This invention is related to electrochemical cells, more particularly, to rechargeable lithium ion electrochemical cells.
Rechargeable lithium batteries are often utilized in applications where high energy density is a requirement. A lithium battery may contain one lithium electrochemical cell, but more commonly it consists of several lithium electrochemical cells in series or parallel, or a combination of such connections.
Lithium electrochemical cells are frequently packaged in cylindrical containers, or are button shaped, or laminar, sometimes referred to as thin profiled cells, packaged and sealed in multi-layered polymer laminates.
The electro-active particles of the negative electrode of a lithium electrochemical cell, are usually but not necessarily, graphite particles or carbonaceous particles of similar nature, which are capable of reversibly intercalating lithium ions. Other particulate substances which are capable of reversibly intercalating lithium, can also be utilized as negative-active particles. Lithium metal or lithium alloy, subject to certain conditions, may also be used as negative electrode material. The most commonly used electro-active particles in the positive electrode of lithium batteries are particles of lithiated transition metal oxides and sulphides, however, any other similar substance capable of reversibly intercalating lithium in its structure can be used. The electrolyte of a lithium cell is a non-aqueous liquid or a polymer containing mobile or dissociable lithium ions, or it can be a lithium salt containing glassy material, which is liquid at the temperature of operation of the lithium ion cell. The electrolyte of the cell is conductive for lithium ions but is an insulator with respect to electrons. The electrodes of a lithium electrochemical cell are usually separated from one another by some form of a separator. Negative and positive current collectors located adjacent the appropriate electrodes, provide electrical leads for charging and discharging the lithium ion electrochemical cell.
The electrolyte of a lithium electrochemical cell or battery, for obvious reasons, has an important role in the working of the cell, thus there are many known types of electrolytes utilized in lithium batteries. The electrolyte may be a non-aqueous, organic liquid having a lithium salt dissolved therein. The advantage of a liquid electrolyte is that the mobility of lithium ions is usually higher in a liquid than in a solid, however, the organic liquid may be lost by seepage if the container is punctured or damaged. The organic liquid is frequently an organic carbonate, or mixtures of such, but there are many other known organic compounds which have the required properties. Another frequently utilized form of electrolyte is a solid polymer layer bearing dissociable lithium compounds, such as for example, described in U.S. Pat. No. 5,436,091, issued to Shackle et al. on Jul. 25, 1995. It is noted that such polymer electrolyte layers frequently play the role of an electrode separator as well. In yet another form of electrolyte an inert porous polymer and a curable or polymerizable absorbent gelling compound are combined, and the combination is impregnated with an organic liquid containing a lithium salt, either before or after polymerization of the absorbent gelling compound. Such an electrolyte system is described, for example, in U.S. Pat. No. 5,681,357, issued to Eschbach et al. on Oct. 28, 1997.
It is also known to have an electrode paste comprising electro-active particles mixed with polymer electrolyte precursors and a lithium compound, which is subsequently fully polymerised to form an ionically conductive electrode layer. The electrode paste may additionally contain an ion conducting binder, such as for instance, a fluoropolymer. In the case of preparing a positive electrode or cathode layer containing cathode particles, in addition to polymerised electrolyte particles bearing a lithium compound and binders, fine carbon can also be added for electrical conduction. Fauteux et al. in U.S. Pat. No. 4,925,752, issued on May 15, 1990, teach a cathode paste made of a mixture of particles of vanadium oxide, polyethylene oxide, fine carbon, a lithium salt, propylene carbonate and a radiation curable acrylate. The cathode paste may be overlain with a curable, lithium salt containing electrolyte layer which also separates the cathode layer electrically from the negative electrode or anode, and the layers bearing polyethylene oxide and radiation curable acrylate are polymerized to form a tightly adherent layered cell assembly.
There are known rechargeable lithium cells in which the electrolyte particles are mixed with the electro-active particles and an ion conducting binder to form an electrode-electrolyte mixture which is then separated by means of a porous separator from the other electrode, such that electronic short circuiting between the electrodes is avoided but without hindering the passage of lithium ions between the electrodes.
Other known lithium ion electrochemical cell structures can be prepared by forming a negative electrode or anode slurry composed of carbonaceous electro-active particles, an ion conducting fluoropolymer dispersed in a low boiling point solvent and dibutyl phthalate as plasticizer, a positive electrode or cathode slurry composed of lithium ion bearing positive-active particles, an ion conducting fluoropolymer dispersed in a low boiling point solvent, dibutyl phthalate and electroconducting carbon particles, and an electrolyte slurry made of an ion conducting fluoropolymer dispersed in a low boiling point solvent and dibutyl phthalate, each cell component forming a separate entity. An example of this method is described in U.S. Pat. No. 5,756,230, issued to Feng Gao et al. on May 26, 1998, the obtained slurries are spread to form layers, the low boiling point solvent allowed to evaporate, the dibutyl phthalate is then extracted thereby leaving porous polymeric web structures which are subsequently assembled into lithium electrochemical cell precursors. Another example of such method is U.S. Pat. No. 5,571,634, issued to Gozdz et al. on Nov. 5, 1996, wherein the separator is comprising a PVDF copolymer and an organic plasticizer, each electrode is composed of appropriate electro-active particles dispersed in a PVDF copolymer matrix, and the each layer, namely, the negative electrode layer, the positive electrode layer and the electrolyte layer forms a flexible self-supporting cell element. It is noted that neither the ion conducting matrix carrying the electro-active particles, nor the separator element, contain any lithium bearing compound at the time of assembling the lithium cell precursor layers. Furthermore, the ion conducting matrix comprised in the electrodes is in the form of a laminate in which the ion conducting particles are randomly distributed, without any specific structural form, such as filaments.
It is noted that one of the conventional electrolyte systems utilized in rechargeable laminar lithium batteries is a combination of a solid, lithium ion conducting polymer electrolyte layer with an organic liquid solution having a dissolved lithium salt therein. The lithium compound in the solid polymer is usually but not necessarily, the same lithium compound that is dissolved in the organic solution.
It can be seen that in all the above discussed lithium electrochemical cells the role of the electrolyte is to allow dissociable lithium ions of various nature to be available for electrolytic movement and conduction in the proximity of the electro-active particles. Such objectives are frequently achieved by cell component layers being relatively tightly packed together. It is, however, known that the thickness of cathode and anode layers may change during cycling the cell through charging and discharging steps. Furthermore, the layers may also delaminate in small areas for different reasons. It is thus desirable to provide some indigenous elasticity between the electro-active and electrolyte particles and layers, and at the same time maintain good contact within and between the electrode layers.
A new electrode having a mixture of electrode-electrolyte particles, for rechargeable lithium batteries has been found, comprising electro-active particles intermixed with pliable, solid, ion conducting, polymer filaments containing a first dissociable lithium compound, said solid polymer electrolyte filaments having adhesive surfaces, thus forming a matted layer of interlinked filaments and adhering particles held together, and also having voids between the interlinked pliable filaments, which are subsequently impregnated with a non-aqueous solution containing a second dissociable lithium compound. The resulting mixture in the shape of a matted layer is placed between a current collector and an inert, porous separator having preferably, a multiplicity of polymer layers.
The rechargeable lithium electrochemical cell in one embodiment has a negative electrode comprising negative active particles intermixed with pliable, solid, ion conducting, polymer filaments containing a first dissociable lithium compound, said pliable, solid, polymer filaments having adhesive surfaces, said negative active particles adhering to the surface of the pliable, solid, ion conducting polymer filaments, which thereby form a matted layer of interlinked filaments and adhering negative active particles, having voids, and the matted layer is subsequently impregnated with a non-aqueous solution containing a second dissociable lithium compound. The matted layer comprising a first particulate mixture of interlinked, pliable, solid, ion conducting polymer filaments and adhering negative active particles, impregnated with a non-aqueous, lithium compound containing solution, is placed between the negative electrode current collector and one face of a multi-layered inert, porous polymer separator. A positive electrode having a matted layer comprising a second mixture of interlinked, pliable, solid, ion conducting, polymer filaments containing a first lithium compound and having adherent surfaces, and particles further comprising positive active particles and fine, electron conducting carbon, is placed between the other face of the multi-layered inert porous separator and a positive electrode current collector. The voids in the matted layer of the positive electrode are also impregnated with the second lithium compound containing non-aqueous solution.
In the rechargeable lithium battery of another embodiment of the invention, the multi-layered, inert, porous, polymer separator between the matted layers of the negative and positive electrodes, is coated on at least one of its faces with a porous layer of a solid polymer containing a first dissociable lithium compound, and the ion conducting polymer is partially filling the pores of said multi-layered, inert porous polymer separator. The ion conducting polymer coating the multi-layered, inert, porous separator, and partially filling the pores of the separator, has the same composition as the interlinked, pliable, ion conducting, solid polymer filaments having adhesive surfaces, comprised in the matted layers of the positive and negative electrodes of the rechargeable lithium electrochemical cell.