers and 0.160 g of carbon black in 6 ml of water to form an aqueous slurry containing 8.3% by weight of FeNCN, 2.5% by weight of CMC and 3.5% by weight of conductive agent.

The components of the negative electrode material, namely the metal carbodiimide or the metal cyanamide (I), the binder and the conductive agent, represent about 14.3% by weight of the total weight of the aqueous slurry.

The aqueous slurry was homogeneously mixed and ground for about 40 min in a planetary ball-mill such as the one defined in example 1. It was then tape casted on a 22 pm thick aluminium foil, dried at room temperature for about 12h and finally at about 120°C under vacuum for another 12h. The final mass loading of FeNCN on the negative electrode was about

1.2 mg/cm ² and the thickness of the negative electrode material was 17 pm.

The negative electrode material included 57.9% by weight of FeNCN, 17.3% by weight of CMC and 24.8% by weight of conductive agent

The resulting negative electrode was mounted in an electrochemical cell in which the counter-electrode was composed of lithium metal, the electrolyte was a 1 M solution of LiPF ₆ in a mixture of ethylene carbonate, propylene carbonate and dimethyl carbonate (1 : 1 : 3), said mixture further including about 1% in volume of vinylene carbonate and about 5% in volume of fluoroethylene carbonate, and the separator was a Whatman GF/D borosilicate glass fiber disk. The cell was sealed and then a reducing current corresponding to a rate of C/10 (which corresponds to 0.1 Li ⁺ per hour) was applied to it.

Figure 3 shows the variation in potential E (in volts (V) vs. Li ⁺ /U) as a function of the equivalent of the number x of Li ⁺ ions inserted [Eq(x)] during the first ten charging/discharging cycles and figure 4 shows the specific capacity (in mAh.g ^"1 ) (curve with solid triangles during the charge and curve with empty triangles during the discharge) and the coulombic efficiency (in %) (curve with empty stars) as a function of the number n of cycles.

Figure 3 demonstrates an irreversible phenomenon only during the 1st cycle. Then, during the following galvanostatic cycles, the insertion of Li ⁺ into the iron carbodiimide FeNCN is perfectly reversible. In addition, it is shown that it is possible to reversibly insert about 2.0 lithium ions at most into the iron carbodiimide FeNCN.

Figure 4 shows long cycling stability both on charge and on discharge (i.e. over more than 110 cycles), a high specific capacity of about 600 mAh.g ^"1 and a coulombic efficiency of 96%).

Example 3

Use of ZnNCN as an active material in a negative electrode of a lithium battery according to the invention

ZnNCN was prepared according to the method described in Liu et al. [X. Liu, PhD Dissertation, 2002, http://sylvester.bth.rwth- aachen.de/dissertationen/2002/207/02_207.pdf] or by metathesis reaction of [Zn(NH ₃ ) ₄ ] ²⁺ and cyanamide in aqueous solution described in Krott et al. (M. Krott, PhD Dissertation, 2009, http://publications.rwth- aachen.de/record/51589/files/Krott_Manuel.pdf). 0.579 g of ZnNCN were mixed with 0.173 g of carboxymethyl cellulose

(CMC), 0.088 g of vapor grown carbon fibers and 0.16 g of carbon black in 6 ml of water to form an aqueous slurry containing 8.3% by weight of ZnNCN, 2.5% by weight of CMC and 3.5% by weight of conductive agent.

The components of the negative electrode material, namely the metal carbodiimide or the metal cyanamide (I), the binder and the conductive agent, represent about 14.3% by weight of the total weight of the aqueous slurry.

The aqueous slurry was homogeneously mixed and ground for about 40 min in a planetary ball-mill such as the one defined in example 1. It was then tape casted on a 18 prn thick copper foil, dried at room temperature for about 12h and finally at about 120°C under vacuum for another 12h.

The negative electrode material included 57.9% by weight of ZnNCN, 17.3% by weight of CMC and 24.8% by weight of conductive agent

The final mass loading of ZnNCN on the negative electrode was about 1.5 mg/cm ² and the thickness of the negative electrode material was about 17 prn. The resulting negative electrode was mounted in an electrochemical cell in which the counter-electrode was composed of lithium metal, the electrolyte was a 1 M solution of LiPF ₆ in a mixture of ethylene carbonate, propylene carbonate and dimethyl carbonate (1 : 1 : 3), said mixture further including about 1% in volume of vinylene carbonate and about 5% in volume of fluoroethylene carbonate, and the separator was a Whatman GF/D borosilicate glass fiber disk. The cell was sealed and then a reducing current corresponding to a rate of C/10 (which corresponds to 0.1 Li ⁺ per hour) was applied to it. Figure 5 shows the variation in potential E (in volts (V) vs. Li ⁺ /U) as a function of the equivalent of the number x of Li ⁺ ions inserted [Eq(x)] during the first four charging/discharging cycles and figure 5 shows the specific capacity (in mAh.g ^"1 ) (curve with solid triangles during the charge and curve with empty triangles during the discharge) and the coulombic efficiency (in %) (curve with empty stars) as a function of the number n of cycles.

Figure 5 demonstrates a moderate irreversible phenomenon during the 1st cycle. Then, during the following galvanostatic cycles, the insertion of Li ⁺ into the zinc carbodiimide ZnNCN is perfectly reversible. In addition, it is shown that it is possible to reversibly insert about 1.5 lithium ions at most into the zinc carbodiimide ZnNCN.

Figure 6 shows long cycling stability both on charge and on discharge (i.e. over more than 60 cycles), a moderate specific capacity of about 300 mAh.g ^"1 and coulombic efficiency of 97%.

Example 4

Use of Cr ₂ (NCN) ₃ as an active material in a negative electrode of a lithium battery according to the invention

Cr ₂ (NCN) ₃ was prepared according to the method described in Tang et al. [Angew. Chem. Int. Ed., 2010, 49, 4738] by solid metathesis reaction of CrCIs and ZnNCN at 550°C. 0.579 g of Cr ₂ (NCN) ₃ were mixed with 0.173 g of carboxymethyl cellulose (CMC), 0.088 g of vapor grown carbon fibers and 0.16 g of carbon black in 6 ml of water to form an aqueous slurry containing 8.3% by weight of Cr ₂ (NCN) ₃ , 2.5% by weight of CMC and 3.5% by weight of conductive agent.

The components of the negative electrode material, namely the metal carbodiimide or the metal cyanamide (I), the binder and the conductive agent, represent about 14.3% by weight of the total weight of the aqueous slurry.

The aqueous slurry was homogeneously mixed and ground for about 40 min in a planetary ball-mill such as the one defined in example 1. It was then tape casted on a 18 prn thick copper foil, dried at room temperature for about 12h and finally at about 120°C under vacuum for another 12h. The negative electrode material included 57.9% by weight of

Cr ₂ (NCN) ₃ , 17.3% by weight of CMC and 24.8% by weight of conductive agent

The final mass loading of Cr ₂ (NCN) ₃ on the negative electrode was about 1.5 mg/cm ² and the thickness of the negative electrode material was about 17 pm. The resulting negative electrode was mounted in an electrochemical cell in which the counter-electrode was composed of lithium metal, the electrolyte was a 1 M solution of LiPF ₆ in a mixture of ethylene carbonate, propylene carbonate and dimethyl carbonate (1 : 1 : 3), said mixture further including about 1% in volume of vinylene carbonate and about 5% in volume of fluoroethylene carbonate, and the separator was a Whatman GF/D borosilicate glass fiber disk. The cell was sealed and then a reducing current corresponding to a rate of C/10 (which corresponds to 0.1 Li ⁺ per hour) was applied to it.

Figure 7 shows the variation in potential E (in volts (V) vs. Li ⁺ /U) as a function of the equivalent of the number x of Li ⁺ ions inserted [Eq(x)] during the first seven charging/discharging cycles and figure 8 shows the specific capacity (in mAh.g ^"1 ) (curve with solid triangles during the charge and curve with empty triangles during the discharge) and the coulombic efficiency (in %) (curve with empty stars) as a function of the number n of cycles. Figure 8 demonstrates an irreversible phenomenon during the 1st cycle. Then, during the following galvanostatic cycles, the insertion of Li ⁺ into the metal carbodiimide or the metal cyanamide Cr ₂ (NCN) ₃ is reversible. In addition, it is shown that it is possible to reversibly insert about 4 lithium ions at most into the chromium carbodiimide Cr ₂ (NCN) ₃ .

Figure 8 shows long cycling stability both on charging and on discharging (i.e. over more than 15 cycles), a high specific capacity of about 500 mAh.g ^"1 and coulombic efficiency of about 98%.

Comparative example 5

Use of conductive carbon as an active material in a negative electrode of a lithium battery 0.35 g of commercial carbon Y50A were mixed with 0.35 g of vapor grown carbon fibers 0.3 g of carboxymethyl cellulose (CMC) in 6 ml of water to form an aqueous slurry containing 10% by weight of carbon (as an active material and a conductive agent) and 4.3% by weight of CMC.

The components of the negative electrode material, namely carbon and the binder, represent about 14.3% by weight of the total weight of the aqueous slurry.

The aqueous slurry was homogeneously mixed and ground for about 40 min in a planetary ball-mill such as the one defined in example 1. It was then tape casted on a 18 prn thick copper foil, dried at room temperature for about 12h and finally at about 120°C under vacuum for another 12h.

The negative electrode material (which is not part of the invention) included 70% by weight of carbon as an active material and a conductive agent and 30% by weight of CMC.

The resulting negative electrode was mounted in an electrochemical cell in which the counter-electrode was composed of lithium metal, the electrolyte was a 1 M solution of LiPF ₆ in a mixture of ethylene carbonate, propylene carbonate and dimethyl carbonate (1 : 1 : 3), said mixture further including about 1% in volume of vinylene carbonate and about 5% in volume of fluoroethylene carbonate, and the separator was a Whatman GF/D borosilicate glass fiber disk. The cell was sealed and then a reducing current corresponding to a rate of C/10 was applied to it. Figure 9 shows the specific capacity (in mAh.g ^"1 ) (curve with solid triangles during the charge and curve with empty triangles during the discharge) as a function of the number n of cycles.

Figure 9 demonstrates a high irreversible phenomenon during the 1st cycle (reduction of the specific capacity by more than 96%). It also shows a lack of cycling stability especially on discharging and a low specific capacity of about 5 mAh.g ^"1 .

This example shows that the conducting agent and/or the binder used in the electrode material of the invention are not contributing to the obtained capacities since the specific capacity of a mixture of conducting agent and binder represents less than 1-2% of the capacities obtained for the negative electrode materials of the invention based on carbodiimide or cyanamide metals.

Example 6

Use of PbNCN as an active material in a negative electrode of a lithium battery according to the invention

PbNCN was prepared according to the method described in Liu et al. [Z. Anorg. Allg. Chem., 2000, 626, 103-105 by the reaction between aqueous solutions of cyanamide and lead acetate. 0.579 g of PbNCN were mixed with 0.173 g of carboxymethyl cellulose

(CMC), 0.088 g of vapor grown carbon fibers and 0.16 g of carbon black in 6 ml of water to form an aqueous slurry containing 8.3% by weight of PbNCN, 2.5% by weight of CMC and 3.5% by weight of conductive agent.

The components of the negative electrode material, namely the metal carbodiimide or the metal cyanamide (I), the binder and the conductive agent, represent about 14.3% by weight of the total weight of the aqueous slurry.

The aqueous slurry was homogeneously mixed and ground for about 40 min in a planetary ball-mill such as the one defined in example 1. It was then tape casted on a 18 prn thick copper foil, dried at room temperature for about 12 h and finally at about 120°C under vacuum for another 12h. The negative electrode material included 57.9% by weight of PbNCN, 17.3% by weight of CMC and 24.8% by weight of conductive agent

The final mass loading of PbNCN on the negative electrode was about 1.5 mg/cm ² and the thickness of the negative electrode material was about 17 Mm.

The resulting negative electrode was mounted in an electrochemical cell in which the counter-electrode was composed of lithium metal, the electrolyte was a 1 M solution of LiPF ₆ in a mixture of ethylene carbonate, propylene carbonate and dimethyl carbonate (1 : 1 : 3), said mixture further including about 1% in volume of vinylene carbonate and about 5% in volume of fluoroethylene carbonate, and the separator was a Whatman GF/D borosilicate glass fiber disk. The cell was sealed and then a reducing current corresponding to a rate of C/10 (which corresponds to 0.1 Li ⁺ per hour) was applied to it. Figure 10 shows the variation in potential E (in volts (V) vs. Li ⁺ /U) as a function of the equivalent of the number x of Li ⁺ ions inserted [Eq(x)] during the first seven charging/discharging cycles and figure 11 shows the specific capacity (in mAh.g ^"1 ) (curve with solid triangles during the charge and curve with empty triangles during the discharge) and the coulombic efficiency (in %) (curve with empty stars) as a function of the number n of cycles.

Figure 11 demonstrates an irreversible phenomenon during the 1st cycle. Then, during the following galvanostatic cycles, the insertion of Li ⁺ into the metal carbodiimide or the metal cyanamide PbNCN is reversible. In addition, it is shown that it is possible to reversibly insert about 3 lithium ions at most into the lead carbodiimide PbNCN.

Figure 11 shows cycling stability both on charging and on discharging (i.e. over 65 cycles) at different cycling rates, a low specific capacity of about 65 mAh.g ^"1 and a coulombic efficiency of about 97%. Example 7

Use of Fe0.eCo0. ₂ NC as an active material in a negative electrode of a lithium battery according to the invention

Fe0.sCo0.2NCN was prepared by mixing Fe(NCNH) ₂ and Co(NCNH) ₂ (80/20) with the LiCI-KCI eutectic salt (melting point = 352 °C). Then the mixture was put in a Schlenk-tube and heated for one week at 370°C. The salt cake obtained was dissolved with water and a product remained in the filter crucible. The product was washed with water and acetone. In the end, the brown powder was dried in vacuum. 0.579 g of Fe ₀ .8Co ₀ . ₂ NCN was mixed with 0.173 g of carboxymethyl cellulose (CMC), 0.088 g of vapor grown carbon fibers and 0.160 g of carbon black in 6 ml of water to form an aqueous slurry containing 8.3% by weight of Fe0.sCo0.2NCN, 2.5% by weight of CMC and 3.5% by weight of conductive agent. The components of the negative electrode material, namely the metal carbodiimide or the metal cyanamide (I), the binder and the conductive agent, represent about 14.3% by weight of the total weight of the aqueous slurry.

The aqueous slurry was homogeneously mixed and ground for about 40 min in a planetary ball-mill commercialized under the brand name Pulverisette 7 by the firm Fritsch. It was then tape casted on a 18 prn thick copper foil, dried at room temperature for about 12h and finally at about 120°C under vacuum for another 12h.

The negative electrode material included 57.9% by weight of Fe0.sCo0.2NCN, 17.3% by weight of CMC and 24.8% by weight of conductive agent.

The final mass loading of Fe0.sCo0.2NCN on the negative electrode was about 1.5 mg/cm ² and the thickness of the negative electrode material was about 17 pm.

The resulting negative electrode was mounted in an electrochemical cell in which the counter-electrode was composed of lithium metal, the electrolyte was a 1 M solution of LiPF ₆ in a mixture of ethylene carbonate, propylene carbonate and dimethyl carbonate (1 : 1 : 3), said mixture further including about 1% in volume of vinylene carbonate and about 5% in volume of fluoroethylene carbonate, and the separator was a Whatman GF/D borosilicate glass fiber disk. The cell was sealed and then a reducing current corresponding to a rate of 4C (which corresponds to 4 Li ⁺ per hour) was applied to it.

Figure 12 shows the specific capacity (in mAh.g ^"1 ) (curve with solid triangles during the charge and curve with empty triangles during the discharge) and the coulombic efficiency (in %) (curve with empty stars) as a function of the number n of cycles. Up to 15 cycles, 82% of the initial capacity is maintained with about 98% coulombic efficiency.