Conductive Carbons for Lithium Ion Batteries

Abstract

Disclosed herein are cathode formulations comprising a lithium ion-based electroactive material, and a carbon black, in which the carbon black is selected from one of: (i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (La) of at least 30 Å, as determined by Raman spectroscopy; (ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m2; and (iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (La) of at least 35 Å, as determined by Raman spectroscopy. Also disclosed are cathodes comprising the cathode formulations, electrochemical cells comprising the cathodes, and methods of making the cathode formulations and cathodes.

Description

FIELD OF THE INVENTION

Disclosed herein are cathode formulations comprising conductive carbons, e.g., carbon black, for use in lithium ion batteries, pastes comprising such conductive carbons, and methods for preparing the same.

BACKGROUND

The lithium ion battery industry is facing pressure related to ever increasing requirements for improved energy density and reduced cost. Some of the directions pursued involve development of new cathode compositions that typically operate at higher voltages and modification of existing compositions through coating and/or doping. Coating or doping of existing compositions can enable operation in wider potential/voltage range and thus enable reversible lithiation/delithiation of larger fractions of stoichiometric amounts of lithium stored in these materials. For example, the theoretical capacity of compositions such as LiCoO₂ (LCO) is close to 300 mAh/g, based on the stoichiometric amount of lithium stored. However, the practical capacity is often limited by the mechanical and chemical stability of LCO and is limited to ˜50% of the theoretical capacity. New electroactive materials, new electrolytes, and additives to electrolytes, are directions currently pursued by the industry. Such new materials, although possibly capable of operating at higher voltages, may compromise cycle life and durability.

Accordingly, there remains a need for continued development of new cathode formulations.

SUMMARY

One embodiment provides a cathode formulation comprising:

a lithium ion-based electroactive material;

carbon black having an OAN ranging from 100 to 250 mL/100 g,

wherein the carbon black has a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy.

Another embodiment provides a cathode formulation comprising:

a lithium ion-based electroactive material;

carbon black having an OAN ranging from 100 to 300 mL/100 g,

wherein the carbon black has a surface energy of less than or equal to 10 mJ/m².

Another embodiment provides a cathode formulation comprising:

a lithium ion-based electroactive material;

carbon black having an OAN ranging from 100 to 300 mL/100 g,

wherein the carbon black has a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

Another embodiment provides a method of making a cathode, comprising:

combining particles comprising carbon black, a lithium ion-based electroactive material, and a binder in the presence of a solvent to produce a paste;

depositing the paste onto a substrate; and

forming the cathode,

wherein the carbon black is selected from one of:

• • (i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy;
  • (ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m²; and
  • (iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

Another embodiment provides a cathode paste containing particles comprising a lithium ion-based electroactive material and a carbon black, wherein the paste further comprises:

a binder; and

a solvent,

wherein the carbon black is selected from one of:

• • (i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy;
  • (ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m²; and
  • (iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1A is a TEM image of a base carbon black (without heat treatment);

FIG. 1B is a TEM image of the base carbon black after heat treatment at 1400° C. (Sample A);

FIG. 2 shows cyclic voltammograms for a base carbon black, and heat-treated carbon blacks Sample A and Sample B, as outlined in Example 2;

FIG. 3 shows cyclic voltammograms for comparing commercially available carbon blacks commonly used in the Li-ion battery industry versus heat-treated carbon blacks Sample A and Sample B, as outlined in Example 2;

FIG. 4A is a plot showing the correlation between oxidation current and OAN at 4.5 V (positive sweep) for different carbon blacks, as outlined in Example 2;

FIG. 4A is a plot showing the correlation between oxidation current and graphitic domain size (L_a Raman) at 4.5 V (positive sweep) for different carbon blacks, as outlined in Example 2;

FIG. 5A is a comparison plot of voltage versus discharge capacity for base carbon black, Sample A, with LiFePO₄ as the active material, as outlined in Example 3;

FIG. 5B is a comparison plot of capacity retention versus C-rate for base carbon black and Sample A, with NCM-111 as the active material, as outlined in Example 3;

FIG. 5C is a comparison plot of capacity retention versus C-rate for base carbon black and Sample A, with LiNi_0.5Mn_1.5O₄ as the active material, as outlined in Example 3; and

FIG. 6 is a plot illustrating the difference in rate performance of Base carbon black before and after graphitization with different cathode chemistries for different charging cut-off voltages, as outlined in Example 4.

DETAILED DESCRIPTION

Composite cathode formulations typically contain an electroactive component, a binder, and conductive additives. While much of the development to improve the performance of lithium ion batteries centers on the electroactive and electrolyte components, a frequently neglected component of the cathode formulation is the conductive additive, with respect to improvements in chemical and electrochemical properties. Conductive additives function to impart a necessary level of electrical conductivity to the composite cathode and minimize area specific impedance of the whole system. Area specific impedance (ASI) can be affected not only by efficiency of the conductive additive to conduct electrons, but also the morphology of the layer affecting mass transport within the electrode (ionic conductivity).

Today, the most commonly used conductive additives are carbon blacks having certain surface area specifications and other properties. Grades with surface areas ranging from 40-70 m²/g are currently the standard used in the industry, which per gram is 100+times higher than the surface area of the active material in the cathode. Even at very low weight loading of the conductive additive, the surface area of conductive additive is comparable and frequently much higher that of the active cathode responsible for storing energy. For example, for a composite cathode comprising active phase (surface area=0.2 m²/g) at 94 wt. % and carbon black (surface area=50 m²/g) at 3 wt. %, the carbon black would contribute 1.5 m²/g_cathode to the total surface area of 1.8 m²/g_cathode−more than 80% of the total surface area in the electrode. It follows that any degradation of the conductive additive, e.g., via parasitic reactions on the carbon, such as electrolyte oxidation and carbon corrosion, can cause cell degradation and failure. It has been reported in the lithium ion battery community that postmortem analysis of Li-ion cells operated at high voltages revealed almost a complete disappearance of conductive carbon after a certain number of cycles, which can be a significant factor in increased cell impedance and ultimate failure.

The surface of carbon black does not perfectly terminate with graphitic carbon layers, but frequently has other atoms or functional groups attached to it. Most commonly, functional groups on the carbon black surface contain oxygen and hydrogen. Without wishing to be bound by any theory, it is believed that electrochemical corrosion of carbon (e.g., via conversion to CO₂) initiates at those imperfections (e.g., either at surface functional groups and/or the amorphous phase) and then propagates to the rest of the carbon black particle. Reactivity of the carbon black surface toward electrochemical reactions with electrolyte are also believed to be a function of the surface imperfections that could act as high energy sites for adsorption, which can facilitate electron transfer reactions.

Not wishing to be bound by any theory, the mechanism of electron conduction between carbon black aggregates is believed to occur through tunneling phenomena, the probability of which is the exponential function of junction separation. Surface functional groups could act as a buffer to increase separation at the junction and thus impart a negative impact on electronic conductivity of the carbon black. Increasing the amount of graphitic termination on the carbon black surface can reduce this separation, which could in turn improve electronic conductivity.

Disclosed herein are cathode formulations comprising conductive carbon blacks that, through the provision of certain properties, can have a beneficial impact on power performance in lithium ion batteries. One embodiment provides a cathode formulation comprising a lithium ion-based electroactive material and a carbon black having an OAN ranging from 100 to 300 mL/100 g. Carbon black consists of primary particles fused into aggregates that are the smallest units of carbon black. The structure of carbon black (measured by oil absorption, “OAN”) roughly correlates with number of primary particles in the aggregate. High OAN (high structure) carbon blacks can provide improved electrical conductivity at low loading due to the lower critical volume fraction required for percolation. In one embodiment, the carbon black has an OAN ranging from 100 to 250 mL/100 g, or an OAN ranging from 100 to 200 mL/100 g. OAN can be determined according to ASTM-D2414.

In one embodiment, the carbon black has a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy, where L_a is defined as 43.5×(area of G band/area of D band). The crystallite size can give an indication of the degree of graphitization where a higher L_a value correlates with a higher degree of graphitization. Raman measurements of L_a were based on Gruber et al., “Raman studies of heat-treated carbon blacks,” Carbon Vol. 32 (7), pp. 1377-1382, 1994, which is incorporated herein by reference. The Raman spectrum of carbon includes two major “resonance” bands at about 1340 cm⁻¹ and 1580 cm⁻¹, denoted as the “D” and “G” bands, respectively. It is generally considered that the D band is attributed to disordered sp² carbon and the G band to graphitic or “ordered” sp² carbon. Using an empirical approach, the ratio of the G/D bands and the L_a measured by X-ray diffraction (XRD) are highly correlated, and regression analysis gives the empirical relationship:

L _a=43.5×(area of G band/area of D band),

in which L_a is calculated in Angstroms. Thus, a higher L_a value corresponds to a more ordered crystalline structure. In another embodiment, the carbon black has a crystallite size of at least 35 Å, at least 40 Å, at least 45 Å, or at least 50 Å.

In one embodiment, a higher % crystallinity (obtained from Raman measurements as a ratio of D and G bands) may also indicate a higher degree of graphitization. In one embodiment, the carbon black has a % crystallinity (I_D/I_G) of at least 40%, as determined by Raman spectroscopy.

In one embodiment, a higher degree of graphitization can be indicated by lower surface energy values, which are typically a measure of the amount of oxygen on the surface of carbon black, and thus, its hydrophobicity. Surface energy can be measured by Dynamic Water Sorption (DWS). In one embodiment, the carbon black has a surface energy (SE) less than or equal to 10 mJ/m², e.g., less than or equal to 9 mJ/m², less than or equal to 7 mJ/m², less than or equal to 6 mJ/m², less than or equal to 5 mJ/m², less than or equal to 3 mJ/m², or less than or equal to 1 mJ/m².

In one embodiment, selected BET surface areas can provide increased charge acceptance and cycleability. BET surface area can be determined according to ASTM-D6556. In one embodiment, the carbon black has a BET surface area ranging from 25 to 800 m²/g, e.g., a BET surface area ranging from 25 to 700 m²/g, from 25 to 500 m²/g, from 25 to 200 m²/g, or from 25 to 100 m²/g. In another embodiment, the BET surface area ranges from 130 to 700 m²/g, e.g., from 130 to 500 m²/g, from 130 to 400 m²/g, from 130 to 300 m²/g, from 200 to 500 m²/g, from 200 to 400 m²/g, or from 200 to 300 m²/g.

In one embodiment, the carbon black is a heat-treated carbon black. A “heat-treated carbon black” is a carbon black that has undergone a “heat treatment,” which as used herein, generally refers to a post-treatment of a carbon black that had been previously formed by methods generally known in the art, e.g., a furnace black process. The heat treatment can occur under inert conditions (i.e., in an atmosphere substantially devoid of oxygen), and typically occurs in a vessel other than that in which the carbon black was formed. Inert conditions include, but are not limited to, an atmosphere of inert gas, such as nitrogen, argon, and the like. In one embodiment, the heat treatment of carbon blacks under inert conditions, as described herein, is capable of reducing the number of defects, dislocations, and/or discontinuities in carbon black crystallites and/or increase the degree of graphitization.

In one embodiment, the heat treatment (e.g., under inert conditions) is performed at a temperature of at least 1000° C., at least 1200° C., at least 1400° C., at least 1500° C., at least 1700° C., or at least 2000° C. In another embodiment, the heat treatment is performed at a temperature ranging from 1000° C. to 2500° C. Heat treatment “performed at a temperature” refers to one or more temperatures ranges disclosed herein, and can involve heating at a steady temperature, or heating while ramping the temperature up or down, either continuously or stepwise.

In one embodiment, the heat treatment is performed for at least 15 minutes, e.g., at least 30 minutes, at least 1 h, at least 2 h, at least 6 h, at least 24 h, or any of these time periods up to 48 h, at one or more of the temperature ranges disclosed herein. In another embodiment, the heat treatment is performed for a time period ranging from 15 minutes to at least 24 h, e.g., from 15 minutes to 6 h, from 15 minutes to 4 h, from 30 minutes to 6 h, or from 30 minutes to 4 h.

In one embodiment, the carbon black is present in the cathode formulation in an amount ranging from 0.5% to 10% by weight, e.g., and amount ranging from 1% to 10% by weight, relative to the total weight of the formulation.

In one embodiment, the carbon black is selected from one of:

(i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy;

(ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m²; and

(iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

In one embodiment, the electroactive material is present in the cathode formulation in an amount of at least 80% by weight, relative to the total weight of the formulation, e.g., an amount of at least 90%, an amount ranging from 80% to 99%, or an amount ranging from 90% to 99% by weight, relative to the total weight of the formulation. The electroactive material is typically in the form of particles. In one embodiment, the particles have a D₅₀ particle size distribution ranging from 100 nm to 30 μm, e.g., a D₅₀ ranging from 1-15 μm. In one embodiment, the particles have a D₅₀ ranging from 1-6 μm, e.g., from 1-5 μm.

In one embodiment, the electroactive material is a lithium ion-based compound. Exemplary electroactive materials include those selected from at least one of:

• • LiMPO₄, wherein M represents one or more metals selected from Fe, Mn, Co, and Ni;
  • LiM′O₂, wherein M′ represents one or more metals selected from Ni, Mn, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, and Si;
  • Li(M″)₂O₄, wherein M″ represents one or more metals selected from Ni, Mn, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, and Si (e.g., Li[Mn(M″)]₂O₄); and
  • Li_1+x(Ni_yCo_1−y−zMn_z)_1−xO₂, wherein x ranges from 0 to 1, y ranges from 0 to 1 and z ranges from 0 to 1.

In one embodiment, the electroactive material is selected from at least one of LiNiO₂; LiNi_xAl_yO₂ where x varies from 0.8-0.99, y varies from 0.01-0.2, and x+y=1; LiCoO₂; LiMn₂O₄; Li₂MnO₃; LiNi_0.5Mn_1.5O₄; LiFe_xMn_yCo_zPO₄ where x varies from 0.01-1, y varies from 0.01-1, z varies from 0.01-0.2, and x+y+z=1; LiNi_1−x−yMn_xCo_yO₂, wherein x ranges from 0.01 to 0.99 and y ranges from 0.01 to 0.99; and layer-layer compositions containing an Li₂MnO₃ phase or a LiMn₂O₃ phase.

In one embodiment, the electroactive material is selected from at least one of Li₂MnO₃; LiNi_1−x−yMn_xCo_yO₂ wherein x ranges from 0.01 to 0.99 and y ranges from 0.01 to 0.99; LiNi_0.5Mn_1.5O₄; Li_1+x(Ni_yCo_1−y−zMn_z)_1−xO₂, wherein x ranges from 0 to 1, y ranges from 0 to 1 and z ranges from 0 to 1; and layer-layer compositions containing at least one of an Li₂MnO₃ phase and an LiMn₂O₃ phase. Cathodes are the performance limiting component in Li-ion batteries because their capacity (˜160 mAh/g) does not match the anode capacity (320 mAh/g for graphite). It has been discovered that the use of certain Mn rich formulations as active materials can result in cathodes having a capacity approaching 280 mAh/g, and a gravimetric energy around 900 Wh/kg. However, these materials have low charge and discharge rate capabilities, causing them to lose their energy advantage even at moderate discharge rates of 2 C. Another drawback of these materials is that they display a wide voltage swing from 4.8 to 2.0V during discharge.

Accordingly, one embodiment provides a mixture of active materials comprising: a nickel-doped Mn spinel, which has a high and flat discharge voltage around 4.5 V and a high power capability; and a layer-layer Mn rich composition, which makes it possible to increase discharge voltage and power capability. In one embodiment, the nickel-doped Mn spinel has the formula LiNi_0.5Mn_1.5O₄, and the layer-layer Mn rich composition contains a Li₂MnO₃ or a LiMn₂O₃ phase, and mixtures thereof.

In one embodiment, the cathode formulation further comprises a binder. Exemplary binder materials include but are not limited to fluorinated polymers such as poly(vinyldifluoroethylene) (PVDF), poly(vinyldifluoroethylene-co-hexafluoropropylene) (PVDF-HFP), poly(tetrafluoroethylene) (PTFE), polyimides, and water-soluble binders such as poly(ethylene) oxide, polyvinyl-alcohol (PVA), cellulose, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrrolidone (PVP), and copolymers and mixtures thereof. Other possible binders include polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluoro rubber and copolymers and mixtures thereof.

Another embodiment provides a cathode formulation comprising, consisting essentially of, or consisting of:

a lithium ion-based electroactive material and a carbon black,

wherein the carbon black is selected from one of:

• • (i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy;
  • (ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m²; and
  • (iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

Another embodiment provides a cathode formulation comprising, consisting essentially of, or consisting of:

a lithium ion-based electroactive material, a carbon black, and a binder,

wherein the carbon black is selected from one of:

• • (i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy;
  • (ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m²; and
  • (iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

In one embodiment, the cathode formulation can take the form of a paste or slurry in which particulate electroactive material and carbon black are combined in the presence of a solvent. In another embodiment, the cathode formulation is a solid resulting from solvent removal from the paste/slurry.

In one embodiment, the formulation is a particulate cathode formulation. In one embodiment, “particulate” refers to a powder (e.g., a free-flowing powder). In one embodiment, the powder is substantially free of water or solvent, such as less than 10%, less than 5%, less than 3%, or less than 1% water or solvent.

In one embodiment, the carbon black is homogeneously interspersed (uniformly mixed) with the electroactive material, e.g., the lithium-ion based material. In another embodiment, the binder is also homogeneously interspersed with the carbon black and electroactive material.

In one embodiment, the carbon black has a substantially reduced amount of defects (e.g., oxygen-containing groups, junction separation) that can give rise to detrimental oxidation or corrosion. In one embodiment, cyclic voltammetry in the 3.5-4.5 V range can provide an indication of reduced amount of defects. In one embodiment, the carbon black provides a cyclic voltammogram (positive sweep) with substantially no oxidation current in the 3.5-4.5 V range.

Another embodiment method of making a cathode, comprising:

combining particles comprising carbon black, a lithium ion-based electroactive material, and a binder in the presence of a solvent to produce a paste;

depositing the paste onto a substrate; and

forming the cathode,

wherein the carbon black is selected from one of:

• • (i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy;
  • (ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m²; and
  • (iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

In one embodiment, the one embodiment, the paste is the product of combining particles comprising electroactive material with carbon black and binder in the presence of a solvent. In one embodiment, the paste has a sufficiently high solids loading to enable deposition onto a substrate while minimizing the formation of inherent defects (e.g., cracking) that may result with a less viscous paste (e.g., having a lower solids loading). Moreover, a higher solids loading reduces the amount of solvent needed.

The particles can be combined in the solvent in any order so long as the resulting paste is substantially homogeneous, which can be achieved by shaking, stirring, etc. The particles can be formed in situ or added as already formed particles having the domain sizes disclosed herein. “Solvent” as used herein refers to one or more solvents. Exemplary solvents include e.g., N-methylpyrrolidone, acetone, alcohols, and water.

In one embodiment, the method comprises depositing the paste onto a current collector (e.g., an aluminum sheet), followed by forming the cathode. In one embodiment, “forming the cathode” comprises removing the solvent. In one embodiment, the solvent is removed by drying the paste either at ambient temperature or under low heat conditions, e.g., temperatures ranging from 20° to 100° C. The method can further comprise cutting the deposited cathode/AI sheet to the desired dimensions, optionally followed by calendaring.

Another embodiment provides a cathode paste containing particles comprising a lithium ion-based electroactive material and a carbon black, wherein the paste further comprises:

a binder; and

a solvent,

wherein the carbon black is selected from one of:

• • (i) a carbon black having an OAN ranging from 100 to 250 mL/100 g and a crystallite size (L_a) of at least 30 Å, as determined by Raman spectroscopy;
  • (ii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a surface energy of less than or equal to 10 mJ/m²; and
  • (iii) a carbon black having an OAN ranging from 100 to 300 mL/100 g and a crystallite size (L_a) of at least 35 Å, as determined by Raman spectroscopy.

In one embodiment, the cathode paste consists essentially of, or consists of, the lithium ion-based electroactive material, the carbon black, the binder, and the solvent.

One embodiment provides a cathode comprising the cathode formulation. The cathode can further comprise a binder and a current collector. In one embodiment, the active material is a high voltage cathode with the charging cut-off voltage of 4.95 V versus Li-metal reference electrode. In one embodiment, the cathode has a thickness of at least 10 μm, e.g., a thickness of at least 30 μm. Another embodiment provides an electrochemical cell comprising the cathode, such as a lithium ion battery.

In one embodiment, an electrochemical cell comprising the disclosed cathode materials provides one or more of improved power performance in lithium ion battery cathodes, improved inertness toward carbon corrosion oxidation, and/or improved inertness toward carbon and/or electrolyte oxidation.

EXAMPLES

Example 1

This Example describes the preparation of highly graphitized carbon blacks via direct resistive heat treatment of a base carbon black. Carbon black samples were processed in an electrothermal fluidized bed reactor operated in continuous mode. Carbon black was fed and discharged from the reactor at a flow rated based on achieving target reactor residence time of between 30 minutes and 4 hours. Nitrogen gas was introduced through a distributor to fluidize the carbon black. Direct resistive heating was applied by passing direct current through the carbon black bed in the annulus between a central electrode and the cylindrical reactor wall. The reactor temperature was set to target range of 1200-2000° C. by adjusting the electrical power input. Carbon black Samples A and B were obtained by direct resistive heat treatment of a base carbon black at approximately 1400° C. and 2000° C., respectively, under inert atmosphere of nitrogen. Table 1 below summarizes physical characteristics of resulting powders.

TABLE 1
Physical properties of Base Carbon Black before and after heat treatment
Property	Value	Base Carbon Black	Sample A	Sample B
Heat treatment	° C.	na	1400	2000
temperature
N2 BET SA	m2/g	54	53	51
OAN	ml/100 g	147	135	132
SEP	mJ/m2	17	0	0
La Raman	Angstroms	21	38	95
Crystallinity	%	33	46	68

The base carbon black has almost no microporosity (indicated by the same values for N₂ BET SA and STSA, not shown here), and any impact of heat treatment on the N₂ BET surface area is negligible. In a similar fashion, heat treatment has a minimal effect on the OAN. This data indicates that morphology of the carbon black, defined through the size of the primary particles and their arrangement, is not affected by heat treatment. FIGS. 1A and 1B show TEM images before and after heat treatment at 1400° C. The difference in morphology before and after heat treatment at 1400° C. is not discernible in TEM images.

From Table 1, it can be seen that heat treatment has an impact on surface properties of carbon black and its bulk crystallinity. Surface energy values (SEP values in the Table 1) were obtained from Dynamic Water Sorption (DWS) measurements, and are mostly indicative of the amount of oxygen on the surface of carbon black. SEP values increase from 17 mJ/m² for the untreated base carbon black to below detection limit after heat treatment at 1400° C. for Sample A.

Raman measurements were used to capture lateral size of crystalline domains, in the table represented by L_a Raman values. Initially, small crystal domains of the base carbon black had L_a Raman values on the order of 21 Å, which increased to 38 Å at 1400° C. (Sample A) and finally to 95 Å after treatment at 2000° C. (Sample B). Simultaneously, % crystallinity, also determined from Raman measurements as the ratio of D and G-bands, increases from 33%, typical for conventional carbon blacks, to 46% at 1400° C. (Sample A) to 68% after heat treatment at 2000° C. (Sample B). Consequently, when progressing from base carbon black to the 2000° C. heat-treated material (Sample B), the degree of crystallinity more than doubles, i.e. the amount of amorphous phase is reduced by 50%.

Example 2

The impact of different levels of graphitization on the electrochemical stability of carbon and its activity toward the electrolyte oxidation was measured in the presence of standard Li-ion battery electrolyte (with 1% vinylene carbonate (VC) additive) in coin cell configuration.

Electrodes slurries were prepared by mixing 80 wt. % carbon black, 20 wt. % PVDF at 8% solids loading in N-methyl pyrrolidinone (NMP) for 30 minutes in a SPEX 8000 mill using two zirconia media.

A typical slurry was made by mixing the following: 768 mg carbon black, 2.22 g solution of PVDF (Solvay 1030, 8.3 wt %) in NMP and 8.98 g NMP. The slurry was coated on 17 microns thick aluminum foil with an automated doctor blade coater (MTI technologies), resulting in final carbon loadings of 1-1.6 mg/cm².

Disc electrodes were cut (15 mm diameter), dried at 100° C. under vacuum for 4 h, weighed and assembled into 2032 coin-cells under inert atmosphere of an Ar filled glove box. Lithium metal foil was used as the reference and counter electrodes, Whatman FG/40 17 mm discs were used as separator, the electrolyte was 0.1 mL EC:DMC:EMC (1:1:1), 1% VC, LiPF6 1M, less than 20 ppm moisture contents (BASF/Novolyte), and the working electrode was 80 wt. % carbon black+20 wt. % PVDF. Carbon black oxidation was tested by cyclic voltammetry at 10 mV/s from 2-5V with an EG&G 2273 potentiostat.

FIG. 2 shows cyclic voltammograms for carbon blacks with different degree of graphitization, i.e., base black and heat-treated Samples A and B. For base carbon black, as is typical for conventional carbon blacks used in current Li-ion cathodes, a pseudocapacitive feature is observed in the positive sweep in the range of 3.5-4V, most likely due to the oxidation of the carbon black. This oxidation, although not entirely irreversible, results in carbon corrosion and consequently the degradation in Li-Ion battery cells. Electrolyte oxidation is associated with irreversible oxidation currents at the positive limit of the voltammetric sweep; here on potentials >4.75 V.

From FIG. 2, it can be seen that heat treatment has the biggest impact on the carbon oxidation manifested as suppression of pseudocapacitive features in 3.5-4 V range. This pseudocapacitive feature is significantly reduced on carbon black treated at 1400° C., but it was not entirely eliminated. After heat treatment at 2000° C., the pseudocapacitive feature is almost entirely absent from the surface.

Without wishing to be bound by any theory, it is believed that this pseudocapacitive feature indicates the presence of oxygenated surface functional groups or surface imperfections. Upon heat-treatment, the decrease of this feature may indicate very small (e.g., below the detection limit of DWS measurements) amounts of oxygen or surface imperfections after removal of oxygenated surface functional groups. After a 2000° C. heat treatment, oxygen is almost entirely absent from the surface and the temperature was sufficient to smooth out most of the surface imperfections, resulting in close to ideal graphitic termination of the carbon black surface. Consequently, it is believed that the increased level of graphitization beneficially impacts carbon black electrochemical stability at relevant cathode potentials in Li-Ion battery cells which should, in turn positively affect cycle life and durability of Li-Ion systems, e.g., at high charging cut-off voltages.

The graphitized carbon blacks were then compared to commercially available carbon blacks currently used as standard conductive additives in lithium ion battery industry, namely Super P® conductive carbon black (TIMCAL Graphite and Carbon), Denka acetylene black (DENKA), and high surface area Ketjenblack® EC300 conductive carbon black (AkzoNobel). A comparison of cyclic voltammograms of each black is shown in FIG. 3, under the same conditions as in FIG. 2.

As can be seen from FIG. 3, all competitive carbon black grades produce an onset of oxidation current at approximately 3.3V, well within the range of operation of typical lithium ion battery cathodes. Oxidation currents on competitive carbon black grades most likely result from a superimposition of carbon oxidation and electrolyte decomposition. In the entire potential range, oxidation currents on competitive grades significantly exceed oxidation currents from the present graphitized carbon blacks. A summary of physical properties for commercial grade carbon blacks are provided in Table 2.

TABLE 2
Physical properties of competitive carbon blacks, standard conductive
additives in Li-Ion battery industry
Property	Value	SuperP	Denka Black	Ketjen EC300
N2 BET SA	m2/g	60.4	66.1	853
OAN	ml/100 g	287	218	360
SEP	mJ/m2	0.7	1.8	11.1
La Raman	Angstroms	21.1	33.5
Crystallinity	%	26	24

FIGS. 4A and 4B show correlations between oxidation currents in cyclic voltammetry at 4.5V, positive sweep, as a function of OAN (FIG. 4A) and L_a Raman (FIG. 4B). It can be seen from FIG. 4A that OAN has a dramatic impact on oxidation currents as measured by cyclic voltammetry. High OAN (high structure) carbon blacks typically provide improved electrical conductivity at low loading due to the lower critical volume fraction required for percolation, but high OAN comes with the penalty of more difficult slurry processing and unfavorable rheology. Further, based on FIG. 4A, high OAN blacks also come with the penalty of more significant oxidation activity in the presence of Li-Ion battery electrolyte. Increased graphitization (FIG. 4B) also has a systematic impact on the size of graphitic domains, as evidenced by increased L_a Raman values.

FIGS. 3, 4A, and 4B provide evidence of improved electrochemical stability of graphitized carbon black grades with optimal morphology in a typical Li-Ion battery electrolyte, which may benefit Li-ion battery cell durability and cycle life, e.g., for high voltage Li-ion battery systems.

Example 3

This Example describes experiments demonstrating improved power capability of cathodes comprising the graphitized carbon blacks disclosed herein.

Li-ion cathode formulations were prepared with different electroactive cathode materials by using electrode preparation methods similar to that of Example 2 in the amounts shown in Table 3. The cathode materials were assembled into a Li-ion pouch cells using Li metal anode and EC:DMC 1:1, LiPF6 1M electrolyte. The cells were subjected to charge-discharge tests on Maccor series 4000 battery cycler with increasing discharge currents expressed in C-rate, where C is the inverse of the discharge time in hours (ex: 1 C is 1 h, 0.1 C is 10 h discharge).

TABLE 3
Formulations for different cathode materials
	PVDF		Electrode
Cathode	binder	Conductive Additive	loading, mg/cm2
LiFePO4, 91%	5 wt. %	4 wt. %	7
NCM 111,* 94%	3 wt. %	3 wt. %	11
LiNi0.5Mn1.5O4, 85%	5 wt. %	10 wt. %	7
*LiNi0.33CO0.33Mn0.33O2

FIG. 5A shows a plot of voltage versus discharge capacity for LiFePO₄. With LiFePO₄, the cathode system with the lowest charging-cut-off voltage, the difference between the base black and heat treated material is almost not discernible at different discharge currents. This difference becomes pronounced with higher voltage NCM system (FIG. 5B), and even further amplified with Ni-rich spinel cathode with the charging cut-off voltage of 4.95 V (FIG. 5C).

Without wishing to be bound by any theory, these differences may be attributed to the confluence of two factors: (i) improved intrinsic electrical conductivity of heat treated material with increase in level of graphitization, and (ii) suppression of interfacial charge transfer phenomena between carbon black and electrolyte resulting in suppression of carbon oxidation/corrosion and electrolyte decomposition. Suppression of interfacial charge-transfer phenomena potentially result in preservation of pristine carbon black surface beneficial for electron tunneling and conduction, consequently minimizing area specific impedance and improving the power capability of the Li-Ion composite cathode.

Example 4

This Example describes the effect of the graphitized carbon blacks disclosed herein on the cycle life performance. The cells described in Example 3 were subjected to continuous charge-discharge cycling at constant rate of 1 C, and the change in capacity of the cells was recorded as a function of cycle number.

FIG. 6 shows the cycle life performance before and after heat treatment with LiNi_0.5Mn_1.5O₄ cathode. As can be seen from FIG. 6, after 250 cycles, the electrode containing heat treated material shows less capacity fade, while base carbon black based cell shows much more pronounced decay. This behavior may be linked to the smaller oxidation current observed by cyclic voltammetry on the heat treated materials, and demonstrates the benefit of oxidation resistance on cycle-life.

The use of the terms “a” and “an” and “the” are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Claims

1. A cathode formulation comprising: a lithium ion-based electroactive material;carbon black having an OAN ranging from 100 to 250 mL/100 g,wherein the carbon black has a crystallite size (La) of at least 30 Å, as determined by Raman spectroscopy, and a % crystallinity (ID/IG) of at least 40%, as determined by Raman spectroscopy.

2. A cathode formulation comprising: a lithium ion-based electroactive material;carbon black having an OAN ranging from 100 to 300 mL/100 g,wherein the carbon black has a surface energy of less than or equal to 10 mJ/m2, and a % crystallinity (ID/IG) of at least 40%, as determined by Raman spectroscopy.

3. A cathode formulation comprising: a lithium ion-based electroactive material;carbon black having an OAN ranging from 100 to 300 mL/100 g,wherein the carbon black has a crystallite size (La) of at least 35 Å, as determined by Raman spectroscopy.

4. The cathode formulation of claim 1 or 2, wherein the carbon black has a crystallite size (La) of at least 35 Å, as determined by Raman spectroscopy.

5. The cathode formulation of 3, wherein the carbon black has a % crystallinity (ID/IG) of at least 40%, as determined by Raman spectroscopy.

6. The cathode formulation of any one of claims 1-3, wherein the carbon black has a surface energy less than or equal to 10 mJ/m2.

7. (canceled)

8. The cathode formulation of any one of claims 1-3, wherein the carbon black has a BET surface area ranging from 25 to 800 m2/g.

9. The cathode formulation of any one of claims 1-3, wherein the carbon black is a heat-treated carbon black.

10-20. (canceled)

21. An electrochemical cell comprising the cathode formulation of any one of claims 1-3.

22-24. (canceled)