Patent Application
20170054143
The present invention relates to a graphene/HE-NCM composite for lithium ion battery; as well as a method for preparing said graphene/HE-NCM composite, an electrode material and a lithium ion battery comprising said graphene/HE-NCM composite.
Because of the high discharge capacity and low cost comparing to conventional cathode materials, Li-rich layered oxide compounds HE-NCM (xLi2MnO3.(1−x)LiNiyCozMn1-y-zO2 (0<x<1, 0<y<1, 0<z<1)) are considered to be the most prospective candidate of the next generation cathode materials. However, the electrochemical performances at high current density of this kind of material still need to be improved. What's more, the side reactions of electrode with currently commonly used electrolytes are inevitable because of the high cut off voltage of this kind of material. As previously reported, simple mechanical mixing of electrode materials and graphene (Gra) was effective in improving the rate capability as well as some other electrochemical performances. For example, Jiang etc. synthesized a graphene wrapped HE-NCM cathode material by simply mechanically mixing graphene with HE-NCM cathode material. In the hybrid cathode material, the graphene sheets serve as efficient electronically conductive frameworks benefiting from their 2D structure and outstanding electronic conductivity. The polarization of pristine HE-NCM can be effectively alleviated with the help of graphene, leading to improved high-rate capability and cyclability.
The enhanced electrochemical performance via graphene/HE-NCM composite is from the improved electronic conductivity. Thus obtaining high conductivity is the key to design the structure of the graphene/HE-NCM composite. In principal, a good design has characterizations of uniform distribution of graphene in HE-NCM particles, good contact of HE-NCM particles with graphene sheets and maximum usage of graphene. Besides, the method to achieve the designed structure is also a challenge. Generally, there are several methods to produce graphene/HE-NCM composite material. One is simply mechanically mixing pristine powder and graphene solution. The product by this method cannot be mixed very well because of the lack of interaction of graphene and inorganic particles. The other is reducing graphene oxide (GO)/inorganic composite material using strong reductants (such as hydrazine and sodium borohydride), solvothermal reduction in caustic solvents or special atmosphere under rather high temperature. Transitional metals are prone to be reduced via this method due to the strong reducing medium. What's more, it is difficult for these two methods to prepare graphene/HE-NCM composite materials on a large scale.
It is therefore an object of the present invention to provide a graphene/HE-NCM composite with uniform distribution of graphene in a facile, low-cost method. Said composite is prepared by thermal decomposition of graphene oxide/HE-NCM at low temperature in a short time, wherein neither special atmosphere nor high temperature is employed. All these endow this method suitable for large-scale production of HE-NCM composite material.
This object is achieved by a method for preparing a graphene/HE-NCM composite, wherein more than one HE-NCM particles of the formula (1)
xLi2MnO3.(1−x)LiNiyCozMn1-y-zO2 (1),
wherein 0<x<1, 0<y<1, and 0<z<1,
are in electrical contact with each other via one or multiple graphene flakes, said method including:
Another object of the present invention is to provide a graphene/HE-NCM composite for lithium ion battery with enhanced electrochemical performance due to improved conductivity of composite.
This object is achieved by the graphene/HE-NCM composite prepared by the method according to the present invention.
According to another aspect of the invention, an electrode material is provided, which comprises the graphene/HE-NCM composite according to the present invention.
According to another aspect of the invention, a lithium ion battery is provided, which comprises the graphene/HE-NCM composite according to the present invention.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
The present invention relates to a method for preparing a graphene/HE-NCM composite, wherein more than one HE-NCM particles of the formula (1)
xLi2MnO3.(1−x)LiNiyCozMn1-y-zO2 (1),
wherein 0<x<1, 0<y<1, and 0<z<1,
are in electrical contact with each other via one or multiple graphene flakes,
said method including:
In an embodiment of the method according to the present invention, the index x in the formula (1) can be from 0.3 to 0.7; the index y can be from 0.2 to 0.8; and the index z can be from 0.1 to 0.5.
a) Dispersing HE-NCM in a Solution of GO
GO sheets can be prepared by a modified Hummer's method (D. C. Marcano, et al., Improved Synthesis of Graphene Oxide, ACS Nano, 2010, 4, 4806).
The concentration of the solution of GO is not particularly limited, since all the solvent will be removed in step b).
The HE-NCM particles can be dispersed preferably by ultrasonication in a solution, preferably an aqueous solution of graphene oxide. The solvent of the GO solution is not particularly limited. Any other solvents within which GO is soluable and which are inert to the reactants and easy to be removed in step b) can also be used. The dispersion method is not particularly limited. Any other dispersion methods, such as high speed mechanical mixing, can also be used.
In an embodiment of the method according to the present invention, based on the HE-NCM particles, 1-20 wt. %, preferably 1-10 wt. % of graphene oxide is used.
b) Drying the Dispersion
In an embodiment of the method according to the present invention, the dispersion can be dried by lyophilization. It is believed by the inventors that lyophilization is favorable for the further exfoliation of graphene oxide. After lyophilization, homogenously mixed GO/HE-NCM composite can be obtained because of the interaction between the HE-NCM particle and the oxygen-containing group, such as —OH, —COOH, —O—, in the GO.
The blueshift of the peaks of the chemical bonds in HE-NCM particles as shown in
c) Thermal Decomposition of the GO/HE-NCM Composite
In an embodiment of the method according to the present invention, the temperature in the c) thermal decomposition can be in a range of 300-350° C.
In an embodiment of the method according to the present invention, the duration of the c) thermal decomposition can be in a range of 5 min-3 h, preferably 10 min-2 h, more preferably 30 min-1 h.
The atmosphere of the c) thermal decomposition is not particularly limited.
In an embodiment of the method according to the present invention, the c) thermal decomposition can be carried out under an ambient atmosphere.
In an embodiment of the method according to the present invention, the c) thermal decomposition can be carried out under an inert atmosphere, such as N2 or Ar, or under a reducing atmosphere, such as H2, or in their combination, such as H2/Ar.
In an embodiment of the method according to the present invention, HE-NCM particles may be used in any shape of particles, for example, spherical, sheet-like, or irregular particles. Further, the lithium-rich layered oxide particle may be in a form of primary particles or secondary particles. The size of the lithium-rich layered oxide particle can be any commonly used sizes in the art; for primary particles, for example, 50 nm to 800 nm, or 100 nm to 500 nm. HE-NCM particles used in the invention may be prepared by traditional preparation processes, such as the co-precipitation process.
The present invention further relates to the graphene/HE-NCM composite prepared by the method according to the present invention.
In an embodiment of the graphene/HE-NCM composite according to the present invention, more than one HE-NCM particles can be in electrical contact with each other via a common graphene flake.
In an embodiment of the graphene/HE-NCM composite according to the present invention, more than one HE-NCM particles can be in electrical contact with each other via multiple graphene flakes.
In an embodiment of the graphene/HE-NCM composite according to the present invention, one or more HE-NCM particles can be partially or completely wrapped by one or multiple graphene flakes. In particular, HE-NCM particles can be wrapped by graphene flakes in the following ways:
More than one HE-NCM particles can be partially or completely wrapped together by one or multiple graphene flakes, as shown in
Alternatively or additionally, one or more HE-NCM particles together with one or more HE-NCM particles which are already partially or completely wrapped by one or multiple graphene flakes, are partially or completely wrapped by one or multiple graphene flakes, as shown in
Alternatively or additionally, a HE-NCM particle is partially or completely wrapped by one or multiple graphene flakes, as shown in
Alternatively or additionally, one or multiple graphene flakes by which one or more HE-NCM particles are partially or completely wrapped, are in electrical contact with the other one or multiple graphene flakes by which the other one or more HE-NCM particles are partially or completely wrapped. In this case, more than one HE-NCM particles are in electrical contact with each other via multiple graphene flakes.
In particular, one or more than one graphene flakes can also cover one particle and the average diameter or length of this graphene flake is in the range of the size of the particle, so that the particle is covered on its outer side by one or more than one graphene flakes. Flakes of this size are covered preferably one particle and the one or by multiple flakes covered particle has an enhanced outer side surface conductivity. If particles of that type covered are touching each other, the electrical conductivity of the arrangement of particles is quite enhanced and the flakes outside on one particle touches the other flake on the outer side of the other particle, giving a commonly joint conductive skin on the outside of the particles and still leaving enough space for electrolyte in between the particles, so that the space for this electrolyte does fill at least 25%, better 35-48% of the empty space between the coated particles.
In an embodiment of the graphene/HE-NCM composite according to the present invention, at least one fifth, preferably at least one fourth, more preferably at least one third, particularly preferably at least half of the surface of a HE-NCM particle is wrapped by one or multiple graphene flakes.
In an embodiment of the graphene/HE-NCM composite according to the present invention, the graphene/HE-NCM composite when x=0.5; y=1/3; z=1/3 shows a first discharge capacity of at least 180 mAh/g, preferably at least 190 mAh/g, more preferably at least 200 mAh/g, at 1 C.
The present invention further relates to an electrode material, which comprises the graphene/HE-NCM composite according to the present invention.
The present invention further relates to a lithium ion battery, which comprises the graphene/HE-NCM composite according to the present invention.
The following non-limiting examples illustrate various features and characteristics of the present invention, which is not to be construed as limited thereto.
GO was synthesized by a modified Hummer's method. Firstly, graphite flakes were put into concentrated H2SO4 (98%), and then KMnO4 was gradually added under ice water bath. Subsequently, the ice bath was heated to 35° C. and maintained for 30 min. After the addition of 100 mL deionized water, the temperature of the reaction mixture was increased to 98° C. Then the suspension was further treated with H2O2 (3%) until there were no gas bubbles. After that, the suspension was washed with HCl solution (3%) and then deionized water. Finally, the graphene oxide was obtained after centrifugation.
HE-NCM particles were prepared by sintering M(OH)2 (M=Ni, Co, Mn) and LiOH.H2O in a molar ratio of Ni(OH)2:Co(OH)2:Mn(OH)2:LiOH.H2O=1:1:4:9 at 900° C. for 10 hours in a muffle furnace, after M(OH)2 was prepared by a coprecipitation method with respective transition metal sulphate and sodium hydroxide.
HE-NCM particles were dispersed in aqueous solution of GO by ultrasonication. The weight ratio between GO and HE-NCM is 1:99. Then, a homogenously mixed GO/HE-NCM composite was obtained after lyophilization. And then, a graphene/HE-NCM composite was obtained by thermal decomposition of the GO/HE-NCM composite in air at a temperature of 350° C. for 60 min.
HE-NCM particles were dispersed in aqueous solution of GO by ultrasonication. The weight ratio between GO and HE-NCM is 3.5:96.5. Then, a homogenously mixed GO/HE-NCM composite was obtained after lyophilization. And then, a graphene/HE-NCM composite (
HE-NCM particles were dispersed in aqueous solution of GO by ultrasonication. The weight ratio between GO and HE-NCM is 3.5:96.5. Then, a homogenously mixed GO/HE-NCM composite was obtained after lyophilization. And then, a graphene/HE-NCM composite was obtained by thermal decomposition of the GO/HE-NCM composite in air at a temperature of 300° C. for 60 min.
HE-NCM particles were dispersed in aqueous solution of GO by ultrasonication. The weight ratio between GO and HE-NCM is 10:90. Then, a homogenously mixed GO/HE-NCM composite was obtained after lyophilization. And then, a graphene/HE-NCM composite was obtained by thermal decomposition of the GO/HE-NCM composite in air at a temperature of 350° C. for 60 min.
Structural and Electrochemical Evaluation:
Transmission Electron Microscopy (TEM) was employed to characterize the structure of the graphene/HE-NCM composite obtained from Example 4 (
A mixture of active material (HE-NCM particles with or without graphene wrapped), carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10 was homogenized in N-methyl-2-pyrrolidone (NMP) as a solvent to form a slurry. The slurry was then uniformly coated on an aluminum foil, dried at 100° C. under vacuum for 10 h, pressed and cut into 12 mm cathode discs. Coin cells (CR2016) were assembled using metallic Li as the counter electrode, Celgard 2400 as the separator, and 1 mol L−1 LiPF6 as the electrolyte, in an Ar-filled glove box. The cycling performances of the cells were evaluated by using a Land CT2001A battery tester between 2.0V and 4.8V versus Li/Li+.
The graphene/HE-NCM composite obtained from Example 4 delivered a discharge capacity of 266 mAh/g at the rate of 0.1 C under 30° C. comparing to 261 mAh/g of the pristine material (HE-NCM particles without graphene wrapped). The first cycle efficiency (FCE) of the graphene/HE-NCM composite was 87%, much higher than 81% of the pristine material (
In the rate test, the graphene/HE-NCM composite obtained from Example 4 delivered much better capacity retention under large current density. In the rate of 1 C, the graphene/HE-NCM composite showed a discharge capacity of 186 mAh/g, while the pristine material showed 170 mAh/g. When increasing the weight ratio of graphene, thermal decomposition temperature or time, the capacity at high current density increases. The average capacity of 5 cycles at each current density for all samples are summarized in Table 1.
The cyclability was also improved because of the reduction of the polarization due to the suppression of side reactions in the composite material. The capacity retention after 50 cycles at 0.1 C for the graphene/HE-NCM composite obtained from Example 4 was 90%, comparing to 82% for the pristine material (
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. The attached claims and their equivalents are intended to cover all the modifications, substitutions and changes as would fall within the scope and spirit of the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2014/072197 | 2/18/2014 | WO | 00 |
