Positive electrode plate and method of forming slurry for positive electrode plate

The present application is based on, and claims priority from, Taiwan Application Serial Number 106143522, filed on Dec. 12, 2017, the disclosure of which is hereby incorporated by reference herein in its entirety.

The technical field relates to a positive electrode plate of a lithium battery, and in particular it relates to the slurry of the positive electrode plate and a method of forming the same.

Recently, lithium batteries have been widely applied in several types of electronic devices, including cell phones, personal digital assistants, and the like. These lithium battery may be improved by reducing their overall size even further, reducing their weight, and prolonging their lifespan. In addition, lithium batteries may be applied not only in electronic devices, but also in other large devices. This may take the form of the type of removable battery sets that are installed in electronic devices, electric cars, home power servers, and other power storage systems.

Specifically, a lithium battery has a higher power density than that of a lead battery, a nickel-cadmium battery, and other general batteries. According to every theory of charging and discharging, the composition of the positive electrode, the negative electrode, and/or the electrolyte of the battery can be adjusted to enhance the battery's performance. For example, the hyper branched polymer can be introduced into the electrode to efficiently reduce the risk of thermal runaway when a short circuit occurs inside the battery. As such, the safety of the battery can be enhanced. However, the hyper branched polymer is detrimental to the discharge performance of the battery. Even if the conductive powder is further doped, the conductive effect lost by introducing the hyper branched polymer cannot be compensated for.

Accordingly, a novel electrode composition is called for that takes into account both the safety of the battery and the conductivity of the electrode.

One embodiment of the disclosure provides a positive electrode plate, including: 0.1 to 1 part by weight of the hyper branched polymer; 80 to 97.79 parts by weight of active material; and 0.1 to 6 parts by weight of carbon nanotube, wherein the hyper branched polymer, the carbon nanotube, and the active material are bonded by 0.01 to 1 parts by weight of coupling agent.

One embodiment of the disclosure provides a method of forming slurry for a positive electrode plate, including: reacting maleimide compound and barbituric acid to form a hyper branched polymer; mixing 0.1 to 1 part by weight of the hyper branched polymer, 0.01 to 1 part by weight of coupling agent, and 0.1 to 6 parts by weight of carbon nanotube to form a mixture; and adding 80 to 97.79 parts by weight of active material to the mixture, wherein the hyper branched polymer, the carbon nanotube, and the active material are bonded by the coupling agent.

A detailed description is given in the following embodiments.

In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details.

In one embodiment, a method of forming slurry for a positive electrode plate includes reacting maleimide compound and barbituric acid to form a hyper branched polymer. The maleimide compound can be maleimide, bismaleimide, or a combination thereof. In one embodiment, the maleimide includes N-phenyl maleimide, N-(o-methylphenyl)maleimide, N-(m-methylphenyl)maleimide, N-(p-methylphenyl)maleimide, N-cyclohexylmaleimide, maleimide, maleimidophenol, maleimidephenoxy benzocyclobutene, phosphorous maleimide, phosphate-containing maleimide, silicone-containing maleimide, N-(4-tetrahydropyranyl hydroxyphenyl) maleimide, or 2,6-benzhydryl maleimide. In addition, the barbituric acid (BTA) may serve as an initiator, thereby polymerizing the double bond of the maleimide to form the hyper branched polymer.

The bismaleimide has a chemical structure of

in which R is —(CH 2 ) 2 —, —(CH 2 ) 6 —, —(CH 2 ) 8 —, —(CH 2 ) 12 —,

In one embodiment, BTA may serve as an initiator, thereby polymerizing the double bond of the bismaleimide to form the hyper branched polymer. In one embodiment, maleimide and bismaleimide can be mixed in an appropriate ratio, and BTA serves as an initiator to polymerize the mixture to form a hyper branched polymer of the maleimide and the bismaleimide. The maleimide compound and the barbituric acid may have a molar ratio of 97:3 to 60:40. The maleimide compound may self-react if there is not enough barbituric acid, meaning the hyper branched polymer cannot be formed efficiently, negatively affecting the safety of the battery. If there is too much barbituric acid, it cannot completely react with the maleimide compound, and the residual barbituric acid may negatively affect the performance of the battery. The maleimide compound and the barbituric acid can react with each other in a temperature range of between room temperature and 130° C. for a period of 0.5 hours to 48 hours. If the reaction temperature is too low and/or the reaction period is too short, a hyper branched polymer cannot be efficiently formed to provide safety. An operating temperature that is too high and/or a reaction period that is too long may cause the hyper branched polymer to over react and further gel as a 3D network structure, and the network structure cannot be used in the slurry.

In one embodiment, 0.1 to 1 part by weight of the hyper branched polymer, 0.01 to 1 part by weight of coupling agent, and 0.1 to 6 parts by weight of carbon nanotube are mixed to form a mixture. If there is not enough of the coupling agent, the subsequently added active material cannot bond to the hyper branched polymer and the carbon nanotube, thereby lowering the conductivity of the positive electrode plate. If there is too much of the coupling agent, the free coupling agent may over react with the hyper branched polymer, making it difficult to manufacture the positive electrode and negatively affecting the performance of the battery. In one embodiment, the coupling agent is

(CH 2 ═CH—R 2 ) n —Si(OR 1 ) m , or a combination thereof. R 1 is H or C 1-3 alkyl group; R 2 is C 2-4 alkylene group or carbonyl group; m=1-3; n=1-3; m+n=4; p=1-2; q=1-2; and p+q=3. In one embodiment, the coupling agent is allyltrimethoxysilane or (3-aminopropyl)trimethoxysilane. If there is not enough carbon nanotube, a conductive network cannot be formed between the active material and the active material, between the hyper branched polymer and the subsequently added active material, or between the hyper branched polymer and the hyper branched polymer, thereby increasing battery impedance. Too much of the carbon nanotube may depress the content ratio of the subsequently added active material, thereby lowering the capacitance per area of the positive electrode. In one embodiment, the carbon nanotube can be single wall carbon nanotube, a multi-wall carbon nanotube, or a combination thereof. The carbon nanotube may have a length that is between 1 micrometer and tens of micrometers, and a diameter of 0.5 to 3 nm. A carbon nanotube that is too short cannot overcome the distance between particles to form an effective conductive network. A carbon nanotube that is too long may penetrate through the separator film after the assembly of the battery, thereby causing internal short circuits and a high self-discharge rate of the battery. If the diameter of the carbon nanotube is too small, the specific surface area of the carbon nanotube will be too high. For ease of preparing the positive electrode slurry, the carbon nanotube amount should be reduced so that the conductive network is incomplete. In addition, a carbon nanotube that sustains too much stress (during the roller compacting of the electrode) can be easily broken. If the diameter of the carbon nanotube is too large, this may increase the volume of the positive electrode, thereby lowering the compaction density of the electrode.

80 to 97.79 parts by weight of active material is added to the mixture. In one embodiment, the active material includes LiCoO 2 , LiNiO 2 , LiMnO 2 , LiMn 2 O 4 , Li(Ni a Co b Mn c )O 2 (0<a<1, 0<b<1, 0<c<1, a+b+c=1), LiNi 1-y Co y O 2 (0<y<1), LiCo 1-y Mn y O 2 (0<y<1), LiNi 1-y Mn y O 2 (0<y<1), Li(Ni a Co b Mn c )O 4 (0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn 2-z Ni z O 4 (0<z<2) LiMn 2-z Co z O 4 (0<z<2), LiCoPO 4 , LiFePO 4 , or a combination thereof. In one embodiment, the active material is LiCoO 2 . The active material is bonded to the hyper branched polymer and the carbon nanotube by the coupling agent, and the hyper branched polymer is bonded to the carbon nanotube by the coupling agent. In other words, the hyper branched polymer, the carbon nanotube, and the active materials have bondings therebetween. The bondings can be silicon-containing segments, phosphorous-containing segments, or sulfur-containing segments, depending on the type of the coupling agent. In one embodiment, the hyper branched polymer, the coupling agent, the carbon nanotube, and the active material can be mixed simultaneously.

In one embodiment, 0.5 to 6 parts by weight of conductive carbon powder can be further added to the mixture. Too much conductive powder may depress the active material content ratio, thereby lowering the capacitance per area of the positive electrode. The conductive carbon powder can be carbon powder with a high specific surface area, such as Super P or KS-6 commercially available from Timcal. In one embodiment, the conductive carbon powder has a diameter of 0.03 μm to 10 μm. Too small the conductive carbon powder has a surface area that is too large, which not only decreases the other material content ratio and leads to a loss of capacitance of the positive electrode, but also may result in an incomplete conductive network, reducing the conductivity of the positive electrode. If the size of conductive carbon powder is too large, the materials of the positive electrode plate cannot stack closely, thereby increasing the porosity of the positive electrode plate and increasing electron transport resistance in the positive electrode plate.

In one embodiment, 1.5 to 6 parts by weight of binder can be further added to the mixture. Too much binder may increase the resistance of the positive electrode plate. In one embodiment, the binder can be polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyamide, melamine resin, or a combination thereof.

In one embodiment, a further 25 to 60 parts by weight of solvent can be added to the mixture to adjust the viscosity of the slurry. Too much solvent may result in the slurry being too thin to form a coat. The solvent can be N-methyl-2-pyrrolidone (NMP), ethanol (EtOH), dimethylacetamide (DMAc), γ-butyrolactone (GBL), or a combination thereof.

The slurry is coated on a metal foil, such as copper foil or aluminum foil, dried, and then cut to an appropriate size to serve as a positive electrode plate. A negative electrode plate such as lithium electrode is collocated with the positive electrode plate. An electrolyte (e.g. liquid state, gel state, or solid state) is disposed between the two electrode plates, and then sealed to complete a battery. For details of the negative electrode plate, the electrolyte, and the battery type for being collocated with the positive electrode plate of the disclosure, please refer to U.S. Pat. No. 5,292,601.

Below, exemplary embodiments will be described in detail so as to be easily realized by a person having ordinary knowledge in the art. The inventive concept may be embodied in various forms without being limited to the exemplary embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout.

Synthesis Example 1 (Hyper Branched Polymer)

2 parts by mole of bismaleimide (BMI) and 1 part by mole of barbituric acid (BTA) were added to N-methyl-2-pyrrolidone (NMP), wherein the total weight of BMI and BTA and the weight of NMP had a ratio of 7:93. The mixture was stirred at room temperature for about 12 hours to obtain a hyper branched polymer.

Synthesis Example 2 (Hyper Branched Polymer and Carbon Nanotube)

The hyper branched polymer was formed by the method of Synthesis Example 1. Single-wall carbon nanotube (SWCNT, TECO Nanotech Co., Ltd.) was then gradually added to the hyper branched polymer to form a composition of the hyper branched polymer and the carbon nanotube.

Synthesis Example 3 (Hyper Branched Polymer, Carbon Nanotube, and Coupling Agent)

The hyper branched polymer was formed by the method of Synthesis Example 1. SWCNT and coupling agent (allyltrimethoxysilane, commercially available from Sigma Aldrich) were then gradually added to the hyper branched polymer to form a composition of the hyper branched polymer, the carbon nanotube, and the coupling agent. The composition contents of Synthesis Example 1 to 3 are tabulated in Table 1.

Comparative Example 1

93.2 parts by weight of the active material LiCoO 2 (R767, solid content=74% to 88%, commercially available from Hunan Ruixiang New Materials Limited Company) was added into a planetary ball mill, and then mixed and dispersed at 25° C. for 60 minutes. Subsequently, 1.05 parts by weight of the conductive carbon powder Super P (Commercially available from Timcal) and 3 parts by weight of the conductive carbon powder KS-6 (Commercially available from Timcal) were added into the planetary ball mill, and then mixed and dispersed at 25° C. for 60 minutes. Subsequently, 2.4 parts by weight of binder PVDF (HSV900, commercially available from Kynar) was added into the planetary ball mill, and then mixed and dispersed at 25° C. for 90 minutes. Subsequently, 35 parts by weight of the solvent NMP was added into the planetary ball mill, and then mixed and dispersed at 25° C. for 20 minutes to obtain slurry. The slurry was coated on a surface of an aluminum foil, and then dried under vacuum and roller compacted to obtain a positive electrode plate. The coating amount of the slurry was 20 mg/cm 2 .

Comparative Example 2

0.3 parts by weight of the hyper branched polymer in Synthesis Example 1 was added into a planetary ball mill, and then mixed and dispersed at 25° C. for 10 minutes. Subsequently, 93.2 parts by weight of the active material LiCoO 2 (R767) was added into the planetary ball mill, and then mixed and dispersed at 25° C. for 60 minutes. Subsequently, 1.05 parts by weight of the conductive carbon powder Super P and 3 parts by weight of the conductive carbon powder KS-6 were added into the planetary ball mill, and then mixed and dispersed at 25° C. for 60 minutes. Subsequently, 2.4 parts by weight of the binder PVDF was added into the planetary ball mill, and then pulped at 25° C. for 90 minutes. Subsequently, 35 parts by weight of the solvent NMP was added into the planetary ball mill, and then pulped at 25° C. for 20 minutes to obtain slurry. The slurry was coated on a surface of an aluminum foil, and then dried under vacuum and roller compacted to obtain a positive electrode plate. The coating amount of the slurry was 20 mg/cm 2 .

Comparative Example 3

0.35 parts by weight of the composition of the hyper branched polymer and the carbon nanotube in Synthesis Example 2 was added into a planetary ball mill, and then mixed and dispersed at 25° C. for 10 minutes. Subsequently, 93.2 parts by weight of the active material LiCoO 2 (R767) was added into the planetary ball mill, and then mixed and dispersed at 25° C. for 60 minutes. Subsequently, 1.05 parts by weight of the conductive carbon powder Super P and 3 parts by weight of the conductive carbon powder KS-6 were added into the planetary ball mill, and then mixed and dispersed at 25° C. for 60 minutes. Subsequently, 2.4 parts by weight of the binder PVDF was added into the planetary ball mill, and then pulped at 25° C. for 90 minutes. Subsequently, 35 parts by weight of the solvent NMP was added into the planetary ball mill, and then pulped at 25° C. for 20 minutes to obtain slurry. The slurry was coated on a surface of an aluminum foil, and then dried under vacuum and roller compacted to obtain a positive electrode plate. The coating amount of the slurry was 20 mg/cm 2 .