Armed lithium metal anodes with functional skeletons
Introduction
With the never-ever stoppable development and advancement of both daily life and industrialization in modern society, the desire for constructing high-power/energy-density devices and large-scale electric grids is becoming more and more intense [[1], [2], [3], [4]]. Such a reality promotes the booming research studies on energy storage technologies, among which lithium (Li) metal batteries (LMBs) as a significantly potential candidate have been revived and widely studied in recent years [[5], [6], [7], [8], [9], [10], [11], [12], [13]]. The renaissance of research studies on LMBs can be ascribed to the superior advantages of the engaged ‘holy grail’ anode [[14], [15], [16], [17], [18], [19], [20]], namely, the Li metal anode (LMA), which has a light gravimetric density (0.534 g/cm3), the lowest redox potential (−3.040 V vs. standard hydrogen electrode), and a high theoretical capacity (3860 mAh/g). However, before the practical utilization of LMBs, several inherent defects of the LMA must be resolved [15,[21], [22], [23]]. One such issue is the notorious dendrite growth due to the uncontrollable deposition of Li, which will pierce through the separator and cause the great safety hazard such as short circuit of batteries [[24], [25], [26]]. Another typical challenge is the ultra-active nature of metallic Li, which makes it to spontaneously react with electrolytes to form a well-known solid electrolyte interphase (SEI) [[27], [28], [29]]. Owing to the limitations of battery chemistries at present, the SEI with insufficient mechanical and chemical stability suffers repeated breakage repair process, leading to the continuous loss of both Li and electrolytes [27,[30], [31], [32]]. Consequently, this dilemma will result in low coulombic efficiency and instant battery failure [[33], [34], [35]]. In addition, the ‘hostless’ feature of the LMA makes it experience severe volumetric effect during Li plating/stripping process that induces internal stress within batteries and incurs the deterioration of battery performances [[36], [37], [38]].
To tackle the aforementioned challenges of the LMA, tremendous efforts have been devoted, and numerous strategies have been raised. A plenty of research studies focus on constructing a stable SEI by introducing functionalized electrolytes and additives [[39], [40], [41]]. However, the reality of undesirable exhaustion of the electrolyte and Li source remains unchanged, and dendrites will sooner or later grow to break down the artificially designed SEI. Therefore, some research studies propose the direct deployment of protective interlayers and solid-state electrolytes that own high mechanical strength and excellent electrochemical stability against the LMA [[42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]]. Such materials can physically restrict the dendrites to puncture the separator but allow the formation of dendrites because they do not fundamentally change the electrochemical properties of the LMA. Compared with the aforementioned strategies, designing three-dimensional (3D) skeletons/hosts for the LMA is thought to be a more promising and powerful choice [[55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66]], which are supposed to have positive influence on mechanical properties, dendrite suppression, and charge/mass transfer processes for the LMA. In detail, when accommodating the LMA into appropriate skeletons, the undesired volumetric change of the hostless LMA during battery operation can be effectively suppressed because the deformation of Li is well constrained and stabilized in the skeleton. In addition, the 3D skeletons with a relatively high surface area or abundant porosity can help reduce the effective local current density, meanwhile being capable of homogenizing the distribution of Li-ion flux (e.g. Li-affinity surface groups to concentrate the Li-ion, channel-like structure to guide the Li-ion transfer) [67,68]. On the basis of Sand’s time (Eq. (1)) [69], all these are beneficial for suppressing the formation of Li dendrites and promoting the uniform deposition/dissolution of Li during battery operation. More importantly, surficial engineering and nanotechnology can further enable the skeletons with the ability to reduce the Li nucleation barrier, achieve the selective deposition of Li, and to regulate the chemical component/microstructure of SEI layers via strong chemical interaction with Li.
In this contribution, we classify the skeletons of the LMA in terms of material chemistries into five categories: carbon skeletons, metallic skeletons, alloy skeletons, polymer skeletons, and novel-type skeletons. Particularly, we compare 3D skeletons, lithiophilic skeletons, and heteroatom doping/modification functional skeletons with commercial current collectors in terms of Li growth behavior (Fig. 1). We review the function mechanisms, advantages, and prospects of each type of skeleton and compare the effects of various skeleton design toward the electrochemical performance of the LMA. Finally, we propose material design strategies that could eventually lead to practical applications of LMBs.
Section snippets
Carbon skeletons
Carbon materials owing to their high electronic conductivity, low density, hierarchical structures, inexpensive cost, and abundance are regarded as ideal candidate skeletons for the LMA. Several kinds of carbon skeletons with functionalized structures and components have been developed, including biomass-derived carbon skeletons, graphene, carbon nanotubes (CNTs)/nanofibers, hierarchical porous carbon, and heteroatom-doped carbon. Note some carbon materials with a high specific surface area
Metallic skeletons
In the research studies for developing ideal LMA skeletons, metallic skeletons that are widely used as current collectors for anode materials have caught great attention owing to their high conductivity and strong mechanical properties. Moreover, numerous studies demonstrate the development of varied metallic skeletons by processing architectures and introducing composite materials with the aim of achieving the homogenous deposition of Li.
Alloy skeletons
Skeletons can enable the LMA with better stability and cyclic performance, while these inactive materials no doubt will increase the mass and volume of the electrode, reducing the energy density of batteries. In addition, extra infusion or electrodeposition process is needed for harvesting the skeleton supported LMA. To address the aforementioned considerations, directly alloying Li with skeleton materials is a promising strategy [198,199].
Polymer skeletons
In most research studies, skeletons are designed with excellent electronic conductivity, which makes such skeletons suffer from avoidably Li deposition on the conductive surface of the skeleton rather than totally within the skeleton even at the situation that uses lithiophilic seeds for guiding Li plating behavior. Such a dilemma promotes the development of insulative skeletons, especially the polymers [[215], [216], [217], [218]]. Liu et al. [219] designed an insulating polymer skeleton
Lithiophilic gradient skeletons
As discussed previously, the properties of scaffolds such as lithiophilicity and conductivity have a great impact on the nucleation and growth of Li, making gradient materials composed of differentiated lithiophilicity or/and conductivity promising skeletons for guiding the specific deposition of Li [108,[221], [222], [223], [224], [225]]. In terms of gradient materials for the LMA, Zhang et al. [226] provided an excellent reference by designing a lithiophilic-lithiophobic gradient protective
Novel skeleton materials
In addition to skeletons mentioned previously, novel skeleton materials with robust mechanical strength or/and superior Li affinity have been developed and applied in stabilizing the LMA, including the tough ceramic/clay materials, MOFs, carbides, and so on. For instance, MXene is a graphene-like material with metallic electronic conductivity, fast Li-ion transport ability, and abundant Li nucleation sites, which can serve as an ideal skeleton for the high-performance LMA. Li et al. have used Ti
Conclusion and outlook
In this work, we have reviewed various types of skeleton materials made of carbon, metal, alloy, polymer, ceramic, and MOF that have showed great promise in stabilizing the LMA by reducing the local current density, relieving the volumetric changes, homogenizing the Li-ion distribution, diminishing the nucleation barrier, or buffering the dendrite growth. Owing to the construction of those functional skeletons, advances have been made in both the half-cell and full-cell of LMBs at the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge financial support from the National Natural Science Foundation of China (grant no. 51722210, 51972285, U1802254, 11904317, and 21902144), the Natural Science Foundation of Zhejiang Province (grant no. LD18E020003, LY17E020010, and LQ20E030012), and the Innovation Fund of the Zhejiang Kechuang New Materials Research Institute (grant no. ZKN-18-P05).
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