High purity swcnt additives for lithium ion battery performance enhancement

Abstract

The present invention relates to a lithium ion battery cell having a cathode comprising an active material and about 1-5% by weight of single-walled carbon nanotubes (SWCNTs) as a conductive additive, wherein the SWCNTs have an inorganic impurity content of less than 5% by weight.SOLUTION: The presently invented LiB cathode improves performance characteristics, such as higher capacity, lower cell resistance, and greater capacity retention with cycling of a fully assembled cell, compared to LiB cells that use only conventional conductive carbon additives in the cathode.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to the use of purified single-walled carbon nanotubes (SWCNTs) as a conductive additive in lithium-ion battery cathodes to significantly improve the performance characteristics of the fully assembled cell compared to lithium-ion battery cells that use only conventional conductive carbon additives, such as carbon black, in the cathode. [Background technology]

[0002] Li-ion batteries (LiBs) are commonly used in consumer electronics and are attractive for use in hybrid gas-electric and all-electric vehicles. However, widespread vehicle application requires improvements in battery performance. Specifically, improvements in energy density, power density, weight reduction, and reliability are desired. Particularly attractive are thinner and / or lighter electrode materials with lower electrical resistance, more efficient ionic transport capabilities, and sufficient mechanical strength for battery use.

[0003] There is a growing demand for the development of battery chemistries that offer improved performance metrics beyond those currently achieved with LiB technology. The US DOE has set performance targets for LiB cells, including a capacity of 350 Wh / kg (750 Wh / L), at least 1,000 cycles of continuous operation, and a life expectancy of 10 years or more. At the battery module and pack level, targets are equally high, including fast charging (80% state of charge) in 15–30 minutes and battery pack cost reductions of $125–$80 per kWh. To achieve these highly ambitious targets, new battery chemistries will be crucial, particularly for the cathode formulation of such energy storage devices. Conventional LiB configurations consist of cathodes with Li-metal oxides as the active material (e.g., Li-NiMnCo-Oxide (NMC), Li-NiCoAl-Oxide (NCA), Li-Fe-Phosphate (LFP), Li-Cobalt-Oxide (LCO), and Li-Manganese-Oxide (LMO)). These oxides are particulate in nature and require the inclusion of a polymeric binder, such as polyvinylidene fluoride (PVDF), to form a cohesive electrode coating on the current collector. PVDF binders are insulating. Furthermore, Li metal oxides are resistive. Therefore, conductive additives, such as carbon black, are included in the cathode formulation to lower the overall electrode resistance. All of these components are mixed with a suitable solvent, such as N-methyl-2-pyrrolidone (NMP), isopropanol (IPA), or water, and processed into a slurry for coating applications. While the inclusion of the binder and conductive additives is necessary, these components do not contribute to the capacity of the LiB cell. In fact, common practice is to incorporate these inert additives at the lowest practical level to maximize the concentration of active material. The result of such tailored cathode formulations is increased energy density.

[0004] Carbon nanotubes (CNTs) are mainly sp 2CNTs are one-dimensional, cylindrical graphitic nanostructures composed of hybridized carbon atoms. CNTs are classified according to the number of cylindrical graphitic sheets, or "walls," that make up each individual nanotube structure. Nanotubes with one wall are called single-wall carbon nanotubes (SWCNTs) and are distinguished from nanotubes with two or more walls, called multi-wall carbon nanotubes (MWCNTs). MWCNTs with two walls are also called double-wall carbon nanotubes (DWCNTs). Both SWCNT and MWCNT types of CNTs have nanometer-sized diameters and micron-sized lengths, providing a high surface area-to-volume ratio and resulting in high aspect ratio properties. The typical diameter of an individual SWCNT is 5 to 50 times smaller than that of an individual MWCNT, resulting in a significantly higher aspect ratio for SWCNTs than for MWCNTs.

[0005] New materials are needed that can act as beneficial additives to the cathode of LiB cells to enhance performance metrics such as higher capacity, lower cell resistance, higher charge and discharge rate capabilities, and greater capacity retention with cycling, enabling the next generation of advanced lithium-ion batteries. [Brief explanation of the drawings]

[0006] [Figure 1] Images of slot-die roll-to-roll coating of LiB cathode material containing SWCNTs as conductive additives on Al foil are shown: (A) coating for LiB energy cells, and (B) coating for LiB power cells. [Figure 2] 1 is a set of two charts showing the performance improvement of a type 18650 LiB energy cell containing purified SWCNTs as a conductive additive compared to a LiB cell with a conventional carbon black conductive additive (Super-P® or “SP”, Imerys Graphite & Carbon, Terrebonne, Quebec, Canada). [Figure 3]1 is a chart comparing the life cycle capacity of Type 18650 LiB energy cells containing 6 wt. % standard carbon black (SP) conductive additive (labeled control), 1 wt. % purified SWCNTs + 1 wt. % carbon black (SP), and 2 wt. % purified SWCNTs. [Figure 4] A set of two charts showing the improved charge and discharge capacity performance achieved in LiB power cells when using different concentrations of purified SWCNTs as the conductive additive compared to conventional carbon black additive (standard cell with 6 wt% SP). [Figure 5] 1 is a graph showing the difference in discharge capacity performance improvement achieved in LiB power cells when using as-received SWCNTs (CNTs) or purified SWCNTs as conductive additives compared to conventional carbon black (SP) additives. [Figure 6] 1 is a chart showing the reduction in cell impedance achieved in a LiB power cell when using different concentrations of purified SWCNTs as a conductive additive compared to a conventional carbon black additive (standard cell with 6 wt. % SP). [Figure 7] A set of two charts showing the improved discharge rate performance at increasing discharge currents of (A) LiB power cells containing purified SWCNTs as conductive additives compared to conventional carbon black additives (6 wt% SP). (B) [Figure 8] 1 is a chart showing the improved discharge rate performance at 20 A discharge current for LiB power cells containing 2 wt % and 3 wt % purified SWCNTs as a conductive additive compared to a cell containing carbon black additive (standard cell with 6 wt % SP). [Figure 9] 1 is a chart showing a comparison of the discharge rate performance at 20 A discharge current of a LiB power cell containing 2 wt % purified SWCNTs as a conductive additive and a LiB power cell also containing 2 wt % as-received SWCNT additive. [Figure 10]1 is a chart showing the improved life cycle performance (normalized discharge capacity) of a representative LiB power cell containing 2 wt% purified SWCNTs as a conductive additive compared to a cell containing carbon black additive (6 wt% for standard cell). [Figure 11] 1 is a chart showing superior initial life cycle performance (normalized discharge capacity) of a LiB power cell containing 2 wt. % purified SWCNTs + 2 wt. % carbon black (SP) as the conductive additive compared to a cell containing 2 wt. % as-received SWCNTs + 2 wt. % carbon black (SP) as the conductive additive. [Figure 12] A series of charts showing the improvement in LiB parameter performance for charge capacity (top), discharge capacity (middle), and cell impedance (bottom) when SWCNTs are included as a conductive additive to reduce or completely replace carbon black content. The "asterisk" in Figure 12 indicates the best performance in each category of energy or power cells. [Figure 13] 1 is a chart showing the superior initial life cycle performance (normalized discharge capacity) of a LiB power cell containing 2 wt. % highly purified SWCNTs as a conductive additive compared to cells containing 2 wt. % purified SWCNTs or as-received SWCNTs as a conductive additive, each with 2 wt. % carbon black (SP). [Figure 14] 1 is a chart showing a comparison of the discharge rate performance at 20 A discharge current of LiB power cells containing 2 wt % of various grades of SWCNTs, namely, as-received SWCNTs, purified SWCNTs, and highly purified SWCNTs, as conductive additives (each with 2 wt % SP). [Figure 15] 1 is a chart showing the life cycle performance (normalized discharge capacity) of LiB power cells containing as-received MWCNTs as a conductive additive and cells containing 2 wt. % of as-received SWCNTs, purified SWCNTs, or highly purified SWCNTs (each with 2 wt. % SP) as a conductive additive, or cells containing no CNTs (6 wt. % SP). [Figure 16] 1 is a chart showing a comparison of the discharge rate performance at 20 A discharge current of a LiB power cell containing MWCNTs as a conductive additive with cells containing 2 wt % of various grades of SWCNTs (each with 2 wt % SP) as a conductive additive or no CNTs (6 wt % SP). DETAILED DESCRIPTION OF THE INVENTION

[0007] The present inventors have discovered that the use of highly pure and well-dispersed conductive carbon nanotubes, particularly single-walled carbon nanotubes (SWCNTs), as a conductive additive material in Li-ion battery cathode formulations enhances the performance characteristics of fully assembled LiB cells. In contrast to conventional carbon black particles, the high aspect ratio of carbon nanotubes provides good contact between the active metal oxides, enabling conductive percolation at lower incorporation levels. Control of the cathode morphology and porosity is essential. Low porosity results in low surface area, high diffusion coefficients, and short response times (Chang Kyoo Park et al., Bull. Korean Chem. Soc. 2011, Vol. 32, No. 3, pp. 836-840). To achieve the desired performance enhancement, the SWCNTs must be highly pure with <5 wt.% inorganic impurities and have a diameter of 400 to approximately 1300 m. 2 / g, preferably 800 to about 1300m 2 / g BET specific surface area They found that the tubes needed to be highly graphitized with minimal defects (evidenced by a Raman G / D ratio >20 when measured using a 532 nm laser). Incorporating conductive SWCNTs with these specific criteria into the cathode formulation of LiB cells would provide the necessary performance improvements of increased capacity, increased rate capability, increased cycle life, and reduced impedance, enabling next-generation battery systems that meet the set target criteria.

[0008] The present inventors have discovered that the use of SWCNTs, rather than MWCNTs, as a conductive additive material in Li-ion battery cathode formulations enhances the performance characteristics of fully assembled LiB cells. The advantage of using SWCNTs over MWCNTs as a conductive additive in LiB cathode formulations is due to the higher aspect ratio and greater surface area afforded by SWCNTs. SWCNTs typically have aspect ratios 5-50 times larger than MWCNTs. SWCNTs are typically several hundred to thousands of meters larger than MWCNTs. 2 / g This combination of higher aspect ratio and greater surface area offered by SWCNTs makes these nanotubes more attractive than MWCNTs for use in the present invention as conductive carbon additives.

[0009] LiB cells typically contain a carbon-based anode, a cathode, and an electrolyte containing a lithium salt in an organic solvent.

[0010] The present invention relates to a LiB cell having a cathode comprising an active material and about 1-5 weight percent (wt%) single-walled carbon nanotubes (SWCNTs) as a conductive additive, wherein the SWCNTs have an inorganic impurity content of less than 5 wt%, or less than 4.5 wt%.

[0011] As used herein, "about" refers to ±10% of the recited value.

[0012] Unless otherwise specified, % in this application refers to % by weight (wt).

[0013] The cathode active material of the LiB of the present invention generally contains a material selected from the group consisting of Li-NiMnCo-Oxide (NMC), Li-NiCoAl-Oxide (NCA), Li-FePhosphate (LFP), Li-Cobalt-Oxide (LCO), Li-Manganese-Oxide (LMO), and any combination thereof. The LiB cathode of the present invention generally contains 91 to 99 wt. % of the active material, preferably 92 to 98 or 92 to 97 wt. %.

[0014] The cathode of the LiB of the present invention can contain a polymer binder. For example, polyvinylidene fluoride (PVDF) can be used as the binder in the cathode. The binder can be present in the LiB cathode in an amount of 0.5 to 8 wt %, preferably 1 to 6 wt %, or more preferably 1 to 2 wt %.

[0015] In one embodiment, the LiB cathode includes a polymer binder.

[0016] In one embodiment, the LiB cathode does not contain a polymer binder. In this embodiment, the carbon nanotubes in the LiB cathode composition function as a binder to hold together the active oxide particles and any other components, such as carbon black. As noted above, polymer binders do not contribute to the capacity of LiB cells, and it is advantageous to minimize or completely eliminate the amount of polymer binder in the cathode composition.

[0017] In one embodiment, the SWCNTs are purified and contain ≦5%, or ≦4.5%, or ≦4%, or ≦3%, or ≦2%, or ≦1% inorganic impurities by weight. By comparison, commercially available SWCNTs typically contain greater than 10% by weight (e.g., 10-25%) inorganic impurities, including iron (Fe), nickel (Ni), silicon (Si), and / or other metals. These metallic impurities are usually present as residues of the manufacturing process.

[0018] In one embodiment, the cathode comprises about 0.5-4.5 wt% SWCNTs. In another embodiment, the cathode comprises about 1-4.5 wt%, or 1.5-4.5 wt%, or 2-4 wt% SWCNTs. For example, the cathode comprises about 1, 2, 3, or 4 wt% SWCNTs.

[0019] In one embodiment, the cathode comprises SWCNTs as the only conductive additive.

[0020] In one embodiment, the cathode includes SWCNTs as a conductive additive and does not include a substantial amount of carbon black, e.g., the cathode does not include more than 0.1 wt. % carbon black or is free of carbon black.

[0021] In another embodiment, the cathode further comprises a second conductive additive such as carbon black. Acetylene black, a highly crystalline type of carbon black, can be used as the second conductive additive.

[0022] In one embodiment, the ratio of the two conductive additives SWCNT to carbon black in the cathode can be in the range of 1:3 to 3:1, preferably in the range of 1:2 to 2:1, and more preferably about 1:1.

[0023] The amount of conductive carbon additive in the cathode depends on the requirements of the application in which the battery will be used. Power cells have a higher conductive carbon content (4-10 wt%) and a lower active material content, which allows for lower cell resistance, but at the cost of lost storage capacity. Energy cells have a lower conductive carbon content (2-4 wt%) and a higher active material content, which allows for higher storage capacity, but at the cost of lost rate capability.

[0024] In one embodiment, the total content of SWCNTs and carbon black in the cathode is 1% to 8% by weight, preferably 2% to 6% by weight.

[0025] In one embodiment, the cell is a power cell, and the total content of SWCNTs and carbon black in the cathode is 4 wt% to 10 wt%, preferably 4 wt% to 8 wt%, more preferably 4 wt% to 6 wt%.

[0026] In one embodiment, the cell is an energy cell, and the total content of SWCNTs and carbon black in the cathode is 2 wt % to 4 wt %, preferably 2 wt % to 3 wt %, more preferably about 2 wt %.

[0027] For example, a LiB power cell cathode may contain about 92 wt% NMC, about 2 wt% PVDF, about 3 wt% SWCNTs, and about 3 wt% carbon black.

[0028] For example, a LiB power cell cathode may contain about 94% by weight NMC, about 2% by weight PVDF, about 2% by weight SWCNTs, and about 2% by weight carbon black.

[0029] For example, a LiB power cell cathode may contain about 94 wt% NMC, about 2 wt% PVDF, and about 4 wt% SWCNTs. In one embodiment, the cathode contains less than 0.1 wt% carbon black. In another embodiment, the cathode contains no carbon black.

[0030] For example, a LiB energy cell cathode may contain about 96 wt% NMC, about 2 wt% PVDF, and about 2 wt% SWCNTs. In one embodiment, the cathode contains less than 0.1 wt% carbon black. In another embodiment, the cathode contains no carbon black.

[0031] For example, a LiB energy cell cathode may contain about 96% by weight NMC, about 2% by weight PVDF, about 1% by weight SWCNTs, and about 1% by weight carbon black.

[0032] In one embodiment, the cathode has a thickness of at least 100 μm.

[0033] The present invention utilizes purified SWCNTs to either supplement or replace carbon black materials as conductive additives in the cathode of LiB cells. Due to their smaller diameter, SWCNTs have a significantly higher aspect ratio than multi-walled carbon nanotubes (MWCNTs). Therefore, one of the challenges faced when processing high-aspect-ratio SWCNTs into solution is overcoming substantial van der Waals interactions to achieve stable dispersion of SWCNTs. A common approach is to utilize suitable dispersants or surfactants to minimize nanotube aggregation and re-bundling after processing in solution. However, the presence of such agents adversely affects intrinsic CNT properties, such as high electronic conductivity, and adds additional post-processing time and cost to remove the dispersant from the processed CNTs.

[0034] This invention describes the processing of purified, high-aspect-ratio SWCNTs into stable dispersions without the use of dispersants or surfactants. SWCNT aggregates and large bundles are sufficiently reduced in size by high-shear processing to remain suspended and dispersed in the carrier solvent for extended periods without separation or settling. Mechanically processed SWCNT dispersions are stable in that the dispersed nanotubes show no aggregation or settling after standing for at least 24 hours, or for at least several weeks at room temperature.

[0035] Because neither dispersants nor surfactants are used in preparing the SWCNT dispersion, the cathode material composition of the present invention is completely free of these substances. If such substances were present in the cathode material, they could adversely affect the performance of the cathode in an assembled LiB cell. By eliminating these substances from the SWCNT dispersion process, it is ensured that these substances are not present in the cathode material and therefore will not cause such performance loss when the cathode material is used in an assembled LiB cell.

[0036] The effectiveness of the treatment to stable dispersion of SWCNTs according to the present invention is manifested in the superior cycling and rate performance of LiB cells assembled with treated SWCNTs as additives in the cathode compared to cells containing MWCNTs as additives.

[0037] The SWCNTs of the present invention are preferably metallic SWCNTs or semiconducting SWCNTs, or a mixture of metallic and semiconducting SWCNTs. Overall, this mixture of high aspect ratio, high purity metallic and semiconducting SWCNTs has relatively high electrical conductivity. Furthermore, the SWCNTs contain less than 5% by weight of detectable metal and other impurities, and have a Raman G / D integrated peak area ratio of at least 20, preferably >30, >40, >50, or >60, as measured using a 532 nm laser. Purified SWCNTs can be purified to 400 mW. 2 / g~approx. 1300m 2 / g, preferably 800m 2 / g~approx. 1300m 2 / g, more preferably 900m 2 / g~approx. 1300m 2 / g, or 1000 to approximately 1300 m 2 / g, or 1100 to approximately 1300 m 2 / g BET specific surface area.

[0038] The use of high aspect ratio purified SWCNTs generally allows for a reduction in the total conductive additive content in the cathode compared to using carbon black alone, resulting in lower overall levels, particularly for LiB power cells. This feature not only increases capacity, but also allows for a higher proportion of active material, resulting in better utilization of the cell capacity compared to standard LiB cells. This design allows for improved capacity (energy density) at a given impedance and power density. Conversely, in conventional LiB power cell structures, as the ratio of conductive carbon to active material increases, capacity (energy density) decreases, impedance becomes lower, and power density becomes higher.

[0039] The resulting performance improvements achieved in the LiB cells of the present invention containing SWCNT additives are significant in terms of increased capacity, reduced impedance, improved capacity retention at high discharge rates, and superior life cycle characteristics compared to the performance of LiB cells with standard carbon black conductive additives. Furthermore, these multiple important performance enhancements are significantly enhanced by incorporating purified SWCNTs into the LiB electrode. Similar performance improvements could not be achieved using MWCNTs as the conductive additive. Commercially available SWCNTs can contain 10-25 wt% inorganic impurities and various amounts of amorphous carbon. Incorporation of the same amount of as-received SWCNTs as purified SWCNTs into the LiB cathode attenuated the LiB cell's performance enhancements compared to LiB cells with the same amount of purified SWCNTs but with less than 5 wt% metal impurities, resulting in a 4% smaller capacity gain, a 10% lower high discharge rate, and a faster capacity fade with cycling (8% capacity fade after 150 cycles compared to 5% capacity fade for purified SWCNTs). Incorporation of highly purified SWCNTs with less than 1 wt% metal impurities as conductive additives in the cathode at the same amount as purified SWCNTs further enhanced LiB performance over that achieved with purified SWCNTs.

[0040] The present invention is further illustrated by the following examples, which should not be construed as limiting the scope of the invention to the specific procedures or products described therein. [Example]

[0041] Example 1. Preparation of SWCNT-NMP dispersion Dispersions of SWCNTs in N-methyl-2-pyrrolidone (NMP) were prepared using as-received commercially available SWCNTs (approximately 14 wt %, typically with 10–25 wt % inorganic impurities), purified SWCNTs (<5 wt %, typically with 2–4 wt % inorganic impurities), and highly purified SWCNTs (≤1 wt % inorganic impurities).

[0042] Three grams of each SWCNT type was combined with 1 L of NMP and treated by bath sonication with impeller mixing for 1 hour. After this premixing process, the SWCNTs were dispersed using a high shear process. The typical viscosity of the resulting SWCNT-NMP dispersion was 0.1 s. -1 The shear rates were in the range of 80,000–90,000 cP. The three resulting dispersions, each representing a different SWCNT type, were stable at room temperature for at least 24 hours without separation or sedimentation.

[0043] Example 2. Incorporation into a cathode formulation Formulation A: Purified SWCNTs only (no carbon black) In Formulation A, purified SWCNTs (with <5 wt.% inorganic impurities) were used as the conductive additive at 2 wt.% and 4 wt.% for LiB energy and LiB power cells, respectively, to completely replace the conventional carbon black additive. The purified SWCNT-NMP dispersion prepared in Example 1 was mixed with a commercially available Li metal oxide active material designated NMC111-Ni 0.33 Mn 0.33 Co 0.33 The cathode slurry mixture was then incorporated into a cathode compound mixture containing 2% by weight of PVDF binder dissolved in NMP solvent. The compound mixture was then loaded into a 60 L mixer capable of both high shear and planetary mixing and processed overnight under vacuum. The resulting viscosity of the processed cathode slurry mixture with a solids content of 19% by weight was 26,000 cP, as measured using a Brookfield RV instrument with spindle #4 at 2 rpm.

[0044] Formulation B: Purified SWCNT + carbon black (SP) In Formulation B, purified SWCNTs were used as a conductive additive to partially replace and reduce the amount of conventional carbon black additive. Purified SWCNTs (<5 wt% inorganic impurities) were incorporated in various amounts: 1 wt% SWCNTs + 1 wt% carbon black (Super-P® or "SP", Imerys Graphite & Carbon, Terrebonne, Quebec, Canada) for LiB energy cells, and 2 wt% and 3 wt% SWCNTs with 2 wt% and 3 wt% carbon black for LiB power cells, respectively. Thus, in all cases, carbon black was present in each formulation, but in a reduced amount compared to standard LiB cells, which typically contain approximately 4–10 wt% carbon black for LiB power cells. Also, 2 wt% PVDF binder dissolved in NMP solvent was used in the formulation mixtures.

[0045] Formulation C: As-received SWCNT + carbon black (SP) In Formulation C, as-received commercial SWCNTs (approximately 14 wt. % inorganic impurities) were used as the conductive additive to partially replace and reduce the amount of conventional carbon black additive. As-received commercial SWCNTs (approximately 14 wt. % inorganic impurities) were incorporated into the LiB power cell at a level of 2 wt. % SWCNTs + 2 wt. % carbon black. This formulation was otherwise identical to Formulation B with 2 wt. % purified SWCNTs + 2 wt. % carbon black described above and was prepared to compare the effect of SWCNT purity on the performance characteristics of LiB cells. Additionally, 2 wt. % PVDF binder dissolved in NMP solvent was used in the formulation mixture.

[0046] Formulation D: Standard LiB cell: Carbon black control (SP) The standard LiB energy cells were formulated to contain 2 wt.% carbon black, and the standard LiB power cells were formulated to contain 6 wt.% carbon black, which served as their respective control cells. The formulation mixtures also contained 2 wt.% PVDF binder dissolved in NMP solvent.

[0047] Table 1 summarizes all the above prepared formulations A to D and their compositions.

[0048] [Table 1]

[0049] Table 2 shows the difference in inorganic (metallic) impurity content between one batch each of as-received commercial SWCNTs, purified SWCNTs, and highly purified SWCNTs used as conductive additives in LiB cells.

[0050] [Table 2]

[0051] Example 3. Coating of cathode formulation onto Al foil Freshly processed cathode formulations A to D prepared in Example 2 were coated onto both sides of a roll of battery-grade Al foil (15 μm thick) by slot die coating using a Dynacoat lab coater and dryer (Frontier Industrial Tech Inc.). The wet coating was then dried in the 8-zone convection oven drying chamber of the slot die coater at temperatures throughout the zones ranging from 93.3 to 127 °C.

[0052] For the energy cells, the resulting dry electrode thickness was approximately 300 μm on average, but after calendering it was reduced to approximately 170 μm. The coating weight per side was approximately 25 mg / cm. 2 and 3 g / cm 3This corresponds to a density exceeding 1000 . Figure 1(A) shows a typical wet cathode coating on Al foil for a LiB energy cell.

[0053] For the power cell, the resulting dry electrode thickness was approximately 195 μm, which decreased to approximately 98 μm after calendering. The coating weight per side was approximately 13 mg / cm. 2 This is also 3g / cm 3 This corresponds to a density exceeding 1000 . Figure 1(B) shows a typical wet cathode coating for a LiB power cell.

[0054] Example 4. Type 18650 cell assembly For full-cell assembly, the LiB cathodes containing the SWCNT additive prepared in Example 3 were split and each was combined with a standard graphite intercalation anode composed of BTR918 graphite powder (BTR New Energy Materials, China), carbon black conductive additive (SuperP®), carboxymethyl cellulose (CMC) gelling agent, and styrene butadiene rubber (SBR) as a binder. This anode formulation was then slot-die coated onto battery-grade Cu foil to produce the anode electrode. Each cathode was paired with an anode and assembled into a Type 18650 cell filled with LiPF6 electrolyte. Using each of the fabricated cathodes described above, Type 18650 cells were assembled for operation and testing as complete LiB energy and power cells.

[0055] Example 5. Performance Comparison of Purified SWCNTs and Carbon Black as Conductive Additives in LiB Energy Cells The capacity of a LiB is the total ampere-hours available when the battery is discharged at a specific discharge current (designated as C-rate) from 100% state of charge to a cut-off voltage. Discharge capacity is calculated by multiplying the discharge current (in amperes) by the discharge time (in hours). In this example, all LiB cells were discharged from an initial voltage of 4.2 V to a cut-off voltage of 3.0 V.

[0056] FIG. 2 shows performance data comparison between a standard LiB energy cell containing 96 wt. % NMC, 2 wt. % binder, and 2 wt. % carbon black in the cathode and a LiB energy cell of the present invention with 96 wt. % NMC, 2 wt. % binder, and 2 wt. % SWCNTs in the cathode.

[0057] By replacing 2 wt% of carbon black additive with 2 wt% of SWCNTs, a 5% increase in cell capacity and a 6% decrease in cell ESR were observed.

[0058] A change in electrode formulation typically results in a corresponding change in one performance criterion of the LiB cell. What was unique in this case was achieving simultaneous performance improvements of both increased capacity and decreased impedance from the same cathode formulation containing 2 wt% SWCNTs. The inclusion of 1 wt% SWCNTs and 1 wt% carbon black reduced the cell ESR by 4.5% compared to the standard cell with 2 wt% carbon black.

[0059] As shown in Figure 3, the increase in cell capacity due to the use of 2 wt% purified SWCNTs as a conductive additive was not only observed initially but also maintained throughout the life cycle. The LiB cell containing 1 wt% SWCNTs and 1 wt% carbon black exhibited similar initial cell discharge capacity to the standard cell with 2 wt% carbon black, demonstrating better cell capacity retention during cycling compared to the standard cell. The cell with 6 wt% carbon black exhibited a rapid capacity drop within the first few cycles, followed by a more gradual decrease in capacity. A less rapid capacity drop within the first few cycles was observed in the cell with 1 wt% carbon black and 1 wt% SWCNTs. Conversely, the cell with 2 wt% SWCNTs but no carbon black did not exhibit a rapid capacity drop within the first few cycles. Instead, only a gradual decrease in capacity was observed.

[0060] Example 6. Performance Comparison of Purified SWCNTs and Carbon Black as Conductive Additives in LiB Power Cells LiB power cells were assembled from cathodes incorporating purified SWCNTs as a conductive additive. As shown in Figure 4, the LiB power cell of the present invention, equipped with a cathode containing an optimized amount of 2 wt% purified SWCNTs and 2 wt% carbon black, exhibited a charge capacity increase of approximately 9.6% and a discharge capacity increase of 6.7% over a standard LiB power cell equipped with a cathode containing 6 wt% carbon black. Smaller increases in capacity and discharge capacity were also observed with amounts of 3 wt% purified SWCNTs and 3 wt% carbon black in the cathode, and with amounts of 4 wt% purified SWCNTs without carbon black in the cathode. This result indicates that the addition of purified SWCNTs to the cathode is beneficial, and that the total content and ratio of SWCNTs and carbon black can be optimized to achieve the greatest increase in both charge and discharge capacity.

[0061] As shown in Figure 5, the addition of 2 wt% as-received SWCNTs + 2 wt% carbon black to the cathode of a LiB power cell increased the discharge capacity compared to a standard LiB cell with only carbon black. However, this capacity increase was only 2.4% compared to the 6.7% increase achieved with 2 wt% purified SWCNTs + 2 wt% carbon black.

[0062] The unique performance characteristics previously observed in LiB energy cells were replicated in LiB power cells, where again, every increase in cell capacity was accompanied by a simultaneous decrease in cell ESR. As shown in the impedance plot in Figure 6, a maximum -3% decrease in cell impedance was achieved with a cathode formulation of 2 wt% purified SWCNTs and 2 wt% carbon black, again resulting in the greatest increase in cell capacity. Similarly, cells with cathodes containing 3 wt% purified SWCNTs, 3 wt% carbon black, and 4 wt% purified SWCNTs also exhibited lower ESR, although not to the same extent as cells with 2 wt% purified SWCNTs and 2 wt% carbon black.

[0063] In addition to exhibiting superior capacity and impedance characteristics, the discharge rate performance at high current increments of the LiB power cells containing the purified SWCNT additive was significantly improved compared to the standard cells. Figure 7 shows the advantageous high-current discharge performance characteristics of the LiB cells with 2 wt.% purified SWCNTs + 2 wt.% carbon black. These cells exhibit a 50% increase in capacity and significantly less capacity fade than the standard carbon black cells at a high discharge current of 20 A.

[0064] A comparison of the discharge curves between the LiB power cells at a discharge current of 20 A is shown in Figure 8. Improved high-current performance of the LiB power cells containing purified SWCNT additives was observed in cells with either 2 wt% purified SWCNTs + 2 wt% carbon black or 3 wt% purified SWCNTs + 3 wt% carbon black. Specifically, the most improved 20 A discharge performance from the LiB power cells was achieved with a lower total conductive carbon content (4 wt% vs. 6 wt%).

[0065] Comparing the 20 A discharge rate behavior of LiB cells assembled from two grades of SWCNT additive (purified SWCNT and as-received SWCNT) reveals differences in performance, as shown in Figure 9. The power cell incorporating purified SWCNT as an additive (light gray profile) exhibited superior high current discharge rate capability to that of the LiB cell with as-received SWCNT additive (black profile).

[0066] A comparison of life cycle data for LiB power cells is shown in Figure 10. Compared to the standard reference cell with 6 wt% carbon black, the increase in initial capacity of the cell containing 2 wt% purified SWCNT additive + 2 wt% carbon black was maintained even after approaching 1,000 cycles at high currents.

[0067] Figure 11 compares the early life cycle behavior of LiB cells assembled from two grades of SWCNT additive. The results show that power cells incorporating as-received SWCNTs as the conductive additive (open symbols) exhibit a rapid decline in capacity after a given amount of cycling compared to cells with purified SWCNTs as the conductive additive (solid symbols).

[0068] Example 7. Improved performance of LiB energy and power cells in terms of charge capacity, discharge capacity, and cell impedance upon inclusion of SWCNTs as a conductive additive Figure 12 summarizes the results of Examples 1–6. Within each of the two categories of power and energy cells, the cells exhibiting the best performance in terms of charge capacity, discharge capacity, and impedance are marked with an asterisk. Adding purified SWCNTs to the cathode formulation of Li-ion battery cells in the amounts listed allows for increased cell capacity and simultaneous reduction in cell impedance, or similar performance characteristics, compared to cells using conventional carbon black conductive additives. These significant performance improvements are achieved with lower conductive additive content than is typical for cells with standard carbon black additives, allowing for an increased amount of active material in the electrode formulation, potentially further increasing the capacity of LiB cells. The performance improvements of LiB cells with purified SWCNT additives are maintained throughout lifecycle testing. Furthermore, these significant improvements in LiB performance depend on the purity level of the SWCNTs, as higher purity SWCNT additives result in better LiB cell performance.

[0069] Example 8. Comparison of specific energy and power density of LiB power cells A battery's nominal energy per unit mass is referred to as its specific energy or gravimetric energy density. Specific energy is a property of the LiB chemistry and packaging and is calculated by dividing the battery's energy in Wh (watt-hours) by the packaged cell's mass in kg. Similarly, specific power, or gravimetric power density, is the maximum usable power per unit mass of the battery and is calculated by dividing the battery's power output in W (watts) by the packaged cell's mass in kg.

[0070] The performance data summarized in Table 3 quantifies the twofold gain in specific energy density and specific power density of LiB power cells achieved by incorporating purified SWCNTs into standard LiB cells containing only carbon black as the conductive additive. The values ​​in the table are given to compare cells with and without the SWCNT additive, and the absolute values ​​depend on the cell design and active material used.

[0071] [Table 3]

[0072] As shown in Table 4, average increases in cell performance were observed for all formulations containing purified SWCNTs. Specific energy improvements ranged from 22% to 46% with the largest increase observed for 2 wt% SWCNT + 2 wt% carbon black. Specific power improvements at high C-rates ranged from 3% to 4.4% with the largest increase observed for 3 wt% SWCNT + 3 wt% carbon black.

[0073] [Table 4]

[0074] Example 9. Performance Comparison of As-Received, Purified, and Highly Purified SWCNTs as Conductive Additives in LiB Power Cells 2 wt% SWCNTs purified to greater than 99 wt% (i.e., containing 1 wt% or less inorganic impurities), referred to as highly purified SWCNTs, were also used as conductive additives along with 2 wt% SP in the cathode of LiB power cells. Figure 13 compares the early life cycle behavior of LiB power cells with cathodes containing the highly purified SWCNT grade to the other two investigated grades of SWCNT additive, as-received SWCNTs and purified SWCNTs. The cycling results indicate that the best LiB cell performance was achieved using the highly purified SWCNTs as the additive. After 400 consecutive cycles, the LiB cell containing the highly purified SWCNT additive exhibited nearly 90% capacity retention, a value higher than that achieved with either the purified SWCNTs or the as-received SWCNT conductive additive.

[0075] Comparing the 20 A discharge rate behavior of LiB cells with cathodes containing two grades of SWCNT additive with reduced inorganic impurities, namely purified SWCNT and highly purified SWCNT, reveals differences in performance, as shown in Figure 14. The high rate results reflect the cycling performance, with the LiB power cell incorporating highly purified SWCNT as an additive exhibiting the highest current discharge rate capability, even better than a similar LiB power cell with purified SWCNT as an additive.

[0076] Example 10. Performance Comparison of MWCNTs and SWCNTs as Conductive Additives in LiB Power Cells The use of 2 wt.% MWCNT as a conductive additive and 2 wt.% SP in the cathode of LiB power cells was investigated to compare the effectiveness of various CNT types as conductive additives. Figure 15 compares the early life cycle behavior of LiB cells assembled with MWCNT additive to individual cells containing three grades of SWCNT additive: as-received SWCNT, purified SWCNT, and highly purified SWCNT. Results show that LiB power cells incorporating as-received MWCNT as an additive (dashed and dotted lines) exhibited the fastest capacity fade after the first cycling, retaining less than 85% of their capacity after 400 cycles compared to cells with the three different grades of SWCNT, all of which exhibited superior cycling performance compared to cells containing MWCNT as an additive.

[0077] Figure 16 compares the 20 A discharge rate behavior of LiB power cells with cathodes containing MWCNT additives with the corresponding high-current discharge curves from cells containing various grades of SWCNT as additives. Power cells incorporating MWCNT as additives exhibited the lowest high-current discharge rate capability, inferior to similar LiB cells containing all grades of SWCNT additive.

[0078] While several embodiments of the present invention have been described in the examples above, those skilled in the art will recognize that various modifications can be made without departing from the scope of the present invention. Accordingly, other embodiments are within the scope of the following claims.

Claims

[Claim 1] 1. A lithium-ion battery cell having a cathode comprising an active material and 1-5 wt. % single-walled carbon nanotubes (SWCNTs) as a conductive additive, wherein the active material is lithium metal oxide or lithium iron phosphate, and the SWCNTs have an inorganic impurity content of less than 5 wt. %.

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