Li-Ion has the potential to become the preferred energy-storage solution for most HEV applications as it retains greater potential for low cost and high performance than NiMH, and may have similar longevity.
Typically, the higher the level of hybridisation, the more stringent the requirements on the energy-storage device in terms of power, energy, and duty cycle. To date, the micro-hybrid architectures are being equipped with Lead-Acid batteries, the moderate and strong hybrids with NiMH batteries, and the mild architectures, buses, plug-in, and delivery vehicles—all still at very low volumes—with multiple energy-storage devices including Lead-Acid, NiMH and Li-ion batteries, as well as ultracapacitors.
Table 1 lists the various Hybrid Electric Vehicle (HEV) architectures that are being developed by automakers around the world.
Having conducted on-site interviews with all major automakers in the last year, it is estimated that the HEV market will grow from about 385,000 units in 2006 to 1.1 million units by 2010 (Figure 1).
The optimal Li-ion-battery design for HEVs is yet to be created. Developers are balancing their work on performance, cost, reliability, and safety issues to come up with the best overall solution. There are at least four cell configurations under development for HEV applications:
1. Spirally wound designs in a cylindrical metal case
2. Spirally wound designs in an elliptic metal case
3. Spirally wound elliptic designs in a soft-pouch packaging, and
4. Stacked-electrode assemblies in a soft-pouch packaging
The first two configurations are the most common and there is no significant difference between them. The cylindrical design is more uniform and relatively easier to manufacture, while the elliptic design, although suffers from lack of uniformity at the narrow side of the spiral, does provide a larger cell container area, which facilitates packaging and heat removal.
The soft-pouch configurations are less promising for automotive applications than for other applications, at least in the shorter term, for the following reasons:
• The seal may not remain hermetic for 10 years due to the combined temperature fluctuations, vibrations, and electrochemical cycling of the HEV application
• Although the cells are lighter, much of the weight advantage is lost at the pack level due to the requirement for compression bars to hold the cells in place
• The design does not tolerate internal gassing, which increases the requirements for cell material purity and dryness, thereby increasing the cost
• Securing the integrity of the tabs and terminal and the cell-to-cell connections against vibrations is more challenging due to the lack of rigidity in the configuration
• The external compression bars are likely to cause non-uniform compression of the stack, which would lead to variations in current densities and susceptibility to local shorts
To provide compression in a pouch cell, some developers use a gelled electrolyte, an approach that, while valid, carries a penalty due to the associated increase in cell impedance, particularly at low temperatures. Other developers rely only on external compression, an approach that is subject to the concerns expressed above.
Li-ion batteries can be designed with a variety of solid positive active materials that are able to intercalate Li+ cations into and out of the cathode structure. The choice of cathode material is the single most critical design choice for automotive battery as it has an impact on safety, life, power capability, energy content and cost.
Currently, three classes of cathode materials are under development, as single components as well as in blends, as follows:
LiMn2O4-based Cathodes: These materials, known as spinels due to their spinel crystal structure, are used by some Japanese and Korean developers. They have the potential for low cost and offer higher thermal stability than other compounds. They can support high charge and discharge rates, but their typical specific capacity of 110mAh/gram is somewhat lower than that of the nickel- and cobalt-based cathodes. However, the main challenge to their use in HEV applications is their modest calendar and cycle lives.
LiNiCoMO2-based Cathodes: These compounds, which feature a layer structure, generally deliver specific capacities of 150 to 180 mAh/gram. The materials exhibit good power capability and long cycle life. However, they have lower thermal decomposition thresholds and thus, evolving oxygen can react violently during abuse. Additionally, they are more expensive than LiMn2O4 (current price is over US$ 35/kg)—and long-term cost projections, at likely nickel-metal prices of US$ 10 to US$ 15 per kg, are expected to be not less than US$ 20/kg.
LiFePO4-based Cathodes: The LiFePO4 material has a stable crystal structure that intercalates Li+ reversibly at about 3.4V against the Li potential. LiFePO4 exhibits lower energy density, and rather poor electronic and ionic conductivity. The major interest in LiFePO4-based cathodes is for use in large cell applications, since it is inherently stable in overcharge and high-temperature conditions and does not release oxygen in either condition. The main challenge to its application in HEV Li-ion batteries is to obtain sufficient rate capabilities without significant sacrifice in energy or a significant increase in cost. Recently the compound has been successfully utilised in power-tool batteries made by A123Systems. In these batteries the poor ionic conductivity has been counteracted by using cathodes consisting of nano-particles, and the poor electronic conductivity has been offset by the use of carbon coatings, and possibly metal doping.
Beyond the choice of cathode active material, mechanical cell configuration, and packaging, the variations in other cell components between the different key developers is less significant. Table 2 lists the key developers of HEV batteries and their cell designs based on available information. The variety of approaches to cell design for HEVs is an indication of its low level of maturity, which is characteristic of early development. We expect developers to retain only a few technologies as the product matures.
The Li-ion battery technology that now dominates much of the portable battery business has matured enough over the last five years to be considered for short-term implementation in HEV applications. Its power density is 50-100 percent greater than that of existing HEV NiMH batteries and projected data from laboratory testing suggest a similar cycle life (but as yet less proven calendar life) to that of NiMH. For a given application, Li-ion technology will offer a battery that is about 20 percent smaller and 30 percent lighter than the existing NiMH batteries, a notable, if not overwhelming, advantage.
The basic chemistry and design of Li-ion HEV cells are quite similar to those of small consumer cells. This suggests that the essential manufacturing processes used in the production of HEV batteries should be well understood. The manufacture of Li-ion cells is known to require a higher level of process control and precision than that of most other types of battery and, as a result, scrap rates tend to be higher with Li Ion.
Most producers of small Li-ion batteries have experienced product recalls and/or production shut-downs due to reliability issues and/or safety incidents. Projecting this experience to the much larger HEV cells, which have thinner electrodes and more demanding life and reliability requirements, it is safe to predict that scaling-up cell production from the current early pilot levels will be slow and costly.
The most difficult questions concerning Li-ion batteries relate to reliability and safety in the field. The awareness of battery safety has increased over the past year as a result of the massive recall of portable Li-ion batteries in notebook computers and cell phone applications. The issue of Li-ion battery safety includes at least two points: i) ensuring that a cell and a battery built to specifications are able to withstand certain abuse conditions without causing unsafe situations and ii) ensuring that cells built outside the design specifications will not cause unsafe situations, at least in normal usage. In both cases, there is no simple answer since regarding the first point it is not clear what level of abuse (high temperature, high voltage, external crash) is appropriate for testing, and regarding the second point, it is difficult to ascertain how much design margin is needed to ensure that even cells that are out of specification will operate safely. The HEV Li-ion battery will include a string of 50 to 100 serially connected individual cells operating at between 150 and 300V. Furthermore, HEV batteries will have to function in harsher thermal and mechanical environments than computer batteries, which are used predominately in offices. Thus, considering the high level of incidents in the somewhat tame environment of computer batteries, hybrid-vehicle manufacturers and their battery suppliers are understandably cautious. Yet the questions of product safety and reliability cannot be answered until field introduction occurs, which is expected to start within the next two years, albeit slowly.
Considering the current rate of improvement and investment in the technology, it is likely that Li-ion will eventually become the preferred energy-storage solution for most HEV applications as it retains greater potential for low cost and high performance than NiMH, and may have similar longevity. We are less confident about using the technology—at least with the common nickel- or manganese-based cathodes—in applications that require full-state-of-charge operation, such as electric vehicles and plug-in HEVs, due to the decreased stability of the nickel- and manganese-based positive-electrode interface at higher cell voltages, which has an obvious impact on safety and life.
The overall challenge for the industry is to produce a single battery design capable of providing power and energy levels sufficiently superior to those of existing NiMH batteries, and at the same time showing potential to meet the life and reliability requirements of the automotive application and match or beat the cost of existing NiMH batteries with reasonable production volumes and time schedule. This means that safety and reliability need to be ensured with no notable cost increase or reduction of the energy and power advantage of Li-ion batteries over NiMH batteries for HEVs.
