Lithium-ion battery and method for preparing the same

Abstract

The present invention discloses a lithium-ion battery comprising an anode electrode with an anode mixture layer coated on at least one surface of a substrate. The anode mixture layer includes graphite as the anode active material. Additionally, the battery includes a cathode electrode comprising lithium nickel cobalt aluminum oxide (NCA) compound as the cathode active material, a separator positioned between the cathode and anode electrodes, and an electrolyte. The anode mixture layer is designed to have a thickness within the range of 135-175 μm, with a corresponding mass loading falling between 0.01-0.05 g / cm2. The invention also discloses a method for preparing the said battery. The battery according to the present invention demonstrates exceptional characteristics such as high-energy density, high-power density, improved safety in application, and an extended cycle life.

Description

[0001] LITHIUM-ION BATTERY AND METHOD FOR PREPARING THE SAME

[0002] TECHNICAL FIELD

[0003] This invention relates to a lithium-ion battery and a method for preparing the same.

[0004] BACKGROUND OF THE INVENTION

[0005] Lithium-ion (Li-ion) batteries find widespread applications in various devices such as electric vehicles, laptops, mobile phones, and energy storage systems, owing to their remarkable energy and power densities. However, achieving optimal energy and power densities necessitates careful consideration of the design of electrode properties, with particular focus on fine-tuning the tortuosity of the electrode.

[0006] The conventional lithium-ion battery consists of cathode and anode electrodes separated by a separator to prevent short circuits. While the cathode electrode has received much attention for improving battery efficiency, the significance of the anode electrode shouldn't be overlooked. However, research on anode electrode characteristics like porosity, thickness, and loading content has been more common than studies on its tortuosity.

[0007] The tortuosity of the electrode is a complex aspect of design that determines the extent to which the electrolyte must navigate around or penetrate the porous structure of the electrode in order to reach the active material, where electrochemical reactions take place. Typically, conductive particles within a porous matrix contain open gaps or pores that hold the active material in position. The tortuosity of the electrode structure directly impacts the flow rate of the electrolyte through these pores to reach the active material.

[0008] In materials with high tortuosity, the electrolyte has to travel a longer distance to access the active material. This can potentially slow down the rate of ion transport and compromise the overall performance of the battery. On the other hand, when the tortuosity is minimized, the electrolyte can more easily reach the active material, facilitating faster rates of ion transport and thereby enhancing the battery's performance.

[0009] Therefore, to optimize the performance of lithium-ion batteries, careful consideration of the electrode structure's tortuosity and the optimization of its porous architecture become crucial. These steps are essential for unlocking the batteries' full potential and meeting the escalating demands of modern energy storage applications.

[0010] Here are examples of patent documents that address the enhancement of battery electrode tortuosity: JP 2009-199730 A discloses a non-aqueous electrolyte secondary battery with an electrode group consisting of a positive and negative electrode plate wrapped around a current collector. The electrode group, containing an active material for lithium-ion storage, is enclosed in a case with a non-aqueous electrolyte. An insulation separation layer provides insulation and comprises a high tortuosity layer and a low tortuosity layer. The high tortuosity layer, positioned on the negative electrode plate side, has a tortuosity ranging from 2.0 to 4.0, while the low tortuosity layer has a tortuosity ranging from 1.2 to 2.2.

[0011] US 9,065,093 B2 describes electrodes with low tortuosity pores, along with their associated systems and methods. The electrode consists of a matrix material containing high- tortuosity pores with tortuosity of at least 2. Additionally, there are numerous low-tortuosity pores with tortuosity less than 1.5, which extend through the matrix material from the electrode's first external geometric surface to its second external geometric surface. The electrode possesses a total porosity ranging from 20-60%, and the low-tortuosity pores occupy 20-80% of the total porosity. These electrodes enable the production of batteries with high energy densities, high power, or both.

[0012] SUMMARY OF THE INVENTION

[0013] The present invention aims at the development and improvement of optimized graphite anode electrode properties for lithium-ion batteries, especially when combined with a high nickel ratio cathode electrode.

[0014] The invention relates to a lithium-ion battery and a method for its preparation. More specifically, this invention focuses on enhancing the characteristics of the graphite anode electrode, such as achieving an appropriate tortuosity and enabling high mass loading. This invention is particularly advantageous for lithium-ion batteries, especially those utilizing a cathode with a high nickel ratio, such as 88% or above.

[0015] In the first aspect, the present invention discloses a lithium-ion battery comprising: an anode electrode comprising an anode mixture layer coated on at least one surface of a substrate, the anode mixture layer comprising graphite as an anode active material; a cathode electrode comprising lithium nickel cobalt aluminum oxide (NCA) compound as a cathode active material; a separator positioned between the cathode electrode and the anode electrode; and an electrolyte,

[0016] Preferably, the anode mixture layer has a thickness ranging from 135-175 m at a mass loading ranging from 0.01-0.05 g / cm2.

[0017] In another aspect, the invention discloses a method for preparing the lithium-ion battery according to the present invention. The method comprises the steps of preparing the anode electrode by coating the anode mixture layer onto the substrate, drying the coated substrate, and calendering the coated substrate to have a predetermined thickness. It is preferable to conduct the calendering by using a pressure ranging from 0.2-9.5 kg / m2and a speed ranging from 1-3 m / min.

[0018] The lithium-ion battery incorporating the developed anode electrode, as described in the present invention, offers notable advantages. It demonstrates high-energy density and high-power density, resulting in enhanced performance. Additionally, it provides improved application safety and an extended battery cycle life.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Fig. 1 depicts an image obtained from a field emission scanning electron microscope (FESEM) showing the surface morphology of the graphite powder particles according to the present invention at a 2000x magnification.

[0021] Fig. 2 is a graph obtained from an X-ray diffraction (XRD) analyzer showing a crystal structure of the graphite powder particles according to the present invention.

[0022] Figs. 3(a) - 3(h) are images obtained from FESEM showing the surface morphology of the graphite anode electrodes calendered at different pressures at a magnification of lOOOx (Figs. 3(a), 3(c), 3(e), and 3(g)), and 2500x (Figs. 3(b), 3(d), 3(f), and 3(h)), wherein:

[0023] Figs. 3(a) and 3(b) show the surface morphology of the graphite anode electrode calendered at a pressure of 2.1 kg / m2,

[0024] Figs. 3(c) and 3(d) show the surface morphology of the graphite anode electrode calendered at a pressure of 4.3 kg / m2,

[0025] Figs. 3(e) and 3(f) show the surface morphology of the graphite anode electrode calendered at a pressure of 6.7 kg / m2, and

[0026] Figs. 3(g) and 3(h) show the surface morphology of the graphite anode electrode calendered at a pressure of 9.2 kg / m2.

[0027] Figs. 4(a) - 4(1) are cross-sectional views obtained from FESEM showing the surface morphology of graphite anode electrodes calendered at different pressures, at a magnification of 250x (Figs. 4(a), 4(d), 4(g), and 4(j))), 500x magnification (Figs. 4(b), 4(e), 4(h), and 4(k)), and l,000x magnification (Figs. 4(c), 4(f), 4(i), and 4(1)), wherein:

[0028] Figs. 4(a), 4(b), 4(c) show the surface morphology of the graphite anode electrode calendered at pressures of 2.1 kg / m2,

[0029] Figs. 4(d), 4(e), 4(f) show the surface morphology of the graphite anode electrode calendered at pressures of 4.3 kg / m2,

[0030] Figs. 4(g), 4(h), 4(i) show the surface morphology of the graphite anode electrode calendered at pressures of 6.7 kg / m2, and

[0031] Figs. 4(j), 4(k), 4(1) show the surface morphology of the graphite anode electrode calendered at pressures of 9.2 kg / m2.

[0032] Figs. 5(a) - 5(f) are graphs that show the differential capacity (dQ / dV) versus voltage (V) and internal resistance (IR) of the batteries according to the present invention with different characteristics of the graphite anode electrodes coupled with NCA cathode as full-cell battery, at a voltage in a range of 3.0-4.2 V.

[0033] Figs. 6(a) and 6(b) are plotted graphs that show the specific capacity and voltage polarization of the batteries according to the present invention with different characteristics of the graphite anode electrodes, at a current density in a range of 0.1C to 5C, wherein:

[0034] Fig. 6(a) shows the specific capacity versus cycle number of the batteries, and Fig. 6(b) shows differential cell voltage (dV) versus cycle number of the batteries.

[0035] Figs. 7(a) and 7(b) are plotted graphs that show the specific capacity and voltage polarization of the batteries according to the present invention with different characteristics of the graphite anode electrodes, at a current density in range of 0.1C to 5C, wherein:

[0036] Fig. 7(a) shows the specific capacity versus cycle number of the batteries, and Fig. 7(b) shows differential cell voltage (dV) versus cycle number of the batteries.

[0037] Figs. 8(a) - 8(d) are plotted graphs that show stability properties of the batteries according to the present invention with different characteristics of the graphite anode electrodes, wherein:

[0038] Fig. 8(a) shows the capacity retention versus cycle number of the batteries, Fig. 8(b) shows the specific capacity versus cycle number of the batteries, Fig. 8(c) shows differential cell voltage (dV) versus cycle number of the batteries, and

[0039] Fig. 8(d) shows accumulated capacity of the batteries.

[0040] Figs. 9(a) - 9(d) are graphs showing differential capacity (dQ / dV) versus voltage (V) at different cycles of the batteries according to the present invention with different characteristics of the graphite anode electrodes.

[0041] Figs. 10(a) - 10(e) displays contour plots of in-operando XRD for spectra graphite phase transition on different graphite anode electrode characteristics according to the present invention, wherein:

[0042] Fig. 10(a) shows a phase transition of the graphite anode electrode calendered at a pressure of 2.1 kg / m2with a corresponding charge-discharge profile, Fig. 10(b) shows a phase transition of the graphite anode electrode calendered at a pressure of 9.2 kg / m2with a corresponding charge-discharge profile, and Figs. 10(c) - 10(e) show results from using Rietveld refinement technique and corresponding charge-discharge profile.

[0043] Figs. 11(a) - 11(d) are graphs showing the cell volume and lattice parameters changes of batteries with different characteristics of the graphite anode electrodes, which were monitored by in-operando XRD measurement.

[0044] Figs. 12(a) and 12(b) are graphs showing the gas evolution and corresponding charge-discharge profile as a function of time during the formation and charge-discharge process of batteries with different characteristics of the graphite anode electrodes, which were detected by in situ differential electrochemical mass spectrometry (DEMS) measurement.

[0045] Fig. 13 is a graph showing a surface temperature versus time of batteries with different characteristics of the graphite anode electrodes, which were measured according to T6 impact test of UN38.3 standard.

[0046] DETAILED DESCRIPTION

[0047] Any aspects shown herein shall encompass the application to other aspects of the present invention as well, unless specified otherwise.

[0048] Any tools, devices, methods, materials, or chemicals mentioned herein, unless specified otherwise, mean the tools, devices, methods, materials, or chemicals generally used or practiced by a person skilled in the art, unless explicitly specified as special or exclusive tools, devices, methods, or chemicals for the present invention. The terms “comprise(s)”, “consist(s) of’, “have / has”, “contain(s)”, and “include(s)” are open-end verbs. For example, any method which “comprises”, “consists of’, “has”, “contains”, or “includes” one component or multiple components or one step or multiple steps is not limited to only one component or one step or multiple steps or multiple components as specified, but also encompass components or steps that are not specified.

[0049] The inventors of the present invention have made a remarkable discovery regarding the influence of the graphite electrode's tortuosity on the quantity of Li within the lithiated graphite phase, predominantly as a result of diffusion limitations. It has been observed that electrodes with low tortuosity exhibit more efficient utilization of graphite compared to those with high tortuosity. Furthermore, this study provides valuable insights into the atomic-level energy storage mechanisms of the involved materials. As a significant aspect, the invention highlights that modifying the tortuosity of the anode electrode can profoundly impact the performance of lithium-ion batteries (LIBs), particularly when combining Ni-rich cathodes with graphite anodes.

[0050] In one aspect, the present invention relates to a lithium-ion battery comprising: an anode electrode comprising an anode mixture layer coated on at least one surface of a substrate, the anode mixture layer comprising graphite as an anode active material; a cathode electrode comprising lithium nickel cobalt aluminum oxide (NCA) compound as a cathode active material; a separator positioned between the cathode electrode and the anode electrode; and an electrolyte.

[0051] Preferably, the anode mixture layer has a thickness ranging from 135-175 pm at a mass loading ranging from 0.01-0.05 g / cm2. More preferably, the mass loading of the anode mixture layer ranges from 0.02-0.03 g / cm2.

[0052] According to a preferred embodiment, graphite is a natural graphite, and graphite has an average particle size ranging from 10-20 pm and has crystallite size (LC) ranging from 330-360 angstrom (A).

[0053] The particle size range of 10-20 pm is considered suitable based on a preferred embodiment. Although reducing the particle size has a positive impact on density and subsequently improves loading, there is a drawback to be considered. The reduction in particle size leads to an increase in irreversible reactions during the first cycle, ultimately resulting in a decrease in density. Additionally, it negatively affects the ability to conduct Li ions, leading to a decrease in power density. Moreover, a crystallite size (LC) ranging from 330-360 angstrom (A) is considered a suitable range for obtaining electrodes that can efficiently store energy. Preferably, the anode mixture layer has a porosity ranging from 25-50%, more preferably in a range of 30-45%.

[0054] According to the present invention, the lithium nickel cobalt aluminum oxide compound has a formula Li(NiaCobAlc)O2, whereby a > 0.88, 0 < b < 1 , 0 < c < 1 and the sum of a, b, and c is 1. For example, the lithium nickel cobalt aluminum oxide compound may be LiNi0.8sCo0.09Al0.03O2.

[0055] The anode and cathode electrodes of this invention may include a conductive material and a binder. The binder and the conductive material can be chosen from any materials commonly used in electrode preparation within the expertise of a skilled individual in this field.

[0056] For example, the conductive material options may consist of carbon black, super-P carbon black, acetylene black, channel black, furnace black, thermal black, carbon nanotube, nanocarbon, or a mixture of these materials.

[0057] Regarding the binder for the cathode electrode, suitable options may include polyvinylidene fluoride (PVDF), poly(3,4-ethylenedioxythiophene), polytetrafluoroethylene, or a mixture of these binders. As for the anode electrode, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), or a combination of these binders may be used.

[0058] In a preferred embodiment, the electrolyte suitable for use in this invention may comprise a combination of a lithium salt, a carbonate solvent, and an additive.

[0059] For examples, the lithium salt may be selected from lithium hexafluorophosphate (LiPFe), lithium oxalyldifluoroborate (LiODFB), lithium tetrafluoroborate (LiBF- , lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClCL), or a mixture thereof.

[0060] The carbonate solvent may be ethyl carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), or a mixture thereof.

[0061] As an example, the carbonate solvent may be a mixture of ethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. In an exemplary embodiment, the invention uses a mixture comprising ethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate with a volume ratio of 1 : 1 : 1.

[0062] Additionally, the additive can be vinylene carbonate (VC), propane sultone (PS), or other suitable additives known in the field. Another aspect of the present invention pertains to a method for preparing the lithium-ion battery with the aforementioned characteristics.

[0063] Specifically, the method of this invention comprises the steps of preparing the anode electrode by coating the anode mixture layer onto the substrate, drying the coated substrate, and calendering the coated substrate to have a predetermined thickness. Preferably, the calendering is carried out by using a pressure ranging from 0.2-9.5 kg / m2, more preferably 2.0-9.3 kg / m2, and a speed ranging from 1-3 m / min.

[0064] In a preferred embodiment, the drying of the coated substrate is carried out at a temperature ranging from 20-25 °C at a dew point ranging from -35 to -45 °C.

[0065] In the context of battery manufacturing, the calendering process involves compressing a dried electrode that has been formed by coating and drying an electrode mixture layer or slurry. The purpose of calendering, as described in the present invention, is to reduce the thickness of the electrode. This reduction in thickness leads to a decrease in electrode porosity, an enhancement in particle contact, and ultimately an improvement in the energy or power density of the electrode. Furthermore, the calendering process also contributes to the improvement of the tortuosity of the anode electrode.

[0066] By employing the graphite anode electrode design described in the present invention, the tortuosity anisotropy of the graphite can be determined within a range of 1-10, taking into account both through-plane tortuosity and in-plane tortuosity. If the tortuosity anisotropy of the graphite value exceeds 10, it will notably enhance the energy density; however, the power density will suffer to an extent that renders it unacceptable. Therefore, the range of 1-10 is considered optimal for achieving a battery with the appropriate balance of energy density and power density.

[0067] The substrate suitable for using in this invention may be aluminum foil or copper foil. Preferably, the substrate for preparing the anode electrode may be copper foil.

[0068] The present invention is applicable to the production of various types of lithium-ion batteries, including button batteries or cylindrical batteries.

[0069] To provide a more comprehensive understanding of the present invention, examples of the anode electrode and batteries incorporating it will be described in detail. The experimental results mentioned subsequently will be accompanied by relevant drawings. It is important to note that these drawings are provided for illustrative purposes and do not intend to restrict the scope of the invention in any manner. Examples

[0070] Fabrication of 18650 Li-ion cells:

[0071] The fabrication process for an 18650 cylindrical battery was carried out in a dry room, where the dew point was set at -40°C for battery cell fabrication and -55°C for electrolyte injection. All material powders, including lithium nickel cobalt aluminum oxide ( LiNi0.8sCo0.09Al0.03O2) with a BET surface area of 0.3 m2g (obtained from Gelon Corporation), natural graphite, Super-P carbon black, PVDF, and CMC / SBR, were stored in a vacuum oven at 80°C for 24 hours before usage.

[0072] Using a planetary vacuum mixer, the anode slurry was prepared for the anode electrode. The graphite, carbon black, and CMC / SBR binder were mixed in a ratio of 95.3:1:1.2:2.4 wt.% in deionized water, resulting in an approximate solid content of 49.3%. For the cathode slurry, NCA, carbon black, and PVDF were mixed in a ratio of 95.2:2.4:2.4 wt.% and dissolved in N-methyl-2-pyrrolidone (NMP) with a solid content of approximately 67.6%.

[0073] Subsequently, the well-mixed anode and cathode slurries were coated onto copper foil and aluminum foil, respectively, using a roll-to-roll automatic coating system equipped with a built-in 120°C drier. The coated anode electrode had a thickness ranging from 180-230 pm, while the coated cathode electrode had a thickness of 228 pm. The active mass loading contents for the coated anode and cathode electrodes were approximately 0.02034 and 0.0305 g / cm2, respectively.

[0074] Then, the dried coated anode electrodes obtained from the previous steps were rolled using the MSK-E2300B, which is an Automatic Battery Electrode Rolling Press System. This process was carried out under various preparation conditions to achieve the desired thickness and porosity of the electrodes.

[0075] The graphite anode electrode underwent compression at various pressures ranging from 2.0-9.3 kg / m2, resulting in pressed electrodes with thicknesses ranging from 139-175 pm. Detailed characteristics of all the pressed graphite anode electrode samples are provided in Table 1.

[0076] Table 1 presents the different samples of graphite anode electrodes, namely Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4, along with the corresponding pressing pressures of 2.1 kg / m2, 4.3 kg / m2, 6.7 kg / m2, and 9.2 kg / m2, respectively. CMC and SBR were used as binders for these samples. Table 1

[0077] Based on the information provided in Table 1, the porosity of the graphite anode electrodes can be calculated using the following Equation (1).

[0078] % porosity Equation (1) wherein mareai represents mass loading; co represents mass fractions; p represents density;

[0079] AM represents active material;

[0080] CM represents conductive material; and

[0081] B represents binder. The anode and cathode were then assembled together, with a tri-layer polypropylene / polyethylene / polypropylene (PP / PE / PP) film separator placed in between. For the electrolyte, a commercial carbonate-based electrolyte (EJN01, Gelon Corporation) containing IM LiPFe in a mixture of EC:DMC:EMC (1:1:1, v / v) with 1.2% vinylene carbonate (VC) and 1.35% propane sultone (PS) additives was used. 30 wt.% of IM LiPFe in FEC was mixed with the electrolyte to be used in the graphite / NCA 18650 cells.

[0082] All electrolyte preparation were conducted inside a glovebox located in the dry room with a dew point temperature of -55°C. A total of 5.2 g of the prepared electrolyte was injected into each cell. To ensure proper conditioning, all 18650 cells were aged at 1.5V for 24 hours prior to conducting electrochemical performance testing.

[0083] Furthermore, a formation procedure was implemented for two cycles, involving voltage ranges of 3.0-4.2 V and a current step starting at 0.04C, 0.06C, 0.08C, and 0.1C rates.

[0084] Measurements

[0085] Physicochemical Characterizations:

[0086] Graphite and electrode morphologies were analyzed using a field-emission scanning electron microscope (FESEM). The NCA powder particles used in this analysis consisted of a lithium nickel cobalt aluminum oxide composition with a high nickel ratio. The ratio of nickel to cobalt to aluminum in these NCA powder particles was found to be 88.95:8.67:1.27.

[0087] The crystallography of the powder particles was determined using powder X-ray diffraction (XRD), Bruker New D8 Advance diffractometer. Cu K radiation was employed to generate diffraction patterns in the 2 range of 10-70°.

[0088] A single layer pouch cell coupled with D8 Advance XRD machine and Metrohm Autolab electrochemical workstation (PGSTAT 302 N) were used for in-operando XRD / electrochemistry measurements during the charging / discharging. Mo Ka-radiation and charge-discharge tests were used to gather the diffraction patterns every 6 minutes at the 2 range of 7-55°. The Rietveld refinement analysis was done using TOPAS software, version 5.0 (Bruker AXS). In-operando XRD can be used to map the phase diagram of batteries with regard to their charge state and to relate the various phases to the voltage profile. Electrochemical Evaluation:

[0089] The battery was charged using the constant current and constant voltage (CCCV) method, with a target voltage of 4.2 V and a charging rate of 0.1C (1C equivalent to 2,300 mA). The battery was discharged to 3.0 V at a rate of 0.1C. These charging and discharging processes were conducted to determine the cell capacity of the NCA 18650 cylindrical configuration.

[0090] The rate capability test involved a fast charging and discharging protocol. The test aimed to evaluate the battery's performance under different charge and discharge current densities. During the test, the battery was subjected to CCCV charging, with target voltages of 4.2 V, at various current densities including 0.1, 0.25, 0.5, 0.75, 1C, 2C, 3C, and 5C. Following each charge, the battery was discharged using constant current (CC) mode at a rate of 0.5C until it reached a voltage of 3.0 V. Additionally, the rate capability test also included an assessment of the battery's performance under different discharge current densities. The battery was charged using CCCV method to 4.2 V at a rate of 0.5C and subsequently discharged using CC mode at current densities of 0.1, 0.25, 0.5, 0.75, 1C, 2C, 3C, and 5C until it reached a voltage of 3.0 V. Capacity retention was assessed using an asymmetric protocol involving CCCV charging at a rate of 0.5C and CC discharging at 1C for a total of 800 cycles. At the end of each cycle, the cell voltage was held with a constant current of 0.05C during the charge phase. This protocol was implemented to evaluate the battery's ability to maintain its capacity over a prolonged period of cycling.

[0091] The efficiency of the exemplary 18650 cylindrical battery, prepared following the previous steps, was evaluated using an electrochemical technique. The test results are presented in conjunction with the accompanying drawings and are explained as follows.

[0092] Test results

[0093] Study on surface morphological and structural properties:

[0094] Fig. 1 presents a top-view image obtained from FESEM, illustrating the surface morphology of the graphite powder particles utilized as the anode's active material. The analysis revealed that the graphite powder particles exhibit a non- spherical shape, deviating from a perfectly spherical form. Furthermore, the average particle size of the graphite powder particles was measured to be approximately 10-20 pm.

[0095] Fig. 2 shows a graph generated by an XRD analyzer, illustrating the crystal structure of the graphite powder particles. The XRD pattern of the graphite materials reveals distinct peaks corresponding to planes 002, 100, 101, 004, and 110. These peaks indicate the presence of an orderly arrangement of the crystal structure of the graphite materials.

[0096] Study on electrode properties:

[0097] The investigation focused on the samples of the graphite anode electrodes, i.e., Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4. These samples underwent a calendaring process at various pressures to reduce thickness and porosity, enhance particle interactions, and improve the energy density of the battery cells.

[0098] Figs. 3 and 4 present both top-view and cross-sectional images of the graphite anode electrodes at different calender pressures. These images were captured at different magnifications. The analysis revealed that the surface morphology of the graphite anode electrode exhibited a non-spherical shape, characteristic of graphite materials, with noticeable gaps between particles.

[0099] Upon observation, it became evident that increasing the calender pressure during the process resulted in a reduction in thickness. This increased compaction, leading to a denser packing effect of the coating layer and higher coating density of the electrodes.

[0100] When analyzing the thickness of all graphite anode electrodes (Graphite T-l to Graphite T-4), it is evident from Fig. 4 that the thickness decreases as the calendering process intensifies. The measured thicknesses of the anode mixture layer are as follows: Graphite T-l (Fig. 4(a)- 4(c)) is 174 pm, Graphite T-2 (Fig. 4(d)-4(f)) is 152 pm, Graphite T-3 (Fig. 4(g)-4(i)) is 145 pm, and Graphite T-4 (Fig. 4(j)-4(l)) is 139 pm. Correspondingly, the different porosities are 45%, 37%, 34%, and 31%, respectively (as shown in Table 1).

[0101] Furthermore, the tortuosity anisotropy of all graphite anode electrodes was calculated through the generalized Bruggeman using the following Equation (2):

[0102] T=£'aEquation (2) wherein

[0103] T represents tortuosity;

[0104] £ represents porosity; and (1 represents Bruggeman exponent.

[0105] The Bruggeman exponent for graphite particles is determined to be 0.666 for both the X and Y in-plane directions and 1.949 for the through-plane direction, attributed to the non- spherical shape of the graphite particles.

[0106] Based on the calculations, the in-plane tortuosities of Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4 are found to be 1.702, 1.938, 2.051, and 2.181, respectively. The through-plane tortuosities of Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4 are determined as 4.741, 6.943, 8.187, and 9.802, respectively. It is observed that the through-plane tortuosity increases exponentially due to the longer planar alignment of graphite particles along the transport direction.

[0107] Furthermore, the NCA electrode exhibits a thickness of 120 pm, corresponding to a porosity of 22%. The Bruggeman exponent for spherical NCA particles is 0.5 in all three directions: X (in-plane), Y (in-plane), and Z (through-plane). Therefore, the tortuosity of the NCA electrode is calculated to be 2.132.

[0108] Study on electrochemical performance of 18650 LIB cells using different graphite anode electrode configurations:

[0109] To assess the result of varying characteristics of graphite anode electrodes on electrochemistry, 18650 cylindrical LIB cells with NCA as the cathode material and graphite as the anode material were studied. The relevant information pertaining to the graphite and NCA electrodes can be found in Table 3.

[0110] Table 3: Properties of Graphite and NCA Electrodes

[0111] Referring to Fig. 5, the graphs in Fig. 5(a) depict the differential capacity (dQ / dV) versus voltage (V) for the 18650 lithium-ion battery (LIB) cells using the anode electrode samples Graphite T-l to T-4. To mitigate the influence of IR (internal resistance) or voltage shift on the dQ / dV vs. V plot, all cells were standardized to have an IR value within the range of 50-51 mQ. By analysing the dQ / dV results alongside the constant IR, the kinetics of the LIB cells can be examined (see Fig. 5(b)).

[0112] The 18650 battery cells were designed with a voltage range of 3.0-4.2 V and a capacity ranging from approximately 2.1-2.3 Ah. The discharge capacities at 0.1C for LIB cells with Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4 cells were found to be 2074, 2251, 2319, and 2374 mAh per cell, respectively. Notably, reducing the thickness of the graphite electrode from 180 to 145 pm resulted in a 14.5% increase in discharge capacity.

[0113] To evaluate the results further, Fig 5(c) shows the dQ / dV plot in the low voltage region of the full-cell potential range (3.0 to 4.2 V). The phase change from stage 1' to stage 4 of the Li / Li+graphite plateau is shown by the first peak at 3.4 V. Comparatively, the first peak for the Graphite T-4 cell shifted significantly to a higher voltage zone during the charging process, suggesting a slower Li-ion intercalation due to higher tortuosity. The Li-ion deintercalation peaks during the discharging process at 3.3 V showed minimal variation among all the graphite conditions (see Fig. 5(e)), indicating various Li-ion intercalation and deintercalation properties within the graphite anode electrode.

[0114] Furthermore, the NCA redox peaks, including the phase transitions at the Hl-M, M-H2, and H2-H3 positions, remained consistent across all samples. This suggests that the de- / intercalation or phase transition of the NCA structure is not affected by graphite porosity.

[0115] Referring to Fig. 6, rate capability tests were performed on multiple graphite tortuosity cells to gain insights into the de- / intercalation properties of graphite. The rate capability and voltage polarization results for current densities ranging from 0.1C to 5C are presented in Fig. 6(a) and Fig. 6(b), respectively.

[0116] At lower current densities (0 . 1 C), all samples exhibited similar behavior, with average specific capacities nearing 175 mAh / g. Moreover, the results demonstrated that the graphite anode electrode with lower tortuosity exhibited a higher specific capacity and a lower voltage (V) across all C-rates compared to the electrodes with higher tortuosity. This difference in performance can be attributed to the higher surface area per geometric surface area in the lower- tortuosity electrode, which enables more efficient solid-phase polarization. Conversely, the higher- tortuosity electrode can lead to greater solid-phase polarization due to a relatively lower surface area per geometric surface area. As a result, the flux at the particle surface of the electrode with higher tortuosity is larger.

[0117] In a similar manner, referring to Fig. 7, the rate capability and voltage polarization results of the fast-discharging protocol at current densities ranging from 0.1C to 5C for different graphite electrode properties are presented. Figs. 7(a) and 7(b) illustrate the specific capacity and dV for different C-rates.

[0118] Across the C-rates of 0.1C to 2C, all samples exhibit comparable specific capacities and dV values. Specifically, at 0.1C, 0.2C, 0.5C, 0.7C, 1.0C, and 2.0C rates, the specific capacities are measured to be 190.89, 181.75, 178.45, 172.86, and 171.63 mAh / g, respectively.

[0119] During the delithiation process, stage 1 transitions into stage 2L near the edge of the graphite particle. The rest of the highly lithiated stages of the graphite particle can be balanced by stage 2L's increased Li-ion diffusion. However, due to a significant liquid-phase polarization at high current density, the high- tortuosity electrode exhibits lower specific capacity and greater V at the high current densities (3C and 5C).

[0120] The stabilities of all graphite electrode samples were evaluated to examine the impact of graphite porosity using a 0.5C-rate CCCV charging and l.OC-rate CC discharging protocol, with corresponding check-ups at C / 10. Figs. 8(a) to 8(d) present the retention, specific capacity, relative voltage polarization (AV), and accumulated capacity plotted against the cycle number, respectively.

[0121] In Fig. 8(a), the capacity retention at the 800thcycle demonstrates insignificant differences among all graphite conditions. This can be attributed to the sufficiently high n to p ratio (n / p is 1.25) of all samples. The retention values at the 800thcycle for Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4 are 72.3%, 73.9%, 74.7%, and 75.5%, respectively.

[0122] Fig. 8(c) illustrates the relative voltage polarization (AV), which indicates the growth of internal resistance during cycling. All samples exhibit a similar increase in AV over cycles. However, the initial AV values at 1C for Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4 cells are 0.349 V, 0.360 V, 0.381 V, and 0.429 V, respectively. The higher electrode tortuosity leads to higher internal impedance due to limitations in Li-ion diffusion and the significant concentration gradient of the salt across the electrodes.

[0123] To assess the degradation mechanism of the materials during cycling, the dQ / dV versus cell voltage at C / 10 for the 1stcycle and their corresponding check-ups at C / 10 for the 56th, 109th, 212th, and 515thcycle were investigated, as depicted in Fig. 9.

[0124] As observed in Figs. 9(a) to 9(d), all redox peaks, including the Hl-M, M-H2, and H2-H3 phase transitions for all samples, undergo changes with cycling. The reduction in peak intensity and voltage shift correspond to the structural degradation of the cathode material. These observations confirm that the porosity of the graphite electrode does not impact the degradation of the cathode material.

[0125] Study on in-operando XRD of full pouch cells:

[0126] In order to gain a better understanding of the effects of graphite phase transitions on various characteristics of the graphite anode electrode, in-operando XRD analysis was conducted on both Graphite T-l and Graphite T-4 cells. Referring to Fig. 10, Figs. 10(a) and 10(b) depict the evolution of graphite phases LixCi2, LiCi2, and LiCe on the contour plot in the 2theta range of 10°-14°.

[0127] During Li-ion intercalation, graphite undergoes phase transitions based on the lithium concentration. The initial phase that emerges is LixCi2, which lacks in-plane order. The Li content of LixCi2 steadily increases until reaching a peak at x = 0.86, corresponding to a voltage of 3.54 V, as shown in Fig. 10(c). Subsequently, it declines to almost 0% weight, Rietveld at x = 0.7, corresponding to a voltage of 3.70 V. The trends observed are similar for both samples.

[0128] However, the LiCi2 phase experiences a significant increase in Li content as the LixCi2 phase declines. Fig. 10(d) demonstrates that the LiCi2 phase reaches its maximum value at x = 0.48, corresponding to a voltage of 3.96 V. Consequently, Graphite T-l exhibits a higher Li content in the LiCi2 phase compared to the Graphite T-4 cell. The utilization of the low-tortuosity electrode is greater than that of the high-tortuosity electrode due to solid-diffusion limitations. The pore- wall flux at the particle surface of the high-tortuosity electrode is larger due to a lower interfacial area per geometric area (A x Leiectrode). The lower pore-wall flux can impede the transport of Li-ions from the surface to the canter of the particle. Thus, the graphite utilization is influenced by the design of the porous electrode. Moreover, the fully lithiated state of graphite intercalation, known as the LiCe phase, appears as the LiCn phase decreases below x = 0.45, as depicted in Fig. 10(e). The Li content of the LiCe phase in the Graphite T-4 cell is slightly higher than that in the Graphite T-l cell, corresponding to the limitation of Li-ion availability from the NCA cathode at specific UCV (upper cutoff voltage).

[0129] To investigate the influence of graphite porosity on the crystallographic changes of the NCA structure, in-operando XRD analysis was conducted on both Graphite T-l and Graphite T-4 cells. The analysis focused on three selected 2theta regions, namely the (003), (101), and (104) diffraction planes.

[0130] The hexagonal (003) plane represents the c-axis, which consists of the NCA's transition metal and lithium layer. During Li-ion deintercalation or charging (x = 1 to x = 0.5), the d-spacing of the c lattice expands due to the strong electrostatic attraction between the NiCoO2 interslab. This expansion is observed as a decrease in 2theta (according to Bragg's law) and indicates a phase shift from Hl to H2. The reduction in shape (x = 1 to x = 0.26) is caused by the significant amount of Li exiting the structure, resulting in a decrease in the electrostatic repulsion of the transition metal due to the formation of a partial covalent bond (Ni4+-O2‘).

[0131] The (101) plane represents the a-axis, and the peak shifts towards higher 2theta values due to lattice contraction resulting from the oxidation of Ni2+ / Ni3+to Ni4+, leading to a reduction in ionic radius.

[0132] Regarding the (104) plane, the trend is similar to that of the (003) plane, but there may be an overlapping peak originating from the current collector or separator.

[0133] Rietveld refinement was utilized to determine the structural parameters of the studied samples. Fig. 11, particularly Figs. 11(a) and 11(b), illustrates the unit cell volume of Graphite T-l and Graphite T-4 cells . At x= 1 , the unit cell volume of Graphite T- 1 is measured to be 97.878 A, which is slightly higher than that of Graphite T-4, which is 96.834 A. This larger cell volume in Graphite T-l confirms a higher degree of Li content in the NCA structure.

[0134] The changes in lattice parameters (a, c) are shown in Figs. 11(c) and 1 l(d. The Li extraction and insertion processes exhibit slight differences between the two samples.

[0135] Tables 4 and 5 provide information on the contraction and expansion of the a lattice, elongation and shrinkage of the c lattice, and the corresponding changes in cell volume during the charging and discharging processes. Table 4: Percentage of Lattice Parameter Change during Charging and Discharging of Graphite T-l / NCA Cells

[0136] Table 5: Percentage of Lattice Parameter Change During Charging and Discharging of Graphite T-4 / / NCA cells.

[0137] From Table 4 and Table 5, it can be observed that there are variations in the contraction and expansion of a lattice, elongation and shrinkage of a lattice, and cell volume during the charge and discharge processes.

[0138] For both Graphite T-l and Graphite T-4 cells, the lattice parameter (a) gradually contracts by 1.69188% and 1.63321% during the charging process, respectively, and then expands by 1.77959% and 1.76887% on discharge, respectively.

[0139] The lattice parameter (c) of both samples demonstrates a significant elongation during the initial stage of 1.37010% and 1.38755%, respectively. However, towards the end of the charge process, there is a rapid shrinking, resulting in a shrinkage of 3.82308% and 3.84850% for Graphite T-l and Graphite T-4 cells, respectively. The changes in lattice parameters during Li-ion extraction and insertion show similarities between Graphite T-l and Graphite T-4 cells.

[0140] The cell volume experiences a change of 5.631% and 5.287% during intercalation for Graphite T-l and Graphite T-4 cells, respectively. During deintercalation, the cell volume changes by 6.195% and 5.841% for Graphite T-l and Graphite T-4 cells, respectively. This indicates that the cell volume in Graphite T-l cells undergoes a slightly greater change compared to Graphite T-4 cells.

[0141] Study on in situ differential electrochemical mass spectrometry (DEMS):

[0142] A study on in situ differential electrochemical mass spectrometry (DEMS) or online gas detection was conducted on Graphite T-l and Graphite T-4 paired with NCA 18650 cylindrical cells to examine the influence of tortuosity on gas evolution during the cell creation and chargedischarge processes.

[0143] Fig. 12 illustrates the gas evolution with associated voltage profiles for Graphite T-l and Graphite T-4 cells (refer to Figs. 12(a) and 12(b), respectively). Both systems produce similar gases, including ethylene (C2H4), carbon dioxide (CO2), carbon monoxide (CO), and hydrogen (H2). Notably, H2 gas experiences significant evolution in the voltage range of 3.2 to 3.5 V, attributed to the reduction in moisture contamination during the initial formation cycle.

[0144] As the charging process progresses, gases such as CO2, CO, C2H4, and H2 evolve simultaneously. CO2 and H2 gases are produced by the four-electron reaction of FEC (fluoroethylene carbonate) and the one-electron reaction of FEC. The ring opening of EC (ethylene carbonate) at the carbonyl carbon atom contributes to the formation of CO and C2H4. Both Graphite T-l and Graphite T-4 exhibit similar gas evolution behavior throughout the cycle.

[0145] During the charging process up to 4.2 V, no gas generation is observed. However, as the cell voltage reaches 4.3 V and continues up to 4.6 V, the evolutions of CO2, CO, and H2 commence and continue to progress. The evolution of CO2, CO, and H2 gases is thus potentialdependent. It is worth noting that the CO2 evolution at high potential (4.4 V vs. Li / Li+) arises from the oxidation of Li2CO3 impurities as well as the breakdown of FEC. Consequently, the tortuosity of the electrode has no impact on gas formation during the operation of the Li-ion battery.

[0146] Study on Battery Safety:

[0147] Ensuring the safety of lithium-ion batteries is of utmost importance. In accordance with the UN38.3 standard, a comprehensive safety test was conducted on all 18650 battery cells to assess their safety during shipment. The tests included an altitude simulation (Tl), a heat test (T2), and a shock test (T4), all performed on the same set of samples. Prior to testing, ten battery cells for each graphite condition were fully charged to 4.2 V (100% State of Charge (SOC)). The summarized test results are presented in Tables 6-8. Table 6: Attitude Simulation Test (Tl) Results

[0148] Table 7: Thermal Test (T2) Results

[0149]

[0150] Table 8: Shock Test (T4) Results

[0151] As a result of the safety tests, there were no instances of leakage or disintegration observed in any of the graphite conditions, including Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4. Tables 6-8 provide the weight and voltage loss before and after the tests. In all cases that met the criteria, both the weight and voltage loss were found to be less than 1%.

[0152] To simulate the impact and crushing scenario of a battery cell, an impact test (T6) was conducted. Five battery cells from each graphite condition were charged to 3.6 V (50% State of Charge). Importantly, no electrolyte leaks or explosions occurred in any of the cells, including Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4.

[0153] In Fig. 13, the recorded surface temperature responses of all graphite samples are plotted against time. It can be observed that for the Graphite T-l, Graphite T-2, Graphite T-3, and Graphite T-4 cells, the surface temperature of the crushed cells increased to 84°C, 98.5°C, 109.8°C, and 113.5°C, respectively, before eventually cooling down to room temperature after 10 minutes. The increase in temperature was caused by the exothermic reaction of the primary anode SEI (Solid Electrolyte Interphase) breakdown. These results suggest that the tortuosity of the graphite material has an impact on the heat generation during anode SEI decomposition.

[0154] Battery Demonstration:

[0155] The present invention includes a demonstration of a battery utilizing different graphite anode electrode characteristics. The demonstration involved subjecting a sample of the Graphite T-2 to a test by assembling a LIB pack with 15 cells arranged in four parallel groups connected in series, resulting in a configuration of 55V and 8500 mAh. This battery pack was successfully integrated into the Ninebot Gokart Pro Lamborghini 2022 Edition.

[0156] By utilizing LIB batteries with graphite anode electrodes according to the present invention, the go-karts demonstrated impressive performance capabilities. They were able to accelerate to a top speed of 40 km / h in just 5 seconds, showcasing the high-power potential of the batteries. Additionally, these batteries exhibited fast charging capabilities, with a full recharge time of only 37 minutes. The successful demonstration of the battery pack in the go-kart highlights its suitability for high-power applications. This provides promising insights for future studies, specifically in optimizing the electrochemical performance for applications such as energy storage systems (ESS) and electric vehicles (EVs).

[0157] Summary of the Experimental Results:

[0158] The investigation focused on graphite anode electrodes with different tortuosities, and several electrochemical tests were conducted to evaluate their performance. The dQ / dV plot, rate capability tests for quick charging and fast discharging, and in-operando XRD analysis provided insights into the influence of graphite tortuosity on Li-ion diffusion and phase transitions.

[0159] The results confirmed that the tortuosity of the graphite electrodes significantly affects Li-ion diffusivity. Low-tortuosity electrodes exhibited higher graphite utilization due to a soliddiffusion limitation. The stability results of all graphite properties showed minimal differences among the samples.

[0160] In situ DEMS observations revealed that the evolution of gases, including H2, CO2, CO, and C2H4, during the formation process and at high state of charge (SOC), was similar for all graphite conditions. Therefore, tortuosity did not play a role in the electrolyte degradation mechanism.

[0161] The final testing, which included altitude simulation (Tl), heat test (T2), shock test (T4), and impact test (T6), successfully met the requirements specified by UN38.3 standards. However, the impact test (T6) showed that the surface temperature response of a shattered cell was influenced by the tortuosity of the graphite anode electrode.

[0162] The testing results clearly confirm that the inclusion of the graphite anode electrode, as proposed in the present invention, provides significant advantages in optimizing the energy and power density of lithium-ion batteries. Importantly, these advantages are achieved without compromising the cycle life of the batteries.

[0163] BEST MODE OF THE INVENTION

[0164] Best mode of the invention is as described in the detailed description of the invention.

Claims

WHAT IS CLAIMED IS:

1. A lithium-ion battery comprising: an anode electrode comprising an anode mixture layer coated on at least one surface of a substrate, the anode mixture layer comprising graphite as an anode active material; a cathode electrode comprising lithium nickel cobalt aluminum oxide (NCA) compound as a cathode active material; a separator positioned between the cathode electrode and the anode electrode; and an electrolyte, wherein the anode mixture layer has a thickness ranging from 135-175 pm at a mass loading ranging from 0.01-0.05 g / cm2.

2. The lithium-ion battery according to claim 1, wherein the mass loading ranges from 0.02-0.03 g / cm2.

3. The lithium-ion battery according to claim 1, wherein graphite is a natural graphite.

4. The lithium-ion battery according to claim 1, wherein graphite has an average particle size ranging from 10-20 pm.

5. The lithium-ion battery according to claim 1, wherein graphite has liquid crystals (LC) ranging from 330-360 angstrom (A).

6. The lithium-ion battery according to claim 1, wherein the anode mixture layer has a porosity ranging from 25-50%.

7. The lithium-ion battery according to claim 1, wherein the lithium nickel cobalt aluminum oxide compound has a formula Li(NiaCobAlc)O2, whereby a > 0.88, 0 < b < 1 , 0 < c < 1 and the sum of a, b, and c is 1.

8. A method for preparing the lithium-ion battery according to claim 1, comprising the steps of preparing the anode electrode by coating the anode mixture layer onto the substrate, drying the coated substrate, and calendering the coated substrate to have a predetermined thickness, wherein the calendering is carried out by using a pressure ranging from 0.2-9.5 kg / m2and a speed ranging from 1-3 m / min.

9. The method according to claim 8, wherein the drying of the coated substrate is carried out at a temperature ranging from 20-25°C at a dew point ranging from -35 to -45 °C.

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