FIELD OF THE INVENTION

The present invention relates to polyphenothiazine-type polymers and their synthesis, as well as to electrochemical cells comprising said polymer as a cathode material, such as for example in lithium ion rechargeable batteries.

BACKGROUND OF THE INVENTION

The use of standard conductive polymers such as for example polyaniline,

poiypyrrole or polythiophene as stand-alone cathode materials is in general limited by the requirement of high positive charge density, which should be dispersed over the length of the polymer chain during the full oxidation of the material.

Despite good cyclability, most standard conductive polymers fail to display a satisfying high charge accommodation during electrochemical cycling in a battery, as shown in Novak (P. Chem. Rev. 1997, 97, 207-281).

Alternative polymers with electronic conductivities comparable to known standard conductive polymers such as polyaniline, poiypyrrole or polythiophene, but at the same time having higher theoretical and/or practical charge accommodation capacities and processability are highly desirable, provided that the production costs of such polymers is comparable with their inorganic counterparts commercially used nowadays.

There is therefore a need for alternative, electrically conductive polymers that display higher charge accommodation capacities and can be synthesized in an economically feasible way.

In this context, Mital et al (J. Chem. Soc. Section C, 1971 , 1875) described a method for synthesizing an electrically conductive poly-phenothiazine by polycondensation of thiophenol derivatives and halogenated anilines. The resulting polymers were studied as potential low-band semiconductors, but however the synthetic method described in Mital et al used expensive monomers and a cumbersome methodology.

While the use of inorganic, carbonaceous filler materials such as graphite is widespread for cost reasons in electrochemical cells, there remains the problem that such materials undergo significant volumetric change during cycling, i.e. expansion during charge and contraction during discharge. Furthermore, materials such as carbon or other redox active materials and can easily be degraded.

There is therefore a need for cathodic materials that display lesser volumetric change during cycling of the electrochemical cell, and/or adapt to such changes in a forgiving manner.

SUMMARY OF THE INVENTION

The present invention provides for a method for synthesizing an electrically conductive, preferably redox active, phenothiazine-type polymer from a polyaniline, comprising the steps of a. optionally reacting polyaniline, preferably in emeraldine form, with a source of chalcogen in a solvent to form a suspension of polyaniline in leucoemeraldine form, b. optionally removing the solvent from the suspension of a polyaniline in leucoemeraldine form to form a powder of a polyaniline in

leucoemeraldine form, c. heating the powder of a polyaniline in leucoemeraldine form in the presence of a catalyst and a chalcogen to form an phenothiazine-type polymer and d. optionally doping the phenothiazine-type polymer with a protic acid.

The present invention further provides for the phenothiazine-type polymer obtained by said method.

Moreover, the present invention provides for an electrochemical cell comprising a. a cathode comprising a phenothiazine-type polymer, optionally mixed with a conductive material, b. an anode comprising elemental lithium and c. an electrolyte composition, preferably a non-aqueous and aprotic electrolyte composition.

The presence of electronegative elements such as chalcogens, and in particular sulphur, selenium and tellurium on, or within, the polymeric chain of the

phenothiazine-type polymer is believed to help in the derealization of the positive charges and thereby helps to increase the overall electric charge an electrochemical cell such as a battery is able to hold in a reversible manner.

DESCRIPTION OF THE INVENTION

The present invention shall now be discussed in more detail and with reference to the following figures:

Figure 1 is a representation of the electrochemical response for the

phenothiazine-type polymer of the present invention, comparing the electrochemical response for the polymer dried at 140°C (olive, labelled 1) and 160°C (black, labelled 2).

Figure 2 is a representation of the specific charge (capacity per mass unit) of a polyaniline (red, bottom) and the phenothiazine-type polymer (black, top) of the present invention versus the number of cycles in batteries based on lithium metal anodes.

The present invention describes the synthetic protocol for phenothiazine-type polymers and their applications as electroactive materials in electrochemical devices, more particularly in lithium-based electrochemical devices (batteries). Due to electronic conduction, reversible redox processes in highly oxidizing environments (cathodic regime in a battery) and procesability, these conducting polymers can be used either as active cathodic materials or as conductive binder for inorganic, or mineral, based cathodic materials such as LiFeP0 ₄ or LiCo0 ₂ .

It is a general object of this invention to provide a synthetic path for

polyphenothiazine-type polymers and their applications as conductive, redox active materials in lithium based batteries.

Soluble conducting polymers can be used as alternative to

carbonaceous conductive materials and coatings. The advantages of conductive polymers over traditional carbonaceous coatings are non reductive behaviour to participate to the overall battery capacity by its own redox component and an ability to adapt to volumetric changes of the inorganic cathode materials used as main component in lithium based batteries.

The present invention provides for a method for synthesizing an phenothiazine-type polymer from a polyaniline, comprising the steps of a. optionally reacting a

polyaniline, preferably in emeraldine form, with a source of chalcogen in a solvent to form a suspension of polyaniline in leucoemeraldine form, b. optionally removing the solvent from the suspension of a polyaniline in leucoemeraldine form to form a powder of a polyaniline in leucoemeraldine form, c. heating the powder of a polyaniline in leucoemeraldine form in the presence of a catalyst and a chalcogen to form an phenothiazine-type polymer and d. optionally doping the phenothiazine-type polymer with a protic acid to form a doped phenothiazine-type polymer .

The preferably one-pot synthetic method consists in the mixing of polyaniline polymer in neutral emeraldine form with a source of chalcogen such as sulphur in low oxidation state.

The present invention provides for a one-pot synthetic method in the sense that the source of chalcogen, which is preferably in an oxidation state below 0, used in step a., also furnishes the chalcogen of step c. which then reacts with two adjacent benzene rings of the polymeric chain of the polyaniline in its leucoemeraldine form in a ring-closing reaction to form the phenothiazine-type polymer.

The degree of incorporation of chalcogen into the polymeric chain of the

phenothiazine-like polymer can be ascertained by elemental analysis of the phenothiazine-like polymer. Using the synthetic method of the present invention, it is possible to arrive at an incorporation level of up to 65 percent, in most cases between 40 to 60 percent, when compared to the case where all the benzene rings of the polyaniline are linked by a chalcogen atom, i.e. an incorporation level of 100%.

The present method for synthesizing a phenothiazine-type polymer from a polyaniline may be carried out with different starting materials in the sense that the polyaniline can be used in either its emeraldine form, i.e. as emeraldine salt or emeraldine base, or in its leucoemeraldine form, i.e leucoemeraldine base.

For cost reasons, the method for synthesizing a phenothiazine-type polymer from a polyaniline is preferably carried out by starting with from a polyaniline in its

emeraldine form, which is reacted with a source of chalcogen with reducing character, where the chalcogen in the source of chalcogen is preferably in an oxidation state below 0.

The source of chalcogen acts as a reducing agent reduces the polyaniline in emeraldine form to a polyaniline in leucoemeraldine form while it is itself oxidized to elemental chalcogen.

In the alternative where polyaniline in its leucoemeraldine form becomes available at reasonable cost, it is also possible to start from a polyaniline in its leucoemeraldine form, or in other words from step c. of the method for synthesizing a phenothiazine- type polymer from a polyaniline. The step a. of optionally reacting a polyaniline in emeraldine form with a source of chalcogen preferably in a solvent to form a suspension, of polyaniline in

leucoemeraldine form may be carried out in a suitable reaction vessel, such as for example a flask equipped with a stirrer, or a rotary evaporator.

The solvent may be any suitable solvent such as m-cresol, DMAC, DMF, DMSO, NMP, water, and is most preferably H ₂ 0.

The reduction of emeraldine takes place at ambient temperature and can be visually monitored by change in colour from blue to clean yellow-gray solution or suspension.

No particular precautions such as air or moisture exclusion are required

The source of chalcogen may be a source of sulphur, selenium, tellurium, or polonium where the chalcogen is in a low oxidation state, and preferably is in an oxidation state below 0, and more preferably in an oxidation state between -2 and 0.

The source of chalcogen may be a source of sulphur or selenium, since sulphur and selenium are more readily available, and is most preferably a source of sulphur.

The source of chalcogen may be added in excess molar amounts, to maximize the incorporation level of the chalcogen into the polyaniline.

Convenient sources of sulphur are ammonium polysulfide solution or polymeric sulphur. Ammonium polysulfide plays a dual role as reducing agent for emeraldine base to leucoemeraldine with subsequent oxidation of S _n ^2~ to S _n .

The source of sulphur may be a polysulfide such as for example salts of polysulfide having the general formula (I)

M(2/x) ^X+ Sn ^2" (I)

where x can be either 1 or 2, and n may be an integer from 1 to 10, is preferably less than 5, and more preferably is 1 , 2 or 3.

Suitable sources of sulphur may be for example ammonium polysulfide (NH ₄ ) ₂ S ₃ and solutions, preferably aqueous solutions thereof or to a lesser extent sodium sulfide Na ₂ S.

The source of sulphur may alternatively be polymeric sulphur in any available allotropes of sulphur.

The step of b. removing the solvent from the suspension of a polyaniline in

leucoemeraldine form to form a powder of a polyaniline in leucoemeraldine form can be carried out in a suitable reaction vessel, such as for example a flask equipped with a stirrer or a rotary evaporator. Other methods include placing the suspension of polyaniline in a dessicator equipped with dessicants such as freshly calcined quicklime or dry silica.

The solvent is preferably removed by evaporation, and more preferably by

evaporation under vacuum alone or vacuum combined with heat.

In the case the solvent is water, the vacuum may be adjusted to a pressure of less than 100 mbar, or between 100 and 5 mbar, and the temperature can be adjusted to 80°C. In this case, the removing of the essentially all the water can be achieved after heating for 1 hour.

Removing the solvent from the suspension of a polyaniline in leucoemeraldine form yields a particulate aggregate, such as for example a powder, of polyaniline in leucoemeraldine form that may be ground into a finer powder for further processing.

The step of c. heating the powder of a polyaniline in leucoemeraldine form in the presence of a catalyst and a chalcogen to form a phenothiazine-type polymer may be carried out in a suitable sealed reaction vessel, such as for example an ampoule or vial. As already explained, in some embodiments of the invention, the chalcogen of step c. is the product of the oxidation of the source of chalcogen in step a.

The heating of the powder of a polyaniline in leucoemeraldine form is achieved by heating the polyaniline in leucoemeraldine form to a temperature of from 50°C to 250°, preferably to a temperature of from 100°C to 250°C, more preferably to a temperature of 175°C to 225°C, most preferably to a temperature of 200 °C.

The appropriate duration for heating the polyaniline in leucoemeraldine form in the presence of a catalyst to form a phenothiazine-type can be determined by a change in colour from yellow to black, which may be accomplished after heating for 1 to 5 hours, preferably after 2 hours.

The catalyst may be a Lewis acid, such as for example FeCI ₃ or AICI ₃ or iodine. Preferably the catalyst is iodine.

The phenothiazine-type polymer is formed by electrophilic addition of chalcogen, preferably of sulphur, selenium or tellurium to the aromatic system, (more details?) The electrophilic addition of sulphur, selenium or tellurium to the aromatic system is assisted by catalytic amounts of iodine, FeCI ₃ or AICI3.

After the formation of the phenothiazine-type polymer, reaction byproducts such excess sulphur, selenium or tellurium and the catalyst may be removed in an intermediate washing step in which the resulting solid block of phenothiazine-type polymer is re-grinded and then washed with excess aromatic solvent such as benzene or toluene, which is preferably heated to a temperature of from 50°C to 80°C.

The step of d. optionally doping the phenothiazine-type polymer with a protic acid to form a doped phenothiazine-type polymer may be achieved by washing the.

phenothiazine-type polymer obtained in step c with an solution of a protic acid.

Suitable protic acids are mineral acids such as for example HF, HBr, HCI, H2S04, H3PO4, H3BO3, HCIO4 . Preferably the mineral acid is hydrochloric acid.

Preferably, the molarity of the solution of the protic acid is of from 0.01 to 1 molar, more preferably of from 0.8 to 0.12 molar.

The method according to the present invention may further comprise the step of e. heating the phenothiazine-type polymer to a temperature of 100°C to 200°C, preferably to a temperature of 120°C to 160°C, more preferably to a temperature of 120°C to 140°C under vacuum (less than 1 mbar) for 1 to 10 hour, preferably 4-8 hours.

The heating of the phenothiazine-type polymer to a temperature of 100°C to 200°C leads to an unexpected augmentation of capacity of the phenothiazine-type polymers, which of course is highly desirable for the use of the phenothiazine-type polymers in electrochemical cells, and more particularly in lithium-based

electrochemical cells.

The polymer in the doped state (HCI) shows an electronic conductivity similar to the parent emeraldine salt (10 ^"2 -10 ^"1 S/cm) and a temperature dependency of conductivity typical for semiconductors with mixed intra-chain conduction and inter-chain hopping mechanism.

The polyphenothiazine polymers exhibit redox capacities and cyclability in excess of those reported for the parent polyaniline or standard conductive polymers.

The electrical conductivity, electrochemical stability in highly oxidizing regime

(cathodic regime) and redox reversibility could promote these polymers as

alternative cathode materials or conducting fillers additives for cathode assemblies.

The polymeric chain of the phenothiazine-type polymer obtained through the method above comprises block structures of chemical structure (I) and/or chemical structure (II).

Structure (I) Structure (II)

In chemical structure (I), the n may be any positive integral, and preferably ranges of from 1 to 10 ⁶ , and more preferably ranges of from 10 to 10 ⁵ , and X may be a chalcogen, and preferably is sulphur, selenium, tellurium or polonium, and more preferably is sulphur or selenium.

In chemical structure (II), the m may be any positive integral, and preferably ranges of from 1 to 10 ⁶ , and more preferably ranges of from 10 to 10 ⁵ , and X may be a chalcogen, and preferably is sulphur, selenium, tellurium or polonium, and more preferably is sulphur or selenium.

The block structures of chemical structure (I) and/or chemical structure (II) may be distributed randomly in the polymeric chain of the phenothiazine-type polymer of the present invention.

Because the block structures of chemical structure (I) and/or chemical structure (II) are the result of the incorporation of chalcogen into the polymeric chain of the polyaniline, an incorporation level of chalcogen of for example 40 to 60 percent means that in the polymeric chain of the phenothiazine-type polymer the block structures of chemical structure (I) and/or chemical structure (II) are linked by the residual polyaniline segments devoid of chalcogen. Alternatively, the phenothiazine-type polymer of the present invention could also be described as a random block polymer comprising block structures, or blocks, of polyaniline, chemical structure (I) and/or chemical structure (II).

The block structures of polyaniline may comprise one aniline residue and preferably of from 1 to 10 ⁶ , and more preferably ranges of from 10 to 10 ⁵ aniline residues.

The electrochemical cell of the present invention comprises a. a cathode comprising a phenothiazine-type polymer, optionally mixed with a conductive material, b.an anode comprising elemental lithium and can electrolyte composition, preferably a non-aqueous and aprotic electrolyte composition, and may further comprise a separator.

In one embodiment of the present invention, the electrochemical cell of the present invention comprises a. a cathode comprising, essentially consisting of, the

phenothiazine-type polymer, b.an anode comprising elemental lithium and can electrolyte composition, wherein the phenothiazine-type polymer is doped with a protic acid to form a doped phenothiazine-type polymer. In the case where the cathode essentially consists of the phenothiazine-type polymer, the phenothiazine- type polymer acts as the cathodic material in the electrochemical cell and provides an electronically conductive path for the external circuit on the cathodic side.

In another embodiment of the present invention, the electrochemical cell of the present invention comprises a. a cathode comprising, preferably essentially consisting of, a phenothiazine-type polymer in a non-doped state mixed with a carbonaceous conductive material, b.an anode comprising elemental lithium and can electrolyte compound, wherein the carbonaceous conductive material is chosen among graphite or graphene, and/or mixtures thereof. For cost reasons, graphite may be preferable to graphene. In the case where the cathode comprises a

phenothiazine-type polymer mixed with a carbonaceous conductive material, the phenothiazine-type polymer mixed with a carbonaceous conductive material is preferably left un-doped, i.e. is not is doped with a protic acid to form a doped phenothiazine-type polymer. In the case where the cathode comprises a

phenothiazine-type polymer mixed with a carbonaceous conductive material, the carbonaceous conductive material acts as a conductive filler and the phenothiazine- type polymer acts as the redox active cathodic material. The role of the

carbonaceous additive is to compensate for the drop in the electronic conduction of the phenothiazine-type polymer in its fully reduced form around 2.8V vs. lithium.

The carbonaceous conductive material may be present in an amount of 10 to 20 weight percent, based on the totai weight of the phenothiazine-type polymer and the carbonaceous conductive material.

In yet another embodiment of the present invention, the electrochemical cell of the present invention comprises a. a cathode comprising, preferably essentially consisting of, a phenothiazine-type polymer mixed with a lithium-based conductive material, b.an anode comprising elemental lithium and can electrolyte compound, wherein the phenothiazine-type polymer is doped with a protic acid to form a doped phenothiazine-type polymer and wherein the lithium-based conductive material may be chosen among lithium manganese spinel, lithium nickel manganese cobalt, lithium iron phosphate, lithium nickel cobalt aluminium oxide, lithium cobalt oxide and/or mixtures of two or more thereof. In the case where the cathode comprises a phenothiazine-type polymer mixed with a lithium-based conductive material, the phenothiazine-type polymer acts as an electrically conductive binder for the inorganic lithium-based conductive material which in this case is the cathodic material.

The inorganic lithium-based conductive material may be present in an amount of 80 to 95 weight percent, preferably 86 to 93 weight percent, based on the total weight of the electrically conductive phenothiazine-type polymer and the lithium-based conductive material.

The electrolyte may either be a solid electrolyte composition or a liquid electrolyte composition, within the typical temperature range of -20°C to 60°C used for consumer electronics. However, a person skilled in the art will know how to choose the electrolyte such that it does not undergo phase change within the intended use and corresponding temperature range.

Suitable liquid electrolyte compositions may be chosen from non-aqueous, aprotic electrolytes such as for example solutions of lithium fluoroalkyl phosphates or lithium hexafluorophosphate in carbonate derivatives such as ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate; or solutions of lithium fluoroalkyl phosphates or lithium hexafluorophosphate in acetonitrile, ethyl acetate, n-methyl pyrrolidone or tetrahydrofurane. Preferably the liquid electrolyte composition is a 1 M solution of lithium fluoroalkyl phosphates in a 1 :1 mixture of ethylene carbonate and dimethyl carbonate.

EXPERIMENTS

Synthesis

The starting polyaniline emeraldine base was synthesised based on available literature (J. Am. Chem. Soc; 2005; 127; 16770-16771)

500mg of the emeraldine base were mixed with 0.5 ml_ 20% aqueous ammonium polysulfide solution to form a suspension. The suspension was stirred under ambient conditions for 2 hours. The suspension was then placed under vacuum at 80°C for 1 hour, and the resulting powder was grinded and placed in an ampoule with a catalytic amount of iodine (35mg).The ampoule was heated to 200 °C for 3 hours. After that, the solid had changed colour from yellow to black. The resulting solid block was grinded and washed with excess hot benzene to remove excess sulphur followed by a washing step with diluted 0.1 molar HCI for acid doping. Due to non-stoichiometric sulphur reaction the yield of 121 % (604mg) is reported based on the total amount of the starting polyaniline.

BATTERY TYPE 1

A current collector made of titanium with a diameter of 13 mm was covered with a thin layer of graphite oxide solution in water and heated up 180°C for 1 hour when a thin layer of conductive carbon is formed. The conductive carbon layer plays the role of a more reliable electrical interface between the cathodic material and metal collector without the possible complexities associated with the insulating oxide layer inherently formed on the metal.

The dried polymer in its non-protonated state (170mg) was mixed with conductive carbon (SuperP, 30mg) and ball-milled for 30 minutes at 30 oscillations per second. A sample of the powder was pressed on the titanium collector at a pressure of 8-10 tones/cm2. The amount of cathodic composite was in 5-20 mg range. A lithium foil (Alfa Aesar 99.9% 1.5mm) was used as reference electrode and anode material. The physical separator is a laminated polypropylene/polyethylene membrane (Celgard) and glass wool disks for the liquid electrolyte retention. For each battery, LP30 (Merck) was used as electrolyte. The rate of charge and discharge was about 40 to 00 A/Kg of active mass in the potential range of 2.8 to 4.1 V. Over- and under potentials lead to degradation of the polymer via overoxidation or reductive degradation.

Electrochemical cells having the polymer mixed with 10-20% graphite as

cathode, metal lithium as anode and LF 30 commercial electrolyte. Typical capacities of 180 Ah/kg, substantially higher than the typically accepted value for polyaniline of 110 Ah/kg, were observed at average voltage of 3.4V. The battery capacity after 100 cycles drops to 145Ah/kg and to 120Ah/kg after 200 cycles. The cycling was performed between 4.2 and 2 or 2.5V versus lithium.

BATTERY TYPE 2

Alternatively, the polymer-acid doped was used as conductive filler and binder for LFP (lithium iron phosphate). Functional reversible batteries with a maximum of 164 Ah/kg were made with LFP as main component and a loading of 14% polymer. No additional additives were needed. Lower polymer loadings (10 respectively 7%) have led to a maximum capacity of 121Ah/kg.

Thus, the polymeric materials could find applications as stand-alone cathodic materials with capacities comparable to inorganic oxides, conductive filler for cathodic applications and conductive support for solid electrolytes upon further derivatization.