Among high-capacity anodes, lithium metal anodes are perfect candidates for the design of rechargeable batteries due to their extremely high theoretical specific capacity, the lowest negative potential, and low gravimetric density.
Study: Nano Architectured Halloysite Nanotubes Enable Advanced Composite Separator for Safe Lithium Metal Batteries. Image Credit: Immersion Imagery/Shutterstock.com
However, these lithium metal anodes suffer from uncontrollable lithium dendrites growth leading to their piercing through polymer separators, inducing safety concerns that limit their applications. An article published in the journal Chemical Engineering Journal discussed the fabrication of nano halloysite nanotube-based high-performance poly (vinyl alcohol) composite separators (OPVA/NHNTs separator) to enhance the safety and electrochemical performance in lithium metal batteries.
Compared to Celgard and control OPVA separators, the fabricated OPVA/NHNTs separator showed high ion conductivity and Young's modulus, retarding the growth of lithium dendrites, and maintaining the electrochemical properties in lithium metal batteries.
Among OPVA/halloysite nanotubes (HNTs) and OPVA/NHNTs separator, the one with high Young’s modulus retarded the lithium dendrite’s growth. Moreover, ion conductivity was a critical parameter that promoted the even distribution of lithium ions and suppressed the formation of lithium whiskers.
The present work confirmed that nanomaterials have a critical role in suppressing lithium dendrite growth, paving a new path towards fabricating high-performing polymer composite separators in lithium metal batteries.
Strategies to Achieve Lithium Metal Batteries with High Safety
Lithium-ion batteries (LIBs) could not meet the growing demand for energy density for specific applications in electric vehicles and large-scale energy storage systems. Hence these LIBs were replaced by lithium metal batteries having high energy density by replacing graphite with lithium metal as anode.
Lithium metal has a high theoretical specific capacity of 3860 milliamperes per gram, a lower anode potential of -3.04 volts, and a low gravimetric density of 0.53 grams cubic centimeters. Lithium metal serves as an ideal anode material in designing high-performing lithium metal batteries.
The solid electrolyte interphase (SEI) layer present on the surface of the lithium anode, generated by the reaction between lithium salt, lithium metal, and electrolytes, may cause surface defects on the interfacial layer between the electrolyte and anode. These defects cause higher electron concentration leading to the reduction of lithium ions at the surface during charging processes resulting in the growth of lithium dendrites that penetrates through commercial polymer separators.
To enhance the safety of lithium metal batteries, strategies such as the employment of advanced separators and maintaining stable anion mobility in the battery systems were proposed for the suppression of lithium dendrite growth. Many reported works to date were designed based on employing electrolyte additives, modified polymer separators (organic/inorganic), synthetic electrolytes, and hierarchical current collectors.
NHNTs-Enabled Advanced Composite Separator for Safe Lithium Metal Batteries
In the present work, a composite separator with high ion conductivity and Young’s modulus was fabricated to enable the construction of lithium metal batteries with enhanced electrochemical properties and safety. The main aim of this work is to suppress the penetration of lithium dendrites through the separator in these batteries.
Although previous works reported the use of various artificial inorganic materials with high costs, halloysite nanotubes are natural minerals employed in the present work via chemical etching to form NHNT composites having larger pores on the interior side. The chemical etching removed the inner aluminum, and the porosity improved the ion conductivity.
The separator based on natural inorganic nanotube-based nano architectures of the present work revealed the criticality of ion conductivity and elasticity on lithium dendrite growth inhibition. Furthermore, the PVA polymer matrix used in this work was nontoxic to human health as well as environmentally friendly.
The nano architectures used in the present work were derived from natural ceramics, reducing the need for additional raw materials and post-processing. Compared to previously reported works that used anode hosts, artificial membranes, and inorganic material-coated commercial separators, the present method using NHNT-based separators simplified the process, reducing the overall production cost of lithium metal batteries.
To conclude, NHNT-based composite separators with high ion conductivity and Young’s modulus were fabricated to achieve lithium metal batteries with high safety and electrochemical performance. The OPVA/NHNTs composite separator showed high ion conductivity and Young’s modulus, suppressing the lithium whiskers growth compared to the OPVA/HNTs counterparts.
The results revealed that ion conductivity and elastic strength are critical parameters in realizing dendrite-free lithium metal batteries. Consecutively, the polymer-based Celgard and OPVA separators with poor ion conductivity and Young’s modulus or separators with low ion conductivity and high Young’s modulus could not prevent the penetration of lithium dendrites through the separator in these batteries.
Thus, the present work verified the role of nanomaterials in suppressing lithium dendrite growth, paving a new path for nanoscience towards the fabrication of high-performing polymer composite separators, meeting the pre-requisites of safety and electrochemical performances in lithium metal batteries.
Wang, W., Yuen, A. C. Y., Yuan, Y., Liao, C., Li, A., Kabir, I. I., Kan, Y et al. (2022). Nano Architectured Halloysite Nanotubes Enable Advanced Composite Separator for Safe Lithium Metal Batteries. Chemical Engineering Journal. https://www.sciencedirect.com/science/article/pii/S1385894722039778?via%3Dihub