Electrochemical cells are presently the preferred method of providing cost effective portable power for a wide variety of consumer devices. The consumer device market dictates that only a handful of standardized cell sizes (e.g., AA or AAA) and specific nominal voltages (typically 1.5 V) be provided. Moreover, more and more consumer electronic devices, such as digital still cameras, are being designed with relatively high power operating requirements. As has been the practice within the market, consumers often prefer and opt to use primary batteries for their convenience, reliability, sustained shelf life and more economical per unit price as compared to currently available rechargeable (i.e., secondary) batteries.
Within this context, it is readily apparent that design choices for primary (i.e., non-rechargeable) battery manufacturers are extremely limited. For example, the necessity of using specified nominal voltages significantly limits the selection of potential electrochemical materials, and the use of standardized cell sizes restricts the overall available internal volume available for active materials, safety devices and other elements typically expected in such consumer products. What's more, the variety of consumer devices and the range of operating voltages for those devices make smaller nominal voltage cells (which can be provided separately or in series, thereby giving device makers more design options) more versatile as compared to higher voltage electrochemical pairings typically associated with secondary batteries. Thus, 1.5 V systems, such as alkaline or lithium-iron disulfide systems, are far more prominent than others, such as 3.0 V and higher lithium-manganese dioxide.
Within the realm of 1.5 V systems, lithium-iron disulfide batteries (also referred to as LiFeS2, lithium pyrite or lithium iron pyrite) offer higher energy density, especially at high drain rates, as compared to alkaline, carbon zinc or other systems. However, current regulatory limitations on the amount of lithium (the preferred electrochemically active material in the anode) make the FR03 (AAA LiFeS2 cells) and FR6 (AA LiFeS2 cells) sizes the most significant cell sizes for this chemistry within the consumer market.
The design considerations for 1.5V electrochemical systems (e.g., alkaline v. lithium-iron disulfide, etc.) are significantly different. For example, alkaline and nickel oxy-hydroxide systems rely on an aqueous and highly caustic electrolyte that has a propensity for gas expansion and/or leakage, leading to very different approaches in terms of selection of internal materials and/or compatibility with containers and closures. In rechargeable 1.5 V systems (note that lithium-iron disulfide systems are not currently considered suitable for consumer-based rechargeable systems), various highly specialized electrochemical and/or electrolyte compositions may be used to best accommodate lithium ion charge/discharge cycling. Here, such high cost components are not a key design concern because secondary systems typically sell for a higher retail price than their primary battery equivalents. Moreover, the discharge mechanisms, cell designs and safety considerations are, by and large, inconsequential and/or inapplicable to primary systems.
Even with the inherent advantages of lithium-iron disulfide cells for high power devices (as compared to primary alkaline cells), LiFeS2 cell designs must strike a balance between the cost of materials used, the incorporation of necessary safety devices and the overall reliability, delivered capacity and intended use of the designed cell. Normally, low power designs emphasize the quantity of active materials, while high power designs focus more on configurations to enhance discharge efficiency. For example, a jellyroll design maximizes the surface area between the electrodes and allows for greater discharge efficiencies, but in doing so, might sacrifice capacity on low power and low rate discharges because it uses more inactive materials, such as separator and current collector(s) (both which occupy internal volume, thereby requiring removal of active materials from the cell design).
In addition to improved capacity, cell designers must also consider other important characteristics, such as safety and reliability. Safety devices normally include venting mechanisms and thermally activated “shutdown” elements, such as positive thermal circuits (PTCs). Improvements to reliability primarily focus on preventing internal short circuits. In both instances, these characteristics ultimately require elements that occupy internal volume and/or design considerations that are usually counterproductive to cell internal resistance, efficiency and discharge capacity. Moreover, there are additional challenges because transportation regulations limit the percent amount of weight lithium batteries can lose during thermal cycling, meaning that cell designs for smaller container sizes like AA and AAA can only lose milligrams of total cell weight (usually by way of evaporation of the electrolyte). Plus, the reactive and volatile nature of the non-aqueous, organic electrolyte severely limits the universe of potential materials available (particularly with respect to interactions between the electrolyte and cell closure, separator and/or current collector(s) provided within the cell) as compared to other electrochemical systems.
Ultimately, maximizing the amounts of active materials in lithium-iron disulfide batteries while maintaining optimal properties, particularly with respect to the cathode, may be the most difficult challenge. As noted above, the jellyroll electrode assembly is the preferred configuration in LiFeS2 systems. In order to accommodate iron disulfide most effectively, the iron disulfide is mixed into slurry with conductors and binders and then coated onto a metallic foil current collector, while the lithium is most effectively provided without a current collector. Lastly, the separator is a thin polymeric membrane whose thickness is preferably minimized to reduce the inactive inputs into the cell.
Because the reaction end products occupy more volume than the inputs, the electrode assembly swells as the battery discharges. In turn, swelling creates radial forces that can cause unwanted bulging of the cell container, as well as short circuits if the separator is compromised. Previous means of handling these problems include using strong (often thicker) materials for the cell housing and inactive components within the cell. However, thicker inactive materials limit the internal volume available and thicker, more rugged electrodes were previously deemed not necessarily desirable because they allow for fewer winds possible in the jellyroll, resulting in less surface area between the electrodes and the expectation of comparatively lower performance at higher drain rates.
A number of other approaches have been taken to strike an appropriate balance between optimal internal volume utilization and acceptable LiFeS2 cell capacity/performance. For example, a possible solution for problems created by swelling, disclosed in U.S. Pat. No. 4,379,815, is to balance cathode expansion and anode contraction by mixing one or more other active materials (such as CuO, Bi2O3, Pb2Bi2O5, P3O4, CoS2) with pyrite, although these additional materials can negatively affect the discharge characteristics of the cell, and the capacity and efficiency of the overall cell may also suffer.
Other means of improving discharge capacity in LiFeS2 cell contemplate the use of thinner separators and/or specific cathode coating mixes and coating techniques, as disclosed in U.S. Patent Publication Nos. 20050112462 and 20050233214.
U.S. Pat. Nos. 6,849,360 and 7,157,185 discloses the use of a specific cathode coating formulation and an anode provided as pure lithium (or a lithium-aluminum alloy) to obviate the need for an anode current collector. The amount of anode and cathode are then provided at specified ratio of anode to cathode interfacial active materials (i.e., the theoretical interfacial input capacity ratio) in order to enhance LiFeS2 cell high rate performance.
U.S. Patent Publication Nos. 20090074953, 20090070989 and 20080050654 and Chinese Patent Application No. 200410026754.0 all disclose cathodes that may be pertinent to a LiFeS2 cell.