ABSL Power Solutions

The current and future status of Lithium-ion batteries for UUVs

By David Goodwin and Nick Russel

Abstract

Lithium-ion (Li-ion) rechargeable batteries increasingly are replacing older battery technologies in UUVs. This paper explores the reasons behind this trend, highlighting the significant advantages of Lithium-ion in terms of specific power and energy (W/kg and Wh/kg), increased cycle- and calendar-life, superior charge-retention, improved reliability, maintenance-free operation and, above all, reduced 'through-life costs'. Lithium-ion technology is compared with lead-acid, silver zinc, nickel cadmium and nickel metal hydride batteries. Specific examples of Lithium-ion batteries used in UUVs are presented. Further developments in Lithium-ion technology are discussed, which promise significant increases in specific energy (Wh/kg) in the near term, and finally issues concerning the international transportation of lithium batteries are highlighted.

Introduction

In recent years the development and deployment of Unmanned Underwater Vehicles has created profound challenges for power and energy source providers. A number of trends in the demand for energy storage systems for UUVs have become evident:

• The need for an increased number of payloads, requiring more energy and reducing the space and weight budget available for the battery.
• The need for UUVs to dive to ever increasing depths, increasing vehicle weight, and further reducing the weight available for the battery.
• Vehicles need to travel at faster speeds, further increasing the power required from the battery.
• Vehicles are required to tackle longer missions, requiring the battery to provide more energy.

As a result UUV batteries need to be smaller, lighter, more powerful and last longer than ever before. Traditional battery technologies have been found wanting. Typically UUVs have relied on lead acid (Pb-Acid), silver zinc (AgO-Zn) nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries as their energy sources.

Traditional Battery Technologies for UUVs

A brief review of the traditional battery technologies reveals significant weaknesses and limitations for UUVs of the future:

Lead Acid Batteries:

• Exhibit low specific energy (Wh/kg), typically up to 30 Wh/kg, this makes them too heavy for the demands of today's UUV.
• Exhibit low volumetric energy density (Wh/l), typically up to 100 Wh/l, this makes them too large.
• Capacity (Ah) reduces significantly at low temperature.
• Hydrogen is generated during the charge cycle, which means that charging cannot take place within a sealed environment.
• Voltage compensation is needed when charging at elevated temperature, otherwise service life can be drastically reduced.

Silver Zinc Batteries:

• Once 'activated' by flooding the cells with electrolyte, they have a poor 'wet stand' life expectancy.
• Cycle life (number of missions) is typically less than 50, and can be even lower when high discharge rates are required.
• Capacity is significantly reduced at temperatures below 10C.
• Individual cell failures require frequent replacement and maintenance.
• Charging times are extended because of the need to 'purge' the system of potassium hydroxide

Nickel Cadmium Batteries:

• Also suffer from low specific energy, typically no more than 50 Wh/kg, and low volumetric energy density, no more than 140 Wh/l.
• Capacity is significantly reduced at low temperature.
• Charge retention is poor (typically 20-25% of capacity is lost per month), requiring at least a 'top-up' charge prior to deployment.
• After low 'Depth-of-Discharge' (DoD) conditioning cycles are required to limit the 'memory effect', which can significantly reduce the available energy from the system.
• Cadmium, as a heavy metal, is causing significant environmental concerns, leading to the replacement of NiCd batteries in many applications.

Nickel Metal Hydride batteries have similar limitations (other than cadmium content) to those of NiCd, however they do exhibit slightly better specific energy (up to 80 Wh/kg) and volumetric energy density (up to 250 Wh/l).

The Development of Lithium-ion (Li-ion)

The development of Li-ion technology in the 1980's and 90's transformed the battery industry. Earlier attempts to harness lithium (the lightest metallic element) for rechargeable batteries (primary batteries had used lithium metal since the early 1970's) failed because it proved to be too difficult to re-plate lithium in the anode on the charge cycle, causing very poor cycle-life and worse, because of dendrite growth between the electrodes, significant safety risks.

The breakthrough to Li-ion, was enabled by the invention of transition metal oxide cathodes doped with lithium ions. This material, patented by AEA Technology, provided an alternative source of lithium ions, which could now be cycled efficiently between two intercalation electrodes. In the 1990's AEA licensed twenty-three battery manufacturers worldwide with its technology, enabling Li-ion batteries to become accepted as the preferred power source for nearly all consumer electronic devices.

Figure 1: Li-ion Discharge Cycle

Figure 1 above illustrates a typical Li-ion system, in this case characterising the discharge cycle. The most important fact to grasp is that under normal operation there is no metallic lithium present in the system. The lithium ions 'shuttle' between the two electrodes (the anode being a carbon intercalation compound and the cathode typically a lithiated metal oxide, such as cobalt, LiCoO2).

Li-ion batteries succeeded in the consumer electronics field because of their dramatic impact on the size and weight of portable equipment. The improvements offered by Li-ion batteries are best illustrated in Figure 2, which compares Li-ion with the more traditional battery technologies in terms of Wh/kg and Wh/l (size and weight). In comparison with Pb-acid, Li-ion batteries can be as much as a quarter of the size and weight.

Figure 2: Energy Density Comparison

AEA's strategy following its licensing activity of the 1990's has been to focus its efforts on extending the benefits of Li-ion technology beyond consumer electronics into more specialist fields, particularly portable military equipment, space vehicles and other aerospace applications. It has had significant success in these fields, for example by supplying all the batteries and chargers for the radios and data terminals required by the British Army's new digital battlefield communications System, BOWMAN, and supplying virtually all the Li-ion batteries used in space satellites by ESA.

These markets require higher specification cells and management systems than those required in consumer electronics. To support the transfer of the technology into these applications AEA established a joint venture (AGM Batteries Ltd.) with two of its licensees (Japan Storage Battery Co. and Mitsubishi Materials) and set up an advanced Li-ion cell manufacturing plant in Scotland in 1999. Increasingly AEA is seeking to support UUV manufactures with its technology.

Advantages of Li-ion

In addition to size and weight reductions, this technology has many other characteristics, which make it ideal for UUV batteries:

• It has excellent charge retention. Its capacity loss over 12 months storage is typically 10-15% (remember NiCd batteries lose 20-25% per month!)
• Cycle-life exceeds 1000 on 100% depth-of-discharge (NiCd and Pb-Acid can be as low as 200).
• Service-life can extend to 10-15 years.
• The technology is tolerant of low depth-of-discharge, so that at 50%, cycle life can be well over 2000, without the need for disruptive conditioning cycles, as in the case of NiCd batteries.
• Fast charge is possible. Normal charging can be achieved in 5 hours from 100% depth-of-discharge. A 2-3 hour charge is also achievable.
• Since no gas is generated during charge, charging in a sealed environment is possible*.
• AEA's Li-ion technology is also capable of safe operation over a very wide temperature range (-50C to +70C).
• The technology also lends itself to many 'smart' options, such as 'fuel gauging' and the recording of its 'life history'.

(* provided a safety venting mechanism is incorporated in case of external abuse)

Furthermore the absence of lead, cadmium and metallic lithium means that environmental issues are significantly reduced for disposal at the end of life. However AEA has established a recycling facility, which means that useful materials, such as cobalt, copper and aluminium will be recovered.

Above all, however, the extended cycle- and service-life and the elimination of maintenance requirements will significantly reduce the through-life cost of batteries to the vehicle operator.

UUV Examples

AEA has provided batteries for a number of UUVs in the recent past, for both large vehicles for extended mission times and smaller vehicles for short duration missions such as mine clearance, where high power is required rather than high energy.

Figure 3 illustrates a typical battery module for a large AUV. In this case the battery was constructed from a large array of nominal 5 Ah Li-ion cells. Each module was made up of 33 cells in parallel and 28 in series, providing a nominal 165 Ah and 100 VDC (16.5 kWh). Four of these modules provided the overall energy package for the AUV, a total of 66 kWh. With a total weight of 550 kg, the achieved specific energy was 120 Wh/kg. Figure 4 shows how the cells were arranged within the cylindrical profile of the pressure vessel.

Figure 3: AUV Li-ion Battery Module

Figure 4: Cell Stack

Future Developments

The challenge for battery manufacturers is to continue the rapid pace of technology improvements seen in this industry in the last decade. AEA is at the forefront of this effort, striving for even better specific energy. Current Li-ion technology is reaching the end of its development cycle.

Figure 5: Capacity Development of 18650 Cells

Figure 5 illustrates this, showing how the capacity of a consumer 18650 cell has increased from 1 Ah in the early 1990's to 2.4 Ah today, but will peak at around 2.5 Ah. AEA's Li-ion 'D' cell capacity has mirrored the progress shown in Figure 5, increasing from 4.5 Ah in 1999 to 6 Ah in 2004 and expected to peak at 7 Ah in 2005. The latter will take specific energy, at the cell level, up to approximately 170 Wh/kg.

The limitation of current Li-ion systems is that the LiCoO2 used in the cathode becomes 'unstable' once half the lithium ions have been transferred to the anode on charge. Consequently current Li-ion systems contain twice as much lithium as can be safely utilised. This limits the maximum available specific energy.

To overcome this limitation it is necessary to utilise the lithium in the system more efficiently. AEA is currently working with FMC Corp. to develop cell technology based on the use of stabilized lithium metal powder (SLMP).

SLMP Technology

SLMP will provide an alternative source of lithium ions in the cell, resulting in a significant increase in specific energy. SLMP can be used in conjunction with:

• Existing LiCoO2 system
• Non-lithiated cathode materials
• New anode materials

By using SLMP in our hard carbon anode and using our existing LiCoO2 cathode an increase in specific energy of 16% is predicted (2005). Combining SLMP with partial or non-lithiated cathode materials, such as V6O13 or V3O8, will yield a 25% increase in specific energy (2006) and 75% in capacity (for the same space envelope), and finally SLMP should exploit the potential capacity of new anode materials, which can be optimised for cell performance rather than for low irreversible capacity loss. The expectation is that we can double specific energy by 2010, with further improvements beyond. Figure 7 illustrates the AGM development road map for the 'D' sized cell, indicating a 60 Wh+ capability in the longer term, compared with today's 25 Wh.

Figure 6: Development Road Map

Battery Design

In parallel with cell developments that can be expected to keep Li-ion expanding its capabilities over the next 10 years, AEA is focusing on the design of the next generation of batteries for UUVs to simplify and speed up the adoption of the technology in this arena. The historic approach to developing Li-ion batteries for UUVs has been to design a customised package based on a specific power and energy budget. This approach has a number of limitations:

• As the power and energy demands of the UUV change, the existing battery may need to be re-designed.
• Any re-design will have significant cost and timescale implications.
• Changes will often require re-qualification, and almost certainly re-testing for international transport (more on this later).

To minimise these issues, AEA is developing a 'modular' approach to the design of Li-ion batteries for UUVs based on the AGM range of advanced cylindrical Li-ion cells. The concept is illustrated in Figure 8.

Figure 7: Modular Batteries for UUVs

In this illustration a 'module' will have a nominal voltage of 3.7 VDC. The capacity will depend upon the internal diameter of the pressure vessel, whether the 'module' takes up half or all of the cross-section and therefore the number of cells per 'module'. The advantages of this approach are:

• An individual 'module' can be qualified and tested for transportation.
• Additional 'modules' can be added to build up extra capacity or voltage without incurring significant re-design cost.
• 'Modules' can be upgraded as new cells are developed with the additional capacity discussed in the previous section.
• A module can have high-power or high-energy characteristics depending upon which AGM cell is chosen for the build.

This flexibility, combined with the new advances in Li-ion technology, will provide UUV manufacturers with a tremendous opportunity to extend vehicle mission times, increase payloads, dive deeper and move more quickly than ever before.

In a parallel activity, AEA is developing (under contract to SEA Ltd, prime contractor for the MoD's BAUUV programme) an extremely powerful predictive modeling tool. By using Virtual Test Bed software, AEA will be able to simulate a UUV's power consumption for a given mission and predict energy source performance. This activity will model a range of different energy sources, however it will be particularly powerful in the design phase of 'modular' Li-ion batteries for specific UUVs.

International Transportation of Lithium Batteries

Since 1999, Li-ion cells and batteries have been defined for international transport as lithium batteries. A number of domestic and international transport bodies regulate the transport of lithium batteries. For example IATA and ICAO regulate international air transport, European road and rail transport is regulated by ADR and RID respectively, sea transport by IMO and within the US the DoT regulates all modes of transport.

Each body publishes its own rules, which then become the legal requirements. These regulations reflect the UN’s Model Regulations on the Transport of Hazardous Goods. These regulations are determined by an international committee of experts convened by the UN, and are reviewed regularly. The last major change occurred with the publication of the UN's 12th edition of the regulations in 2001, which came into law with the publication of the mode-specific regulations in 2003.

The rules are too detailed to go into in any depth here, however there are a couple of key 'issues' to bear in mind:

• Any Li-ion battery with more than 8g of 'lithium equivalent' must be transported as Class 9 goods (i.e. hazardous), and are therefore subject to specific packaging regulations depending upon whether the batteries are shipped alone, with equipment or 'in' equipment.
• All Li-ion batteries need to be 'type-tested' in accordance with the latest edition of the UN's 'Manual of Tests and Criteria' prior to transport.

The '8-gram rule', means that any UUV battery over ~100 Wh will need to be shipped as Class 9 goods (virtually all UUV batteries!). The UN 'Manual of Tests and Criteria' requires the testing of 16 batteries. The tests include altitude simulation, thermal cycling, shock, vibration, external short circuit, an impact test on individual cells, overcharge and forced discharge. For large batteries this can be expensive and time consuming. However there are some important exceptions:

• The road, rail and sea regulators will allow the transport of non-tested batteries with additional packaging requirements, provided the production run is not more than 100 batteries.
• The airfreight regulators will allow the transport of non-tested 'prototype' batteries (no more than 12) with the approval of the appropriate authority of the State of Origin. In the case of the UK this would be the CAA and the US the DoT.

It is the consignor’s responsibility to ensure that all batteries conform to the latest legal requirements for each intended mode of transport. You are strongly advised to consult your battery manufacturer early in the design process to ensure compliance. AEA employs a dedicated logistics co-ordinator to ensure it stays abreast of the latest legal requirements.

If this seems a daunting prospect, it is intended that AEA's 'modular' design concept will simplify life for the UUV manufacturer by enabling batteries to be assembled from fully tested and approved 'modules'.

Conclusion

Li-ion batteries have particular advantages for the UUV community. Their superior specific energy enables UUV manufacturers significantly to increase mission times, vehicle speeds and operating depths. In addition their superior cycle- and service-life, minimal maintenance requirements and high reliability will reduce 'through-life' costs. Characteristics such as excellent charge retention and the ability to charge within a pressure hull, will also enhance UUV operational capabilities.

New materials are being developed, which will make possible further significant improvements to the specific energy and power of Li-ion batteries. Moreover, as the 'modular' concept develops, it will be a relatively simple task to upgrade battery performance by incorporating Li-ion cells with even higher specific energy or power, and allow UUV manufacturers to avoid significant re-design and re-qualification costs.

Acknowledgements

The authors gratefully acknowledge the assistance provided Jas Singh of SEA Ltd, and all the employees of AEA Technology Battery Systems who contributed to the development work described in this paper.