Alternative anodes

A high capacity anode such as silicon or lithium metal is required to realize the cost reduction as illustrated in Fig. 4. Both of them offer an order of magnitude higher charge storage capacities than graphite. The challenges associated with silicon are well documented. Silicon experiences a volume expansion of ∼300% during lithiation that may lead to particle disintegration. ^{Reference McDowell, Lee, Nix and Cui25,Reference Szczech and Jin26} Even more challenging is the unstable interface between the lithiated silicon electrode and battery electrolyte. The large volume change creates mechanical instability at the interfacial layer. Any fracture will lead to fresh electrode surfaces that need to be passivated again which consumes charge and leads to rapid cyclable charge loss. Significant worldwide effort has been devoted to tackling these issues by using designed nanostructures to manage this mechanical failure mechanism. ^{Reference McDowell, Lee, Nix and Cui25–Reference Xin, Qingliu, Juchuan, Xingcheng, Lott, Wenquan, Sheldon and Ji29} However, active lithium loss at the unstable interface remains a persistent challenge. The first demonstration of high capacity silicon anode will likely be in portable electronics where the sensitivity to specific energy is greater than cycle life. To impact EV applications, further progress is needed to scale up nanostructured materials, improve interfacial stability through electrolyte innovation, demonstrate performance in larger format cells, achieve longer cycle and calendar life, and finally, pass abuse tolerance tests.

The challenges associated with the use of lithium metal anodes are also very well known which include (i) the formation of lithium dendrites that can lead to shorting of the batteries as well as loss of active lithium due to electrode pulverization; and (ii) the low charge efficiency due to the high reactivity between metallic lithium and the organic electrolytes. A recent review offered a comprehensive account of the approaches attempting to address these challenges. ^{Reference Xu, Wang, Ding, Chen, Nasybulin, Zhang and Zhang30} They can be categorized as (i) mechanical suppression; (ii) the use of electrolyte additives; and (iii) using single ion conductors. For mechanical suppression, a separator or solid electrolyte layer with high sheer modulus (∼10 Ga) is used to combat dendrite growth since lithium metal is a soft material with a Young's modulus of only 4.9 GPa. ^{Reference Monroe and Newman31} For electrolyte additives, compounds such as CO₂, ^{Reference Aurbach, Zaban, Gofer, Ely, Weissman, Chusid and Abramson32,Reference Aurbach, Weissman, Zaban and Chusid33} HF, ^{Reference Shiraishi, Kanamura and Takehara34} vinylene carbonate, and fluoroethylene carbonate, ^{Reference Mogi, Inaba, Jeong, Iriyama, Abe and Ogumi35,Reference Stark, Ding and Kohl36} are reduced on the electrode surface to form an effective SEI. More recently, Cs⁺ was found to suppress dendrite growth and a “self-healing electrostatic shield” mechanism was proposed to explain the effect. ^{Reference Ding, Xu, Graff, Zhang, Sushko, Chen, Shao, Engelhard, Nie, Xiao, Liu, Sushko, Liu and Zhang37} Finally, single ion conductors that effectively mitigate concentration gradients can significantly reduce dendrite growth. ^{Reference Sadoway, Huang, Trapa, Soo, Bannerjee and Mayes38,Reference Bouchet, Maria, Meziane, Aboulaich, Lienafa, Bonnet, Phan, Bertin, Gigmes, Devaux, Denoyel and Armand39} The most noteworthy approach is the use of ceramic solid electrolytes. These electrolytes offer both mechanical strength and single ion conductivity. In fact, cycle life of over ten thousand charge/discharge cycles has been demonstrated in solid-state thin-film lithium batteries. ^{Reference Bates, Dudney, Neudecker, Ueda and Evans40} However, using the same principle in a bulk form remains challenging and will be discussed in more detail later.

Alternative cathodes

With a lithium metal anode, Fig. 5 projects that a high capacity lithium transition metal oxide will enable a cost target of 100 $/kWh. If lithium metal is viable, then sulfur and oxygen would be even more desirable cathode materials. They offer 3–5x energy density with minimal cost attributed to the material. For these reasons, both systems are subjects of intensive research efforts. The sulfur cathode chemistry has progressed rapidly in recent years. ^{Reference Barghamadi, Kapoor and Wen41–Reference Wang, Zeng, Zhou, Yin, Li, Cheng, Gentle and Lu44} The employment of nanostructured carbon scaffold to contain sulfur effectively addresses the insulating nature of the sulfur material and serves to confine the soluble polysulfide species and mitigate the well-known crossover mechanism that has plagued Li–S technology. ^{Reference Ji and Nazar45,Reference Ji, Lee and Nazar46} As a result, cycling stability has been markedly improved. However, capacity loss due to sulfur redistribution persists. ^{Reference Ji and Nazar45,Reference Wan, Pu and Ai47–Reference Zhang49} In addition, the crossover mechanism can in principle only be eliminated with the use of a solid-state separator. Finally, electrode performance is a strong function of electrode materials loading levels. Thicker electrodes, which are essential to higher cell specific energy, often lead to reductions in specific capacity, rate capability, and cycling stability. ^{Reference Pope and Aksay50}

The progress and challenges of lithium–air batteries have been reviewed recently. ^{Reference Balaish, Kraytsberg and Ein-Eli51–Reference Christensen, Albertus, Sanchez-Carrera, Lohmann, Kozinsky, Liedtke, Ahmed and Kojic56} The oxygen cathode functions like a fuel cell electrode except that the discharge product is Li₂O₂, which is a solid and accumulates in the electrode. Challenges of lithium–air chemistry include: (i) the stability of the electrolyte in the presence of Li₂O₂; (ii) the reaction rate of Li₂O₂ formation and decomposition due to the involvement of oxygen catalysis; and (iii) the influence of contaminants from ambient air such as water and carbon dioxide. A recent analysis shows that Li–air cells offer limited advantages over advanced lithium-ion batteries due to these complexities. ^{Reference Gallagher, Goebel, Greszler, Mathias, Oelerich, Eroglu and Srinivasan24} Breakthroughs in cathode catalysis and microstructural and system designs are needed to realize a lithium–air battery with the rate capability, cycle life, and energy efficiency sufficient for EV applications.

Other high specific energy nonaqueous systems

In addition to lithium-based systems, there has been recent effort in investigating cell chemistries based on other cations. Specifically, intercalation reactions involving multivalent cations such as Mg²⁺ have been a focus for the Joint Center for Energy Storage Research, a research hub supported by the U.S. Department of Energy. ^{Reference Van Noorden57} Compared to lithium metal, magnesium metal electrode has a higher volumetric capacity and cycles reversibly without the formation of dendrite which stimulated intense research interest. However, the search for suitable electrolytes and high energy density cathode materials is still in its infancy. MgFeSiO₄ was shown to deliver 300 mA h/g at 2.4 V, showing great promise in delivering competitive specific energy to lithium-ion batteries. ^{Reference Orikasa, Masese, Koyama, Mori, Hattori, Yamamoto, Okado, Huang, Minato, Tassel, Kim, Kobayashi, Abe, Kageyama and Uchimoto58} Likewise, a high rate aluminum battery was reported recently. With an ionic liquid-based electrolyte, a cycle life of 7500 cycles was demonstrated but the low specific energy makes the concept more suitable for grid storage rather than EVs. Finally, the development of room temperature sodium-ion batteries has continued to progress, with most of the anode and cathode materials being analogs of lithium counterparts. ^{Reference Lin, Gong, Lu, Wu, Wang, Guan, Angell, Chen, Yang, Hwang and Dai59} A cell chemistry that can deliver competitive specific energy and cycle life to lithium-ion has yet to emerge. Nevertheless, these recent advancements may stimulate increased interests in these under-explored chemistries which may eventually lead to higher specific energies.

Advanced pack designs and battery management systems

The approaches outlined above focus on developing long life, high specific energy battery chemistries to drive down cost and weight. In order for this to be effective, the K _w factor in Eq. (8) needs to remain comparable to that for lithium-ion battery cells currently used. The K _w factor, around 150% for lithium-ion batteries, represents the weight overhead on the system level that provides thermal, mechanical, and electrical protections for the battery cells. ^{Reference Wagner, Lakshmanan and Mathias12} Lithium-ion batteries use high-energy redox couples operating with flammable organic carbonate electrolytes. As a result, the total combustible energy in a lithium-ion cell can be an order of magnitude greater than the electrochemical energy. ^{Reference Harmon, Gopalakrishnan and Mikolajczak60} The thermal system is tasked to maintain temperature distribution in a very narrow range to minimize degradation of the cells. The electronic control system estimates the state of charge and health of the battery and provides safe shutdown functions. As the specific energy of cells increases, the K _w factor will likely increase. The high specific energy demands better structural protection to avoid intrusion and deformation; consequently, additional guarding is required. In fact, it is a common practice to treat the battery pack as if it were a fuel tank. ^{Reference Gabrielli61} Despite the conservative system designs, there have been several vehicle fire incidents that have raised safety concerns for consumers. These incidences illustrate the challenges of addressing abuse tolerances by relying on system protection. ^{Reference Smith62} Advanced cell monitoring methods that can directly detect physical and chemical changes inside lithium-ion cells are being developed to assist with preventing these safety incidents and to improve utilization of battery capacities. Recently, a major initiative, the Advanced Management and Protection of Energy Storage Devices program, was launched by the Advanced Research Projects Agency for Energy (ARPA-E) of the U.S. Department of Energy to create a new generation of Battery Management System (BMS) sensing, controls, and power systems that can unlock greater performance, safety, and reliability from battery chemistries. ⁶³

In summary, intensive research effort is underway to develop battery chemistries with specific energies far greater than those of lithium-ions. Candidates for both the cathode and the anode materials use reaction mechanisms beyond intercalation chemistry. The challenge of the coupled mechanical–chemical degradation mechanisms in these batteries is far greater than those encountered with lithium-ion batteries, which demands continued innovation in materials and systems design. The risk of achieving specific energy targets set in Fig. 3 while maintaining calendar and cycle life remains high. There is a great need to consider approaches other than driving up specific energies of nonaqueous batteries to improve the chance to reach the cost and range targets necessary to facilitate mass adoption of EVs.

High energy density flow batteries

Flow batteries have enjoyed a recent renaissance due to their perceived advantages for grid storage. ^{Reference Liu, Zhang, Yang, Lemmon, Imhoff, Graff, Li, Hu, Wang, Xiao, Xia, Viswanathan, Baskaran, Sprenkle, Li, Shao and Schwenzer67–Reference Dunn, Kamath and Tarascon72} In flow batteries, the anode and cathode active materials are usually dissolved in electrolyte solvents, which flow through the electrode surface where charge transfer takes place. An ion exchange membrane is needed to allow the passage of ions while preventing crossover of the electroactive species. For grid storage, the most important metrics are cost, cycle life, and round trip efficiency. Specific energy, on the other hand, is not as critical since most of the applications are not weight or volume constrained. As a result, most of the redox couples, such as the vanadium redox battery, have specific energies <25 Wh/kg. ^{Reference Dunn, Kamath and Tarascon72} With the acceptance of having a lower cell voltage, aqueous electrolytes are often used due to their low cost relative to nonaqueous electrolytes.

Flow batteries can offer unique advantages for vehicle applications as well. For example, the battery can be electrically recharged or, alternatively, the catholyte and anolyte can be replaced at a fueling station, providing rapid charging which overcomes a major obstacle to EV adoption. While hydrogen fueling for FCVs can be fast, flow batteries can deliver much higher round trip efficiency assuming FCVs' hydrogen source is generated by electrolysis of water. In addition, the flow battery has the potential to tolerate degradation of the replaceable fluid, assuming purification is not prohibitively costly. However, for flow batteries to be suitable for EV applications, their specific energy has to be increased.

To increase specific energy, cell voltage or cell capacity needs to increase, either separately or together. For flow batteries, cell capacity is often limited by the solubility of the reactants. Consequently, it is important to search for flow systems with very high concentrations of active materials. ^{Reference Dunn, Kamath and Tarascon72} Alternatively, a solid electrode can replace one of the solutions as in the well-known example of the zinc–bromine battery. ^{Reference Liu, Zhang, Yang, Lemmon, Imhoff, Graff, Li, Hu, Wang, Xiao, Xia, Viswanathan, Baskaran, Sprenkle, Li, Shao and Schwenzer67,Reference Wang, Luo, Li, Wei, Li and Yang68} The zinc electrode functions as a traditional plating/dissolution electrode during battery cycling. However, reversible operation of the metal anode requires maintaining stability of the electrode dimension and preventing the formation of dendrites. Despite decades of research, a reversible metal electrode that can undergo thousands of cycles while maintaining dimensional stability remains elusive, although research continues to progress in this area. ^{Reference Parker, Chervin, Nelson, Rolison and Long73}

Another approach to improve energy density is to formulate the solid electrode material as particles suspended in an electrolyte. Recent work demonstrated that common lithium-ion battery materials could be formulated as a stable slurry with sufficient electrical conductivity to function as active materials. ^{Reference Duduta, Ho, Wood, Limthongkul, Brunini, Carter and Chiang71} Compared to a traditional lithium-ion battery, a semisolid flow battery greatly reduces the number of steps for battery manufacturing, and system costs can be significantly reduced. Many challenges remain for semisolid flow batteries, including high parasitic pumping power loss due to friction, and the instability of the electrode/electrolyte interface due to the shear force. The last issue is particularly relevant for lithium-ion anode materials whose surface require the protection of the SEI. The SEI layer has to remain mechanically robust to survive the shear force during the flow of the particles. Research into material systems that are free of this issue is critically needed.

Aqueous batteries

The lack of flammable materials is a unique advantage for batteries using water-based electrolytes, if hydrogen gas evolution during operation is avoided. Until the invention of lithium-ion batteries, rechargeable aqueous batteries dominated the market. Despite their long history, only lead–acid, nickel–cadmium, nickel–metal hydride (Ni–MH), and silver–zinc have reached mass adoption. ^{Reference Beck and Ruetschi74} For automotive applications, lead–acid batteries powered the early automobiles in the 1910s before ICEs were introduced. Amazingly, lead–acid batteries remained the power source of choice for General Motor's EV1 when it was introduced in the 1990s. ^{Reference Brost75}

Research in rechargeable aqueous batteries of high specific energy has stagnated in recent years. ^{Reference Beck and Ruetschi74} This fact is largely driven by the success of lithium-ion batteries in the portable electronics market. With higher specific energy and longer cycle life than nickel–metal hydride, lithium-ion batteries have dominated recent research efforts in energy storage.

If aqueous batteries are to compete with lithium-ion batteries for EV applications, they need to demonstrate both higher specific energy and longer cycle life. Most aqueous batteries do not offer greater than 100 Wh/kg specific energy, ^{Reference Beck and Ruetschi74} which is the lower bound as illustrated in Fig. 2. To increase specific energy, one approach is to expand the redox stability window of water so that redox couples with potentials higher than oxygen or lower than hydrogen evolution can be utilized. In fact, for the Ni–MH battery, the nickel oxyhydroxide electrode potential (0.49 V in 1 M of OH⁻ solution) is very close to oxygen evolution as shown by nickel's Pourbaix diagram. ^{Reference Beverskog and Puigdomenech76} As a result, the electrode is rarely fully charged before oxygen evolution takes place. Similarly, the metal hydride electrode operates very close to the hydrogen evolution potential. As a result, corrosion of metal hydride often limits battery life. If the electrolyte window can be widened, metal hydrides such as Mg₂NiH₄ may be used with a capacity of 2.5 times of state-of-the-art LaNi₅H₆, significantly increasing cell specific energy. ^{Reference Cheng, Liang, Tao and Chen77} Consequently, research approaches for widening the aqueous electrolyte stability window through the suppression of water decomposition are critically needed. Alternatively, a double membrane configuration was proposed recently that promises operation of the anode and cathode reactions at vastly different pH values thus creating a zinc–cerium aqueous battery with a voltage of 3.08 V. ^{Reference Gu, Gong, Yan and Yan78}

The other issue that has plagued aqueous batteries is their poor cycle life. For example, lead–acid batteries degrade rapidly when cycled at full depth of discharge. This poor cycle life is rooted in its chemistry. The electrode reactions of lead–acid battery are not intercalation based and involve major changes in composition, microstructure, dissolution, and phase transformation. These changes make it difficult to maintain the dimensional stability of the electrode and lead to performance decay. ^{Reference Ruetschi79} In contrast, aqueous lithium-ion batteries such as LiFePO₄/Li₅Ti₂(PO₄)₃ have demonstrated exceptional cycle life due to the use of two intercalation electrode reactions; however, cell specific energy is low due to the limited capacity and low cell voltage. ^{Reference Luo, Cui, He and Xia80} Searching for new redox reactions in aqueous electrolytes represents a major research opportunity. In particular, advancements in nanoengineering may enable better control of the complex battery reactions to offer long cycle life, such as with the lead–acid battery chemistry. As an example, nanoengineering approaches have been extensively used in lithium-ion battery research and have shown potential for enabling conversion and alloy type reactions. ^{Reference Cheng, Liang, Tao and Chen77,Reference Trevey, Gross, Wang, Liu and Vajo81–Reference Li, Wang, Chen and Huang83}

Batteries with built-in safety mechanisms

Lithium-ion batteries primarily rely on the battery management system to ensure their safety; however, there have been continuous attempts to improve the inherent safety of these batteries. The approaches include: (i) redox shuttle for over-charge protection. A compound soluble in the electrolyte is designed to be oxidized at potentials just beyond the upper limit of electrode charging potential. The oxidation product then migrates to the negative electrode surface where it is re-oxidized. This mechanism thus creates a chemical short inside the battery, while preventing unwanted irreversible reactions that may lead to battery safety incidents and cell failure. ^{Reference Chen, Qin and Amine84} (ii) Mechanisms to cut off electronic or ionic flow at high temperatures or in the event of an internal short. Separators are designed to melt at elevated temperatures that drastically reduce the porosity of the separator and prevent further flowing of ions. ^{Reference Roth, Doughty and Pile65} (iii) Addition of flame retardants to the electrolyte. Phosphorous or halogen-containing compounds have been extensively studied. ^{Reference Nagasubramanian and Fenton64} However, the amount of fire retardant to be effective also tends to negatively impact battery performance, with reduction at the negative electrode being a leading cause.

The effectiveness of using any built-in mechanism depends on the relative rates between the on-set of the mechanism and that of thermal runaway. Consequently, detailed modeling and testing are necessary whenever a change is made to cell materials or formats. Finally, the majority of the research work has focused on mitigating thermal runaway or flammability. Developing inherent safety mechanism in response to mechanical abuse remains a less explored endeavor and represents a major research opportunity.

Batteries with essentially no combustible organic materials

All solid-state batteries have been a persistent research topic for the past two decades ^{Reference Quartarone and Mustarelli85,Reference Takada86} and features prominently on the roadmap of Japan's NEDO. ¹⁵ A clear advantage is that these batteries are free of volatile, flammable organic solvents. Consequently, the total combustible energy is significantly lower than that of commercial lithium-ion batteries. Additional advantages of solid-state batteries include high energy density and long calendar and cycle life. For example, it is possible to use lithium metal as the negative electrode material, which greatly increases cell energy density. ^{Reference Trevey, Gilsdorf, Stoldt, Lee and Liu87} It is also possible to use cathode materials such as FeS₂ which normally does not cycle reversibly due to material dissolution into the nonaqueous electrolyte solution. ^{Reference Yersak, Macpherson, Kim, Le, Kang, Son, Kim, Trevey, Oh, Stoldt and Lee88} The stable interface between electrode material and electrolyte results in few parasitic reactions in solid-state batteries and contribute to the long life of the battery. Comprehensive reviews of solid-state batteries have been published recently. ^{Reference Quartarone and Mustarelli85,Reference Takada86} This article only discusses issues that are relevant to EV energy storage.

Early solid-state batteries used thin-film electrode and electrolyte layers. An example was the Li/LIPON/LiCoO₂ cell developed at Oak Ridge National Laboratory. ^{Reference Bates, Dudney, Neudecker, Ueda and Evans40,Reference Bates, Dudney, Lubben, Gruzalski, Kwak, Yu and Zuhr89} The electrolyte layer, LIPON, prepared by sputtering Li₃PO₄ in a nitrogen atmosphere, is a nitrogen-doped lithium phosphate, and has a conductivity of ∼10⁻⁶ S/cm. The low conductivity mandates the use of a very thin electrolyte layer to minimize the resistance. Despite the outstanding cycling stability of these batteries (>10,000 cycles demonstrated), the actual energy density of the device is very low since most of the battery is made of substrates and packaging materials.

Fabricating a bulk version of the solid-state battery has proven to be challenging. First, a solid-state ion conductor with much higher conductivity needs to be developed. For reference, common lithium-ion battery electrolytes have conductivities of ∼10⁻² S/cm. ^{Reference Xu90} There are several electrolytes, including La_0.5Li_0.5TiO₃, Li_3.25Ge_0.25P_0.75S₄, Li₇P₃S₁₁, and Li₂S–SiS₂–Li₃PO₄ with conductivities of >10⁻³ S/cm ^{Reference Takada, Aotani and Kondo91–Reference Inaguma, Chen, Itoh, Nakamura, Uchida, Ikuta and Wakihara94} and a recent development of Li₁₀GeP₂S₁₂ with a conductivity of >10⁻² S/cm. ^{Reference Kamaya, Homma, Yamakawa, Hirayama, Kanno, Yonemura, Kamiyama, Kato, Hama, Kawamoto and Mitsui95} Many of these phases use highly polarizable elements such as sulfur to enhance ionic conductivity. Unfortunately, the presence of these elements also gives rise to electronic conductivities as well as limited electrochemical stability window. One solution to this dilemma is to use electrochemical materials with very high capacities but with work potentials within the stability window. Recent work demonstrated that cathode materials such as TiS₂, ^{Reference Trevey, Stoldt and Lee96} FeS₂, ^{Reference Yersak, Macpherson, Kim, Le, Kang, Son, Kim, Trevey, Oh, Stoldt and Lee88} and a polymer composite made by the reaction between polyacrylonitrile and sulfur show stable cycling with a sulphide-based electrolyte. ^{Reference Trevey, Gilsdorf, Stoldt, Lee and Liu87} It is thus important to develop the right electrolyte/electrode combination so that interfacial redox stability can be ensured.

An even greater challenge for bulk solid-state batteries is the large interfacial resistance. Ideally, the electrode is composed of a solid–solid composite of active materials and the electrolyte where (i) all the active materials are connected electrically, perhaps with the assistance of an electrically conducing additive; (ii) all the electrolyte materials are connected to ensure continuous ion conducting pathways; and (iii) the interface between the active material and the electrolyte is free of voids to provide a continuous electrochemical interface. The third requirement is particularly challenging, in contrast to batteries with liquid electrolytes where the liquid readily permeates into the pores of the electrodes and can manage volume change associated with the charge–discharge cycle. Current fabrication methods are not conducive to forming composites as described above. In general, powders of electroactive material, electrolyte, and conducting additives (most likely carbon black) are mixed thoroughly and pressed into a pellet. Residual porosity is usually unavoidable and there is no mechanism to ensure the formation of the ideal structure described above. Breakthroughs in materials processing are critically needed in this area.

Finally, all-solid-state batteries have to deal with the volume change and mechanical stress experienced during operation. Lithium-ion battery materials are known to experience varying degrees of volume change. For example, graphite expands by ∼13% when it is fully lithiated to a composition of LiC₆ while LiCoO₂ expands by ∼1.9% when it reaches a composition of Li_0.5CoO₂. ^{Reference Koyama, Chin, Rhyner, Holman, Hall and Chiang97} These volume changes are minor compared to the materials of higher capacities. As mentioned previously, Si expands by ∼300% while S expands by 80% ^{Reference Li, Zheng, Yang, Seh, Liu and Cui98} when it transforms into Li₂S. In an all-solid-state battery, it is highly unlikely any appreciable amount of volume change is acceptable, to prevent crack formation that may lead to catastrophic failure. Consequently, the need for electrode materials with very small volume changes is of paramount importance for solid-state batteries.

Multifunctional energy storage systems

If energy storage chemistries and architecture can provide inherent safety, then multifunctional designs become possible. These energy storage devices could provide structural functions, including load transfer and energy absorption.

The concept of structural power dates back to the early 2000s, when various systems were evaluated under DARPA's Synthetic Multifunctional Materials Program. For example, prismatic cells were used as parts of the wing for an unmanned air vehicle, which achieved record flight time. ^{Reference Christodoulou and Venables99} In this case, the battery served as both the power source and as part of the structure. To achieve this, the mechanical strength of the battery was increased by external reinforcement. An alternative approach is to make the battery itself truly structural by changing the material composition and structure of battery electrodes and separator. This approach is exemplified by the work of Snyder et al. ^{Reference Snyder, Wetzel and Watson100} Both mechanical and electrochemical performance goals were considered in the battery design which dictated the material choices. For example, the polymeric electrolyte layer requires high ionic conductivity; however, the ionic conductivity of a polymeric material often correlates inversely with its mechanical strength. Nonetheless, this truly multifunctional approach represents a new paradigm in material design and engineering and holds great potential in realizing weight and volume savings on the system level. Research efforts in this area persist to this day. ^{Reference Asp101–Reference Ekstedt, Wysocki and Asp103} For example, chopped carbon and glass fibers were used as fillers to the polymer matrix in both the electrode and electrolyte region. ^{Reference Liu, Sherman and Jacobsen104} A battery that is load bearing with a tensile modulus of 3.1 GPa was fabricated, which achieved an energy density of 35 Wh/kg. Similarly, when highly elastic polymers are used to form the battery matrix, elastic batteries can be fabricated showing a reversible elastic strain of over 35%. More recently, carbon fiber composites have been used to construct an electrochemical capacitor and its integration as the roof of an automobile has been suggested. ^{Reference MacKenzie105}

If the targeted application is for automobiles, multifunctional design can be considered on multiple length scales. In addition to material and cell level solutions discussed above, pack and vehicle level opportunities are possible. For example, mechanical testing results of cylindrical lithium-ion battery cells show an energy absorption density of 0.27 J/g before cell failure. ^{Reference Sahraei and Wierzbicki106} The cell can be viewed as a steel tube filled with packed powders immersed in liquids. Likewise, a water filled steel tube has been shown to absorb energy of nearly 4 J/g. ^{Reference Chen, Surani, Kong, Punyamurtula and Qiao107} On the pack level, a recent design ^{Reference Ginsberg108} builds battery cells into subunits, which are connected together using structural hinges. Upon mechanical impact, the battery pack can deform without sacrificing the mechanical integrity of the battery cell. The deformation absorbs part of the impact energy and contributes to vehicle occupant safety. In other words, the energy storage system, instead of being protected by additional vehicle structure, was able to participate in vehicle crash management and helped to protect vehicle occupant. Such additional functionality presents a powerful design opportunity that a vehicle equipped with an ICE does not have. The ability of the battery to absorb crash energy represents novel pathways to whole vehicle level optimization. A basic requirement for utilizing this mechanism is to ensure the inherent safety of the energy storage system.