CATHODE MIX CONTAINING IMPROVED EFFICIENCY AND ENERGY DENSITY OF ELECTRODE FIELD OF THE INVENTION The present invention relates to a cathode mix with improved electrode efficiency and energy density. More specifically, the present invention relates to a cathode mix comprising, as a cathode active material, lithium iron phosphate (LiFe(P1-XO4)) wherein a molar fraction (1-x) of phosphorus (P) is in the range of 0.910 to 0.999 to allow operational efficiency of the cathode active material to be leveled to a lower operational efficiency of an anode active material and improve energy density of the cathode active material, BACKGROUND OF THE INVENTION Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long life span and low self-discharge are commercially available and widely used. The lithium secondary batteries generally use a carbon material as an anode active material. Also, the use of lithium metals, sulfur compounds, silicon compounds, tin compounds and the like as the anode active material have been considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCo02) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNi02) as the cathode active material has been considered. LiCoO2 is currently used owing to superior physical properties such as cycle life, but has disadvantages of low stability and high-cost due to use of cobalt, which suffers from natural resource limitations, and limitations of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of short cycle life. In recent years, methods to use lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphate is largely divided into LixM2(PO4)3 having a NASICON structure and LiMPO4 having an olivine structure, and is found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 is the most widely known NASICON structure compound, and LiFePO4 and Li(Mn, Fe)PO4 are the most widely known olivine structure compounds. Among olivine structure compounds, LiFePQ4 has a high output voltage of 3.5 V and a high theoretical capacity of 170 mAh/g, as compared to lithium (Li), and exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe as an ingredient, thus being highly applicable as the cathode active material for lithium secondary batteries. However, such an olivine-type LiFePO4 has an operational efficiency of about 100%, thus making it difficult to control with the operational efficiency of an anode. In this regard, by imparting equivalent operational efficiency to a cathode and an anode in batteries, inefficient waste of the electrodes can be minimized. For example, in the case where an anode having efficiency of about 100% is used for a battery, the battery can exert 100% efficiency, while when a cathode having 100% efficiency and an anode having 90% efficiency are used for a battery, the battery can exert only 90% efficiency. As a result, 10% of the efficiency of the cathode is disadvantageously wasted. For example, in the case of generally-used carbon-based anode active materials, about 10-20% irreversible capacity are generated upon initial charge/discharge including the first charge and its reversible capacity is only about 80 to 90%. Accordingly, when a material having an efficiency of 100% is used as a cathode active material, the electrode material is disadvantageously wasted in direct proportion to the irreversible capacity of about 10 to 20%. In addition, when an anode active material having relatively low efficiency is used, an amount of anode active material used should be increased, depending on a higher efficiency of a cathode, which disadvantageously entails an increase in manufacturing costs. On the other hand, in order to impart 100%> efficiency to a battery using a cathode having 100%) efficiency, an anode having about 100% efficiency should be used. In this case, the selection range of an anode active material is disadvantageously narrowed. However, to date, there is no technology suggesting a method for controlling efficiency of LiFeP04 as a cathode active material. In addition, there is an increasing need for a breakthrough that can considerably improve electrical conductivity of LiFeP04 and solve Li+ diffusion problems thereof via improvement in initial IR drop and Li+ diffusion properties. SUMMARY OF THE INVENTION Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved. As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have discovered that control of molar fraction (1-x) of phosphorus (P) in high-efficiency lithium iron phosphate to the range of 0.910 to 0.999 enables leveling of efficiency of the cathode active material to a lower operational efficiency of an anode active material, minimization in waste of electrode efficiency and thus ultimate maximization of efficiency and capacity of electrodes and batteries, and that controlling Fe valence enables improvement in IR drop and rate properties, improvement in charge/discharge plateau potential and thus maximized increase in energy density. Based on this discovery, the present invention has been completed. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a graph showing results of XRD/ND refinement assay in Experimental Example 2; FIG. 2 is an image showing results of HRTEM structural analysis in Experimental Example 2; FIG. 3 is a graph showing results of Fe valence analysis using Mossbauer effects in Experimental Example 2; and FIG. 4 is a graph showing discharge results in Experimental Example 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a cathode mix for lithium secondary batteries, comprising a cathode active material having a composition represented by the following Formula I, LiFe(P1-xO4) (I) wherein molar fraction (1-x) of phosphorus (P) is in the range of 0.910 to 0.999 to allow operational efficiency of the cathode active material to be leveled to a lower operational efficiency of an anode active material and improve energy density of the cathode active material. As mentioned hereinbefore, LiFePO4 has an operational efficiency of about 100%. Accordingly, when an anode active material having lower efficiency is used as an anode active material, electrode materials are required, in order to cause the anode active material to have reversible capacity, comparable to a cathode active material, thus disadvantageously entailing an increase in manufacturing costs. In this regard, the inventors of the present invention discovered that initial operational efficiency can be relatively reduced by controlling molar fraction (1-x) of phosphorus (P) in the range of 0.910 to 0.999. In accordance with this discovery, although an anode active material having lower operational efficiency is used, operational efficiency of a cathode active material can be leveled to that of the anode active material. Accordingly, the present invention enables minimization of electrode material waste and thus considerable decrease in manufacturing costs, and secures desired efficiency and capacity of batteries, thus being highly advantageous in view of manufacturing processes. In addition, the present invention solves problems associated with irreversible capacity of anode active materials and widens selection range of anode active material used in combination with cathode active material, when taking into consideration battery efficiency. Furthermore, general LiFePO4 contains only Fe with a valence of 2+, while LiFeP(1-x)O4 wherein molar fraction (1-x) of phosphorus (P) is in the range of 0.910 to 0.999 in accordance with the present invention has a decreased molar fraction of phosphorus (P) and thus contains both Fe2+ and Fe 3+. When a metal present in the structure of an active material has a mixed valence {e.g. Fe2+/Fe3+), electrical conductivity and Li+ diffusion-associated ionic conductivity are increased and overall rate properties are thus considerably improved, as compared to when the metal has a single valence. The present inventors have discovered that the cathode active material of the present invention inhibits IR drop upon charge/discharge and enhances discharge profile, without causing any structural variation, and thus ultimately increases energy density of batteries. As used herein, the term "an anode active material having lower operational efficiency" refers to a material having operational efficiency lower than that the compound of Formula I, the cathode active material, and includes all anode active materials having lower efficiency and anode active materials having decreased operational efficiency, as compared to cathode active materials due to irreversible capacity generated therein upon initial charge/discharge including the first charge, although they have theoretical capacity comparable to cathode active materials. The anode active material has an operational efficiency lower than 100%, preferably, of 90 to 98%, more preferably, of 90 to 95%. For example, such an anode active material is preferably a carbon-based material capable of exerting high discharge capacity. Any carbon-based material may be used without particular limitation so long as it permits reversible intercalation/deintercalation of lithium ions. The carbon-based material may be a crystalline carbon-based compound, an amorphous carbon-based compound, or a combination thereof A representative example of the crystalline carbon-based compound is graphite. The graphite-based crystalline carbons include potato- or mesocarbon microbead (MCMB)-shape artificial graphite, natural graphite surface-treated to obtain a flat edge, and the like. In addition, the amorphous carbon-based compound is a material comprising carbon atoms having an amorphous crystal structure and examples thereof include non-graphitizable carbon (hard carbon) prepared by subjecting phenol or furan resins to pyrolysis and graphitizable carbon (soft carbon) prepared by carbonizing coke, needle coke or pitch. In a preferred embodiment, the carbon material may be natural or artificial graphite which has high capacity and high energy density owing to superior density and conductivity and thus exhibits superior output and rate properties. More preferably, the carbon material may be mesocarbon microbeads (MCMBs) which are optical anisotropic spherical particles prepared by heating coke, pitch or the like at about 400°C. In addition, as examples of the carbon materials that can be used in the present invention, mention may be made of LiyFe2O3(0≤ y≤ 1), LiyWO2(0≤ y≤ 1), SnxMe1- xMe'yOz (Me: Mn, Fe, Pb, Ge; Me': A1, B, P, Si, Group I, II and III elements of the Periodic Table, halogens; 0 (Table Removed) As can be seen from Table 1 below, charge/discharge efficiency in each cycle can be adjusted to a level lower than 100% by controlling the amount of P present in LiFeP04 to a level lower than 1. [Experimental Example 2] The cathode active material obtained in Preparation Example 4 was subjected to XRD and the batteries prepared in Example 4 and Comparative Example 1 were subjected to ND (neutron) refinement assay, HRTEM structural analysis, and Fe valence analysis using Mossbauer effects. The results thus obtained are shown in FIGs. 1 to 3. As can be seen from the figures, the cathode active material in accordance with the present invention underwent no structural variation and maintained its single crystal olivine-structure containing no impurities, although the molar fraction of P is lower than 1. In addition, variation in voltage upon 0.5C discharge was measured and the results thus obtained are shown in FIG. 4. As can be seen from FIG. 4, the battery (LiFe(P(1-x)O4); x = 0.015) of the present invention underwent lower initial IR drop and expressed discharge profiles at a higher potential, as compared to the battery (LiFe(PO4) of Comparative Example 1. This indicates considerable improvement in ionic conductivity and electrical conductivity and thus considerable improvement in energy density of batteries. This behavior is considered because Fe2+ and Fe3+ coexist in the cathode active material, when taking into consideration that a dominant amount of Fe and a small amount of Fe3+ were measured in Fe valence analysis using Mossbauer effects, as shown in FIG. 3. In this regard, as apparent from FIGs. 1 and 2, a single phase containing no impurity was observed in XRD/ND refinement and HRTEM, which indicates coexistence of Fe / in the olivine structure. INDUSTRIAL APPLICABILITY As apparent from the above description, in accordance with the present invention, molar fraction (1-x) of phosphorus (P) in LiFePO4, the high-efficiency cathode active material, is controlled to the range of 0.910 to 0.999, thereby allowing operational efficiency of the cathode active material to be leveled to operational efficiency of an anode active material, maximizing usable efficiency of batteries, minimizing electrode waste and thus reducing manufacturing costs of batteries. In addition, controlling a Fe valence leads to improvement in IR drop and rate properties as well as charge/discharge plateau potential, thus realizing fabrication of superior batteries with an increased energy density. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. WHAT IS CLAIMED IS: 1. A cathode mix for lithium secondary batteries, comprising a cathode active material having a composition represented by the following Formula I: LiFe(P1-xO4) (I) wherein a molar fraction (1-x) of phosphorus (P) is in the range of 0.910 to 0.999, to allow operational efficiency of the cathode active material to be leveled to a lower operational efficiency of an anode active material and improve energy density of the cathode active material. 2. The cathode mix according to claim 1, wherein the anode active material has operational efficiency of 90 to 98%. 3. The cathode mix according to claim 2, wherein the anode active material is a carbon-based material. 4. The cathode mix according to claim 1, wherein the molar fraction (1-x) of phosphorus (?) is in the range of 0.955 to 0.995, 5. The cathode mix according to claim 1, wherein operational efficiency of the anode active material is leveled to 95 to 99.9%. 6. A cathode lithium secondary battery comprising an electrode to which the cathode mix according to any one of claims 1 to 5 is applied on a current collector. 7. A lithium secondary battery comprising the cathode according to claim 6.