Surface-Functionalized Separator for Stable and Reliable Lithium Metal Batteries: A Review

Abstract

Metallic Li has caught the attention of researchers studying future anodes for next-generation batteries, owing to its attractive properties: high theoretical capacity, highly negative standard potential, and very low density. However, inevitable issues, such as inhomogeneous Li deposition/dissolution and poor Coulombic efficiency, hinder the pragmatic use of Li anodes for commercial rechargeable batteries. As one of viable strategies, the surface functionalization of polymer separators has recently drawn significant attention from industries and academics to tackle the inherent issues of metallic Li anodes. In this article, separator-coating materials are classified into five or six categories to give a general guideline for fabricating functional separators compatible with post-lithium-ion batteries. The overall research trends and outlook for surface-functionalized separators are reviewed.

Environment-Friendly Li-Rich Molecular Sieve Composite Separator with Abundant Lewis Base Sites for Enhancing the Lithium-Ion Transport Property

“…Among these promising candidates, lithium metal batteries (LMBs) have received widespread attention. Compared with the graphite anode (372 mAh g –1 ) most widely used in LIBs, the metallic lithium (Li) anode of LMBs possesses an ultrahigh theoretical specific capacity (3860 mAh g –1 ) and the lowest redox potential (−3.04 V vs the standard hydrogen electrode). − Li metal anodes with multiple advantages could be equipped with a series of high-energy cathode materials to adapt to specific work environments, mainly including Li–LiMO 2 (M = Ni, Co, Mn), Li–air (oxygen and carbon dioxide), and Li–sulfur batteries. , Nevertheless, the practical commercialization of LMBs is hindered by the rapid capacity decay, poor cycle life, and serious safety issues, which are mostly caused by the propagation of uncontrollable lithium dendrites during the lithium metal plating and stripping processes. − …”

“…With the continuous accumulation of lithium dendrites and volume expansion effect inside the battery, the weak separator would be punctured, and thus, the anode and cathode would be connected to cause the short-circuit accidents. In this context, various functional materials are combined with polyolefin-based separators to essentially address the lithium dendrite issues in LMBs. , On the one hand, the inorganic ceramic nanomaterials including SiO 2 , Al 2 O 3 , and TiO 2 are coated on the surface of the polyolefin separator to improve its wettability with the liquid electrolyte and block the growth of lithium dendrites. − For example, Zhu et al presented a special SiO 2 -based nanoshield design for separators to effectively resist lithium dendrites and avoid the short-circuit of LMBs . This strategy could validly extend the life of the charging test to more than 110 h and be applied to various material systems.…”

Size Effect of a Piezoelectric Material as a Separator Coating Layer for Suppressing Dendritic Li Growth in Li Metal Batteries

“…Li metal has a high specific capacity (3860 mAh g −1 ) and the lowest reduction potential (−3.04 V versus the standard hydrogen electrode at room temperature). These advantages render it the ultimate candidate as a next-generation rechargeable battery anode material [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. However, Li metal suffers from a huge volume change during repeated Li deposition/dissolution processes [ 8 , 13 ].…”

“…Among all the strategies, the introduction of functional separators has emerged as one of the most efficient strategies for enhancing LMB performance. The separator performs three essential functions, i.e., providing an ion path, serving as an electrolyte reservoir, and serving as a physical barrier between anode and cathode [ 12 , 30 , 31 , 32 , 33 ]. Considering the roles of the separator, separator modification can be regarded as a promising strategy for enhancing the cycle performance and for solving the safety issues of LMBs by inducing a uniform Li ion flux and by improving thermal/mechanical stability.…”

“…Therefore, many attempts have been devoted to making the LMBs feasible by depositing various functional materials on the separator. However, further research is still required to achieve long cycle life and to reduce parasitic reactions occurring in LMBs [ 12 , 19 ].…”