chemistries—sodium sulfur, lead oxide or lead acid, and lithium iron phosphate—Bradwell explained that it is important to look not just at raw material costs, but at total battery costs. The elemental cost of sodium sulfur is tiny, less than $0.10/kilowatt-hour (kWh), whereas that for lead acid is much higher, between $10 and $30/kWh, and lithium iron phosphate costs come in around $1/kWh. Looking at the total cost of the battery paints a different picture. Sodium sulfur batteries cost $300 to $500/kWh, while a lead acid battery costs between $100 and $300 kWh and a lithium iron phosphate battery costs between $300 and $1,000/kWh (Wadia et al., 2011). The Advanced Research Projects Agency-Energy goal is $100/kWh (Figure 6-1).
Electrode material costs are not the only driver of battery cost. Processing costs to produce high-purity starting materials, battery assembly, and the expense of secondary components such as the electrolyte, current collector, current lead, housing, and safety features can be important cost factors. High-purity sodium and lithium iron phosphate are expensive to produce. In contrast, if the cost of lithium were to increase by a factor of 10, it would have little effect on the price of a lithium battery, said Bradwell.
Supply issues can be a concern. The United States has substantial reserves of lithium brines, and China, Canada, Brazil, and Australia have reserves of lithium carbonate. Bolivia has the largest resources of lithium brines, but their profitability is uncertain. There are strategic concerns about lithium supplies, but lithium ranks at the low end of supply risk in terms of minable reserves.
“Cost and natural abundance alone do not appear to be major limitations for lithium,” said Bradwell. “There are opportunities for better understanding the system and the processing costs and materials. In the academic world, people get very excited about making something that works and works well, but for these broad-scale applications, we should be thinking about assembly, processing, manufacturability of the system, and scalability.” He added that there is an opportunity for possible collaboration between those who are inventing the chemistries and batteries and the experts who could ultimately help build and deploy these systems.
FIGURE 6-1 Costs of raw electrode material affect final battery costs.
SOURCE: Adapted from Wadia et al. (2011, p. 1596, in Bradwell 2011a).
New approaches to making batteries can use lower-cost processes and assembly methods. In addition, it may be possible to develop batteries that use lower-purity, and therefore lower-cost, materials. There may also be ways of improving lithium extraction processes to enable the use of lowerquality ores or brines.
While the conventional metrics for battery storage are energy density and power density, the key metrics for grid-based storage are cost, lifespan, and energy effciency, said Bradwell. Looking at energy density, lithium and sodium sulfur batteries score well, while pumped hydroelectric power compares poorly. However, for grid storage, pumped hydro dominates (Figure 6-2) because of its low cost to deploy, long lifespan, and its greater than 70 percent effciency. Bradwell noted that there is a nice relationship between the cost of a technology and its deployed capacity. At a cost of between $100 and $200/kWh, adoption takes off.
Other promising battery technologies include flow batteries, magnesium batteries, and zinc manganese oxide batteries. Lead acid batteries, a technology that has been around for a long time, also have the potential to contribute to grid-based storage.
Challenges must be solved for all of these new battery chemistries. Dendrite formation, a particular problem with zinc batteries, occurs when metal is deposited on a battery electrode, extends across the electrode, and eventually shorts the battery. Electrode cracking and deterioration is still a challenge that arises when material intercalates within the structure of the electrode. This is a major mechanism for electrode failure in batteries. Nanostructured materials may solve this problem, but these materials are more expensive to use.
Interfacial film growth caused by the slow reaction with electrolytes is another major challenge. The electrolyte in a