Ionically conductive polymer electrolytes

Details Ionically conductive polymer electrolytes Ionically conductive polymer electrolytes

2002-07-02

2004-12-07

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S310000, C429S314000, C429S316000, C429S317000

Reexamination Certificate

active

06828065

ABSTRACT:

BACKGROUND OF THE INVENTION
A. Field
The present invention relates generally to a range of polymer electrolytes characterized by high ionic conductivity at room temperature and below, improved stability, and ability to be formed in very thin film configuration, for use in lithium ion batteries, and to methods of manufacturing lithium ion batteries comprising such polymer electrolytes.
B. Prior Art
A high energy density rechargeable battery system is currently a highly sought technology objective because of the proliferation of power-consuming portable electronics that demand increasingly greater energy levels, as well as more interest in practical electric-powered vehicles with significantly improved range presently unavailable from lead acid batteries. As a result, lithium rechargeable batteries are the focus of intense investigation around the world. Table I, below, describes the available rechargeable lithium systems which are either in commercial production or under development today. The lithium solid-state polymer electrolyte battery (system 3 in the Table) would be the ideal system for such high power-consumption applications owing to its true flexibility and energy density together with a capability of very high cycle life. However, in its present stage of development, this otherwise enviable system is not viable at temperatures below 60° C.
TABLE 1
Performance Characteristics of Lithium Rechargeable Batteries
Self-
Energy
Density
Voltage
Discharge
Cycle
System
Wh/kg
Wh/liter
(V)
(%/month)
Life
Electrolyte
1
Lithium Ion
100-120
260-280
3.6
10-12
500-800
Liquid
(Organic)
2
Lithium Ion
100-120
260-280
3.6
<8
2500
Solid-Liq.
Polymer
3
Lithium Polymer
250-300
350-400
3.6
<<1
>>1000
Solid
(Organic)
(Note: In Table 1, Wh/kg is specific energy (gravimetric); Wh/liter is energy density (volumetric).)
The lithium ion liquid electrolyte battery (system 1) is presently the only commercial chemistry described in Table 1. No generic lithium ion chemistry exists since each manufacturer has its own chemistry containing different positives, different negatives, binders, electrolyte and formation process. These are major factors influencing cycle life and the charge and discharge profiles. The most common lithium sink (i.e., place where the ion inserts) negative electrodes in a lithium ion battery are carbon-type insertion compounds, while layered metal oxides of the LiMO
2
type (where M═Ni or Co) or spinel lithium manganese oxides of the LiMn
2
O
4
type are currently used as preferred lithium source positive electrodes. These electrodes are usually calendared onto metallic current collectors (which are about 25 to 50 microns thick). The overall process of these batteries may be written as:
As indicated by the above cell reaction, charge and discharge proceed via intercalation of lithium ions into the carbon and metal oxide structure, respectively. Cell voltage at full charge is usually 4.2 volts while cell voltage on discharge is 2.6 volts.
A microporous polypropylene or polyethylene separator separates the two electrodes from shorting electrically, and liquid organic solvents containing a lithium salt as the electrolyte which is usually absorbed into the separator material and portions of the electrode provides high ionic conductivity (10
−3
to 10
−2
S/cm) and ease of migration of ions between the electrodes of the cell. These batteries are commonly used in portable computers, cellular telephones and camcorders, among other applications. The specific energy and energy density of the lithium ion battery is usually about 125 Wh/kg and 260 Wh/l, respectively. The packaged battery, usually in a hard plastic case, has a much lower energy density than the individual cell (approximately 20% lower). The cycle life (i.e., the number of times the battery can be recharged) of this battery is about 500 to 800 cycles, the self-discharge (i.e., loss of capacity on standing) per month is about 10%, and the cost is currently about $1.00 per Watt-hour of energy. These batteries can be manufactured in near fully automated, high volume production. Although lithium ion battery technology is being commercialized very heavily, numerous safety issues have arisen. For example, cells that are abused under crush test or high temperature test have been known to explode and ignite.
To overcome the disadvantages inherent in liquid electrolytes, and to obtain superior long-tem storage stability, an interest has arisen in solid polymeric electrolytes in which ion mobility is achieved through coordination by sites on the polymer chain of electrolyte ions, thus promoting electrolyte dissolution and salt dissociation. An all-solid-state battery using an ionically conductive polymer membrane as the electrolyte would have several attractive features. It could be produced in virtually any shape and size, in thin film power cells or thick film energy cells, by automated fabrication techniques, as well as made reasonably rugged and leakproof, with low self-discharge, and have high open-circuit potential using a lithium metal anode. Such features represent a unique combination of properties and give rise to the possibility of using them as either secondary or primary devices across a wide range of applications.
Polyethylene oxide (PEO), a polymer examined extensively for the present application, is able to form stable complexes with a number of salts. Because of its low ionic conductivity of about 10
−9
to 10
−8
S/cm at ambient temperature, batteries using this material were found to require being operated at a temperature of 100° C. or higher. A major problem observed with PEO-based electrolytes at temperatures below 60° C. is their high crystallinity and associated low ion mobility. The crystalline structure of many polymers, including PEO, results in a weaker structure. In recent years, many radically different approaches have been taken to improve the conductivity of PEO and PEO-based polymers, which have also led to the proposal of other polymers for this purpose. These approaches included modification of existing polymers, synthesis of new polymers, forming composite polymers with ceramic materials, using plasticizer salts to increase ion transport and mobility of the cation, using plasticizing solvents in the polymer again to increase the ionic character of the cation, and other approaches. Several review articles describe these approaches in detail, e.g., “Technology Assessment of Lithium Polymer Electrolyte Secondary Batteries,” by M. Z. A. Munshi, Chapter 19 in Handbook of Solid State Batteries and Capacitors, Ed. M. Z. A. Munshi (World Scientific Pub. Singapore), 1995, and A. Hooper, M. Gauthier, and A. Belanger, in: “Electrochemical Science and Technology of Polymers—2, ” Ed. R. G. Linford (Elsevier Applied Science, London), 1987.
These approaches have not resulted in adequate conductivity enhancements on the polymer electrolytes desired for battery operation at room temperature. As a result, another approach has been taken in which plasticizing solvents or low molecular weight polymers are added to the polymer electrolyte to increase ionic conductivity of the PEO-based polymer electrolyte. The purpose of the latter is to increase the ionic mobility and concentrations of the charge carriers in the solid polymer electrolyte by enhancing the dissociation of the lithium salt. It is believed that the lithium ion is also solvated to the solvent molecule and participates in enhancing the ionic mobility. Many electrolyte composites incorporating low molecular weight polymers or liquid organic solvents have been prepared and have demonstrated high room temperature conductivity approaching those of the typical non-aqueous liquid electrolytes.
For example, Kelly et al, in J. Power Sources, Vol. 14, page 13 (1985) disclosed that adding 20 mole percent of liquid polyethylene glycol dimethyl ether polymer (PEGDME) to solid PEO polymer results in an increase in the ionic conductivity of the final plasticized polymer from 3×10
−7
S/cm to 10
−4
S/cm at 40

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