When Gaston Planté invented the lead–acid battery more than 160 years ago, he could not have foreseen it spurring a multibillion-dollar industry. Despite an apparently low energy density—30 to 40% of the theoretical limit versus 90% for lithium-ion batteries (LIBs)—lead–acid batteries are made from abundant low-cost materials and nonflammable water-based electrolyte, while manufacturing practices that operate at 99% recycling rates substantially minimize environmental impact (1). Nevertheless, forecasts of the demise of lead–acid batteries (2) have focused on the health effects of lead and the rise of LIBs (2). A large gap in technological advancements should be seen as an opportunity for scientific engagement to expand the scope of lead–acid batteries into power grid applications, which currently lack a single energy storage technology with optimal technical and economic performance.
In principle, lead–acid rechargeable batteries are relatively simple energy storage devices based on the lead electrodes that operate in aqueous electrolytes with sulfuric acid, while the details of the charging and discharging processes are complex and pose a number of challenges to efforts to improve their performance. This technology accounts for 70% of the global energy storage market, with a revenue of 80 billion USD and about 600 gigawatt-hours (GWh) of total production in 2018 (3). Lead–acid batteries are currently used in uninterrupted power modules, electric grid, and automotive applications (4, 5), including all hybrid and LIB-powered vehicles, as an independent 12-V supply to support starting, lighting, and ignition modules, as well as critical systems, under cold conditions and in the event of a high-voltage battery disconnect (3). Although the principle of operation has not changed, manufacturers have improved this technology by optimizing performance of the electrodes and active components mainly for application in vehicles. Future performance goals include enhanced material utilization through more effective access of the active materials, achieving faster recharging rates to further extend both the cycle life and calendar life and to reduce their overall life cycle cost with a direct impact on the implementation of grid storage systems.
The constant dissolution and redeposition of the cell’s active materials, over each charge–discharge cycle, creates a situation where both positive and negative electrode morphology and microstructure are constantly changing (see first the figure). These structural changes enable the corrosion of electrode grids typically made of pure lead or of lead-calcium or lead-antimony alloys and affect the battery cycle life and material utilization efficiency. Because such morphological evolution is integral to lead–acid battery operation, discovering its governing principles at the atomic scale may open exciting new directions in science in the areas of materials design, surface electrochemistry, high-precision synthesis, and dynamic management of energy materials at electrochemical interfaces. This understanding could have a direct impact on battery life, as preserving the overall electrode surface area ensures effective charge–discharge processes.
These efforts must take into account the complex interplay of electrochemical and chemical processes that occur at multiple length scales with particles from 10 nm to 10 µm (see the second figure) (5). The active materials, Pb and PbO2, are traditionally packed as a self-structured porous electrode. When discharged, Pb2+ ions quickly react with the available sulfuric acid in the electrolyte and nucleate insoluble PbSO4 crystals. During charging, PbSO4 must be converted back to Pb and PbO2, which is a thermodynamically and kinetically more demanding process given the poor solubility of the PbSO4 crystals. The intricate relationship between acid concentration gradients within the electrode pores and lead sulfate dissolution rates underscores the challenge of improving the battery’s ability to recharge at fast rates.
All of these processes occur in competition with the thermodynamically favored but undesired water-splitting reactions that evolve O2 and H2 gases. Lead and lead dioxide are poor catalysts for these reactions and have high overpotentials that kinetically limit these processes unless fast charging occurs with high voltages. However, metal and ionic impurities in electrodes and electrolyte facilitate electrolysis of water and its loss (5). The requirement for a small yet constant charging of idling batteries to ensure full charging (trickle charging) mitigates water losses by promoting the oxygen reduction reaction, a key process present in valve-regulated lead–acid batteries that do not require adding water to the battery, which was a common practice in the past.
Some of the issues facing lead–acid batteries discussed here are being addressed by introduction of new component and cell designs (6) and alternative flow chemistries (7), but mainly by using carbon additives and scaffolds at the negative electrode of the battery (4), which enables different complementary modes of charge storage (supercapacitor plus faradaic Pb charge–discharge). These electrodes also offer a rigid, unreactive, and conductive electrode backbone that prolongs cycle life.
At the positive electrode, identification of a material that can withstand the high electrode potentials and harsh acidic environment remains a problem to be solved. Utilization of bipolar electrodes can reduce the amount of lead used for structural components (electrode grid), immediately improving material utilization, but challenges with corrosion and cost-effective manufacturing are still a limiting factor. Implementation of battery management systems, a key component of every LIB system, could improve lead–acid battery operation, efficiency, and cycle life.
Perhaps the best prospect for the unutilized potential of lead–acid batteries is electric grid storage, for which the future market is estimated to be on the order of trillions of dollars. For that reason, the low cost of production and materials, reduced concerns about battery weight, raw material abundance, recyclability, and ease of manufacturing make it an attractive solution if technical barriers can be addressed. At a current spot price below $2/kg and an average theoretical capacity of 83 ampere hours (Ah)/kg (which includes H2SO4 weight and the average contribution from Pb and PbO2 active materials) that rivals the theoretical capacity of many LIB cathode materials (8), lead–acid batteries have the baseline economic potential to provide energy storage well within a $20/kWh value (9).
Despite perceived competition between lead–acid and LIB technologies based on energy density metrics that favor LIB in portable applications where size is an issue (10), lead–acid batteries are often better suited to energy storage applications where cost is the main concern. In reality, LIB technology has been more detrimental to nickel–metal hydride and nickel-cadmium battery markets (3). The increased cost, small production rates, and reliance on scarce materials have limited the penetration of LIBs in many energy storage applications.
The inherent concern surrounding lead–acid batteries is related to the adverse health and environmental effects of lead (11). More effective mitigation is feasible with application of known practices, strict government regulations, and improved training and engineering controls, which would further increase the already impressive recycling rate of 99% (12). Also, many serious safety and health concerns exist as part of LIB manufacturing and operation, including the carcinogenic potential of Ni and Co oxide components of cathode materials, the production of highly toxic organofluorophosphate neurotoxins as a consequence of thermal runaway events (battery fire and explosion) (8, 13) and potential contamination of the environment with toxic organofluorine by-products arising from electrolytes and additives (14).
As with any technology, many of the associated risks can be limited with proper management of materials, good manufacturing practices, and committed waste management. The 99% recycling rate of lead–acid batteries (12) and stringent regulations on Pb environmental emissions greatly minimize the risk of Pb release to the environment. Alternatively, the lack of economically feasible recycling solutions to LIB technology in the short term, combined with the expected increase in the number of battery cells that are approaching their end of life, aggravate the potential for environmental contamination from discarded LIB systems. Accidental inclusion of LIBs in lead battery recycling has proven hazardous, and better safety and recyclinge protocols are needed.
The range of tools and methods developed over the past 30 years, both experimentally and theoretically, are readily applicable to further develop and elucidate the science of lead–acid batteries. These topics would greatly benefit from further engagement from U.S. National Laboratories and across academia (15). Leveraging our current scientific knowledge and an established manufacturing industry with admirable safety and recycling records would ensure strong economic, technical, and environmental support for lead–acid batteries to continue serving as part of a future portfolio of energy storage technologies.
Acknowledgments: The approach applied to develop structure-function correlations was funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. The research efforts were supported by the Lead Battery Science Research Program through a Cooperative Research and Development Agreement. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. We thank E. Coleman, D. Strmcnik, M. Zorko, C. Ferels, N. Chaudhari, and in memoriam Stefan Djokic for support in experiments.
Recent Comments