This invention concerns electrochemically addressable lithium insertion electrode systems for electrochemical cells using non-aqueous organic electrolytes, quasi-solid gel electrolytes, solid electrolytes, or the like and in particular the use of said electrolytes in combination with porous electrode materials, i.e. doped or non-doped nanoparticles or sub-microparticles of lithium insertion materials and redox active compounds in the electrolyte. This invention also concerns the configuration of the electrochemical cell containing the redox active compounds.
Electrochemical cells, as illustrated in
The lithium insertion materials in commercial electrochemical cells comprise 2˜25 wt. %, typically 10 wt. % conductive additives. These conductive agents do not participate in the redox reactions and therefore represent inert mass reducing the specific energy storage capacity of the electrode. This situation is especially severe as the lithium insertion material or its de-intercalated state has very poor electronic conductivity.
For instance, pioneering work by Padhi et al (J. Electrochem. Soc. 144, 1188 (1997).) first demonstrated reversible extraction of Li from the olivine-structured LiFePO4, however 25 wt. % acetylene black was added. This is also illustrated in JP 2000-294238 A2 wherein a LiFePO4/Acetylene Black ratio of 70/25 is used.
U.S. Pat. No. 6,235,182 and WO Pt. No. 9219092 disclose a method for coating insulators with carbon particles by substrate-induced coagulation. This method involves the adsorption of polyelectrolyte compound and subsequent coagulation of carbon particle on the substrate to form an adhesive carbon coating. For high quality carbon coating, the size of carbon particle is very dependent on the dimension of substrate and the amount of carbon used is still remarkable.
International patent application WO 2004/001881 discloses a new route for the synthesis of carbon-coated powders having the olivine or NASICON structure by mixing the precursors of carbon and said materials and subsequent calcinations. Nevertheless, it is still necessary to have 4˜8 wt. % of coated carbon to exploit the invention fully.
It has been discovered that the presence of some redox active compounds in the electrolyte forms an electrochemically addressable electrode system. As illustrated in
The cell is composed of two compartments, where the cathodic compartment comprises a cathodic lithium insertion material and p-type redox active compound(s) in the electrolyte; the anodic compartment comprises an anodic lithium insertion material and n-type redox active compound(s) in the electrolyte. These two compartments are separated by a separator and the redox active compounds are confined only in each compartment.
Compared to the whole electrode system, the redox active compounds do not occupy any extra volume of the whole electrode system. Hence with respect to prior art, the present invention allows reducing greatly the volume of the conductive additives resulting in a much improved energy storage density.
It is therefore an object of the invention to provide a means to avoid or minimize the amount of the conductive additives required for the operation of an ion insertion battery. It is also an object of the invention to provide a rechargeable electrochemical cell having higher energy density.
The invention relates therefore to a rechargeable electrochemical cell as defined in the claims.
As used herein, the term “lithium insertion material” refers to the material which can host and release lithium or other small ions such as Na+, Mg2+ reversibly. If the materials lose electrons upon charging, they are referred to as “cathodic lithium insertion material”. If the materials acquire electrons upon charging, they are referred to as “anodic lithium insertion material”.
As used herein, the term “p-type redox active compound” refers to those compounds that present in the electrolyte of cathodic compartment of the cell, and act as molecular shuttles transporting charges between current collector and cathodic lithium insertion material upon charging/discharging. On the other hand, the term “n-type redox active compound” refers to the molecules that present in the electrolyte of anodic compartment of the cell, and act as molecular shuttles transporting charges between current collector and anodic lithium insertion material upon charging/discharging.
The invention will be better understood below with a detailed description including different embodiments.
This is illustrated by the following figures
Right chart: Galvanostatic charge/discharge cycle; electrolyte solution 1 M LiPF6 in EC/DMC. Curve A: Electrode from LiFePO4 surface-derivatized with Z-907Na/SWCNT mg/cm2) charging rate C/5. Curve B (dashed line): electrode from carbon-coated LiFePO4 (Nanomyte BE-20, 2.28 mg/cm2) charging rate C/50.
As illustrated in
The relevant materials used in the cathodic electrode system comprise a cathodic lithium insertion material and a p-type redox active compound dissolved in the electrolyte of the cathodic compartment.
Preferred Cathodic Lithium Insertion Materials used Herein are:
Doped or non-doped oxides LiMO2 where M is one or more elements selected from M=Co, Ni, Mn, Fe, W, V, LiV3O8 or mix of them; phosphor-olivines as LiMPO4 where M is one or more elements selected from M=Fe, Co, Mn, Ni, VO, Cr and mix of them and spinels and mixed spinels as LixMn2O4 or Li2CoxFeyMnzO8, etc., nano- or sub-microparticles. The particle size ranges from 10 nm to 10 micrometer, preferably 10˜1000 nm.
Preferred p-Type Redox Active Compounds have the Following Structure:
n=0 to 20
R′=alkyl(C1 to C20) or H
R′=alkyl(C1 to C20) or H
R′=alkyl(C1 to C20) or H
R′=alkyl(C1 to C20) or H
B12R1R2R3R4R5R6R7R8R9R10R11R12
R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12 can be
R′=alkyl(C1 to C20) or H
The relevant materials used in the anodic electrode system comprise an anodic lithium insertion material and an n-type redox active compound dissolved in the electrolyte of the anodic compartment.
Preferred anodic lithium insertion materials used herein are: Doped or non-doped TiO2, SnO2, SnO, Li4Ti5O12 nano- or sub-microparticles. The particle size ranges from 10 nm to 10 micrometer, preferably 10-500 nm.
Preferred n-type redox active compounds have the following structure:
or
R=alkyl(C1 to C20) or H
The separator used herein can be solid electrolyte-fast lithium ion conductor, such as Lithium Phosphorus Oxynitride (LiPON), 70Li2S.30P2S5, etc. or ceramic nanofiltration membrane, which allows the transport of lithium ions through it, but prohibits the permeation of the redox active compounds.
In one embodiment of the invention, the rechargeable electrochemical cell comprises:
In a preferred embodiment, the rechargeable electrochemical cell according to the invention comprises:
In a particularly preferred embodiment of the rechargeable electrochemical cell of the present invention, the cathodic electrode comprising binder, conductive additives, and doped or non-doped LiMPO4, wherein M=Fe, Mn, Co in first electrode compartment, having p-type redox active compound dissolved therein; and the second electrode comprising binder, conductive additives, and anodic lithium insertion material.
In this embodiment, the electronic conductivity of the cathodic lithium insertion materials is very poor, and the presence of p-type redox active compound makes the treated electrode system much more electrochemically addressable.
The invention is illustrated in the following EXAMPLES.
LiFePO4 powder with particle size distribution of 200˜700 nm was mixed with PVDF in weight ratio of 95:5. A 1.0 cm×1.0 cm electrode sheet comprising 10 μm thick same was used as working electrode, with lithium foil as counter and reference electrodes for electrochemical test. The three electrodes were separated to three compartments by glass frits and filled with EC+DMC (1:1)/1M LiPF6 electrolyte. In the LiFePO4 electrode compartment, 0.1M MPTZ was dissolved therein.
LiFePO4 powder with particle size distribution of 200˜700 nm was mixed with PVDF and acetylene black in weight ratio of 95:5. A 1.0 cm×1.0 cm electrode sheet comprising 10 μm thick same was used as working electrode, with lithium foil as counter and reference electrodes for electrochemical test. The three electrodes were separated to three compartments by glass frits and filled with EC+DMC (1:1)/1M LiPF6 electrolyte. In the LiFePO4 electrode compartment, 0.032 M Os(mobpy)3Cl2 and Os(mbpy)3Cl2 was dissolved therein. The volume of electrolyte in cathodic compartment is 30 μl.
It has been discovered that the presence of some redox active polymer compounds covered on active material forms an electrochemically addressable electrode system. As illustrated in
The cell is composed of two compartments, where the cathodic compartment comprises a cathodic lithium insertion material and p-type redox active polymer compound(s); the anodic compartment comprises an anodic lithium insertion material and n-type redox active polymer compound(s), which can also act as binder. These two compartments are separated by a separator. Compared to the whole electrode system, the redox active polymer do not occupy any extra volume of the whole electrode system. Hence with respect to prior art, the present invention allows reducing greatly the volume of the conductive additives resulting in a much improved energy storage density. The polymer redox material is not soluble in the working electrolyte so the use of a special separator as described in the European patent application 06 112 361.8 is not necessary.
According to the present invention, a redox active molecule is attached to the polymer backbone, either by covalent bonding or quternization. A suitable polymer may be selected from polyvinyl pyridine, polyvinyl imidazole, polyethylene oxide, polymethylmethacrylate, polyacrylonitrile, polypropylene, polystyrene, polybutadiene, polyethyleneglycol, polyvinylpyrrolidone, polyaniline, polypyrrole, polythiophene and their derivatives. Preferred polymer is polyvinyl pyridine.
A redox active centre may an organic compound or a metal complex having suitable redox potential as that of the battery material.
In preferred configuration the redox active metal complex or organic compound (D) is of the type given below,
D-π(Ral)q- (1)
wherein π represents schematically the π system of the aforesaid substituent, Ral represents an aliphatic substituent with a saturated chain portion bound to the π system, and wherein q represents an integer, indicating that π may bear more than one substituent Ral.
The π system π may be an unsaturated chain of conjugated double or triple bonds of the type
wherein p is an integer from 0 to 20.
or an aromatic group Rar of from 6 to 22 carbon atoms, or a combination thereof.
wherein R1, R′1 is an alkyl from 1 to 10 carbon atoms, x≧0, and 0<n<5.
According to a preferred embodiment, D is selected from benzol, naphtaline, indene, fluorene, phenantrene, anthracene, triphenylene, pyrene, pentalene, perylene, indene, azulene, heptalene, biphenylene, indacene, phenalene, acenaphtene, fluoranthene, and heterocyclyc compounds pyridine, pyrimidine, pyridazine, quinolizidine, quinoline, isoquinoline, quinoxaline, phtalazine, naphthyridine, quinazoline, cinnoline, pteridine, indolizine, indole, isoindole, carbazole, carboline, acridine, phenanthridine, 1,10-phenanthroline, thiophene, thianthrene, oxanthrene, and derivatives thereof, optionally be substituted.
According to a preferred embodiment, D is selected from structures of formula (1-11) given below:
in which each of Z1, Z2 and Z3 is the same or different and is selected from the group consisting of O, S, SO, SO2, NR1, N+(R1′)(1″), C(R2)(R3), Si(R2′)(R3′) and P(O)(OR4), wherein R1, R1′ and R1″ are the same or different and each is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxy groups, alkoxyalkyl groups, aryl groups, aryloxy groups, and aralkyl groups, which are substituted with at least one group of formula —N+(R5)3 wherein each group R5 is the same or different and is selected from the group consisting of hydrogen atoms, alkyl groups and aryl groups, R2, R3, R2′ and R3′ are the same or different and each is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxy groups, halogen atoms, nitro groups, cyano groups, alkoxyalkyl groups, aryl groups, aryloxy groups and aralkyl groups or R2 and R3 together with the carbon atom to which they are attached represent a carbonyl group, and R4 is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxyalkyl groups, aryl groups, aryloxy groups and aralkyl groups.
Preferred embodiments of, structure (10) for D may be selected from structures (12) and (13) below:
Alternatively a redox active centre may be a metal complex having suitable redox potential as that of the battery material.
These aims are achieved by using, as a ligand, an organic compound L1 having a formula selected from the group of formulae (14) to (27)
wherein at least one of substituents —R, —R1, —R2, —R3,
—R′, —R1′, —R2′, —R3′, —R″ comprises an additional π system located in conjugated relationship with the primary π system of the bidentate or respectively tridentate structure of formulae (14) to (27).
In preferred compounds L1, the said substituent is of the type
—R=π(Ral)q
wherein π represents schematically the π system of the aforesaid substituent, Ral represents an aliphatic substituent with a saturated chain portion bound to the π system, and wherein q represents an integer, indicating that π may bear more than one substituent Ral.
The π system π may be an unsaturated chain of conjugated double or triple bonds of the type
wherein p is an integer from 0 to 8.
or an aromatic group Rar of from 6 to 22 carbon atoms, or a combination thereof.
The presence of an aromatic group is preferred, since it is less sensitive to oxidation than a long chain of conjugated double or triple bonds.
Among suitable aromatic groups, there are monocyclic aryls like benzene and annulenes, oligocyclic aryls like biphenyle, naphthalene, biphenylene, azulene, phenanthrene, anthracene, tetracene, pentacene, or perylene. The cyclic structure of Rar may incorporate heteroatoms.
In metal complexes as redox active centers, the preferred ligands coordinated to the metal, according to the invention are organic compounds L1 having a formula selected from the group of formulae (14) to (27)
wherein at least one of the substituents —R, —R1, —R2, —R3,
—R′, —R1′, —R2′, —R3′, —R″ is of formula (2), (3) or (4)
wherein p is an integer from 0 to 4,
wherein q is an integer from 0 to 4,
wherein Rar is a monocyclic or oligocyclic aryl from C6 to C22,
wherein -Ral is H, —R1, (—O—R1)n, —N(R1)2, —NHR1,
wherein R1, R′1 is an alkyl from 4 to 10 carbon atoms, x≧2 and 0<n<5 and
wherein the other one(s) of substituent(s) —R, —R1, —R2, —R3, —R′, —R1′, —R2′, —R3′, —R″ is (are) the same or a different substituents of formula (1), (2) or (3), or is (are) selected from —H, —OH, —R2, —OR2 or —N(R2)2, wherein R2 is an alkyl of 1 to 20 carbon atoms.
The resulting compound is an organometallic complex of a metal Me selected from the group consisting of Ru, Os and Fe, comprising as a ligand a compound L and L1 as described herein before, said complex being of formula
MeL1L(Z)2 (5)
if L and L1 are the same or different from a compound of formulas (15), (16), (18), (20), (21), (22) or (23), (24), (25), (26), (27), (28)
and of formula
MeL2LZ (6)
if L is from a compound of formula (15), (16), (18), (20), (21), (22), (23) or (24), (25), (26), (27), (28) and L2 is a compound of formula (17) or (19)
wherein Z is selected from the group consisting of H2O, Cl, Br, CN, NCO, NCS and NCSe and
wherein in L at least one of substituents R, R′, R″ comprises a π system in conjugated relationship with the π system of the bidentate, respectively the tridentate structure of formulae (14) to (28),
and wherein the other one(s) of substituents R, R′, R″ is (are) the same or a different substituent including a π system, or is (are) selected from H, OH, R2, (OR2)n, N(R2)2, where R2 is an alkyl of 1-20 carbon atoms and 0<n<5.
and of formula
MeL1(L2)(L3) (7)
wherein L1, L2 and L3 are the same or different from a compound of formula (14), (15), (16), (18), (20), (21), (22), (23), (24), (25), (26), (27) or (28)
and of formula
Me(L1)(L2) (8)
wherein L1 and L2 may be same or different, and at least one of substituents R, R′, R″ comprises a π system in conjugated relationship with the π system of the tridentate structure of formulae (17) to (19),
and wherein the other one(s) of substituents R, R′, R″ is (are) the same or a different substituent including a 7 system, or is (are) selected from H, OH, R2, (OR2)n, N(R2)2, where R2 is an alkyl of 1-20 carbon atoms and 0<n<5.
LiFePO4 was synthesized by a variant of solid state reaction [17] employing FeC2O4.2H2O and LiH2PO4 as precursors. Their stoichiometric amounts were mixed and ground in a planetary ball-milling machine for 4 h. Then the powder was calcined in a tube furnace with flowing Ar—H2 (92:8 v/v) at 600° C. for 24 h. After cooling down to room temperature, the sample was ground in agate mortar. The BET surface area of the powder was ca. 5 m2/g with an average particle size of 400 nm. X-ray diffraction confirmed the phase purity. The BET surface area of the powder was ca. 5 m2/g with an average particle size of 400 nm.
Synthesis of 10-(12′-bromododecyl)phenoxazine. Sodium hydride (55% dispersion in mineral oil; 119 mg, 4.97 mmol) was stirred in dry THF under argon atmosphere. Phenoxazine (500 mg, 2.73 mmol) was added to a stirred suspension of the sodium hydride in THF. The mixture was stirred to form phenoxazine N-sodium salt for 2 hours at 50° C. 1,12-dibromododecane (8962 mg, 27.3 mmol) was added to the solution and stirred vigorously for 24 hours at room temperature. The mixture was filtered and evaporated under reduced pressure. The excess 1,12-dibromododecane was recovered from the mixture by Kugelohr distillation (163° C., 0.1 mmHg). 10-(12′-bromododecyl)phenoxazine was distilled at 225° C. by Kugelohr distillation. The material was kept in inert atmosphere. The product was identified by 1H NMR spectrum. 1H NMR (400 MHz; CDCl3); δ (ppm); 6.80 (2H, Ar—H), 6.67 (4H, Ar—H), 6.48 (2H, Ar—H), 3.49 (2H, t), 3.44 (2H, t), 1.87 (2H, m), 1.67 (2H, m), 1.42 (16H, m).
Poly(4-(-(10-(12′-dodecyl phenoxazine)pyridinium)-co-4-vinylpyridine). To a solution of poly(4-vinylpyridine) (number average molecular weight; 160,000) (173 mg) in 15 ml DMF was added LiTFSI (260 mg) and 10-(12′-bromododecyl)phenoxazine (111 mg, 0.26 mmoles). The solution was mechanically stirred at 50° C. for 36 h. The solution was cooled to room temperature and then diethyl ether was added slowly to obtain a precipitate of Poly(4-(-(10-(12′-bromododecyl phenoxazine)pyridinium)-co-4-vinylpyridine). The solid was collected by a vacuum filtration and dried under vacuum at 35° C. for 8 hrs. This redox polymer is insoluble in common organic solvents hampering the characterization of this material. The molar ratio of pyridine to phenoxazine was 1/6; the polymer is further abbreviated PPV-POA (1/6).
The polymer PVP-POA(1/6) was stirred with γ-butyrolactone for several hours until a viscous slurry was obtained. This slurry was further mixed with LiFePO4 powder while the proportion of PVP-POA(1/6) in the solid mixture with LiFePO4 was 10 wt %. This slurry was stirred again overnight. The mixing and homogenization was sometimes also promoted by sonication in ultrasound bath. The resulting homogeneous slurry was then doctor-bladed onto F-doped conducting glass (FTO) and dried at 100° C. The typical film mass was ca. 1 mg/cm2. Blank electrodes from pure PVP-POA(1/6) were prepared in the same way for reference experiments. In this case, the typical film mass was 0.1 to 0.2 mg/cm2.
Electrochemical experiments employed an Autolab PGSTAT 30 potentiostat. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, v:v). The reference and counter electrodes were from L1-metal.
Obviously, the phenoxazine, which is covalently bonded to a polymer backbone, acts as a mediator, providing holes to interfacial charge transfer of LiFePO4. The long (C12) aliphatic chain grants sufficient swinging flexibility to the redox mediator, so that it can reach the olivine surface. We suggest calling this effect as “polymer wiring”. Its advantage over molecular wiring [15] consists in the fact, that the amount of redox material can be easily increased above the monolayer coverage. This would allow running larger currents, as the process is not limited by the speed of cross-surface hole percolation. The polymer wiring thus resembles the redox targeting [16]. However, the electrochemical cell employing polymer wiring does not require any molecular separator between the cathode and anode, which would prevent undesired transport of the redox-targeting molecule to the other electrode [16]. Hence, the polymer wiring seems to be the optimum strategy for enhancement of the electrochemical activity of virtually insulating materials like LiFePO4. It combines the advantages of both approaches: (i) fixed redox species near the LiFePO4 surface and (ii) larger amount of available redox species for wiring. The latter fact is also beneficial for the electrode stability, as the system is less sensitive to imperfections in the adsorbed monolayer of redox relay [15].
At high scan rates, such as 50 mV/s, the polymer wiring is, however, not fast enough for charging of LiFePO4 to a significant capacity. For the electrode in
However, this charge balance changes in favor for charging of LiFePO4 at slower scan rates.
It has been discovered that some amphiphilic redox active molecules interact to SWCNT can further anchor with the surface of electrode active material such as LiFePO4 (olivine). The assembly of redox molecule and SWCNT thus covers the surface of the active material, forming an electrochemically addressable electrode system. For cathodic lithium insertion material upon positive polarization the donor redox active compound (D) will be oxidized at current corrector and charges (holes) will be transported from the current collector to the lithium insertion material by the oxidized form of the redox active compound (D+). As the redox potential of the redox active compound is higher or matches closely the Fermi level of the lithium insertion material, D+ will be reduced by the lithium insertion material. Electrons and lithium ions will be withdrawn from it during battery charging. By contrast, during the discharging process, the oxidized species are reduced at current collector and charges (electrons) are transported from the current collector to the lithium insertion material by the redox active compound (D). Lithium ions and electrons are injected into the solid, as the redox potential of the redox active compound is lower or matches closely the Fermi level of the lithium insertion material.
The cell is composed of two compartments, where the cathodic compartment comprises a cathodic lithium insertion material and redox active compound(s); the anodic compartment comprises an anodic lithium insertion material and redox active compound(s). These two compartments are separated by a separator. Compared to the whole electrode system, the redox active adsorbate does not occupy any significant extra volume of the whole electrode system. Hence with respect to prior art, the present invention allows reducing greatly the volume of the conductive additives resulting in a much improved energy storage density. The redox adsorbate is not soluble in the working electrolyte so the use of a special separator as described in the European patent application 06 112 361.8 is not necessary.
According to the present invention, a redox active molecule is attached to the SWCNT backbone by non-covalent bonding. A redox active centre (D) may be an organic compound or a metal complex having suitable redox potential as that of the battery material. In preferred configuration the redox active metal complex or organic compound (D) is localized between the SWCNT surface and the surface of electrode active material.
SWCNT-D-[M] (1)
Wherein [M] represents schematically the electrode material
As used herein, the term “donor-type redox active compound” refers to those compounds that are present in the cathodic compartment of the cell, and act as molecular relay transporting charges between current collector and cathodic lithium insertion material upon charging/discharging. On the other hand, the term “acceptor-type redox active compound” refers to the molecules that present in the anodic compartment of the cell, and act as molecular relay transporting charges between current collector and anodic lithium insertion material upon charging/discharging.
A redox active centre may be an organic compound or a metal complex having suitable redox potential as that of the lithium insertion material.
In preferred configuration the redox active metal complex or organic compound (D) is of the type given below,
D-π(Ral)q
wherein π represents schematically the π system of the aforesaid substituent, Ral represents an aliphatic substituent with a saturated chain portion bound to the π system, and wherein q represents an integer, indicating that π may bear more than one substituent Ral.
The π system π may be an unsaturated chain of conjugated double or triple bonds of the type
wherein p is an integer from 0 to 20.
or an aromatic group Rar of from 6 to 22 carbon atoms, or a combination thereof.
wherein R1, R′1 is an alkyl from 1 to 10 carbon atoms, x≧0, and 0<n<5.
According to a preferred embodiment, D is selected from benzol, naphtaline, indene, fluorene, phenantrene, anthracene, triphenylene, pyrene, pentalene, perylene, indene, azulene, heptalene, biphenylene, indacene, phenalene, acenaphtene, fluoranthene, and heterocyclyc compounds pyridine, pyrimidine, pyridazine, quinolizidine, quinoline, isoquinoline, quinoxaline, phtalazine, naphthyridine, quinazoline, cinnoline, pteridine, indolizine, indole, isoindole, carbazole, carboline, acridine, phenanthridine, 1,10-phenanthroline, thiophene, thianthrene, oxanthrene, and derivatives thereof, optionally be substituted.
According to a preferred embodiment, D is selected from structures of formula (1-11) given below:
in which each of Z1, Z2 and Z3 is the same or different and is selected from the group consisting of O, S, SO, SO2, NR1, N+(R1′)(1″), C(R2)(R3), Si(R2′)(R3′) and P(O)(OR4), wherein R1, R1′ and R1″ are the same or different and each is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxy groups, alkoxyalkyl groups, aryl groups, aryloxy groups, and aralkyl groups, which are substituted with at least one group of formula —N+(R5)3 wherein each group R5 is the same or different and is selected from the group consisting of hydrogen atoms, alkyl groups and aryl groups, R2, R3, R2′ and R3′ are the same or different and each is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxy groups, halogen atoms, nitro groups, cyano groups, alkoxyalkyl groups, aryl groups, aryloxy groups and aralkyl groups or R2 and R3 together with the carbon atom to which they are attached represent a carbonyl group, and R4 is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxyalkyl groups, aryl groups, aryloxy groups and aralkyl groups.
Preferred embodiments of, structure (10) for D may be selected from structures (12) and (13) below:
Alternatively a redox active centre may be a metal complex having suitable redox potential as that of the lithium insertion material.
These aims are achieved by using, as a ligand, an organic compound L1 having a formula selected from the group of formulae (14) to (27)
wherein at least one of substituents —R, —R1, —R2, —R3,
—R′, —R1′, —R2′, —R3′, —R″ comprises an additional π system located in conjugated relationship with the primary π system of the bidentate or respectively tridentate structure of formulae (14) to (27).
In preferred compounds L1, the said substituent is of the type
—R=π(Ral)q
wherein π represents schematically the π system of the aforesaid substituent, Ral represents an aliphatic substituent with a saturated chain portion bound to the π system, and wherein q represents an integer, indicating that π may bear more than one substituent Ral.
The π system π may be an unsaturated chain of conjugated double or triple bonds of the type
wherein p is an integer from 0 to 8.
or an aromatic group Rar of from 6 to 22 carbon atoms, or a combination thereof.
The presence of an aromatic group is preferred, since it is less sensitive to oxidation than a long chain of conjugated double or triple bonds.
Among suitable aromatic groups, there are monocyclic aryls like benzene and annulenes, oligocyclic aryls like biphenyle, naphthalene, biphenylene, azulene, phenanthrene, anthracene, tetracene, pentacene, perylene or pyrene. The cyclic structure of Rar may incorporate heteroatoms.
In metal complexes as redox active centers, the preferred ligands coordinated to the metal, according to the invention are organic compounds L1 having a formula selected from the group of formulae (14) to (27)
wherein at least one of the substituents —R, —R1, —R2, —R3,
—R′, —R1′, —R2′, —R3′, —R″ is of formula (1), (2) or (3)
wherein p is an integer from 0 to 4,
wherein q is an integer from 0 to 4,
wherein Rar is a monocyclic or oligocyclic aryl from C6 to C22,
wherein -Ral is H, —R1, (—O—R1)n, —N(R1)2, —NHR1,
wherein R1, R′1 is an alkyl from 4 to 10 carbon atoms, x≧2 and 0<n<5 and
wherein the other one(s) of substituent(s) —R, —R1, —R2, —R3, —R′, —R1′, —R2′, —R3′, —R″ is (are) the same or a different substituents of formula (1), (2) or (3), or is (are) selected from —H, —OH, —R2, —OR2 or —N(R2)2, wherein R2 is an alkyl of 1 to 20 carbon atoms.
The resulting compound is an organometallic complex of a metal Me selected from the group consisting of Ru, Os and Fe, comprising as a ligand a compound L and L1 as described herein before, said complex being of formula
MeL1L(Z)2 (I)
if L and L1 are the same or different from a compound of formulas (15), (16), (18), (20), (21), (22) or (23), (24), (25), (26), (27), (28)
and of formula
MeL2LZ (II)
if L is from a compound of formula (15), (16), (18), (20), (21), (22), (23) or (24), (25), (26), (27), (28) and L2 is a compound of formula (17) or (19)
wherein Z is selected from the group consisting of H2O, Cl, Br, CN, NCO, NCS and NCSe and
wherein in L at least one of substituents R, R′, R″ comprises a π system in conjugated relationship with the π system of the bidentate, respectively the tridentate structure of formulae (14) to (28),
and wherein the other one(s) of substituents R, R′, R″ is (are) the same or a different substituent including a π system, or is (are) selected from H, OH, R2, (OR2)n, N(R2)2, where R2 is an alkyl of 1-20 carbon atoms and 0<n<5.
and of formula
MeL1(L2)2 (3)
wherein L1 and L2 are the same or different from a compound of formula (14), (15), (16), (18), (20), (21), (22), (23), (24), (25), (26), (27) or (28)
and of formula
Me(L2)(L2) (4)
wherein L2 may be same or different, in L2 at least one of substituents R, R′, R″ comprises a π system in conjugated relationship with the π system of the tridentate structure of formulae (17) and (19),
and wherein the other one(s) of substituents R, R′, R″ is (are) the same or a different substituent including a π system, or is (are) selected from H, OH, R2, (OR2)n, N(R2)2, where R2 is an alkyl of 1-20 carbon atoms and 0<n<5.
LiFePO4 was synthesized by a variant of solid state reaction [15] employing FeC2O4.2H2O and LiH2PO4 as precursors. Their stoichiometric amounts were mixed and ground in a planetary ball-milling machine for 4 h. Then the powder was calcined in a tube furnace with flowing Ar—H2 (92:8 v/v) at 600° C. for 24 h. After cooling down to room temperature, the sample was ground in agate mortar. The BET surface area of the powder was ca. 5 m2/g with an average particle size of 400 nm. X-ray diffraction confirmed the phase purity. The Ru-bipyridine complex, NaRu(4-carboxylic acid-4′-carboxylate(4,4′-dionyl-2,2′bipyridine)(NCS)2, coded as Z-907Na was synthesized as described elsewhere [16]. Single walled carbon nanotubes were grown by catalytic laser ablation method. The average diameter of tubes was determined by Raman and Vis-NIR spectroscopy to be ca. 1.3-1.4 nm. Other chemicals were from commercial sources and were used as received.
SWCNT were dispersed with solutions of surfactants (either pyrene butanoic acid in dimethylformamide (DMF) or Z-907Na in acetonitrile+tert-butanol (1:1) (AN/t-BuOH) by sonication. The optimized synthetic protocol for Z-907Na was as follows: 9 mg of SWCNT was sonicated for 2 hours with 10 mL of 6·10−4 M Z-907Na in acetonitrile+t-butanol (1:1). The resulting black-brown solution was centrifuged at 5000 rpm for 1 hour, while ca. 4 mg of undissolved carbon remained as a sediment. This working solution (abbreviated further as Z-907Na/SWCNT) was stable for at least weeks at room temperature without precipitation. Hence, the solution contained ca. 5 mg of dispersed SWCNT (417 μmol) and 6 μmol of Z-907Na (molar ratio C/Z-907Na≈70). The olivine LiFePO4 (200 mg) was mixed with several portions (0.5-0.7 mL) of this working solution. At the initial stages, the supernatant turned to colorless within several seconds after mixing. After each addition of the Z-907Na/SWCNT solution, the slurry was centrifuged, supernatant separated and a next portion of the solution was added. This procedure was repeated until the supernatant did not decolorize. The total amount of applied solution was 1.5 mL. Finally the powder was washed with AN/t-BuOH and dried at room temperature. The same synthetic protocol was also adopted also for surface derivatization of LiFePO4 with pyrenebutanoic acid/SWCNT.
Electrodes were prepared by mixing the powder of surface derivatized LiFePO4 with 5 wt % of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrolidone. The resulting homogeneous slurry was then doctor-bladed onto F-doped conducting glass (FTO) and dried at 10° C. overnight. Alternatively the slurry was coated on alumina current collector and dried at 100° C. overnight. The typical film mass was 1.5-2 mg/cm2. Blank electrodes from pure LiFePO4 were prepared in the same way for reference experiments. A second reference material was a carbon-coated LiFePO4 (Nanomyte BE-20 from NEI Corporation, USA).
The electrode was assembled in the electrochemical cell with Li reference and counter electrodes or alternatively in the Swagelok cell with Li negative electrode.
Vis-NIR spectra were measured at Varian Cary 5 spectrometer in 2 mm glass optical cells. The measurement was carried out in transmission mode with integrating sphere. Electrochemical experiments employed an Autolab PGSTAT 30 potentiostat. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, v:v). The reference and counter electrodes were from Li-metal.
The effect of molecular wiring was recently applied to the LiFePO4 electrode material, which can be wired by 4-(bis(4-methoxyphenyl)amino)benzylphosphonic acid [20]. In this case, the cross-surface hole percolation was followed by interfacial charging and discharging of LiFePO4 with Li+ ions [20]. Our data confirm that the hole-transport wiring is possible also with the Z-907Na complex, while a similar anodic current (exceeding 0.2 mA/cm2) can be wired to the LiFePO4 electrode at 0.1 V/s. The formal redox potential of Z-907Na adsorbed on inert TiO2 surface was about 3.5 V vs. Li/Li+ [19,21], which is just sufficient for the anodic wiring of LiFePO4 (redox potential 3.45 V vs. Li/Li+) but not for cathodic wiring [20]. Our data on
However, this situation changes dramatically, if the surface derivatization is carried out with the starting LiFePO4 powder instead of the doctor-bladed porous film.
The exceptional properties of our Z-907Na/SWCNT electrode are further demonstrated by galvanostatic charging/discharging cycle.
The applied amount of working solution Z-907Na/SWCNT (1.5 mL; 6×10−4 mol/L Z-907Na) gives the upper limit of the adsorbed Z-907Na to be 0.9 μmol and the amount of adsorbed carbon (in the form of SWCNT) to be 6.3 μmol per 200 mg of LiFePO4 (See Experimental Section). The concentration of elemental carbon from SWCNT was, therefore, less than 0.04 wt % in the final solid material). From the BET surface area of LiFePO4 we can calculate that the surface coverage of Z-907Na is equivalent to about one molecule per 2 nm2. This is not far from the monolayer coverage, if we take into account the usual dimensions of Ru-bipyridine molecules [22].
The unprecedented activity of the electrode composite of LiFePO4/Z-907Na/SWCNT is obviously due to the presence of carbon nanotubes, which can quickly transport the charge mediated by Z-907Na complex towards the olivine surface. This beneficial role of carbon nanotubes even promotes the cathodic process. This is almost absent in sole molecular wiring, due to low driving force of the redox process in Z-907Na for the reduction of Li1-xFePO4 back to the starting stoichiometric composition (
Number | Date | Country | Kind |
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06112361.8 | Apr 2006 | EP | regional |
PCT/IB2006/053832 | Oct 2006 | IB | international |
PCT/IB2006/053833 | Oct 2006 | IB | international |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2007/051246 | 4/6/2007 | WO | 00 | 12/8/2008 |