|
|
| Fig. 1: Chemical stability vs. ionic conductivities for various SEs and carbonate liquid electrolyte. [7] (Image source: H. Prout, after Chen et al. [7]) |
The world's increasing shift towards electrification has driven up demand for energy storage, with batteries playing a pivotal role in applications such as transportation and consumer electronics. With the rise of electric vehicles and growing electronics markets, battery demand is set to skyrocket by nearly 10x from 2022 to 2035, with a demand of approximately 0.5 TWh in 2022 to over 5.7 TWh in 2023. [1] To date, lithium-ion batteries have been the favored battery technology due to their high energy density, wide ranging utility, and relatively low costs. However, LIBs are not without challenges. The majority of lithium ion batteries used today are made with liquid electrolytes which are inherently dangerous due to their flammability and potential for thermal runaway. [2] In response to the safety concerns associated with liquid electrolyte batteries (LEBs), solid-state batteries (SSBs) have emerged as a promising alternative. SSBs replace the flammable, liquid electrolyte with a nonflammable, solid electrolyte. [2] This report explores the feasibility of SSBs as a replacement for LEBs.
One of the primary functions of an electrolyte in a battery is to facilitate the flow of ions between the cathode and anode. Hence, much attention has been paid to assessing the ability of solid electrolytes (SEs) to facilitate the flow of ions, which is measured in ionic conductivity. The ionic conductivity of an electrolyte directly influences the battery's power output, rate capability, and overall performance. Traditional liquid electrolytes (LEs) have a ionic conductivity on the order of 10-3 - 10-2 S/cm. [3] After many years of research, ionic conductivities on this level have been achieved for SEs. [3] In fact, a 2011 study out of Japan reported a lithium superionic conductor with an ionic conductivity of .012 S/cm, which exceeds that of many liquid electrolytes. [4] Achieving ionic conductivity in SEs that is comparable to LEs has been highly encouraging to the potential commercialization of SSBs. However, despite these developments, SSBs still face issues that prevent their widespread adoption, primarily at their solid/solid interfaces, including high impedance and low stability. [5]
Electrochemical impedance in a battery refers to the opposition that the battery's components, particularly the electrolyte and interfaces between electrodes and electrolyte, present to the flow of current during electrochemical processes. It plays a critical role in the energy efficiency and power capability of a battery. [6] Even though a SE can exhibit high ionic conductivity, the interfaces between a SE and electrode can have low ionic conductivity. [6] This is one of the reasons that overall electrochemical impedance in a battery can be low, even when the ionic conductivity of a SE is high. Electrochemical interface reactions depend on more than just the properties of the electrolyte - they are also affected by the lattice structure of the interacting materials, grain boundary diffusion properties, and physical characteristics such as contact and conformity between materials. [2] For many SEs, interfacial impedance has been shown to be prohibitively high and/or increase over time, as a battery is cycled. [6] This affects the potential lifespan of a SSB, as increasing impedance would degrade the performance of the battery over time during use.
In addition to high interfacial impedance, SEs exhibit challenges associated with stability, including chemical, electrochemical, mechanical, and thermal stability. [7] Chemical stability refers to the ability of the battery to maintain its composition and structure in ambient air conditions during fabrication and storage. [7] Some SSB formulations are not chemically stable in ambient air conditions, and some degrade in an unsafe manner, such as sulfide SEs which can release highly-toxic hydrogen sulfide gas. [7] Moreover, each SE has an electrochemical stability window: the voltage range where reactions at both the anode and cathode are stable. [8] The upper and lower limit of this voltage range prevents the widespread usage of certain SE chemistries, such as sulfide SEs which have narrow windows. [8] Mechanical stability refers to the battery's ability to withstand the physical stress that occurs during the electrochemical reactions. [7] Volume changes have been shown to create significant challenges that may not be able to be overcome by cell packing and may require innovative architecture to overcome. [9] Lastly, while solid electrolytes are much safer than liquid, thermal runaway can still occur. [10]
With each important property for the commercial success of SSBs, there is a tradeoff. While individual SEs may be able to meet one of the necessary performance requirements, no single SE formulation has been able to achieve the high ionic conductivity, low interfacial impedance, and high stability in all aspects that is required for a successful SSB. Fig. 1 shows the range of ionic conductivity vs. electrochemical stability window for various SEs, compared to the performance of carbonate liquid electrolyte. This exemplifies that no SE matches the performance of liquid electrolyte on both ionic conductivity and stability, emphasizing that SEs are not yet commercially viable.
In summary, today's SSBs cannot replace LEBs due to the many challenges still associated with their performance. While there have been recent successes in achieving the ionic conductivity required for a SSB, the high impedance and lack of stability prohibits the widespread adoption of SSBs. Barring a breakthrough in technology that drastically decreases the impedance and improves stability, solid state batteries should not be considered for commercial applications.
© Haley Prout. 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] C. Xu, et al., "Future Material Demand For Automotive Lithium-Based Batteries," Commun. Mater. 1, 99 (2020).
[2] C. Chen, et al., "Interface Aspects in All-Solid-State Li-Based Batteries Reviewed," Adv. Energy Mater. 11, 2003939 (2021).
[3] N. Wu and H. Yang, "Ionic Conductivity and Ion Transport Mechanisms of Solid-State Lithium-Ion Battery Electrolytes: A Review," Energy Sci. Eng. 10, 1643 (2022).
[4] N. Kamaya, et al., "A Lithium Superionic Conductor," Nat. Mater. 10, 682 (2011).
[5] Y. Xiao, et al., "Understanding Interface Stability in Solid-State Batteries," Nat. Rev. Mater. 5, 105 (2020).
[6] L. Xu, et al., "Interfaces in Solid-State Lithium Batteries," Joule 2, 1991 (2018).
[7] R. Chen, et al., "Approaching Practically Accessible Solid-State Batteries: Stability Issues Related to Solid Electrolytes and Interfaces," Chem. Rev. d120, 6820 (2020).
[8] A. Banerjee, et al., "Interfaces and Interphases in All-Solid-State Batteries with Inorganic Solid Electrolytes," Chem. Rev. 120, 6878 (2020).
[9] R. Koerver, et al., "Chemo-Mechanical Expansion of Lithium Electrode Materials on the Route to Mechanically Optimized All-Solid-State Batteries," Energy Environ. Sci. 11, 2142 (2018).
[10] R. Chen, et al., "The Thermal Stability of Lithium Solid Electrolytes with Metallic Lithium," Joule 4, 812821 (2020).