WO2023170449A1

WO2023170449A1 - Active material for cathode of lithium-ion battery, cathode comprising said active material, and method for preparing said cathode

Info

Publication number: WO2023170449A1
Application number: PCT/IB2022/052135
Authority: WO
Inventors: Montree SAWANGPHRUK; Chanikarn TOMON; Sangchai SARAWUTANUKUL; Nutthaphon PHATTHARASUPAKUN; Nichakarn ANANSUKSAWAT; Kan HOMLAMAI; Nattanon JORALEECHANCHAI; Worapol TEJANGKURA; Chonticha JANGSAN
Original assignee: Vidyasirimedhi Institute Of Science And Technology (Vistec)
Priority date: 2022-03-10
Filing date: 2022-03-10
Publication date: 2023-09-14

Abstract

The present invention relates to an active material for a cathode of a lithium-ion battery comprising a mixture of lithium manganese oxide (LMO) of a formula LiMn₂O₄ and lithium nickel manganese cobalt oxide (NMC) of a formula Li(Ni_xMn_yCo_z)0₂, whereby 0.7 ≤ x <1, 0 < y < 1, 0 < z < 1 and the sum of x, y, and z is 1 and a mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is in a range of 1 : 1-2. Moreover, the invention further relates to a battery cathode comprising such active material and a method for preparing such cathode.

Description

ACTIVE MATERIAL FOR CATHODE OF LITHIUM-ION BATTERY, CATHODE COMPRISING SAID ACTIVE MATERIAL, AND METHOD FOR PREPARING SAID CATHODE

TECHNICAL FIELD

Chemical technology related to an active material for a cathode of a lithium-ion battery, a cathode comprising said active material, and a method for preparing said cathode.

BACKGROUND OF THE INVENTION

Energy storage devices currently play a highly important role, particularly in the rapid development of technologies and industries, such as electronics technology and electric vehicle industry. A high-efficiency energy storage device which is commonly used at present is battery, especially that used in electronic devices which is usually a rechargeable secondary battery.

There are various types of secondary battery, such as lead-acid battery, nickel-cadmium (Ni-Cd) battery, nickel-metal hydride (Ni-MH) battery, and lithium-ion (Li-ion) battery. The Li-ion battery provides high energy density per weight and capacity, compared to other types of battery. Moreover, it can be used at high voltage with a nominal voltage as high as 3.6-3.7 V. Great attention has therefore been given to the improvement of lithium-ion battery’s efficiency and cycle life.

The improvement of battery efficiency in general is focused on four main components of the battery, i.e., a cathode consisting of lithium oxide compound, an anode consisting of carbon compound, a separator, and an electrolyte solution. Nevertheless, lithium metal oxide contained in the cathode is a key factor in improving the battery’s efficiency and stability.

The cathode of the currently available lithium-ion battery consists of various materials, i.e., lithium cobalt oxide (LCO), lithium iron phosphate (LFP), lithium manganese oxide (LMO), lithium nickel cobalt aluminium oxide (NCA), and lithium nickel manganese cobalt oxide (NMC). Among these materials, the demand for the NMC material is increasing considerably in the market as it provides high energy storage capacity, especially the NMC material with a high mole ratio of nickel (Ni), compared to the mole ratio of manganese (Mn) and cobalt (Co), for example, the mole ratio of Ni : Mn : Co of 8 : 1 : 1 (NMC 811), which is a ratio that provides maximum capacity at approximately 200 mAh/g. Moreover, the NMC material also contains a small amount of cobalt material, which is an expensive material. Therefore, the use of the Ni-rich NMC material provides high energy storage at a reasonable price, making it suitable for the industrial- scale production in the future.

Although the NMC material with a high nickel ratio provides high capacity due to the large amount of nickel, there are still limitations as the capacity fading can occur after a longterm usage and the NMC material has low thermal stability. These are mainly caused by the instability of the layered structure, for example, an excessive lattice expansion/contraction in the R(-)3m structure caused by the interlayer repulsive force of metal oxide (MOe slab). This excessive expansion/contraction can result in a structure distortion. Moreover, a reconstruction of the R(-)3m structure to the Fm(-)3m rock salt structure can also occur, wherein the rock salt structure is a structure which is difficult to be reversed, thus causing a loss of energy storage. Moreover, after multiple uses, the NMC material can experience a reduction of Ni⁴⁺ to Ni²⁺, whose size is similar to that of lithium ion, which causes a replacement of lithium ion with Ni²⁺ called cation mixing which results in a loss of lithium storage space. The reduction of Ni⁴⁺ to Ni²⁺ causes oxygen to be released, which is the main cause of thermal runaway and can lead to battery explosion.

In order to solve the aforementioned problems of the NMC material, an improvement is made by mixing the NMC material with other stable materials, such as LCO material and LMO material having the spinel structure which are materials with high thermal stability and a stable structure under a voltage range of 3.0-4.5 V. Mixing of the spinel material can thus solve the problems caused by the NMC material as it reduces the capacity fading and increases the thermal stability, thus reducing danger or explosion risk of the battery. Examples of the prior art related to this improvement are the following patent documents.

WO 2011113515 Al discloses a cathode of a lithium-ion battery comprising a mixture of NMC, with a mole ratio of Ni : Mn : Co of 1 : 1 : 1 (NMC 111), and LMO in a ratio of NMC 111 to LMO of 9 to 1, 3 to 7, 7 to 3, 6 to 4, and 4 to 6. The mixture of NMC 111 and LMO can increase the stability in application without capacity fading after 250 cycles.

US 9,446,963 B2 discloses a cathode active material for a lithium-ion battery comprising a mixture of a first LMO material prepared using a spray-dry technique, and a second LMO material prepared using a co-precipitation technique, wherein the LMO material may include an NMC 111 material.

Although the above patent documents improve the cathode active material for the lithium-ion battery by using a mixture of the NMC and LMO materials, all such documents focus on improving the cathode efficiency by using a mixture of NMC with a low nickel ratio (that is, NMC 111), which gives low energy storage of approximately 160 mAh/g. There is no prior art focusing on improving the efficiency of the cathode active material by using a mixture of NMC with a high nickel ratio (such as NMC 811) which provides energy storage of up to 200 mAh/g approximately. Although the prior arts can improve the cathode active material and increase the battery stability, the obtained battery still provides low energy storage, making it inappropriate for using with technology that requires high energy storage.

SUMMARY OF THE INVENTION

The first object of the present invention is to solve the capacity-fading problem when using a battery with a cathode comprising the lithium nickel manganese cobalt oxide (NMC) material.

Another object of the present invention is to improve the cathode active material to obtain high battery capacity after being assembled to the lithium-ion battery, long cycle life, increased application cycle, and good stability and energy density.

The above objects of the invention can be achieved by developing the cathode active material comprising a mixture of the NMC material, which has a high mole ratio of nickel (Ni) compared to the mole ratio of manganese (Mn) and cobalt (Co), and the lithium manganese oxide (LMO) material in order to increase the stability, extend the cycle life of the lithium-ion battery, and increase the capacity, compared to the battery using the cathode comprising only the LMO material.

In one aspect, an active material for a cathode of a lithium-ion battery according to the present invention comprises a mixture of lithium manganese oxide (LMO) of a formula Li Mni04 and lithium nickel manganese cobalt oxide (NMC) of a formula Li(Ni_xMn_yCo_z)O2, whereby 0.7 < x <1, 0 < y < 1, 0 < z <l and the sum of x, y, and z is 1 and a mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is in a range of 1: 1-2.

In another aspect, the present invention relates to a cathode of a lithium-ion battery comprising the active material according to the present invention, a binder, and a conductive material.

Furthermore, the present invention relates to a method for preparing s cathode of a lithium-ion battery comprising the steps of (i) preparing a mixture of the active material according to the present invention, the binder, and the conductive material, and (ii) coating the mixture obtained from step (i) onto a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is images obtained from a scanning electron microscope (SEM) at a l,000x magnification showing the characteristic of the surface of the cathode active material comprising the mixture of lithium nickel manganese cobalt oxide (NMC) of the formula Li(Ni0.8Mn0.1Co0.1)O2 (NMC 811) and lithium manganese oxide of the formula LiMn2O4 (EMO) at different mass ratios, wherein

Fig. 1(a) shows the characteristic of the surface of the active material at the mass ratio of NMC 811 : LMO of 1 : 1,

Fig. 1(b) shows the characteristic of the surface of the active material at the mass ratio of NMC 811 : LMO of 1 : 2, and

Fig. 1(c) shows the characteristic of the surface of the active material at the mass ratio of NMC 811 : LMO of 2 : 1.

Fig. 2 is images obtained from an energy dispersive X-ray spectroscopy (EDX) used in conjunction with the scanning electron microscope (SEM) which show the dispersion of different elements on the cathode active material comprising the mixture of NMC 811 and LMO at different mass ratios, wherein

Fig. 2(a) shows an analysis of the elements present in the active material sample in Fig. 1(a) (the mass ratio of NMC 811 : LMO being 1 : 1),

Fig. 2(b) shows an analysis of the elements present in the active material sample in Fig. 1(b) (the mass ratio of NMC 811 : LMO being 1 : 2), and

Fig. 2(c) shows an analysis of the elements present in the active material sample in Fig. 1(c) (the mass ratio of NMC 811 : LMO being 2 : 1).

Fig. 3 is graphs obtained from an X-ray diffraction (XRD) analyzer showing the crystal structures of the cathode active material comprising the mixture of NMC 811 and LMO at different mass ratios and a comparative cathode active material, wherein

Fig. 3(a) shows the crystal structure of the comparative cathode active material which is LMO,

Fig. 3(b) shows the crystal structure of the cathode active material which is NMC 811, and Fig. 3(c) shows the crystal structure of the cathode active material comprising the mixture of NMC 811 and LMO at the mass ratio of 1 : 1, 1 : 2, and 2 : 1.

Fig. 4 is a graph showing the charge-discharge profile of the battery using the cathode comprising the active material comprising the mixture of NMC 811 and LMO at different mass ratios and the comparative battery at 0.1C current.

Fig. 5 is graphs showing the characteristic of the reaction or the oxidation-reduction of the battery using the cathode comprising the active material comprising the mixture of NMC 811 and LMO at different mass ratios and the comparative battery through the dQ/dV graph analysis, wherein

Fig. 5(a) shows the dQ/dV graph of the comparative battery using the cathode active material which is LMO,

Fig. 5(b) shows the dQ/dV graph of the comparative battery using the cathode active material which is NMC 811 ,

Fig. 5(c) shows the dQ/dV graph of the battery using the cathode active material which is the mixture of NMC 811 and LMO at the mass ratio of NMC 811 : LMO of 1 : 1,

Fig. 5(d) show's the dQ/dV graph of the battery using the cathode active material which is the mixture of NMC 811 and LMO at the mass ratio of NMC 811 : LMO of 1 : 2, and

Fig. 5(e) show's the dQ/dV graph of the battery using the cathode active material which is the mixture of NMC 811 and LMO at the mass ratio of NMC 811 : LMO of 2 : 1.

Fig. 6 is graphs showing the stability and the coulombic efficiency at different cycles of the battery using the cathode comprising the active material comprising the mixture of NMC 811 and LMO at different mass ratios and the comparative battery, wherein

Fig. 6(a) shows the stability and the coulombic efficiency of the comparative battery using the cathode active material which is LMO,

Fig. 6(b) shows the stability and the coulombic efficiency of the comparative battery using the cathode active material which is NMC 811,

Fig. 6(c) shows the stability and the coulombic efficiency of the battery using the cathode active material which is the mixture of NMC 811 and LMO at the mass ratio of 1 : 1 , Fig. 6(d) shows the stability and the coulombic efficiency of the battery using the cathode active material which is the mixture of NMC 811 and LMO at the mass ratio of 1 : 2, and

Fig. 6(e) shows the stability and the coulombic efficiency of the battery using the cathode active material which is the mixture of NMC 811 and LMO at the mass ratio of 2 : 1.

Fig. 7 is a graph showing the capacity retention at different cycles of the battery using the cathode comprising the active material comprising the mixture of NMC 811 and LMO at different mass ratios and the comparative battery.

DETAILED DESCRIPTION

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

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,” “contain(s),” and “include(s)” are open-end verbs. For example, any method which “comprises,” “consists of,” “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.

Throughout the present invention, the term “about” is used to indicate that any values appearing or shown herein may be varied or deviate. Such variation or deviation may be caused by equipment error, method used to determine values, or a person using the equipment or carrying out such method.

In the present invention, the disclosure described includes the drawings and the examples of experiments shown are intended for assisting the explanation of various aspects of the invention and for understanding only and do not limit the scope of the present invention. It is anticipated that a person of ordinary skill in the art may be able to modify, add or improve the embodiment in various manners without departing from the scope of the appended claims.

The present invention is aimed at developing the cathode active material of the lithium-ion battery by using a mixture of two materials, that is, lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide (NMC), which has a high mole ratio of nickel (Ni), compared to the ratio of manganese (Mn) and cobalt (Co). Studies and experiments are carried out in order to obtain an optimal mass ratio of LMO to NMC to acquire the active material which, when used as a component of the battery cathode, provides the battery with high capacity, long cycle life, increased application cycle, good stability, and energy density, and particularly, high capacity per weight (in Wh/kg).

The cathode active material of the lithium-ion battery according to the present invention comprises the mixture of lithium manganese oxide (LMO) of the formula LiMn2O4 and lithium nickel manganese cobalt oxide (NMC) of the formula Li(Ni_xMn_yCo_z)O2 , whereby 0.7 < x <1, 0 < y < 1, 0 < z <1 and the sum of x, y, and z is 1 and a mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is in a range of 1 : 1-2.

Preferably, the lithium nickel manganese cobalt oxide (NMC) has the formula Li(Ni0.8Mn0.1Co0.1)O2 and the mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is 1 : 2.

The lithium nickel manganese cobalt oxide (NMC) suitable for the present invention has a particle size in a range of 5-15 pm. Particularly preferred is a particle size in a range of 8-10 pm.

The lithium manganese oxide (LMO) suitable for the present invention has a single particle size in a range of 400 nm -1 pm. Particularly preferred is a particle size in a range of 400-700 nm.

In another aspect, the present invention develops the cathode of the lithium-ion battery comprising the active material according to the present invention with the characteristic as mentioned above, the binder, and the conductive material.

The binder may be selected from polyvinylidene fluoride (PVDF), poly(3,4-ethylene dioxythiophene) (PEDOT), polytetrafluoroethylene (PTFE), and a mixture thereof. The conductive material may be selected from carbon black, acetylene black, super P, and a mixture thereof.

According to a preferred embodiment of the invention, a weight ratio of active material to binder to conductive material is in a range of 90-98 to 1-5 to 1-5.

Furthermore, the present invention develops the method for preparing the cathode of the lithium-ion battery, the method comprising the steps of: (i) preparing the mixture of the active material with the characteristics as mentioned above, the binder, and the conductive material, and

(ii) coating the mixture obtained from step (i) onto a substrate.

The binder and the conductive material for preparing the mixture of the active material in step (i) can be selected from the list above and an example of preferred substrate is aluminium.

Preferably, the weight ratio of active material to binder to conductive material is in a range of 90-98 to 1-5 to 1-5.

Preferably, the preparation of the mixture according to step (i) is carried out by mixing in a presence of a solvent. The solvent is, for example, N-methylpyrrolidone solution.

According to a preferred embodiment of the present invention, step (ii) is carried out with a coating thickness ranging from 200-270 pm.

The method for preparing the cathode of the present invention may also comprise step

(iii) of drying the coated substrate. For example, the coated substrate may be heated at a temperature ranging from 100-180°C.

The obtained cathode comprising the active material according to the present invention is particularly preferred for the production of various types of lithium-ion battery, such as a cylindrical battery.

The present invention will now be described in more detail by citing the examples of the invention and the test results, which will be mentioned hereinafter with reference to the accompanying drawings which do not limit the scope of the invention in any way.

Example

An example cylindrical battery 18650 used in the test was prepared as follows.

1. Preparation of the cathode

The preparation of the cathode was performed by mixing 90-150 g polyvinylidene fluoride (PVDF), which serves as a binder, with 500-1,500 g N-methylpyrrolidone solution and stirring for 10-60 minutes under vacuum. Then, 90-150 g carbon material was added and stirred for 10-60 minutes under vacuum. Then, 1,500-2,500 g lithium nickel manganese cobalt oxide which is NMC 811 was added, followed by 500-1 ,500 g N-methylpyrrolidone solution, and stirred until homogeneous using an automatic mixer for approximately 1 hour. Then, 1,500-2,500 g lithium manganese oxide (LMO) of the formula LiMn-204 was added, followed by an additional 200-500 g N-methylpyrrolidone solution. Then, the mixed solution was stirred until homogeneous using the automatic mixer for approximately 6-24 hours. The mixture was then coated onto an aluminium sheet used as a substrate using an automatic coater with a coating thickness of 200-270 gm and a drying temperature of l()0-180°C.

2. Preparation of the anode

The preparation of the anode was performed by mixing 30-50 g carboxymethyl cellulose, which serves as a binder, and 50-100 g ethanol in 500-1,000 g deionized water using the automatic mixer and stirring using a large paddle at a speed of 50-100 rpm and a small paddle at 2,000-5,000 rpm for 1-2 hours under vacuum. Then, 20-50 g carbon material was added to the solution and the stirring was continued for 20-60 minutes under vacuum. Then, 50-100 g ethanol was added to the solution and the stirring was continued for 30-60 minutes under vacuum. Then, 1,500-2,000 g graphite material was added and stirred for 1-2 hours under vacuum. Then, 50-100 g styrene-butadiene rubber, which serves as another binder, and 500-1,000 g deionized water were added and stirred for 1 hour under vacuum. Then, 500-1,000 g additional deionized water was added and stirred until homogeneous under vacuum. Then, the mixture was coated onto a copper sheet using the automatic coater with a coating thickness of 50-150 pm and a drying temperature of 100-130°C.

3. Battery assembly

The cathode and the anode obtained from steps 1 and 2 were assembled into an 18650 cylindrical battery. The assembly started with calendering the cathode and the anode using an automatic calendaring machine with a pressure of 2- 10 tons to obtain the thickness of the cathode and the anode of about 100-160 and 50-120 pm, respectively. Then, the cathode and the anode were cut into about 5-6 cm in width and about 55-70 cm in length using an automatic cutter. Then, the head portion of the cathode was welded with an aluminium strip using a welding machine and the end portion of the anode was welded with a nickel strip using the welding machine as well. Then, the electrodes were then wound together using an automatic winding machine with a ceramic film between the two electrodes to prevent a short circuit. The wound electrodes were then loaded into a cylindrical battery case. The case containing the electrodes was then subjected to a case grooving process. Then, a battery cap was welded to the electrodes inside the battery case before filling with 3-6 g electrolyte per one battery in an atmosphere- controlled chamber with the humidity and oxygen levels lower than 0.1 ppm. The electrolyte solution used was lithium hexafluorophosphate which was dissolved in a solution mixed with ethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. Then, the battery was then charged using an automatic battery charger before being wrapped with a polyvinyl chloride (PVC) sheet at a temperature of about 120-160°C in a belt oven to obtain the 18650 cylindrical battery.

Test results

The exemplary 18650 cylindrical battery obtained above was analyzed using various techniques and tested for its efficiency by comparing it to a battery using the cathode active material which is the NMC 811 material and a battery using the cathode active material which is a conventional LMO material. The test results are explained hereinafter in conjunction with the accompanying drawings.

Study on the characteristic of the surface and the elements which are the composition of the cathode active material

Fig. 1 is images obtained from the scanning electron microscope (SEM) showing the characteristic of the surface of the cathode active material comprising the mixture of NMC 811 and LMO at the mass ratio of 1 : 1 (Fig. 1(a)), 1 : 2 (Fig. 1(b)), and 2 : 1 (Fig. 1(c)). The figure shows a mix of the NMC 811 material, which is a sphere with a particle size in a range of about 8-12 pm, and the LMO material, which is an octahedral crystal with a single particle size in a range of about 400 nm - 1 pm, with a presence of carbon particles and polyvinylidene fluoride particles and a gap between the particles.

Figs. 1(a) and 1(b) indicate that when the amount of LMO is increased, the gap between the particles is reduced as the smaller LMO particles fill the gap between the NMC 811 particles and, in Fig. 1(c), when the amount of NMC 811 is increased, the surface of the obtained cathode active material is composed of small particles, which are relatively rounder, and the carbon material is found in the gap of the polyvinylidene fluoride mixture. It is observed that there is only a small gap between the particles of different materials. This reduced gap results in a good and more continuous electron and lithium ion (Li+) transfer within the electrodes. Furthermore, if the gaps or pores are reduced, an area where a side reaction can take place and cause deterioration of the battery will also be reduced.

Fig. 2 shows the analysis of the elements dispersing in the cathode active material of Fig. 1 through the energy dispersive X-ray spectroscopy (EDX) used in conjunction with the scanning electron microscope (SEM).

According to Figs. 2(a) and 2(b) which use the mass ratio of NMC 811 : LMO of 1 : 1 and 1 : 2, respectively, a dispersion of manganese (Mn), nickel (Ni), and cobalt (Co) is found on the surface of the cathode active material. When the amount of LMO is increased, the mass ratio of manganese to nickel (Mn/Ni) is increased from 1.65 and 5.69, respectively. Furthermore, in Fig. 2(c), a dispersion of Mn, Ni, and Co is also found on the surface of the cathode active material. It is also found that the elements are dispersed in an orderly manner, with the measured ratio of manganese to nickel of 1.05 by weight.

Study on the crystal structure and the quantity of elements which are the composition of the cathode active material

Fig. 3(a) shows the crystal structure of the LMO cathode active material having solely the Fd(-)3m space group spinel structure at planes (111), (311), (222), (400), (511), (440), and (531).

Fig. 3(b) shows the crystal structure of the NMC 811 cathode active material having the R(-)3m structure at planes (003), (101), (006/012), (104), (015), (107), (018/110), (113), and (021/116).

Fig. 3(c) shows the crystal structure of the cathode active material comprising the mixture of NMC 811 and LMO at the mass ratio of 1 : 1, 1 : 2, and 2 : 1 with a presence of the crystal planes of both LMO and NMC 811 materials. The circle represents the LMO crystal structure, and the inverted triangle represents the NMC 811 crystal structure.

Furthermore, a study was conducted to identify the elements and determine the quantity of the elements which are the composition of the active material comprising the mixture of NMC 811 and LMO at different mass ratios and the active materials which are the NMC 811 material and a conventional LMO material by using an X-ray fluorescence spectrometry (XRF) technique. The study results are shown in Table 1.

Table 1 shows the ratios of different elements present in the cathode active material.

Table 1

According to Table 1, it can be seen that the cathode active material which is the LMO material does not contain Ni and Co as a component, whereas the cathode active material which is the NMC 811 material has the measured ratios of Ni/Mn and Co/Mn of 9.58 and 0.86, respectively, which are close to the theoretical values obtained from a calculation, which are 8 and 1, respectively.

Furthermore, upon analysis of three examples of the cathode active material comprising the mixture of NMC 811 and LMO using the XRF, it was found that when the mass ratio of NMC 811 is increased, the ratios of Ni/Mn and Co/Mn are likely to be increased respectively. The analysis results obtained from the XRF are similar to the theoretical values obtained from a calculation as shown in Table 1.

Study on the charge-discharge efficiency and profile in the first cycle of the battery

The battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at different mass ratios and the battery using the cathode which is the NMC 811 and LMO materials were subjected to a test at 0.1C current using a battery tester.

According to Fig. 4, it can be seen that the batteries using the cathodes which are the NMC 811 and LMO materials provide a markedly different capacity: the battery using the NMC 811 cathode provides a higher capacity of 2,279 mAh/cell, while the battery using the LMO cathode provides a capacity of about 1,261 mAh/cell. After subjecting the battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at different mass ratios to the test, it was found that the obtained graph has a mixed characteristic of the graph of the battery using the NMC 811 and the graph of the battery using the LMO. If the NMC 811 ratio is higher (for example, NMC 811 : LMO = 2 : 1), the characteristic of the chargedischarge graph will be similar to the NMC 811 graph. However, if the LMO ratio is higher (for example, NMC 811 : LMO = 1 : 2), the characteristic of the charge-discharge graph will be similar to the LMO graph. Furthermore, the capacity also varies directly with the ratio of the mixture: when the NMC 811 ratio is increased, the capacity in the first cycle is increased as well. The capacity of the battery using the cathode with the mass ratio of NMC 811 : LMO of 2 : 1, 1 : 1, and 1 : 2 is approximately 2133, 1926, and 1866 mAh, respectively. Study on the capacity of the battery versus the mass ratios of NMC 811 and LMQ

In general, when the LMO is increased, the battery’s capacity will become lower, but the battery will have a higher nominal voltage. The capacity and the nominal voltage vary directly with the obtained energy density. Therefore, in order to obtain a preferred energy density, it is necessary to determine optimal mass ratios of NMC 811 and LMO mixed in the cathode active material of the battery. The study results are as shown in Table 2.

Table 2 shows the properties of the battery using the cathode comprising the active material which is a mixture of NMC 811 and LMO at different mass ratios and the batteries using the cathodes which are the NMC 811 and LMO materials. Table 2

The test results in Table 2 demonstrate that although the battery using the NMC 811 cathode alone provides the highest capacity, the energy density is lower than the battery using the cathode which is the mixture of NMC 811 and LMO at the ratio of 2: 1 (189.3 Wh/kg) as the capacity takes into account only the capacity per battery cell, whereas the energy density also takes into account variants of the nominal voltage and the cell weight. When the battery is used in different devices, the reduced weight of the battery cell is very important and useful in a commercial aspect. The battery using the cathode active material comprises the mixture of NMC 811 and LMO at the mass ratio of 2 : 1 provides the energy density of up to 189.3 Wh/kg, higher than the battery using the cathode active material which is the NMC 811 material alone.

Study on the reaction of the NMC material with the LMO material

Fig. 5 shows the characteristic of the reaction or the oxidation-reduction of such mixture through the dQ/dV graph analysis.

Fig. 5(a) shows the characteristic of the reaction of the LMO material. It was found that the reduction-oxidation peak at 3.7-3.8 V indicates a redox reaction of Mn²⁺ with Mn³⁺, while a redox couple at a voltage of about 4.1-4.2 V indicates the oxidation-reduction reaction of Mn³⁺ and Mn⁴⁺.

Fig. 5(b) shows the characteristic of the reaction of the NMC 811 material, wherein the redox peak at a voltage ranging from 3.6-4.2 V indicates the redox reaction of Mn³⁺/ Mn⁴⁺ and Ni²⁺/Ni³⁺/Ni⁴⁺. Furthermore, the redox peak at different positions further indicates a structural phase transition as follows: the first peak at about 3.7 V indicates a phase transition from hexagonal 1 (Hl) to monoclinic (M), i.e., Hl

M, and, subsequently, the peak at about 3.8-3.9 V indicates a phase transition from M

H2 and the peak at 4.2 V from H2 - H3.

Figs. 5(c) and 5(d) show the characteristic of the reaction of the mixture of NMC 811 and LMO. It was found that the mixture presents a peak that shows the characteristics of both NMC 811 and LMO. If any material has a larger amount, the characteristic of such material will become more apparent, for example, the mixture of NMC 811 and LMO at the ratio of 1 : 2. Furthermore, Fig. 5(d) shows the peak characteristic that is very similar to LMO, but the oxidation-reduction peak of NMC 811 is presented at a voltage of about 3.6 V.

Therefore, it can be concluded from the experiment results that mixing the two materials together allows the characteristics of both materials to be presented and causes a synergistic effect of the invented battery.

Study on the stability and the coulombic efficiency of the battery

Fig. 6 shows a stability and coulombic efficiency test of the battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at different mass ratios and the batteries using the cathodes which are the NMC 811 and LMO materials after several cycles of application at 1C current.

Figs. 6(a) and 6(b) show the test results of the batteries using the cathodes which are LMO and NMC 811, respectively. The test results show that the battery using the LMO material has a lower capacity but a much better stability than the battery using the NMC 811 material by 34%.

Fig. 6(d) shows the test results of the battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at the mass ratio of 1 : 2. It was found that the capacity in the first cycle was 1,673 mAh. When reaching the 150^th cycle, the capacity started to fade. When reaching the 200^th cycle, it was found that the capacity clearly faded until there was 1,130 mAh capacity left.

Figs. 6(c) and 6(e) show the test results of the battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at the mass ratio of 1 : 1 and 2 : 1, respectively. It was found that the capacity at both mass ratios decreased only slightly. The capacity in the first cycle was about 1,800-1,900 mAh and the battery could still maintain a good capacity. After 400 cycles, the capacity decreased to 1,300-1,400 mAh, approximately.

According to Figs. 6(a) to 6(d), it can be seen that all battery examples have high coulombic efficiency of about 100%, which indicates that the battery using the cathode active material according to the present invention can maintain a good coulombic efficiency.

Comparison of the battery's capacity retention

Fig. 7 shows the capacity retention of the battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at different mass ratios and the batteries using the cathodes which are the NMC 811 and LMO materials at different cycles.

According to the graph in Fig. 7, it can be seen that the battery using the cathode active material which is NMC 811 alone has a distinctly reduced capacity at the 450^th cycle which drops to 40%, suggesting a low battery cycle life. As for the capacity retention of the battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at the mass ratio of 1 : 2 at the 200^th cycle, it was found that the capacity is also reduced considerably to 70%. On the contrary, the capacity retention of the battery using the cathode comprising the active material which is the mixture of NMC 811 and LMO at the mass ratio of 1 : 1 and 2 : 1 still provides a high capacity of up to 80% at the 450^th cycle, higher than the battery using the cathode which is the NMC 811 material alone. This suggests that the battery using the cathode which is the active material according to the present invention provides better capacity and stability at the same number of cycles.

Hence, it can be concluded from the test results above that the battery using the cathode comprising the active material at suitable ratios of NMC 811 to LMO according to the present invention can substantially increase the stability and extend the cycle life of the battery, as well as providing high energy density. The properties in terms of stability, long cycle life, and energy density are crucial and extremely necessary for the commercial and industrial application of battery. BEST MODE OF THE INVENTION

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

Claims

WHAT IS CLAIMED IS: An active material for a cathode of a lithium-ion battery comprising: a mixture of lithium manganese oxide (LMO) of a formula LiMn₂O₄ and lithium nickel manganese cobalt oxide (NMC) of a formula Li(Ni_xMn_yCo_z)O2, whereby 0.7 < x <1, 0 < y < 1, 0 < z <l and the sum of x, y, and z is 1 and a mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is in a range of 1 : 1-2. The active material according to claim 1, wherein the lithium nickel manganese cobalt oxide (NMC) has a formula Li(Ni0.8Mn0.1Co0.1)O2. The active material according to claim 1, wherein the mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is 1 : 2. The active material according to claim 1, wherein the lithium nickel manganese cobalt oxide (NMC) has a particle size in a range of 5-15 pm. The active material according to claim 1, wherein the lithium manganese oxide (LMO) has a single particle size in a range of 400 nm-1 pm. A cathode of a lithium-ion battery comprising: an active material comprising a mixture of lithium manganese oxide (LMO) of a formula LiMn2O4 and lithium nickel manganese cobalt oxide (NMC) of a formula Li(Ni_xMn_yCo_z)O2, whereby 0.7 < x <1, 0 < y < 1, 0 < z <l and the sum of x, y, and z is 1 and a mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is in a range of 1 : 1-2; a binder, and a conductive material. The cathode according to claim 6, wherein the lithium nickel manganese cobalt oxide (NMC) has a formula Li(Ni0.8Mn0.1Co0.1)O2. The cathode according to claim 6, wherein the mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is 1 : 2. The cathode according to claim 6, wherein the lithium nickel manganese cobalt oxide (NMC) has a particle size in a range of 5-15 pm. The cathode according to claim 6, wherein the lithium manganese oxide (LMO) has a single particle size in a range of 400 nm-1 pm.

11. The cathode according to claim 6, wherein the binder is selected from polyvinylidene fluoride (PVDF), poly(3,4-ethylene dioxythiophene) (PEDOT), polytetrafluoroethylene (PTFE), and a mixture thereof.

12. The cathode according to claim 6, wherein the conductive material is selected from carbon black, acetylene black, super P, and a mixture thereof.

13. The cathode according to claim 6, wherein a weight ratio of active material to binder to conductive material is in a range of 90-98 to 1-5 to 1-5.

14. A method for preparing a cathode of a lithium-ion battery comprising the steps of:

(i) preparing a mixture of an active material, a binder, and a conductive material, and

(ii) coating the mixture obtained from step (i) onto a substrate wherein the active material comprises a mixture of lithium manganese oxide (LMO) of a formula LiMn2O4 and lithium nickel manganese cobalt oxide (NMC) of a formula Li(Ni_xMn_yCo_z)O2, whereby 0.7 < x <1, 0 < y < 1, 0 < z <l and the sum of x, y, and z is 1 and a mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is in a range of 1 : 1-2.

15. The method according to claim 14, wherein the lithium nickel manganese cobalt oxide (NMC) has a formula Li(Ni0.8Mn0.1Co0.1)O2.

16. The method according to claim 14, wherein the mass ratio of lithium manganese oxide (LMO) to lithium nickel manganese cobalt oxide (NMC) is 1 : 2.

17. The method according to claim 14, wherein the lithium nickel manganese cobalt oxide (NMC) has a particle size in a range of 5-15 pm.

18. The method according to claim 14, wherein the lithium manganese oxide (LMO) has a single particle size in a range of 400 nm-1 pm.

19. The method according to claim 14, wherein the binder is selected from polyvinylidene fluoride, poly(3,4-ethylene dioxythiophene), polytetrafluoroethylene, and a mixture thereof.

20. The method according to claim 14, wherein the conductive material is selected from carbon black, acetylene black, super P, and a mixture thereof.

21. The method according to claim 14, wherein the substrate is aluminium.

22. The method according to claim 14, wherein a weight ratio of active material to binder to conductive material is in a range of 90-98 to 1-5 to 1-5.

23. The method according to claim 14, wherein step (i) is carried out by mixing using N-methylpyrrolidone solution as a solvent.

24. The method according to claim 14, wherein step (ii) is carried out with a coating thickness ranging from 200-270 pm. 25. The method according to claim 14 further comprising step (iii) of drying the coated substrate.

26. The method according to claim 25, wherein step (iii) is carried out by heating the coated substrate at a temperature ranging from 100-180°C.

27. A lithium-ion battery comprising the cathode according to claim 6. 28. The lithium-ion battery according to claim 27 which is a cylindrical battery.