In Situ Plating of Porous Mg Network Layer to Reinforce Anode

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

In Situ Plating of Porous Mg Network Layer to Reinforce Anode Dendrite Suppression in Li-Metal Batteries Fulu Chu,†,‡ Jiulin Hu,‡ Jing Tian,‡ Xuejun Zhou,‡ Zheng Li,*,† and Chilin Li*,‡ †

School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China

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S Supporting Information *

ABSTRACT: Li dendrite suppression enables a highly reversible Li-metal battery. However the strategy to smooth Li surface, especially under long-term cycling and high current density, is still a big challenge. Here, we propose a facile additive strategy to reinforce Li dendrite inhibition in a range of ether and carbonate electrolytes. Dissoluble Mg(TFSI)2 is employed as a degradable electrolyte additive, leading to in situ plating of porous Mg network when contacting reductive Li atoms. Mg adatoms with extremely low diffusion energy barrier as lithiophilic sites enable a smooth or flaky morphology of Li surface even under a high current density of 2 mA/cm2 and high capacity of 6 mAh/cm2. Mg-salt additive significantly extends the cycling life of Li||Cu asymmetric cells up to 240 and 200 cycles for carbonate and ether electrolytes, respectively. Under a current density as high as 5 mA/cm2, the Li|| Cu cell based on ether system can still survive up to 140 cycles with a small voltage hysteresis close to 60 mV. The hysteresis can be reduced to below 25 mV for lasting 200 cycles at 1 mA/cm2. This additive strategy provides a facile solution to in situ construction of conductive anode−electrolyte interface with low interface resistance for alleviating uneven Li nucleation. KEYWORDS: Li-metal batteries, dendrite suppression, electrolyte additive, Mg(TFSI)2, anode modification

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INTRODUCTION In the development of new energy sources, current secondary lithium-ion battery (LIB) based on graphite as the anode material is an excellent alternative because of its high operating voltage, low internal resistance, and minimal memory effect.1 However, in view of the urgent demand for mobile systems such as electric vehicles and large-scale stationary grid storage devices, rechargeable batteries with higher energy density deserve to be further explored. Rechargeable lithium-metal battery (LMB) is among the most promising candidates in view of the lowest reduction potential (−3.04 V vs the standard hydrogen electrode) and highest theoretical specific capacity (3860 mAh/g) of Li metal.2,3 In addition, LMBs enable the Li− O2, Li−S, and Li−fluoride (Li−MFx) systems based on Li-free conversion cathodes with a potentially high energy density exceeding 500 Wh/kg.4,5 Unfortunately, up to now, its practical application is still hindered by issues such as the needle- or mosslike deposition (so-called dendrite) issue of Li metal, mainly stemming from repeated uneven plating/stripping of Li and unstable formation/decomposition of a solid electrolyte interface (SEI) at the anode surface during the charge/ discharge process. Because of this pernicious effect, LMBs are surrounded by many knotty technological hurdles, such as low Coulombic efficiency (CE), insufficient cycle life, and flammable safety hazard.6 Besides, for today’s LIBs, Li cannot © XXXX American Chemical Society

be exclusively and smoothly embedded in graphite anodes. Some Li thin films can also be deposited on the anode in the form of Li metal due to the low Li-intercalation potential of graphite (0.2 V vs Li/Li+), especially under the conditions such as fast or excessive charging and low temperatures.7 Hence, suppressing the growth of Li dendrites can not only achieve the fast commercialization of high-energy-density LMBs but also contribute to a vital upgrade in terms of security for current LIBs. SEI is an ionically conductive but electronically insulating layer, which is usually formed by the natural reaction of electrolyte components with surface Li atoms at the anode.2 It can minimize further reactions between Li and electrolyte. However, SEI formed naturally is not robust enough to bear anode volume expansion arising from repeated Li deposition and extraction, leading to the disruption of SEI integrity and uneven current distribution that accelerate the propagation of Li dendrite. The dendrite growth in turn promotes the increases of anode surface area and roughness, resulting in the difficulty of SEI repair/healing and ultimate deterioration of the electrode system.2,8 Therefore, artificial SEI layers are Received: January 18, 2018 Accepted: March 23, 2018 Published: March 23, 2018 A

DOI: 10.1021/acsami.8b00989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) XRD pattern of Li anode after soaking in the mixture of EC/DMC (1:1 by volume) with 1 M LiPF6 and 25 mM Mg(TFSI)2 as solutes for 6 h. The standard patterns of Li (JCPDS 15-0401), Mg (JCPDS 35-0821), and Li2CO3 (JCPDS 72-1216) are also listed as references. (b) EDX element mapping of Mg, F, N, and S on a soaked Li anode surface.

Na- and K-ion batteries.33,34 The self-healing electrostatic shield requires a low Cs+ concentration, which is easy to be consumed and cannot guarantee an SEI of high mechanical strength, likely leading to a relatively low Coulombic efficiency (76.6%).27 As an additive to form Li-rich alloy, In precursor is expensive and Al3+ is likely to complex with excess anions to form large-sized anion or neutral species (e.g., AlCl4− or Al(OH)3) due to its high polarity.32,35 Inspired by the success of Mg batteries with dendrite-free Mg anode even in the Mg−Li dual-salt systems,36,37 we will attempt to employ a typical Mg salt, e.g., magnesium bis(trifluoromethanesulfonyl)imide Mg(TFSI)2, as a dissoluble electrolyte additive in this work. Mg2+ has a standard reduction potential 0.67 V higher than that of Li+/Li, and therefore it is chemically reduced to form Mg metal when contacting with Li.36 The precoating of Mg avoids the nucleation of subsequent Li at the hot spots of the original Li surface and protects the Li anode against corrosion by electrolyte. It enables an effective suppression of Li dendrite propagation and long-life Li plating/ stripping under an optimized concentration of Mg salt. Mg additive takes effect in a range of electrolyte systems based on typical ether or carbonate solvents. The Li||Cu asymmetric cell based on Mg(TFSI)2-reinforced ether electrolyte presents a low-voltage hysteresis of