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| Fig. 1: Solid-State Battery (Source: Wikimedia Commons) |
Solid-state electrolytes have garnered a lot of attention in recent years due to their potential to revolutionize the battery and energy storage industries. Typically, Lithium-ion batteries contain a liquid electrolyte composed of a lithium salt in an organic solvent. However, due to concerns about flammability and the ongoing global need for improved battery efficiency, the world is turning to new alternative battery types. Solid-state batteries are strong candidates for taking over the energy storage industry. [1]
In a battery, the electrolyte is the substance that provides the mechanism to transport ions from one electrode to the other. As ilustrated in Fig. 1, ions in the electrolyte move to the battery's anode, react there, and release electrons that then move across a wire towards the cathode, therefore generating a current.
It is important to select an electrolyte that will be safe to use in batteries for all types of purposes, whether small-scale or large-scale. One of the main motivations for exploring the capabilities of solid-state electrolytes in batteries is that liquid electrolyte solvents currently in use can be quite dangerous and unstable accidentally short-circuiting or overcharging. The liquid-electrolyte battery can spark a fire or explode. [2] By being in solid form, SSEs prevent such an event from happening.
The most notable advantage that SSEs possess is their improved energy density. Energy density is defined as the amount of energy stored in a system per unit volume. The unit is typically given as Wh L-1, or J m-3. However, most research papers seem to define energy density in terms of mass (which is synonymous with specific energy). The units they therefore use are Wh kg-1. Current lithium-ion batteries - which have a liquid electrolyte, a graphite anode, and a lithiated transition metal cathode - have theoretical energy densities of 350-400 Wh kg-1, although their energy densities in practice are closer to 100-220 Wh kg-1. [2] These ranges are insufficient to meet high demands for energy storage due to advances in technology.
Solid-state electrolytes can provide batteries with higher energy density: for instance, lithium-sulfur batteries have been found to possess a theoretical energy density of approximately 2600 Wh kg-1, and lithium-air batteries having an astounding theoretical ED of 11680 Wh kg-1. This latter number is close to the energy density of petrol, which stands at 13000 Wh kg-1. [3]
Solid-state batteries are also said to benefit from improved cycle life. Cycle life is defined as the number of cycles (battery charges and discharges, with 100% depth of discharge) a battery can carry out before its capacity drops to 80% of its initial specified capacity. [4] Li-ion batteries currently have 500-1000 cycles, which can seem large, but in contrast with the performance of SSEs it becomes relatively small. [4] A report has stated that thin-film batteries using LiPON solid electrolyte have already achieved 10,000 cycles with a Li-metal anode, which proves that SSEs have excellent performance. [3]
Solid-state electrolytes in batteries have a lot of the advantages needed to become widely applicable in energy storage and next-generation batteries. Currently, SSEs are seeking to vastly improve electric vehicles, due to the improved safety features as well as higher energy density and cycle life. [1] They are also being implemented in a wide array of systems, including photovoltaic cells, sensors, fuel cells, and supercapacitors. [2]
One of the challenges that solid-state batteries have faced which have hindered their ability to revolutionize the energy industry earlier on is that they tend to have lower ionic conductivity than liquid electrolytes, as ions can move in a liquid easier and faster than in a solid. [2] However, research is being done to tackle this issue. It has also been discovered that halide- and hydride-type electrolytes have unexpectedly high ionic conductivities. [1]
Another challenge when it comes to SSBs has to do with the interfacial reactions between the SSE and the electrodes. [5] An interesting approach to tackle this problem is machine learning, with algorithms being developed to predict the best electrolyte materials based on interfacial reactions with the electrodes in the system. [6]
© Jad Fidawi. 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] Z. Gao et al., "Promises, Challenges, and Recent Progress of Inorganic Solid-state Electrolytes for All-Solid-State Lithium Batteries," Adv. Mater. 30 1705702 (2018).
[2] L. Fan et al., "Recent Progress of the Solid-State Electrolytes for High-energy Metal-Based Batteries," Adv. Energy Mater. 8, 1702657 (2018).
[3] A. Manthiram, X. Yu, and S. Wang, "Lithium Battery Chemistries Enabled by Solid-State Electrolytes," Nat. Rev. Mater. 2, 16103 (2017).
[4] J. L. Garcia, "Electric Power Systems" in Cubesat Handbook, ed. by C. Cappelletti, S. Battistini, and B. Malphrus (Academic Press, 2020).
[5] T. Famprikis et al., "Fundamentals of Inorganic Solid-State Electrolytes for Batteries," Nat. Mater. 18, 1278 (2019).
[6] W. Fitzhugh et al., "A High-Throughput Search for Functionally Stable Interfaces in Sulfide Solid-State Lithium Ion Conductors," Adv. Energy Mater. 9, 1900807 (2019).
