Boosting the electrochemical performance of 3D composite lithium metal anodes through synergistic structure and interface engineering
Graphical abstract
Introduction
Traditional lithium ion batteries (LIBs) based on reversible intercalation/decalation of lithium ions are approaching their energy density limits and not meeting the ever-increasing demand of today’s electric power and energy storage applications including electric vehicles and portable devices [[1], [2], [3]]. In pursuing next-generation batteries with significantly higher energy density, alternative systems beyond lithium intercalation chemistry have been developed such as Li–S [4] and Li–O2 [5,6] systems based on Li metal plating/stripping electrochemistry. In these systems, Li metal, which possesses fascinating advantages of an ultrahigh specific capacity (3860 mAh g−1), a very low redox potential (−3.040 V versus standard hydrogen electrode) and a small gravimetric density (0.534 g cm−3), has been widely considered to be the ultimate anode material to replace graphite anodes [[7], [8], [9], [10]]. However, practical usage of lithium metal anodes (LMAs) has been challenged by the issues of low Coulombic efficiency (CE) and Li dendrite growth, originating from the highly reactive nature of Li metal in organic electrolyte and an unstable solid electrolyte interphase (SEI), as well as nontrivial volume change during Li metal plating/stripping [[11], [12], [13]]. In order to solve these problems, a large number of strategies, for example, adding electrolyte additives to facilitate SEI formation [14,15], constructing artificial SEI films to protect LMAs from electrolyte attack [[16], [17], [18], [19]], modifying separators to homogenize Li ion flux [20,21], improving lithiophilicity to reduce lithium nucleation overpotential [[22], [23], [24], [25], [26]], have been developed and effectively prolonged the lifespan of LMAs. However, these strategies are usually effective at relatively low current densities and cycling capacities, and need further improvements for high-rate battery applications. Structure-engineered composite electrodes with embedded hosts which can stabilize Li plating/stripping process are demanded for high-rate, high-capacity and long-lifespan LMAs [[27], [28], [29], [30], [31], [32], [33]].
Three-dimensional (3D), highly porous current collectors and skeletons have been introduced into LMAs as 3D hosts to improve electrochemical performance. These 3D hosts can effectively suppress lithium dendrite growth not only by accommodating huge volumetric change but also by reducing the effective current density during Li plating/stripping processes [12,34]. However, most of these structured LMAs are initially Li-free, and require pairing with Li-containing cathodes, which results in a lack of Li to offset irreversible consumption of lithium during SEI formation and later cycling. In order to match Li availability at both ends of the cell when including Li-free cathodes, such as in high-energy-density S and O2 cathodes, electrochemical plating of lithium into these 3D hosts prior to cell assembly is necessary [35,36], which usually causes uneven Li deposition, and also makes this process infeasible for battery manufacturing industry. Therefore, pre-storing of Li metal into 3D hosts is important for obtaining a composite lithium metal electrode. In this regard, Cui et al. are pioneers. They developed a facile and effective method to fabricate composite LMAs by infiltrating molten Li metal into 3D hosts [29]. Using this method, a variety of 3D highly porous scaffolds, such as nickel foam [37], carbon cloth/paper [30,38] and carbonized wood/polymer [28], have been employed to produce 3D composite LMAs. Compared with hostless Li metal electrodes, such 3D composite LMAs are capable of confining Li metal within 3D matrices, and maintaining a relatively constant electrode dimension, thus addressing the issue of large volume change associated with Li plating/striping, and realizing dendrite suppression and stable cycling. Nevertheless, most of reported 3D hosts have lithiophobic surfaces that need lithiophilicity conversion, and therefore, an extra phase, such as ZnO [39,40], SnO2 [41] and Si [29], have frequently been introduced to enhance lithiophilicity. The modification process always demands complicated facilities, e.g., chemical vapor deposition and atomic layer deposition, which are often labour-intensive, time-consuming and highly expensive. More importantly, such semiconducting phases lower electronic conductivity at the host-Li interface, which was usually overlooked in previous studies. In this manner, development of novel and simple strategies for constructing a lithiophilic and highly conductive interface on 3D hosts is of tremendous importance for fabricating 3D composite LMAs. In addition to a lithiophilic interface, the electrode structure of 3D composite LMAs is another important factor that contributes significantly to overall performance. However, it is a great challenge to control the electrode structure of 3D composite LMAs during the fabrication procedure, since the infusion process of molten Li metal is completed in a spontaneous manner within a short time period. Thus, in order to boost the electrochemical performance of 3D composite LMAs, there is a need for an alternative way to regulate the electrode structure after the melt-infusion process. So far, very little attention has been paid to the pretreatment of 3D composite LMAs.
In this study, we propose a synergistic structure and interface engineering strategy, which is simple and easy to conduct without the use of complicated instruments, to construct a newly designed 3D composite LMA for boosting its electrochemical performance. Specifically, a lithiophilic nanoporous gold (NPG) film with hierarchical micro/nano-porosity was initially coated on the surface of carbon fibers (CF), which reacts with molten Li and converts to a AuLi3 film during molten Li infusion process. Due to the fact that the AuLi3@CF scaffold was completely buried in Li metal layer after the melt-infusion process, a fraction of Li metal was then stripped from the 3D composite LMA (Li–AuLi3@CF) in order to expose the upper porous AuLi3@CF structure. We term this treatment process as “pre-stripping” (PS) of 3D composite LMAs. The as-designed 3D composite LMA (PS–Li–AuLi3@CF) possesses some remarkable advantages: (i) the pre-stripped AuLi3@CF scaffold is highly porous and conductive, which promotes rapid and homogeneous electron/ion transport at the PS-Li-AuLi3@CF/electrolyte interface, allowing fast electrode kinetics; (ii) the surface of the pre-stripped scaffold, namely the AuLi3 phase, is highly lithiophilic, which can bring about a much lower Li nucleation overpotential, enabling suppression of Li dendrite growth; (iii) the pre-stripped scaffold has a relatively large surface area, which can significantly reduce local current densities during cycling, thus enlarging the Sand’s time [42] and retarding Li dendrite formation; (iv) the released void space of pre-stripped scaffold provides extra room to accommodate Li deposition, alleviating the huge volume change during Li plating/stripping. Owing to the above mentioned merits, the PS-Li-AuLi3@CF based 3D composite LMAs exhibit outstanding electrochemical performance. In Li|Li symmetric cells, PS-Li-AuLi3@CF based 3D composite LMAs can run for 1800 h without cell failure at a current density of 0.5 mA cm−2. PS-Li-AuLi3@CF|LiFePO4 (LFP) cells show an excellent capacity retention of 96.1% with a CE of 99.2% after 1000 cycles at 5C. Furthermore, PS-Li-AuLi3@CF|sulfur/polyacrylonitrile (SPAN) cells deliver a capacity retention of 70.6% with a high CE of 99.8% after 1000 cycles at 2C.
Section snippets
Fabrication of NPG@CF scaffolds
The fabrication procedure of NPG@CF scaffolds consists of two steps. Firstly, a commercial CF (Toray Carbon Fiber Paper, Japan) paper was cut into rectangular pieces with a size of 1 × 6 cm2 and successively washed with acetone, dilute hydrochloric acid solution, ethanol, and distilled water in an ultrasonic bath for 20 min to remove impurities on the CF surface, and then dried in a vacuum oven at 60 °C. The AuSn alloy film was electrodeposited onto the pre-treated CF with an area of 1 cm2
Results and discussion
Fig. 1a illustrates the fabrication process of PS-Li-AuLi3@CF electrodes. CF paper was chosen in this study as a 3D host due to its lightweight as well as high electrical conductivity and porosity. However, the pristine CF surface is intrinsically lithiophobic, which makes it difficult to spontaneously wet with molten Li metal. Therefore, in order to enhance the surface lithiophilicity of CF paper, we firstly adopted a facile method developed previously in our group to coat a nanoporous gold
Conclusion
In summary, we have demonstrated a synergistic structure and interface engineering strategy for 3D composite LMAs, namely “Li–AuLi3@CF” electrodes, which are fabricated by Li melt-infusion into 3D porous CF paper scaffold modified with NPG that converts to AuLi3 after molten Li infusion, through a simple and effective pre-stripping (PS) protocol to initially expose a portion of AuLi3@CF scaffold, yielding “PS-Li-AuLi3@CF” electrodes. Compared with Li foil and Li–AuLi3@CF electrodes, the
Notes
The authors declare no competing financial interests.
Author contribution statement
Yuanmao Chen: Conceptualization, Investigation, Validation, Data curation, Writing-Original draft preparation. Xi Ke: Supervision, Conceptualization, Methodology, Data Curation, Writing-Reviewing and Editing. Yifeng Cheng: Investigation. Mouping Fan: Investigation. Wenli Wu: Data curation. Xinyue Huang: Data curation. Yaohua Liang: Visualization. Yicheng Zhong: Visualization. Zhimin Ao: Software. Yanqing Lai: Validation. Guoxiu Wang: Writing-Review and Editing. Zhicong Shi: Conceptualization,
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 research was supported by the National Key R&D Program of China (2018YFB0104200).
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