Research Article www.acsami.org
Creasable Batteries: Understanding Failure Modes through Dynamic Electrochemical Mechanical Testing Aaron J. Blake,‡,†,⊥ Ryan R. Kohlmeyer,*,§,†,⊥ Lawrence F. Drummy,† Jacob S. Gutiérrez-Kolar,∥ Jennifer Carpena-Núñez,§,† Benji Maruyama,† Reza Shahbazian-Yassar,∥ Hong Huang,‡ and Michael F. Durstock*,† †
Soft Materials Branch, Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base, Ohio 45433, United States ‡ Department of Mechanical and Materials Engineering, Wright State University, Dayton, Ohio 45435, United States § National Research Council, Washington, D.C. 20001, United States ∥ Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, Michigan 49931, United States S Supporting Information *
ABSTRACT: Thin-film batteries that can be folded, bent, and even repeatedly creased with minimal or no loss in electrochemical performance have been demonstrated and systematically evaluated using two dynamic mechanical testing approaches for either controlled bending or creasing of flexible devices. The results show that mechanically robust and flexible Li-ion batteries (Li4Ti5O12//LiFePO4) based on the use of a nonwoven multiwalled carbon nanotube (MWNT) mat as a current collector (CC) exhibited a 14-fold decrease in voltage fluctuation at a bending strain of 4.2%, as compared to cells using traditional metal foil CCs. More importantly, MWNTbased full-cells exhibited excellent mechanical integrity through 288 crease cycles, whereas the foil full-cell exhibited continuously degraded performance with each fold and catastrophic fracture after only 94 folds. The enhancements due to MWNT CCs can be attributed to excellent interfacial properties as well as high mechanical strength coupled with compliancy, which allow the batteries to easily conform during mechanical abuse. These results quantitatively demonstrate the substantial enhancement offered in both mechanical and electrochemical stability which can be realized with traditional processing approaches when an appropriate choice of a flexible and robust CC is utilized. KEYWORDS: carbon nanotube current collectors, Li-ion battery, bendable battery, creasable battery, in situ mechanical testing
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INTRODUCTION Flexible energy storage is becoming a necessity to power the next generation of portable and flexible electronic devices such as sensors, smart skins for human performance monitoring, radio frequency ID tags, and wearable electronics. Traditional Li-ion batteries cannot meet these evolving demands mainly due to their metal foil current collectors (CCs). While metal foils are intrinsically flexible, they unfortunately cannot withstand repeated bending or creasing without buckling or tearing, a requirement of flexible devices. Additionally, metal foil CCs contribute to a significant proportion of the battery’s total weight (anywhere from 15% to 80%)1,2 and are prone to corrosion in liquid electrolyte over time,3 which reduces overall energy density and degrades performance. These issues have led the battery research community to seek alternate CCs, such as carbon nanotubes (CNTs),4 graphene,5,6 textiles,7,8 and paper,9−14 which have led to significant advances in flexible batteries.15 CNTs in particular, owing to their light weight, high © XXXX American Chemical Society
mechanical strength, chemical stability, and good electrical conductivity, have recently drawn significant interest for use in Li-ion batteries.2,16−22 While promising progress is being made, the development of f lexible, bendable, and creasable devices that can maintain their performance while being continuously exposed to these extremely harsh and dynamic mechanical conditions is still a significant challenge. In general, there is a conspicuous lack of knowledge in the community regarding how to design and fabricate mechanically flexible and highly robust batteries than can survive hundreds or even thousands of flex, bend, or crease cycles. In large part, this is due to the fact that current flexible battery testing methods (including LED luminescence,5,19,23−28 three-point bend,29 and static folding/ bending)24,28 have lacked in establishing a meaningful, Received: November 19, 2015 Accepted: January 7, 2016
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DOI: 10.1021/acsami.5b11175 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Characterization of the MWNT mat. (a) Photograph of the MWNT mat. (b) Uniaxial tensile stress−strain curves for Al, Cu, and MWNT CCs (inset: photograph of typical sample failure of the CCs, demonstrating the elastic behavior of the MWNT mat). (c) SEM and (d) TEM images of the MWNT mat.
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quantitative comparison between cell mechanics and electrochemical characteristics. Here, we report the development of an automated mandrel bending test designed to systematically examine the relationship between mechanical and electrochemical properties of flexible batteries. Contrary to the majority of characterization techniques used for flexible batteries, this method enables repetitive application of a known radius of curvature, and therefore a known strain, across the entire flexible device. While limited reports on mandrel bend tests are available, they lack in physical significance by either implementing a static measurement30,31 or isolating the bending strain to a small area,24 leaving the remainder of the device unstrained. Our approach to dynamic fatigue testing addresses these issues and can also be utilized for many other devices in the growing field of flexible electronics. In the same vein, we have implemented a dynamic crease test during electrochemical cycling that subjects the flexible cell to a recurring condition of extreme mechanical strain. Together, these two in situ mechanical tests are utilized to gain a fundamental understanding of the failure modes (e.g., interfacial delamination, interlayer separation, and fracturing of the CC) afflicting traditional Li-ion battery architectures. We find that when a correct choice of CC is selected traditional processing approaches can be utilized to fabricate batteries that, when exposed to continuous and severe mechanical deformation for hundreds of cycles, maintain their characteristics with negligible loss of electrochemical performance.
EXPERIMENTAL METHODS
Electrode and Cell Fabrication. Electrode material slurries were prepared by mixing the LiFePO4 cathode or the Li4Ti5O12 anode together with graphite powder and a PVDF binder solution in NMP. The mass ratio of active material:graphite:binder was 70:20:10. Both LiFePO4 and Li4Ti5O12 slurries were applied to MWNT mat or metal foil CCs by a doctor blade (Gardco Inc.) method using a 6 mil path depth and subsequently dried at 120 °C under vacuum for at least 12 h. Electrode samples were punched to 9.5 mm diameter discs. On average, the weights of Li4Ti5O12 and LiFePO4 in each electrode disc ranged from 2.0 to 2.5 mg on both metal foil and MWNT mat CCs corresponding to coating thicknesses between 50 and 60 μm. Electrode samples were assembled in 2325 coin cell configuration under an argon environment (
