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| Fig. 1: Dendrites penetrating layers of a lithium ion battery. (Source: Wikimedia Commons |
Solid State Batteries (SSBs) are a class of batteries that utilize a solid electrolyte rather than the liquid solutions used in commercial lithium-ion batteries (LIBs) today. Liquid electrolytes are comparatively heavy and less stable at higher temperatures, so swapping them out with solid electrolytes and pairing them with lithium metal anodes would allow for broader use cases and energy densities over 500 Wh/kg, whereas the average LIB tops out around 270 Wh/kg. [1] The implementation of SSBs would be especially useful in markets such as the emerging Electric Vertical Takeoff and Landing (EVTOL) industry that hopes to address the significant CO2 emissions of the aviation industry. However, SSBs are far from hitting the market due to a tangled web of decisions regarding how to choose and prepare the proper cathodic and anodic materials that will have stable and efficient interfaces with the electrolyte, and how to ensure sustainable manufacturability and scale up the process to be economically viable.
Pure lithium-metal anodes have a theoretical capacity of 3862 mAh/g, which is nearly 10x greater than the widely used graphite anodes with a capacity of 372 mAh/g. [2] Lithium metal anodes are also advantageous over other chemistries because they do not experience a volume change as lithium ions flow back and forth between the electrodes, whereas anode containing other materials such as silicon, the alternative with the next highest theoretical capacity, experience a volume change of 320%. [2] This would unsurprisingly lead to large internal stresses and other safety hazards, making it a poor choice for an anodic material. However, lithium metal has its barriers to adaption as well; it is a very reactive metal, so great care must be taken to select a solid electrolyte material that has a nonreactive and safe interface with the anode while also having decent ionic conductivity so as not to limit the sky-high capacity. The electrolyte would also ideally prevent the growth of dendrites - spiky tendrils that form as batteries cycle and can lead to structural damage. (See Fig. 1)
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| Table 1: Comparison of various anode materials' maximum specific capacity and volume change due to charge/discharge cycles; data from Mukhopadhyay and Sheldon. [2] |
Regarding ionic conductivity, most liquid electrolytes offer about 0.5-1 S/cm (Siemens are the SI measurement for conductivity, essentially the inverse of ohms for resistance) at room temperature. [3] Meanwhile, most solid electrolytes have conductivities several orders of magnitude lower; for example, most standard solid polymer electrolytes muster about 1.0 × 10-4 Siemens/cm while some sulfide solid electrolytes have been reported to reach up to .025 S/cm. [4,5] More research is needed to get these solid electrolytes on par with their liquid counterparts.
Once the battery chemistry is decided upon, the manufacturability of the battery will still present a significant hurdle. New megawatt-scale processing techniques, different from the established and relatively streamlined lithium-ion manufacturing processes, will need to be designed and vetted. They will need to match the < 1% variation in capacity found in commercial batteries today, as individual battery cell homogeneity is a major factor for battery pack life. [6] It is already remarkably expensive to set up a battery manufacturing facility. According to Mauler et al., the plant investment required to reach the minimum efficient scale (values in literature range from 0.2 to 7.1 GWh of annual production) and to allow for the exploitation of economies of scale will significantly increase by hundreds of millions of dollars as production techniques get more advanced (and expensive). [7] Thus, developing and refining a new manufacturing procedure for SSBs that can compete with the already prohibitively expensive costs of manufacturing lithium-ion batteries will be a considerable challenge.
This paper explored some of the main barriers to commercial viability of solid- state batteries: battery chemistry selection and manufacturability. Based on current progress, it may likely be over a decade before solid-state batteries can be widely implemented as researchers work to untangle the confounded variables of safety, cost, and performance.
© Andrew Sleugh. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. Albertus et al., "Challenges for and Pathways Toward Li-Metal-Based All-Solid-State Batteries," ACS Energy Lett. 6, 1399 (2021).
[2] A. Mukhopadhyay and B. W. Sheldon, "Deformation and Stress in Electrode Materials for Li-Ion Batteries," Prog. Mater. Sci. 63, 58 (2014).
[3] M. K. Jha and C. Subramaniam, "Characterization of Microsupercapacitors," in Microsupercapacitors, ed. by K. Kobashi and K. Laszczyk (Woodhead Publishing, 2022).
[4] X. Wu et al, "Electrolyte for Lithium Protection: From Liquid to Solid," Green Energy Environ. 4, 360 (2019).
[5] Y. Zhao, et al, "Solid Polymer Electrolytes with High Conductivity and Transference Number of Li Ions for Li-Based Rechargeable Batteries," Adv. Sci. 8, 2003675 (2021).
[6] L. Xie et al., "A Facile Approach to High Precision Detection of Cell-to-Cell Variation for Li-Ion Batteries," Sci. Rep. 10, 7182 (2020).
[7] L. Mauler, R.Duffner, and J. Leker, "Economies of Scale in Battery Cell Manufacturing: The Impact of Material and Process Innovations," Appl. Energy 286, 116499 (2021).
