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Improving the High-Voltage Li2FeMn3O8 Cathode by Chlorine Doping Jiaqi Dai, Lihui Zhou, Xiaogang Han, Marcus Carter, and Liangbing Hu* Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States ABSTRACT: High-capacity and high-voltage cathode materials are desirable for high-energy-density lithium ion batteries. Among various cathode materials, Li2FeMn3O8 is attractive due to its high working voltage, low toxicity, and low cost. However, its superior electrochemical properties are significantly limited by the intrinsic defects in the Li2FeMn3O8 cathode, which makes the theoretical working voltage (4.9 V) and capacity (148 mAh/g) hard to reach. In this paper, we demonstrated that Cl doping can effectively increase the capacity and working voltage of the Li2FeMn3O8 cathode. Xray photoelectron spectroscopy reveals that Cl doping reduced the valence state and increased the electron binding energy in cations and thus increased the voltage and enhanced the capacity of the Li2FeMn3O8 cathode. Our results also indicate that Cl doping can be a promising low-cost method to improve the electrochemical performance of various oxide cathode materials, including LiCoO2 and LiMn2O4. KEYWORDS: lithium ion batteries, high-voltage cathode, chlorine doping, Li2FeMn3O8
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of LFMO is around 148 mAh/g.21 Moreover, LFMO delivers a working potential of 4.8 V,20,21 and it would be highly beneficial for practical applications if the working voltage could be further increased. General methods of improving cathode electrochemical performances include doping,23,24 nanostructure constructions,25,26 and surface modifications.27,28 In terms of stabilizing the cycling performance and improving the capacity, various kinds of spinel-coated layered Li-rich cathodes have been reported by many pioneering works.29−35 As for increasing the working potential, elemental doping is very effective. For example, chromium,36,37 boron,38 fluorine,39 and chlorine doping have been reported as effective dopants to improve the electrochemical performances of various electrode materials for LIBs,40−43 including LiMnPO4, LiNi0.8Co0.15Al0.05O2, and LiFePO4. However, its effect on increasing the cathode voltage is rarely reported. Cl ions more easily lose electrons than oxygen ions; Cl insertion into oxide cathodes will, therefore, influence the distribution of electrons between various ions and the oxidation state of transition metals, which would facilitate more efficient Li ion insertion and extraction.44 In this study, we report a chlorine-doped LFMO cathode, which is synthesized by simple solid-state reactions. We observed the successful doping of Cl ions into the LFMO, which increased the working voltage to 5 V and first cycle capacity to 130 mAh/g. Moreover, utilizing X-ray photoelectron spectroscopy (XPS) technology, we demonstrated that
INTRODUCTION The development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has become increasingly attractive as an effective solution to the growing need for an already limited supply of fossil fuels. Recent advances in vehicle technologies, for example, lightweight materials and high-efficiency engines, have made EVs more competitive and reliable. However, EVs are still not comparable to traditional vehicles in terms of driving ranges and durability, due to the limitations of battery technologies. Increasing the energy density of lithium ion batteries (LIBs) is a key solution to improve the driving ranges of EVs, and has always been an alluring target for research in LIBs. General approaches include reducing the current collector weight,1−3 raising the electrode capacity,4−8 and increasing the working voltage of the cathode. The application of high-voltage cathodes was limited by the low decomposition voltage of traditional liquid electrolytes.9−11 With the emergence of solid-state electrolytes12 and advances in highvoltage electrolytes,13−15 improving the energy density by increasing the cell voltage has become possible. Many spinel structured oxides have been reported as highvoltage cathodes for LIBs, for example, Li2NiMn3O8 (4.7 V),16,17 Li2CrMn3O8 (4.8 V),18 Li2.02Cu0.64Mn3.34O8 (4.9 V),19 Li2FeMn3O8 (4.8 V),20,21 and LiCoMnO4 (5.1 V).22 Compared with cathodes containing nickel, chromium, and cobalt, Li2FeMn3O8 (LFMO) is more attractive due to its low cost, abundance, and nontoxicity. Typically, LFMO has a spinel structure which provides a stable framework and 3D pathways for Li diffusion. LFMO has two reversible plateaus at ∼4.1 V and ∼4.9 V vs Li/Li+, caused by the valence change of Mn3+ to Mn4+ and Fe3+ to Fe4+, respectively.20 The theoretical capacity © XXXX American Chemical Society
Received: January 8, 2016 Accepted: April 5, 2016
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DOI: 10.1021/acsami.6b00288 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) XRD pattern of Li2FeMn3O8−0.5xClx with different chlorine concentrations [(a-a) x = 0; (a-b) x = 0.67; (a-c) x = 1.0; (a-d) x = 1.5], (a, inset) comparison of (440) peak locations, and (b) comparison of their lattice constants.
Figure 2. SEM images of Li2FeMn3O8−0.5xClx with various chlorine contents: (a) x = 0; (b) x = 0.67; (c) x = 1.0; (d) x = 1.5. a Hitachi SU-70 scanning electron microscope using a Schottky field emission gun. To prepare the electrodes for battery testing, the cathode powder was mixed with carbon black using hand grinding. Then a 2 wt % solution of poly(vinylidene fluoride) (PVDF) in N-methyl-2pyrrolidone (NMP) was added to the powder mixture. The proportions of carbon black and PVDF were both 10 wt %. The mixture was hand-ground for 1 h to obtain a homogeneous slurry. Then the slurry was coated onto aluminum foil using a doctor blade (the as-coated thickness was 30 μm). The resulting electrode sheet was then dried in a vacuum oven at 90 °C for 12 h and then punched into round disks for battery assembly. The electrode was assembled into coin cells with lithium metal as the counter electrode and a highvoltage LiPF6/FEC (fluoroethylene carbonate) electrolyte.48 The cells were cycled at a 0.5C rate in a potential range of 3.5−5.3 V vs Li/Li+ using Bio-Logic SAS battery testers (SP-150 and VMP3).
the improvement in the voltage and capacity was due to an increase in the electron binding energy of various ions in the LFMO cathode after Cl doping.
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EXPERIMENTAL SECTION
We use Li2FeMn3O8−0.5xClx to denote Cl-doped LFMO, where x represents the nominal atomic ratio of Cl added in the synthesis, and varies from 0 to 1.5 for the different sample groups. Li2FeMn3O8−0.5xClx nanopowder was prepared by a combustion method using glycine and nitrate as the combustion agents.45−47 Lithium nitrate, cobalt nitrate, iron nitrate, lithium chloride, and manganese nitrate were mixed according to stoichiometric molar ratios and dissolved in deionized (DI) water. Glycine was then dissolved in DI water and added to the nitrate solution under stirring. The molar ratio21 of glycine to nitrate was 1:2. The mixture solution was heated to 300 °C, and a sudden ignition occurred after the complete evaporation of water, leaving black powders. The final product was obtained by calcining these black powders at 700 °C for 2 h in air. X-ray diffraction (XRD) patterns were measured with a Bruker D8 Advance X-ray diffraction system (Cu Kα radiation, λ = 1.54060 Å). The XPS data were collected by a Kratos Axis 165 analyzing system. The morphology of the cathode powders before and after Cl doping was observed with
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RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of Cl-doped LFMO with various dopant concentrations (a−e). The diffraction peaks matched the cubic phase Li2FeMn3O8 (JCPDS no. 48-0258), which was in agreement with Hiroo Kawai’s results.22 B
DOI: 10.1021/acsami.6b00288 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) XPS survey spectrum of Li2FeMn3O7.25Cl1.5. Each peak is labeled with its corresponding states. (b) Detailed plot showing the Cl 2p XPS spectrum. (c) Detailed plot showing the Mn 3s XPS spectrum. (d) Detailed plot showing the Mn 2p XPS spectrum. The shoulder is marked with a short black line.
Compared with the undoped LFMO (x = 0), samples with various Cl contents showed no differences in XRD patterns (b−e). This result indicates that Cl was doped into the lattice site instead of forming impurities, or the impurities formed were too little to be detected. Figure 1a (inset) also shows the shift of peak locations. As the Cl content increases, all the peaks, especially those at higher angles, shift toward lower angles. The shifting of the peaks indicates changes in the lattice constant. A shrinking 2θ value is a result of growing d spacing, which also reflects an expanding interlattice distance, according to Bragg’s law.49 To confirm the changes in the lattice constant, a Rietveld refinement was performed on all samples. Figure 1b shows the lattice constant of cubic Li2FeMn3O8−0.5xClx with various Cl contents (x values). The lattice constant of standard Li2FeMn3O8 is plotted as a single round dot in the figure for comparison. Both the standard LFMO (JCPDS no. 48-0258) and the undoped samples (x = 0) used in this study were synthesized in the same conditions. However, the standard LFMO was annealed in oxygen for 2 days after the synthesis, which ensured that almost all the possible vacancies were fully occupied. The dramatic difference between the undoped samples and the standard LFMO was a result of oxygen deficiency, which was also strong evidence proving the existence of abundant oxygen vacancies in the undoped LFMO. As Cl enters the lattice, these oxygen vacancies will be gradually filled up. Cl ions have a larger size than oxygen
ions. Therefore, the occupation of Cl on oxygen sites expands the lattice with an increasing concentration of Cl until, finally, the lattice constant exceeds that of a perfect LFMO crystal. These results not only prove that Cl was doped into a host site, but also reveal that the host site was the intrinsic oxygen vacancy. The scanning electron microscopy (SEM) images of Li2FeMn3O8−0.5xClx with various Cl contents are shown in Figure 2. The undoped LFMO particles did not show any specific shapes. The particle size was about 200 nm; however, most of the small particles grew on each other, forming secondary particles which were larger than 10 μm (Figure 2a). With an increase in the chlorine content, small crystal seeds started to grow and develop into individual large crystal particles with an octahedral shape (Figure 2b). Particles with higher chlorine content (Figure 2c,d) showed clear facets and edges resulting from a typical cubic crystal growth with strong direction preferences.50 The well-defined octahedral shape indicates that the crystals were well developed.51 In addition, an increasing chlorine content was correlated with increasing particle size. For example, the particle size of Li2FeMn3O7.5Cl ranged from 500 nm to 1 μm, but for Li2FeMn3O7.25Cl1.5, it was about 2 μm. The change in morphology and particle size was a direct result of chlorine doping. Oxygen, manganese, and iron ions build up the framework of LFMO. The lower occupancy on the oxygen sites generally obstructs the crystal growth. Upon doping, chlorine C
DOI: 10.1021/acsami.6b00288 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Cyclic voltammetry, (b) rate performance, (c) cycling performance, and (d) voltage profile comparison of Li2FeMn3O8−0.5xClx with different chlorine concentrations.
difference between oxygen and chlorine. Similar effects are also found on Fe, which can be identified by the distinct shakeup satellite about 8 eV above its main peak. This evidence suggests that the chlorine ions reduced the valence state of the adjacent ions, which increases the energy barrier of the redox reactions. In addition, the binding energy of Mn 2p electrons in the studied specimen is larger than that of the previously reported undoped LFMO.52 In conclusion, the doping of chlorine ions increases the energy barrier for the redox reactions by reducing the valence state and increasing the electron binding energy of the cations, which leads to a higher working voltage of the electrodes. Figure 4a shows the cyclic voltammetry (CV) curve comparisons of the Li2FeMn3O8−0.5xClx cathode with various Cl contents. The test cell consisted of the Li2FeMn3O8−0.5xClx cathode slurry-coated on aluminum foil, a high-voltage LiPF6/ FEC electrolyte, and lithium metal as the counter electrode. The scanning range was 3−5.3 V with a rate of 0.1 mV/s. All the samples showed signature peaks around 4.3 and 4.9 V which resulted from the valence change of Mn3+ to Mn4+ and Fe3+ to Fe4+, respectively. The sample with the higher chlorine concentration shows a smaller polarity, which indicates better reversibility. Figure 4b compares the rate performances of the Li2FeMn3O8−0.5xClx cathode with various Cl contents. Despite the difference in specific capacity, cathodes with different chlorine concentrations behave similarly in the rate performance. After cycling at a high rate (5C), the cathode capacity
ions get into oxygen sites (vacancies), which provides more “building blocks” to construct the crystal framework. Therefore, the particles grow larger and the octahedral shape becomes more apparent when the chlorine content in Li2FeMn3O8−0.5xClx increases. XPS analysis was performed on Li2FeMn3O7.25Cl1.5 to investigate how chlorine doping influences ions adjacent to oxygen vacancies. Figure 3 shows the total XPS survey spectrum and the spectra of Cl 2p electrons, Mn 3s electrons, and Mn 2p electrons of the specimen. The XPS survey spectrum indicates that the sample surface consisted of C, O, Fe, Mn, and Cl. The presence and atomic percentage of Li are unable to be confirmed or calculated from the survey spectrum due to the overlap of the Li 1s signal with the Fe 3p signal (Figure 3a). Disregarding the lack of Li information, the relative atomic percentage of the rest of the elements was confirmed and calculated. As direct evidence of chlorine doping, the Cl 2p signal can be found in the survey spectrum, and the atomic percentage is 0.5% (Figure 3b). Figure 3c shows the separation in the multiplet spin states of the Mn 3s signal, which indicates that the majority of Mn is in the 4+ state in the specimen. However, a shoulder found on the low-binding-energy side of the Mn 2p signal indicates that there exists a small amount of Mn in a lower oxidation state (Figure 3d). Given the fact that the distinct shakeup satellite for Mn2+ at 647 eV cannot be found, the lower oxidation state Mn is most likely to be Mn3+. The valence-state drop of Mn compensates for the valence-state D
DOI: 10.1021/acsami.6b00288 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Density, Solid-State Li-Ion Batteries”). We acknowledge Dr. Kang Xu from the Army Research Lab, Dr. Eric Wachsman from the University of Maryland, and Dr. Venkataraman Thangadurai from the University of Calgary for their valuable suggestions. Mr. Greg Hitz from the University of Maryland, Dr. Karen Gaskell from the Maryland Surface Analysis Center, and the Maryland NanoCenter (AIMLab) are also graciously acknowledged.
recovers 87.5%, 87.0%, and 79.4% of its initial capacity, respectively. Concluding from the results, chlorine doping has little effect on the rate performance improvement. Figure 4c shows the cycling performances of Li2FeMn3O8−0.5xClx with various chlorine concentrations. The cells were cycled between 3.5 and 5.3 V, which exceeded the stability window of the electrolyte.53 Therefore, the capacity gradually faded during charging/discharging as a result of the slow decomposition of both the electrolyte and electrode. With an increase of the chlorine concentration, the capacity increases and then drops. Li2FeMn3O8−0.5xClx (x = 1.5) achieved the best performance among all the tested compositions. This trend can be explained by the increased crystal structure instability due to the overdistorted lattice generated by the excessive amount of chlorine. With a low chlorine content, the distortion caused by intrinsic oxygen vacancies is neutralized by dopant insertion, thus stabilizing the crystal structure. However, when the chlorine content exceeds a critical value, the excessive dopants will overdistort the lattice, which leads to the structural breakdown. Figure 4d shows the voltage profiles of Li2FeMn3O7.5Cl and Li2FeMn3O8. Both samples show two reversible plateaus for Mn3+/Mn4+ and Fe3+/Fe4+ redox couples, respectively; however, the charging/discharging voltage of the chlorine-doped LFMO is higher. The specific capacity of LFMO was about 130 mAh/g, which was 2.6 times the undoped LFMO capacity and approached 90% of the theoretical capacity (Figure 4d). The electrochemical performance suggests that chlorine doping can effectively increase the specific capacity and working voltage of the LFMO cathode.
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CONCLUSION Chlorine-doped Li2FeMn3O8 high-voltage cathodes with various dopant concentrations were successfully synthesized using a simple combustion method. The influence of chlorine doping on the performance of the LFMO cathodes was investigated with XRD, SEM, and battery cycling tests. The performance enhancement was studied using XPS. Our results show that chlorine ions filled up the intrinsic oxygen vacancies in LFMO crystals upon doping, which led to the lattice expansion, valence-state reduction, and electron binding energy increase of cations adjacent to the vacancies. Thus, the energy barrier for the redox reactions of the cathode was increased. Accordingly, the electrochemical performance was effectively improved by chlorine doping. The Li2FeMn3O7.5Cl cathode showed superior performance over the undoped LFMO with a specific discharge capacity of 130 mAh/g at 0.5C and a working voltage of 5 V. The specific capacity was 2.6 times the undoped LFMO capacity and approached 90% of the theoretical capacity. Our results reveal the electrochemical improvement in the LFMO to be due to chlorine doping. This simple, lowcost fabrication method can also be applied on other spinel cathodes, following a similar mechanism.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy ARPA-E RANGE (entitled “Safe, Low-Cost, High-EnergyE
DOI: 10.1021/acsami.6b00288 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.6b00288 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
