Elsevier

Energy Storage Materials

Volume 15, November 2018, Pages 148-170
Energy Storage Materials

Perspectives for restraining harsh lithium dendrite growth: Towards robust lithium metal anodes

https://doi.org/10.1016/j.ensm.2018.03.024Get rights and content

Abstract

Lithium (Li) metal is regarded as a “Holy Grail” anode for next-generation high-energy-density rechargeable batteries due to its high volumetric (2046 mA h cm−3) and gravimetric specific capacity (3862 mA h g−1) as well as the lowest reduction potential (−3.04 V vs. standard hydrogen electrode). However, undesirable dendrite growth and repeated destruction/formation of the solid electrolyte interphase (SEI) on Li metal anode during the long-term charging/discharging cycles have limited the practical applications of Li metal batteries. In this review, we summarize the strategies to restrain Li dendrites through electrolyte modification, multifunctional barriers, composite metallic lithium electrode, and 3D current collectors. The Li metal anode protection can be achieved by efficiently regulating the diffusion and distribution behavior of Li ions and electrons. The further exploration on rational integration of these strategies is highly expected to afford more fundamental understanding and engineering applications to practical Li metal batteries.

Introduction

The combustion of fossil fuel is the main energy source for current transportations and electric powers, while induces air and water pollutions. Therefore, renewable energy sources are strongly considered in households, industries and transportations. Many efforts have been devoted to explore advanced energy materials and related high-energy-density storage systems [1], [2]. Lithium (Li) ion battery (LIB) is among the most prosperous batteries currently and has been widely applied in electric vehicles, portable devices, robots, and other power tools [3], [4]. However, conventional LIBs fail to meet the continuously increasing demand for high-end devices with higher energy density due to its relatively low theoretical capacity of cathode and anode materials. Advanced electrode materials are critically demanded to further enhance the energy/power density of next-generation lithium batteries [5], [6].

Lithium metal anode has a high theoretical specific capacity (3862 mA h g−1) and the lowest reduction potential of −3.04 V (vs. standard hydrogen electrode) among all the metallic electrodes that affords a high working voltage and then high-energy-density for full cells [2], [7], [8], [9], [10], [11]. Therefore, lithium metal secondary batteries have been considered as one of the most attractive next-generation energy storage systems [12], [13], [14], [15]. Once lithium metal anode is paired with oxygen or sulfur cathode, Li-oxygen [16], [17], [18], [19], [20] with very high energy density are achieved. Different from the classical layered graphite anode in LIBs, Li metal is a “hostless” anode. The working mechanism for Li anode is Li+ plating/stripping from the anode instead of intercalation/de-intercalation in graphite anode during charge/discharge process. Uniform plating and stripping of Li ions on the anode surface are highly expected.

Nevertheless, the actual cases for Li metal plating/stripping is very complex. Currently, the growth of lithium dendrites during charge/discharge process is almost unavoidable, which not only breaks solid electrolyte interfacial (SEI) film and leads to the generation of “dead Li” with low Coulombic efficiency (CE), but also induces safety hazards (like internal short circuit, combustion/explosion of full cells) (Fig. 1). This seriously hinders the practical applications of high-energy-density rechargeable batteries with lithium metal anode [21], [22], [23], [24], [25], [26].

During the past several decades, the suppression of Li dendrites has been widely investigated (Fig. 2). The cycling performance of Li metal has been significantly enhanced based on the regulation of initial nucleation and growth of Li metal during plating process [27]. Herein the strategies to suppress dendrite formation are classified into four categories: i) The direct improvement on the stability and flexibility of solid electrolyte interphase (SEI) film on the anode surface to suppress dendrite growth. Li metal is thermodynamically unstable towards in all dipolar aprotic solvents (e.g. ethers and carbonates) used in most electrolytes, as indicated by Aurbach and Zaban [28]. The components deciding the properties of SEI film are regulated by the electrolyte additives. ii) Multifunctional barriers. Advanced battery separators with thermal stability, high ionic conductivity, dendrite detection, and other functions are proposed to enhance the safety of lithium metal batteries. In addition, the solid electrolytes with high mechanical modulus are considered as alternative mechanical barriers, which have been applied to prevent dendrite growth as well. iii) The Li metal can be further plating/stripping into 3D structured scaffolds. This is beneficial to overcome volume expansion and improve electrochemical performance. iv) 3D current collectors are employed to uniformly deposit Li metal and guide Li metal plating.

In this review, we briefly summarize recent investigation of high performance lithium metal anodes based on the aforementioned four aspects. The recent efforts to enhance the ion conductivity, stabilize the SEI layer, develop the emerging strategies to characterize the morphology of ion deposition are also involved. The perspectives for Li dendrite suppression is also included.

Section snippets

Regulation on liquid electrolyte recipe

Lithium metal is thermodynamically unstable in most organic solvents. Therefore, a passivation film (named as SEI) is always obtained on Li metal electrode once the fresh Li metal meets organic electrolytes [29], [30]. The growth of Li dendrites is a typical crystal growth in organic solution. The sharp tip of Li dendrites can puncture the SEI film, which renders fresh Li exposed to electrolyte again with the formation of new SEI films. The repeated processes not only consume organic

Multifunctional separators

In rechargeable batteries, separators with electrolyte play an important role to avoid the direct contact of electrodes and enable rapid ionic transference [130], [131]. The functionality of the separators is affected by various properties. For instance, a low thickness of separators is beneficial to achieve full cells with high energy density. The homogeneity in the thickness facilitates uniform ion distribution in a work battery, which is pivotal to dendrite-free Li+ deposition on anode

Tailoring lithium metal anodes

For a metallic Li electrode, the modification itself is the most effective strategy to fundamentally handle its dendrite growth and volume change issues. This section will focus on the method to understand the Li plating revolution behavior and summarize the possible solutions to its dilemma.

Building novel current collectors

The Cu foil is usually used as current collectors for anode of LIBs. However, it is not appropriate for LMBs because of the uneven deposition of Li metal on planar Cu foil [201], [202]. Modified current collectors are proposed to handle the heterogeneous distribution of current density on the current collectors, include designing 3D current collectors and modifying the surface of current collector.

Regulation on Li nucleation and growth

Several optimal characteristics include high specific capacity, low bulk density, and low reduction potential render Li metal among the most promising anode materials for high-energy-density batteries. However, the growth of lithium dendrite and volume expansion have been regarded as unavoidable barriers for practical applications of LMBs. Although many valuable progresses have been achieved on Li anode research, more advances on this topic are still necessary.

Based on the Volmer-Weber theory

Suppression of other metal dendrites

Metals are natural candidates as anode materials in high-energy-density battery systems by virtue of the various advantages such as the abundant reserves in earth, low cost, and high potential capacity. Metallic anodes except lithium, for example, sodium (Na) [238], magnesium (Mg) [239], zinc (Zn) [240], potassium (K) [241], and aluminum (Al) [35], [242] have gained much attention. Particularly, zinc (Zn)-, magnesium (Mg)-, and aluminum (Al)-based secondary batteries are regarded as

Conclusion and perspective

The recent progresses of solving the problems derived from Li metal dendrites is reviewed for next generation high-energy-density rechargeable batteries. Strategies are proposed in mitigating the formation of lithium dendrites along with the side reactions related to lithium metal and minimizing the volume change during cycling. Fig. 23 concludes the strategies to protect Li anode based on different parts of batteries, including tailoring the anode structure, designing anode/electrolyte

Acknowledgments

This work is supported by National Key Research and Development Program (2016YFB0100508&2016YFA0202500) and National Natural Science Foundation of China (21676160).

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