Lithium batteries (batteries containing metallic lithium as the negative electrode active material) are becoming increasingly popular as portable power sources for electronic devices that have high power operating requirements. Common consumer lithium batteries include lithium/manganese dioxide (Li/MnO2) and lithium/iron disulfide (Li/FeS2) batteries, which have nominal voltages of 3.0 and 1.5 volts per cell, respectively.
Battery manufacturers are continually striving to design batteries with more discharge capacity. This can be accomplished by minimizing the volume in the cell taken up by the housing, including the seal and the vent, thereby maximizing the internal volume available for active materials. However, there will always be practical limitations on the maximum internal volume.
Another approach is to modify the internal cell design and materials to increase the discharge capacity. How to best accomplish this can depend at least in part on the discharge requirements of the devices to be powered by the batteries. For devices with low power requirements, the quantity of active materials tends to be very important, while for devices with high power requirements, discharge efficiencies tend to be more important. Lithium batteries are often used in high power devices, since they are capable of excellent discharge efficiencies on high power discharge.
In general, battery discharge efficiency decreases rapidly with increasing discharge power. Therefore, for high power providing high discharge efficiency is a priority. This often means using designs containing less active materials, thus sacrificing capacity on low power and low rate discharge. For example, for good high power discharge efficiency, high interfacial surface area between the negative electrode (anode) and positive electrode (cathode) relative to the volume of the electrodes is desirable. This is often accomplished by using a spirally wound electrode assembly, in which relatively long, thin electrode strips are wound together in a coil. Unless the electrode compositions have a high electrical conductivity, such long, thin electrodes typically require a current collector extending along much of the length and width of the electrode strip. The high interfacial surface area of the electrodes also means that more separator material is needed to electrically insulate the positive and negative electrodes from each other. Because the maximum external dimensions are often set for the cells, either by industry standards or the size and shape of the battery compartments in equipment, increasing the electrode interfacial surface area also means having to reduce the amount of active electrode materials that can be used.
For batteries that are intended for both high and low power use, reducing cell active material inputs in order to maximize high power performance is less desirable than for batteries intended for only high power use. For example, AA size 1.5 volt Li/FeS2 (FR6 size) batteries are intended for use in high power applications such as photoflash and digital still camera as well as general replacements for AA size 1.5 volt alkaline Zn/MnO2 batteries, which are often used in lower power devices. In such situations it is important to maximize both high power discharge efficiency and cell input capacity. While it is generally desirable to maximize the electrode input capacity in any cell, the relative importance of doing so is greater in cells for lower power usage.
To maximize the active material inputs in the cell and mitigate the effects thereon of increasing the electrode interfacial surface area, it is desirable to use separator materials that take up as little internal volume in the cell as possible. There are practical limitations to doing so. The separator must be able to withstand the cell manufacturing processes without damage, provide adequate electrical insulation and ion transport between the anode and cathode and do so without developing defects resulting in internal short circuits between the anode and cathode when the cell is subjected to both normal and anticipated abnormal conditions of handling, transportation, storage and use.
Separator properties can be modified in a number of ways to improve the strength and resistance to damage. Examples are disclosed in U.S. Pat. Nos. 5,952,120; 6,368,742; 5,667,911 and 6,602,593. However, changes made to increase strength can also adversely affect separator performance, based in part on factors such as cell chemistry, electrode design and features, cell manufacturing process, intended cell use, anticipated storage and use conditions, etc.
For certain cell chemistries maximizing the amounts of active materials in the cell can be more difficult. In lithium batteries, when the active cathode material reacts with the lithium to produce reaction products having a total volume greater than that of the reactants, swelling of the electrode assembly creates additional forces in the cell. These forces can cause bulging of the cell housing and short circuits through the separator. Possible solutions to these problems include using strong (often thicker) materials for the cell housing and inert components within the cell, further limiting the internal volume available for active materials in cells with such active materials compared to cells with lower volume reaction products. For Li/FeS2 cells another possible solution, disclosed in U.S. Pat. No. 4,379,815, is to balance cathode expansion and anode contraction by mixing another active material with the FeS2. Such active cathode materials include CuO, Bi2O3, Pb2Bi2O5, P3O4, CoS2 and mixtures thereof. However, adding other active materials to the cathode mixture can affect the electrical and discharge characteristics of the cell.
Just as battery manufacturers are continually trying to improve discharge capacity, they are also continually working to improve other battery characteristics, such as safety and reliability; making cells more resistant to internal short circuits can contribute to both. As is clear from the above discussion, changes made to improve resistance to internal short circuits can be counterproductive in maximizing discharge capacity.
The pyrite or iron disulfide (FeS2) particles utilized in electrochemical cell cathodes are typically derived from natural ore which is crushed, heat treated, and dry milled to a particle size of 20 to 30 microns. The fineness of the grind is limited by the reactivity of the particles with air and moisture. As the particle size is reduced, the surface area thereof is increased and is weathered. Weathering is an oxidation process in which the iron disulfide reacts with moisture and air to form iron sulfates. The weathering process results in an increase in acidity and a reduction in electrochemical activity. Small pyrite particles can generate sufficient heat during oxidation to cause hazardous fires within the processing operation. Prior art iron disulfide particles utilized can have particles sizes which approach the final cathode coating thickness of about 80 microns due to the inconsistencies of the dry milling process.
The dry milling process of iron disulfide is typically performed by a mining company or an intermediate wherein large quantities of material are produced. The processed iron disulfide is shipped and generally stored for extended periods of time before it can be used by the battery industry. Thus, during the storage period, the above-noted oxidation and weathering occur and the material degrades. Moreover, the large iron disulfide particles sizes can impact processes such as calendering, causing substrate distortion, coating to substrate bond disruption, as well as failures from separator damage.