Article www.acsaem.org
Cite This: ACS Appl. Energy Mater. 2019, 2, 4167−4174
Enhanced Electrochemical and Safety Performance of Lithium Metal Batteries Enabled by the Atom Layer Deposition on PVDF-HFP Separator Wei Wang, Yao Yuan, Junling Wang, Yan Zhang, Can Liao, Xiaowei Mu, Haibo Sheng, Yongchun Kan,* Lei Song, and Yuan Hu* State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China Downloaded via UNIV FRANKFURT on July 26, 2019 at 03:08:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Lithium metal batteries, due to their unique advantages, such as being lightweight and having the lowest anode potential and the highest theoretical specific capacity, have been regarded as a promising candidate for next-generation electrical energy storage. Nevertheless, the development and practical application of lithium metal batteries has been seriously hindered by the safety issues induced by lithium dendrites. Here, the atom layer deposition (ALD) technique is introduced to fabricate Al2O3 on the surface of a PVDF-HFP membrane to endow it with higher thermal stability, great affinity with electrolytes, improved ion conductivity, and higher Young’s modulus. Employing LiFePO4|Li cells, the ALD100/PH separator imparts batteries with the best cycling performances and rate capacity among various separators. The SEM images of morphology after cycling indicate that ALD100/PH separators with extremely high Young’s modulus and ion conductivity can suppress the growth of lithium dendrites. Moreover, the ALD100/ PH separator shows more than 1300 h of stable operation at a current density of 0.5 mA cm−2, exhibiting the capability against metallic lithium and the potential for application in the field of lithium metal batteries. Thus, this interesting ALD technique, capable of a feasible fabrication procedure and with desirable advantages, may be used to make commercial or as-prepared separators more advanced and may be a potential candidate for future rechargeable battery systems, including Li−S, Li−O2, and other metal batteries. KEYWORDS: ALD technique, PVDF-HFP, safety, lithium dendrites, lithium metal batteries
■
batteries,6−10 especially for lithium metal batteries.11,12 In the case of the suppression of lithium dendrites, the previous work reported that electrolytes with high ionic conductivity and stable anion mobility can prevent anion depletion and slow down the generation of lithium dendrites.13 Moreover, an adequate Young’s modulus of membranes, separators, and protective layers can mechanically suppress the uncontrollable growth of dendritic lithium.14−16 Therefore, fabricating advanced separators capable of high ion conductivity and Young’s modulus can be regarded as an efficient way to achieve safer lithium metal batteries. Actually, the most widely employed separators are polyolefin-based separators, including polypropylene and polyethylene, which exhibit high flammability and low thermal stability. Once under high temperature conditions, polyolefin separators will heat shrink and may result in the direct contact of cathodes and anodes, which may lead to an internal short circuit and even explosion.17−20 Moreover, other properties of
INTRODUCTION Nowadays, lithium metal has been recognized as the most promising metal for the next generation of lithium-ion batteries.1−3 Lithium metal as an ideal anode material in terms of energy density can deliver a high theoretical capacity (3860 mAh g−1) and possesses the lowest redox potential of −3.04 V vs H+/H and gravimetric density (0.53 g cm−3). Nevertheless, the further practical application of lithium metalbased batteries has been hindered by the fatal flaw of the dentritic and mossy lithium and low Coulombic efficiency, leading to unsatisfactory electrochemical performance and safety concerns.4 Usually, lithium dendrites are more likely to grow at the surface position where the current density is locally concentrated, concurrent with the generation of SEI layer. The reduplicative broken and formation of the SEI membrane may cause the constant consumption of electrolytes and salts, which makes the capacity fade and Coulombic efficiency low. Furthermore, the repeated growth of lithium dendrites will eventually pierce through the separator and cause internal short circuits.4,5 Up to now, extensive efforts have been devoted to improving the safety and electrochemical capability of lithium ion © 2019 American Chemical Society
Received: February 22, 2019 Accepted: May 30, 2019 Published: May 30, 2019 4167
DOI: 10.1021/acsaem.9b00383 ACS Appl. Energy Mater. 2019, 2, 4167−4174
Article
ACS Applied Energy Materials
Figure 1. (a−c) Digital images of ALD100/PH separators with good foldability and twistability under a room temperature of 25 °C. The SEM images of the cross-section of (d) PH separator and (e, f) ALD100/PH with low and high resolution. The scale bar of (d, e, i, j, and k) is 1 μm; the scale bar in (f and g) is about 200 and 30 nm, respectively. (h) The EDS data and elements mapping of ALD100/HP separator with (i) SEM image, (j) aluminum, and (k) oxygen.
least three highlights could make this ALD decorated PVDFHFP separator advanced and novel. (1) Different from the traditional coating method, the ALD technique can uniformly deposit inorganic materials on the surface of all interfaces throughout the whole separators with a limited thickness increase, which can efficiently protect the polymer matrix and thus improve the thermal shrinkage properties. (2) In incorporation with Al2O3 layers, ALD-coated PVDF-HFP separators are anticipated to display a higher compatibility with polar electrolytes and great ion conductivity, thus achieving improved electrochemical performance. Moreover, the enhanced ion conductivity can meanwhile reduce the current density during the cycling process and may slow down the growth rate of lithium dendrites. (3) Compared to pristine PVDF-HFP separators, Al2O3-coated PVDF-HFP separators usually possess a high Young’s modulus, which can play a key role in preventing the growth of lithium dendrites and realize advanced lithium metal batteries with higher safety and great capacity.
separators such as poor wettability, low electrolyte uptake capability, and low ion conductivity would also effectively damage the electrochemical properties of LIBs.21,22 Therefore, it is of great importance to develop advanced separators with high thermal stability and great affinity with polar electrolyte for lithium metal batteries. Several works such as modification of novel polymer separators and ceramic separators are proposed to enable state-of-the-art lithium-ion batteries or lithium metal batteries with great electrochemical capacity and high safety.23,24 Among these, inorganic materials, for instance, SiO2,25 TiO2,26 Al2O3,27 and MgO,28 are also well-studied routes to enhance the thermal stability and wettability of polymer-based separators. Some advanced techniques such as atom layered deposition (ALD) are also used to fabricate novel separators with high safety. Jung et al. prepared the Al2O3coated PP separators with a low thickness of
