Capacity Fading Mechanism of the Commercial 18650 LiFePO4

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Capacity Fading Mechanism of The Commercial 18650 LiFePO-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 A. Stach, and Jian Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13060 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Capacity Fading Mechanism of The Commercial 18650 LiFePO4Based Lithium Ion Batteries: An In-Situ Time-Resolved HighEnergy Synchrotron XRD Study Qi Liu1, 2†, Yadong Liu1† , Fan Yang1,2, Hao He1, Xianghui Xiao3, Yang Ren3,Wenquan Lu4, Eric Stach5, and Jian Xie*,1 1

Department of Mechanical and Energy Engineering, Purdue School of Engineering and

Technology, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202, USA 2

School of Mechanical Engineering, Purdue University, West Lafayette, IN, 47097

3

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South

Cass Avenue, Lemont, IL, 60439, USA 4

Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass

Avenue, Lemont, IL, 60439, USA 5

Department of Materials Science and Engineering, School of Engineering and Applied Science,

University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104-6272

†These authors contributed equally to this work. *Corresponding author: [email protected]

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 solidsolution behavior, while 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


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1. Introduction Lithium-ion batteries, which were first commercially introduced into the market in 1991 by Sony1, 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 Liion battery applications2-4 and, so far, the performance of LiFePO4 has been greatly improved5-7. The combination of a graphite anode and the LiFePO4 cathode is still the good choice for most manufactures producing LiFePO4-based cells due to the factors of safety, cost, and cycling stability8-11. The advancement of portable electronics and EVs/HEVs demands batteries with longterm 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 non-intrusive 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 degradation12-18. 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 findings9, 11-12, 18-22. However, for the characterization methods mentioned above, the structural information was obtained via ex situ methods in a non-



continuous 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 process23-27. 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 dualphase solid-solution behavior in the LiFePO4 cathode that explains the remarkable rate capability of the LiFePO4 in commercial battery cells23. 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 laboratories9, 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 applications). The failed cells along with the pristine


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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. 2. Experiment 2.1. Battery. The commercial cells used for this study were 18650-type LiFePO4 cells (APR18650M 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 BT2000 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 < 0.02 A, which is regarded as 100% state of charge (SOC) or 0% depth of discharge (DOD). After a short rest (i.e. 5 min), the cell was discharged at a 1.0 C rate until the voltage reached 2.00 V, which is the discharge cutoff voltage of the cell and 100% DOD. After a short rest (i.e. 5 min), the cell was charged to 3.60 V again. The charge/discharge cycle continued until the cells failed. The electrochemical impedance was measured using an 8-channel Solartron 1470E Multistat (Solartron, England). The impedances were taken at 2.80 V at different cycle numbers on the same cell. The impedance was measured in the frequency range of 10 mHz~1 MHz with amplitude of 5.0 mV.



2.3 Characterization. The morphology of the LiFePO4 cathode in the failed cells along with the pristine cells was characterized using a high-resolution TEM (JEOL-2100F, Japan). The highenergy synchrotron beamline of the Advanced Photon Source 11-ID-C (Argonne National Laboratory, IL) with a fixed high-energy X-ray beam (λ = 0.10804 Å, E = 115 keV, beam size = 0.2 mm × 0.2 mm) was used to in situ monitor the microstructure changes of A123 18650 LiFeO4 cells during normal cycles. An A123 18650 LiFeO4 cell was placed on the stage of the beamline, and the high-energy synchrotron beam penetrated through the cell while the cell was cycled using a Maccor battery test system (Tulsa, OK) with the same charge/discharge protocol mentioned in the electrochemical characterization part of Experimental, while two dimensional (2D) diffraction patterns of the corresponding crystalline phases of each component in the cell (i.e. graphite, LiFePO4, and stainless steel case) were recorded simultaneously during cycling. The diffraction data was collected every 34 seconds, and 106 diffraction patterns were obtained throughout the charge/discharge cycles. The obtained 2D diffraction patterns were calibrated using a standard CeO2 sample and converted to 1D patterns using Fit2D software. The observed diffraction patterns, which contained multiple phases, were fitted by general structure analysis software (GSAS). 3. Results and discussion. 3.1 Electrochemical characterization. The A123 18650 LiFePO4 cells were cycled under normal conditions (100% SOC and 100% DOD) at a 1.0 C rate (corresponding to current of 1.1 A). The charge/discharge curves of an A123 18650 cell under normal cycling are presented in Figure 1a, which shows the cell performance at different cycle numbers ranging from the 1st to the 2500th cycle. The observed voltage profiles are typical for LiFePO4/graphite full cells; the charge plateau was 3.4 V and the average discharge voltage was about 3.2 V. The cycle


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performance of A123 18650 cells under normal cycle conditions is shown in Figure 1b. Notably, the repetitive cycling resulted in capacity fade, while the degradation rate was almost constant from the 1st cycle to the 2500th cycle. Compared with the rate performance at the beginning of the cycling test, the rate performance of the A123 18650 commercial cell after the 2500th cycle significantly decreased (Figure 1c). Initially at the 1st cycle, the capacity retention of the cell slightly decreased from 100% at 0.1 C, to 99.4% at 1C, and to 98.9% at 5 C. Meanwhile, at the 2500th cycle, the capacity retention of the cell constantly decreased from 100% at 0.1C, 98.8% at 0.2C, 97.3% at 0.5C, 96.1% at 1C, 94.3% at 2C, 93.6 % at 3 C, and finally to 92.2 % at 5C. Electrochemical impedance spectra of the commercial 18650 cell at the 1st and 2500th cycle are shown in Figure 1d. Here EIS was employed to monitor the changes of the interphase between the electrode and electrolytes (i.e. solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI)), contact resistance, charge transfer, and double layer and Li+ ion diffusion in the LiFePO4 cathode inside the commercial 18650 cell. The results were fitted using the model (inset of Fig. 1d) as we reported before34-35 and the fitting results are listed Table S1 in the supporting information. R0, which includes the Ohmic resistance from electrodes, electrolyte, current collectors, separator, and contact resistance between these components, typically can be obtained from the intercept of the high frequency loop on the x axis. RSEI and CSEI refer to the resistance and capacitance of the SEI layer separately for the high frequency loop. Rct and Cdl stand for the charge-transfer resistance and double-layer capacitance in the electrode, respectively, which corresponds to the medium frequency arc. W refers to the Warburg diffusion impedance, which is related to the diffusional (or mass transfer) impedance of electrochemical systems. It represents semi-infinite linear diffusion within the cell and is represented by the following expression36:



W = σ/ω1/2 - jσ/ω1/2 where σ is the Warburg coefficient, which can be determined by fitting to an equivalent circuit model which includes a Warburg impedance; j is the imaginary number and ω is the angular frequency36. The Warburg diffusion impedance could reflect the diffusion of Li ions in the electrode37 in correspondence to the slopping line following the second arc and related by the following equation38:

‫ܦ‬௅௜ =

1 ܸ௠ ݀‫ ܧ‬ଶ ൤൬ ൰ ൬ ൰൨ 2 ‫ݔ݀ ߪܵܨ‬

where Vm is the molar volume, F is the Faraday constant, S is the surface area of the electrode, dE/dx is the slope of the electrode potential E vs. composition x. Clearly, the contact and charge transfer conductivity of the battery cell remains unchanged even after the 2500th cycle, but the Warburg coefficient “σ” changed from 1.982 at the 1st cycle to 3.096 at the 2500th cycle, which indicates that the Li+ diffusion coefficient decreased significantly with the increased cycle number. The decreased Li+ diffusion coefficient might be due to the unexpected morphology changes of the LiFePO4 nanoparticles, which will be discussed in the following paragraphs.


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Z' (Ω Ω) Figure 1 (a) Voltage profiles at 1C for the A123 LiFePO4 commercial cell at different cycle numbers; (b) The capacity retention of an A123 18650 cell with cycle numbers under normal charge/discharge conditions at a 1C rate. (c) The 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.

3.2 Morphology of LiFePO4 cathode.


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The morphologies of LiFePO4 particles were studied using TEM as shown in Figure 2. Notably, the TEM images of the LiFePO4 in a fresh cell show that most particles exhibit a sphere-like morphology (Figure 2a), though a small number of particles with different geometric shapes could also be found. The size of the LiFePO4 nanoparticles ranges from 20 nm to 100 nm with an average particle size around 62.5 nm ± 20 nm (Figure 2b). The TEM images of the LiFePO4 in the cell after 2500 cycles (this cell and the fresh cell are from the same batch) show that the LiFePO4 particles exhibit a flake-like morphology (Figure 2c). Meanwhile, the particle size of the LiFePO4 nanoparticles ranges from 20 nm to 240 nm with the mean size around 110.4 nm (Figure 2d). The apparent difference of the TEM results in the 1st cycle vs. the 2500th cycle directly show that the repetitive cycle process causes both the morphology and size of the LiFePO4 particles to continually change. We expect that the LiFePO4 nanoparticles should either grow or agglomerate with the increased cycle numbers. Finally, the flake-like LiFePO4 nanoparticles should have a significant impact on the electrochemical performance of the commercial LiFePO4 cells. As the particles grow larger, the Li+ ion transport may become diffusion limited and result in the decreased Li+ diffusion coefficient as mentioned above (Figure 1d).

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20 15 10 5 0

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Figure 2. TEM images of a LiFePO4 cathode in commercial A123 18650 cells: (a) Fresh cells, (c) after 2500 cycles; Particle size distribution of LiFePO4 particles: (b) Fresh cells, (d) after 2500 cycles.

3.2 In situ HESXRD investigation. In order to further study the capacity fade mechanism of LiFePO4 phases during normal cycles, the HESXRD technique was used to monitor the microstructure evolution of the electrodes in the A123 18650 LiFePO4 cell during a normal cycle. Importantly, all synchrotron data were obtained from the same cell. The contour plot of the diffraction patterns obtained at the 2500th cycle is shown in Figure 3a; meanwhile, the voltage profile of the A123 18650 cell during the 2500th cycle is shown in Figure 3b for reference. During the course of the experiment, the x-ray source was off several times for very short period of time, leading to some missed data, shown as blank areas in Figure 3a. Notably, the diffraction pattern evolution with time is nearly symmetric along the 100% SOC, which indicates that the microstructure evolution of the electrodes is reversible, similar to that in the 1st charge/discharge process23. The reversible XRD pattern evolution directly confirms that even after 2500 cycles, the Li ion insertion and deinsertion process of the LiFePO4 cathode still follows the same


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mechanism. Hence, in the following, we will focus only on the structural evolution of the electrode in the discharge process.

Figure 3. (a) Contour plot of peak intensities as a function of reaction time throughout the 2500th charge/discharge cycle; (b) voltage profile of the A123 18650 cell during the 2500th cycle. All the structure information of the commercial A123 18650 cell is shown in the same figure. The main peaks of LiFePO4 and FePO4 XRD patterns are located between 2θ=1.0º and 2.8º. For the graphite anode, the peak at 2θ=1.84º (at 100% DOD) is ascribed to the diffraction from the (002) graphite planes while the peak at 2θ=1.75º (at 0% DOD) is attributed to the lithiated graphite. The d-spacing of the graphite layers can be calculated from the graphite (002) peak, and the plot of the d-spacing value of graphite (002) as a function of the DOD at a 1.0 C rate during the 1st and 2500th cycle is shown in Figure 4. During the discharge process, the Li+ ion deinserted from the graphite layers, while the d-spacing of graphite decreased. For the 1st cycle, the d-spacing decreased constantly until 50% DOD, which corresponds to the phase transformation from LiC6 to LiC12. Upon reaching a 50% DOD, the d-spacing decreased sharply at the second stage of the discharging process, which corresponds to the phase transformation



from LiC12 to graphite39. Importantly, we found that after the 2500th cycle, the phase transformation behaviour in the anode was almost identical to that observed during the 1st cycle. These results indicate that the graphite anode is very stable, and the structure collapse did not occur before the 80% capacity loss for the commercial A123 18650 cell. Hence, in the following discussion, we will focus on the structure evolution of the cathode materials during the long-time cycling.

1 st 2500th

3.52 D-spacing (Å)

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3.48 3.44 3.40 3.36

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Figure 4. Plot of the D-spacing value of graphite (002) as a function of DOD at a 1 C rate during 1st and 2500th cycles. Empty circles correspond to 1st and filled circles correspond to the 2500th cycle.

XRD patterns between 2θ = 1.0º and 2.8º during discharge process at 1.0 C rate as a function of DOD are shown in Figure 5. The expected diffraction peaks (2θ) at 1.21º, 2.06º, 2.21º, and 2.47º are ascribed to the (200), (020), (301), and (311) planes of the LiFePO4; the peaks (2θ)


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at 1.25º, 2.11º, 2.27º, and 2.53º result from the (200), (020), (301), and (311) planes of the FePO4. XRD pattern evolution during the 1st cycle is shown in Figure 5a. During the discharge process, lithium ions intercalated into the FePO4, forming LiFePO4, and deintercalate from the lithiated graphite, forming graphite. As a result, the diffraction peaks of FePO4 will transform to LiFePO4 correspondingly. Finally, at the end of the DOD, all FePO4 phases disappear, which indicates that the FePO4 phase has completely transformed to the LiFePO4 phase. XRD pattern evolution during the 2500th cycle is shown in Figure 5b. Obviously, similar phase transformation behaviour has been found in the 2500th cycle and also been found in other different cycles, as shown in Figure S1 (supporting information). However, compared to Figure 5a and Figure 5b, it is worthy to notice that the peaks of the FePO4 didn’t disappear at 100% DOD after the 2500th cycle, the existence of FePO4 peaks indicate that there were not enough active lithium ions to let FePO4 transfer back to LiFePO4 at the end of discharge. The compared results confirm that the loss of the active lithium ion sources is obvious with increased cycling. The peak area of an XRD pattern is proportional to the mass of the corresponding crystalline materials. As seen in Figure 5, the (200) reflection of LiFePO4 is isolated from the (200) reflection of FePO4, which makes them the best candidates to establish the phase fraction (weight) of LiFePO4 and FePO4 during different stages. Consequently, the residual amount of FePO4 at 100% DOD could be obtained by using the Rietveld analysis and Gaussian fits to the (200) pair peaks. The capacity retention, as obtained from the normalized phase fraction of LiFePO4 in the fully discharged state (i.e. 0% SOC) at different cycles, is compared with those obtained from the electrochemical data. As shown in Table 1, the capacity retention calculated from XRD data coincides well with that obtained from the electrochemical data. In addition, the phase fraction (weight) of the residual FePO4, which does not participate in the intercalation reaction during



discharge, is proportional with that of the loss of capacity. Obviously, the appearance of the FePO4 phase can be linked to the decrease of the available lithium source during cycling.

(200) (200)

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Figure 5. XRD patterns between 2θ = 1.0º and 2.8º during discharge process at a 1C rate as a function of DOD throughout the (a) 1st and (b) 2500th cycles.

Table 1 Capacity retention after cycling at a 1C rate Sample

1st cycle

After 500 cycles

After 1000 cycles

After 1500 cycles

After 2000 cycles

Capacity retention 100 97.4 90.2 89.2 86.1 after cycle1 (%) Capacity retention 100 97.5 91.2 88.2 85.6 after cycle2 (%) 1 Capacity retention vs cycling number obtained from the electrochemical data

After 2500 cycles 79.3 79.8

2

Capacity retention vs cycling number obtained from the normalized phase (weight) fraction of LiFePO4 (µLFP) and FePO4 (µFP) in the fully discharge state (100% DOD). The ratio is determined by the equation: R = µLFP / (µLFP + µFP).

In our data analysis, the GSAS program was used to fit the observed diffraction patterns. Excellent fitting results were obtained with the GSAS and the pattern taken in situ at the fully charged state and fully discharged state after 2500 cycles and are presented in Figure 6. At the fully charged state (Figure 6a), only the FePO4 phase (heterosite) could be found; the refined unit cell volume was 272.0 Å3. While at the fully discharged state (Figure 6b), both the LiFePO4 phase (triphylite) and the FePO4 phase (heterosite) could be determined, and the refined unit cell volumes were 290.1 Å3 for the LiFePO4 phase and 271.7 Å3 for the FePO4 phase. The detailed fitting results clearly show that physical deterioration of LiFePO4 cathode did not occur after the 2500th cycle.



Background Observed Calculated Difference

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LFP FP

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Figure. 6. Synchrotron x-ray diffraction patterns taken in situ after 2500 cycles at (a) the fully charged state and (b) the fully discharged state.


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Up to now, to a certain extent, there has been no consensus as to whether or not the physical deterioration of the LiFePO4 cathode and the graphite anode will occur with cycling. Some modeling work suggests that bonds between the two neighboring graphite sheets break after long term charge/discharge cycling, which causes the partial collapse of the graphite sheets, resulting in the increased diffusion resistance for Li+ in the graphite layers and is the cause of the capacity fading40-41. But our in situ experiment clearly reveals that the degradation of both graphite anode materials (Figure 4) and LiFePO4 cathode materials (Figure 6) in the A123 commercial 18650 cells did not occur before 80% capacity loss. However, with the increased cycle numbers, the amount of inactive FePO4, which did not participate in the (de)lithiation of FePO4, will continue to increase, (in other words, the amount of active FePO4 decreased with the cycle number), indicating that the loss of active lithium ion is the primary source of capacity fading (Figure 5 and Table 1). This also suggests that these Li+ ions are trapped within the FePO4 with possible very small amount of Li+ ion lost due to repeated reformation of SEI layer over the graphite surface. Meanwhile, though the microstructure of LiFePO4 nanoparticles is very stable during the repetitive cycling process, with the increasing cycle number, not only the particle size but also the morphology of the cathode materials has significantly changed (Figure 2). This unexpected morphology change in the cathode materials will undoubtedly significantly affect the electrochemical performance of LiFePO4 commercial batteries. 3.4 Unit-cell volume evolution during the charge/discharge process with different cycling numbers. Another important finding in this study is that the unit cell volumes of both LiFePO4 and FePO4 changed with the increasing cycling numbers. As reported previously23, 42-45, there exists solid-solution reaction in the cathode side of the commercial cells; the proposed dual-



phase solid-solution mechanism may explain the remarkable rate capability of LiFePO4 in the fresh commercial cells45. However, after the 2500th cycle, it is clear that the deterioration of the LiFePO4/FePO4 microstructure did not happen (Figure 6), but something else must have changed, which possibly could be the lithium ion insertion/deinsertion behavior of the LiFePO4 cathode. Here, in the following discussion, we designate LiFePO4 as Li1-xFePO4 (Li-rich triphylite) and FePO4 as LiyFePO4 (Li-poor heterosite). As it can be seen in Figure 7a, during the 1st discharge process, between 0% and ~15% DOD, there exists only one phase, LiyFePO4, and the unit cell volumes of LiyFePO4 (Li-poor heterosite) keep decreasing, suggesting that the LiyFePO4 was undergoing a single-phase solid-solution reaction. During the 15% and the 80% DOD, two phases, Li1-xFePO4 and LiyFePO4, coexist and experience a dual-phase solid-solution reaction45. The unit-cell volume difference between the Li-poor phase and the Li-rich phase was about ∆V= 5.6%. After 80% DOD, only one phase, Li1-xFePO4, was observed and underwent another singlephase solid-solution reaction. The detailed Li-ion insertion/deinsertion behavior has been discussed in our previous paper23. The unit cell volume evolution during the discharge process at the 2500th cycle is shown in Figure 7b. Compared with that in the 1st cycle, the unit cell volume evolution behaves very differently. First, due to the appearance of inactive FePO4 and the loss of the active lithium ion source, after 80% DOD, the FePO4 did not completely transfer to LiFePO4 due to the phase transformation. Second, during the time period corresponding to the voltage plateau of the discharge curve, the unit cell volume difference between the Li-poor phase and the Li-rich phase was about ∆V= 6.3%, which is close to that in the ideal two-phase reaction mechanism (∆V= 6.5%)15. The apparent difference of Li ion insertion/deinsertion behavior shown in Figure 7a and Figure 7b suggests that the decay of solid solution behavior is obvious. Since the reaction mechanism is found to be affected by particle size46, we suggest that the


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decay of the solid-solution behavior might be caused by the unexpected morphology and size changes of the LiFePO4 cathode as well as the loss of active lithium ions in the system. Initially, the LiFePO4 nanoparticles are smaller and can be saturated with lithium ions and transformed first, which partially exhibits a solid solution reaction behavior. However, with the increasing cycling numbers, the LiFePO4 nanoparticles will either grow or agglomerate. As the particles become larger, Li+ ion transport may become diffusion limited, and phase-separation may occur44; consequently, the solid solution reaction transform path may be suppressed. Compared with the two-phase reaction, the solid-solution reaction transformation path is much more facile and homogenous45. Consequently, considering that the existence of solid-solution behavior is associated with the rate performance of LiFePO4 cathode, we can expect that the decay of the solid solution behavior might partially explain why the rate performance of the commercial cell becomes worse after long-time cycling (Figure 1c). (15% DOD)

(80% DOD)

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272 0

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Figure 7. Unit cell volume evolutions during the discharge process at (a) 1st cycle; (b) 2500th cycle.

4. Conclusion Here, we report a new in situ HESXRD technique to study the capacity fade mechanism of commercial 18650 LiFePO4-based Lithium ion batteries. This new technique has been successfully used to study the in-situ structure changes of both cathode and anode materials at different cycle numbers. Our in situ experiment results show that (1) physical deterioration of the LiFePO4 cathode and the graphite anode is unlikely to happen until the cell reaches 80% of initial capacity; (2) loss of the active lithium ion source in the system is the primary cause of the capacity fade, while the appearance of the inactive FePO4 phase is proportional to the decrease of the available lithium ion source during cycling; (3) lithium ion insertion/deinsertion behavior in the LiFePO4 cathode has significantly changed. With the increased cycle numbers, the decay of the solid solution behavior is obvious, which is associated with the degradation of the rate performance of LiFePO4 at the higher cycling


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numbers. Consequently, we believe that this new in situ XRD technique will undoubtedly be a powerful tool for other systems. Author contributions J.X. proposed the concept and conceived the mechanism. Q.L. and Y.L designed the experiment, carried out the electrochemical work, and analysed the electrochemical data with the help of F.Y. and H.H. Q.L., Y.L., X. X., and Y.R. performed the in situ HESXRD work. Q.L and J.X. wrote the manuscript. All authors discussed the results and reviewed the manuscript. Competing financial interests The authors declare no competing financial interests. Acknowledgements Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under Contract No. DE-AC0206CH11357. This research was also carried out in part at the Center for Functional Nanomaterials at Brookhaven National Laboratory (U.S. DOE contract DE-AC0298CH10886). Supporting Information XRD patterns of 500th, 1000th, 1500th, 2000th cycles during discharge process at 1C rate and summary table of electrochemical impedance spectra fitting results



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(15% DOD)

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Capacity Fading Mechanism of the Commercial 18650 LiFePO4

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