Patent Number 6,936,382 B2
Inventors: Mikhaylik, Skotheim and Trofimov
Assignee: Moltech Corporation
Summarized by Donald Georgi
Significant media commentary chastises the battery industry for not keeping pace with the continual size reduction of semiconductors, exemplified by Moore’s Law. In April 1965, Gordon Moore did not presented his statement as a ‘law’ but only projected that chip density would double annually. And he did not imply that it would happen forever. Despite newer electronic innovations which have extended the performance of electronics such as adjustable processor clocking techniques, battery energy has not grown to provide 12 hours of continual laptop movies on long flights or two week talk time on cellphones.
Those who do not deal with electrochemistry have an easy time with this chastisement, but in reality, technical people working all over the world understand the challenges. Increasing energy density involves much more than shrinking the real estate of electrodes and electrolytes, enhancing the ratio of active battery materials to ancillary packaging. There are fundamental laws of physics which are explained in electrochemistry as upper bounds, first as the confines of theoretical energy density and then something less in the real world - practical energy density. It boils down to obeying the immutable facts of physics. Electrochemists fine tune the chemistries and packages to move practical energy density to higher levels, but eventually, the maximum theoretical ‘roof’ cannot be punched through.
To look for greater battery performance in the future, one approach is to find anode/cathode materials which have higher theoretical energy densities and find ways to make practical batteries from them. The characteristics of the materials limit performance. Sodium-sulfur has a high 792 Wh/kg specific energy but must operate at 300-350 0C. Nuclear batteries operate for decades, but they have the radiation problems. Researchers constantly try to find better, cheaper and safer battery materials in novel combinations.
In the 1980s, Dr. Terje Skotheim and his group at the Brookhaven National Laboratory identified conductive polymers which provided the basis for sulfur-based cathode materials for rechargeable lithium batteries. In 1988, he formed Moltech Corporation to pursue development of these batteries. The fundamental reason for pursuing Lithium-sulfur is its significantly greater theoretical energy density than that of Lithium-ion. The quest is not without difficulty witnessed by the ensuing 18 years of R & D which have not yet produced a commercial cell which lives up to its performance expectations, can be competitively produced and is intrinsically safe.
In the meantime, while pursuing the Lithium-sulfur battery, Moltech purchased the rechargeable battery business from Energizer, resold the Nickel-cadmium and Nickel-metal hydride operations to the Chinese and shut down the Lithium-ion operations in Florida. In 2002, the Lithium-sulfur operations were transferred to the new organization Sion Power Corporation. Moltech still exists, owns the patents and the Lithium-ion plant facilities, but focuses attention on Sion to succeed in bringing the battery technology to market. As the technology has progressed, Sion batteries have been able to demonstrate powering a tablet notebook for a whole day and provide the power source for a state-of-the-art unmanned aerial vehicle.
First, the significantly higher theoretical energy density makes the pursuit worthwhile. Add to that an ability to provide high currents in excess of 3C and operation over a wide temperature range from -600 C to +650 C. The cell operates at room temperature and self discharge is less than 15 %/month. Initial cycle life data on small cells showed good cycle life to over 400 charge/discharge cycles. Cell Voltage ranges from an initial 2.4 Volts to 2.0 Volts in the 25-100% discharge range. The unique profile suggests straightforward ways to provide battery fuel gauging. The electrolyte and cathode system eliminated the formation of lithium dendrites which are a safety concern in Lithium-ion cells. If subjected to over discharge, characteristics of the electrolyte increase cell impedance significantly, reducing damage and signalling the over discharge state. Normal operation returns after the over discharge condition is removed. If over charged, the cell inherently creates a chemical shuttle shunting over charge currents, allowing charge rates to 3C without electronic controls. In the construction, no heavy metals are used, making disposal less complicated than batteries with elements such as lead or cadmium.
Without limitations, the above advantages appear to make Lithium-sulfur the hands down favorite to power our next generation of portable equipment. Unfortunately, the past 18 years of development time signals a difficult challenge. One of those challenges is in the lithium metal in the anode. A concentrated quantity of lithium inherently represents a problem if the cell is damaged or if a thermal runaway occurs. As batteries become popular for hybrid transportation, the ability to survive crashes and object penetrations must be part of the batteries’ inherent safety.
A second component of the challenge has been to extract as much of the theoretical capacity as possible. In the electrochemistry, a complex number of states of sulfur polymers are created. Over the years, Sion researchers have unfolded the relationships and by 2004 had increased sulfur utilization from 46% to over 90 %. Sulfur which had been providing only 800 mAh produced 1500 mAh with the improvement. At the present level of understanding, there may be enough capacity to field a commercial cell.
Interactivity is another challenge in the quest for new battery technology. With early Lithium-sulfur cells in the 2001-2003 time frame, cycle life was improved from 100 to 450 cycles. Then as cell size was increased, cycle life fell off, requiring new understanding to add high cycle life to the bigger cells.
A challenge not peculiar to battery technology is moving from development to production. Sion/Moltech has made great efforts in structuring the development program to pave the way for both cost effective and reliable construction. In developing the mechanical properties of the anodes and cathodes, comparable tensile strength has been created so that standard lithium-ion winders can be used.
The ability to provide less complex charging and fuel gauging is projected to be another advantage of the chemistry, but again, the full implementation is required to add the full seal of approval to Lithium-sulfur.
As understanding has led to improvements, Moltech has obtained patents so that now it has a portfolio of over 50 patents which relate to everything from electrolytes to stabilized anodes to lithium batteries. These patents probably tell the story of the progression over the years. Describing all would be the basis for a technical book.As an overview, BD has chosen to look at Moltech’s 50th patent which describes the full battery rather than component improvements .
Patent Number 6,936,382 B2: Lithium Batteries
This patent is an continuation of an earlier patent number 6,569,573 B1 with the same name and having most of the materials repeated from this earlier patent. Of the earlier patents’ 24 claims, the latest concentrates only on 16 which build on mediator improvements.
The new abstract leaves out the details of the earlier patent which described the chemical form of a mediator used in the electrolyte. The scope includes a lithium anode with a sulfur containing cathode and appropriate electrolytes.
The background highlights earlier work on a primary battery with a sulfur loaded cathode and another battery by Yamin using a porous carbon cathode impregnated with sulfur. It is noted that sulfur utilization is only 50%. Lithium-thyonyl chloride primary batteries are catalyzed with halide additives; BrCl additives and iodine increase output Voltage and capacity. The present invention adds safer, higher energy and environmentally compatible technology to both primary and secondary batteries.
The primary or secondary battery has a solid lithium anode, a sulfur containing cathode and nonaqueous electrolyte with lithium salts, nonaqueous solvents and capacity enhancing reactive components. The capacity enhancing reactive components can have an anion receptor or an electron transfer mediator. The anion receptor, electron transfer mediator and sulfur containing materials are described in detail. The cell energy density is specified at 1000, 1200 and 1500 Wh/kg. Charge discharge cycles are increased by 30% beyond the previous 30.
The capacity enhancing electrolyte components are described in detail. Separations by categories of anion receptors and electron transfer mediators are presented. Cathode active materials are also specified. They may include sulfides, conductive fillers and binders with examples of preparation methods for cathodes. Separators between anode and cathode are specified and structures for both low and high current batteries are described.
Following the description, a group of 17 examples of preparing solutions, cathode and anode preparation and cell construction are presented. Variations of polysulfide chemistries are prepared and used in test cells which consist of coated cathodes, lithium foil anodes and electrolytes combined into a polyolefin separator. Following construction, testing provided data on the performance of various cells from the standpoint of specific capacity improvement with 30 charge/discharge cycles being the end point at which to measure the cumulative specific capacity to prove the increase in performance of the transfer mediators.
Of the 16 claims, the last 15 build from the first. In claim number one, an electrochemical cell with lithium anode, sulfur containing cathode and nonaqueous electrolyte form the basis of the claim. The electrolyte has lithium salts, nonaqueous solvents and capacity enhancing reactive components which are generally described by electron transfer media formulae. The unique part of this entire claim rests with the electron transfer mediator chemistry which in the examples listed in the patent result in various percentage increases in the cumulative specific capacity after 30 cycles of operation.
Claims 2, 3 and 4 identify charactersitics of the electron transfer media.
Claims 5, 6 and 7 identify the performance increase of the first charge-discharge cycle and the 30 cycle capacities.
Claims 8 and 9 identify makeup of the solvents and salts.
Claims 10 and 11 describe the makeup of the sulfur containing materials.
Claim 12 describes the material of the lithium anode.
Claim 13 states that the cell has an energy density greater than 1000 Wh/kg.
Claims 14 and 15 apply the mediator chemistry to primary and secondary cells.
Claim 16 describes general steps of making the cell from a solid lithium anode, a solid cathode with sulfur-containing material, and nonaqueous electrolyte with the capacity enhancing components which have chemical descriptions.
The patent chemistries appear to be steps in building the performance of larger discharge capacity cells which from earlier data show a reduction in cycle life with added discharge capacity. The data of the patent which shows improved performance at 30 cycles may only be a stepping stone to greater cycle life.
Whether the company goal is to achieve comparable cycle life to Lithium-ion chemistry or to build product with more limited cycle life and greater energy density is yet to be seen. Certainly special applications can be identified which could accept a premium price for low volume, low cycle life cells with the greater energy density.