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LiNi0.5Mn1.5O4 (LNMO) Li-ion cells stored at elevated temperature
Journal of Electroanalytical Chemistry 758 (2015) 33–38
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jeac
Failure mechanism for high voltage graphite/LiNi0.5Mn1.5O4 (LNMO) Li-ion cells stored at elevated temperature D.S. Lu a,b,⁎, L.B. Yuan a, J.L. Li a, R.Q. Huang a, J.H. Guo a, Y.P. Cai a,b,⁎ a b
Institute of chemistry and environment, South China Normal University, Guangzhou 510006, China Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 17 May 2015 Received in revised form 15 October 2015 Accepted 16 October 2015 Available online 17 October 2015 Keywords: LNMO/graphite cell Elevated temperature Storage Mn dissolution Decomposition of electrolyte
a b s t r a c t Graphite/LNMO cells show severe capacity fade after being stored for one week at 55 °C in the fully discharged state. The failure mechanism of the cell has been investigated by electrochemical methods and physical analysis techniques (XRD, SEM, FTIR and ICP-OES). Independent electrochemical analysis of anode and cathode extracted from the ET-stored cell suggests that both electrodes have significant capacity loss. It was observed that capacity loss of the aged cathode can be recovered by charging at a constant high potential (4.9 V vs. Li/Li+), while that of the aged anode cannot be recovered with a constant potential charge (10 mV vs. Li/Li+). Capacity fade for the LNMO cathode is attributed to sluggish kinetics of Li+ intercalation/deintercalation during cycling. Ex-situ surface analysis of the electrode reveals that the sluggish electrochemical kinetics is related to the formation of inactive surface layer on the cathode. Failure of the graphite electrode may result from Mn deposition and subsequent dissolution of the SEI layer on the anode. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Rechargeable lithium-ion batteries are well-suited for electric and hybrid vehicles (EV and HEV) because of their relatively high energy and power densities in comparison to other cell chemistries [1]. LiNi0.5Mn1.5O4 (LNMO) spinel is an attractive cathode candidate for next generation lithium-ion batteries as it offers high power capability with an operating voltage of ~ 4.7 V and a capacity of ~ 135 mAh/g [2–4]. However, its application to EV/HEV is plagued by severe capacity fade at elevated temperature, because the EV/HEV batteries requirement for survival temperature is up to 66 °C [1]. There are several known reasons for the failure of the electrode material at elevated temperature, such as the local dissolution of the LNMO induced by HF [5,6]: 4LiNi0:5 Mn1:5 O4 þ 8Hþ →2λ MnO2 þ Mn2 þ þ Ni2 þ þ 4Liþ þ 4H2 O þ 2Ni0:5 Mn1:5 O4:
ð1Þ
The electrochemically inert protonated λ-MnO2 on the surface of the active mass leads to the sluggish kinetics of Li+ intercalation/ deintercalation during cycling. Another source of failure is the anodic decomposition of the electrolyte on the LNMO electrode at the high potential [7,8]. The products of the anodic oxidation form the cathode–electrolyte interface (CEI). The CEI is composed of inorganic species such as LiF and LixPFyOz or LixBFyOz as well as organic species such as polyethers and carbonates. The thickening of the CEI during cycling ⁎ Corresponding authors. E-mail address: [email protected] (D.S. Lu).
http://dx.doi.org/10.1016/j.jelechem.2015.10.018 1572-6657/© 2015 Elsevier B.V. All rights reserved.
causes a rapid rise in the impedance of the electrode. A third source of failure is the breakdown of the electrically conductive network in the LNMO electrode caused by the swelling of the PVDF binder at elevated temperature [9]. However, all these results mentioned above are from the investigations of Li/LNMO half cells at elevated temperature. In this work, Graphite/LNMO full cells in a fully discharge state were stored at open circuit for one week at 55 °C. After storage the electrodes were extracted and analyzed by electrochemical methods and ex-situ surface analysis in order to study the failure mechanism. 2. Experimental 2.1. Materials Two-electrode 2032-type coin cells with PP separators (Celgard Inc.) were used for electrochemical measurements. Composite LiNi0.5Mn1.5O4 electrodes containing active material (84%), conductive carbon and PVDF binder deposited on Al foil were prepared for the cathode. Composite graphite electrodes on Cu current collectors are comprised of graphite (91%), super P carbon and PVDF binder. The electrolyte solution (Li-battery grade) is a mixture of ethylene carbonate (EC), ethyl–methyl carbonate (EMC), 3:7(v/v) and 1.0 M LiPF6. 2.2. Cell cycling The protocol used to test cycling performance of Graphite/ LiNi0.5Mn1.5O4 cells has been described in our previous work [10]. The cells were first charged to 4.9 V at a constant current(C/20, C/10 or C/5
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D.S. Lu et al. / Journal of Electroanalytical Chemistry 758 (2015) 33–38
rate) (CC mode) followed by a constant voltage step at E = 4.9 V until the current decreased to 10% of the applied charging current (CV mode), and then the cells were discharged to 3.5 V with the same constant current (CC mode). After charge or discharge, the cells were allowed to rest for 20 min at open circuit and the open circuit voltage (OCV) were recorded at the end of the rest period. First cycle was at C/20, followed 2 cycles were at C/10 and the remaining 22 cycles were at C/5. The first 15 cycles were performed before storage. After storage, the cells were cooled to room temperature (RT) and subjected to another 10 cycles. The storage experiments were conducted at open circuit for one week at 55 °C (ET). The cells were sealed with epoxy resin and transferred to storage after being fully discharged to 3.5 V.
2.3. Electrochemical diagnostics To understand the source of capacity fading for Graphite/LNMO cells stored at 55 °C, the ET-stored cells were opened in the discharge state and the graphite anodes and LNMO cathodes were extracted and rinsed with anhydrous dimethyl carbonate (DMC) three times and followed by vacuum drying overnight at room temperature. Some of these washed electrodes were used to assemble Li/graphite and Li/LNMO cells. The cells were cycled from 3.5 V to 4.9 V for the Li/LNMO cell and between 10 mV–1.5 V for Li/graphite cell using different CC–CV charge–discharge protocols. All cells were assembled/disassembled in an Ar filled glove box and all electrochemical measurements were carried out using a multichannel Arbin instruments' battery tester at 25 °C (RT).
Fig. 2. Charge–discharge curves of LNMO/Graphite cell before and after storage.
3. Results and discussion 3.1. Cycling performance of Graphite/LNMO cell
A series of physical diagnostics were carried out with the remaining pieces of washed graphite anodes and LNMO cathodes. Surface morphology of the electrodes was characterized by scanning electron microscopy (SEM, JEOL5900) and surface analysis was done by FTIR in attenuated total reflectance (ATR) mode with a Bruker Tenser IFS 66/S using a ZnSe crystal plate. X-ray diffraction (XRD) was carried out on the LNMO cathode with a Rigaku Rint-2200 machine using Cu Kα radiation. XRD data were obtained at 2θ = 10–80°, with a scan rate of 5°/min. Mn and Ni content of the ET-stored graphite anode was determined by ICP-OES. For comparison, these measurements were also performed on the electrodes extracted from the cells before storage.
Fig. 1 shows cycling performance of LNMO/Graphite cell before and after storage at ET. The cell has stable cycling performance and high columbic efficiency (~98%) before storage, but loses more than 40% of the initial capacity and has poor cycling stability after storage at ET. The poor cycling performance after storage at ET is a significant concern for this battery system, the remainder of the discussion will be geared towards understanding the mechanism responsible for this capacity fade. Voltage profiles from the 1st, 4th, 8th, 10th, 11th, 14th, 18th and 20th cycle for the cell before and after storage at ET are compared in Fig. 2. After ET storage, its discharge capacity and charge efficiency drop to only 69 mAh/g and 44%, respectively. During subsequent cycling, the capacity for charging and discharging greatly decreases, but the charge efficiency gradually increases from 44% to 98%, close to the value before storage. In addition, it can be seen that the ET-stored cell has higher charge plateaus and lower discharge plateaus. These plateaus almost do not shift during further cycling. These results
Fig. 1. Cycling performance of LNMO/Graphite cell at RT before and after storage. (The first 5 formation cycles at RT are not shown in the Figure.)
Fig. 3. The first charge–discharge curves of LNMO/Graphite cell before and after storage. Insets: The dQ/dv plots for the two charge–discharge curves.
2.4. Physical diagnostics
D.S. Lu et al. / Journal of Electroanalytical Chemistry 758 (2015) 33–38
35
In order to study the formation of the surface layer, Fig. 3 shows the first charge–discharge curves of LNMO/Graphite cell before and after storage. Before storage, OCV (open circuit voltage) of LNMO/Graphite cell is about 3.7 V, it can be seen from Fig. 3 that the OCV value drops to only 0.012 V after storage. The potentials of LNMO cathode and graphite anode extracted from the ET-stored cell are 3.09 V (vs. Li/Li+) and 3.16 V (vs. Li/Li+), respectively. It shows that the cell undergoes severe self-discharge during storage at ET. The self-discharge may be related to dissolution of LNMO and SEI on the anode at ET. The dissolution of LNMO may undergo a proton–lithium-ion-exchange process similar to that of spinel LiMn2O4 [11]: HF
LiNi0:5 Mn1:5 O4 þ xLiþ → Li1þx Mn1:5‐y O4 þ yMn2þ þH
þ
þ 0:5Ni2þ →þ Lix Hz Mn1:5‐y O4 ‐Li
ð2Þ
In parallel with reaction (2), the decomposition reaction of SEI occurs on the anode [11]: ðCH2 OCO2 LiÞ2 →Li2 CO3 þ CO2 þ C2 H4 þ 0:5O2 Fig. 4. Variation of OCV vs. cycle number for LNMO/Graphite cell before and after storage. The values of the OCV were recorded in the fully charged or discharged state.
shows that a surface layer may form on the electrodes of the ET-stored cell in the first charge process, leading to an increase in cell impedance and capacity fade.
ð3Þ
Reaction (2) and (3) result in formation of the electrochemically inert surface layers on LNMO and graphite, and the surface layers cause an increase in the impedance of anode and cathode. Generally, the dissolved SEI on the carbon anode can be reformed by reductive decomposition of the electrolyte solution when the electrode
Fig. 5. Influence of CC and CV charge step on charge capacity of Li/LNMO cells using (a) a LNMO cathode extracted from an abused graphite/LNMO cell and (b) a fresh LNMO cathode. (c) Typical charge–discharge curves for Li/LNMO cells containing a fresh cathode and a cathode from an abused graphite/LNMO cell.
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is lithiated again [11,12]. This means that the reformation of SEI should be similar to the formation of SEI on fresh anode. However, it can be seen from Fig. 3 that the first charge curves of LNMO/Graphite cell before and after storage are quite different. Two new peaks at 1.1 V and 2.5 V are shown in the dQ/dV (differential capacity) plot for the first charge curve after storage. The peak at 1.1 V can be assigned to the reduction of Mn2+ → Mn and the peak at 2.5 V corresponds to the deposition of Ni (Ni2+ → Ni) on the graphite anode [13]. Ni deposits on the anode surface just hinder intercalation/deintercalation of Li+ whereas Mn deposits on the anode can catalyze electrolyte decomposition [13–15]. Consequently, the initial charge capacity (155 mAh/g, as shown in Fig. 3) becomes very large with the Mn deposition followed by dramatic electrolyte decomposition, and the initial charge efficiency is very low (44%). As a comparison, the first charge capacity and coulombic efficiency for the cell before storage are 120 mAh/g and 73%, respectively. This shows that the reduction decomposition of the electrolyte on graphite covered with Mn deposits is quite different from SEI formation. During successive cycling, the gradually increasing Mn and the decomposition products lead to an increase in the anode impedance. Fig. 4 shows dependence of OCV of the cell on cycle number before and after storage. Before storage, the OCV in fully discharged and charged state remain almost unchanged during cycling and the values are about 3.7 V and 4.7 V, respectively. This reveals that the cell impedance is stable in these 10 cycles. After storage, the OCV in fully charged state is maintained at 4.7 V during cycling, while that in fully discharged state increases from 3.7 V to 3.83 V and tends to increase gradually in subsequent cycling. This indicates that the ET-stored cell cannot be fully discharged at the applied current density due to its high impedance. 3.2. Cathode diagnostics A cathode extracted from the above ET-stored graphite/LNMO cell was used to prepare a Li/LNMO cell. The Li/aged LNMO cell was cycled at RT with a CC–CV mode. The cell was charged to 4.9 V using a CC mode at a C/5 rate followed by a constant voltage step (CV mode) at E = 4.9 V until the current decreased to 10% of the applied charging current and then discharged to 3.5 V by CC at C/5. For comparison, a Li/LNMO cell containing a fresh LNMO cathode was also cycled using the above protocol. In Fig. 5a, the influence of CC and CV charge step on charge capacity of the aged LNMO electrode is presented. The initial charge capacity of the electrode is 96 mAh/g and it shows an abnormal charge efficiency of 139%, indicating that the aged LNMO electrode can't be fully discharged when it is in the ET-stored graphite/LNMO cell and the LNMO electrode still retains a capacity of about 37 mAh/g before it is charged. This result is consistent with that of OCV in discharged state in Fig. 4. In the 2nd cycle, charge capacity of the aged LNMO electrode increases to 144 mAh/g and the electrode's charge efficiency
decreases to 92%. 70.8% of the total charge capacity, 102 mAh/g, is obtained by CC charge step and the remainder (42 mAh/g) is from CV charge step. In subsequent cycling, the charge capacity from CC step decreases from 102 mAh/g to 48 mAh/g and the capacity obtained by CV step increases from 42 mAh/g to 91 mAh/g. The charge capacity from CV step complements the charge capacity obtained by CC step, so the total charge capacity and charge efficiency remains almost unchanged. It can be seen from Fig. 5b that the fresh LNMO electrode has stable cycling performance. Its charge capacity and coulombic efficiency
Fig. 6. XRD patterns of LNMO cathode before and after storage.
Fig. 8. FTIR spectra of fresh LNMO electrode and LNMO cathodes before and after storage.
Fig. 7. SEM images of LNMO cathodes (a) before and (b) after storage.
D.S. Lu et al. / Journal of Electroanalytical Chemistry 758 (2015) 33–38
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Fig. 9. (a) Cycling performance and (b) typical charge–discharge curves of Li/graphite cell constructed with fresh graphite electrode and graphite electrode extracted from an ET-stored graphite/LNMO cell.
are maintained at 132 mAh/g and 99%, respectively. Most of the total charge capacity is from CC charge step and only 7.5% of that is obtained by CV step. As can be seen from the above results, discharge capacity of the aged LNMO electrode is very close to that of fresh LNMO electrode because capacity loss of the aged electrode can be recovered by CV charge step. However, prolonged potentiostatic charging at a high potential (4.9 V) can result in lower charge efficiency due to electrochemical oxidation of the electrolyte. Typical charge–discharge curves for Li/fresh LNMO cell and Li/aged LNMO cell are depicted in Fig. 5c. The aged LNMO electrode has a higher charge plateau and a lower discharge plateau, which is characteristic of an increase in electrode impedance. These above results show that deterioration of the ET-stored cathode is due to its sluggish kinetics of Li+ intercalation/deintercalation. XRD patterns of LNMO cathodes before and after storage are compared in Fig. 6. This result shows no evidence for degradation of the active material or accumulation of new solid phases after storage. Fig. 7 presents SEM images of LNMO electrodes under the two conditions. As seen from a comparison of Fig. 7a and b, a surface layer may form on the LNMO cathode after storage. To further understand the surface layer, FTIR measurements were performed on a fresh LNMO electrode and LNMO cathodes before and after storage. The measured FTIR spectra are shown in Fig. 8. The peaks at 838, 877, 1174 and 1400 cm−1 are assigned to PVDF binder [16]. In case of the electrode before storage, polyethylene carbonate (PEC) due to oxidation of EC was detected at 839, 1775 and 1803 cm− 1 [4,16–18]. For the ET-stored electrode, a new broad peak assigned to PEC appears in the range of 900–1120 cm−1 [16–17] and the peaks at 1775 and 1803 cm−1 are obviously increased in intensity, indicating that the thickness of PEC layer on the electrode increases after storage. The thick PEC layer leads to sluggish electrochemical kinetics of the ET-stored electrode.
has stable cycling performance and high coulombic efficiency at RT after the formation of SEI. However, the aged graphite electrode shows very low initial discharge capacity (134 mAh/g) and charge efficiency (57%). During cycling at CC mode, the discharge capacity of the
3.3. Anode diagnostics A graphite electrode extracted from the above ET-stored graphite/ LNMO cell and a fresh graphite electrode were used to prepare Li/graphite cells. The Li/graphite cells were cycled between 10 mV and 1.5 V at a CC mode(C/5, 10 cycles) followed by a CC–CV mode(C/5, 4 cycles). The CV charge at 10 mV continued until the current decreased to 10% of applied charge current. In Fig. 9a, the initial discharge capacity and charge efficiency of the fresh graphite electrode are 310 mAh/g and 93.6%, respectively. In subsequent 9 cycles, the electrode retains the initial capacity and its charge efficiency is maintained at about 99.5%. These results indicate that the fresh graphite electrode
Fig. 10. SEM images of graphite anodes (a) before and (b) after storage.
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Capacity loss of the aged LNMO electrode can be recovered by charging at a constant high potential (4.9 V vs. Li/Li+), while that of the aged graphite electrode cannot be recovered with a constant potential charge (10 mV vs. Li/Li+). Acknowledgments The authors acknowledge the financial support by Natural Science Foundation of Guangdong Province (Grant no. 10451063101006346) and the National Natural Science Foundation (Grant no. 21571069), Young Teacher Training Plan of Guangdong Universities (2008) and Undergraduates' Innovating Experimentation Project of South China Normal University (2014086). References
Fig. 11. FTIR spectra of fresh graphite anode and graphite anodes before and after storage.
aged graphite electrode only fluctuates slightly and its charge efficiency gradually increases from 57% to 98%. These show that (1) the graphite anode in the above ET-stored graphite/LNMO cell loses about 60% of its reversible discharge capacity; (2) electrochemical reduction of the electrolyte occurs on the aged graphite electrode and leads to low charge efficiency. Upon utilizing CC charge mode followed by CV charge mode the discharge capacity of the aged graphite electrode increases from 134 mAh/g to 214 mAh/g and then decreases to 184 mAh/g after only 4 cycles, indicating that most of capacity loss of the aged graphite electrode cannot be recovered with an additional CV charge step. Typical charge–discharge curves for fresh graphite and aged graphite electrode are depicted in Fig. 9b. The aged graphite electrode has higher discharge plateau and lower charge plateau. Thus the aged graphite electrode, much like the aged LNMO electrode, has an increase in impedance after storage at ET which contributes to the increase in the impedance of the ET-stored graphite/LNMO cell. SEM images of graphite anodes before and after storage are shown in Fig. 10. Obviously, there is a thick deposit on the surface of the ET-stored graphite anode (Fig. 10b). Fig. 11 presents FTIR spectra of fresh graphite anode and graphite anodes under the two conditions. For the electrode before storage, lithium alkyl carbonate (CH2OCO2Li)2, the major component of the SEI layer on the graphite anode was detected at 1624, 1440, 1300, 1045 and 848 cm−1 [19]. In case of the ET-stored electrode, intensity of the FTIR signals increases greatly and several new absorption bands appear at 900–1150 and 1300–1550 cm−1. The bands at 900– 1150 cm−1 are assigned to LixPFyOz, which is from oxidation of electrolyte salt LiPF6 [20,21]. The peaks at 1300–1550 cm−1 are due to Li2CO3 [19], one of the decomposition products of (CH2OCO2Li) 2 [18]. It can be inferred from above results that the thick deposit on the ET-stored electrode contains Li2CO3 and LixPFyOz. In addition, the ICP-MS results show that the ET-stored anode contains 0.05 mg Mn and 0.02 mg Ni, while the graphite anode before storage contains two orders of magnitude lower concentrations of Mn and Ni. The deposited Mn and Ni may result in failure of the graphite anode due to continuous decomposition of the electrolyte on the electrode during cycling [13,15]. So Mn and Ni in graphite anode have attracted increasing attention in recent years [22–24]. 4. Conclusions Graphite/LNMO cell suffers severe capacity fading after storage for one week in fully discharged state at ET. Both the graphite anode and the LNMO cathode contribute to capacity loss of the ET-stored cell. The oxidation of EC and the dissolution of LNMO lead to the formation of an inactive surface film on the electrode, which increases the impedance of intercalation/deintercalation of Li+ during cycling. Mn deposit and dissolution of the SEI layer result in failure of the graphite electrode.
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Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jeac
Failure mechanism for high voltage graphite/LiNi0.5Mn1.5O4 (LNMO) Li-ion cells stored at elevated temperature D.S. Lu a,b,⁎, L.B. Yuan a, J.L. Li a, R.Q. Huang a, J.H. Guo a, Y.P. Cai a,b,⁎ a b
Institute of chemistry and environment, South China Normal University, Guangzhou 510006, China Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 17 May 2015 Received in revised form 15 October 2015 Accepted 16 October 2015 Available online 17 October 2015 Keywords: LNMO/graphite cell Elevated temperature Storage Mn dissolution Decomposition of electrolyte
a b s t r a c t Graphite/LNMO cells show severe capacity fade after being stored for one week at 55 °C in the fully discharged state. The failure mechanism of the cell has been investigated by electrochemical methods and physical analysis techniques (XRD, SEM, FTIR and ICP-OES). Independent electrochemical analysis of anode and cathode extracted from the ET-stored cell suggests that both electrodes have significant capacity loss. It was observed that capacity loss of the aged cathode can be recovered by charging at a constant high potential (4.9 V vs. Li/Li+), while that of the aged anode cannot be recovered with a constant potential charge (10 mV vs. Li/Li+). Capacity fade for the LNMO cathode is attributed to sluggish kinetics of Li+ intercalation/deintercalation during cycling. Ex-situ surface analysis of the electrode reveals that the sluggish electrochemical kinetics is related to the formation of inactive surface layer on the cathode. Failure of the graphite electrode may result from Mn deposition and subsequent dissolution of the SEI layer on the anode. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Rechargeable lithium-ion batteries are well-suited for electric and hybrid vehicles (EV and HEV) because of their relatively high energy and power densities in comparison to other cell chemistries [1]. LiNi0.5Mn1.5O4 (LNMO) spinel is an attractive cathode candidate for next generation lithium-ion batteries as it offers high power capability with an operating voltage of ~ 4.7 V and a capacity of ~ 135 mAh/g [2–4]. However, its application to EV/HEV is plagued by severe capacity fade at elevated temperature, because the EV/HEV batteries requirement for survival temperature is up to 66 °C [1]. There are several known reasons for the failure of the electrode material at elevated temperature, such as the local dissolution of the LNMO induced by HF [5,6]: 4LiNi0:5 Mn1:5 O4 þ 8Hþ →2λ MnO2 þ Mn2 þ þ Ni2 þ þ 4Liþ þ 4H2 O þ 2Ni0:5 Mn1:5 O4:
ð1Þ
The electrochemically inert protonated λ-MnO2 on the surface of the active mass leads to the sluggish kinetics of Li+ intercalation/ deintercalation during cycling. Another source of failure is the anodic decomposition of the electrolyte on the LNMO electrode at the high potential [7,8]. The products of the anodic oxidation form the cathode–electrolyte interface (CEI). The CEI is composed of inorganic species such as LiF and LixPFyOz or LixBFyOz as well as organic species such as polyethers and carbonates. The thickening of the CEI during cycling ⁎ Corresponding authors. E-mail address: [email protected] (D.S. Lu).
http://dx.doi.org/10.1016/j.jelechem.2015.10.018 1572-6657/© 2015 Elsevier B.V. All rights reserved.
causes a rapid rise in the impedance of the electrode. A third source of failure is the breakdown of the electrically conductive network in the LNMO electrode caused by the swelling of the PVDF binder at elevated temperature [9]. However, all these results mentioned above are from the investigations of Li/LNMO half cells at elevated temperature. In this work, Graphite/LNMO full cells in a fully discharge state were stored at open circuit for one week at 55 °C. After storage the electrodes were extracted and analyzed by electrochemical methods and ex-situ surface analysis in order to study the failure mechanism. 2. Experimental 2.1. Materials Two-electrode 2032-type coin cells with PP separators (Celgard Inc.) were used for electrochemical measurements. Composite LiNi0.5Mn1.5O4 electrodes containing active material (84%), conductive carbon and PVDF binder deposited on Al foil were prepared for the cathode. Composite graphite electrodes on Cu current collectors are comprised of graphite (91%), super P carbon and PVDF binder. The electrolyte solution (Li-battery grade) is a mixture of ethylene carbonate (EC), ethyl–methyl carbonate (EMC), 3:7(v/v) and 1.0 M LiPF6. 2.2. Cell cycling The protocol used to test cycling performance of Graphite/ LiNi0.5Mn1.5O4 cells has been described in our previous work [10]. The cells were first charged to 4.9 V at a constant current(C/20, C/10 or C/5
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rate) (CC mode) followed by a constant voltage step at E = 4.9 V until the current decreased to 10% of the applied charging current (CV mode), and then the cells were discharged to 3.5 V with the same constant current (CC mode). After charge or discharge, the cells were allowed to rest for 20 min at open circuit and the open circuit voltage (OCV) were recorded at the end of the rest period. First cycle was at C/20, followed 2 cycles were at C/10 and the remaining 22 cycles were at C/5. The first 15 cycles were performed before storage. After storage, the cells were cooled to room temperature (RT) and subjected to another 10 cycles. The storage experiments were conducted at open circuit for one week at 55 °C (ET). The cells were sealed with epoxy resin and transferred to storage after being fully discharged to 3.5 V.
2.3. Electrochemical diagnostics To understand the source of capacity fading for Graphite/LNMO cells stored at 55 °C, the ET-stored cells were opened in the discharge state and the graphite anodes and LNMO cathodes were extracted and rinsed with anhydrous dimethyl carbonate (DMC) three times and followed by vacuum drying overnight at room temperature. Some of these washed electrodes were used to assemble Li/graphite and Li/LNMO cells. The cells were cycled from 3.5 V to 4.9 V for the Li/LNMO cell and between 10 mV–1.5 V for Li/graphite cell using different CC–CV charge–discharge protocols. All cells were assembled/disassembled in an Ar filled glove box and all electrochemical measurements were carried out using a multichannel Arbin instruments' battery tester at 25 °C (RT).
Fig. 2. Charge–discharge curves of LNMO/Graphite cell before and after storage.
3. Results and discussion 3.1. Cycling performance of Graphite/LNMO cell
A series of physical diagnostics were carried out with the remaining pieces of washed graphite anodes and LNMO cathodes. Surface morphology of the electrodes was characterized by scanning electron microscopy (SEM, JEOL5900) and surface analysis was done by FTIR in attenuated total reflectance (ATR) mode with a Bruker Tenser IFS 66/S using a ZnSe crystal plate. X-ray diffraction (XRD) was carried out on the LNMO cathode with a Rigaku Rint-2200 machine using Cu Kα radiation. XRD data were obtained at 2θ = 10–80°, with a scan rate of 5°/min. Mn and Ni content of the ET-stored graphite anode was determined by ICP-OES. For comparison, these measurements were also performed on the electrodes extracted from the cells before storage.
Fig. 1 shows cycling performance of LNMO/Graphite cell before and after storage at ET. The cell has stable cycling performance and high columbic efficiency (~98%) before storage, but loses more than 40% of the initial capacity and has poor cycling stability after storage at ET. The poor cycling performance after storage at ET is a significant concern for this battery system, the remainder of the discussion will be geared towards understanding the mechanism responsible for this capacity fade. Voltage profiles from the 1st, 4th, 8th, 10th, 11th, 14th, 18th and 20th cycle for the cell before and after storage at ET are compared in Fig. 2. After ET storage, its discharge capacity and charge efficiency drop to only 69 mAh/g and 44%, respectively. During subsequent cycling, the capacity for charging and discharging greatly decreases, but the charge efficiency gradually increases from 44% to 98%, close to the value before storage. In addition, it can be seen that the ET-stored cell has higher charge plateaus and lower discharge plateaus. These plateaus almost do not shift during further cycling. These results
Fig. 1. Cycling performance of LNMO/Graphite cell at RT before and after storage. (The first 5 formation cycles at RT are not shown in the Figure.)
Fig. 3. The first charge–discharge curves of LNMO/Graphite cell before and after storage. Insets: The dQ/dv plots for the two charge–discharge curves.
2.4. Physical diagnostics
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In order to study the formation of the surface layer, Fig. 3 shows the first charge–discharge curves of LNMO/Graphite cell before and after storage. Before storage, OCV (open circuit voltage) of LNMO/Graphite cell is about 3.7 V, it can be seen from Fig. 3 that the OCV value drops to only 0.012 V after storage. The potentials of LNMO cathode and graphite anode extracted from the ET-stored cell are 3.09 V (vs. Li/Li+) and 3.16 V (vs. Li/Li+), respectively. It shows that the cell undergoes severe self-discharge during storage at ET. The self-discharge may be related to dissolution of LNMO and SEI on the anode at ET. The dissolution of LNMO may undergo a proton–lithium-ion-exchange process similar to that of spinel LiMn2O4 [11]: HF
LiNi0:5 Mn1:5 O4 þ xLiþ → Li1þx Mn1:5‐y O4 þ yMn2þ þH
þ
þ 0:5Ni2þ →þ Lix Hz Mn1:5‐y O4 ‐Li
ð2Þ
In parallel with reaction (2), the decomposition reaction of SEI occurs on the anode [11]: ðCH2 OCO2 LiÞ2 →Li2 CO3 þ CO2 þ C2 H4 þ 0:5O2 Fig. 4. Variation of OCV vs. cycle number for LNMO/Graphite cell before and after storage. The values of the OCV were recorded in the fully charged or discharged state.
shows that a surface layer may form on the electrodes of the ET-stored cell in the first charge process, leading to an increase in cell impedance and capacity fade.
ð3Þ
Reaction (2) and (3) result in formation of the electrochemically inert surface layers on LNMO and graphite, and the surface layers cause an increase in the impedance of anode and cathode. Generally, the dissolved SEI on the carbon anode can be reformed by reductive decomposition of the electrolyte solution when the electrode
Fig. 5. Influence of CC and CV charge step on charge capacity of Li/LNMO cells using (a) a LNMO cathode extracted from an abused graphite/LNMO cell and (b) a fresh LNMO cathode. (c) Typical charge–discharge curves for Li/LNMO cells containing a fresh cathode and a cathode from an abused graphite/LNMO cell.
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is lithiated again [11,12]. This means that the reformation of SEI should be similar to the formation of SEI on fresh anode. However, it can be seen from Fig. 3 that the first charge curves of LNMO/Graphite cell before and after storage are quite different. Two new peaks at 1.1 V and 2.5 V are shown in the dQ/dV (differential capacity) plot for the first charge curve after storage. The peak at 1.1 V can be assigned to the reduction of Mn2+ → Mn and the peak at 2.5 V corresponds to the deposition of Ni (Ni2+ → Ni) on the graphite anode [13]. Ni deposits on the anode surface just hinder intercalation/deintercalation of Li+ whereas Mn deposits on the anode can catalyze electrolyte decomposition [13–15]. Consequently, the initial charge capacity (155 mAh/g, as shown in Fig. 3) becomes very large with the Mn deposition followed by dramatic electrolyte decomposition, and the initial charge efficiency is very low (44%). As a comparison, the first charge capacity and coulombic efficiency for the cell before storage are 120 mAh/g and 73%, respectively. This shows that the reduction decomposition of the electrolyte on graphite covered with Mn deposits is quite different from SEI formation. During successive cycling, the gradually increasing Mn and the decomposition products lead to an increase in the anode impedance. Fig. 4 shows dependence of OCV of the cell on cycle number before and after storage. Before storage, the OCV in fully discharged and charged state remain almost unchanged during cycling and the values are about 3.7 V and 4.7 V, respectively. This reveals that the cell impedance is stable in these 10 cycles. After storage, the OCV in fully charged state is maintained at 4.7 V during cycling, while that in fully discharged state increases from 3.7 V to 3.83 V and tends to increase gradually in subsequent cycling. This indicates that the ET-stored cell cannot be fully discharged at the applied current density due to its high impedance. 3.2. Cathode diagnostics A cathode extracted from the above ET-stored graphite/LNMO cell was used to prepare a Li/LNMO cell. The Li/aged LNMO cell was cycled at RT with a CC–CV mode. The cell was charged to 4.9 V using a CC mode at a C/5 rate followed by a constant voltage step (CV mode) at E = 4.9 V until the current decreased to 10% of the applied charging current and then discharged to 3.5 V by CC at C/5. For comparison, a Li/LNMO cell containing a fresh LNMO cathode was also cycled using the above protocol. In Fig. 5a, the influence of CC and CV charge step on charge capacity of the aged LNMO electrode is presented. The initial charge capacity of the electrode is 96 mAh/g and it shows an abnormal charge efficiency of 139%, indicating that the aged LNMO electrode can't be fully discharged when it is in the ET-stored graphite/LNMO cell and the LNMO electrode still retains a capacity of about 37 mAh/g before it is charged. This result is consistent with that of OCV in discharged state in Fig. 4. In the 2nd cycle, charge capacity of the aged LNMO electrode increases to 144 mAh/g and the electrode's charge efficiency
decreases to 92%. 70.8% of the total charge capacity, 102 mAh/g, is obtained by CC charge step and the remainder (42 mAh/g) is from CV charge step. In subsequent cycling, the charge capacity from CC step decreases from 102 mAh/g to 48 mAh/g and the capacity obtained by CV step increases from 42 mAh/g to 91 mAh/g. The charge capacity from CV step complements the charge capacity obtained by CC step, so the total charge capacity and charge efficiency remains almost unchanged. It can be seen from Fig. 5b that the fresh LNMO electrode has stable cycling performance. Its charge capacity and coulombic efficiency
Fig. 6. XRD patterns of LNMO cathode before and after storage.
Fig. 8. FTIR spectra of fresh LNMO electrode and LNMO cathodes before and after storage.
Fig. 7. SEM images of LNMO cathodes (a) before and (b) after storage.
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Fig. 9. (a) Cycling performance and (b) typical charge–discharge curves of Li/graphite cell constructed with fresh graphite electrode and graphite electrode extracted from an ET-stored graphite/LNMO cell.
are maintained at 132 mAh/g and 99%, respectively. Most of the total charge capacity is from CC charge step and only 7.5% of that is obtained by CV step. As can be seen from the above results, discharge capacity of the aged LNMO electrode is very close to that of fresh LNMO electrode because capacity loss of the aged electrode can be recovered by CV charge step. However, prolonged potentiostatic charging at a high potential (4.9 V) can result in lower charge efficiency due to electrochemical oxidation of the electrolyte. Typical charge–discharge curves for Li/fresh LNMO cell and Li/aged LNMO cell are depicted in Fig. 5c. The aged LNMO electrode has a higher charge plateau and a lower discharge plateau, which is characteristic of an increase in electrode impedance. These above results show that deterioration of the ET-stored cathode is due to its sluggish kinetics of Li+ intercalation/deintercalation. XRD patterns of LNMO cathodes before and after storage are compared in Fig. 6. This result shows no evidence for degradation of the active material or accumulation of new solid phases after storage. Fig. 7 presents SEM images of LNMO electrodes under the two conditions. As seen from a comparison of Fig. 7a and b, a surface layer may form on the LNMO cathode after storage. To further understand the surface layer, FTIR measurements were performed on a fresh LNMO electrode and LNMO cathodes before and after storage. The measured FTIR spectra are shown in Fig. 8. The peaks at 838, 877, 1174 and 1400 cm−1 are assigned to PVDF binder [16]. In case of the electrode before storage, polyethylene carbonate (PEC) due to oxidation of EC was detected at 839, 1775 and 1803 cm− 1 [4,16–18]. For the ET-stored electrode, a new broad peak assigned to PEC appears in the range of 900–1120 cm−1 [16–17] and the peaks at 1775 and 1803 cm−1 are obviously increased in intensity, indicating that the thickness of PEC layer on the electrode increases after storage. The thick PEC layer leads to sluggish electrochemical kinetics of the ET-stored electrode.
has stable cycling performance and high coulombic efficiency at RT after the formation of SEI. However, the aged graphite electrode shows very low initial discharge capacity (134 mAh/g) and charge efficiency (57%). During cycling at CC mode, the discharge capacity of the
3.3. Anode diagnostics A graphite electrode extracted from the above ET-stored graphite/ LNMO cell and a fresh graphite electrode were used to prepare Li/graphite cells. The Li/graphite cells were cycled between 10 mV and 1.5 V at a CC mode(C/5, 10 cycles) followed by a CC–CV mode(C/5, 4 cycles). The CV charge at 10 mV continued until the current decreased to 10% of applied charge current. In Fig. 9a, the initial discharge capacity and charge efficiency of the fresh graphite electrode are 310 mAh/g and 93.6%, respectively. In subsequent 9 cycles, the electrode retains the initial capacity and its charge efficiency is maintained at about 99.5%. These results indicate that the fresh graphite electrode
Fig. 10. SEM images of graphite anodes (a) before and (b) after storage.
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Capacity loss of the aged LNMO electrode can be recovered by charging at a constant high potential (4.9 V vs. Li/Li+), while that of the aged graphite electrode cannot be recovered with a constant potential charge (10 mV vs. Li/Li+). Acknowledgments The authors acknowledge the financial support by Natural Science Foundation of Guangdong Province (Grant no. 10451063101006346) and the National Natural Science Foundation (Grant no. 21571069), Young Teacher Training Plan of Guangdong Universities (2008) and Undergraduates' Innovating Experimentation Project of South China Normal University (2014086). References
Fig. 11. FTIR spectra of fresh graphite anode and graphite anodes before and after storage.
aged graphite electrode only fluctuates slightly and its charge efficiency gradually increases from 57% to 98%. These show that (1) the graphite anode in the above ET-stored graphite/LNMO cell loses about 60% of its reversible discharge capacity; (2) electrochemical reduction of the electrolyte occurs on the aged graphite electrode and leads to low charge efficiency. Upon utilizing CC charge mode followed by CV charge mode the discharge capacity of the aged graphite electrode increases from 134 mAh/g to 214 mAh/g and then decreases to 184 mAh/g after only 4 cycles, indicating that most of capacity loss of the aged graphite electrode cannot be recovered with an additional CV charge step. Typical charge–discharge curves for fresh graphite and aged graphite electrode are depicted in Fig. 9b. The aged graphite electrode has higher discharge plateau and lower charge plateau. Thus the aged graphite electrode, much like the aged LNMO electrode, has an increase in impedance after storage at ET which contributes to the increase in the impedance of the ET-stored graphite/LNMO cell. SEM images of graphite anodes before and after storage are shown in Fig. 10. Obviously, there is a thick deposit on the surface of the ET-stored graphite anode (Fig. 10b). Fig. 11 presents FTIR spectra of fresh graphite anode and graphite anodes under the two conditions. For the electrode before storage, lithium alkyl carbonate (CH2OCO2Li)2, the major component of the SEI layer on the graphite anode was detected at 1624, 1440, 1300, 1045 and 848 cm−1 [19]. In case of the ET-stored electrode, intensity of the FTIR signals increases greatly and several new absorption bands appear at 900–1150 and 1300–1550 cm−1. The bands at 900– 1150 cm−1 are assigned to LixPFyOz, which is from oxidation of electrolyte salt LiPF6 [20,21]. The peaks at 1300–1550 cm−1 are due to Li2CO3 [19], one of the decomposition products of (CH2OCO2Li) 2 [18]. It can be inferred from above results that the thick deposit on the ET-stored electrode contains Li2CO3 and LixPFyOz. In addition, the ICP-MS results show that the ET-stored anode contains 0.05 mg Mn and 0.02 mg Ni, while the graphite anode before storage contains two orders of magnitude lower concentrations of Mn and Ni. The deposited Mn and Ni may result in failure of the graphite anode due to continuous decomposition of the electrolyte on the electrode during cycling [13,15]. So Mn and Ni in graphite anode have attracted increasing attention in recent years [22–24]. 4. Conclusions Graphite/LNMO cell suffers severe capacity fading after storage for one week in fully discharged state at ET. Both the graphite anode and the LNMO cathode contribute to capacity loss of the ET-stored cell. The oxidation of EC and the dissolution of LNMO lead to the formation of an inactive surface film on the electrode, which increases the impedance of intercalation/deintercalation of Li+ during cycling. Mn deposit and dissolution of the SEI layer result in failure of the graphite electrode.
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