Lithium (Li) metal batteries (LMBs) have received extensive research attention in recent years because of their high energy density. However, uncontrollable Li dendrite growth deteriorates the battery life and brings about severe safety hazards. The rational design of battery separators is an effective approach to regulate uniform Li metal deposition towards boosted cycle life and safety of LMBs. Herein, we review the recent research progress concerning this issue, including mechanically strengthened separator fabrication, functional separator construction towards regulated Li ion deposition, and flame-retardant separator design. Moreover, the key issues and prospects of optimal design of separators are clarified for future development. This minireview is expected to bring new insight into developing advanced separators for long-life and safe LMBs.Get more news about aluminium cnc machining parts discount,you can vist our website!
As one of the most mature clean energy storage devices, Li-ion batteries have been widely used in various portable electronic products and electric vehicles [1,2]. Nevertheless, with the urgent need for long cruising range (>300 km) and long service life in electric vehicles, there is an increasing demand for higher energy density in rechargeable batteries [3,4,5]. To achieve this goal, it is of great importance to develop electrode materials with higher capacity. Li metal has an ultrahigh theoretical capacity of 3860 mAh g−1, a low mass density of 0.534 g cm−3, and an extremely low electrochemical reduction potential (−3.04 V vs. standard hydrogen electrode), and has been regarded as the most promising anode material [6,7]. However, the formation of an unstable solid electrolyte interface (SEI), the large volume change during Li plating/stripping cycles, and the uncontrollable growth of Li dendrites deteriorate the cycle life and safety of Li metal batteries (LMBs), seriously hindering their practical applications [8,9,10].
Various strategies have been proposed to address the above challenges in LMBs, including constructing functional artificial SEI [11,12,13,14], electrolyte engineering [15,16,17], separator modification [18,19,20,21,22,23], solid-state electrolyte design [24,25], and the construction of 3D composite Li metal anodes [26,27,28,29]. Among these strategies, separator modification plays an important role, because it would result in little change in the volume/mass of the battery, and thus would have little effect on the energy density of the Li metal battery. In addition, separators can be prepared at large scale, which helps to reduce the cost. The most widely used separators, such as polyethylene (PE) and polypropylene (PP), have the characteristics of porosity and electrolyte wettability. However, the low melting point of traditional separators leads to weak thermal stability and deterioration of mechanical properties as the temperature exceeds the melting point. Some nanofiber membranes with high porosity and outstanding thermal stability, such as polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF), can replace the above-mentioned separators, but suffer from poor mechanical strength due to weak physical interaction between the fibers. Therefore, by changing the composition and structure of the separator, the transport path of the Li ion can be adjusted, which would be beneficial in regulating Li nucleation and deposition behaviors. In addition, the functional separators can improve the mechanical strength to suppress Li dendrite growth and prevent piercing caused by Li dendrites.
The designing and optimization of separators, including modifying separators with functional polymers [30,31], carbon materials [32,33], metal particles [34,35], and solid electrolytes [36,37], have been widely studied. Given the increasing research activity on the optimal design of Li battery separators in recent years, a timely and comprehensive review of this interesting and sustainable research area is highly desirable. Here, in this minireview, we discuss the recent research progress of mechanically strengthened separator fabrication, functional separator construction towards regulated Li ion deposition, and flame-retardant separator design. Furthermore, current limitations and challenges of functional separator design in LMBs, as well as future research directions, are considered.
Strengthening separators has been deemed an effective strategy to block the growth of Li dendrites and prevent the short-circuit of LMBs. However, the Li dendrite is sharp and has a high Young’s modulus to pierce commercial separators such as polypropylene (PP) or polyethylene (PE) films . Naturally, designing separators with a Young’s modulus greater than 6 GPa is a straightforward and convenient strategy to protect the separator from punctures [39,40].
Coating commercial separators with rigid layers is recognized as a promising way to elevate the puncture strength of the separator. Inorganic materials possessing strong mechanical properties can be coated on the surface of commercial separators to improve the separator strength. Inspired by the shield design, Zhu et al.  proposed inhibiting Li dendrite growth by designing a nano-shield separator consisting of SiO2 nanoparticles with 500 nm diameter and commercial PP film (Figure 1a,b). By combining theoretical calculations and experiments, they found that a carved shield with a radius comparable to that of the puncturing tip can efficiently distribute interfacial stresses and mitigate short-circuits in LMBs (Figure 1c). Further, the effect of the nano-shield was attributed to a shift in the growth orientation of the Li dendrites and the tortuous growth pathway induced by the SiO2 nanoparticles coating. At a current density of 0.5 mA cm−2, the voltage plateau of the blank separator abruptly dropped from around 60 mV to roughly 10 mV after about 23 h, indicating that the cell was internally shortened by Li dendrites (Figure 1d). The cell with the nano-shield protected separator achieved a battery life of more than 110 h without short-circuiting, which is approximately five times longer than that of the cell with the conventional blank separator (Figure 1d).