High-Performance Structural Batteries

Graphical AbstractView Large Image Figure ViewerDownload Hi-res image Download (PPT) Structural batteries, i.e., batteries designed to bear mechanical loads, are projected to substantially increase system-level specific energy, resulting in electric vehicles with 70% more range and unmanned aerial vehicles (UAVs) with 41% longer hovering times.1Carlstedt D. Asp L.E. Performance analysis framework for structural battery composites in electric vehicles.Compos. B. Eng. 2020; 186: 107822Crossref Scopus (12) Google Scholar,2Hollinger A.S. McAnallen D.R. Brockett M.T. DeLaney S.C. Ma J. Rahn C.D. Cylindrical lithium-ion structural batteries for drones.Int. J. Energy Res. 2020; 44: 560-566Crossref Scopus (9) Google Scholar By storing energy and bearing mechanical loads, structural batteries reduce the amount of conventional structural materials required by devices. Two approaches to enable this concept have emerged since the first structural-battery prototypes were reported in 2004.3Thomas J.P. Qidwai M.A. Mechanical design and performance of composite multifunctional materials.Acta Mater. 2004; 52: 2155-2164Crossref Scopus (141) Google Scholar One approach emphasizes monofunctional materials with decoupled functions; i.e., one material bears loads, another electrochemically stores energy. The other approach focuses on using multifunctional materials with coupled functions; i.e., materials that bear loads and electrochemically store energy. By performing a meta-analysis on reported structural batteries, we show here that decoupled structural batteries (relying on monofunctional materials) generally achieve higher elastic moduli and specific-energy values than coupled structural batteries (relying on multifunctional materials). We use the equation for flexural rigidity to demonstrate that decoupled structural batteries also have a fundamental advantage because they position load-bearing components on their outermost surfaces; i.e., the casing. This design choice gives decoupled structural batteries greater flexural rigidity than their coupled counterparts, which distribute load-bearing components throughout their volumes. Our analysis suggests that next-generation structural batteries should look to energy-dense aluminum–air and zinc–air batteries. We observe that decoupled structural batteries generally outperform their coupled counterparts by plotting specific-energy values versus corresponding elastic moduli of reported structural batteries (Figure 1).2Hollinger A.S. McAnallen D.R. Brockett M.T. DeLaney S.C. Ma J. Rahn C.D. Cylindrical lithium-ion structural batteries for drones.Int. J. Energy Res. 2020; 44: 560-566Crossref Scopus (9) Google Scholar, 3Thomas J.P. Qidwai M.A. Mechanical design and performance of composite multifunctional materials.Acta Mater. 2004; 52: 2155-2164Crossref Scopus (141) Google Scholar, 4Meng C. Muralidharan N. Teblum E. Moyer K.E. Nessim G.D. Pint C.L. Multifunctional structural ultrabattery composite.Nano Lett. 2018; 18: 7761-7768Crossref PubMed Scopus (22) Google Scholar, 5Thakur A. Dong X. Printing with 3D continuous carbon fiber multifunctional composites via UV-assisted coextrusion deposition.Manufacturing letters. 2020; 24: 1-5Crossref Scopus (9) Google Scholar, 6Moyer K. Meng C. Marshall B. Assal O. Eaves J. Perez D. Karkkainen R. Roberson L. Pint C.L. Carbon fiber reinforces structural lithium-ion battery composite: multifunctional power integration for CubeSats.Energy Storage Mater. 2020; 24: 676-681Crossref Scopus (25) Google Scholar, 7Liu P. Sherman E. Jacobsen A. Design and fabrication of multifunctional structural batteries.J. Power Sources. 2009; 189: 646-650Crossref Scopus (121) Google Scholar, 8Thomas J.P. Qidwai S.M. Pogue III, W.R. Pham G.T. Multifunctional structure-battery composites for marine systems.J. Compos. 2012; 47: 5-26Google Scholar, 9Galos J. Best A.S. Mouritz A.P. Multifunctional sandwich composites containing embedded lithium-ion polymer batteries under bending loads.Mater. Des. 2020; 185: 108228Crossref Scopus (19) Google Scholar, 10Zhang Y. Ma J. Singh A.K. Cao L. Seo J. Rahn C.D. Bakis C.E. Hickner M.A. Multifunctional structural lithium-ion battery for electric vehicles.J. Intell. Mater. Syst. Struct. 2017; 28: 1603-1613Crossref Scopus (23) Google Scholar, 11Ladpli P. Nardari R. Kopsaftopoulos F. Chang F.K. Multifunctional energy storage composite structure with embedded lithium-ion batteries.J. Power Sources. 2019; 414: 517-529Crossref Scopus (34) Google Scholar, 12Xu J. Liu B. Hu D. State of charge dependent mechanical integrity behavior of 18650 lithium-ion batteries.Sci. Rep. 2016; 6: 21829Crossref PubMed Scopus (91) Google Scholar Despite the growing number of papers published in this field, few publications report full-cell testing and provide specific energy and elastic modulus values. To our knowledge, the dataset shown in Figure 1 is nearly comprehensive based on our review of the literature. For some of the data shown, we had to directly contact authors to get specific-energy values because such information was either not reported or could not be calculated from given data. We choose elastic modulus as the defining mechanical property of structural batteries for this study because most reports include this key mechanical value. For reports that provide stiffness (units of N mm–1) or flexural rigidity (units of N m2) instead of elastic modulus, we calculate approximate elastic-modulus values using beam-bending equations and the given outer dimensions of reported structural batteries. For comparison, we include the specific energy and bending modulus (estimated as an elastic modulus for Figure 1) of an 18650 lithium-ion (Li-ion) battery.12Xu J. Liu B. Hu D. State of charge dependent mechanical integrity behavior of 18650 lithium-ion batteries.Sci. Rep. 2016; 6: 21829Crossref PubMed Scopus (91) Google Scholar Some of the coupled structural batteries have lower elastic moduli than a conventional, cylindrical Li-ion battery. We also observe the expected tradeoff in specific energy and stiffness for decoupled structural batteries—the stiffer the battery, the lower the specific energy because monofunctional stiffening components increase battery mass without adding energy. We note that the structural battery reported in Thomas et al.8Thomas J.P. Qidwai S.M. Pogue III, W.R. Pham G.T. Multifunctional structure-battery composites for marine systems.J. Compos. 2012; 47: 5-26Google Scholar (Figure 1) is for marine applications where volumetric energy density is more desirable than gravimetric. This design preference explains its low specific energy with respect to other decoupled structural batteries. We also note that we report specific energy at the cell level, which takes into account all materials found in the reported structural batteries. We use the term “cell level” because if most of the reported structural batteries were integrated into large, practical battery packs, they would likely require thermal management systems that would decrease their system-level specific energy values. Coupled structural batteries achieve lower performance because they use structural electrolytes that reduce ionic conductivity and/or carbon-fiber current collectors that have high ohmic resistances.4Meng C. Muralidharan N. Teblum E. Moyer K.E. Nessim G.D. Pint C.L. Multifunctional structural ultrabattery composite.Nano Lett. 2018; 18: 7761-7768Crossref PubMed Scopus (22) Google Scholar, 5Thakur A. Dong X. Printing with 3D continuous carbon fiber multifunctional composites via UV-assisted coextrusion deposition.Manufacturing letters. 2020; 24: 1-5Crossref Scopus (9) Google Scholar, 6Moyer K. Meng C. Marshall B. Assal O. Eaves J. Perez D. Karkkainen R. Roberson L. Pint C.L. Carbon fiber reinforces structural lithium-ion battery composite: multifunctional power integration for CubeSats.Energy Storage Mater. 2020; 24: 676-681Crossref Scopus (25) Google Scholar, 7Liu P. Sherman E. Jacobsen A. Design and fabrication of multifunctional structural batteries.J. Power Sources. 2009; 189: 646-650Crossref Scopus (121) Google Scholar These multifunctional materials decrease electrochemical performance but are used to increase mechanical performance with the supposition that the tradeoff is worthwhile. In contrast, decoupled structural batteries use battery materials optimized for electrochemical performance and protect those materials with a thick layer of rigid, lightweight casing. Figure 1 shows that the decoupled approach is currently more effective. However, coupled structural batteries use relatively new multifunctional materials that show potential for improvement. Decoupled structural batteries also achieve higher flexural-rigidity values than those for comparable coupled versions because of how each version distributes its load-bearing components (Figure 2). Flexural rigidity is the product of elastic modulus and second moment of area, or the combined effect of material properties and the placement of those materials within a structure. Second moment of area is a measure of how material is distributed with respect to a defined plane and axis (Equation 1).Ix=∬y2dxdy(Equation 1) To compare the flexural-rigidity values achieved by the different approaches, we assume that hypothetical versions of both types have the same outer dimensions and use the same type and quantity of structural and battery materials (Figure 2A). The coupled structural battery uses a structural electrode and/or current collector, which is commonly a metal foam/mesh or network of carbon fibers with a thin casing. The decoupled version uses a thick casing and/or current collector (current collectors do not store energy, so we consider such a design to be in the decoupled category). Under these assumptions, both batteries theoretically have equal elastic moduli along the z axis (the neutral surface lies on the x–z plane, where the z axis is pointing out of the page). If we assume that the battery materials carry a negligible load in comparison to the structural materials, with or without the use of a structural electrolyte, we can transform their cross sections to calculate second moment of area (Figure 2B). Using Equation 1 and Figure 2B, we observe that the decoupled version’s second moment of area is larger because more of its transformed area lies further from the neutral surface than that of the coupled version. The decoupled structural battery will therefore always have a higher flexural rigidity than the coupled version. The flexural rigidity of the decoupled version is maximized because all the structural materials are positioned at the outermost surface. We note that using an electrochemically optimized battery material that also has a high elastic modulus will improve flexural rigidity, assuming that the battery material is well connected to the structural material. However, if methods to increase the elastic modulus of a battery material compromise electrochemical performance and increase mass and/or volume, the modifications may be ultimately detrimental as suggested by Figure 1. While this flexural-rigidity analysis generally reflects current structural-battery designs and testing practices, the analysis fails to encompass all possible testing conditions and future designs. Currently, most researchers create structural batteries in the shape of beams,3Thomas J.P. Qidwai M.A. Mechanical design and performance of composite multifunctional materials.Acta Mater. 2004; 52: 2155-2164Crossref Scopus (141) Google Scholar,4Meng C. Muralidharan N. Teblum E. Moyer K.E. Nessim G.D. Pint C.L. Multifunctional structural ultrabattery composite.Nano Lett. 2018; 18: 7761-7768Crossref PubMed Scopus (22) Google Scholar,10Zhang Y. Ma J. Singh A.K. Cao L. Seo J. Rahn C.D. Bakis C.E. Hickner M.A. Multifunctional structural lithium-ion battery for electric vehicles.J. Intell. Mater. Syst. Struct. 2017; 28: 1603-1613Crossref Scopus (23) Google Scholar,11Ladpli P. Nardari R. Kopsaftopoulos F. Chang F.K. Multifunctional energy storage composite structure with embedded lithium-ion batteries.J. Power Sources. 2019; 414: 517-529Crossref Scopus (34) Google Scholar plates,6Moyer K. Meng C. Marshall B. Assal O. Eaves J. Perez D. Karkkainen R. Roberson L. Pint C.L. Carbon fiber reinforces structural lithium-ion battery composite: multifunctional power integration for CubeSats.Energy Storage Mater. 2020; 24: 676-681Crossref Scopus (25) Google Scholar, 7Liu P. Sherman E. Jacobsen A. Design and fabrication of multifunctional structural batteries.J. Power Sources. 2009; 189: 646-650Crossref Scopus (121) Google Scholar, 8Thomas J.P. Qidwai S.M. Pogue III, W.R. Pham G.T. Multifunctional structure-battery composites for marine systems.J. Compos. 2012; 47: 5-26Google Scholar, 9Galos J. Best A.S. Mouritz A.P. Multifunctional sandwich composites containing embedded lithium-ion polymer batteries under bending loads.Mater. Des. 2020; 185: 108228Crossref Scopus (19) Google Scholar or rods2Hollinger A.S. McAnallen D.R. Brockett M.T. DeLaney S.C. Ma J. Rahn C.D. Cylindrical lithium-ion structural batteries for drones.Int. J. Energy Res. 2020; 44: 560-566Crossref Scopus (9) Google Scholar,5Thakur A. Dong X. Printing with 3D continuous carbon fiber multifunctional composites via UV-assisted coextrusion deposition.Manufacturing letters. 2020; 24: 1-5Crossref Scopus (9) Google Scholar and measure their mechanical properties in tension and bending. Other less common testing conditions include shear loading. Additionally, while most reported coupled structural batteries position their load-bearing elements throughout their volumes, future coupled structural batteries could move all load-bearing elements to their outermost surfaces, which would improve flexural-rigidity values. In summary, our analysis shows that decoupled structural batteries generally outperform coupled versions based on existing full-cell prototypes. Challenges remain, however, for both types of structural batteries. For example, most reported structural batteries use Li-ion chemistries (238 Wh kgcell–1) that can experience thermal runaway if damaged by mechanical loads (Table 1). Unfortunately, more energy-dense, Li-based chemistries such as Li–sulfur and Li–air can also experience thermal runaway. If battery-casing materials are reaching fundamental performance limits by relying on lightweight metals or carbon-fiber composites, then damage-tolerant battery materials with higher volumetric and gravimetric energy densities must be used to advance structural-battery performance. We therefore recommend using energy-dense aqueous metal–air batteries that cannot experience thermal runaway, such as primary (single-use) aluminum–air (900 Wh kgsys–1 at the system level) and secondary (electrically rechargeable) zinc–air (400 Wh kgsys–1).13Hopkins B.J. Shao-Horn Y. Hart D.P. Suppressing corrosion in primary aluminum–air batteries via oil displacement.Science. 2018; 362: 658-661Crossref PubMed Scopus (62) Google Scholar,14Hopkins B.J. Chervin C.N. Sassin M.B. Long J.W. Rolison D.R. Parker J.F. Low-cost green synthesis of zinc sponge for rechargeable, sustainable batteries.Sustain. Energy Fuels. 2020; 4: 3363-3369Crossref Google ScholarTable 1Data Used to Create Figure 1YearChemistryApproachSpecific energy (Wh kgcell–1)Elastic modulus (MPa)Ref.aRef. is an abbreviation for reference.2018Ni–Fecoupled1.47,00042020Li-ioncoupled2428952020Li-ioncoupled351.862009Li-ioncoupled353,10072012Li-iondecoupled4516,40082004Li-iondecoupled958,37032020Li-iondecoupled997,41092017Li-iondecoupled10232,700102004Li-iondecoupled1204,07032020Li-iondecoupled1287,21092019Li-iondecoupled1319,600112019Li-iondecoupled21477522016Li-iondecoupled238aRef. is an abbreviation for reference.507bThe last entry, a conventional 18650 Li-ion battery, uses values for specific energy and elastic modulus from Hollinger et al.2 and Xu et al.,12 respectively.2,12a Ref. is an abbreviation for reference.b The last entry, a conventional 18650 Li-ion battery, uses values for specific energy and elastic modulus from Hollinger et al.2Hollinger A.S. McAnallen D.R. Brockett M.T. DeLaney S.C. Ma J. Rahn C.D. Cylindrical lithium-ion structural batteries for drones.Int. J. Energy Res. 2020; 44: 560-566Crossref Scopus (9) Google Scholar and Xu et al.,12Xu J. Liu B. Hu D. State of charge dependent mechanical integrity behavior of 18650 lithium-ion batteries.Sci. Rep. 2016; 6: 21829Crossref PubMed Scopus (91) Google Scholar respectively. Open table in a new tab We thank Dr. James P. Thomas at the U.S. Naval Research Laboratory for helpful conversations regarding structural batteries. B.J.H. holds provisional patents related to aluminum–air batteries: US patent nos. 2019/0123407 A1 and 2019/0326603 A1. D.R.R., J.F.P., and J.W.L. hold patents related to zinc electrodes: US patent nos. 9802254, 10008711, and 10720635, EU patent no. 2926395, and China patent no. 104813521. B.J.H., D.R.R., J.F.P., and J.W.L. hold a provisional patent related to zinc electrodes: US patent no. 62/876114.