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The latest progress of solid-state lithium metal batteries
Author: Paul Albertus
Corresponding author: Nancy J. Dudney, Jagjit Nanda
Correspondence Unit: Oak Ridge National Laboratory, USA
Compared with current lithium-ion systems, solid-state batteries using lithium metal anodes have the potential to achieve better performance (specific energy>500 Wh/kg, energy density>1500 Wh/L), safety, recyclability, and potential Lower cost (<$100/kWh). These improvements are critical to the widespread adoption of electric cars and trucks, and may lead to the development of short-haul electric aviation. The expectations for solid-state batteries are high, but there are still many challenges to overcome in terms of materials and processing.
On May 15, 2020, Oak Ridge National Laboratory (ORNL) held a 6-hour national online seminar to discuss the latest progress and major obstacles to the realization of solid-state lithium metal batteries. The seminar included more than 30 experts from national laboratories, universities and companies, all of whom have been engaged in solid-state battery research for many years. The consensus of the participants is that although the latest developments in solid-state batteries are exciting, there are still many problems that need to be solved urgently. Our goal is to examine the problem and identify the most urgent needs and the most important opportunities. Participants identified the advantages and disadvantages of solid-state batteries based on sulfides, oxides and polymers, and identified common scientific gaps between different chemical methods. Addressing these common scientific gaps may reveal the most promising systems for future adoption. Figure 1 summarizes the following main findings of the seminar: I. Blanks in materials science, II. Blanks in processing science, and III. Blanks in design engineering.
Figure 1. A schematic diagram that outlines the realization of specific vacancies for competitive solid-state batteries. The 2020 ORNL seminar focuses on specific challenges in the fields of materials science, process science and design engineering.
In order to supplement the seminar’s discussion and evaluation of the latest developments, the organizers conducted a literature analysis of solid-state batteries. Figure 2 shows the number of peer-reviewed publications over time from 2000 to 2020. In the past ten years, the number of documents and the speed of publication have greatly increased. In order to ensure a representative point of view, more than a dozen recent review articles are based on their emphasis on analyzing the key technical areas of the development of solid-state batteries (see Figure 2B). Analysis shows that researchers have made significant progress in the discovery of new materials, but the integration of these materials into actual devices has lagged behind. The lack of relevant prototype battery data may be due to insufficient attention to processing science and solid-state mechanics, and the difficulty of a single PI research model to overcome all the challenges brought by the production of high-quality prototype batteries.
Figure 2. Literature analysis of solid-state batteries shows (a) the number of peer-reviewed publications from 2000 to 2020 (keywords: "lithium" and "solid-state batteries*", Web of Science) and (b) radar chart based on recent The analysis of 12 review articles compares the level of activity in key technical areas of solid-state batteries.
Lithium metal anode is indispensable for all batteries considered in the seminar, but its research is relatively minimal. To fill up the scientific gaps in the optimization process of lithium metal anodes, the following questions need to be answered:
1) When Li is deposited and stripped by solid electrolyte, what defect generation/annihilation process will occur in Li film (thickness less than 30 μm)?
2) What conditions (such as rate, temperature, applied pressure and cycle) will change the deposition and exfoliation behavior of Li?
3) What are the stress relaxation mechanisms of Li, and how do they change with the type and size of the stress field, mechanical boundary conditions and strain rate?
4) How do defects such as grain boundaries, dislocation density, element impurities and alloying elements change the properties and cycle performance of lithium metal anodes?
5) Do you need a Li seed layer to deposit Li templated or provide mechanical flexibility to improve cycle stability?
6) How does the interphase area formed by the reaction or addition at the lithium/solid electrolyte interface control the transport?
In recent years, a lot of information has been learned about the failure of the lithium/solid electrolyte interface. (1) The effective passivation of the interface can reduce the consumption of Li, (2) The high modulus solid electrolyte formed by the dense and smooth interface suffers fewer problems, (3) the higher fracture toughness inhibits the formation of short circuits. Cracks, and (4) higher electronic resistivity alleviates the reduction of Li+ in the solid electrolyte. But there are still the following important issues:
1) What promotes the electrochemical stability of Li or kinetics restricts the passivation of Li?
2) What mechanisms can be used to enhance the performance of solid electrolytes within an appropriate length range, improve stability and suppress failure/fatigue?
3) How do the bulk properties of solid electrolytes and their surface chemistry/uniformity (such as current uniformity) affect the lithium cycle?
4) How does the positive electrode affect the Li negative electrode interface during battery cycling?
In order to obtain the highest energy density, the positive electrode must be the largest component in the battery. For example, suppose the positive electrode is used as a mechanical support and battery substrate. In that case, the current collector, electrolyte and lithium negative electrode can all be coated in the form of a thin coating, as shown in Figure 3. In traditional slurry-cast positive electrodes, the organic binder is sufficient to form a free-standing positive electrode, or polymer electrolytes can be added to fill the voids and promote Li+ transport. Composite positive electrodes can also be bonded, melted or sintered to improve interfacial contact. These steps complicate the process, but can ensure the formation of a mechanically strong solid-solid interface. The key is to make a positive electrode that will (i) withstand stress during cycling and (ii) provide sufficient electron and ion transport at low stack pressures (<1 MPa). Some of the key scientific challenges related to this topic are as follows:
1) How to enhance the reaction kinetics and mechanical properties of the dense single-phase cathode in all charged states through defects and microstructure engineering?
2) How to use texture and grain structure to improve reaction kinetics and reduce the rupture of solid electrolytes?
3) What is the relative pressure and electrochemical driving force that the active cathode material bears? Are there any ingredients that can provide a more uniform response?
4) Which cathode design principle will maintain the elastic cathode-electrolyte interface in close contact during cycling?
Figure 3. A schematic diagram of an ideal high-energy solid-state battery stack, which includes a thin positive electrode current collector, a thick positive electrode, a thin electrolyte separator, a thin lithium negative electrode that expands during charging, and a thin negative electrode current collector.
Advanced processing methods create opportunities for the development of new and improved materials, while traditional methods cannot provide such opportunities. Although many challenges related to materials and interfaces remain unsolved, understanding processing barriers can save a lot of time and effort. Advanced material processing can also open up new directions for solid-state batteries or accelerate the development of current materials.
The following example illustrates how a unique treatment method can be used to form close contact between the positive electrode and the solid electrolyte. When simple cold pressing is difficult to achieve, sintering is often required to form a good interface contact between the positive electrode and the solid electrolyte, especially for oxide solid electrolytes. As long as the binders do not damage electrochemical and mechanical properties, they can be used to densify and fuse active materials and solid electrolytes at lower temperatures. Research on rapid thermal annealing (for example, radiant heating of carbon ribbon) may open up new practical processing directions. The interface of uniform contact can also be achieved by filling or coating the interface with a liquid that cures later. For example, a porous 3D positive electrode can be backfilled with a liquid precursor, which is heat-treated to form a solid ionic gel; other novel and solvent-free processing routes can also be studied to form tight interfacial contact.
The processing process determines the microstructure and mechanical properties of the solid electrolyte. There are well-known methods (for example, precipitation hardening, phase change toughening, and tempering) to reinforce structural ceramics and glass materials, but for solid electrolytes, similar mechanisms have not been reported. A scientific gap in the processing of solid-state batteries is to determine whether there is a mechanism to enhance thin solid electrolytes and thick positive electrodes without hindering transportation. The method of avoiding inert components is particularly attractive for maintaining high specific energy. In addition to providing a naturally smooth surface, the advantages of glass and amorphous electrolytes include good chemical stability and ductility. New knowledge is needed to effectively process thin amorphous materials.
At present, most solid-state batteries are of stacked design, and their cathode composite materials are derived from a typical lithium-ion architecture. An exception is the use of 3D template cathodes or recent 3D porous solid electrolytes formed by freeze casting or ablation of sacrificial components. The other is the so-called "2.5D" design, which consists of a 2D lithium negative electrode sheet and a 3D composite positive electrode. These designs have been adopted to increase the interface area and reduce the local current density at the electrode-electrolyte contact. This design method is very promising, but it faces how to scale up economically and efficiently. Alternative architectures can also achieve mechanically robust structures and interfaces. The reason for studying this type of structure is to worry that maintaining a high stack pressure (≥5MPa) on solid-state batteries may require external mechanical fixing devices, thereby offsetting the advantages of batteries in terms of mass ratio energy/energy density. Advanced manufacturing technology is critical to the preparation of better solid-state battery structures. Many interesting questions on this subject should be considered:
1) Is it possible to ingeniously guide the volume change to provide internal compression at the material interface to replace the larger external pressure?
2) Are there other ways to temper the positive electrode and its internal interface to enhance and resist fracture?
3) How to design composite materials to maintain their structure and internal connectivity during the cycle?
4) How does volume change during cycling affect the performance and packaging requirements of solid-state batteries?
5) How to reduce the chemical reaction between the positive electrode and the solid electrolyte during the densification and circulation process?
Although the reasons for the poor circulation of lithium and solid electrolytes are becoming clear, the solution is still unclear. Applying a large external pressure is not a feasible solution. Issues that help understand lithium metal anodes include: (i) comparing the Li cycle performance of full batteries and Li/Li batteries, (ii) comparing cycle performance at different temperatures (including the temperature at which Li melts), and (iii) evaluation The influence of various impurities in lithium; (iv) Compare lithium from different sources. For example, Li sources may include commercially rolled and passivated ultra-thin Li, vacuum-grown Li films, and electrochemically grown Li in a negative-electrode-free battery configuration. In addition, the choice of using a lithium alloy anode or providing a lithium deposition framework (for example, nano-silver-carbon composite) cannot be ignored. However, these solutions will sacrifice overall energy density. For Li anode research, this means that both the metallic Li and the solid electrolyte should be very thin, and there is almost no excess capacity to maintain side reactions.
The solid-state positive electrode is the key to ensuring that the performance meets or exceeds the lithium-ion battery. For the battery with the highest energy density, the active positive electrode should occupy the largest part of the battery. In this way, the composite positive electrode should act as a physical carrier. This has always been a key obstacle to the practicality of solid-state batteries. In most designs, the positive electrode cannot provide enough energy at room temperature and a reasonable stack pressure. Hybrid designs using standard lithium-ion battery positive electrodes with liquid or gel electrolytes have been tested, but they are also limited by interface reactions and poor transportation.
Scientifically speaking, solid-state cathode is also a basic research topic, but it needs to solve many key interface science challenges. For example, it is necessary to solve the influence of volume change, interface integrity and phase connectivity on ion and electron migration in order to develop a positive electrode (1~10 mA/cm2 and >3 mAh/cm2) that can provide the required current density and areal capacity . Research on the properties, stress and fatigue effects, and stress relaxation mechanisms of the solid-state interface between different materials will provide insights that can be applied to other interfaces in solid-state batteries. Research on solid-state cathodes must also address scientific gaps in materials, processes, and battery architecture.
For a successful solid-state cathode, battery materials and solid-state electrochemistry experts should work closely with materials mechanics and processing experts to (i) minimize and alleviate cyclic stress, (ii) determine the mechanism and architecture of reinforcing materials and interfaces, and (iii) ) Utilize materials and processes to form a direct ion transport path across the thickness of the positive electrode, and (iv) reduce or eliminate the need for external stack pressure. By studying the model anode interface, using positive cycling conditions, determining chemical/mechanical data for computational modeling, and developing advanced characterization tools to characterize the interface during and after cycling, the research progress can also be improved.
Compared with the current state-of-the-art lithium-ion systems, lithium metal solid-state batteries have the potential to provide advantages in terms of energy density, safety, cost, and recycling. However, the development of solid-state batteries still faces several challenges, including (i) improving the control of materials and interfaces, (ii) solving processing challenges and costs, (iii) showing performance beyond advanced lithium-ion batteries; and (iv) maintaining The optimal stacking pressure of the solid-state battery pack does not affect the cost and energy density.
It is an ambitious task to achieve complete solid-state batteries that meet the performance, cost, and manufacturability required by electric vehicles in the next 5 years or even 10 years, especially when the best solid electrolyte and cathode chemistry are still unclear. Targeting less application requirements such as consumer electronics batteries may be achieved in the short term, but it may divert people's attention from the most challenging problems of large-scale electric vehicles and grid-scale deployment. Recently, many powerful scientific researches have been carried out, and can be extended to mechanical, transportation and battery-level measurements. In addition, work including statistics and process control needs to be carried out to promote repeatability between different groups and institutions. Attention should also be paid to battery failure mechanisms and large-scale organized comprehensive data analysis.
All in all, solid-state batteries have broad prospects in high-energy batteries for electric vehicles and other applications. Despite the huge potential, success depends on solving key challenges in materials science, processing science, and practical full battery manufacturing. This article outlines several key challenges and hopes that they can encourage and inspire solutions and ultimately realize high-energy solid-state batteries.
Paul Albertus et al. Challenges for and Pathways toward Li-Metal-Based All-Solid-State Batteries. ACS Energy Lett. 2021, DOI:10.1021/acsenergylett.1c00445
Corresponding Authors
Nancy J. Dudney - Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States; Orcidhttp://orcid.org/0000-0001-7729-6178; Email: dudneynj@ornl.gov
Jagjit Nanda - Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States; Orcidhttp://orcid.org/0000-0002-6875-0057; Email: nandaj@ornl.gov
Authors
Paul Albertus - Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States; Orcidhttp://orcid.org/0000-0003-0072-0529
Venkataramani Anandan - Ford Motor Company, Dearborn, Michigan 48121, United States
Chunmei Ban - Mechanical Engineering Department, University of Colorado, Boulder, Colorado 80309, United States; Orcidhttp://orcid.org/0000-0002-1472-1496
Nitash Balsara - Department of Chemical and Biomolecular Engineering and Lawrence Berkeley National Laboratory, Materials Sciences Division, University of California, Berkeley, California 94720, United States; Orcidhttp://orcid.org/0000-0002-0106-5565
Ilias Belharouak - Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States; Orcidhttp://orcid.org/0000-0002-3985-0278
Josh Buettner-Garrett - Solid Power, Inc., Louisville, Colorado 80027, United States
Zonghai Chen - Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States; Orcidhttp://orcid.org/0000-0001-5371-9463
Claus Daniel - Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
Marca Doeff - Lawrence Berkeley National Laboratory, Energy Storage and Distributed Resources Division, University of California, Berkeley, California 94720, United States; Orcidhttp://orcid.org/0000-0002-2148-8047
Bruce Dunn - Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States; Orcidhttp://orcid.org/0000-0001-5669-4740
Stephen J. Harris - Lawrence Berkeley National Laboratory, Energy Storage and Distributed Resources Division, University of California, Berkeley, California 94720, United States
Subramanya Herle - Applied Materials Inc., 3225 Oakmead Village Drive, B12_2F6, Santa Clara, California 95054, United States
Eric Herbert - Department of Materials Science and Engineering, Michigan Technological University, Houghton, Michigan 49931, United States
Sergiy Kalnaus - Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
Joesph A. Libera - Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
Dongping Lu - Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States; Orcidhttp://orcid.org/0000-0001-9597-8500
Steve Martin - Department of Materials Science & Engineering, Iowa State University of Science & Technology, Ames, Iowa 50011, United States; Orcidhttp://orcid.org/0000-0002-6472-509X
Bryan D. McCloskey - Department of Chemical and Biomolecular Engineering and Lawrence Berkeley National Laboratory, Materials Sciences Division, University of California, Berkeley, California 94720, United States; Orcidhttp://orcid.org/0000-0001-6599-2336
Matthew T. McDowell - Woodruff School of Mechanical Engineering and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States; Orcidhttp://orcid.org/0000-0001-5552-3456
Y. Shirley Meng - Department of NanoEngineering and Materials Science and Engineering Program, University of California San Diego, La Jolla, California 92093, United States; Orcidhttp://orcid.org/0000-0001-8936-8845
Jeff Sakamoto - Department of Materials Science and Engineering and Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
Ethan C. Self - Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States; Orcidhttp://orcid.org/0000-0001-6006-6317
Sanja Tepavcevic - Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
Eric Wachsman - Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States; Orcidhttp://orcid.org/0000-0002-0667-1927
Chunsheng Wang - Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States; Orcidhttp://orcid.org/0000-0002-8626-6381
Andrew S. Westover - Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States; Orcidhttp://orcid.org/0000-0003-0151-8832
Jie Xiao - Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States; Orcidhttp://orcid.org/0000-0002-5520-5439
Thomas Yersak - Chemical and Materials Systems Laboratory, General Motors Global R&D, Warren, Michigan 48092-2031, United States; Orcidhttp://orcid.org/0000-0001-8275-7960
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