METHOD FOR PREPARING HYBRID CAPACITOR CATHODE SLURRY Technical Field of the Invention The present invention relates to the technical field of super capacitors, in particular to a method for preparing hybrid capacitor cathode slurry. Background of the Invention Due to their heavy-current charge/discharge performance, ultra-long cycle life and high energy density, super capacitors, also referred to as electrochemical capacitors, have drawn great attention among various power storage devices and have been widely used in portable digital devices, mobile communication, hybrid buses, wind power variable-pitch systems, etc. Recently, the super capacitors have showed a promising prospect in the fields of port machinery energy feedback, rail transit, military equipment and other power storage equipment. At present, as for the electrode material of super capacitors, active carbon is mainly used as the electrode power storage material. However, due to the limitations to the energy density, more and more attention has been paid on Li-ion battery cathode material with stable performance and rapid charge/discharge speed, among which, the spinel lithium titanate is most prominent due to "zero-strain" structure and excellent cycle stability and becomes a research hotspot of the new generation of super capacitor cathode material. Due to poor electrical conductivity, the spinel lithium titanate needs to be manufactured to submicron or even nanoscale material during the production. In this case, although the change in granularity of this material may decrease the ion resistance of Li* during the charge storage and thus improve the heavy-current charge/discharge performance of products, this material is difficult to be coated during the actual production because of the significant increase of agglomeration between particles, degraded homogeneity of the electrode material with the conductive agent, the binder, etc, and decreased 1 affinity of the electrode material with a current collector. Consequently, the electrochemical performance and production efficiency of super capacitors are decreased. Summary of the Invention To solve the problem of poor homogeneity and coating performance of capacitor lithium titanate cathode slurry in the prior art, the present invention provides a method for preparing hybrid capacitor cathode slurry. The method has simple steps and high operability and is suitable for industrial production. The capacitor cathode slurry prepared by the method has homogeneous and stable components and good coating performance. To achieve the above objective, the present invention employs the following technical solutions. A method for preparing hybrid capacitor cathode slurry is provided, including the following steps of: (1) preparing the following components by mass percentage: 5-10% of conductive agent, 3-5% of dispersant, 3-5% of binder and 80-89% of lithium titanate, 100% in total, and preparing de-ionized water that is 40-60% of the total mass of the conductive agent, the dispersant, the binder and the lithium titanate, where the formulation of the slurry is improved herein so that the whole formulation is environmentally friendly, moderate in viscosity and good in coating performance; (2) adding the dispersant into the de-ionized water that is 40-60% of the total amount of the de-ionized water, stirring uniformly and then standing for 12-24h to obtain dispersed mother liquor; (3) adding the conductive agent into the dispersed mother liquor and stirring for 0.5-1h in vacuum, adding the lithium titanate after the material is cooled to the room temperature and stirring for 0.5-1 h under the same conditions, adding the binder and the remaining de-ionized water after the material is cooled to the room temperature and stirring for 0.5-1h under the 2 same conditions, and cooling the material to the room temperature to obtain hybrid slurry; and (4) dispersing the hybrid slurry at a high speed for 20-35min to obtain the hybrid capacitor cathode slurry. The time duration for dispersing at a high speed is very critical. If dispersing lasts too long, the viscosity of the slurry will be increased; or otherwise, the dispersibility will become poor. Stirring at a high speed provides high efficiency and also allows the components to be dispersed fully, thereby improving the homogeneity and stability of the slurry. Preferably, the conductive agent is one or more of conductive carbon black, carbon nanotube and graphene. Preferably, the conductive agent is a mixture of conductive carbon black and graphene, or a mixture of conductive carbon black, carbon nanotube and graphene. Preferably, the mass ratio of conductive carbon black to graphene is (3-5): (1-3), and the mass ratio of conductive carbon black to carbon nanotube to graphene is (2-4): (1-3): (1-3). Preferably, the dispersant is sodium carboxymethylcellulose. Generally, the conductive agent is hydrophobic and difficult to disperse and prone to agglomerate in water. In the present invention, as sodium carboxymethylcellulose is used as the dispersant, the carboxyl group in the sodium carboxymethylcellulose allows the surfaces of the conductive agent particles to be negatively charged during dispersing at a high speed, and forms double electric layers on the surfaces of the conductive agent particles. When approaching to one another, the two encapsulated conductive agent particles are forced to separate from one another because of the mutual exclusion of like charges, so that the dispersibility of the conductive agent is improved. In addition, the settlement of the slurry is less likely to occur as sodium carboxymethylcellulose may improve the stability of the slurry. Preferably, the binder is SBR emulsion. Preferably, the technological parameters of the stirring in vacuum in step 3 (3) include: degree of vacuum of 0.09-0.1 MPa and stirring rate of 100-250 r/min. Preferably, the rotating speed during dispersing at a high speed in step (7) is 3000-8000 r/min. Therefore, the present invention has the following beneficial effects: (1) the formulation of the slurry is optimized and improved, so that the whole formulation is environmentally friendly, moderate in viscosity and good in coating performance; (2) the addition of various components in several steps and the high-speed dispersion ensure the homogeneity of the slurry, and the addition of sodium carboxymethylcellulose as the dispersant effectively solves the problem that the conductive agent particles are difficult to disperse; and (3) the present invention has simple steps and high operability and is suitable for industrial production. Detailed Description of the Invention The present invention will be further described as below by specific embodiments. In the present invention, unless otherwise specified, the percentage is a unit of weight, and all equipment and raw materials are commercially available or commonly used in this industry. Unless otherwise stated, the methods in the following embodiments are conventional ones in the art. Embodiment 1 (1) The following raw materials are prepared by mass percentage: 5% of conductive agent, 3% of dispersant, 3% of binder and 89% of lithium titanate, and de-ionized water that is 40% of the total mass of the conductive agent, the dispersant, the binder and the lithium titanate is prepared, wherein the conductive agent is a mixture obtained by mixing conductive carbon black and graphene at a mass ratio of 3: 1, the dispersant is sodium carboxymethylcellulose, and the binder is SBR emulsion. 4 (2) The dispersant is added into the de-ionized water that is 40% of the total amount of the de-ionized water, and the mixture is stirred uniformly and then stands for 12h to obtain dispersed mother liquor. (3) The conductive agent is added into the dispersed mother liquor and stirred for 0.5h in vacuum; the lithium titanate is added after the material is cooled to the room temperature and then stirred for 0.5h under the same conditions; the binder and the remaining de-ionized water are added after the material is cooled to the room temperature and then stirred for 0.5h under the same conditions, and the material is cooled to the room temperature to obtain hybrid slurry, wherein the technological parameters of the stirring in vacuum in include: degree of vacuum of 0.09 MPa and stirring rate of 100 r/min. (4) The hybrid slurry is dispersed at a high speed of 3000 r/min for 20min to obtain the hybrid capacitor cathode slurry. Embodiment 2 The difference between this embodiment and Embodiment 1 is that the conductive agent is a mixture obtained by mixing conductive carbon black and graphene at a mass ratio of 2: 1, and the remaining is completely the same. Embodiment 3 The difference between this embodiment and Embodiment 1 is that the conductive agent is a mixture obtained by mixing conductive carbon black and graphene at a mass ratio of 5: 3, and the remaining is completely the same Embodiment 4 (1) The following raw materials are prepared by mass percentage: 10% of conductive agent, 5% of dispersant, 5% of binder and 80% of lithium titanate, and de-ionized water that is 60% of the total mass of the conductive agent, the dispersant, the binder and the lithium titanate is prepared, wherein the conductive agent is a mixture obtained by mixing conductive carbon black, 5 carbon nanotube and graphene at a mass ratio of 2: 1: 1, the dispersant is sodium carboxymethylcellulose, and the binder is SBR emulsion. (2) The dispersant is added into the de-ionized water that is 60% of the total amount of the de-ionized water, and the mixture is stirred uniformly and then stands for 24h to obtain dispersed mother liquor. (3) The conductive agent is added into the dispersed mother liquor and stirred for 1h in vacuum; the lithium titanate is added after the material is cooled to the room temperature and then stirred for 1h under the same conditions; the binder and the remaining de-ionized water are added after the material is cooled to the room temperature and then stirred for 1h under the same conditions, and the material is cooled to the room temperature to obtain hybrid slurry, wherein the technological parameters of the stirring in vacuum in include: degree of vacuum of 0.1 MPa and stirring rate of 250 r/min. (4) The hybrid slurry is dispersed at a high speed of 8000 r/min for 35min to obtain the hybrid capacitor cathode slurry. Embodiment 5 The difference between this embodiment and Embodiment 4 is that the conductive agent is a mixture obtained by mixing conductive carbon black, carbon nanotube and graphene at a mass ratio of 3: 2: 2, and the remaining is completely the same Embodiment 6 The difference between this embodiment and Embodiment 4 is that the conductive agent is a mixture obtained by mixing conductive carbon black, carbon nanotube and graphene at a mass ratio of 4: 3: 3, and the remaining is completely the same Embodiment 7 (1) The following raw materials are prepared by mass percentage: 6% of 6 conductive agent, 4% of dispersant, 4% of binder and 86% of lithium titanate, and de-ionized water that is 50% of the total mass of the conductive agent, the dispersant, the binder and the lithium titanate is prepared, wherein the conductive agent is conductive carbon black, the dispersant is sodium carboxymethylcellulose, and the binder is SBR emulsion. (2) The dispersant is added into the de-ionized water that is 50% of the total amount of the de-ionized water, and the mixture is stirred uniformly and then stands for 18h to obtain dispersed mother liquor. (3) The conductive agent is added into the dispersed mother liquor and stirred for 0.8h in vacuum; the lithium titanate is added after the material is cooled to the room temperature and then stirred for 0.8h under the same conditions; the binder and the remaining de-ionized water are added after the material is cooled to the room temperature and then stirred for 0.8h under the same conditions, and the material is cooled to the room temperature to obtain hybrid slurry, wherein the technological parameters of the stirring in vacuum in include: degree of vacuum of 0.95 MPa and stirring rate of 200 r/min. (4) The hybrid slurry is dispersed at a high speed of 6000 r/min for 30min to obtain the hybrid capacitor cathode slurry. Embodiment 8 The difference between this embodiment and Embodiment 7 is that the conductive agent is carbon nanotube, and the remaining is completely the same Embodiment 9 The difference between this embodiment and Embodiment 7 is that the conductive agent is graphene, and the remaining is completely the same After being subjected to coating, rolling, punching, laminating, injecting and forming processes, the cathode slurry prepared in embodiments 1-9 of the present invention is manufactured into lithium titanate hybrid capacitors of the 7 same specification. The produced lithium titanate hybrid capacitors are tested in terms of internal resistance, cycle life, energy density and other performance parameters, and the results of tests are as shown in the following table. Monomer DC Capacity energy internal Voltage retention Group energy ine range ratio after density resistance (V) 10000 times (Wh/L) (mO) of cycles (%) Embodiment 1 13.1 3.11 1.4-2.8 85 Embodiment 2 14.5 2.8 1.4-2.8 89 Embodiment 3 14.9 2.64 1.4-2.8 86 Embodiment 4 14.3 2.62 1.4-2.8 85 Embodiment 5 14.1 2.71 1.4-2.8 86 Embodiment 6 14.5 2.64 1.4-2.8 87 Embodiment 7 13.8 2.63 1.4-2.8 88 Embodiment 8 14.2 2.85 1.4-2.8 86 Embodiment 9 14.6 2.67 1.4-2.8 85 It can be seen from the above table that the lithium titanate cathode slurry capacitors prepared by the methods of the present invention have high energy density and prominent electrochemical performance, which indicates that the slurry prepared by the present invention has good homogeneity and stability and is advantageous to the improvement of the electrochemical performance of capacitors. The foregoing embodiments are merely preferred solutions of the present invention and not intended to limit the present invention in any form. Other variations and transformations may be made without departing from the technical solutions recorded in the claims. It will be understood that the term "comprise" and any of its derivatives (eg comprises, comprising) as used in this specification is to be taken to be inclusive of features to which it refers, and is not meant to exclude the presence of any additional features unless otherwise stated or implied. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge. 8 It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications in its scope. 9