关键词: 隔膜, 功能化改性, 锂离子电池, 电化学性能, 表征技术

Abstract:

The design functions of lithium-ion batteries are tailored to meet the needs of specific applications. It is crucial to obtain an in-depth understanding of the design, preparation/ modification, and characterization of the separator because structural modifications of the separator can effectively modulate the ion diffusion and dendrite growth, thereby optimizing the electrochemical performance and high safety of the battery. Moreover, the development and utilization of various characterization techniques are critical and essential in bridging the intrinsic properties of separators and their impacts on the electrochemical performance, which guide the functional modification of the separators. In this review, we systematically summarized the recent progress in the separator modification approaches, primarily focusing on its effects on the batteries' electrochemical performance and the related characterization techniques. Herein, we provide a brief introduction on the separators' classification that mainly includes (modified) microporous membranes, nonwoven mats, and composite membranes; thereafter, we discuss the basic requirements that facilitate the use of membranes as separators, such as good wettability with electrolyte, high permeability for ions, and several intrinsic properties including good thermal stability, electronic insulation, excellent (electro)chemical stability, high mechanical strength, and appropriate thickness/porosity. We then highlight the factors that affect the batteries' performance from the viewpoints of ion diffusion, dendrite growth, and safety, along with the modification approaches. Specifically, the separator should possess high ionic conductivity and uniform ion transmission, which can be achieved by adjusting its composition and through surface modifications. The severe dendrite growth, especially in lithium-metal batteries, could be inhibited by controlling the pore structures, increasing affinity between separator and metal anode, constructing artificial solid electrolyte interphase (SEI), adopting high strength separator, as well as smart design of the separator. The safety issue, which is a major concern that limits battery applications, could be mitigated by increasing the separator's mechanical strength, thermal stability, and shutting the batteries down below thermal runaway temperature through various functionalization approaches. More importantly, the characterizations of the separators' structure, and their mechanical, thermal, and electrochemical properties are systematically summarized, including scanning electron microscope (SEM)/atomic force microscope (AFM) for surface morphology observation, focused ion beam scanning electron microscopic (FIB-SEM)/X-ray tomography (X-ray CT) for 3D structure detection, mercury intrusion porosimetry (MIP)/Brunauer-Emmett-Teller (BET)/Gurley number measurement for pore structure analysis, contact angle and climbing behavior of electrolyte in separators for wettability measurements, characterizations of the separator's tensile behavior, puncture behavior and compression behavior, thermo-gravimetric analysis (TGA)/differential scanning calorimetry (DSC)/infrared thermography (FLIR) for thermal properties test, and the electrochemical methods for determining the separator's electrochemical stability, ionic conductivity, internal resistance, lithium-ion transference number, cycle/rate performance, as well as self-discharge characteristic. These characterizations provide theoretical and practical basis for the rational design of functional separators and optimization of the electrochemical performance of lithium-ion batteries. Finally, we provide the perspectives on several related issues that need to be further explored in this research field.

Key words: Separator, Functional modification, Lithium-ion battery, Electrochemical performance, Characterization technology