|Various PHEV targets and battery cell and pack potential mapped to a Ragone plot. Click to enlarge.|
A report from the Institute of Transportation Studies at UC Davis provides an overview of the current state of several battery chemistries—including NiMH and Li-ion—and their abilities to meet the goals and subsequent requirements for plug-in hybrid electric vehicles (PHEVs).
The authors do not intend for the report to be a definitive analysis of the technologies, but a primer for battery non-experts, and as a way to inform electric-drive interest groups, “including researchers, policymakers, companies, advocates and critics” about fundamental battery issues “to facilitate more grounded debates about the present and future of electric-drive vehicles, including plug-in hybrid vehicles.”
The report highlights four main conclusions.
Battery goals or requirements for PHEVs are contingent on a number of assumptions, and differ greatly based on those assumptions.
The “true” requirements of PHEV technology will depend on consumers’ driving and recharging behaviors as well as their valuation of different PHEV designs and capabilities. In turn, producer and consumer behavior alike can be shaped by government regulation, e.g., California’s ZEV mandate. Thus, while the USABC (and others) provides a useful benchmark for the future of PHEV battery technology, there may be a role for less ambitious PHEV designs, such as blended PHEV conversions, as well as Toyota’s demonstration of a PHEV Prius using NiMH.
Battery development is constrained by inherent tradeoffs among the five main battery attributes: power, energy, longevity, safety and cost. No battery currently meets all of the USABC’s PHEV goals for these attributes. Attempts to simultaneously optimize power, energy, longevity, and safety will increase battery cost.
Readers must be careful to understand the complex trade-offs among these attributes and among battery technologies. Certainly we must avoid assembling the best performances from different battery technologies on different drive cycles in different vehicles as an indication of the current state of battery technology.
Li-Ion chemistries are better suited than NiMH to meet USABC’s PHEV battery design goals of high power and energy density.
However, despite Li-Ion’s potential, the technology is not yet firmly established for automotive applications, and development must overcome issues of longevity and safety—and the resulting tradeoffs with performance—in order to achieve commercial success.
Li-Ion technology is following multiple paths of development. An overview table in the report illustrates eight different directions, each using different electrode materials in efforts to optimize power, energy, safety, life, and cost performance.
It is important, the authors note, not to generalize attributes about one battery—e.g, Toyota’s concerns about safety with its LCO battery—to all Li-Ion batteries.
In addition, Table 5 also demonstrates the complexity and uncertainty of selecting a single technological “winner” among advanced automotive battery chemistries.
|Click to enlarge.|
Axsen, Jonn, Andrew F. Burke, Kenneth S. Kurani (2008) Batteries for Plug-in Hybrid Electric Vehicles (PHEVs): Goals and the State of Technology circa 2008. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-08-14