Elsevier

Nano Energy

Volume 76, October 2020, 105068
Nano Energy

Stabilizing lithium metal anode by molecular beam epitaxy grown uniform and ultrathin bismuth film

https://doi.org/10.1016/j.nanoen.2020.105068Get rights and content

Highlights

  • A compact and ultrathin bismuth film is deposited on Li metal surface by molecular beam epitaxy method.

  • The Bi-coated Li anode offers good ionic conductivity, lithiophilicity, chemical stability and air/moisture tolerance.

  • The LixBi-rich surface layer formed on Bi-coated Li anode inhibits parasitic reactions and leads to homogeneous Li plating.

Abstract

Lithium metal anode is regarded as an attractive option for next-generation high-energy-density rechargeable batteries. However, the unstable solid-electrolyte interphases between lithium anode and electrolyte lead to uncontrolled growth of lithium dendrites. Herein, we describe the surface reconstruction of Li anode by molecular beam epitaxy deposition of an ultrathin and compact bismuth film. During Li plating process, the electrochemical active bismuth film is prone to form a close-knit lithiophilic lithium-bismuthide (LixBi) alloy layer through electrochemical alloying with lithium metal. Benefiting from its high ionic conductivity, lithiophilic characteristics, high chemical stability, good mechanical properties and air/moisture tolerance, the LixBi-rich layer efficiently inhibit the parasitic reactions between Li anode and electrolyte, and thus favor a dendrite-free Li plating/stripping process, as verified by electrochemical tests and in-depth analyses. Symmetric cells with Bi-coated Li electrodes show a stable and dendrite-free cycling behavior at 1.0 mAh cm2 for 300 h and superior rate performance up to 5.0 mA cm−2. When Bi-coated lithium anodes are paired with a LiNi0.5Co0.2Mn0.3O2 cathode, a stable cycling life for over 300 cycles at 0.5C with a high capacity retention rate of 97% is obtained. This work provides new insights and strategies for the construction of Li-rich alloy layers on lithium metal surface via molecular beam epitaxy that enables dendrite-free and high-performance lithium metal batteries.

Graphical abstract

A compact and ultrathin bismuth film is deposited on Li anode surface by molecular beam epitaxy method, leading to the in-situ formation of a lithiophilic LixBi alloy interphase after lithium plating. The as-formed LixBi-rich layer exhibits favorable charge transfer characteristics, mechanical properties, electrochemical stability and air/moisture tolerance, enabling uniform Li nucleation and dendrite-free homogeneous Li electrodeposition.

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Introduction

Rechargeable lithium-ion batteries (LIBs) based on intercalation chemistries are approaching their energy-density ceiling due to the inherent limited capacity (372 mAh g−1 for graphite anode), which cannot fully satisfy the ever-increasing demands for electric vehicle and smart grid storage [1]. As a lightweight anode material, metallic lithium has currently regained tremendous attention, owing to its high theoretical specific capacity (3860 mAh g−1) and low reduction potential (3.04 V vs. standard hydrogen electrode, SHE) [[2], [3], [4], [5]]. Despite many advantages, the batteries based on metallic Li anodes are still plagued with three severe issues that hinder their practical applications. (1) Metallic lithium anode is susceptible to undesired/uncontrolled growth of lithium dendrites, due to the uneven distribution of Li ions on the electrode surface during Li plating [6,7]. The accumulated dendritic Li could penetrate separators between electrodes, eventually causing short-circuit of batteries and serious safety issue [8,9]. (2) Li dendrites are easy to fall off from the current collectors during Li stripping process, which causes the formation of “dead Li” and the depletion of fresh Li. In addition, the formation of dead Li at the edge of an electrode exposed to pressure-free space also causes the increased Li consumption during cycling [10]. (3) Lithium anode are easy to react with nonaqueous electrolytes and form an unstable solid electrolyte interphase (SEI) on the surface [11]. The rupture and reconstructions of SEI are difficult to avoid during the repeated plating/stripping process, leading to the exhaustion of limited Li and lean electrolyte in practical Li metal batteries [12].

To circumvent the above issues regarding lithium metal anode, several strategies have been proposed to suppress dendritic Li growth and stabilize Li metal anode, including: (1) optimizing the SEI compositions by adding electrolyte additives to reinforce the SEI film [13,14]; (2) utilizing high-modulus solid-state electrolytes to block dendrite growth [[15], [16], [17]]; (3) modifying the separator with functional compositions or surface groups to homogenize Li-ion flux and achieve uniform Li electrodeposition [[18], [19], [20]]; (4) precoating the Li surface with artificial protective layers to prevent direct contact between Li metal and electrolytes [[21], [22], [23], [24], [25], [26]]; Although these strategies have boosted the performances of Li anode to a certain extent, further efforts/optimizations are still required to enable long cycle life and dendrite-free lithium deposition towards safe and high-energy lithium metal batteries. For instance, owing to the gradual loss of electrolyte additives, the SEI film on lithium surface formed by the introduction of electrolyte additives is not capable of continuously suppressing lithium dendrite growth during long-term cycling [[27], [28], [29]]. On the other hand, the utilization of solid-state electrolyte is hindered by their fragility feature and high interfacial resistance with electrodes [30,31]. Besides, the issues of separator modification associated with poor mechanical strength and sacrificed ionic conductivity still remain unsolved [32]. Compared to the other three strategies, the utilization of an artificial protective layer is a promising choice to continuously suppress dendrite growth and overcome the surface inhomogeneity and mechanical instability of electrolyte-derived SEI film, ensuring long-term stable Li stripping/plating. Nevertheless, artificial protective layer usually undergoes the compositional evolution and large volume variation during cycling, which may result in the degradation of protective effect. Therefore, it is challenging to construct a chemically stable and compositionally homogeneous interfacial layer that can effectively regulate homogeneous deposition of Li and meanwhile reduce parasitic side reactions during long-term cycling.

Thin film deposition technologies have been proposed to stabilize the surface of Li metal, including magnetron sputtering [33], atomic layer deposition (ALD) [34] and molecular layer deposition (MLD) [35]. Promoted by controlling the thickness and compositions of protective layer at atomic level, the cycling performance and rate capability of Li metal batteries can be greatly enhanced. As a precisely-controllable thin film deposition technology, molecular beam epitaxy (MBE) has prominent advantages for large-scale growth of atomic-scale-uniform film by accurately controlling the molecular beam flux, the growth temperature and the rotation/tilting of sample stage [36]. Herein, we report the deposition of a ultrathin and uniform bismuth film on Li metal surface through MBE technology by thermally evaporating Bi precursor under ultrahigh vacuum. The electrochemically active bismuth film is prone to form a chemically stable LixBi alloy phase layer by in situ alloying with lithium, thus can minimize the detrimental side reaction between Li and electrolyte. Moreover, the ionic conductive LixBi alloy layer offers lithiophilic nucleation sites to guide homogeneous lithium plating, and meanwhile facilitates rapid ion transport through the electrode-electrolyte interface, realizing dendrite-free lithium deposition. Furthermore, the Bi-coated Li anode shows excellent humid resistance even when exposed to air atmosphere with the relative humidity (RH) of 30–35%. As a result, the Bi-coated Li anode exhibits superior cycling stability for over 300 h at 1.0 mA cm−2 and rate performance up to 5 mA cm−2 in symmetric cells. A full cell paired Bi-coated Li anode with LiNi0.5Co0.2Mn0.3O2 cathode can deliver a stable operation life for over 200 cycles.

Section snippets

Results and discussion

As schematically illustrated in Fig. 1a, uniform and ultrathin bismuth film was directly grown on the surface of lithium foil using an ultrahigh-vacuum MBE apparatus equipped with a reflection high-energy electron diffraction (RHEED) gun. The bismuth film was deposited by evaporating high-purity Bi metal source from a large-orifice pocket electron-beam evaporator. To achieve the MBE growth of homogeneous bismuth film, ultrahigh vacuum with basic pressure of lower than 2 × 10−10 Torr is very

Conclusions

In summary, a compact and ultrathin MBE-grown bismuth film on Li metal surface was proposed to guide uniform Li nucleation and achieve dendrite-free Li electrodeposition. Notably, the Bi-coated Li anode possesses outstanding moisture resistance in ambient air, benefiting from the high stability and compositional homogeneity of MBE-grown bismuth film. Upon Li plating, the electrochemically-active Bi film was transformed to lithiophilic LixBi alloy layer by in situ alloying reaction between

CRediT authorship contribution statement

Tao Chen: Writing - original draft. Fanbo Meng: Software. Zewen Zhang: Investigation. Junchuan Liang: Investigation. Yi Hu: Methodology. Weihua Kong: Investigation. Xiao Li Zhang: Resources. Zhong Jin: Conceptualization, Supervision, Writing - review & editing.

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.

Acknowledgements

This work was supported by the National Key R&D Program (2017YFA0208200, 2016YFB0700600), the Fundamental Research Funds for the Central Universities (0205-14380219), the Projects of NSFC (21872069, 51761135104, 21573108), the Natural Science Foundation of Jiangsu Province (BK20180008, BK20170644), and the High-Level Innovation and Entrepreneurship Project of Jiangsu Province of China.

Tao Chen received his Ph.D. degree in Chemical Engineering and Technology at Nanjing University of Science and Technology in June 2015. He worked as a postdoctoral scholar at Nanjing University in the group of Prof. Zhong Jin (2015–2019). He is currently a research fellow at National University of Singapore. His current research focuses on the design and synthesis of nanostructured electrode materials for rechargeable batteries.

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  • Cited by (0)

    Tao Chen received his Ph.D. degree in Chemical Engineering and Technology at Nanjing University of Science and Technology in June 2015. He worked as a postdoctoral scholar at Nanjing University in the group of Prof. Zhong Jin (2015–2019). He is currently a research fellow at National University of Singapore. His current research focuses on the design and synthesis of nanostructured electrode materials for rechargeable batteries.

    Fanbo Meng received his Bachelor's degree in Packaging Engineering from Kunming University of Science and Technology in 2016. He is now pursuing his Ph.D degree under the supervision of Prof. Jin Huang in School of Mechatronic Engineering at Xidian University. His research interests include computational fluid dynamics and 3D printing sintering process.

    Zewen Zhang received his Bachelor's degree from School of Chemistry and Chemical Engineering, Nanjing University in 2018. He is currently pursuing his master degree under the supervision of Prof. Zhong Jin at the same school. His research interests focus on improving properties of lithium metal batteries.

    Junchuan Liang received his B.S. degree from the College of Chemistry and Chemical Engineering of Lanzhou University in 2019. Now he is pursuing his Ph.D. degree under the supervision of Prof. Zhong Jin at School of Chemistry and Chemical Engineering at Nanjing University. His research interest focuses on the application of the 2D nano-materials grown through the MBE.

    Yi Hu received his B.S. degree in Chemistry from Sichuan University in 2014. He has obtained his Ph.D. degree in 2019 under the supervision of Prof. Zhong Jin in School of Chemistry and Chemical Engineering at Nanjing University. His research interests reside in two-dimensional nanomaterials for electrochemical energy storage and photoelectric conversion.

    Weihua Kong received his B.S. degree in Chemistry from Hunan University in 2017. He is now pursuing his M.S degree under the supervision of Prof. Zhong Jin in School of Chemistry and Chemical Engineering at Nanjing University. His research interest is mainly concentrated on the lithium-sulfur and lithium-metal batteries.

    Xiao Li Zhang received her Ph.D. degree from Zhengzhou University in 2011, and worked as a visiting scholar at University of Toronto (2012–2013). Now she is an associate professor in School of Materials Science and Engineering at Zhengzhou University.

    Zhong Jin received his B.S. (2003) and Ph.D. (2008) in chemistry from Peking University. He worked as a postdoctoral scholar at Rice University (2008–2010) and Massachusetts Institute of Technology (2010–2014). Now he is a professor in School of Chemistry and Chemical Engineering at Nanjing University. He leads a research group working on advanced materials and devices for energy conversion and storage.

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