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| Fig. 1: A lithium-ion battery. (Source: Wikimedia Commons) |
As society transitions to wider use of renewable energy and battery-powered technologies advance, high energy-density and efficient batteries are required for reliable electric grids, long-range electrical transposition, and new-age consumer electronics. Lithium-ion batteries (LIBs), shown in Fig. 1, have been the staple for the past several decades. However advancements in their energy density have been slow and their gravimetric densities are approaching the theoretical limit. [1]
Lithium-metal batteries (LMBs) are considered the next-generation battery with high gravimetric capacities, low reduction potentials (-3.04 V vs S.H.E), and high theoretical energy densities. [1] In comparison to LIBs, LMBs achieve their high-energy densities by replacing the anode graphite host material with a thin lithium anode (ideally less than 20 microns). [2] Anode-free LMBs have no lithium at the anode, thus achieving the highest energy densities. The cathode is usually composed of a Li-transition metal oxide intercalation host. However, other high-energy technologies like lithium-sulfur (Li-S) and Li-air batteries rely on lithiated polysulfides and lithium peroxide, respectively.
LMBs are plagued by a multitude of problems, specifically the corrosion of the highly reactive lithium metal and the formation of high surface area lithium deposits at the anode, known as dendrites. [1] These issues have led to several safety concerns and low cyclability. This analysis thus aims to contrast LMB technologies with LIBs to offer a perspective on the current state of LMBs.
Two major metrics to assess the energy systems are volumetric energy density (J/m3) and energy density (J/kg). It is important that energy sources be both compact and light for applications such as transportation and portable electronics. Typical LIBs achieve energy densities of around 8.64 × 105 J/kg and volumetric energy densities of around 2,304 J/m3. [1] On the other hand, LMB technologies such as Li-S and Li-air have theoretical energy densities of 9.36 × 106 J/kg and 1.262 × 107 J/kg, respectively. [1] Additionally, LMB technologies can achieve volumetric energy densities of 7,200 J/m3. [3]
However, due to the variety of failure mechanisms present in LMBs they have been plagued by low cycle life and capacity retention. Coulombic efficiency is the ratio of the discharge capacity to the charge capacity in a given cycle. In order to achieve widespread commercialization and meet the U.S. Department of Energy targets, LMBs must achieve 80-90% capacity retention and Coulombic efficiencies of over 99.95% for 1000 cycles. [3] The highest performing systems composed of liquified gas electrolyte have achieved Coulombic efficiencies of 99.9% for a portion of their cycling. [3] However, traditional solid or liquid electrolytes achieve much lower average Coulombic efficiencies. [3]
LMB technologies like Li-S have been praised for their cost efficiency due to the simple sulfur-based cathode, as opposed to the traditional lithium-metal oxide cathode. [4] While LIB tend to cost above $100 USD per kWh (3.6 × 106 J), some studies have shown that Li-S prices can be as low as $60 USD per kWh. [5] A major issue preventing LMB adoption is safety. Because lithium-metal batteries form dendrites, these lithium filaments can become so long that they make contact with the cathode, short-circuiting the cell, and leading to safety hazards such as fires. [1]
Although LMB technologies are extremely promising in terms of energy density and cost, major failure mechanisms prevent them from obtaining the performance and safety of LIBs. Continued research into LMB technologies could help bridge this gap and push LMBs to widespread commercialization.
© Kenzie Sanroman Gutierrez. 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] X.-B. Cheng et al., "Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review," Chem. Rev. 117, 10403 (2017).
[2] J. Zhang et al., "Research Progress of Anode-Free Lithium Metal Batteries," Crystals 12, 1241 (2022).
[3] G. M. Hobold et al., "Moving Beyond 99.9% Coulombic Efficiency for Lithium Anodes in Liquid Electrolytes," Nat. Energy 6, 951 (2021).
[4] H. Jiang et al., "Low-Cost Biomass-Gel-Induced Conductive Polymer Networks for High-Efficiency Polysulfide Immobilization and Catalytic Conversion in Li-S Batteries," ACS Appl. Energy Mater. 5, 2308 (2022).
[5] M. Zhong et al., "A Cost- and Energy Density-Competitive Lithium-Sulfur Battery," Energy Storage Mater. 41, 588 (2021).
