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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 4622−4629
Capacity Fading Mechanism of the Commercial 18650 LiFePO4‑Based Lithium-Ion Batteries: An in Situ Time-Resolved High-Energy Synchrotron XRD Study Qi Liu,†,‡,§ Yadong Liu,†,‡ Fan Yang,‡,§ Hao He,‡ Xianghui Xiao,⊥ Yang Ren,⊥ Wenquan Lu,|| Eric Stach,# and Jian Xie*,‡ ‡
Department of Mechanical and Energy Engineering, Purdue School of Engineering and Technology, Indiana University−Purdue University Indianapolis, Indianapolis, Indiana 46202, United States § School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States ⊥ X-ray Science Division, Advanced Photon Source, ||Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States # Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104-6272, United States S Supporting Information *
ABSTRACT: In situ high-energy synchrotron XRD studies were carried out on commercial 18650 LiFePO4 cells at different cycles to track and investigate the dynamic, chemical, and structural changes in the course of long-term cycling to elucidate the capacity fading mechanism. The results indicate that the crystalline structural deterioration of the LiFePO4 cathode and the graphite anode is unlikely to happen before capacity fades below 80% of the initial capacity. Rather, the loss of the active lithium source is the primary cause for the capacity fade, which leads to the appearance of inactive FePO4 that is proportional to the absence of the lithium source. Our in situ HESXRD studies further show that the lithium-ion insertion and deinsertion behavior of LiFePO4 continuously changed with cycling. For a fresh cell, the LiFePO4 experienced a dual-phase solid-solution behavior, whereas with increasing cycle numbers, the dynamic change, which is characteristic of the continuous decay of solid solution behavior, is obvious. The unpredicted dynamic change may result from the morphology evolution of LiFePO4 particles and the loss of the lithium source, which may be the cause of the decreased rate capability of LiFePO4 cells after long-term cycling. KEYWORDS: capacity fade mechanism, LiFePO4, 18650 cell, synchrotron XRD, in situ study
1. INTRODUCTION Lithium-ion batteries, which were first commercially introduced into the market in 1991 by Sony,1 have been widely used for many applications including both portable electronics (i.e., cellphones, laptops, etc.) and transportation (i.e., electric vehicles (EVs) and hybrid electric vehicles (HEVs)). Compared to traditional cathode materials such as LiCoO2, LiNiO2, and LiMn2O4, lithium iron phosphate (LiFePO4) has emerged as a promising cathode material for Li-ion battery applications,2−4 and so far, the performance of LiFePO4 has been greatly improved.5−7 The combination of a graphite anode and the LiFePO4 cathode is still the good choice for most manufactures producing LiFePO4-based cells because of the factors of safety, cost, and cycling stability.8−11 The advancement of portable electronics and EVs/HEVs demands batteries with long-term reliability, which has become a critical issue for the utilization of lithium-ion batteries. Elucidating the capacity failure mechanism of the batteries is critically important to designing durable batteries. Some © 2018 American Chemical Society
nonintrusive electrochemical techniques, such as galvanostatic cycling, rate performance test, hybrid pulse power characterization (HPPC), and electrochemical impedance, successfully, to some extent, deduce the source of degradation.12−18 But an in-depth understanding of microstructural changes of the active materials during long-term cycling should be further elucidated via advanced imaging, spectroscopic, and diffraction techniques. Ex situ characterization techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), IR spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) have been developed and applied to study the source of degradation, resulting in many interesting findings.9,11,12,18−22 However, for the characterization methods mentioned above, the structural information was obtained via ex situ methods in a Received: August 29, 2017 Accepted: January 8, 2018 Published: January 8, 2018 4622
DOI: 10.1021/acsami.7b13060 ACS Appl. Mater. Interfaces 2018, 10, 4622−4629
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Voltage profiles at 1C for the A123 LiFePO4 commercial cell at different cycle numbers. (b) Capacity retention of an A123 18650 cell with cycle numbers under normal charge/discharge conditions at a 1C rate. (c) Capacity retention of an A123 18650 cell with different cycling rates at the 1st and 2500th cycles. (d) Electrochemical impedance spectroscopy (Nyquist plot) of an A123 18650 cell at the 1st and 2500th cycles.
applications). The failed cells along with the pristine cells were characterized using TEM for the morphology changes. The cells were in situ characterized and the microstructure of the electrodes was investigated using HESXRD at different cycle numbers. High-energy synchrotron X-rays with photon energy of 115 keV are capable of penetrating through thick samples, which allowed us to probe commercial 18650 LiFePO4 cells without any modifications, and the results are presented herein.
noncontinuous way. Therefore, it is more advantageous to introduce an in situ synchrotron technique that can provide a powerful tool to characterize battery materials in real time and during the real process.23−27 In our previous work, we performed an in situ synchrotron X-ray experiment (XRD and XAS) coupled with an electrochemical cycling experiment for A123 18650 LiFePO4 commercial cells. Using these advanced synchrotron techniques, we have discovered the dual-phase solid-solution behavior in the LiFePO4 cathode that explains the remarkable rate capability of the LiFePO4 in commercial battery cells.23 Meanwhile, using this operando method of synchrotron XRD/XAS coupled with the electrochemical test, we also studied the failure of A123 18650 LiFePO4 commercial battery cells during overcharge and overdischarge conditions27−29 and proposed the failure mechanism. Notably, this approach of using high-flux and high-energy X-ray coupled with cycling can provide in situ measurement of electrode structural changes during cycling. Up to now, few have reported the capacity fade mechanisms in the commercial LiFePO4 batteries8,30−32 under normal cycling conditions, although many commercial electrode materials have been investigated in laboratories.9,11,33 Compared with the lab-made coin cells or pouch cells, it is much more scientific and engineering important to understand the microstructure evolution during a normal cycle in the commercial cells. And, to the best of our knowledge, in situ monitoring of microstructure evolution during the capacity fade process has not been reported yet in either commercial battery cells or lab-made battery cells. In this work, the capacity fade process of commercial 18650 LiFePO4 cells during the normal charge/discharge process was systematically investigated. The commercial 18650 LiFePO4 cells (A123 Systems) were chosen to be cycled under normal cycling conditions until failure when cells reach 80% of the initial capacity (typical criteria for EV
2. EXPERIMENTAL SECTION 2.1. Battery. The commercial cells used for this study were 18650type LiFePO4 cells (APR18650 M 1A 3.3 V 1100 mAh) from A123 Systems (Cambridge, MA). All battery cells used in the experiment were from the same batch, which ensures that all materials and manufacturing conditions are the same. No special modifications were made for these 18650 cells, and each cycling test was repeated on at least three different cells to ensure data reproducibility. 2.2. Electrochemical Characterization. Galvanostat tests were performed using an Arbin BT-2000 Battery Cycler (Arbin, TX). The cells were charged to 3.60 V at a 1.0 C rate (constant current of 1.10 A); the charging continued at a constant voltage of 3.60 V until the current was
