This invention relates to anode material for a non-aqueous electrolyte secondary battery, which consists of carbon material particularly carbon in fiber form, and further relates to a non-aqueous electrolyte secondary battery using such anode material.
Recent electronic technologies have conspicuously progressed so that, e.g., miniaturization and/or light weight of electronic equipments can be realized in succession. Followed by this, also for batteries as portable power supply (supply) (source), there has been still more increased demand of miniaturization, light weight and high energy density.
Hitherto, as the secondary battery of general use, aqueous solution system batteries such as lead battery, or nickel/cadmium battery, etc. are the main current. These batteries can be satisfied to some extent in the cycle characteristic, but it cannot be said that they have satisfactory characteristic in points of battery weight and the energy density.
On the other hand, studies/developments of non-aqueous electrolyte secondary batteries using lithium or lithium alloy as anode have been extensively carried out in recent years. Such batteries have excellent characteristics of high energy density, small self-discharge and light weight, but have the drawback that lithium is crystal-grown in dendrite form at the time of charging followed by development (progress) of the charge/discharge cycle and reaches the cathode resulting in an internal short which limits practical use.
As a battery which solves such problem, non-aqueous electrolyte secondary batteries using carbon material as the anode, which are so called lithium ion secondary batteries, have been proposed and remarked. The lithium ion secondary battery utilizes doping/undoping of lithium into a portion between carbon layers as the anode reaction. Even if a charge/discharge cycle is developed, precipitation of crystal in dendrite form cannot be observed at the time of charging. Thus, such batteries exhibit a satisfactory charge/discharge cycle characteristic.
In this case, there are several carbon materials which can be used as the anode of the lithium ion secondary battery. Among them, material which has been first put into practical use is coke and glass-shaped carbon. These materials are materials having low crystallinity obtained after undergoing heat-treatment at relatively low temperature, and have been commercialized as practical battery by using electrolytic solution mainly consisting of propylene carbon (PC). Further, also in graphite or the like which could not be used as anode when PC is used as main solvent, electrolytic solution mainly consisting of ethylene carbon (EC) is used so that arrival to a usable level has been realized.
As the graphite or the like, graphite in a scale form can be relatively easily obtained. Hitherto, such graphite or the like has been widely used as conductive material for an alkali battery. This graphite or the like advantageously has high crystallinity and high true density as compared to non-easily graphitized carbon material. Accordingly, if the anode is constituted by the graphite or the like, high electrode filling (packing ability) can be obtained and the energy density of the battery increases. From this fact, it can be said that the graphite or the like is greatly expected material as the anode material.
Meanwhile, most of the carbon materials exhibit form such as block form. In the case where such carbon materials are actually used as the battery, they are crushed or pulverized and are used in powder form.
For this reason, even if the structure of carbon material is controlled so that the carbon material takes macro form or micro form by, e.g., physical or chemical treatment (processing), there are actual circumstances where the structure is disturbed by crushing, so its effect cannot be sufficiently obtained.
On the contrary, in the case of carbon in fiber form (carbon fiber) obtained by carbonizing organic material in fiber form, it is easy to relatively control the carbon structure and there is no necessity of crushing. For this reason, such carbon fiber is advantageous when application to the anode is assumed.
The structure of the carbon fiber greatly reflects the structure of organic fiber which is precursor.
As organic fiber, there are organic fibers in which polymer such as polyacrylonitrile, etc. is caused to be material, an organic fibers in which pitch or the like such as petroleum pitch, etc. and mesophase pitch caused to be oriented are caused to be material, etc. These organic fibers all take fiber shape after undergoing fiberforming.
By carbonizing these organic fibers, carbon fibers can be obtained. However, since they are fused when heat-treated at the time of carbonization so that there results broken fiber structure, they are carbonized after infusible processing and are ordinarily implemented to the fiber surface by oxidation, etc.
The carbon fiber obtained in this way has cross sectional structure originating in the organic material fiber structure and exhibits high order structure of, such as, for example, the type oriented in concentrical form which is so called onion-skin type, the radially oriented radial type and isotropically random type, etc. Graphite fibers obtained by graphitizing these carbon fibers have high true density and also have high crystallinity.
However, in the above-described carbon fibers, it cannot be said that there is no problem.
Since, e.g., most of carbon fibers have circular cross section nearly equal to be completely round, in the case where they are filled (packed) into the electrode, the so-called dead space takes place. Under the circumstances where there is increased requirement of high energy density with development of electronic equipments, the above-mentioned dead space constitutes great problem.
Moreover, in a lithium ion secondary battery, since the intercalation reaction is the main anode reaction, it is known that as crystallinity of anode carbon material becomes higher, the capacity becomes large. In the carbon fiber, with respect to the fiber cross sectional structure of the radial type, crystallinity is easy to be improved, whereas crack is easy to take place in parallel to fiber axis by expansion/contraction at the time of charge/discharge and the fiber structure is easy to be broken. Accordingly, in the carbon fiber of the radial type structure, large capacity can be obtained, but reversibility of the charge/discharge cycle is not sufficient.
For this reason, as the anode carbon material, carbon fibers of the random radial type in which the radial structure and the random structure are mixed are the main current. However, since the fiber diameter is small and the cross section takes circular shape, rearrangement of the carbon layer surface is difficult to take place, thus making it possible to have high crystallinity, e.g., as in the case of graphite in the scale form.
Further, in the case of the carbon fiber, since the orientation state of the cross section becomes uneven in the fiber length direction, there is also the inconvenience that crack is apt to take place in the fiber axis direction at the time of crushing and cutting. Since the carbon fiber is not in block form as in the case of the ordinary graphite material, crushing is not required under strong condition. However, it is necessary to finely crush and cut carbon fiber so that a fixed (predetermined) aspect ratio is provided. This crushing/cutting of carbon fiber involves various difficulties as compared to crushing of carbon material in block form. Thus, as previously described, not only crack is easy to take place, but also it is difficult to allow material parameter such as aspect ratio, etc. to be fixed.
From these reasons, there is nothing but to say that the battery made up by using conventional graphitized carbon fiber has insufficient capacity in the existing states and has low industrial reliability.
An object of this invention is to provide more practical carbon in fiber form (carbon fiber) which is high in the electrode filling (packing) ability, is excellent in crystallinity, is easy to be cut, and has less variations (unevenness) of material parameter, and to thereby provide a non-aqueous electrolyte secondary battery of high energy density and high reliability.
The inventors of this application have obtained various findings as the result of the fact that they have energetically and repeatedly studied. This invention has been completed on the basis of these findings, and contemplates to attain the above-described object by implementing various improvements to carbon fiber.
Namely, first of all, the cross sectional shape of the carbon fiber is caused to be such a shape that area replenishment rate (degree) (value obtained by dividing area of cross section of carbon fiber by product of long side and short side of circumscribed rectangle which takes the minimum area in the case where the cross section of the carbon fiber is encompassed by the circumscribed rectangle) satisfies a specific range. 0.8 or more.
Thus, anode material having high electrode filling (packing) ability and having less dead space is obtained.
Moreover, at this time, the cross sectional shape is caused to be such a shape to satisfy a specific range of circularity, whereby the cycle characteristic is further improved.
Secondly, since value of fractal dimension determined by the fractal analysis of cross sectional high order structure of random radial type carbon fiber can be utilized as material parameter for evaluating cross sectional structure, this value is caused to fall within a specific range (from 1.1 to 1.8) to conduct a control such that crystallinity is caused to fall within a reasonable range.
Thus, high capacity carbon fiber which is less in variations of the charge/discharge ability and is satisfactory in the charge/discharge cycle reversibility can be realized.
Thirdly, in the high order structure of the carbon fiber, the central portion is caused to be of radial type structure and the surface layer portion is caused to be of the random radial type structure.
Thus, carbon fiber having both strength tolerable to expansion/contraction at the time of charge/discharge and high capacity can be realized.
Fourthly, the cross sectional shape of the carbon fiber is caused to be of notch structure including notch (cut portion). The notch angle is caused to be 2xc2x0 to 150xc2x0.
Thus, even in the case where carbon fiber of the radial type structure is employed, carbon fiber having high capacity and strength tolerable to expansion/contraction at the time of charge/discharge can be realized.
Fifthly, graphitized carbon fiber having cross sectional portions in the crystal structure different from each other at predetermined period (interval) in the fiber length direction is made up. Then, the graphitized carbon fiber thus obtained is crushed.
Thus, carbon fiber crushed powder having less unevenness and a fixed aspect ratio can be easily made up.