Ahmad A. Pesaran
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Electric Vehicle (EV) sales and adoption have seen a significant growth in recent years, thanks to advancements and cost reduction in lithium-ion battery technology, attractive performance of EVs, governments’ incentives, and the push to reduce greenhouse gases and pollutants. In this article, we will explore the progress in lithium-ion batteries and their future potential in terms of energy density, life, safety, and extreme fast charge. We will also discuss material sourcing, supply chain, and end-of-life-cycle management as they have become important considerations in the ecosystem of batteries for the sustained growth and adoption of EVs. With significant government and private sector investments in research and development, processing, and manufacturing and advances in anodes (lithium and silicon), cathodes (high nickel), designs, supply chain development, and the circularity of lithium-ion batteries, lithium-based batteries are on track to make EVs mainstream, addressing climate concerns of fossil-fueled vehicles.
Electric vehicles (EVs) have been around for a while, but it is only in recent years that their popularity has risen steeply. This has been driven largely by EVs’ attractive features of better driving performance, improved battery energy density, lower lithium-ion battery cost, reduced environmental footprint, and, of course, incentives and deadlines imposed by governments around the world. Bans on sales of gasoline and diesel vehicles by 2030 or 2035 are already on the books in several countries and a few states in the United States. Globally, 3 million EVs were sold in 2020 (a 40% jump relative to the year before), 5.6 million EVs were sold in 2021, and about 10 million EVs were sold in 2022. BloombergNEF is now forecasting EV sales (https://about.bnef.com/electric-vehicle-outlook/) to be 20%–30% higher than the 2019 projections shown in Figure 1.
Figure 1. The annual sales of passenger EVs: past and forecasted. (Source: https://legacy-assets.eenews.net/open_files/assets/2019/05/15/document_ew_02.pdf; used with permission.)
One of the main reasons for this is the improvement in lithium-ion battery technology, which has made EVs more practical and affordable. Lithium-ion batteries are the preferred choice for EVs due to their high energy density, long life, and lower cost. A significant amount of lithium-ion batteries will be needed in the coming years based on the demand in EVs and the renewable grid (Figure 2). This means that materials sourcing, processing, and supply chain are important considerations in the United States.
Figure 2. The global demand for lithium-ion batteries (LiBs) is expected to be over 3,100 GWh by 2030. CAGR: compound annual growth rate. (Source: https://www.mynewsdesk.com/rolandberger/pressreleases/rising-demand-for-lithium-ion-batteries-may-lead-to-shortages-in-raw-material-supply-3173780; used with permission.)
In this article, we will examine the progress in lithium-ion battery technology and the future potential and challenges ahead.
The first commercially available lithium-ion battery was introduced in 1991 by Sony, and since then, it has undergone significant improvements. In the past few years, there has been a notable improvement in the performance and cost of lithium-ion batteries, making them more suitable for EVs, grid and industrial applications, in addition to being the major source for consumer electronics. The basic working principle of lithium ion batteries is that lithium ions move from the negative electrode through an electrolyte to the positive electrode during discharge and back when charging. As a result, unlike many batteries such as lead-acid batteries, lithium-ion batteries could be made from many cathode/anode chemistry pairs. Note that to carry current out of lithium-ion batteries, the cathode materials are coated on aluminum, and the anode materials are coated on copper foils. The solid electrolyte interphase is a passivation layer formed on electrode surfaces (particularly graphite anode) that prevents further reaction with the electrolyte; it plays a role for life and safety. Figure 3 shows a schematic of how the lithium-ion battery works and many of the complements and options that could be used.
Figure 3. Lithium-ion battery operation, potential chemistries, and components.
Note that Figure 3 also shows both mature/commercial technologies, such as the nickel cobalt aluminum cathode, graphite anode, electrolyte (carbonate solvents with LiPF6 salt), and polypropylene separator, and technologies under development, such as the no-cobalt nickel-rich cathodes, silicon or lithium metal anodes, solid electrolytes, and ceramic separators. The significant progress in lithium-ion battery technologies that occurred in the last 10–12 years have led to cost reduction battery packs by 85% reaching today to around US$135/kWh.
Although significant improvement has been made in lithium-ion batteries, still there are challenges that need to be addressed for the mass adoption of EVs. In the last couple of years, material sourcing, supply chain, and end-of-life-cycle management have become important considerations in the United States. Figure 4 depicts those challenges. Significant efforts and investments are underway in the United States and other countries to address these challenges through research, development, deployment, manufacturing, sourcing, and end-of-life management.
Figure 4. Lithium-ion battery technology challenges for mass adoption of EVs.
The energy density for lithium-ion batteries used in EVs has significantly improved in recent years. The energy density refers to the amount of energy that can be stored in a battery per unit volume. The specific energy is the amount of energy that can be stored in a battery per unit mass. These are sometimes used interchangeably. However, as the specific energy becomes large enough, the passenger EVs need batteries with higher energy densities to allow larger spaces/volumes in the vehicle for consumer’s convenience. For heavy-duty vehicles, buses, and long-haul trucks, batteries with higher specific energies are needed. The higher the specific energy, the longer the range an EV can travel on a single charge. When EVs first came onto the market, the range they could travel on a single charge was limited. However, advances in lithium-ion battery technology have made significant improvements in energy density, leading to longer-range EVs. In the early days of EVs, the average energy density of lithium-ion batteries was around 100–120 Wh/kg (for example, for a lithium-iron phosphate cathode with a graphite anode). Today, with further developments, energy densities of up to 300 Wh/kg are becoming more common, with some manufacturers claiming specific energy of over 400 Wh/kg. These improvements in energy density have enabled EVs to travel farther, for example, 300+ miles on a single charge, reducing range anxiety and making them more practical for everyday use. However, it is important to note that higher energy density also comes with its own set of challenges. As the energy density of a battery increases, so does the risk of safety issues such as thermal runaway. Additionally, batteries with high energy density may require more extensive thermal management systems to prevent overheating during charging or use.
In recent years, the direction of technology has been to use more cathode and anode active materials and reduce the amount of inactive materials, for example, by using thinner copper foils, anode foils, and separators. Another trend in the last couple of years has been toward the use of lithium iron phosphate (LFP) cathodes because of being less prone to thermal runaway, resource availability, sourcing, and supply chain issues for nickel and cobalt.
The (cycle and calendar) life of a battery is another crucial factor to consider. The factors that contribute to the capacity and power degradation are the loss of cyclable lithium, solid electrolyte interphase growth, mechanical stress on the electrode, particle cracking, side reactions, active material structure changes, and an increase in resistance. The life of lithium-ion batteries can be affected by factors such as the temperature, depth of discharge, and charging and discharging rates. The cycle life of a battery is determined by the number of charge and discharge cycles it can undergo before its capacity decreases significantly, usually to 70%–80% of the original capacity and power as they may no longer provide the driving performance needed for EVs. The number of cycles is equivalent to how many miles an EV can drive. The calendar life is how long a battery will last in terms of time (years for EVs) regardless of the number of charge/discharge cycles. Figure 5 provides an example of degradation testing of a lithium-ion cell at various temperatures and states of charge. The recent generation of lithium-ion batteries has a long life span of 2,000–4,000+ cycles (depending on the depth of discharge) and about 8–12 years. Lithium iron phosphate batteries can withstand many cycles (5,000+) and have long lives (10+ years).
Figure 5. An example of battery capacity loss cycling between 25% and 95% state of charge at different temperatures.
The latest development in the cycle life of lithium-ion batteries for EVs is the use of advanced electrode materials, prelithiation, and battery management systems (BMSs).
Researchers are developing new electrode materials that can improve the cycle life of lithium-ion batteries. For example, silicon–graphene composite anodes have been shown to significantly improve the stability of anode at liquid interface lead to improve life. Other promising electrode materials include high-nickel cathodes and lithium titanate anodes. Batteries with lithium iron phosphate cathodes can withstand many cycles (5,000+) and have long lives (10+ years).
BMSs are critical for monitoring the performance of cells and the system. Advanced BMSs can monitor battery health and adjust charging and discharging parameters to extend battery life.
Temperature impacts battery performance and life and thus the performance and life of EVs. Low temperatures reduce battery power (acceleration) and energy (range) capabilities. Higher temperature reduces lithium-ion battery power and energy in the long term and impacts safety. So, the EVs require a thermal management system (both cooling and heating) to keep all cells in the desired temperature range specified by battery manufacturers, minimize cell internal temperature inhomogeneity, minimize cell-to-cell temperature differences in a pack, and prevent the battery from going above or below acceptable temperature range and safety limits.
The Tesla Model S uses a nickel-cobalt-aluminum lithium-ion battery pack with a claimed cycle life equivalent to 500,000 miles and a calendar life of 10 years. However, this is based on laboratory testing, and the actual cycle life of the battery can vary depending on usage and environmental conditions. The Nissan Leaf uses an NMC622 lithium-ion battery pack with a claimed calendar life of 10 years and a cycle life equivalent to 100,000 miles, whichever comes first. The Chevrolet Bolt EV uses an NMC622 lithium-ion battery pack with a claimed calendar life of 10 years and a cycle life equivalent to 150,000 miles, whichever comes first. The BMW i3 uses a NMC622 lithium-ion battery pack with a claimed calendar life of seven to eight years and a cycle life equivalent to 100,000 miles, whichever comes first. It is important to note that these are manufacturer claims and that the actual cycle life of a lithium-ion battery can depend on several factors such as the environment, driving habits, and number of charge/discharge cycles, particularly ones with deep discharge. Additionally, as battery technology continues to evolve, it is likely that we will see improvements in cycle life over time.
Safety is a critical concern when it comes to lithium-ion batteries, especially in EVs. There is potential for thermal runaway and fire with them as they potentially contain the ingredients of the fire triangle (fuel, oxygen, and heat). Most of the high-performing lithium-ion batteries use organic carbonate electrolytes that are flammable and could act as fuel. Graphite and silicon anodes could also contribute as low-grade fuels. Most cathodes are oxides and generate oxygen. Heat could come from external short circuits, overcharging, external heating, nail/rod/object penetration, crush, and internal short circuits (due manufacturing defects). When a battery experiences heat and higher temperatures (onset temperature), chemical reactions that are exothermic start to further increase the temperature of the cell, which leads to further heat release. If the heat rejection from the cells is not enough to keep it below certain temperatures (the thermal runaway point), the separator loses its integrity, positive and negative electrodes come in contact with each other, further heat is generated, and the cell can vent electrolyte and reaction gases and catch fire. Peak temperatures during the thermal runaway could be between 400 and 1,200°C. Figure 6 depicts the cascading reactions after a lithium-ion battery experiences an abuse that could lead to thermal runaway, fire, and even explosion.
Figure 6. The thermal runaway due to cascading reactions after a lithium-ion battery experiences an abuse. (Source: Pesaran et al. 2017; used with permission.)
To ensure safety, lithium-ion batteries are designed with safety features such as thermal management systems, heat-absorbing materials, fuses, cell balancing, additives to electrolytes, special separators, and protection circuits. Structurally sound battery enclosures are installed out of EV crash zones, so they do not get damaged during accidents. It is important to characterize how cells and batteries respond to various abuses to understand how much heat is released, what the maximum temperatures are, what kind of gases come out, and when and if a fire is initiated. Accelerating rate calorimeters, differential scanning calorimeters, and various explosion-proof chambers used for nail penetration are used by battery researchers and developers. Researchers and developers are also exploring ways to enhance the safety of lithium-ion batteries further.
Currently, most EV drivers use home charging (mostly 1.6 kW ac level I) that adds four to five miles of range per hour. Drivers who have access to work charging can use 6.2 kW ac level II chargers that can provide 15–25 miles of range per hour (https://blog.evbox.com/ev-charging-levels). Public/private level III fast chargers currently have the power capability of 40–120 kW and can add 50 to 120 miles per hour. Although fast charging is good for around-town driving and short excursion driving, it provides an option for long-distance interstate driving with a large fast charging network.
To increase the adoption of EVs by the public, most future consumers would like to fully charge their vehicle in less than 15 min to be on par with gasoline vehicles. This extreme fast charging to add 200–300 miles in less than 15 min requires a charger with 350+ kW and 400+ V capabilities. During fast charging, thermal management of the battery is essential to make sure battery temperature does not go beyond 50–60 °C to not adversely impact the life and safety. The need for thermal management of the battery and the charger becomes paramount with extreme fast charging. Current commercial lithium-ion batteries are not designed to handle such high rates as lithium plate (on graphite anode) and fast degradation could happen with the current thick electrodes. Researcher and developers are making progress in extreme fast charging of batteries through adaptive electrochemical charging protocols, advanced electrodes with less tortuous paths or microstructure features, and novel electrolytes. As silicon has five to eight times the capacity of graphite anodes, researchers are exploring adding silicon to the graphite anode to various degrees to reduce the thickness of the electrode and prevent lithium plating. However, silicon/graphite anodes have their own set of challenges, including reduced calendar life.
Eventually, barriers to extreme fast charging of lithium-ion batteries will be resolved, and extreme fast charging technologies will become public, so that new drivers will be able to use them in cities and on interstate highways.
The supply chain for lithium-ion batteries is complex and involves multiple stages, from material sourcing (mining) to processing battery-grade material, component production, cell manufacturing, distribution, and finally end-of-life-cycle management. The diagram in Figure 7 shows the circular economy needed for the sustainability of lithium-ion batteries. They require a range of materials, including lithium, cobalt, nickel, manganese, copper, aluminum, electrolytes, and graphite. Ensuring a stable and secure supply chain is crucial for the growth of the EV industry. There are also concerns about where these metals come from; for example, there are environmental concerns about mining and the use of too much water for Li extraction or ethical concerns such as child labor in cobalt mines in the Republic of Congo, which has the largest cobalt resource and produces 60% of global cobalt. China produces and controls more than 70% of cathodes and 90% of graphite anode globally, and this has created concerns in the United States and other industrial countries that their supply of these materials could be distributed with increased internal demand or geopolitics. The United States does not have many cobalt or nickel resources, and currently, the amount of lithium and graphite produced in the United States is limited, although there are sufficient resources available that could be extracted and processed. Researchers are exploring ways to reduce the use of cobalt materials and develop more sustainable alternatives. The National Renewable Energy Laboratory has developed a database of North American companies involved in various segments of the lithium ion battery supply chain (https://www.nrel.gov/transportation/li-ion-battery-supply-chain-database.html). Mining, material extraction, and battery-grade processing are not strong in the United States. To address these issues, the U.S. Congress has provided billions of dollars through the Bipartisan Infrastructure Law of 2021 and the Inflation Reduction Act of 2022 to address the mining and cathode/anode processing by industry in the next three to five years.
Figure 7. The circular economy of lithium-ion batteries. (Source: https://www.nrel.gov/docs/fy21osti/77035; used with permission.)
The end-of-life-cycle management of lithium-ion batteries is an important area that has received significant attention because of supply chain and material sourcing issues. As more EVs are produced, there will be a dramatic increase in the number of end-of-life EV batteries in the coming years. When a battery reaches the end of its life in an EV, it needs to be removed for reuse in other applications or recycled for the recovery of key materials to be reintroduced to make new batteries rather than being disposed. The benefits of reusing and recycling lithium-ion batteries include reducing waste, conserving natural resources, reducing the environmental impact of battery production, and compensating for the costs associated with end-of-life decommissioning.
Studies have shown that at the end of its life, the EV battery could have at least 70% of its initial capacity if it is not damaged or failed. The remaining capacity can be more than sufficient for most energy storage applications; the second-life battery can continue to work for another 10 years or more. Many studies have concluded that end-of-life EV batteries are technically feasible for second-use applications such as the stationary grid, backup power, or other applications. Although there are viable business models for high-value, small, and niche application for second-use batteries (forklifts, powering portable devices, replacing diesel backup generators, and aftermarket replacement packs for plug-in EVs), the economic viability of installing many GWh of second-life batteries is still evolving. The costs associated with the purchasing price of end-of-life batteries, transportation, storage, sorting and testing, remanufacturing, reassembly and repurposing, integration into battery energy storage systems, certification, and installation could impact economic viability. Like new batteries, if economical, second-life batteries could create revenue by providing energy and power for many applications such as solar + storage, renewable energy firming, grid voltage support, demand charge management, providing peaking capacity, and spinning reserve, to name a few. After the second (or third) use, these batteries could provide additional revenue by being recycled for the recovery of their valuable materials.
To recover valuable materials from lithium-ion batteries, there are three major technologies currently in various stages of commercialization: pyrometallurgy, hydrometallurgy, and direct recycling. In addition to these methods, mechanical treatment (through disassembly, crushing, shredding, and separation) to create what is called black mass is a major element of any recycling technology. Pyrometallurgy is the process of high-temperature thermal treatment of batteries in a furnace to extract metals and intermediate compounds that can be further processed to create battery-grade precursors. The hydrometallurgical process uses chemical treatment to extract the key compounds in the black mass, including the lithium compounds. The process uses leaching fluids such as inorganic acid, organic acid, alkali, or even bacteria solutions that dissolve metals in the cathode to salts that can be used as precursors to make new cathodes. Many companies in the United States and around the world are building factories based on hydrometallurgy because of lower capital expenditure and flexibility to directly produce cathodes. In the next few years, several facilities will come online to recycle the onslaught of batteries being retired. Direct recycling involves the recovery of the cathode while maintaining its molecular structure, rather than breaking it down into its constituent metals for reprocessing into a battery-grade cathode. Eliminating these steps makes the prospect of direct recycling most economically viable. With improvement in efficiencies and at-scale production of cathodes of the future, direct recycling factories could be the choice for the future.
There are also new methods being researched and developed, including the use of robotics for more efficient disassembly, ultrasound for improved metals separation, chemical removal of impurities from black mass, and separation through electroplating or electrochemistry that could reduce the cost of recycling in the future.
With the expected growth of EVs, lithium-ion batteries will become even more critical. Lithium-ion batteries are expected to see increased use in passenger vehicles, trucks, medium-/heavy-duty vehicles, and grid energy storage. One of the main factors affecting the energy density of a lithium-ion battery is the choice of cathode material. Current commercial cells for EVs have energy densities of 250 to 300 Wh/kg. Different cathode materials have different energy densities, and researchers are continuously exploring new materials that can improve the energy density of lithium-ion batteries. For example, researchers are investigating the use of high-nickel cathodes, which have a higher energy density than traditional cathodes. The technology is moving in the direction of being able to deliver commercial products with 350 to 450 Wh/kg in the next few years.
The amount of lithium that can be stored in the cathode material also affects the energy density of the battery. Theoretically, a higher amount of lithium stored in the cathode would result in a higher energy density. However, there are limits to how much lithium can be stored in a cathode material without causing safety issues such as thermal runaway.
Another factor that affects the energy density of lithium-ion batteries is the anode material. Graphite anodes can store a limited amount of lithium ions, which limits the energy density of the battery. Researchers are exploring the use of silicon anodes, which can store more lithium ions, thus increasing the energy density of the battery (see below).
The state of the energy density of lithium-ion batteries for EVs is constantly evolving as researchers continue to develop new materials and improve existing ones. While there is still room for improvement, lithium-ion batteries have already made significant progress, enabling the widespread adoption of EVs.
Silicon anodes have been researched as a potential alternative to graphite anodes in lithium-ion batteries for many years. Silicon has an eight-times-higher theoretical specific capacity for lithium-ion storage than graphite, which means that it can store more energy per unit mass. However, silicon could expand 300% or more impacting structural integrity of anode particles, so practically speaking, the capacity of silicon anodes could be not more than three to five times the capacity of graphite (which is 350 mAh/g). However, silicon anodes have faced some challenges in practical implementation due to their tendency to expand and contract during charge and discharge cycles. This can cause the anode to crack and lose its electrical conductivity, leading to reduced battery performance and shorter lifetimes.
Recent research has made progress in overcoming these challenges by using various strategies, such as designing nanostructured silicon materials, coating silicon with carbon or other materials to improve stability, and incorporating silicon into composite anodes with other materials. These approaches have shown promising results in improving the performance and stability of silicon anodes in lithium-ion batteries. Developers have been able to use various containment methods to be able to handle expansion and get more than 1,000 deep cycles with silicon or silicon/carbon composites.
For some applications such as drones or consumer electronics, the cycle life is much more important than the calendar life, so the early use of silicon anodes would be in these applications; however, the calendar life of silicon anode is not near the 8–12 years required for automotive applications.
Silicon anodes still face some technical challenges, but they have the potential to significantly improve the energy density and performance of lithium-ion batteries in the future.
Lithium-sulfur (Li-S) batteries are a promising alternative to lithium-ion batteries, with the potential to offer higher energy density and lower cost. However, there are still several challenges that need to be overcome before they can become a viable commercial option for EVs. Figure 8 depicts the operation of lithium-sulfur batteries.
Figure 8. A diagram showing the components of a typical Li-S cell, which consists of a Li-metal anode, a polymeric separator, a sulfur–carbon blend cathode, and an ether-based electrolyte.
One of the main advantages of Li-S batteries is their high theoretical energy density, which is up to five times higher than that of conventional lithium-ion batteries. This is because sulfur is a very lightweight and abundant element and can store a significant amount of energy when used as the cathode material in a battery.
However, Li-S batteries also have several disadvantages that need to be addressed. One of the biggest challenges is their low cycle life, which refers to the number of charge and discharge cycles a battery can undergo before its capacity decreases significantly. This is because during each cycle, the sulfur cathode undergoes significant structural changes, leading to degradation of the electrode material.
Another challenge is the low power density of Li-S batteries, which means they cannot deliver as much power as lithium-ion batteries. This makes them less suitable for high-performance applications such as EVs.
There have been significant efforts to address these challenges and improve the performance of Li-S batteries. One approach is to develop new sulfur cathode materials that are more stable and can withstand more cycles without degrading. This has led to the development of materials such as sulfur–carbon composites, which have shown improved cycle life and power density.
Another approach is to modify the electrolyte used in Li-S batteries. One promising option is to use ionic liquids, which are salts that are liquid at room temperature. These can improve the stability of the sulfur cathode and extend the cycle life of the battery.
Although there are still challenges that need to be addressed, the potential advantages of Li-S batteries make them an attractive option for energy storage applications. Ongoing research and development efforts are focused on addressing these challenges and unlocking the full potential of Li-S batteries for commercial applications.
Solid-state lithium-ion batteries are an emerging technology that has the potential to overcome some of the limitations of current lithium-ion batteries. Solid-state batteries use a solid electrolyte instead of a liquid electrolyte, which makes them safer, more energy dense, and longer lasting. Solid-state batteries can potentially offer several benefits over traditional lithium-ion batteries, including the following:
Despite these benefits, solid-state lithium-ion batteries are still in the research and development phase, and several challenges need to be overcome before they can be commercialized. One of the main challenges is developing a solid electrolyte that can provide high ionic conductivity while remaining stable over multiple charge and discharge cycles. Another challenge is the fabrication of solid-state batteries at a reasonable cost, as current manufacturing techniques are expensive and complex.
Lithium-ion batteries have come a long way since their introduction in 1991. They have become the preferred choice for EVs due to their high energy density, long life, relatively low cost, and fast charging capabilities. However, there is still room for improvement, and researchers and developers are exploring ways to increase the energy density, improve safety, lower cost, and enhance the extreme fast charging capabilities of lithium-ion batteries. High nickel, no-cobalt cathodes, silicon anodes, lithium anodes, lithium-sulfur, and solid-state electrolytes could push the technologies to achieve 350+ Wh/kg cells. With the expected growth of EVs, material sourcing, supply chain, and end-of-life-cycle management have become important considerations. It is crucial to ensure that the supply chain for lithium-ion batteries is secure and ethical and that the end-of-life-cycle management is sustainable and environmentally friendly. Another important development in the cycle life of lithium-ion batteries is the development of recycling and reuse technologies. By recycling and reusing lithium-ion batteries, the overall demand for new batteries and their materials can be reduced, which can reduce the environmental impact of battery production. With these developments and significant government and private sector investments and addressing supply chain issues, the future of lithium-based batteries and EVs is bright.
This work was authored by the National Renewable Energy Laboratory, operated by the Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under contract no. DE-AC36-08GO28308. Funding was provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Vehicle Technologies Office. The views expressed in this article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.
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Ahmad A. Pesaran (ahmad.pesaran@nrel.gov) is the chief energy storage engineer with the National Renewable Energy Laboratory, Golden, CO 80401, USA.
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