Energy Storage for Automotive Propulsion
Part II

W. F. Howard, Ph.D.
Howard Consulting
Silver Spring, Maryland
rikhoward@aol.com

Most Li-ion batteries found in present EV/HEVs incorporate cathodes with LiCoO2 derivatives and anodes of graphitized carbon: these represent the established production technology. Even the newest generation units, with low Co content (reducing price) and stabilizing Al (less chance for thermal runaway), require expensive (and heavy) safety circuitry. And the cathode material is still the high-cost component: reports by Argonne National Lab and other groups cite the cathode intercalant as >20% of the battery cost. This percentage will drop as Co content is reduced or eliminated.

Spinel was for many years considered a disruptive technology against LiCoO2, and oceans of R&D money broke against the technological cliffs preventing spinel’s adoption into Li-ion batteries. Modified LiMn2O4 has much going for it: low cost, stable raw material sources, safe operation, and only minor health and environmental issues. But the bottom line is performance, and this is where spinel foundered. Capacity is relatively low and operating life generally less than 1000 cycles, non-competitive with LiCoO2. The source of this latter problem was recognized quickly: Mn+3 in spinel is subject to disproportionation, a behavior exacerbated by trace acid in the electrolyte. Mn+2 and Mn+4 thus produced interfere with cell operation, causing unacceptable capacity fade.

Rapid deep-discharge fade and low capacity are incompatible with EV batteries, which require extended lifetimes, but the excellent rate capability of spinel suggests application in HEV cells. For example, LG Chemical has a spinel/Li polymer battery producing 5kW/kg. Although Nissan has ventured into spinel-based cells, it appears that the concerns over limited cycle life, even in pulse usage, restrict spinel’s automotive battery future. Even so, Nissan projects power and energy ratings of 3.5kW/kg and 3kWh/kg, respectively, by 2008. PolyStor and Panasonic determined that physical mixtures of spinel and LiCoO2 derivatives yield a cathode with the best properties of both materials, but the move away from Co has kept this concept from appearing in HEVs.

Lithium metal polymer (LMP) batteries from Avestor (Canada) and Bolloré (France) incorporate Li13V3O8 cathode, PEO-based electrolyte, and Li metal anode. These warm batteries run at 60-80°C and are best suited for EVs (120-140Wh/kg, only 250W/kg). Bolloré, with manufacturing by its BatScap subsidiary, projects their ~200kg battery pack will sell for <1500. Acceptance will depend on the public’s perception of the dangers associated with lithium metal and the willingness of auto manufacturers to assume the attendant liability. LMP units appear better suited for public transport vehicles.

Consider a rocking chair battery with LiCoO2 (LCO) cathode and Li4Ti5O12 (LTO) anode. Although both electrode materials are very fade-resistant, cell voltage is low (~2.3V) and capacity is moderate, probably ruling out usage in EVs. Excellent high rate performance (full charge in three minutes), as reported by Amatucci, ensures that this technology will be thoroughly evaluated for HEV use. Toshiba introduced a similar battery in 2005, claiming only 1% fade per 1000 cycles. This extremely low fade leads to speculation that the cathode incorporates LiFePO4, a combination without overcharge-overdischarge problems, a key feature in the safety-conscious auto industry. Production of nano-LTO continues to be a question: can the process be scaled to tons/week output and the price reduced to competitive levels?

Sony developed a nano-structured SnCoC alloy anode (also described by J. Dahn) paired with Li(Co,Mn,Ni)02 cathodes in 14430 cells that charge to 4.6 V and produce 20% more capacity than LCO-based units. While this anode material has 100% expansion with Li uptake, it does not fragment on cycling and discharges in the 0-0.4V range, thus allowing greater energy per cell than with LTO.

LiFePO4 in a conventional Li-ion cell with carbon anodes yields a relatively low-energy system, but one with extreme stability. Not only is the risk of thermal runaway overcome, a trait common to phosphates, but deep-discharge cycle life may exceed 2000 cycles. Development efforts with LiFePO4 are geared toward laptop-sized batteries, although Valence Technology announced their intention to target automotive applications (possibly with Li3V2(PO4)3, which cycles at 3.5-4.6V and has greater capacity – 195mAh/g – than LiFePO4). There are still difficulties with LiFePO4 production, and until the process variables are controlled, cathode material cost and quality will be problematic.

Finally, a few words about Li-ion electrolytes. Typically the electrolyte is a 1M solution of LiPF6 in organic carbonates. This is a very reactive salt that hydrolyzes readily and contains trace HF, but it is still the preferred candidate for the job. Other Li salts, such as LiBF4, LiClO4, and imide derivatives, cannot withstand the high potential or meet the conductivity demands of 4V Li-ion cells. While LiAsF6 has similar electrochemical characteristics as LiPF6, and is very stable and acid-free, the spectre of arsenic has prevented its acceptance by the battery industry. Lithium bis(oxalato) borate (LiBoB) is also stable at high voltage, but production difficulties and low conductivity have restricted this salt’s market entry. Gel electrolytes are electrochemically almost identical to salt solutions, and provide some advantage in battery construction. Adoption of these formulations has been slowed due to cost considerations.

Air Products & Chemicals recently described a series of salts with cage structures, Li2B12F12-xHx, (x < 4), that offer thermal stability to >450°, overcharge protection to ~ 4.7V, non-reactive, with no residual acid, and 8mS/cm conductivity in organic carbonate solutions. This electrolyte may open the door for widespread spinel usage by reducing acid-assisted Mn dissolution from the cathode, and was shown to reduce fade with layered CoNi cathode materials.The salt does not electrochemically decompose and thus requires an additive (1% LiPF6 was cited) to form an SEI layer on the anode.

Polymer electrolytes are touted as the wave of the future for EV/HEV batteries that are here today. While these are unquestionably safer than liquid electrolytes, polymers require a warm working environment (50-100°C) and may have insufficient conductivity for battery operation at ambient temperatures. Thinner films enhance Li-ion transport, but make for more delicate processing requirements. These parameters suggest EV rather than HEV applications for Li-polymer batteries. Considerable time and money are targeted at developing an electrochemically robust polymer stable in a 4+V environment.

Emerging Technologies

It is unusual to speak of new battery design in a system over a century old, but Firefly Energy claims a major breakthrough with PbH+ cells. By using a carbon foam/PbO composite instead of heavy lead plates, Firefly believes they can boost performance to near NiMH and Li-ion levels, but at one-fifth the cost. Further, the new, lighter batteries will have up to 70% less lead than current cells, and charge time will be reduced. Firefly expects first production in late 2007.

NiMH is also a mature technology, and industry pundits project incremental rather than revolutionary advances. Ovonics/Cobasys confidently predict HEV batteries producing 90Wh/kg and 1kW/kg within three years, attributed to design advances rather than novel chemistry.

No major changes are expected in supercap performance, although there is definite interest in substituting toxic acetonitrile electrolyte solvent for something more benign. Room temperature molten salts are replacement candidates that would extend the life and high-temperature stability of the devices.

It is in Li-ion technology that we find the greatest capacity for change. The field is still evolving: nanoparticulate anode and cathode materials, high-voltage intercalants, ionic liquid electrolytes, cermet and composite anodes, and electrochemically robust polymer electrolytes are all under development. These chemicals have the potential to rewrite the book on Li-ion batteries for the transportation industry. Table 4 shows the properties of these emerging materials and lists barriers to their commercial acceptance.

Li4Ti5O12 is the starting point for nanotechnology in battery materials. This anode intercalant functions at rates up to 50C and provides >150mAh/g capacity (theoretical is 155mAh/g). Further, there is little structural degradation during cell operation, resulting in >2000 cycle lifetimes. Other nanotech practitioners have found excellent reversibility with oxide intercalants, such as MnO2, TiO2, CrO3, and Fe2O3. Micron-sized particles are decidedly inferior in all respects: what accounts for this disparity?

As particle size shrinks, the distance that Li+ must travel within the electrode material to reach all active sites becomes smaller, and the process increasingly resembles a surface reaction. In the extreme case, the particle becomes two-dimensional, precluding crystal distortion. With decreasing impedance, redox reactions accelerate, and near-theoretical performance is attained. This rationale applies to all intercalating battery-active compounds, and may be especially relevant to multi-metal oxide anodes. As a class, many of these high-capacity materials will intercalate Li+, but only with a substantial first-cycle hysteresis: as much as 50% of the Li may be trapped by irreversible structural changes and lost to subsequent cycles.

The major concern over nanomaterials (besides the largely unsubstantiated hue-and-cry of health dangers) is the industry’s ability to scale lab processes from several grams per hour to production levels of tons per week. Spray calcination or plasma techniques resulting in <50nm particles are low-volume, expensive procedures that will require very innovative engineering to attain desired output volumes. Nanoparticles will be especially advantageous in power battery electrodes for high rate performance.

The past few years have seen the emergence of layered cathode materials with general formulae LixMyMnzNiwO2, where x e”1, M is a conductivity aid such as Co or Cu, Mn has a +4 oxidation state and doesn’t participate in the redox reactions, and Ni is present as Ni+2. These compounds are solid solutions of LiMn2O3 and Li(Co/Ni)O2 derivatives and feature an immunity to oxygen release with overcharging, thus are much safer than LCO derivatives. They also have capacities up to 330mAh/g when cycled over a 2.5-4.6V range, producing as much as 1.3kWh/g. Practitioners of this science include Thackeray and Amine at Argonne, Dahn at Dalhousie University, and initial commercial cells from Sanyo and Panasonic.

It is the high charge ceiling that is worrisome, as few, if any, electrolytes are stable above 4.3V. Doping with Cu appears to enhance the conductivity of the delithiated (charged) cathode, relieving the stress on the electrolyte. Another coping mechanism may be ionic liquids (below), which are more resistant to high voltage than organic or polymer solvents. The obvious application for layered cathode materials with extremely high energy is EV batteries, although if these compounds can be successfully nano-engineered, they could be suitable for HEV usage.

Ionic liquids, or room temperature molten salts, are noteworthy for their wide liquid range (as low as -65°C to decomposition at >400°C). They are also non-flammable and, with careful selection of substituents, offer a 5+V voltage window. The leading candidates at this time are asymmetric R4N+TFSI- salt solutions with LiTFSI [LiN(SO2CF3)2], first described by MacFarlane at Monash University. Commercial availability is severely limited, and until success dictates higher demand, prices will be high. Ionic liquids are adaptable to almost any battery market, especially where safety is a premium.

Cermet anodes are metal carbides, nitrides, phosphides, etc., and in general, will accept substantial amounts of Li+ at <1V. Micron-sized powders suffer from the same first-cycle Li retention, and there are no clear routes to nano-cermets that might avoid this problem. Nazar at the University of Waterloo is most active in this speculative area.

Composite anodes, led by Sony’s Sn-based material, are new to the battery arena, and comprise an intimate blend of carbon and an element(s) that easily alloys with Li, such as Sn, Si, Ag, or Cu. These anodes are more tolerant of rapid charging than graphitic carbon, which can lead to deposits of highly reactive dendritic Li, and are less susceptible to large volume changes common to Li alloy formation with single elements. The driving force is high capacity, and therefore lighter batteries, a distinct advantage in the weight-conscious auto industry.

Polymer electrolytes are already in the small-battery market, although there is much development to be done before transferring the technology to EV/HEVs. While safety is obviously enhanced compared to liquid electrolytes, conductivity is low unless the cell is warmed to 40°C or more, and high voltages are incompatible with these polymers, thus shortening cell life. Spraying or coating techniques produce very thin electrolyte membranes, yielding more cells per battery, and therefore more power and energy from the same sized pack, although such films are more fragile than desired. Frech and Glatzhofer (University of Oklahoma) are investigating polyimine derivatives (related to PEO, with N-R replacing O atoms) that show signs of stability at 4.5V, which would throw the window of opportunity wide open for EV/HEV battery usage.

Summary

If the Institute of Information Technology’s prediction is correct – 6 million HEVs produced in 2015, double that in 2020 – the automotive battery industry has major growth ahead of it (especially if you factor in technology-hungry India and China). Although mature energy storage technologies such as lead-acid, Ni metal hydride, and supercaps will always find niche markets, the future of EV/HEV energy storage clearly lies with Li-ion batteries. Such Li-ion attributes as power capability and capacity are superior (and improving!) to those found in alternate energy sources. Advances in the older devices will be incremental, based more on engineering refinements; Li-ion evolution will encompass new chemistries with performance levels and safety far surpassing today’s standards. While Li-ion batteries are still high-cost items, improved processing techniques, increased production, and a highly competitive market will bring prices down, and hybrid vehicles will become increasingly popular.