Feature Articles—November 2009 Issue
Advances in Energy, Power Density Of Lithium-Ion BatteriesEnhancing Battery Designs for Manned And Unmanned Undersea Applications
By Dr. Robert Gitzendanner
Executive Director
Lithium Engineering
Frank Puglia
Director
Research and Development
and
Stuart Santee
Senior Research Scientist
Yardney Technical Products Inc.
Pawcatuck, Connecticut
As undersea applications continue to grow in energy consumption, the battery becomes an enabling technology. Lithium-ion technology offers an efficient mass and volume-minimized solution for such needs.
Lithium-ion technology has performed excellently in a host of commercial, military and aerospace applications. Lithium-ion cells and batteries can be designed to support the high-energy needs of applications such as unmanned underwater vehicles (UUVs) that typically operate for hours, or even days, away from a power generation or charging source. High-energy lithium-ion cells have been demonstrated in large-format, high-capacity designs that provide reliability and efficiency in packaging to meet system-level energy densities.
High-power applications, such as torpedoes and targets, benefit from the excellent rate capability and high efficiency that lithium-ion technologies can provide. These higher rate applications utilize either single-string or multistring battery designs to help distribute the current loads.
Yardney Technical Products has delivered battery systems for both types of applications and continues to develop and improve the technology. Recent improvements have focused on reducing cell impedance for improved rate capability and reduced power fade, increased cycle life, and enhanced cell and battery designs for specific application needs. Yardney continues to advance the technology and demonstrate its applicability to a number of undersea applications.
Manned and Unmanned Vehicles
About four years ago, Yardney delivered an eight-ton, 300-volt, 1.2-megawatt-hour prototype lithium-ion battery to the U.S. Navy Sea, Air and Land Forces (SEALs) for their Advanced SEAL Delivery System (ASDS), a manned submersible. The powering of manned submersibles has demonstrated the viability and quantified cost savings of such batteries, while the three years of feedback from field service has allowed enhancements to subsequent generations.
The new technology changed the way the vessel was used by eliminating the need to fully recharge the battery and allowing for relatively rapid recharges. The preceding silver-zinc battery required more than one day to charge, followed by a cooling period. The lithium-ion battery performed operations with both full and partial recharges.
A desired next-generation enhancement is a cell equalization capability that does not require the battery to be fully charged for voltage balancing to operate efficiently. In addition, the reduction of heat generation during equalization is desirable due to the limited thermal dissipation common with many electrically propelled vehicles.
Lightweight Torpedo
Yardney is also working with the Navy on a joint development effort of a 360-volt, 75-kilowatt battery for a lightweight torpedo. Although this is a high-rate application, the effort has many of the same design concerns as the lower rate ASDS and many other UUVs.
Specifically, the accumulation and dissipation of heat needs to be addressed in both battery design and material selection.
As a high-rate application, this battery typically discharges in five to 15 minutes, limiting the battery’s ability to dissipate any heat generated from the discharge. Careful thermal modeling and temperature monitoring during operation were required to ensure the system remained at safe operating temperatures.
Battery management electronics developed by the Navy provide the opportunity to equalize the battery cells at any state of charge and provide rapid feedback on battery temperatures, currents and all cell voltages during all stages of battery charging and discharging. Such information is critical in the proper care and maintenance of a large lithium-ion system.
High-Power Materials
Though these two applications have very different uses, their commonality is the need for stability at higher temperatures and high states of charge. Both applications are sensitive to energy density and require materials at a high technology readiness level.
In both instances, mixed metal oxides such as lithiated nickel cobalt oxide (LiNiCoO2) and lithiated nickel cobalt aluminum oxide (LiNiCoAlO2) are the best candidates. No other mature cathode material matches the cell energy density of LiNiCoO2. Chemical improvements to the design come from developing and utilizing materials with lower impedance. Material selection, binder type and amount, and selection of conductivity diluents can reduce a cell’s impedance without drastically impacting overall energy density.
Improvements in electrode formulation and fabrication have led to an almost 10 percent reduction in the cell’s impedance without impacting the capacity delivered. Continuing research and development efforts have been focused on establishing next-generation chemistries and designs to support further increases in capability. High-energy systems have been developed that deliver greater than 210 watt-hours per kilogram at the cell level.
High-power cells are supporting continuous discharge rates of 15 times the cell’s capacity (15 C) and delivering greater than 8,000 watts per kilogram.
High rate capability can be further enhanced through the use of electrolyte additives. In addition to improving high-temperature stability, enabling cells and batteries to perform well after prolonged exposure to temperatures as high as 70° Celsius, such additives can reduce the impedance of the system. Such additives have been shown to increase rate capability and reduce power fade on high-rate cycling. Additional high-rate life cycle testing of these cells is currently ongoing in order to demonstrate the life capabilities and reduced power fade of this chemistry.
Design Considerations
Large, high-capacity batteries have been used extensively in undersea applications. They typically have been satisfied by extremely large lead-acid cells, but as power and energy demands continue to increase, lithium-ion cells and batteries offer a significantly reduced mass and volume solution. The replacement of a 300-volt/8,000-ampere-hour lead-acid battery on a submarine with a lithium-ion version would reduce mass by more than 80,000 pounds and volume by up to 1,000 cubic feet.
Historically, such large systems have not been comprised of large series/parallel arrays of smaller cells, and with good reason. Primarily, overall system safety and reliability can be enhanced by being able to truly monitor individual cell voltages, ensure equal utilization of all cells and reduce the number of parts. Placing a higher impedance cell in parallel with a normal, or “good,” cell can lead to excessive heat generation, especially during high-rate discharges. Multiple smaller cells present reliability issues with many more intercell connections to fail, potentially more terminals to leak, and more difficulty in packaging and making the battery design rugged.
There are, of course, disadvantages. Since larger cells are usually more specialized and have higher initial costs, they tend to originate from fewer vendors, reducing competition. They can also have larger safety issues, although incidents with even small laptop batteries have shown that failures can propagate in a small cell battery design and yield catastrophic results.
Several design considerations come into play when developing any large-capacity system. Lithium-ion cells and batteries are suitable for such applications, but must be appropriately designed and managed. For high-power applications, the cell size is limited by thermal and heat dissipation concerns. Simple thermal analysis can demonstrate how thermal management of such systems may be achieved. A cell’s ability to dissipate heat is a function of geometry, and thicker cells become impractical with certain designs, thus the final cell design options are limited. Power distribution buswork can also be a limiting factor, as distribution of high currents can allow for smaller individual cables versus a single string with full current-carrying capability. Thus, these applications may benefit from arrays or medium-sized cells.
High-energy systems are usually low-rate applications and therefore are not typically subjected to the same thermal design limitations as high-power designs. Lithium-ion technology typically operates with a 95 to 97 percent energy efficiency, so with discharges of five hours or greater, the cells and batteries will dissipate the heat generated almost immediately. This means that the cells for these types of applications have almost no natural limitation on cell size, and, in fact, manufacturing and handling concerns become predominant in designing such systems.
System safety must, however, always be a primary concern in designing such systems. Cell voltages, temperatures and currents (on multistring batteries) should be consistently and frequently monitored and maintained within prescribed limits. Battery management electronics are typically responsible for monitoring and controlling the charge, and perhaps discharge, modes of the battery. They are designed to limit maximum cell voltage and deviation and abort operation if temperatures get out of range. These systems should also interface with the application system, at least at the charger and/or power distribution levels, to ensure that unsafe demands are not made on the batteries and to provide backup to any internal protections. Communication and data recording features provide additional information in case out-of-bounds events are encountered, or as a means of providing details for battery state of health assessments.
Conclusions
High-energy and high-power undersea applications will continue to benefit from the energy density, efficiency and flexibility of lithium-ion technologies. Large format, high-capacity cells and batteries are best when specifically designed for the application, with appropriate performance, thermal and safety concerns in place. Continued development of such systems for all types of applications will provide robust systems for all manned and unmanned naval applications.
Acknowledgments
The information presented here would not be possible without the hard work of many people. A special thanks to Joe Wallace, Dick Fallon, Dr. M. Gulbinska and other co-workers at Yardney for their contributions.
Dr. Robert Gitzendanner received his Ph.D. from Cornell University. He has been working on batteries and battery materials for more than 15 years.
Frank Puglia has a Master of Science in chemistry from the University of Rhode Island and has been developing advanced materials for primary and secondary products for more than 18 years.
Stuart Santee is finishing his Master of Science in chemistry from the University of Rhode Island. He has worked on numerous military and aerospace applications, including the 2003 Mars Exploration rovers and the 2011 Mars Science Laboratory.
Sea Technology is read worldwide in more than 110 countries by management, engineers, scientists and technical personnel working in industry, government and educational research institutions. Readers are involved with oceanographic research, fisheries management, offshore oil and gas exploration and production, undersea defense including antisubmarine warfare, ocean mining and commercial diving.