Non-aqueous electrolyte lithium secondary battery

Abstract: A non-aqueous electrolyte lithium secondary battery comprising a negative electrode of spinel-type lithium-titanium oxide with improved overcharge and overdischarge characteristics is disclosed. In this battery, the rechargeable electric capacity of the negative electrode is adjusted to be smaller than that of the positive electrode. In a preferable mode, the negative electrode may further comprise a material which has more negative potential than that of the lithium-titanium oxide, and shows electrochemical reversibility. An electrolyte comprising ethylene carbonate and LiN(CF 3 SO 2 ) 2 or LiN(C 2 F 5 SO 2 ) 2 is preferable.

Description: BACKGROUND OF THE INVENTION The present invention relates to an improvement in performance of a non-aqueous electrolyte lithium secondary battery comprising a negative electrode of a lithium-titanium oxide. As used herein the term "battery" is used to include a single battery cell and an array of battery cells. With the recent rapid development of technology in the field of electronics, electronic appliances have been remarkably miniaturized, and the demands for batteries of smaller size and lighter weight with a high energy density are increasing accordingly. As such, lithium secondary batteries configured with a lithium negative electrode are attracting attention in this field and research and development of these batteries is now briskly conducted on a global scale. However, if metallic lithium is utilized in the negative electrode, the lithium may sometimes be pulverised and arborescent crystals called lithium dendrites may sometimes be formed on the lithium negative electrode with repetition of charge and discharge. In such cases, the shape of the lithium negative electrode may be damaged and a satisfactory charge/discharge life cycle cannot be obtained, and in extreme cases, the lithium dendrites pierce through a separator and cause an inner short-circuiting of the battery. As a means for solving the above-mentioned problem, investigations have been conducted for a negative electrode which include; a lithium-aluminum alloy, a lead-containing alloy capable of intercalating and deintercalating lithium, various carbon materials, an oxide of a transition metal such as niobium pentoxide, an anatase-type titanium oxide, tungsten dioxide, or a ferrous or ferric oxide, each doped with lithium. For the prevention of lithium dendrite formation, carbon or an oxide of a transition metal have been found to be particularly useful, and it has been found that the charge/discharge life cycle of the battery can greatly be improved by employing these materials in the negative electrode. Recently, lithium secondary batteries have been used in various appliances such as a direct-current power source for a mobile unit and as power sources for memory back-up and the like. In order to fully satisfy the required performances, it is extremely important to provide not only sufficient energy density or charge/discharge life cycle, but also high reliability of the battery, in particular, withstanding overcharge and overdischarge characteristics of the battery. However, a combination of the above-specified negative electrode and a conventional positive electrode may not necessarily provide a battery that is capable of withstanding overcharge and overdischarge characteristics. BRIEF SUMMARY OF THE INVENTION As a result of research into using various metal compounds for configuring the negative electrode, the present inventors have found that a lithium-titanium oxide of spinel-type structure which is disclosed in Japanese Laid-Open Patent Hei 6-275263 shows excellent charge/discharge performance, and that a negative electrode comprising this material has excellent stability over a wide potential range which cannot be obtained with any other negative electrode materials. The primary aim of the present invention is to provide a non-aqueous electrolyte secondary battery which has improved ability in withstanding overcharge and overdischarge characteristics of a battery system, which is obtained by incorporating into the negative electrode, a lithium-titanium oxide of a spinel-type structure. The present invention provides a non-aqueous electrolyte lithium secondary battery comprising a negative electrode which comprises at least one spinel-type lithium-titanium oxide, a positive electrode which has a higher potential than that of the spinel-type lithium-titanium oxide, and a non-aqueous electrolyte, wherein the rechargeable electric capacity of the negative electrode is smaller than that of the positive electrode. In a conventional lithium secondary battery which utilizes metal oxides as positive and negative electrodes, it is very difficult to satisfy both withstanding overcharge and overdischarge characteristics at the same time, even if the electric capacity of the positive electrode is kept balanced to that of the negative electrode by some means. The reason for this is that although a number of positive and negative electrodes exist which can function stably in a normal charge/discharge potential range, there are very few materials with a constant stable crystal structure over the very wide potential ranges that cover the overcharge and overdischarge potentials, which greatly exceed the normal charge/discharge potential region. The present inventors have now found that among a number of metal oxides, the lithium-titanium oxide having the spinel structure exists in a very stable state without its crystal structure being destroyed over a very wide potential range from 0 V to 4 V, despite the fact that a plateau potential during the charge/discharge processes is near 1.5 V with respect to metallic lithium (in the following description, "potential" is used by taking the potential of metallic lithium as reference). Since intercalating and deintercalating reactions of lithium ions in and from the spinel-type lithium-titanium oxide crystal proceed almost free from distortion of the crystal lattice of the oxide, it is believed that the spinel-type lithium-titanium oxide crystal not only demonstrates excellent charge/discharge performance but also very strong resistance against potential stress. The present invention utilises this very stable characteristic of the spinel-type lithium-titanium oxide over a wide potential range. The electric capacity of the negative electrode is made smaller than the electric capacity of the positive electrode in the reversible region in order to best utilize such characteristic. By doing so, the potential of the positive electrode does not vary greatly even at the end of the discharging process and remains in the reversible region, and thus the crystal structure is maintained stable without being destroyed. On the other hand, although the potential of the negative electrode is close to the potential of the positive electrode during the discharging process, the negative electrode is not essentially destroyed and exists stably even if the potential of the positive electrode remains as high as 3 V or 4 V, and thus the negative electrode can strongly withstand against overdischarge. Even if the battery is left still holding the discharge circuit in a state close to complete discharge for a long period of time, the performance of the electrode is not deteriorated. In addition, in the charging process, since the electric capacity of the negative electrode is smaller than that of the positive electrode, the potential of the positive electrode does not vary greatly and the charging process is completed by lowering the potential of the negative electrode. At that time, since the negative electrode can maintain its stable state until at least a potential of 0 V, the upper-limit within which the battery can be overcharged can be set higher by at least 0.15 V than the plateau voltage during the charging process, and the reliability for withstanding the overcharge is greatly improved. In the description of the present invention, the term "electric capacity of the positive electrode or the negative electrode" means a reversible and chargeable or dischargeable capacity of the respective electrodes under standard conditions equivalent to an ambient temperature and charge/discharge current, as far as a battery configured with these positive electrode and negative electrode is utilized. Accordingly, the present invention establishes a relationship between the negative electrode with an electric capacity which is substantially equal to the chargeable and dischargeable electric capacity of the battery configured with this electrode, and the positive electrode with an electric capacity which is larger than the above-mentioned capacity. Since these electric capacities vary slightly depending on the employed measuring conditions, the values of the electric capacities of the positive and negative electrodes defined in the present invention are determined in accordance with the values measured under the following conditions. A) The electric capacity of the positive electrode The positive electrode and a counter electrode of, for instance a metallic lithium electrode to be used in configuring a relevant battery are immersed in an electrolyte of a composition identical to that used in the relevant battery, and then charged and discharged at room temperature (25±5°C). First, the positive electrode is charged at a current density of 0.1 mA/cm 2 until completion of deintercalation of lithium from the deintercalatable lithium and then discharged at a current density of 0.1 mA/cm 2 until the potential of the positive electrode drops to 2 V. The value of the discharge electric capacity of the positive electrode measured at that time is determined as the electric capacity of the positive electrode. B) The electric capacity of the negative electrode The negative electrode and a counter electrode of, for instance a metallic lithium electrode to be used in configuring a relevant battery are immersed in an electrolyte of a composition identical to that used in the relevant battery, and then charged and discharged at room temperature (25±5°C). First, the negative electrode is charged at a current density of 0.1 mA/cm 2 until the completion of intercalation of lithium into the intercalatable lithium and then discharged at a current density of 0.1 mA/cm 2 until the potential of the negative electrode rises to 2 V. The value of the discharge electric capacity of the negative electrode measured at that time is determined as the electric capacity of the negative electrode. While the novel features of the present invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal cross-sectional view of a non-aqueous electrolyte lithium secondary battery in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention can be embodied in various modes of embodiments which will be described later, by adopting the above-mentioned fundamental means for solving the problems and their functions and further developing them. First, by adding a material which has more negative potential than that of the spinel-type lithium-titanium oxide and electrochemical reversibility, it is possible to enhance the withstanding overcharge characteristic of the oxide. For instance, a carbon material such as graphite which is electrochemically reversible and capable of intercalating or deintercalating lithium may be added, in addition to other carbon materials which are used as the conductive agent. In the case of adding graphite, since the potential of the graphite to which the lithium is intercalated demonstrates a potential as low as approximately 0 V, even when overcharging of the battery is still continued, lithium is intercalated to the graphite at a charge voltage about 1.5 V higher than the charge voltage of the spinel-type lithium-titanium oxide, and the battery demonstrates a plateau charging voltage which corresponds to the increased electric capacity. Therefore, the upper-limit charging voltage can previously be set below the plateau charging voltage. The potential of the negative electrode is maintained for a while even if it reaches the potential close to 0 V, which decreases the risk of fluctuations in the potential. In addition, when a plateau potential close to 0 V is taken as a signal and a charging voltage above that is cut off, it is possible to prevent any damage attributable to the overcharging. Other than the disclosed graphite, a similar technical advantage is obtained by adding any other carbon material, WO 2 , Fe 2 O 3 , LiFe 5 O 8 , SiO 2 , SnO or the like which also show electrochemical reversibility and demonstrate a more negative potential than the spinel-type lithium-titanium oxide in a lithium intercalating state. The electrolyte, on the other hand, plays an important role in fulfilling the requirement for the satisfactory withstanding overcharge or overdischarge characteristic. Since there is a possibility of lowering the potential of the negative electrode below 1.5 V and around 0 V in the overcharging process, the electrolyte on the negative electrode may sometimes be decomposed. In order to prevent the electrolyte from decomposing, it is preferable to use ethylene carbonate as an organic solvent for the electrolyte. The ethylene carbonate has a function of preventing the electrolyte from decomposing which might occur when the electrolyte reacts with the carbon at a potential around 1 V. In addition, since the ethylene carbonate has a high dielectric constant, it is possible to further improve the reversibility of charge/discharge. The ethylene carbonate is however solid at room temperature, and thus it is preferable to mix it with a low-viscosity solvent, such as propylene carbonate, butylene carbonate, tetrahydrofuran, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl carbonate, dimethyl carbonate or the like. In addition, if an electrolyte is prepared by dissolving LiN(CF 3 SO 2 ) 2 as a solute in this organic solvent which contains ethylene carbonate, the stability of the oxide for the charge/discharge at a temperature as high as 70 to 80°C and against the overcharge are enhanced as a result of synergistic effect. The reason for this is that while LiN(CF 3 SO 2 ) 2 demonstrates high electric conductivity comparable to LiPF 6 , it is very stable at a potential of not more than 3.5 V even at high temperature, and, in addition, it has a preferable affinity for ethylene carbonate. The disclosed LiN(CF 3 SO 2 ) 2 is a little unstable at a potential of greater than 4 V, so that it is preferably used in a battery configured with a positive electrode which has a potential of less than 4 V. An analogous imide is LiN (C 2 F 5 SO 2 ) 2 . Although the electric conductivity of this imide is slightly inferior to LiN(CF 3 SO 2 ) 2 , it demonstrates high stability at an ambient temperature as high as 70 to 80°C, and is also stable at a potential of about 4 V and thus it can be used in a wide potential range. The spinel-type lithium-titanium oxide to be used in the negative electrode of the present invention is represented by the general formula Li 1+X Ti 2-X O 4 (-0.2 ≦ X ≦ 1/3), and can be typically exemplified as Li 0.8 Ti 2.2 O 4 , LiTi 2 O 4 , Li 4/3 Ti 5/3 O 4 (Li 1.33 Ti 1.66 O 4 ) and the like. Any of them is preferable in withstanding overcharge and overdischarge characteristics. In particular, Li 4/3 Ti 5/3 O 4 , which is a defective-type titanium spinel, is the most preferred, as the technical advantage realized by the present invention is the greatest. In the following paragraphs, specific non-limiting examples of the present invention will be described in detail. EXAMPLE 1 Li 4/3 Ti 5/3 O 4 , carbon black for a conductive agent, and a fluorocarbon resin powder as a binder were mixed together in a weight ratio of 90 : 5 : 5, kneaded and molded to give pellets having a thickness of about 0.5 mm and a diameter of 18 mm. The obtained pellets were dried at 250°C and dehydrated to obtain a negative electrode. The negative electrode has an electric capacity of about 40 mAh. Separate from this, a lithium-containing manganese dioxide Li x MnO y which was obtained by adding 0.3 mole of lithium hydroxide to 1 mole of manganese dioxide and subjecting the mixture to a heat treatment at 400°C, carbon black as a conductive agent, and a fluorocarbon resin powder as a binder were mixed together in a weight ratio of 90 : 5 : 5, kneaded, and then molded into pellets having a diameter of 12.0 mm. The obtained pellets were dried at 250°C in a vacuum and dehydrated to obtain a positive electrode. The positive electrode had a charge/discharge potential of about 3 V with respect to that of metallic lithium. The thickness of the positive electrode was adjusted so that the ratio of the electric capacity of the negative electrode to that of the positive electrode is 0.8, 0.9, 1.0, 1.1 or 1.2. In this adjustment, the electric capacity of the positive electrode was taken as an electric capacity down to the discharge potential of 2 V and the electric capacity of the negative electrode was also taken as an electric capacity up to the discharge potential of 2 V.The terminal voltage of a battery configured with these positive and negative electrodes in the charged state was about 1.5 V, and the discharge capacity varied depending on the ratio of the electric capacities of the positive and negative electrodes but was in a range of about 32 to 40 mAh. Coin-type lithium secondary batteries as shown in FIG. 1 were prepared with these positive and negative electrodes. In FIG. 1, each of the coin-type lithium secondary batteries comprises a battery case 1 which also serves as a positive electrode terminal, a sealing disk 2 which also serves as a negative electrode terminal, a gasket 3 made of polypropylene for insulating the case 1 from the sealing disk 2, a separator 4 made of polypropylene nonwoven fabric, a positive electrode 5 and a negative electrode 6. Lithium ions have been electrochemically intercalated to the negative electrode 6 inside the battery by pressing the electrode with metallic lithium. The negative electrode 6 has an electric capacity of about 40 mAh. The positive and negative electrodes and the separator are impregnated with an electrolyte prepared by dissolving LiPF 6 in a equivolumetric mixed solvent of propylene carbonate, ethylene carbonate and 1,2-dimethoxyethane. Each of the batteries has an outer diameter of about 23 mm, and a height of about 3.0 mm. These batteries have symbols of A, B, C, D and E in the ascending order of the electric capacity ratios (0.8, 0.9, 1.0, 1.1 and 1.2 ) of the negative and positive electrodes. EXAMPLE 2 Li 4/3 Ti 5/3 O 4 , a natural graphite having a purity of not less than 99%, carbon black as a conductive agent, and a fluorocarbon resin powder as a binder were mixed together in a weight ratio of 75 : 15 : 5 : 5, kneaded and molded to give pellets having a diameter of 12.0 mm so that the electric capacity of Li 4/3 Ti 5/3 O 4 became 40 mAh. The obtained pellets were dried at 250°C and dehydrated to obtain a negative electrode. The electric capacity attributable to the presently used natural graphite, which intercalates and deintercalates lithium in a reversible manner, is about 5 mAh. A positive electrode of the lithium-containing manganese dioxide Li x MnO y similar to that of Example 1 was employed so that the ratio of the total electric capacity of the negative electrode comprising Li 4/3 Ti 5/3 O 4 and the natural graphite to the electric capacity of the positive electrode becomes 0.9 : 1. Except for the above, the battery has an identical configuration to that of Example 1. This battery has a symbol F. EXAMPLE 3 In this example, the same solvent as that used in Example 1 and LiN(CF 3 SO 2 ) 2 , LiClO 4 and LiBF 4 were used as the solute for preparing the electrolytes. The same configuration as that of B in Example 1 was adopted and these electrolytes were used in configuring the batteries having symbols G, H and I. In order to evaluate the performance of these batteries, the batteries of Example 1 were first discharged with a load resistor of 3 kΩ until the terminal voltages dropped to about 0 V, and then discharged continuously for 30 days while maintaining closed circuit state. Thereafter, they were charged at a constant voltage of 2.5 V and discharged with a load of 3 kΩ until the terminal voltages dropped to 1 V. The results of the electric capacities obtained are shown as capacity maintenance rate compared to the initial electric capacities in Table 1 below. Battery Capacity maintenance rate (%) after continuous discharge with 3kΩ for 30 days Capacity maintenance rate after overcharge at 2.8 V at 60°C for 30 days A 99 99 B 99 98 C 98 93 D 85 83 E 75 68 As clearly shown in Table 1, the batteries D and E whose electric capacities of the negative electrodes are excessive suffer a deterioration in the discharge capacity. The deterioration of the battery E is particularly serious. The cause for such deterioration is that the performance of the positive electrode is deteriorated at the end of the discharge due to a great decrease in the positive electrode potential down to about 0 V. The battery C shows slight deterioration, because the influence of the decrease on the positive electrode potential is scarce at the ratio of 1 : 1 of the electric capacities of the negative and positive electrodes. The batteries A and B whose negative electrode capacities are small do not suffer any deterioration. Subsequently, these batteries were applied with a voltage of 2.8 V at 60°C for consecutive 30 days and discharged with a load resistor of 3 kΩ until the terminal voltages dropped to 1 V. The capacity maintenance rates based on the electric capacities obtained during the process compared to the initial electric capacities are also shown in Table 1. As clearly shown in Table 1, although the batteries A and B whose negative electrode capacities are small suffer only scarce deterioration, a deterioration is recognized with the battery C whose electric capacities of the positive and negative electrodes are equal, and a remarkably large deterioration is observed with the batteries D and E. The cause for such deterioration is that the performance of the positive electrode is deteriorated by an increase in the positive electrode potential during the charging process. Next, the battery B of Example 1 and the battery F of Example 2 were applied with a voltage of 3 V at 60°C for consecutive 30 days, and then discharged with a load resistor of 3 kΩ until the terminal voltage dropped to 1 V. The maintenance rates of the electric capacities after the discharge compared to the initial capacities are shown in Table 2. Battery Capacity maintenance rate (%) after overcharge at 3 V at 60°C for 30 days B 95 F 99 As clearly shown in Table 2, the battery B holds an electric capacity maintenance rate of 95% whereas the battery F holds an electric capacity maintenance rate of 99%, and it is found that the latter has been improved in stability against the overcharge. It is believed that the reason for this improvement may be that the active material lithium ion could be kept stable in the presence of graphite which shows electrochemical reversibility. Subsequently, the batteries G, H and I of Example 3 and the battery B of Example 1 were applied with a voltage of 2.8 V at 80°C for consecutive 10 days, and then discharged with a load resistor of 3 kΩ until the terminal voltage dropped to 1 V. The maintenance rates of the electric capacities after the discharge compared to the initial capacities are shown in Table 3. Battery Capacity maintenance rate (%) after overcharge at 2.8 V at 80°C for 10 days B 85 G 99 H 65 I 70 As clearly shown in Table 3, the battery G which comprises LiN(CF 3 SO 2 ) 2 holds the electric capacity maintenance rate of about 90% whereas the electric capacity maintenance rates of the batteries B, H and I which comprise LiPF 6 , LiClO 4 and LiBF 4 , respectively are considerably decreased. It is believed that the reason for the decrease is due to possible decomposition of the electrolyte during the charging process at a temperature as high as 80°C which adversely influences the battery performance. In addition, as a result of a similar test conducted on a battery which uses LiN(C 2 F 5 SO 2 ) 2 as a solute for the electrolyte, it is found that a substantially equal result to that of the battery G which uses LiN(CF 3 SO 2 ) 2 as a solute for the electrolyte was obtained. In the foregoing specific examples, although Li 4/3 Ti 5/3 O 4 was used as a main ingredient for the negative electrode, a similar advantage will be obtained with any of the previously described other spinel-type lithium-titanium oxides. In addition, although a natural graphite was added to the negative electrode in Example 2, an artificial graphite, an expanded graphite, a hardly graphitizable carbon, and any oxides or compounds, which have a lower potential than the spinel-type lithium-titanium oxides in the charged state and show reversibility in charge/discharge, can effectively be applied to the present invention as an additive to the negative electrode. Moreover, in the case of adding graphite, although the total negative electrode capacity of Li 4/3 Ti 5/3 O 4 and that of graphite is set to smaller than the positive electrode capacity, no problem would result by setting the electric capacity of only Li 4/3 Ti 5/3 O 4 to smaller than the positive electrode capacity as in the case of Example 1. Further, although a lithium-containing manganese composite oxide was used as the positive electrode, there are several cases wherein the oxide has a plurality of plateaux of the discharge voltages and includes some plateaux which lack reversibility in the charge/discharge process. For instance, crystal vanadium pentoxide or the like has discharge plateaux in three steps or more, and if it is discharged until the terminal voltage drops to about 2 V, it substantially loses its reversibility. In such a case, it is preferable to cut off the discharge of the negative electrode at around 2.5 V or around 3 V, by limiting the electric capacity of the negative electrode. As such, in a positive electrode which can maintain its potential in the reversible region, it is possible to use any positive electrode active material. As described previously, in a battery comprising a negative electrode of spinel-type lithium-titanium oxide, the present invention can greatly improve the withstanding overdischarge and overcharge characteristics at an ambient temperature as high as 60 to 80°C, not to mention simple charge/discharge performance, and thus its industrial value is very great.

Claims: A non-aqueous electrolyte lithium secondary battery comprising a negative electrode which comprises at least one lithium-titanium oxide having a spinel structure, a positive electrode which has a higher potential than that of said lithium-titanium oxide, and a non-aqueous electrolyte, wherein the rechargeable electric capacity of said negative electrode is smaller than that of said positive electrode. The non-aqueous electrolyte lithium secondary battery according to claim 1, wherein said negative electrode further comprises a material which has more negative potential than that of said lithium-titanium oxide in charged state and electrochemical reversibility. The non-aqueous electrolyte lithium secondary battery according to claim 2, wherein the material contained in said negative electrode is at least one member selected from the group consisting of carbon, graphite, WO 2 , Fe 2 O 3 , LiFe 5 O 8 , SiO 2 and SnO. The non-aqueous electrolyte lithium secondary battery according to any preceding claim, wherein the non-aqueous electrolyte comprises ethylene carbonate as a solvent. The non-aqueous electrolyte lithium secondary battery according to any preceding claim, wherein the non-aqueous electrolyte comprises LiN(CF 3 SO 2 ) 2 or LiN(C 2 F 5 SO 2 ) 2 as a solute. The non-aqueous electrolyte lithium secondary battery according to claim 5, wherein the non-aqueous electrolyte comprises ethylene carbonate as a solvent and LiN (CF 3 SO 2 ) 2 as a solute. The non-aqueous electrolyte lithium secondary battery according to any preceding claim, wherein said lithium-titanium oxide is a defective-type titanium spinel. The non-aqueous electrolyte lithium secondary battery according to claim 7 wherein the lithium-titanium oxide is Li 4/3 Ti 5/3 O 4 .

Inventors: ASAKA EMI, CHIKAYAMA KOICHI, KOSHIBA NOBUHARU, SANO YOKO, ASAKA, EMI, CHIKAYAMA, KOICHI, KOSHIBA, NOBUHARU, SANO, YOKO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H01M10
IPC codes: H01M4, H01M10