Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Fast Cationic and Anionic Redox Reactions in Li2RuO3‑Li2SO4 Positive Electrode Materials Kenji Nagao, Atsushi Sakuda,* Wataru Nakamura, Akitoshi Hayashi, and Masahiro Tatsumisago Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
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S Supporting Information *
ABSTRACT: For the application of lithium-ion batteries (LiBs) to a large-scale power source for electric vehicles, positive electrode materials with high energy and power densities should be developed. Although Li-excess materials are regarded as high-capacity positive electrodes, there are several issues in the anionic redox reactions, such as oxygen gas evolution and large resistance, which lead to capacity fading and limitation of high-current operation. Here, we report the highcurrent operation of an electrochemical cell using the Li2RuO3−Li2SO4 positive electrode in LiBs. The cell using Li2Ru0.8S0.2O3.2 showed a high capacity of 350 mAh g−1 with an average discharge voltage of 2.9 V vs Li at 25 °C. By introducing an amorphous matrix based on Li2SO4, a high diffusion coefficient was maintained during the whole charge−discharge process, and low charge-transfer resistance was maintained even at a high oxidation state up to 4.2 V vs Li. As a result, extremely high current operation, such as 40 C rate, was achieved. KEYWORDS: lithium-excess positive electrode, amorphous, fast anion redox, high capacity, GITT, charge-transfer reaction synthesized novel Li2RuO3−Li2SO4 positive electrode materials for all-solid-state batteries using a high-energy ball-milling technique.17 The Li2Ru0.8S0.2O3.2 (80:20 Li2RuO3−Li2SO4 (mol %)) positive electrode active material is a nanocomposite where nanosize cation-disordered rock-salt-type Li2RuO3 (Fm3̅m) crystals are dispersed into the amorphous Li2RuO3Li2SO4 matrix. This letter reports the detailed electrochemical analyses on the Li2RuO3-Li2SO4 positive electrodes in electrochemical cells using an organic liquid electrolyte. Evaluating the diffusion and charge-transfer properties of the Li2RuO3-Li2SO4 positive electrode materials by detailed electrochemical techniques, herein, we propose the Li2SO4-based amorphous matrix functioned as an ionic conduction pathway and an inhibitor for the impedance increases in anion redox reaction, which leads to an extremely high current density operation of lithiumion batteries. The detailed crystal structure for the synthesized electrode materials is discussed through XRD and TEM analyses in Figures S1 and S2 in the Supporting Information. Figure S3 shows the scanning electron microscopy images for the synthesized particles. Mechanochemically synthesized Li2RuO3-Li2SO4 materials had a smaller average particle size than the solid-state synthesized Li2RuO3 (C2/c) crystals (approximately 5 μm). With an increase in the Li2SO4 content, larger particles were obtained; the primary particle sizes of Li2RuO3 (Fm3̅m), Li2Ru0.8S0.2O3.2, and Li2Ru0.5S0.5O3.5
L
ithium-ion batteries (LiBs) have been used in various electronic devices since their commercialization in 1991. For the commercial use of electric vehicles, LiBs require high energy and power densities and high safety. Typical intercalation materials have been used as positive electrode active materials.1−3 Recently, Li-excess materials have been regarded as candidate electrode active materials for high energy density batteries because of the cumulative cationic and anionic redox reactions that lead to high capacity.4,5 However, in LiBs using Li-excess electrode materials, there are many challenges, such as oxygen gas evolution, voltage fade over cycles, voltage hysteresis during charge−discharge, and slow kinetics, among others.6−8 Li2RuO3 and its solid solutions are well-known Li-excess layered positive electrode materials.9−11 Oxygen redox reactions become reversible and a high capacity of greater than 220 mAh g−1 is obtained by substituting Sn4+ ions for Ru4+ ions in the layered structure.12,13 Detailed analyses of the anion redox reaction in Li2Ru0.75Sn0.25O3 positive electrode materials have been conducted.14 These electrodes are difficult to operate at high current density owing to the large charge-transfer resistance in the electrode/ electrolyte interface and the low Li+-ion diffusion coefficient in the electrode material particles, especially during the anionic redox reaction.14 Considering the rapid charging/discharging of LiBs in electric vehicles, it is necessary to improve the charge-transfer and kinetic characteristics of Li-excess electrodes. We proposed the incorporation of Li oxyacids, such as Li2SO4, in conventional positive electrode materials, such as LiCoO2, to form amorphous phases.15,16 The presence of an amorphous matrix improves Li+-ion migration in the active materials, which enables high-current operation. Recently, we © XXXX American Chemical Society
Received: December 11, 2018 Accepted: February 25, 2019 Published: February 25, 2019 A
DOI: 10.1021/acsaem.8b02163 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 1. Charge−discharge property of the electrochemical cell at 25 °C. (a) Charge−discharge curves of the cell with Li2Ru0.8S0.2O3.2 positive electrode materials operated under various current densities in the voltage range of 1.6−4.2 V vs Li. The sample loading used was 2.95 mg cm−2. (b) Charge−discharge curves of the cell with Li2Ru0.5S0.5O3.5 positive electrode materials operated under various current densities in the voltage range of 1.6−4.2 V vs Li. The sample loading used was 2.02 mg cm−2. (c) Relationship between cycle number and discharge capacity for the cells based on the Li2Ru0.8S0.2O3.2 and Li2Ru0.5S0.5O3.5 active materials operated at various current densities. (d) Relationship between the obtained capacity and the operation current for the electrochemical cell based on the Li2Ru0.8S0.2O3.2 and Li2Ru0.5S0.5O3.5 active materials. (e) Ragone plots of the electrochemical cells. The energy and power densities were calculated by the weight of the active materials.
were approximately 0.2 μm, 0.5−1 μm, and 1−2 μm, respectively. Figure S4 shows the initial charge−discharge curves of the electrochemical cells using the Li2RuO3-Li2SO4 positive electrode active materials. The cell using the layered Li2RuO3 (C2/c) crystals exhibited two voltage plateaus in the initial charging state showing a capacity of about 280 mAh g−1 because of the cumulative cationic and anionic oxidation reactions described in previous reports.12,18 The cells using the mechanochemically synthesized electrode materials (e.g., disordered Li 2 RuO 3 (Fm3̅ m ), Li 2 Ru 0. 8 S 0 . 2 O 3 . 2 , and Li2Ru0.5S0.5O3.5 positive electrodes) showed an initial charge capacity of 312 mAh g−1, 288 mAh g−1, and 200 mAh g−1, respectively. These capacities were calculated from the total weights including both Li2RuO3 and Li2SO4 components. The obtained capacities were mostly consistent with the theoretical capacity expected for the Li2RuO3 component in the Li2RuO3Li2SO4 structures (329 mAh g−1, 281 mAh g−1, and 196 mAh g−1, respectively). This result indicated that all the Li+ ions in the Li2RuO3 component were extracted during the initial charging state. The incorporation of Li2SO4 in the amorphous matrix would improve the utilization of Li2RuO3 in the charge−discharge processes. Moreover, in the initial discharging process, a much higher capacity than that in the charging process was obtained in all the cells using the mechanochemically synthesized electrode materials. This result meant that excess Li+ ions were inserted into the electrode materials in the discharging state. The same phenomena were also observed in the electrochemical cells with the other electrode materials synthesized via mechanochemistry, such as LiMoO2-Li3NbO4, LiMoO2-LiF, and LiMnO2.19−21 A similar phenomenon was also seen in the Li2RuO3-Li2SO4 positive electrodes in all-solidstate batteries.17 Higher capacity was obtained in the initial discharging process than the initial charging one by the excess
reduction of Ru ions, according to X-ray absorption spectroscopy. In the electrochemical cell using organic electrolytes, almost the same electronic structural change would be expected. As shown in Figures S5 and S6, the cycle performance for the layered and disordered Li2RuO3 positive electrodes was not sufficient, and capacity fading was observed during the cycles. Mechanochemical treatment of Li2RuO3 is effective in achieving high capacity, and incorporation of Li2SO4 has a positive influence for improving cyclability. Moreover, the electrochemical cell using the layered Li2RuO3 did not show any capacity under the higher current density operation of more than 10 C rate. On the other hand, it was possible to operate the electrochemical cell with the Li2Ru0.8S0.2O3.2 positive electrode active material under high current densities at 25 °C, as shown in Figure 1a,c. The cell using Li2Ru0.8S0.2O3.2 exhibited an extremely high discharge capacity of about 350 mAh g−1 with an average discharge voltage of 2.89 V vs Li, which corresponded to the specific energy density of 1060 Wh kg−1 of the positive electrode active material when it was charged/discharged at a constant current density of 0.25 mA cm−2 (0.4 C rate). Figure 1b describes the charge−discharge curves of the cell using Li2Ru0.5S0.5O3.5. It also showed a high discharge capacity of about 223 mAh g−1 with an average discharge voltage of 2.87 V vs Li (specific energy density of 693 Wh kg−1) under the operation of a constant current density of 0.25 mA cm−2 (0.32 C rate). The cell with Li2Ru0.8S0.2O3.2 showed an excellent rate characteristic, and the cell was successfully charged and discharged even at an extremely high current density corresponding to 40 C rate at 25 °C (Figure 1c). The cell with Li2Ru0.8S0.2O3.2 showed a slight capacity deterioration, but the cell with Li2Ru0.5S0.5O3.5 exhibited a better cycle performance up to 95 cycles. Kim and Manthiram reported that the iodide-based amorphous nature of the electrode materials helps to overcome the problems B
DOI: 10.1021/acsaem.8b02163 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 2. Electrochemical analysis of the cell based on the Li2Ru0.8S0.2O3.2 positive electrode materials during the initial cycle. (a) Galvanostatic intermittent titration technique (GITT) curve for the cell based on the Li2Ru0.8S0.2O3.2 positive electrode materials during the initial cycle. The cell was charged or discharged under the constant current density of 0.25 mA cm−2 to the capacity of 20 mAh g−1 and then rested under open circuit conditions for 5 h. (b) Example of alternating current (AC) impedance plots for the cell at the charging point of no. 5. The plots were fitted using the equivalent circuit model. There were five resistant components, including the resistance for ionic conduction in liquid electrolyte (Relect) and resistance of ionic diffusion in electrode bulk (Wdiffusion). In the middle frequency region, several components were observed, such as R1, R2, and R3. Summaries of the GITT-AC impedance results during the initial charging and discharging states are shown in panels c and d, respectively.
impedance results for the cell based on Li2Ru0.8S0.2O3.2 during the initial and second cycles are described in Figures 2 and 3, respectively. Figure 2a shows the GITT curves during the initial charging/discharging states for the electrochemical cell using Li2Ru0.8S0.2O3.2 positive electrode active materials. Voltage polarization during the initial charging process was quite low, even in the later charging states. During the initial charging/discharging states, voltage hysteresis was observed. Figure 2b shows the Nyquist plots for the electrochemical cell, such as at the early charging state (no. 5). The Nyquist plots at the other charging/discharging states in the initial cycle are described in Figure S7. Depressed semicircle and spike behaviors were observed in the high- and low-frequency regions, respectively. The plots were fitted using an equivalent circuit model, as shown in Figures 2b and S9. In the highfrequency region, two components (R1 and R2) were observed; these resistances probably originated from fast electrochemical processes including interface contact with the current collector and electronic conductivity, among others.14 Moreover, charge-transfer resistance (R3) in the electrode/electrolyte interface and Warburg diffusion impedance were observed in the middle- and low-frequency regions, respectively. R3 exponentially increased in the anionic oxidation reaction region in the cell using layered Li2RuO3 positive electrode materials, as shown in Figure S13, which was one of the reasons for the slow kinetics in the anion redox reaction.7,8,14 On the other hand, R3 was maintained at a low value even in
associated with lattice distortions, thereby leading to better cycle performance.22 Similar to this report, the presence of the Li2SO4-based amorphous matrix was effective in enhancing the reversibility. Panels d and e of Figure 1 show the relationships between operation current density and discharge capacity and between energy density and average power density, namely, the Ragone plot. These energy and power densities were calculated based on the weight of the positive electrode active materials. Both of the Li2RuO3-Li2SO4 positive electrode materials showed excellent energy and power densities. It is notable that the presence of the Li2RuO3-Li2SO4 amorphous matrix was necessary for achieving better cycle performance and high current density operation. In order to understand the kinetics and thermodynamics of the Li2RuO3−Li2SO4 positive electrode materials, the galvanostatic intermittent titration technique (GITT) and alternating current (AC) impedance measurement were used. Panels a and b of Figure S7 show the relationship between the voltage and the time when the current was applied in the charging and discharging states, respectively. The linear relationship between V vs t0.5 was obtained for the charging and discharging states. Panels c and d of Figure S6 show the open circuit voltage (OCV) relaxation curves in the charging and discharging states, respectively. After 5 h, the voltage change was hardly observed, which meant that the cell was relaxed in the OCV states. These results were favorable for the calculation of the diffusion coefficient. The GITT and AC C
DOI: 10.1021/acsaem.8b02163 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 3. Electrochemical analysis of the cell based on the Li2Ru0.8S0.2O3.2 positive electrode materials during the second cycle. (a) Galvanostatic intermittent titration technique (GITT) curve for the cell based on the Li2Ru0.8S0.2O3.2 positive electrode materials during the second cycle. The cell was charged or discharged under the constant current density of 0.25 mA cm−2 to the capacity of 20 mAh g−1 and then rested under open circuit conditions for 5 h. (b) Alternating current (AC) impedance plots for the electrochemical cell at various states of charge (SOC) and depths of discharge (DOD). Summaries of the GITT-AC impedance results during the second charging and discharging states are shown in panels c and d, respectively.
calculated from the GITT curves using Fick’s diffusion equation in the spherical model,23−25 as shown in Figure S11. The calculated values of DR−2 for the Li2Ru0.8S0.2O3.2 positive electrode materials during the initial charging and discharging states are summarized in Figure 2c,d. During the whole charging state, DR−2 for Li2Ru0.8S0.2O3.2 was maintained at high values of greater than 10−4 s−1, although DR−2 for the layered Li2RuO3 electrode materials exponentially decreased during high-charging states, where anionic redox reactions mainly occurred, as shown in Figures S13 and S14. In general, the crystal structure greatly affected the diffusion properties in the crystalline electrode active materials. In the layered Li2RuO3 electrode materials, the value of DR−2 underwent complicated changes during the charging state. In Figure S13, there are two minimums for the DR−2 values during the early charging states, which was consistent with a previous report on layered Li2Ru0.75Sn0.25O3 by Tarascon’s group.14 Considering these results, the presence of an amorphous matrix helped the Li+-ion migration in the Li2RuO3-Li2SO4 active materials during the whole charging process. Although further structural analyses are needed to clarify the exact role of Li2SO4-based amorphous matrix for the improvements, we expect that Li+
the anionic oxidation reaction region in the electrochemical cells using Li2Ru0.8S0.2O3.2 positive electrode materials. Almost the same tendency was observed in the Li2Ru0.5S0.5O3.5 positive electrode materials; the detailed results for Li2Ru0.5S0.5O3.5 are shown in Figures S10 and S11. It is notable that the highly ionconducting amorphous Li2RuO3-Li2SO4 matrix contributed to the fast charge-transfer reaction. The change in the diffusion coefficient (DR−2) in the Li2RuO3-Li2SO4 positive electrode materials was investigated using GITT. We calculated the diffusion coefficient values (D) for the Li2Ru0.8S0.2O3.2, and Li2Ru0.5S0.5O3.5 positive electrode materials as reference. For the calculation, the particle sizes for these materials were determined as 0.1 and 0.2 μm from the SEM observation as shown in Figure S3. The calculated diffusion coefficient for the as-synthesized electrodes were 3 × 10−11 and 7 × 10−11 cm2 s−1, respectively. This result indicates that incorporation of Li2SO4 is effective for improving the ionic conductivity of the amorphous electrodes. In this study, the value of the diffusion coefficient (DR−2)14 was used, where D is the diffusion coefficient and R is the particle radius in order to avoid the influence of the distribution in particle radius of the synthesized materials, as shown in Figure S3. DR−2 was D
DOI: 10.1021/acsaem.8b02163 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials ions around the SO42− anion would migrate easily owing to the lower electrostatic attraction force in the amorphous structure as shown in Figure S15. Cubic α-Li2SO4 is one of the famous superionic conductors;26 thus, the presence of SO42− anion might encourage the ionic conduction in the Li2RuO3-Li2SO4 electrodes. The ionic conductivity could be increased by the mixed anion effect in the amorphous materials;27−29 thus, high ionic conductivity was possibly obtained by the incorporation of the Li2SO4 component. Furthermore, the ionic conductivity generally increased with an increase in the Li+-ion concentration in the amorphous structure.30 During the charging process, the high ionic concentration was maintained by the introduction of Li2SO4 even when Li+ ions were extracted, which would lead to retention of the high diffusion coefficient. Moreover, the low interfacial resistance and the high diffusion coefficient allowed for the achievement of high-capacity and high-rate operation. Although the detailed chemical effects of the Li2SO4 on maintaining the low interfacial resistance have not been clarified yet, it is noteworthy that incorporation of Li2SO4 into the typical crystalline positive electrode materials is effective in improving the electrode performance. In the corresponding initial discharge process for the electrochemical cell with Li2RuO3−Li2SO4 positive electrodes, DR−2 and R3 changed differently from the charging process, which was likely why voltage hysteresis between charging and discharging was observed. In the second cycle, the cell based on the Li2Ru0.8S0.2O3.2 positive electrode materials showed a high reversible capacity of greater than 300 mAh g−1 with low voltage hysteresis, which was caused by maintaining the high diffusion property and low interfacial resistance during the entire charge−discharge processes (Figure 3). In the cationic redox region, the resistance decreased with an increase in the state of charge (SOC), while DR−2 was almost unchanged. On the other hand, in the anionic redox region, only a slight decrease in DR−2 was observed with charging during the second charging state, as shown in Figure 3c. In the Nyquist plot, the length of the Warburg tail became longer with the increase in the SOC (Figure 3b). This result clearly revealed that the diffusion coefficient decreased, which was the same tendency as that for the DR−2 value calculated from the GITT curves. However, the degree of the decrease was lower than that of the layered Li2RuO3 positive electrode materials. This was the reason for the high rate of operation in the electrochemical cells using Li2RuO3-Li2SO4 positive electrode materials. In this report, the electrochemical properties of the Li2RuO3-Li2SO4 positive electrode materials were investigated using GITT and AC impedance measurements. Because of the presence of the amorphous matrix with Li2SO4, a high diffusion coefficient and low interfacial resistance were maintained during the overall charge−discharge processes, thereby leading to the reversible operation under an extremely high current density. From the viewpoint of rapid charging/discharging, the diffusion coefficient and interfacial resistance are important. The results reported in this letter are useful for achieving properties in the Li-excess positive electrode materials for high capacity.
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Experimental details, structural evaluation, SEM images, charge−discharge performance, AC impedance analyses, and GITT (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel.: +81-72-254-9333. Fax.: +81-72-254-9910. E-mail:
[email protected]. ORCID
Atsushi Sakuda: 0000-0002-9214-0347 Akitoshi Hayashi: 0000-0001-9503-5561 Funding
This research was financially supported by JSPS KAKENHI Grant No. JP18J14547. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We appreciate Dr. H. Tsukasaki and Prof. S. Mori of Osaka Prefecture University for TEM observation. REFERENCES
(1) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. LixCoO2 (0
