Field of Invention

The present invention relates to a hybrid aqueous energy storage device (battery/supercapacitor).

Technical Background

The continuous economic growths in the 21st century cause the lack of natural resources, environment pollution and accelerate global warming. The human being must master the equivalent relationships among of the economic growth, environmental protection and energy security, which poses to particular urgency to high effectively research and develop the advanced electrochemical power sources, energy-saving techniques and environmental techniques. One of the most promising ways to reduce waste gas emission is introducing clean energy, for example, development of the electric vehicle instead of gasoline-fueled vehicles. Currently, the fuel cell electric vehicle has been widely researched and developed worldwide. For the FCEV, however, it is difficult to satisfy the requirement when its engineering starting, and climbing because of the small power. The fuel cell must combine with a secondary battery or a capacitor with long cycling life, high-energy density and high power. For example, Toyota FCEV used Ni-MH battery as an assistant power source, Honda with an electrochemical double layer capacitor, Mazda with a lead-acid battery, and Nissan with a lithium-ion battery. Among the currently commercialized secondary batteries, Ni-MH and lithium-ion battery have a high energy density but with a drawback of undesirable cycling life and specific capability, the output specific power is limited to 600 W/kg. On other hand, the electrochemical double layer capacitor has a long life over than 10,000 cycles, and a high specific power as high as 1500 W/kg, but with a low energy-density (<5 Wh/kg).

Li-ion battery using two Li-ion intercalated compounds, typically a transition metal oxide positive electrode and a carbon negative material, provides the highest energy density among all secondary batteries. The nonaqueous lithium-ion batteries have been widely used for the portable electric devices, such as note-sized PC, and cell phone etc. However, the drawbacks of low safety and high fabrication cost due to the use of highly toxic and/or flammable organic solvents limite the application as large-scale batteries, especially for electric vehicle (EV) application. In l

order to overcome the safety problem, an aqueous lithium-ion battery was suggested by MoIi Energy Ltd. (WO95/21470) in which two intercalated compounds are used for both the positive and negative electrodes, e.g., LiMn ₂ O ₄ as positive electrode and VO ₂ as negative electrode, but the cycling life is limited. On the other hand, the low power density for the batteries is also an obstacle for EV application. In 2001, Telcordia Technologies Inc. reported (US patent 6252762) a new concept energy storage system which consists of a capacitor electrode and a battery electrode in Li-ion containing nonaqueous electrolyte. For example, a spinel structure Li ₄ Ti ₅ O ₁₂ was used as negative electrode, and the activated carbon as a positive electrode. The system is able to exhibit both high energy storage capability of batteries, high power density and excellent cycling life of supercapacitors.

The hybrid system of the present invention also consists of capacitor electrode and a Li-ion battery electrode in a Li-ion containing aqueous electrolyte solution in which typically an activated carbon is used as negative electrode, and a Li-ion intercalated compound as the positive electrode. The negative electrode stores charge through a reversible non-faradic reaction of Li-ion on the surface of an activated carbon. The positive electrode utilizes a reversible faradic reaction of Li-ion insertion/extraction in LiMn ₂ O ₄ . The charge/discharge process is associated with the transfer of Li-ion between two electrodes, which we defined as "hybrid aqueous lithium-ion cell". The electrode reactions of the hybrid system described in the present invention are different from any hybrid electrochemical surpercapacitors and electrochemical double layer supercapacitors in which the salts in electrolyte will be consumed during the charge process. The ion concentration in the electrolyte will affect the energy density of the hybrid electrochemical surpercapacitors, especially in the organic electrolyte based hybrid system. The hybrid system of the present invention has provided a real green energy storage device with a long cycling life, an appreciate energy density, high power, low cost, low toxicity and high safety, especially for the electric vehicle (EV) application.

References Cited: Foreign Patent Documents WO 95/21470 August 10, 1995 US 6,252,762 June 26, 2001

Description of the Invention

The purpose of the present invention is to provide a real green energy storage device with a long cycling life, an appreciate energy density, high power, low cost, low toxicity and high safety.

In the present invention, a hybrid aqueous energy storage device comprises in contiguity a positive electrode membrane, a negative electrode membrane, a separator membrane interposed therebetween, and an aqueous electrolyte containing cation and anion of an ion species of a dissociable salt. As for the hybrid system of the present invention, the material used for positive electrode is lithium-ion intercalated compounds which can be selected from the group consisting of transition metal oxides, sulfides, phosphates, and fluorides. The negative material of the hybrid system can be selected from material with double layer capacitance behavior, such as activated carbon, mesoporous carbon and carbon nanotubes etc. Moreover, the negative electrode material can also be of composites based on carbon material with high surface area and pseudocapacitive electrode material. The pseudocapacitive electrode can be selected from transition metal oxides, lithium-ion intercalated compounds, conductive polymer and organic polyiadical. The electrolyte containing at least one ion is lithium-ion in aqueous solution.

In the hybrid system of the present invention, a lithium-ion contained aqueous is used. The oxygen evolution occurs on the positive electrode when charged to a definite potential. Typically 4 V lithium-ion intercalated compounds are used, which can be selected from the group consisting of oxides, sulfides, phosphates, and fluorides of transition metal including Mn, Ni, Co, Fe, V. The compound can be LiMn ₂ O ₄ , LiCoθ ₂ , LiCo _1Z3 Ni _1Z3 Mn _1Z3 O ₂ , LiNiO ₂ , LiFePO ₄ , and can be doped by other element M which is at least one selected from the group consisting of Li, Mg, Cr, Al, Co, Ni, Mn, Al, Zn, Cu, La. Typically the doped amount of M is less 50% by molar of total amount of the metal. In view of the cost and safety, LiMn ₂ O ₄ and the other metal element modified LiM _x Mn _2- XO ₄ are most preferred. As the above electrode materials are normally the semiconductor, it is preferred to add electronic conductors which can be carbon black, acetylene black, and graphite. The composite positive membrane also contains at least one binder selected from the group consisting of PTFE, water-solubility rob, and CMC. The weight content of the binder in the composite electrode membrane is less than 20%.

In the hybrid system of the present invention, the negative electrode stores charges through a reversible nonfaradic reaction of cation on the surface of porous carbon material (double layer capacitance). The surface area for these porous carbons over than 1000 m ² /g is preferred. The electronic conductor can be added, and can be carbon black, acetylene black, and graphite. The composite negative membrane also contains at least one binder selected from the group consisting of PTFE, water-solubility rubber, and cellulose. In order to increase the capacitance of the negative electrode, some pseudocapacitance electrode materials which can be selected from the group consisting of LiMn ₂ O ₄ , VO _2, LiV ₃ O ₈ , FeOOH, and polyaniline can also be added. The intercalation potential for these pseudocapacitance material is typically at 2.5-3 V vs. Li/Li ⁺ "

In the present invention, the electrolyte used for this hybrid system can be in liquid or gel state. The electrolyte salts can be the one or the mixed lithium salts, the anion of which is selected from the group consisting of SO ₄ ^2" , NO ₃ ^2" , PO ₄ ^3" , CH ₃ COO ^" , Cl ^"1 and OH ^" . In order to improve its ionic conductivity, the supporting electrolyte salt consisting of the above anion and the other metal cation is preferred to add. The metal cations can be selected from the group consisting of alkaline, alkaline earths, lanthanides, aluminum and zinc ions, such as KCl, K ₂ SO ₄ , and KNO ₃ . The concentration of the electrolyte solution is 0.1 M to 10 M. Some porous materials can also be added to form the gel electrolyte. Such materials can be of porous SiO ₂ , polyvedin (PVA), and polyvinylidene fluoride (PVDF). Moreover, the electrolyte with a pH value over 7 is preferred to assure the utilization of the positive electrode without the evolution of oxygen.

A simplified schematic hybrid cell of the present invention is given in Figure 1. The assembled cell is at first charged. In the charge process, lithium-ion is extracted from the positive electrode into the electrolyte, and then adsorbed to the surface of negative electrode. The opposite electrode reaction occurs in the discharge process. The charge/discharge process is associated with the transfer of Li-ion between two electrodes, which we defined as "hybrid aqueous lithium-ion cell". The electrode reactions of the hybrid system described in the present invention are different from any hybrid electrochemical surpercapacitors or electrochemical double layer supercapacitors in which the salts in electrolyte will be consumed during the charge process, such as, AC/AC, AC/Ni(0H) ₂ , Li ₄ Ti ₅ 0 ₁₂ /AC system. The ion concentration in the electrolyte will affect the energy density of the hybrid electrochemical surpercapacitors, especially for the

organic electrolyte based hybrid system.

In the hybrid system of the present invention, the negative electrode utilizes mainly a reversible non-faradic reaction of Li-ion sorption and de-sorption on the surface of the porous carbon. It is possible to control the charge/discharge potential only by adjusting simply the mass loading ratio of the positive to the negative so as to avoid the oxygen and hydrogen evolution. On other hand, the Li-ion adsorption/de-sorption shows excellent reversibility.

The hybrid cell described in the present invention shows a typical average working voltage of about 1.3 V, and exhibits excellent cycling ability. The hybrid system of the present invention has provided a real green energy storage device with a long cycling life, an appreciate energy density, high power, low cost, low toxicity and high safety, especially for the electric vehicle (EV) application.

The separator membrane used in the hybrid cell of the present invention can be the porous membrane used for the aqueous secondary batteries such as glass fiber membrane used for lead-acid battery, polyethylene membrane in nickel-hydrogen metal battery, and other type of inert electron-insulating, ion-transmissive medium capable of adsorbing electrolyte solution.

The case used for the hybrid cell of the present invention can be plastics, metal, or a composite material of metal and polymer. The shape of the hybrid cell of the present invention can be the cylindrical, prismatic, and button type.

The technologies for the hybrid cell of the present invention integrates both the secondary battery including lithium-ion, nickel-metal hydrogen, and the lead-acid batteries, and the electrochemical supercapacitors. Therefore, all fabrication process can be also applied in the hybrid cell of the present invention.

Brief Description of the Drawings

Figure 1 is a diagrammatic representation of the hybrid cell structure of the present invention; Figure 2 is the graphical representation of the structure of a cylindrin hybrid cell. Figure 3 is a graphical representation of the charge/discharge characteristics of the hybrid cell of the present invention.

Embodiments

A representative embodiment of the present invention may be more particularly fabricated and employed as shown in the following example.

Example 1

A commercial spinel LiMn ₂ O ₄ was used as positive electrode and a commercial activated carbon was used as negative electrode. Composite electrodes were prepared by mixing the active material with acetylene black and PTFE at the following rate: 80/10/10 for LiMn ₂ O ₄ electrode and 85/10/5 for AC electrode. The mixtures thus prepared were cold rolled into films. Then the films were pressed onto a nickel grid (1.2 X 10 ⁷ Pa) that served as a current collector to form composite electrodes. The composite electrodes were dried at 100 ⁰ C for several hours. In this example, the capacities of positive electrode material (LiMn ₂ O ₄ ) and negative electrode material (AC) are 80mAh/g and 40mAh/g respectively and the loads of electrodes are 5 mg/cm ² for positive electrode and 10 mg/cm ² for negative electrode. Both positive composite electrode and negative composite electrode have the same area. A polyethylene membrane used for the commercial Ni-MH battery was used as a separator. The positive composite electrode, negative composite electrode and the separator membrane were stacked together to form a sandwich structure (positive composite electrode/ separator membrane / negative composite electrode). At last, the sandwich structure including positive composite electrode/ separator membrane / negative composite electrode was rolled to form the 2# battery (14 mm in diameter, and 50 mm in length). The typical structure of this hybrid battery was shown in Figure 2. The charge-discharge curve of this hybrid aqueous cell was shown in Figure 3. As shown in Figure 3, the cut-off voltage of this hybrid cell was controlled between 0 ~ 1.8 V with an average work voltage of 1.3 V. The specific capacity of the this hybrid cell was 200 mAh at current density of 1 C and decreased to 190 mAh at current density of 10 C. After 10000 charge-discharge cycles, the retention of capacity of the hybrid cell is 90%.

Example 2

A commercial spinel LiCoO ₂ was used as positive electrode. The else parts of this hybrid cell are same as in Example 1. The preparation of composite electrode and fabrication of hybrid cell is same as the process mentioned in Example 1. In this example, the capacities of positive electrode

material (LiCoO ₂ ) and negative electrode material (AC) are 100 mAh/g and 40 mAh/g respectively and the loads of electrodes are 3.4 mg/cm ² for positive electrode and 10 mg/cm for negative electrode. The cut-off voltage of this hybrid cell was controlled between 0 ~ 1.8 V with an average work voltage of 1.0 V. The specific capacity of the this hybrid cell was 190 mAh at current density of 1 C and decreased to 185 mAh at current density of 10 C. After 10000 charge-discharge cycles, the retention of capacity of the hybrid cell is 91%.

Example 3

A commercial spinel LiCo _1Z3 Ni _1Z3 Mn _1Z3 O ₂ was used as positive electrode. The else parts of this hybrid cell are same as in Example 1. The preparation of composite electrode and fabrication of hybrid cell is same as the process mentioned in Example 1. In this example, the capacities of positive electrode material (LiCo _1Z3 Ni _1Z3 Mn _1Z3 O ₂ ) and negative electrode material (AC) are 100 mAh/g and 40 mAh/g respectively and the loads of electrodes are 4 mg/cm ² for positive electrode and 10 mg/cm ² for negative electrode. The cut-off voltage of this hybrid cell was controlled between 0 ~ 1.8 V with an average work voltage of 1.0 V. The specific capacity of the this hybrid cell was 230 mAh at current density of 1 C and decreased to 210 mAh at current density of 10 C. After 10000 charge-discharge cycles, the retention of capacity of the hybrid cell is 92%.

Example 4

A commercial spinel LiMg ₀₂ Mn ₁ , ₈ O ₄ was used as positive electrode. The else parts of this hybrid cell are same as in Example 1. The preparation of composite electrode and fabrication of hybrid cell is same as the process mentioned in Example 1. In this example, the capacities of positive electrode material (LiMg _C2 Mn _L8 O ₄ ) and negative electrode material (AC) are 78 mAh/g and 40mAh/g respectively and the loads of and electrodes are 5.5 mg/cm ² for positive electrode and 10 mg/cm ² for negative electrode. The cut-off voltage of this hybrid cell was controlled between 0 ~ 1.8 V with an average work voltage of 1.3 V. The specific capacity of the this hybrid cell was 190 mAh at current density of 1 C and decreased to 185 mAh at current density of 10 C. After 10000 charge-discharge cycles, the retention of capacity of the hybrid cell is 91%.

Example 5

A commercial spinel LiMn ₂ O ₄ was used as positive electrode. A mixture of commercial AC and

LiV ₃ O ₈ (the mass ration of AC/LiV ₃ Os is 2/1) was used as negative electrode. The else parts of this hybrid cell are same as in Example 1. The preparation of composite electrode and fabrication of hybrid cell is same as the process mentioned in Example 1. In this example, the capacities of positive electrode material (LiMn ₂ O ₄ ) and negative electrode material (AC/LiV ₃ Os) are all 80 mAh/g and the loads of both electrodes are 10 mg/cm ² . The cut-off voltage of this hybrid cell was controlled between 0 ~ 1.8 V with an average work voltage of 1.2 V. The specific capacity of the this hybrid cell was 300 mAh at current density of 2 C and decreased to 250 mAh at current density of 10 C. After 10000 charge-discharge cycles, the retention of capacity of the hybrid cell is 80%.

Example 6

A commercial AC was used as positive electrode. The else parts of this hybrid cell are same as in Example 1. The preparation of composite electrode and fabrication of hybrid cell is same as the process mentioned in Example 1. In this example, the capacities of positive electrode material and negative electrode material are all 40 mAh/g and the loads of both electrodes are lOmg/cm ² . The cut-off voltage of this hybrid cell was controlled between 0 - 1.0V with an average work voltage of 0.5 V. The specific capacity of this hybrid cell was 100 mAh at current density of 1 C and kept at 100 mAh when the current density increases to 10 C. After 10000 charge-discharge cycles, the retention of capacity of the hybrid cell is 95%.

Table 1 Summary in the electrochemical properties of the hybrid aqueous lithium-ion b cylindrical battery/supercapacitor