Materials and Processing for lithium-ion Batteries

Materials and Processing for Lithium-Ion Batteries

Lithium-ion battery technology is projected to be the leapfrog technology for the electrification of the drivetrain and to provide stationary storage solutions to enable the effective use of renewable energy sources. The technology is already in use for low power applications such as consumer electronics and power tools. Extensive research and development has enhanced the technology to a stage where it seems very likely that safe and reliable lithium-ion batteries will soon be on board hybrid electric and electric vehicles and connected to solar cells and windmills. However, the safety of the technology is still a concern; service life is not yet sufficient, and costs remain high. This paper summarizes the state of the art of lithium-ion battery technology for non-experts. It lists materials and processing for batteries and summarizes the costs associated with them. This paper should foster an overall understanding of materials and processing and highlight the need to overcome the remaining barriers for a successful market introduction.

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INTRODUCTION

Worldwide battery demand, primarily driven by consumer electronics and electric power tools, is projected to rise at a 6.9% annual rate, reaching $73.6 billion.

The effective use of low-emission and emission-free energy sources, such as renewable but intermittent wind and solar energy, requires stationary, high-yield, long-lasting, and low-maintenance electrical energy storage solutions. In Germany, the leading nation in wind energy utilization as part of its overall energy portfolio, 15% of the wind-produced energy was wasted due to the lack of suitable electrical energy storage.

Hybrid electric vehicles (HEVs) and all-electric vehicles (EVs) can help reduce U.S. dependence on foreign oil and will contribute to battery demand in the future. Counting engine efficiencies and electrical energy production, EVs could reduce gasoline consumption to one-fourth of today’s levels and could decrease U.S. dependence on imported oil to one-sixth of current levels.

The focus of the U.S. Department of Energy’s (DOE) Vehicle Technologies Program is on lithium-ion-based electrochemical energy storage due to its electrochemical potential and theoretical capacity. Lithium-ion batteries provide reliable rechargeable storage technology. Developments in this program include lithium-ion, lithium-ion-polymer, and lithium-metal technology.

The DOE’s short-term goals for power-assist HEVs are met or exceeded in eight of eleven areas, showcasing the program's success. These areas include discharge pulse power, regenerative pulse power, available energy, efficiency, cycle life, system weight, system volume, and self-discharge. However, three goals remain unmet: maintaining operational temperatures from -30°C to 52°C, achieving a lifetime of 15 years, and reducing the selling price to between $500 and $800 per system at an output of 100,000 units per year. For plug-in hybrid electric vehicles (PHEVs) in the intermediate term and for EVs in the long term, significant advances still need to be made to meet these goals. Figure 1 illustrates the DOE and the U.S. Advanced Battery Consortium (USABC) goals and milestones for HEV and EV applications.

The DOE program is focused on overcoming technical barriers associated with HEV battery technology, namely cost, performance, safety, and life:

• Cost: Current lithium-ion battery costs per kilowatt remain approximately twice as high as desired. Main costs are attributed to the high costs of raw materials and material processing, as well as costs associated with the cell, packaging, and manufacturing.
• Performance: Performance barriers primarily relate to reduced discharge power in low temperatures and loss of power due to usage and aging.
• Safety: Existing lithium-ion battery technology is not intrinsically safe. Risks such as short circuits, overcharging, over-discharging, crushing, and high temperatures can lead to thermal runaway, fires, and explosions.
• Life: Hybrid engines have an estimated 15-year lifetime, and battery technology must meet this target with a goal of 300,000 charging cycles. While demonstrated cycle life is adequate, calendar life falls short.

Historically, electrochemistry and device engineering have predominated battery development. The performance barriers mentioned above are related to material problems, including issues with low-temperature diffusion. Mechanically induced loss of power due to usage—prompted by crack initiation and growth, followed by fracture, passive surface coating, and other factors—needs to be addressed. Furthermore, advancements in materials and material processing should occur concurrently to minimize costs and ensure safety in battery technologies. Consequently, materials scientists and process engineers are increasingly crucial to achieving reliability, safety, and durability in electrical energy storage.

BATTERY PRINCIPLE AND BASICS

HOW WOULD YOU...

describe the overall significance of this paper?
Lithium-ion battery technology must overcome significant technological, safety, and cost barriers to achieve marketplace success. Historically driven by electrochemical R&D, today the engagement of materials scientists and process engineers is vital to surmounting barriers and comprehending failure mechanisms. This paper aims to educate these professionals to initiate this process.

describe this work to a materials science and engineering professional with no experience in your technical specialty?
Lithium-ion battery technology is poised to be a transformative solution for drivetrain electrification and stationary storage that enables effective use of renewable energy sources. However, safety, service life, and cost pose significant challenges. This paper summarizes the current state of lithium-ion battery technology for nonexperts, fostering understanding among materials scientists and process engineers.

describe this work to a layperson?
Hybrid and all-electric vehicles, alongside renewable wind and solar energy, rely on efficient energy storage solutions. However, the current battery technology faces substantial cost and efficiency challenges to ensure reliability and safety for both mobile and stationary storage. Materials scientists and engineers are dedicated to improving these technologies to create safe, affordable solutions for our energy crisis.

The fundamental working unit of a battery is the electrochemical cell, consisting of a cathode and an anode, separated and connected by an electrolyte. The electrolyte conducts ions but acts as an insulator for electrons. In the charged state, the anode contains a high concentration of intercalated lithium, while the cathode is depleted of lithium. During discharge, a lithium ion leaves the anode, migrating through the electrolyte to the cathode, while its associated electron is collected by the current collector to power an electric device (as illustrated in Figure 2).

The designs of cells and their configuration in modules and packs can vary significantly. To establish a foundational understanding, this paper outlines the primary cell designs and focuses on materials, processing, and manufacturing, with particular emphasis on batteries for transportation.

The electrodes in lithium-ion cells are always solid materials. Cell types can be distinguished based on their electrolytes, which may be liquid, gel, or solid-state components. The electrolytes in gel and solid-state cells serve as structural components, eliminating the need for additional separators for effective electrode separation and short circuit avoidance. Cells can be configured in button, cylindrical, or prismatic forms (see Figure 3). A comprehensive overview of cell forms and materials is provided by J. Besenhard et al.

For low-energy and low-power applications, a cell frequently represents a complete battery. Conversely, for high-energy and high-power applications, such as transportation or stationary storage, multiple cells are packaged into a module, with several modules forming a battery.

Thin-Film Batteries
A special category includes solid-state thin-film batteries. These consist of only solid materials, where the electrolyte is a solid-state ionic glass or crystal, with the components deposited via vapor deposition techniques. This design offers the highest energy density, safety, and abuse tolerance, but is limited to small devices for specialized applications and involves the most expensive production method. A comprehensive review of thin-film battery systems is provided by N.J. Dudney and B.J. Neudecker.

MATERIALS

Cathode Materials
State-of-the-art cathode materials encompass lithium-metal oxides (e.g., LiCoO2, LiMn2O4, and Li(NixMnyCoz)O2), vanadium oxides, olivines (such as LiFePO4), and rechargeable lithium oxides. Layered oxides containing cobalt and nickel are the most extensively studied materials for lithium-ion batteries, showcasing high stability in the high-voltage range; however, cobalt's limited availability and toxicity pose drawbacks for mass production. Manganese offers a low-cost substitution with a high thermal threshold and excellent rate capabilities, albeit with limited cycling behavior. To combine the best properties while minimizing drawbacks, mixtures of cobalt, nickel, and manganese are frequently utilized. Vanadium oxides boast large capacity and excellent kinetics, but lithium insertion and extraction tend to render the material amorphous, limiting cycling behavior. Although olivines are non-toxic and possess moderate capacity with minimal cycling fade, their low conductivity is a significant limitation. Coating methods have been introduced to enhance conductivity, though at the cost of additional processing expenses.

Anode Materials
Anode materials include lithium, graphite, lithium-alloying materials, intermetallics, or silicon. Lithium may appear straightforward but faces cycling behavior issues and dendritic growth that can cause short circuits. Carbonaceous anodes are the most commonly utilized due to their low cost and availability; however, the theoretical capacity (372 mAh/g) pales in comparison to lithium's charge density (3,862 mAh/g). Attempts to improve capacity through novel graphite varieties and carbon nanotubes have incurred high processing costs. Alloy anodes and intermetallic compounds offer high capacities but exhibit drastic volume changes, leading to poor cycling behavior. There are efforts to address volume changes by utilizing nanocrystalline materials and placing the alloy phase (with Al, Bi, Mg, Sb, Sn, Zn, etc.) within a non-alloying stabilization matrix (with Co, Cu, Fe, or Ni). Silicon provides an extremely high capacity (4,199 mAh/g), corresponding to a composition of Si5Li22, yet struggles with cycling behavior and unexplained capacity fading.

Electrolytes
A robust electrolyte is essential for a safe and long-lasting battery. It should withstand existing voltage and high temperatures, possess long shelf life, and provide high lithium ion mobility. Types encompass liquid, polymer, and solid-state electrolytes. Liquid electrolytes are predominantly organic, solvent-based systems containing LiBC4O8 (LiBOB), LiPF6, Li[PF3(C2F5)3], or similar. A key concern is flammability, as optimal solvents have low boiling points and flash points around 30°C, posing a risk of cell venting or explosion. Electrolyte decomposition and highly exothermic side reactions can trigger thermal runaway within lithium-ion batteries; thus, selecting an electrolyte involves balancing flammability with electrochemical performance.

Separators incorporate thermal shutdown mechanisms, and sophisticated thermal management systems are often integrated into modules and battery packs. Ionic liquids are being considered for their thermal stability, yet significant drawbacks persist, such as lithium dissolution from the anode.

Polymer electrolytes consist of ionically conductive polymers, often mixed with ceramic nanoparticles to achieve higher conductivity and increased voltage resistances. Due to their high viscosity and quasi-solid behavior, polymer electrolytes may also hinder the growth of lithium dendrites and could potentially be used with lithium metal anodes.

Solid electrolytes are lithium-ion conductive crystals and ceramic glasses, though they exhibit poor low-temperature performance due to severely reduced lithium mobility in the solid at lower temperatures. Moreover, solid electrolytes necessitate special deposition conditions and temperature treatments, making them costly to implement, albeit they eliminate the need for separators and mitigate the risk of thermal runaway.

Separators
A comprehensive overview of separator materials and requirements is provided by P. Arora and Z. Zhang. The battery separator ensures physical separation between the two electrodes to prevent short circuits. In liquid electrolyte configurations, the separator is composed of a foam material that absorbs the electrolyte, maintaining it in position. It must serve as an electronic insulator while minimizing electrolyte resistance, maximizing mechanical stability, and resisting chemical degradation in the highly electrochemically active environment. Moreover, the separator typically features a safety mechanism termed "thermal shutdown," where elevated temperatures cause it to melt or seal its pores to halt lithium ion transport without compromising mechanical stability. Separators may be synthesized in sheets and combined with electrodes or deposited onto one electrode in situ; the latter method is preferred from a cost perspective, though it entails additional synthesis, handling, and mechanical challenges. Solid-state electrolytes and some polymer electrolytes do not require separators.

PROCESSING AND MANUFACTURING

Battery discharge relies on the diffusion of lithium ions from the anode to the cathode through the current collector, as depicted in Figure 2. This movement is chiefly governed by diffusion processes that provide lithium ions to the surface of the anode, transitioning through the electrolyte, and then diffusing into the cathode. Diffusion represents the most limiting factor in high-current discharge/charge scenarios and also affects performance at low temperatures. Additionally, intercalation and deintercalation processes introduce volume changes in the active electrode materials, and the cycling repetition can induce cracks, ultimately leading to the unusable active electrode material due to disconnection from the current collector or creating a short circuit—and in the case of lithium-metal batteries, posing safety risks from dendritic growth and roughening of the anode.

Advancements in material processing and manufacturing aim to enhance performance and manage unavoidable volume changes, with a focus on composite materials featuring micro- and nanoscaled particles. Nanoparticles can adapt to volume changes while minimizing the risk of crack initiation, and the micro-scaled agglomerates and composites yield minimal diffusion path lengths through the slower-diffusing phases (electrodes). A priority lies in optimizing packing density to maximize active material content, ensuring open porosity for electrolyte access, and maintaining electronic continuity for efficient charge exchange to the current collectors.

Cylindrical cells are manufactured and assembled through several steps. The electrolytes, formed from pastes containing active material powders, binders, solvents, and additives, are fed into coating machines that spread them over current collector foils (aluminum for the cathode and copper for the anode). This is followed by calendaring to achieve uniform thickness and particle size, subsequently slitting to correct widths. The components are then structured into separator-anode-separator-cathode stacks, wound into cylindrical cells, inserted into cylindrical cases, and welded with a conducting tab. Electrolyte is then introduced, requiring the separator to be wetted and thoroughly soaked into the electrodes. This wetting and soaking process represents the slowest step, thus dictating production speed. Required insulators, seals, and safety devices are added and connected. Finally, cells undergo initial charging and testing; they often require venting during the first charge. Initial charging cycles follow extensive protocols designed to enhance performance, cycling behavior, and service life of the cells. Recent initiatives have started combining and hybrid processing approaches, such as direct deposition of separators onto electrodes and rapid heat treatments.

COST ANALYSIS FOR BATTERIES FOR TRANSPORTATION

The battery pack requirements for HEVs differ from those needed for PHEVs and EVs. The DOE’s program production price targets stand at $500 to $800 for HEV battery packs and $1,700 to $3,400 for PHEV battery packs.

Material Needs and Raw Material Cost
The raw material needs and costs stem from a study conducted by L. Gaines and R. Cuenza. A standard cylindrical cell is referred to as the "18650" cell (18 mm wide and 65 mm long) with a total mass of roughly 40 g (including inactive materials and packaging) and a capacity of about 1.35 Ah. The material masses required for HEV and EV battery cells are presented in Table I.

From Table I, one can observe that a cell’s capacity approximately scales with its mass. Though packaging for larger batteries constitutes a smaller fraction than that for smaller batteries, the total mass for a 10 Ah cell is about 325 g, and for a 100 Ah cell, it is approximately 3,430 g. Thus, material cost estimates can be calculated by scaling up the costs of materials in an 18650 cell by a factor of 10 for HEVs and by a factor of 100 for EVs. Most battery designs consist of about 100 cells compartmentalized into several modules (such as 12 × 8, 10 × 10, etc.).

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Table I. Estimated Materials Content of Typical Lithium-Ion Cells (based on Reference 15)

High-Energy (100 Ah) Cell EV
High-Power (10 Ah) Cell HEV
Material/Component
Quantity (g)
Part (%)
Quantity (g)
Part (%)

Anode (dry)
Active material (graphite): 563.6, 16.4%, 14.1, 4.3%
Binder: 69.7, 2.0%, 3.1, 1.0%
Current collector (Cu): 151.9, 4.4%, 41.6, 12.8%

Cathode (dry)
Active material: 1,408.6, 41.0%, 74.4, 22.9%
Carbon: 46.4, 1.4%, 3.2, 1.0%
Binder: 92.9, 2.7%, 6.3, 1.9%
Current collector (Al): 63.0, 1.8%, 19.4, 6.0%

Electrolyte: 618.0, 18.0%, 44.0, 13.5%
Separator: 60.5, 1.8%, 16.4, 5.0%
Rest of Cell
Tabs, end plates, terminal assemblies: 66.2, 1.9%, 32.2, 9.9%
Core: 0.9, 0.0%,
Container: 291.0, 8.5%, 70.1, 21.6%
Total: 3,432.7,
324.8

As an example, the materials costs for a LiCoO2-based 18650 cell (including materials processing) can be estimated at about $1.28 for the entire cell. Materials processing is closely tied to material costs and is therefore included in this section. Processing costs shift dramatically with different materials and can be considered material-specific; however, new processing techniques may lower the current high costs of raw materials.

Manufacturing and Labor Cost
State-of-the-art manufacturing of a cylindrical cell on a production line involves mixing and coating, calendaring and slitting, cutting, winding, tab welding, automated assembly, and inspection, followed by testing, cycling, and packaging. Producing 100,000 units annually requires a total workforce of 76 to 104 individuals, operating on two lines in two shifts. Gaines and Cuenza estimated the labor cost per cell and overhead costs at $0.42 based on a 18650 cell.

Total Cost
The total costs for a 18650 cell accumulate to roughly $1.70. Scaling to HEV batteries results in $1,700 (double the price target). Currently, there is no fixed cost target for EV batteries. However, based on this calculation, a highly uncertain estimate may yield around $17,000 per battery.

This estimate illustrates the significant effort needed to reduce processing costs, material costs, and total material requirements.

CONCLUSION

There is no doubt that lithium-ion cell chemistries provide some of the best opportunities for electrical energy storage in high-power and high-energy applications, such as transportation and stationary storage, due to their electrochemical potential, theoretical capacity, and energy density. Nevertheless, the estimated battery cost for the HEV application example still stands at twice the indicated price target set by the USABC and DOE. Given rising oil prices, a slightly higher price than the target may attain sufficient consumer acceptance for a successful market introduction; nonetheless, the price must decrease further.

Clearly, there are needs in materials development, optimization, and processing. The separation between material and labor costs is evident in the computations presented. However, the complex interrelation of raw material costs and material processing costs prevails, as pure raw materials are rarely employed; instead, material compounds better suited for production and application are utilized. Furthermore, processed raw materials and compounds are prevalent. Thus, new low-cost processing methods for these materials and compounds must be developed to reduce battery "raw material" costs.

Efforts are warranted on hybrid technologies, which would integrate low-cost slurry-based techniques with treatment methods that currently function in separate stages. High-speed treatments, such as radiant processing, require optimization to replace slower furnace procedures. Investment costs and manufacturing times must be minimized to ensure feasibility for battery applications. Additionally, hybrid materials capable of performing multiple functions from existing components should be developed and incorporated into future batteries (e.g., solid or high-viscosity electrolytes that do not require separators, facilitate enhanced lithium exchange behavior, wet electrodes, and maintain strong bonding).

ACKNOWLEDGEMENTS

The author gratefully acknowledges the support from David Howell (Energy Storage R&D Program Manager, Vehicle Technologies Program, Office of Energy Efficiency and Renewable Energy, Department of Energy) and Raymond Boeman (Transportation Program Director, Oak Ridge National Laboratory), guidance from Craig Blue, as well as fruitful discussions with Nancy Dudney and numerous colleagues. This research at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR, was sponsored by the Vehicle Technologies Program for the Office of Energy Efficiency and Renewable Energy.

REFERENCES

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Claus Daniel is with the Materials Processing Group, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, and also with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN. Dr. Daniel can be reached at (865) 241-....