Enthusiasm—and especially expectations—for lithium-ion battery technology applied to hybrids, plug-in hybrids and electric vehicles is demonstrably growing, fueled by the convergence of very substantive technical advances and the clear need to make rapid progress to reduce the consumption of fossil fuels in transportation.
This year’s Advanced Automotive Battery Conference (AABC) and related Symposia, being held this week in Long Beach, California, are enjoying the largest attendance ever. Yet with the enthusiasm comes caution. There are still technical issues to be resolved for automotive applications. At a high level the outstanding issues can be categorized as safety and cost; at a cell level the issues come down to the selection of electrolytes, electrode materials, separators, and design.
Dr. Menahem Anderman, the President of Advanced Automotive Batteries and the organizer of the conferences, projects that lithium-ion battery sales may capture 5% of the hybrid and electric vehicle market by 2010, 17% by 2012 and 36% by 2015. While NiMH batteries would still dominate that market segment, clearly their marketshare would be beginning to decline. A corollary forecast is that plug-in hybrids—assuming that lithium-ion chemistries are required for those applications—are unlikely to reach commercial volumes (more than 20,000 units per year) by 2015.
There are several non-technical wildcards that could accelerate the rate of growth in the adoption of lithium-ion technology. A rapid spike in petroleum costs and/or concern over global warming that translates into government mandates for fuel efficiency would have a major impact. There are also wildcards that could slow the rate of growth. A reduction in oil prices, steady adoption of diesels in the US market, the failure of advanced HEV, PHEV or EV batteries to provide at least 8 years of life in most applications, and the inability of the HEV industry to reduce system costs at the steady rate now projected, could each slow the pace of adoption.
Lithium-ion battery chemistry is attractive because it offers higher power and energy (1.3 to 1.7 times) than that of current NiMH technology. That higher power and energy should result in a lower number of cells being required in the pack (lowering cost); lower heat generation (if impedance is suppressed); a higher usable SOC (state-of-charge) range (yet to be proven); a lower metal cost per kWh (especially for certain cathode candidates); and potentially a lower long-term overall cost.
Electrolytes. There are three basic pathways to a thermal event with a lithium-ion battery: an internal short circuit (which is uncontrollable, outside of trying to ensure that it doesn’t happen through manufacturing quality), an external short circuit, and an overcharge situation.
Researchers at Ube Industries explored whether or not a tailored electrolyte could reduce the flammability of a battery in a failure condition (field or abuse). They found that while a non-flammable solvent could reduce the flammability issue, it also deteriorated battery performance.
Dr. Khalil Amine from Argonne National Laboratory described efforts in improving li-ion battery life using advanced salt and electrolyte additives. Argonne has found that maganese-spinel systems, combined with an alternative salt, could provide excellent calendar life, outstanding safety and low cost. Such a battery could meet the FreedomCar requirements ands could be the most suitable for HEV applications.
Argonne has found that the use of electrolyte additives can stabilize the battery, and reduce heat flow. An advanced Li1+x(Ni1/3CO1/3MN1/3)1-xO2 system offers high power and relatively low cost. This could be improved with the addition of a new electrolyte additive: LiC2O4BF2.
Air Products, probably better known for its industrial gases, presented the results of its work with an electrolyte salt family: the StabiliLife salts. The Li2B12FxH12-x slats provided better cell thermal stability than LiPF6 systems.
Chemetall GmbH described its work with lithium bis(oxalato) borate, known as LiBOB, for enhanced safety and stability.
Cathode materials. The selection of a cathode (the positive electrode) material is one of the areas in which battery makers are striving to differentiate themselves. There are four main candidates for cathode materials, plus blends:
LiNiCoAlO2: The most proven, but the most thermally unstable at high states of charge.
LiNiCoMnO2: This material is gaining momentum, although there is not much durability data yet.
LiMn2O4: Life at elevated temperatures has not yet been solved.
LiFePO4: The most thermally stable, but with lower voltage and lower energy. Modified iron phosphate cathodes are the choice of A123Systems and Valence. Saft, one of the well-established large-format li-ion battery companies, uses a Phostech iron phosphate as its cathode material. (Phostech is the licensor of the standard lithium iron phosphate material developed by Dr. Goodenough at the University of Texas.)
Blends of different materials, hopefully making the whole greater than the sum of the parts, according to Dr. Anderman.
HPL, a spin-off from Dr. Michael Grätzel’s “Laboratoire de photonique et interfaces” at the Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, described its work with lithium manganese phosphate as a new high-voltage li-ion cathode. The LiMnPO4 material offers durability, safe performance and cost effectiveness, according to HPL.
There are four phosphates from which to choose for a cathode material: iron, cobalt, nickel and manganese. According to HPL, manganese is essentially a voltage compromise between the other three, and a nanostructured manganese phosphate can offer high rate performance, yet be safe and durable. HPL noted that Toyota has been working with the manganese cathode material.
In addition to iron phosphates, Valence is looking at vanadium-based oxides as attractive materials when coupled with a phosphate group.
Safety. Safety is a critical issue for an emerging enabling technology such as lithium-ion batteries. As J.B. Stroubel, the CTO for Tesla Motors, noted during his presentation, it doesn't matter how many gasoline fires that cars suffer annually; because electric and hybrid technology both are relatively new, any such event would have a disproportionate impact.
While the prospects of such hazardous events remain relatively high, there is a great deal of work underway to mitigate the hazard risks. USABC, for example, is working on quantifying risk with a hazard risk number, and using that approach as a mechanism to drive mitigating responses from manufacturers.
Safety issues fall into two primary categories: events resulting from abuse, and events resulting from field failure.
Field failure—a safety-related event that occurs during the normal operation of a device resulting from a manufacturing problem—must be made less severe as well as less frequent. The recent spate of battery failures in laptops and the resulting recalls in laptops were field failures.
In their presentation, Tiax argued that new approaches to safety are required, including recognition of the processes associated with field failure as a research area.
The safety “cloud” and other uncertainties in the lithium-ion market will not stop the growth of that market for HEVs, said Dr. Anderman, it will just make its growth slow—slower than many would like to see.
And in that context, he said, “It is smart of the car companies to go slow.”