Electrolysis: processes – compositions used therein – and methods – Utilizing electrolysis to form battery electrode active... – Group ia metal-containing active material
Reexamination Certificate
2000-03-27
2003-09-02
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Utilizing electrolysis to form battery electrode active...
Group ia metal-containing active material
C205S704000, C029S623100
Reexamination Certificate
active
06613213
ABSTRACT:
BACKGROUND OF INVENTION
The invention relates to a method for producing an electrode. More specifically, the invention relates to a method for producing an electrode using microscale or nanoscale materials obtained from hydrogen driven metallurgical reactions.
The miniaturization of electronic components has created widespread growth in the use of portable electronic devices such as cellular phones, pagers, video cameras, facsimile machines, portable stereophonic equipment, personal organizers and personal computers. The growing use of portable electronic equipment has created ever increasing demand for improved power sources that are safe, long-lasting, and are high energy density rechargeable batteries.
There is also an ongoing investigation into the manufacture of batteries suitable for use in electric motor vehicles or hybrid motor vehicles that can operate on electric and combustion power.
A battery or voltaic cell generally includes two chemicals or elements with differing electron-attracting capabilities that are immersed in an electrolytic solution and connected to one another through an external circuit. These two chemicals can be referred to as an electrochemical couple. The reaction that occurs between an electrochemical couple and a voltaic cell is a reduction-oxidization (redox) reaction.
The mechanism by which a battery generates an electric current typically involves the arrangement of chemicals in such a manner that electrons are released from one part of the battery via said redox reaction and made to flow through an external circuit or cell connection to another part of the battery. The element of the battery at which the electrons are released to the circuit is called the anode. During discharge, oxidation reactions occur at the anode. The element that receives the electrons from the circuit is known as the cathode, or the positive electrode. During discharge, reduction reactions occur at the cathode.
At rest, a voltaic cell exhibits a potential difference (voltage) between its two electrodes that is determined by the maximum amount of chemical energy available when an electron is transferred from one electrode to the other. The current that flows from the cell is determined by the resistance of the total circuit, including that of the cell itself. Further, a voltaic cell has a limited energy content, or capacity, that is generally given in ampere-hours and determined by the quantity of electrons that can be released at the anode and accepted at the cathode. When all of the chemical energy of the cell has been consumed (usually because the anode has been completely discharged) the operating voltage falls to zero and will not recover unless the battery can be recharged. The capacity of the cell is determined by the quantity of active ingredients in the electrode.
Presently, the most widely used rechargeable batteries are secondary batteries employing aqueous electrolytes, such as nickel/cadmium and nickel metal-hydride batteries. The half-cell reactions taking place in a nickel/metal-hydride cell may be written as follows:
Anode
MH
x
+xOH
M+xH
2
O+xe
−
[1]
Cathode
Ni(OOH)+H
2
O+e
−
Ni(OH)
2
+OH
−
[2]
It is in effect a rocking chair type electrochemical cell in which hydrogen is transferred from one electrode to the other.
Nickel/metal-hydride cells have similar operating characteristics to nickel/cadmium cells, but the nickel/metal-hydride cells use a metal-hydride anode in place of cadmium.
At the anode of the nickel/metal-hydride cell, a reversible electrode oxidation reaction occurs with OH
−
ions at the surface of the electrode. When the battery is charged, a corresponding reduction reaction occurs at the surface of the electrode in which hydrogen is absorbed into the metal producing a solid metal hydride and a hydroxide ion. The metal expands when absorbing the hydrogen and shrinks when releasing the hydrogen. The increase in volume during the hydriding reaction is a consequence of the volume of the absorbed hydrogen atoms.
The atomic volume of hydrogen in a metal, V
H
, is defined as the increase in the volume of the unit cell of the metal upon the insertion of one hydrogen atom. The expansion of the metal due to the absorption of hydrogen has been directly correlated to electrode corrosion. See Willems J. J. G. and Buschow K. H. J.,
J. Less-Common Metals,
129:13(1987).
There is also a corresponding contraction when hydrogen is removed. The anode is therefore subjected to volumetrically induced strains during charging and discharging cycles. This imposes great mechanical stress on the alloy which, consequently, breaks down into small particles. Furthermore, a large volume change in each charge and discharge cycle increases the flushing action of the electrolyte through the pores and micro-cracks of the electrode, thereby increasing the corrosion rate.
Lithium batteries have also been investigated vigorously as a battery that can ensure a high-energy density. Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells typically include an anode of metallic lithium, a cathode, typically LiMeO
2
(Me═Co, Ni or Mn), and an electrolyte interposed between separated positive and negative electrodes. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically non-aqueous (aprotic) organic solvents.
By convention, the lithium electrode of the cell is defined as the anode and the counter electrode is referred to as the cathode. During use of the cell, lithium ions (Li
+
) are transferred to the “anode” on charging (in reality the Li electrode is acting as a cathode in the charging step). During discharge, lithium ions (Li
+
) are transferred from the Li (anode) to the now positive electrode (cathode). Upon subsequent charge and discharge, the lithium ions are transported between the electrodes. Cells having metallic lithium anode and complex metal oxide cathode are assembled in the charged state.
During discharge, lithium ions from the metallic anode are transported through the liquid electrolyte to the cathode. During charging, the flow of lithium ions is reversed and they are transferred from the “cathode” material through the electrolyte to the lithium “anode”.
However, Li metal anodes have a serious disadvantage as they undergo undesirable morphological changes which not only affect cell performance, but can also constitute a severe safety hazard. The substitution of lithium alloys for metallic Li does not improve the situation as they are subject to severe strain due to the charging and discharging process. This is due to the large volume changes the bulk alloy undergoes when Li is inserted or removed. For example, in the case of tin, the expansion can be as high as 300% upon Li insertion. See K. D. Kepler et al.
Electrochemical and Solid State Letters,
2(7): 307 (1999). Of course, there is a corresponding contraction upon the discharge of Li from the alloy. The stress incurred in charge and discharge of Li rapidly fractures the alloy into smaller particles. As this occurs the particles lose contact with one another and become electrically isolated and inactive. This is believed to be the source of the poor reversibility of such electrodes, which is reflected by rapid capacity decay.
Therefore, there is a need for electrochemical cells, such as lithium, nickel/metal hydride, etc., with electrodes that can better withstand such volumetrically induced strains.
In an effort to avoid the problems associated with metal anodes, the Sony Corporation introduced a lithium ion battery in approximately 1991 where the lithium metal anode was replaced by an anode made of a carbonaceous material. In this battery, lithium metal need not be present at any time. Lithium ions are transported back and forth between a cathode and a carbon intercalation anode. While cycle life and safety was considerably improved relative to Li metal anodes, a substantial penalty was incurred in battery capacit
Adzic Gordana D.
Johnson John R.
McBreen James
Reilly James J.
Vogt Thomas
Bogosian Margaret C.
Brookhaven Science Associates LLC
Phasge Arun S,.
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