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graphite cells for grid operation
Journal of Power Sources 449 (2020) 227502
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Unexpected capacity fade and recovery mechanism of LiFePO4/graphite cells for grid operation Yo Kobayashi *, Hajime Miyashiro, Atsuko Yamazaki, Yuichi Mita Central Research Institute of Electric Power Industry, 2-6-1, Nagasaka, Yokosuka, Kanagawa, 240-0196, Japan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Capacity of LiFePO4/graphite cell tentatively faded by medial SOC operation. � The tentative faded capacity can be restored up to 20% by full SOC operation. � Capacity of LiFePO4 and graphite elec trode did not change after cycle operation. � The tentative capacity fade was derived from inhomogeneous reaction in LiFePO4. � Risk of the inhomogeneous degradation should be considered for the grid use. A R T I C L E I N F O
A B S T R A C T
Keywords: Lithium-ion cells LiFePO4 cathode Graphite anode Capacity fade Load frequency control
The capacity fade mechanism of large-scale LiFePO4/graphite cells (>100 Wh) with shallow state of charge (SOC) cycling was investigated. In the case of shallow SOC operation, for ΔSOC ¼ 20% between SOCs of 40% and 60%, the cells showed capacity fade after 250 days of operation. However, the tentatively lost capacity was recovered by subsequent full SOC cycling operation, and the highest capacity recovery of 20% was recorded at C/ 2 after the recovery cycling. The phenomena cannot be explained by the previously reported negative electrode overhung model or the partial plating of lithium. In this study, we have proposed a model where an in homogeneity of the LiFePO4 planar surface induces the decrease in capacity because of the flat potential oper ating region of LiFePO4, and this decrease in the cell capacity can be restored by full SOC operation. Large capacity energy storage systems connected to the electric grid require the use LiFePO4/graphite cells for medial SOC operation because of the low installation cost and the requirement for rapid response of the battery system. In such cases, we should consider the properties of the electrodes in the cell.
1. Introduction The increased use of renewable energy sources is expected to help reduce CO2 emissions, but renewable energy sources sometimes create instability in the power supply. Li-ion batteries are expected to serve as energy buffers for stabilizing the electric power system [1,2]. The
applicability of an energy storage system depends on the power (MW) to capacity (MWh) ratio, which is defined as P (P ¼ MW/MWh). Promising applications for energy storage systems include (i) nighttime/daytime load leveling (low P), (ii) peak shifting of photovoltaic (PV) power to meet evening peak demands (medium P), and (iii) load frequency con trol (LFC) (high P). Among these applications, LFC is first implemented
* Corresponding author. E-mail address: [email protected] (Y. Kobayashi). https://doi.org/10.1016/j.jpowsour.2019.227502 Received 2 October 2019; Received in revised form 5 November 2019; Accepted 23 November 2019 Available online 13 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 449 (2020) 227502
because of the low installation cost of the storage systems [3]. Addi tionally, the safety requirements for MWh class energy storage systems are stricter than those of electric vehicles, and a LiFePO4/graphite sys tem is expected to be used as the storage system [4,5]. In fact, MWh scale battery system consisted of LiFePO4/graphite was already installed for the primary frequency control [6]. LFC operation requires rapid switching between charging and discharging, with time scales of 30 s to 30 min at relatively high rates (e.g., 2C). Consequently, the resulting operational state of charge (SOC) range is relatively shallow due to the rapid polarity reversal. For example, Alexander et al. estimated the SOC region of battery energy storage for the primary control reserves were between SOC 35% and SOC 65% [7]. However, conventional cycle performance has been evaluated by relatively wide SOC range. Alasdair recently reported that LiFePO4/graphite cell showed larger capacity fade in frequency regulation (FR) use than EV þ FR use [8]. In their report, operated SOC region was between 35% and 50% (ΔSOC ¼ 15%) for FR use, and between 55% and 85% (ΔSOC ¼ 30%) for EV þ FR use. Generally, the capacity fade accelerates at high SOC and wide SOC operation, but reported results were opposite from the assumption. More surprisingly, the discharge resistance of FR cell showed lower than that of EV þ FR cell during service cycles. We should consider different kind of capacity fade mechanism to explain the unexpected results. Herein, we report a mechanism for the capacity fade of an LFC during simulated cycling operation (ΔSOC ¼ 20%, SOC range is between 40% and 60%) in comparison with normal cycling conditions (ΔSOC ¼ 80%, SOC range is between 10% and 90%). We compared the effect of capacity trend based on the operated SOC width. Here, the operation rate was 2C in both ΔSOC width so that the effect of operation rate can be neglected [9].
Fig. 1. Cell testing procedures.
2. Experimental procedure 2.1. Cycling test conditions Commercially available large-scale Li-ion cells (LiFePO4 positive electrode and graphite negative electrode, >100 Wh with metal hous ing) were used for the cycling operation tests. Test cells were produced in the mass production line and has been used for stationary purposes. Due to the contract between the cell production company and us, further specifications cannot be declared. These cells were cycled using a charge/discharge system (Fujitsu Telecom Network) in a thermostatic chamber (Espec, LU-113). The protocols used for testing the cells are shown in Fig. 1. Cell ca pacities were preliminarily measured at rates of C/2, C/5, and C/20 at 25 � C, as shown in Fig. 1[A]. Hereinafter, upper case alphabet in parenthesis denotes step in Fig. 1. Charge/discharge cutoff voltages are 3.5V/2/0V with constant current. The 3rd discharge capacity was used. Next, shallow SOC continuous operation, referred to as LFC simulated cycling (SOC 40% - SOC 60%, at 25 � C and at 35 � C), and normal cycling (SOC 10% - SOC 90% at 45 � C) tests were carried out for approximately 250 days [B]. We also tested normal cycling at 25 � C and at 35 � C. However, capacity retentions of them after 250 days were higher than the LFC simulated cycling in the same period (>80% based on C/20 rate). On the other hand, normal the capacity retention of normal cycling at 45 � C (74% based on C/20) was similar to those obtained LFC simulating cycling (76% at 25 � C operation and 73% at 35 � C operation). Then, we selected 45 � C condition as a normal cycling. The rest time between charging and discharging was 30 min for the normal cycling, and no rest time was set for the LFC simulated cycling conditions. The operation voltage region is based on the capacity calculated from the 2C rate test at 25 � C, as shown in Fig. 2. The cell voltages during charging at 2C to 60% and 90% are defined as V60, and V90, respectively. Similarly, the cell voltages during discharging at 2C to 60% (corresponding to an SOC of 40%) and at 90% (corresponding to an SOC of 10%) are defined as V40 and V10, respectively. The defined cutoff voltages and cycle numbers are shown in Table 1. After the testing the cycling operations, the cell capacities at C/2, C/5, and C/20 rates were compared to the
Fig. 2. Charge/discharge curves and the operating regions during LFC, normal, and recovery cycling. Voltage curves are obtained at 2C-25 � C.
initial cell capacities. All capacities were measured at 25 � C [C]. 2.2. Recovery test conditions The cell performance strongly depended on the test conditions as described in the section 3.1. Two cells were operated between SOC 40% and 60%, and the other was operated between SOC 10% and 90% [B]. Table 1 Test conditions of LFC simulated and normal charge/discharge cycles. Run
Temp. Rate
Rest
SOC range
Cutoff
Cycle number
Duration
LFC-25 � C
25 � C
No rest
40–60%
45700 cycles
250 days
No rest
40–60%
31700 cycles
250 days
30 min
10–90%
V40 ¼ 3.17 V V60 ¼ 3.45 V V40 ¼ 3.17 V V60 ¼ 3.45 V V10 ¼ 3.03 V V90 ¼ 3.48 V
3400 cycles
250 days
2C LFC-35 � C
35 � C 2C
Normal45 � C
2
45 � C 2C
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Journal of Power Sources 449 (2020) 227502
We supposed that the operation region strongly might be affected the cell performance. To clarify the effect on operated SOC, we operated additional cycles in the full voltage range of the cell. The LFC cycled cell that was operated at 25 � C (named [LFC-25 � C]) and the cell cycled under normal conditions (named [Normal-45 � C]) were cycled over the full SOC range (SOC 0% - SOC 100% at 1C) at 25 � C, as shown in Table 2. These cells were cycled 450 times under these conditions. Additionally, the LFC cycled cell operated at 35 � C was set to an SOC of 20% and stored without applying additional current for 40 days (a similar time period as the cells that were cycled 450 times) [D] to clarify the effect of additional cycle operation between SOC 0% and 100%. This test is referred to the recovery test. After the recovery test, the cell capacities were measured at C/2, C/5, and C/20 rates at 25 � C and compared to the initial capacities [E]. 2.3. Temperature measurement and disassembly after recovery
Fig. 3. Relative capacity various conditions.
During the capacity measurement at C/20, we measured the surface temperature of the cell to investigate the conditions inside the cell. To minimize fluctuation of ambient temperatures, the cells were covered with thermally insulated rubber. The detailed measurement procedure for the temperature test was previously reported [10]. A platinum resistance temperature detector was used, and the precision of the temperatures measured is �0.01 � C. After the final capacity check [E], the cells were disassembled in a glove box (Miwa FMG), and the reversible capacities of the positive electrode and the negative electrode were compared to those of a new cell. After one side of the electrode was completely removed, the positive and the negative electrode capacities were measured using laminated aluminum pouch cells. The operation voltage regions ranged between 2.0 and 3.8 V for LiFePO4 and between 0 and 2.5 V for graphite. The capacity was measured using an electro chemical measurement system (Biologic, VMP3) at a C/20 rate. Detailed procedures for the disassembly and reassembly of the cells were previ ously reported [11].
of
the
cells
after
cycling
operations
in
reflected in the degradation of the electrodes that is described below. 3.2. Recovery of cell capacity Fig. 4 shows the results of the capacity trend during the recovery test (SOC 0–100%, 1C at 25 � C). The capacity of the [LFC-25 � C] cell showed significant recovery, while that the [Normal-45 � C] cell showed small recovery. Slight capacity increases in Li-ion cells have been reported and explained by the overhang design of the negative electrode [12,13] and/or partial lithium plating occurring during low-temperature oper ation [14,15]. These previous studies reported slight capacity increases, within several percent at highest. Such kind of significant recovery of capacity has only been reported for Ni/Cd and Ni/MH batteries and is called the “Memory Effect” [18]. Some phenomena resembling the memory effect have been reported for Li-ion batteries, Sasaki et al. and Kondo et al. found a minimal voltage signature in LiFePO4 by the partial charge and discharge at a medial SOC [19,20]. However, the recovery of capacity for cells or electrodes with the memory effect has not been reported in Li-ion batteries. Our results shown in Fig. 4 cannot be explained by these previous models. Since the significant increase in capacity was not observed in the [Normal-45 � C] cell, we attributed the increase in capacity to operation in shallow SOC regions. The relative capacities after the recovery tests were compared in Fig. 5. The [LFC-25 � C] and [Normal-45 � C] cells were cycled between SOC 0% and 100%, while the [LFC-35 � C] cell was stored at an SOC of 20% without further cycling. The capacity increased for both the LFC cycled cells, while the [Normal-45 � C] cell showed little increase after the recovery test. Among these cells, the [LFC-25 � C] cell showed notable improvements in reversible capacity and rate performance.
3. Results and discussion 3.1. Capacity retention after cycling tests The capacity retention values after the cycling tests (corresponds to [C] in Fig. 1) are shown in Fig. 3 depending on testing conditions. These values are percentage of the practical capacity measured for each CRate. The trend of the capacity fade of the cells during cycling were almost linear without any inflection points. Poor rate performance was observed for the cell tested using LFC simulated cycling as shown in Fig. 3. The practical capacities vs. nominal capacity in the fresh cell is 100% at C/20, 98% at C/5, and 97% at C/2, respectively. Therefore, the tested cell showed not so bad rate performance before cycling. The rapid and continuous charging and discharging process in the middle of the SOC range for the LFC operating may lead to the poor rate performance in the LiFePO4/graphite cells. However, this assumption is not exactly Table 2 Cycling and storage conditions for capacity recovery. Run
Temp. Rate
Rest
SOC range
Cutoff
Cycle number
Duration
LFC-25 � C
25 � C
30 min
0–100%
450 cycles
40 days
30 min
0–100%
V0 ¼ 2.00 V V100 ¼ 3.50 V V0 ¼ 2.00 V V100 ¼ 3.50 V
450 cycles
40 days
1C Normal45 � C
25 � C 1C
Storage condition LFC-35 � C 25 � C
SOC 20%
Fig. 4. Capacity trend of the cells for full range cycling (SOC 0–100%) after LFC operation at 25 � C and normal cycling at 45 � C. The operation rate was 1C and temperature was 25 � C. The duration of the test was 40 days.
40 days
3
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Journal of Power Sources 449 (2020) 227502
Fig. 5. Relative capacities after the recovery test. The bars with arrows represent the capacity recovered after the recovery test.
Though partial capacity recovery occurred during storage, as shown for the [LFC-35 � C] cell, the capacity recovery was further enhanced by the cycling at the full SOC (0%–100%) range. As far as we know, the approximately 20% capacity recovery of the [LFC-25 � C] cell operated at a C/2 rate is the highest value reported for Li-ion batteries to date. 3.3. Voltage and temperature analysis before and after capacity recovery To understand the mechanism of the capacity recovery, we compared the charge voltage curves and the differential voltage (dV/ dQ), as shown in Fig. 6. Here, most of the peaks except the additional voltage plateau (marked red arrow) was derived from the change in graphite potential. In the case of [Normal-45 � C] cell, graphite peaks shifted to low SOC after the capacity fade. The phenomena were recognized as a shift of the operation window (SOW) due to the lithium consumption at the graphite [16,17]. On the other hand, In the case of [LFC-25 � C] and [LFC-35 � C] cells, graphite peaks tentatively shifted low SOC after the cycle [C], but inversely shifted high SOC after the recovery test [E]. In addition, the cells after LFC simulated cycling showed an additional voltage plateau at the end of charging (near 3.5 V), as marked by the vertical red arrows. We consider this plateau to be the result of the inhomogeneity inside the cell due to the LFC simulated cycling. If the lithium content x of LixFePO4 in any specific area becomes lower than that of other areas due to this inhomogeneity, the LixFePO4 potential of the specific area increases faster than those of the surrounding areas. This leads to a stepwise increase in cell voltage at the end of charging. We found direct evidence of this inhomogeneous reaction in LiFePO4 by post analysis in other cells after similar shallow ΔSOC operation. In the post analysis, we found in plane inhomogeneity of Li-ion content in LiFePO4 using half-cell capacity check and also XRD mapping. Details are described related paper [21]. In LiFePO4, some intermediate phases have been proposed [22,23]. They were discussed based on the unexpected fast reaction kinetics of two-phase material, and/or origins of charge/discharge hysteresis. The proposed intermediate phase was mainly x � 0.6 in LixFePO4. However, our observed voltage plateau is at the end of charge. In the LiFe PO4/graphite cell, LiFePO4 is known to be fully deintercalated (x � 0 in LixFePO4) at the end of charge. It means the voltage plateau at the end of charge cannot be explained by the proposed intermediate phase such as x � 0.6 in LixFePO4. In addition, our experimental condition is very low rate (C/20). Furthermore, the high voltage plateau appeared just after LFC operation and disappeared after the recovery cycling. There no results about such voltage plateau in the previous reports about inter mediate phases [22,23]. Above these evidences, the observed plateau is not related to the intermediate phase in LiFePO4. On the other hand, the observed voltage steps and dV/dQ peaks except 3.5 V region are derived from the graphite negative electrode
Fig. 6. Charge voltages and dV/dQ of initial state, before and after the recovery tests. Operation condition: C/20 at 25 � C.
[24]. These peaks sifted to lower SOC after the capacity fade and also shifted higher SOC after the capacity recovery. This means that the inhomogeneous reactions in the LiFePO4 positive electrode also affected the capacity of the graphite negative electrode. The peaks in the dV/dQ plots have been frequently used to estimate the capacity of electrodes [25,26]. However, it should be noted that the apparent dV/dQ peaks may be affected not only by the degradation of the electrode but also by the inhomogeneity of the electrode. We should also crosscheck the degradation of the electrodes by dissembling the cell to fully understand the mechanism of the capacity fade. In contrast, the additional voltage plateau near 3.5 V completely disappeared after the recovery test. The capacity between the dV/dQ peaks also increased. During the recovery test, the cell was operated between SOC 0–100%. Under these conditions, all Li-ions are removed from LiFePO4 at the end of charging (x � 0 in LixFePO4) in every cycle; in other words, the whole electrode reached the end of the solid solution region in LiFePO4. This operation forces homogenization of the Li-ions in LiFePO4. These findings indicate that inhomogeneity due to the LFC simulated cycling can disappear by the recovery test. This suggests that the inhomogeneous state is not permanent, and we can recover the reversible capacity of the LiFePO4/graphite cells. On the other hand, there is no change in the voltage profile for the [Normal-45 � C] cell before and after the recovery test. This means that the capacity fade in the [Normal-45 � C] cell is irreversible and agrees with the well-known capacity fade of Li-ion batteries [11,24]. We also compared the cell temperature changes during charging and discharging, as shown in Fig. 7. The heat of polarization is negligibly small for the cycling rate of C/20 [10]. From the comparison of the thermal profiles before and after cycling, we found distinct exothermic heat being produced at the beginning of discharging in the [LFC-25 � C] 4
Y. Kobayashi et al.
Journal of Power Sources 449 (2020) 227502
Fig. 7. Thermal profiles of the cells ([LFC-25 � C], [LFC-35 � C], and [Normal-45 � C]) during charging (a,c, and e) and discharging (b,d, and f) before cycle [A], after cycle [C], and after the recovery test [E]. Operating conditions: C/20 at 25 � C.
and [LFC-35 � C] cells after LFC simulated cycling, as shown by the vertical red arrows in Fig. 7 (b) and (d). The exothermic heat phenomena were accompanied with a voltage plateau near 3.5 V. The corresponding exothermic peaks simultaneously disappeared with the corresponding voltage plateau after the recovery test, meaning that the observed voltage plateau near 3.5 V includes some reactions, which result in the inhomogeneity of the electrodes. Partial domains of LiFePO4 are charged x ¼ 0 in LixFePO4 before the overall electrode area is not fully charged (x > 0 in LixFePO4) at the end of charging after the LFC simulated cycling. In these cases, the reaction at the beginning of intercalation of LixFePO4 from x ¼ 0 is exothermic, as reported previously [27,28]. The exothermic peak observed disappears because the inhomogeneity of LiFePO4 disappears after the recovery test [D].
negative electrode [11,16,17,24]. On the other hand, since the capac ities of the cells operated using the LFC simulated cycling showed the highest recovery, this cannot be explained by simple lithium consump tion at the negative electrode. 3.5. Proposed model for capacity recovery Fig. 8 shows a model we have proposed for the capacity fade and recovery in this study. In the initial capacity check [A], Li-ions move between the positive and negative electrode homogeneously. However, the cell was operated in a restricted SOC range (SOC 40–60%) in the LFC simulated cycling [B]. During this operation, the LiFePO4 positive electrode potential stayed flat (3.42 V vs. Li/Liþ) with a two-phase re gion so that there is no difference in the potential to force the Li-ions to homogenize at the electrode’s planar surface. The reversible capacity temporarily decreased due to the in homogeneity during the capacity test after the LFC simulated cycling [C]. The end voltage after charging was controlled by the lowest lithium content in the positive electrode (x ¼ 0 in LixFePO4). Afterwards, the recovery test was performed, which was operated between SOC 0% and 100% [D]. Under these conditions, the x in LixFePO4 homogeneously reaches near x � 0 at the end of charging, so the inhomogeneity in the LiFePO4 electrode disappeared. As a result, the inhomogeneity of the graphite electrode also disappeared, as shown by the recovered capacity [E]. The capacity recovery was also observed in the long-term storage [LFC-35 � C] cell, as shown in Figs. 5 and 6. This means that some ho mogenization reactions occur only in the relaxed state. However, the recovered capacity is higher for the cell that was cycled between 0 and 100% than that stored at an SOC of 20%. This indicates that charging up to 100% enhances the recovery of the capacity, as mentioned above.
3.4. Capacity retention of positive and negative electrode after disassembly To understand the capacity fade and the recovery of the cells after the LFC simulated cycling, we confirmed the capacity of each electrode after testing the capacity after the recovery tests [E]. The electrode ca pacities, which were obtained from uniform surface areas, were compared before ([A]) and after ([E]) the cycling test in Table 3. The relative capacities of all electrodes were almost similar values (98–100% of the initial values). The electrode capacities did not change after both the cycling operations. As mentioned in 3.3, the capacity fade of the [Normal-45 � C] cell was due to the SOW between the positive and negative electrode due to irreversible lithium consumption at the Table 3 Capacity retention of the positive and negative electrodes before and after the cycling tests. Run
Positive Electrode Retention
Negative Electrode Retention
LFC-25 � C LFC-35 � C Normal-45 � C
100% 99% 98%
100% 99% 100%
4. Conclusions We investigated the difference of the capacity fading between shallow and wide cycle operation using large-scale LiFePO4/graphite 5
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Journal of Power Sources 449 (2020) 227502
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227502. References [1] Norio Takami, Hiroki Inagaki, Yoshinao Tatebayashi, Hidesato Saruwatari, Keizoh Honda, Shun Egusa, J. Power Sources 244 (2013) 469–475. [2] Boucar Diouf, Ramchandra Pode, Renew. Energy 76 (2015) 375–380. [3] Michaela Bauer, Tam T. Nguyen, Andreas Jossen, John Lygeros, Energy Procedia 155 (2018) 32–43. [4] Milad Ghorbanzadeh, Majid Astaneh, Farzin Golzar, Energy 166 (2019) 1194–1206. [5] Peter J. Bugryniec, Jonathan N. Davidson, Denis J. Cumming, Solomon F. Brown, J. Power Sources 414 (2019) 557–568. [6] Fabio Massimo Gatta, Alberto Geri, Regina Lamedica, Stefano Lauria, Marco Maccioni, Francesco Palone, Massimo Rebolini, Alessandro Ruvio, Energy 9 (2016) 887. [7] Zeh Alexander, Marcus Müller, Maik Naumann, C Hesse Holger, Ahdreas Jossen, Rolf Witzmann, Batteries 2 (2016) 29. [8] Alasdair J. Crawford, Qian Huang, C. Michel, W. Kintner-Meyer, Ji-Guang Zhang, David M. Reed, Vincent L. Sprenkle, Vilayanur V. Viswanathan, Daiwon Choi, J. Power Sources 380 (2018) 185–193. [9] Matthieu Dubarry, Cyril Truchot, Bor Yann Liaw, J. Power Sources 258 (2014) 408–419. [10] Yo Kobayashi, Takeshi Kobayashi, Kumi Shono, Yasutaka Ohno, Yuichi Mita, Hajime Miyashiro, J. Electrochem. Soc. 160 (2013) A1415–A1420. [11] Yo Kobayashi, Takeshi Kobayashi, Kumi Shono, Yasutaka Ohno, Yuichi Mita, Hajime Miyashiro, J. Electrochem. Soc. 160 (2013) A1181–A1186. [12] Meinert Lewerenz, Jens Munnix, Schmalstieg Johannes, Stefan Kabitz, Marcus Knips, Dirk Uwe Sauer, J. Power Sources 345 (2017) 254–263. [13] Jorn Wilhelm, Stefan Seidlmayer, Peter Keil, Jorg Schuster, Armin Kriele, Ralph Gilles, Andreas Jossen, J. Power Sources 365 (2017) 327–338. [14] Mathias Petzl, Michael Kasper, Michael A. Danzer, J. Power Sources 275 (2015) 799–807. [15] Thomas Waldmann, Margaret Wohlfahrt-Mehrens, Electrochim. Acta 230 (2017) 454–460. [16] Ira Bloom, Andrew N. Jansen, Daniel P. Abraham, Jamie Knuth, Scott A. Jones, Vincent S. Battaglia, Gary L. Henriksen, J. Power Sources 139 (2005) 295–303. [17] M. Safari, C. Delacourt, J. Electrochem. Soc. 158 (2011) A1436–A1447. [18] R. Huggins, Solid State Ion. 177 (2006) 2643–2646. [19] Tsuyoshi Sasaki, Yoshio Ukyo, Petr Nov� ak, Nat. Mater. 12 (2013) 569–575. [20] Hiroki Kondo, Tsuyoshi Sasaki, Pallb Barai, Venkat Srinivasan, J. Electrochem. Soc. 165 (2018) A2017–A2057. [21] Yo Kobayashi, Hajime Miyashiro, Yuichi Mita, J. Power Sources, (submitted) for publication. [22] Michael Hess, Tsuyoshi Sasaki, Claire Villevieille, Petr Nov� ak, Nat. Commun. 6 (2015) 8169. [23] Yukinori Koyama, Takeshi Uyama, Yuki Orikasa, Takahiro Naka, Hideyuki Komatsu, Keiji Shimoda, Haruno Murayama, Katsutoshi Fukuda, Hajime Arai, Eiichiro Matsubara, Yoshiharu Uchimoto, Zempachi Ogumi, Chem. Mater. 29 (2017) 2855–2863. [24] Hisashi Kato, Yo Kobayashi, Hajime Miyashiro, J. Power Sources 398 (2018) 49–54. [25] Kohei Honkura, Ko Takahashi, Tatsuo Horiba, J. Power Sources 196 (2011) 10141–10147. [26] Hannah M. Dahn, A.J. Smith, J.C. Burns, D.A. Stevens, J.R. Dahn, J. Electrochem. Soc. 159 (2012) A1405–A1409. [27] Atsuo Yamada, Hiroyuki Koizumi, Shin-ichi Nishimura, Noriyuki Sonoyama, Ryoji Kanno, Masao Yonemura, Tatsuya Nakamura, Yo Kobayashi, Nat. Mater. 5 (2006) 357–360. [28] K. Kazuhiko, Yo Kobayashi, Hajime Miyashiro, Gosuke Oyama, Shin-ichi Nishimura, Masashi Okubo, Atsuo Yamada, Chem. Phys. Chem. 15 (2014) 2156–2161. https://doi.org/10.1002/cphc.201301219.
Fig. 8. Proposed model for the capacity fade and recovery after LFC (SOC 40–60% at 2C) operation and the capacity recovery test (SOC 0–100% at 1C).
cells. In the case of cycling tests within shallow state of charge (SOC) windows, denoted as load frequency control (LFC) simulated cycling, the cell capacity fade was mainly due to the inhomogeneous Li-ion content inside the electrodes, and the some of the cell capacity can be recovered by the full range operation. On the other hand, the shift of operation window (SOW) derived from the irreversible lithium con sumption at the graphite was main factor for capacity fade of the cell in the case of cycling with wide SOC window. The positive and negative electrode capacities did not change after the LFC simulated cycling, which provides additional evidence that the capacity fade is due to the inhomogeneity inside the cell. These results show that unexpected rapid capacity fade will occur during shallow SOC cycle operation with LiFePO4/graphite cells. However, we have proposed a way to recover the capacity of the cells used in LFC systems. These findings demonstrate that LiFePO4/graphite cells can achieve long-life operation with suitable SOC management in grid-scale stationary systems. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
6
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Unexpected capacity fade and recovery mechanism of LiFePO4/graphite cells for grid operation Yo Kobayashi *, Hajime Miyashiro, Atsuko Yamazaki, Yuichi Mita Central Research Institute of Electric Power Industry, 2-6-1, Nagasaka, Yokosuka, Kanagawa, 240-0196, Japan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Capacity of LiFePO4/graphite cell tentatively faded by medial SOC operation. � The tentative faded capacity can be restored up to 20% by full SOC operation. � Capacity of LiFePO4 and graphite elec trode did not change after cycle operation. � The tentative capacity fade was derived from inhomogeneous reaction in LiFePO4. � Risk of the inhomogeneous degradation should be considered for the grid use. A R T I C L E I N F O
A B S T R A C T
Keywords: Lithium-ion cells LiFePO4 cathode Graphite anode Capacity fade Load frequency control
The capacity fade mechanism of large-scale LiFePO4/graphite cells (>100 Wh) with shallow state of charge (SOC) cycling was investigated. In the case of shallow SOC operation, for ΔSOC ¼ 20% between SOCs of 40% and 60%, the cells showed capacity fade after 250 days of operation. However, the tentatively lost capacity was recovered by subsequent full SOC cycling operation, and the highest capacity recovery of 20% was recorded at C/ 2 after the recovery cycling. The phenomena cannot be explained by the previously reported negative electrode overhung model or the partial plating of lithium. In this study, we have proposed a model where an in homogeneity of the LiFePO4 planar surface induces the decrease in capacity because of the flat potential oper ating region of LiFePO4, and this decrease in the cell capacity can be restored by full SOC operation. Large capacity energy storage systems connected to the electric grid require the use LiFePO4/graphite cells for medial SOC operation because of the low installation cost and the requirement for rapid response of the battery system. In such cases, we should consider the properties of the electrodes in the cell.
1. Introduction The increased use of renewable energy sources is expected to help reduce CO2 emissions, but renewable energy sources sometimes create instability in the power supply. Li-ion batteries are expected to serve as energy buffers for stabilizing the electric power system [1,2]. The
applicability of an energy storage system depends on the power (MW) to capacity (MWh) ratio, which is defined as P (P ¼ MW/MWh). Promising applications for energy storage systems include (i) nighttime/daytime load leveling (low P), (ii) peak shifting of photovoltaic (PV) power to meet evening peak demands (medium P), and (iii) load frequency con trol (LFC) (high P). Among these applications, LFC is first implemented
* Corresponding author. E-mail address: [email protected] (Y. Kobayashi). https://doi.org/10.1016/j.jpowsour.2019.227502 Received 2 October 2019; Received in revised form 5 November 2019; Accepted 23 November 2019 Available online 13 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 449 (2020) 227502
because of the low installation cost of the storage systems [3]. Addi tionally, the safety requirements for MWh class energy storage systems are stricter than those of electric vehicles, and a LiFePO4/graphite sys tem is expected to be used as the storage system [4,5]. In fact, MWh scale battery system consisted of LiFePO4/graphite was already installed for the primary frequency control [6]. LFC operation requires rapid switching between charging and discharging, with time scales of 30 s to 30 min at relatively high rates (e.g., 2C). Consequently, the resulting operational state of charge (SOC) range is relatively shallow due to the rapid polarity reversal. For example, Alexander et al. estimated the SOC region of battery energy storage for the primary control reserves were between SOC 35% and SOC 65% [7]. However, conventional cycle performance has been evaluated by relatively wide SOC range. Alasdair recently reported that LiFePO4/graphite cell showed larger capacity fade in frequency regulation (FR) use than EV þ FR use [8]. In their report, operated SOC region was between 35% and 50% (ΔSOC ¼ 15%) for FR use, and between 55% and 85% (ΔSOC ¼ 30%) for EV þ FR use. Generally, the capacity fade accelerates at high SOC and wide SOC operation, but reported results were opposite from the assumption. More surprisingly, the discharge resistance of FR cell showed lower than that of EV þ FR cell during service cycles. We should consider different kind of capacity fade mechanism to explain the unexpected results. Herein, we report a mechanism for the capacity fade of an LFC during simulated cycling operation (ΔSOC ¼ 20%, SOC range is between 40% and 60%) in comparison with normal cycling conditions (ΔSOC ¼ 80%, SOC range is between 10% and 90%). We compared the effect of capacity trend based on the operated SOC width. Here, the operation rate was 2C in both ΔSOC width so that the effect of operation rate can be neglected [9].
Fig. 1. Cell testing procedures.
2. Experimental procedure 2.1. Cycling test conditions Commercially available large-scale Li-ion cells (LiFePO4 positive electrode and graphite negative electrode, >100 Wh with metal hous ing) were used for the cycling operation tests. Test cells were produced in the mass production line and has been used for stationary purposes. Due to the contract between the cell production company and us, further specifications cannot be declared. These cells were cycled using a charge/discharge system (Fujitsu Telecom Network) in a thermostatic chamber (Espec, LU-113). The protocols used for testing the cells are shown in Fig. 1. Cell ca pacities were preliminarily measured at rates of C/2, C/5, and C/20 at 25 � C, as shown in Fig. 1[A]. Hereinafter, upper case alphabet in parenthesis denotes step in Fig. 1. Charge/discharge cutoff voltages are 3.5V/2/0V with constant current. The 3rd discharge capacity was used. Next, shallow SOC continuous operation, referred to as LFC simulated cycling (SOC 40% - SOC 60%, at 25 � C and at 35 � C), and normal cycling (SOC 10% - SOC 90% at 45 � C) tests were carried out for approximately 250 days [B]. We also tested normal cycling at 25 � C and at 35 � C. However, capacity retentions of them after 250 days were higher than the LFC simulated cycling in the same period (>80% based on C/20 rate). On the other hand, normal the capacity retention of normal cycling at 45 � C (74% based on C/20) was similar to those obtained LFC simulating cycling (76% at 25 � C operation and 73% at 35 � C operation). Then, we selected 45 � C condition as a normal cycling. The rest time between charging and discharging was 30 min for the normal cycling, and no rest time was set for the LFC simulated cycling conditions. The operation voltage region is based on the capacity calculated from the 2C rate test at 25 � C, as shown in Fig. 2. The cell voltages during charging at 2C to 60% and 90% are defined as V60, and V90, respectively. Similarly, the cell voltages during discharging at 2C to 60% (corresponding to an SOC of 40%) and at 90% (corresponding to an SOC of 10%) are defined as V40 and V10, respectively. The defined cutoff voltages and cycle numbers are shown in Table 1. After the testing the cycling operations, the cell capacities at C/2, C/5, and C/20 rates were compared to the
Fig. 2. Charge/discharge curves and the operating regions during LFC, normal, and recovery cycling. Voltage curves are obtained at 2C-25 � C.
initial cell capacities. All capacities were measured at 25 � C [C]. 2.2. Recovery test conditions The cell performance strongly depended on the test conditions as described in the section 3.1. Two cells were operated between SOC 40% and 60%, and the other was operated between SOC 10% and 90% [B]. Table 1 Test conditions of LFC simulated and normal charge/discharge cycles. Run
Temp. Rate
Rest
SOC range
Cutoff
Cycle number
Duration
LFC-25 � C
25 � C
No rest
40–60%
45700 cycles
250 days
No rest
40–60%
31700 cycles
250 days
30 min
10–90%
V40 ¼ 3.17 V V60 ¼ 3.45 V V40 ¼ 3.17 V V60 ¼ 3.45 V V10 ¼ 3.03 V V90 ¼ 3.48 V
3400 cycles
250 days
2C LFC-35 � C
35 � C 2C
Normal45 � C
2
45 � C 2C
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Journal of Power Sources 449 (2020) 227502
We supposed that the operation region strongly might be affected the cell performance. To clarify the effect on operated SOC, we operated additional cycles in the full voltage range of the cell. The LFC cycled cell that was operated at 25 � C (named [LFC-25 � C]) and the cell cycled under normal conditions (named [Normal-45 � C]) were cycled over the full SOC range (SOC 0% - SOC 100% at 1C) at 25 � C, as shown in Table 2. These cells were cycled 450 times under these conditions. Additionally, the LFC cycled cell operated at 35 � C was set to an SOC of 20% and stored without applying additional current for 40 days (a similar time period as the cells that were cycled 450 times) [D] to clarify the effect of additional cycle operation between SOC 0% and 100%. This test is referred to the recovery test. After the recovery test, the cell capacities were measured at C/2, C/5, and C/20 rates at 25 � C and compared to the initial capacities [E]. 2.3. Temperature measurement and disassembly after recovery
Fig. 3. Relative capacity various conditions.
During the capacity measurement at C/20, we measured the surface temperature of the cell to investigate the conditions inside the cell. To minimize fluctuation of ambient temperatures, the cells were covered with thermally insulated rubber. The detailed measurement procedure for the temperature test was previously reported [10]. A platinum resistance temperature detector was used, and the precision of the temperatures measured is �0.01 � C. After the final capacity check [E], the cells were disassembled in a glove box (Miwa FMG), and the reversible capacities of the positive electrode and the negative electrode were compared to those of a new cell. After one side of the electrode was completely removed, the positive and the negative electrode capacities were measured using laminated aluminum pouch cells. The operation voltage regions ranged between 2.0 and 3.8 V for LiFePO4 and between 0 and 2.5 V for graphite. The capacity was measured using an electro chemical measurement system (Biologic, VMP3) at a C/20 rate. Detailed procedures for the disassembly and reassembly of the cells were previ ously reported [11].
of
the
cells
after
cycling
operations
in
reflected in the degradation of the electrodes that is described below. 3.2. Recovery of cell capacity Fig. 4 shows the results of the capacity trend during the recovery test (SOC 0–100%, 1C at 25 � C). The capacity of the [LFC-25 � C] cell showed significant recovery, while that the [Normal-45 � C] cell showed small recovery. Slight capacity increases in Li-ion cells have been reported and explained by the overhang design of the negative electrode [12,13] and/or partial lithium plating occurring during low-temperature oper ation [14,15]. These previous studies reported slight capacity increases, within several percent at highest. Such kind of significant recovery of capacity has only been reported for Ni/Cd and Ni/MH batteries and is called the “Memory Effect” [18]. Some phenomena resembling the memory effect have been reported for Li-ion batteries, Sasaki et al. and Kondo et al. found a minimal voltage signature in LiFePO4 by the partial charge and discharge at a medial SOC [19,20]. However, the recovery of capacity for cells or electrodes with the memory effect has not been reported in Li-ion batteries. Our results shown in Fig. 4 cannot be explained by these previous models. Since the significant increase in capacity was not observed in the [Normal-45 � C] cell, we attributed the increase in capacity to operation in shallow SOC regions. The relative capacities after the recovery tests were compared in Fig. 5. The [LFC-25 � C] and [Normal-45 � C] cells were cycled between SOC 0% and 100%, while the [LFC-35 � C] cell was stored at an SOC of 20% without further cycling. The capacity increased for both the LFC cycled cells, while the [Normal-45 � C] cell showed little increase after the recovery test. Among these cells, the [LFC-25 � C] cell showed notable improvements in reversible capacity and rate performance.
3. Results and discussion 3.1. Capacity retention after cycling tests The capacity retention values after the cycling tests (corresponds to [C] in Fig. 1) are shown in Fig. 3 depending on testing conditions. These values are percentage of the practical capacity measured for each CRate. The trend of the capacity fade of the cells during cycling were almost linear without any inflection points. Poor rate performance was observed for the cell tested using LFC simulated cycling as shown in Fig. 3. The practical capacities vs. nominal capacity in the fresh cell is 100% at C/20, 98% at C/5, and 97% at C/2, respectively. Therefore, the tested cell showed not so bad rate performance before cycling. The rapid and continuous charging and discharging process in the middle of the SOC range for the LFC operating may lead to the poor rate performance in the LiFePO4/graphite cells. However, this assumption is not exactly Table 2 Cycling and storage conditions for capacity recovery. Run
Temp. Rate
Rest
SOC range
Cutoff
Cycle number
Duration
LFC-25 � C
25 � C
30 min
0–100%
450 cycles
40 days
30 min
0–100%
V0 ¼ 2.00 V V100 ¼ 3.50 V V0 ¼ 2.00 V V100 ¼ 3.50 V
450 cycles
40 days
1C Normal45 � C
25 � C 1C
Storage condition LFC-35 � C 25 � C
SOC 20%
Fig. 4. Capacity trend of the cells for full range cycling (SOC 0–100%) after LFC operation at 25 � C and normal cycling at 45 � C. The operation rate was 1C and temperature was 25 � C. The duration of the test was 40 days.
40 days
3
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Journal of Power Sources 449 (2020) 227502
Fig. 5. Relative capacities after the recovery test. The bars with arrows represent the capacity recovered after the recovery test.
Though partial capacity recovery occurred during storage, as shown for the [LFC-35 � C] cell, the capacity recovery was further enhanced by the cycling at the full SOC (0%–100%) range. As far as we know, the approximately 20% capacity recovery of the [LFC-25 � C] cell operated at a C/2 rate is the highest value reported for Li-ion batteries to date. 3.3. Voltage and temperature analysis before and after capacity recovery To understand the mechanism of the capacity recovery, we compared the charge voltage curves and the differential voltage (dV/ dQ), as shown in Fig. 6. Here, most of the peaks except the additional voltage plateau (marked red arrow) was derived from the change in graphite potential. In the case of [Normal-45 � C] cell, graphite peaks shifted to low SOC after the capacity fade. The phenomena were recognized as a shift of the operation window (SOW) due to the lithium consumption at the graphite [16,17]. On the other hand, In the case of [LFC-25 � C] and [LFC-35 � C] cells, graphite peaks tentatively shifted low SOC after the cycle [C], but inversely shifted high SOC after the recovery test [E]. In addition, the cells after LFC simulated cycling showed an additional voltage plateau at the end of charging (near 3.5 V), as marked by the vertical red arrows. We consider this plateau to be the result of the inhomogeneity inside the cell due to the LFC simulated cycling. If the lithium content x of LixFePO4 in any specific area becomes lower than that of other areas due to this inhomogeneity, the LixFePO4 potential of the specific area increases faster than those of the surrounding areas. This leads to a stepwise increase in cell voltage at the end of charging. We found direct evidence of this inhomogeneous reaction in LiFePO4 by post analysis in other cells after similar shallow ΔSOC operation. In the post analysis, we found in plane inhomogeneity of Li-ion content in LiFePO4 using half-cell capacity check and also XRD mapping. Details are described related paper [21]. In LiFePO4, some intermediate phases have been proposed [22,23]. They were discussed based on the unexpected fast reaction kinetics of two-phase material, and/or origins of charge/discharge hysteresis. The proposed intermediate phase was mainly x � 0.6 in LixFePO4. However, our observed voltage plateau is at the end of charge. In the LiFe PO4/graphite cell, LiFePO4 is known to be fully deintercalated (x � 0 in LixFePO4) at the end of charge. It means the voltage plateau at the end of charge cannot be explained by the proposed intermediate phase such as x � 0.6 in LixFePO4. In addition, our experimental condition is very low rate (C/20). Furthermore, the high voltage plateau appeared just after LFC operation and disappeared after the recovery cycling. There no results about such voltage plateau in the previous reports about inter mediate phases [22,23]. Above these evidences, the observed plateau is not related to the intermediate phase in LiFePO4. On the other hand, the observed voltage steps and dV/dQ peaks except 3.5 V region are derived from the graphite negative electrode
Fig. 6. Charge voltages and dV/dQ of initial state, before and after the recovery tests. Operation condition: C/20 at 25 � C.
[24]. These peaks sifted to lower SOC after the capacity fade and also shifted higher SOC after the capacity recovery. This means that the inhomogeneous reactions in the LiFePO4 positive electrode also affected the capacity of the graphite negative electrode. The peaks in the dV/dQ plots have been frequently used to estimate the capacity of electrodes [25,26]. However, it should be noted that the apparent dV/dQ peaks may be affected not only by the degradation of the electrode but also by the inhomogeneity of the electrode. We should also crosscheck the degradation of the electrodes by dissembling the cell to fully understand the mechanism of the capacity fade. In contrast, the additional voltage plateau near 3.5 V completely disappeared after the recovery test. The capacity between the dV/dQ peaks also increased. During the recovery test, the cell was operated between SOC 0–100%. Under these conditions, all Li-ions are removed from LiFePO4 at the end of charging (x � 0 in LixFePO4) in every cycle; in other words, the whole electrode reached the end of the solid solution region in LiFePO4. This operation forces homogenization of the Li-ions in LiFePO4. These findings indicate that inhomogeneity due to the LFC simulated cycling can disappear by the recovery test. This suggests that the inhomogeneous state is not permanent, and we can recover the reversible capacity of the LiFePO4/graphite cells. On the other hand, there is no change in the voltage profile for the [Normal-45 � C] cell before and after the recovery test. This means that the capacity fade in the [Normal-45 � C] cell is irreversible and agrees with the well-known capacity fade of Li-ion batteries [11,24]. We also compared the cell temperature changes during charging and discharging, as shown in Fig. 7. The heat of polarization is negligibly small for the cycling rate of C/20 [10]. From the comparison of the thermal profiles before and after cycling, we found distinct exothermic heat being produced at the beginning of discharging in the [LFC-25 � C] 4
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Journal of Power Sources 449 (2020) 227502
Fig. 7. Thermal profiles of the cells ([LFC-25 � C], [LFC-35 � C], and [Normal-45 � C]) during charging (a,c, and e) and discharging (b,d, and f) before cycle [A], after cycle [C], and after the recovery test [E]. Operating conditions: C/20 at 25 � C.
and [LFC-35 � C] cells after LFC simulated cycling, as shown by the vertical red arrows in Fig. 7 (b) and (d). The exothermic heat phenomena were accompanied with a voltage plateau near 3.5 V. The corresponding exothermic peaks simultaneously disappeared with the corresponding voltage plateau after the recovery test, meaning that the observed voltage plateau near 3.5 V includes some reactions, which result in the inhomogeneity of the electrodes. Partial domains of LiFePO4 are charged x ¼ 0 in LixFePO4 before the overall electrode area is not fully charged (x > 0 in LixFePO4) at the end of charging after the LFC simulated cycling. In these cases, the reaction at the beginning of intercalation of LixFePO4 from x ¼ 0 is exothermic, as reported previously [27,28]. The exothermic peak observed disappears because the inhomogeneity of LiFePO4 disappears after the recovery test [D].
negative electrode [11,16,17,24]. On the other hand, since the capac ities of the cells operated using the LFC simulated cycling showed the highest recovery, this cannot be explained by simple lithium consump tion at the negative electrode. 3.5. Proposed model for capacity recovery Fig. 8 shows a model we have proposed for the capacity fade and recovery in this study. In the initial capacity check [A], Li-ions move between the positive and negative electrode homogeneously. However, the cell was operated in a restricted SOC range (SOC 40–60%) in the LFC simulated cycling [B]. During this operation, the LiFePO4 positive electrode potential stayed flat (3.42 V vs. Li/Liþ) with a two-phase re gion so that there is no difference in the potential to force the Li-ions to homogenize at the electrode’s planar surface. The reversible capacity temporarily decreased due to the in homogeneity during the capacity test after the LFC simulated cycling [C]. The end voltage after charging was controlled by the lowest lithium content in the positive electrode (x ¼ 0 in LixFePO4). Afterwards, the recovery test was performed, which was operated between SOC 0% and 100% [D]. Under these conditions, the x in LixFePO4 homogeneously reaches near x � 0 at the end of charging, so the inhomogeneity in the LiFePO4 electrode disappeared. As a result, the inhomogeneity of the graphite electrode also disappeared, as shown by the recovered capacity [E]. The capacity recovery was also observed in the long-term storage [LFC-35 � C] cell, as shown in Figs. 5 and 6. This means that some ho mogenization reactions occur only in the relaxed state. However, the recovered capacity is higher for the cell that was cycled between 0 and 100% than that stored at an SOC of 20%. This indicates that charging up to 100% enhances the recovery of the capacity, as mentioned above.
3.4. Capacity retention of positive and negative electrode after disassembly To understand the capacity fade and the recovery of the cells after the LFC simulated cycling, we confirmed the capacity of each electrode after testing the capacity after the recovery tests [E]. The electrode ca pacities, which were obtained from uniform surface areas, were compared before ([A]) and after ([E]) the cycling test in Table 3. The relative capacities of all electrodes were almost similar values (98–100% of the initial values). The electrode capacities did not change after both the cycling operations. As mentioned in 3.3, the capacity fade of the [Normal-45 � C] cell was due to the SOW between the positive and negative electrode due to irreversible lithium consumption at the Table 3 Capacity retention of the positive and negative electrodes before and after the cycling tests. Run
Positive Electrode Retention
Negative Electrode Retention
LFC-25 � C LFC-35 � C Normal-45 � C
100% 99% 98%
100% 99% 100%
4. Conclusions We investigated the difference of the capacity fading between shallow and wide cycle operation using large-scale LiFePO4/graphite 5
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227502. References [1] Norio Takami, Hiroki Inagaki, Yoshinao Tatebayashi, Hidesato Saruwatari, Keizoh Honda, Shun Egusa, J. Power Sources 244 (2013) 469–475. [2] Boucar Diouf, Ramchandra Pode, Renew. Energy 76 (2015) 375–380. [3] Michaela Bauer, Tam T. Nguyen, Andreas Jossen, John Lygeros, Energy Procedia 155 (2018) 32–43. [4] Milad Ghorbanzadeh, Majid Astaneh, Farzin Golzar, Energy 166 (2019) 1194–1206. [5] Peter J. Bugryniec, Jonathan N. Davidson, Denis J. Cumming, Solomon F. Brown, J. Power Sources 414 (2019) 557–568. [6] Fabio Massimo Gatta, Alberto Geri, Regina Lamedica, Stefano Lauria, Marco Maccioni, Francesco Palone, Massimo Rebolini, Alessandro Ruvio, Energy 9 (2016) 887. [7] Zeh Alexander, Marcus Müller, Maik Naumann, C Hesse Holger, Ahdreas Jossen, Rolf Witzmann, Batteries 2 (2016) 29. [8] Alasdair J. Crawford, Qian Huang, C. Michel, W. Kintner-Meyer, Ji-Guang Zhang, David M. Reed, Vincent L. Sprenkle, Vilayanur V. Viswanathan, Daiwon Choi, J. Power Sources 380 (2018) 185–193. [9] Matthieu Dubarry, Cyril Truchot, Bor Yann Liaw, J. Power Sources 258 (2014) 408–419. [10] Yo Kobayashi, Takeshi Kobayashi, Kumi Shono, Yasutaka Ohno, Yuichi Mita, Hajime Miyashiro, J. Electrochem. Soc. 160 (2013) A1415–A1420. [11] Yo Kobayashi, Takeshi Kobayashi, Kumi Shono, Yasutaka Ohno, Yuichi Mita, Hajime Miyashiro, J. Electrochem. Soc. 160 (2013) A1181–A1186. [12] Meinert Lewerenz, Jens Munnix, Schmalstieg Johannes, Stefan Kabitz, Marcus Knips, Dirk Uwe Sauer, J. Power Sources 345 (2017) 254–263. [13] Jorn Wilhelm, Stefan Seidlmayer, Peter Keil, Jorg Schuster, Armin Kriele, Ralph Gilles, Andreas Jossen, J. Power Sources 365 (2017) 327–338. [14] Mathias Petzl, Michael Kasper, Michael A. Danzer, J. Power Sources 275 (2015) 799–807. [15] Thomas Waldmann, Margaret Wohlfahrt-Mehrens, Electrochim. Acta 230 (2017) 454–460. [16] Ira Bloom, Andrew N. Jansen, Daniel P. Abraham, Jamie Knuth, Scott A. Jones, Vincent S. Battaglia, Gary L. Henriksen, J. Power Sources 139 (2005) 295–303. [17] M. Safari, C. Delacourt, J. Electrochem. Soc. 158 (2011) A1436–A1447. [18] R. Huggins, Solid State Ion. 177 (2006) 2643–2646. [19] Tsuyoshi Sasaki, Yoshio Ukyo, Petr Nov� ak, Nat. Mater. 12 (2013) 569–575. [20] Hiroki Kondo, Tsuyoshi Sasaki, Pallb Barai, Venkat Srinivasan, J. Electrochem. Soc. 165 (2018) A2017–A2057. [21] Yo Kobayashi, Hajime Miyashiro, Yuichi Mita, J. Power Sources, (submitted) for publication. [22] Michael Hess, Tsuyoshi Sasaki, Claire Villevieille, Petr Nov� ak, Nat. Commun. 6 (2015) 8169. [23] Yukinori Koyama, Takeshi Uyama, Yuki Orikasa, Takahiro Naka, Hideyuki Komatsu, Keiji Shimoda, Haruno Murayama, Katsutoshi Fukuda, Hajime Arai, Eiichiro Matsubara, Yoshiharu Uchimoto, Zempachi Ogumi, Chem. Mater. 29 (2017) 2855–2863. [24] Hisashi Kato, Yo Kobayashi, Hajime Miyashiro, J. Power Sources 398 (2018) 49–54. [25] Kohei Honkura, Ko Takahashi, Tatsuo Horiba, J. Power Sources 196 (2011) 10141–10147. [26] Hannah M. Dahn, A.J. Smith, J.C. Burns, D.A. Stevens, J.R. Dahn, J. Electrochem. Soc. 159 (2012) A1405–A1409. [27] Atsuo Yamada, Hiroyuki Koizumi, Shin-ichi Nishimura, Noriyuki Sonoyama, Ryoji Kanno, Masao Yonemura, Tatsuya Nakamura, Yo Kobayashi, Nat. Mater. 5 (2006) 357–360. [28] K. Kazuhiko, Yo Kobayashi, Hajime Miyashiro, Gosuke Oyama, Shin-ichi Nishimura, Masashi Okubo, Atsuo Yamada, Chem. Phys. Chem. 15 (2014) 2156–2161. https://doi.org/10.1002/cphc.201301219.
Fig. 8. Proposed model for the capacity fade and recovery after LFC (SOC 40–60% at 2C) operation and the capacity recovery test (SOC 0–100% at 1C).
cells. In the case of cycling tests within shallow state of charge (SOC) windows, denoted as load frequency control (LFC) simulated cycling, the cell capacity fade was mainly due to the inhomogeneous Li-ion content inside the electrodes, and the some of the cell capacity can be recovered by the full range operation. On the other hand, the shift of operation window (SOW) derived from the irreversible lithium con sumption at the graphite was main factor for capacity fade of the cell in the case of cycling with wide SOC window. The positive and negative electrode capacities did not change after the LFC simulated cycling, which provides additional evidence that the capacity fade is due to the inhomogeneity inside the cell. These results show that unexpected rapid capacity fade will occur during shallow SOC cycle operation with LiFePO4/graphite cells. However, we have proposed a way to recover the capacity of the cells used in LFC systems. These findings demonstrate that LiFePO4/graphite cells can achieve long-life operation with suitable SOC management in grid-scale stationary systems. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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