NASICON-Type Mg0.5Ti2

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A NASICON-type Mg0.5Ti2(PO4)3 Negative Electrode Material Exhibits Different Electrochemical Energy Storage Mechanisms in Na-Ion and Li-Ion Batteries Yingying Zhao, Zhixuan Wei, Qiang Pang, Yingjin Wei, Yongmao Cai, Qiang Fu, Fei Du, Angelina Sarapulova, Helmut Ehrenberg, Bingbing Liu, and Gang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14196 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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ACS Applied Materials & Interfaces

A NASICON-type Mg0.5Ti2(PO4)3 Negative Electrode Material Exhibits Different Electrochemical Energy Storage Mechanisms in Na-Ion and Li-Ion Batteries Yingying Zhaoa, Zhixuan Weia, Qiang Panga, Yingjin Weia*, Yongmao Caib, Qiang Fuc, Fei Dua*, Angelina Sarapulovac, Helmut Ehrenbergc, Bingbing Liud, and Gang Chena,d

a

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, P. R. China.

b

College of Science, Northeast Dianli University, Jilin 132012, P. R. China.

c

Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen, Germany. d

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R. China.

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ABSTRACT: A Carbon-coated Mg0.5Ti2(PO4)3 polyanion material was prepared by the sol-gel method and then studied as the negative electrode materials for lithium-ion and sodium-ion batteries. The material showed a specific capacity of 268.6 mAh g-1 in the voltage window of 0.01-3.0 V vs. Na+/Na0. Due to the fast diffusion of Na+ in the NASICON framework, the material exhibited superior rate capability with a specific capacity of 94.4 mAh g-1 at a current density of 5A g-1. Additionally, 99.1 % capacity retention was achieved after 300 cycles, demonstrating excellent cycle stability. By comparison, Mg0.5Ti2(PO4)3 delivered 629.2 mAh g-1 in 0.01-3.0 V vs. Li+/Li0, much higher than that of the sodium-ion cells. During the first discharge, the material decomposed to Ti/Mg nano particles, which were encapsulated in an amorphous SEI and Li3PO4 matrix. Li+ ions were stored in the Li3PO4 matrix and the SEI film formed/decomposed in subsequent cycles, contributing to the large Li+ capacity of Mg0.5Ti2(PO4)3. However, the lithium-ion cells exhibited inferior rate capability and cycle stability compared to the sodium-ion cells due to the sluggish electrochemical kinetics of the electrode.

KEYWORDS: lithium ion battery, sodium ion battery, anode material, magnesium titanium phosphate, electrochemical properties

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INTRODUCTION Over the past decades, the greenhouse effect and limited fossil fuel resources have promoted worldwide interest in the development of sustainable/renewable energy and electrical transportation. One of the most serious bottlenecks of these technologies is energy storage. Lithium ion batteries (LIBs) are widely used in portable electronics. However, the energy density of current automotive LIBs remains far less than that of traditional gasoline engines. Moreover, the limited and unevenly distributed global lithium resources significantly limit the expansion of large-scale LIBs in these fields. The standard electrode potential of Na+/Na0 is only ~ 0.3 V higher than that of Li+/Li0, and sodium compounds are abundant and cheap resources. For these reasons, sodium-ion batteries (SIBs) may be a viable option for large-scale energy storage, where cycle life and cost are more essential factors than energy density. A major focus of current research on LIBs and SIBs is the development of new positive and negative electrode materials with large capacity, high rate capability, and long cycle life. According to Maier et al.1, the lithium storage mechanisms of LIB electrode materials include the intercalation reaction (absorptive mechanism, such as performed by LiCoO2 and graphite)2-3, the heterogeneous reaction (reactive mechanism, such as performed by SnO2 and Si)4-5, and the interfacial reaction (adsorptive mechanism, such as performed by Ti/LiF and Fe/Li3PO4)6-7. Most state-of-the-art sodium storage materials use the intercalation reaction (materials including hard carbon and Na0.44MnO2)8-9 or the heterogeneous reaction (materials such as SnSb and SnS2)10-11. However, very few materials have been reported that can utilize the interfacial Na+ storage reaction. Because Li+ and Na+ can transport in some large tunnel or three-dimensional (3D) frameworks with low activation energy, 3



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NASICON-type materials with the formula unit of AxMy(PO4)3 have attracted recent interest for use in LIBs and SIBs. AxMy(PO4)3 can be viewed as a 3D framework, in which two MO6 octahedra and three PO4 tetrahedra are connected by corner-sharing oxygens. This framework forms one M1 void and three larger M2 voids, which can be occupied by Li and Na ions. The AxMy(PO4)3 compounds have rich compositions, where “A” can be either monovalent (Li+ or Na+) or divalent (Cu2+, Mg2+, or Mn2+) ions, and M can be a transitional metal ion (V5+, Ti4+, or Fe3+). Most of the studied AxMy(PO4)3 are alkaline metal-based, such as A3V2(PO4)3 and ATi2(PO4)3, (A = Li, Na). Of these materials, A3V2(PO4)3 is considered a promising positive electrode for LIBs and SIBs because of its large specific capacity and excellent rate capability12-15, and ATi2(PO4)3 is usually considered as a negative electrode because of its low working voltage vs. Li+/Li and Na+/Na16-19. This work focused on the alkaline earth metal-based NASICON-type material Mg0.5Ti2(PO4)3. First described by Barth et al.20, half of the M1 void in this material is filled by Mg2+ ions. Abarca et al. studied the Li ion and Na ion storage properties of Mg0.5Ti2(PO4)3 in the voltage window of 1.0-4.2 V21. However, only limited capacity was obtained (119 mAh g-1 for LIB and 97 mAh g-1 for SIB at the C/20 rate), which was insufficient for practical applications. In this work, we explored the efficacy of using carbon-coated Mg0.5Ti2(PO4)3 as the negative electrode for LIBs and SIBs. Carbon coating was used to improve the electronic conductivity of Mg0.5Ti2(PO4)3 as applied for many other polyanion electrode materials. The material showed very different electrochemical properties in LIBs and SIBs. A fast Na+ intercalation mechanism was observed for the SIB system, resulting in a specific capacity of 268.6 mAh g-1. In contrast, reversible interfacial Li+ storage and formation/decomposition of the solid electrolyte interface (SEI) film was demonstrated for the LIB system, 4


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resulting in a much larger specific capacity of 629.2 mAh g-1. The detailed reaction mechanisms of Mg0.5Ti2(PO4)3 in the SIB and LIB systems were studied using various electrochemical and spectroscopic techniques. The results provide new insights into understanding effects on the electrochemical performance of a polyanion host material for storage of different alkali-metal ions. EXPERIMENTAL SECTION Preparation of carbon-coated Mg0.5Ti2(PO4)3 The carbon-coated Mg0.5Ti2(PO4)3 was prepared by the sol-gel method. First, magnesium acetate (Mg(CH3COO)2, 99.9%) and ammonium dihydrogen phosphate (NH4H2PO4, 99.9%) were dissolved in de-ionized water at a molar ratio of 1 : 6. Next, 50 ml 0.2 M tetraisopropyltitanate ((CH3CH3CHO)4Ti, 99.9%) ethyl alcohol solution was added dropwise. Next, 0.02 M citric acid was added into this solution to a molar ratio of Mg0.5Ti2(PO4)3 to citric acid of 1 : 1. This viscous solution was heat treated at 80 °C for 12 h, and then dried at 120 °C for 12 h. The dried material was then pre-treated at 350 °C for 5 h, followed by sintering at 800 °C for 12 h under argon flow to obtain the carbon-coated Mg0.5Ti2(PO4)3. Characterization of the material X-ray diffraction (XRD) was performed on a DX-2700B X-ray diffractometer with Cu Kα radiation. The morphological properties of the material were studied by a Hitachi SU8020 scanning electron microscope (SEM). High-resolution transmission electron microscope (HRTEM) was performed on a FEI Tacnai G2 instrument coupled with a BRUKER AXS X-ray energy dispersive spectroscopy (EDX). Raman scattering was studied using a LabRAM HR Evolution Raman spectrometer with Ar-ion laser excitation. The carbon content in the material was determined by thermal gravimetric analysis (TGA) on a SDT Q600-1649 instrument. X-ray photoelectron 5



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spectroscopy (XPS) was performed on an ESCALAB spectrometer using Mg-Kα light source. The formation energy of the electrochemical decomposition products was determined by first principle calculations using the Vienna ab initio simulation package (VSAP)22-23 with the projector augmented-wave (PAW) method24 and the generalized gradient approximation (GGA)25. The energy cut-off was chosen as 450 eV and the k meshes containing at least 172 points in the primitive Brillouin zone.

Electrochemical experiments The electrochemical properties of the material were studied with 2032 coin cells. The positive electrode was composed of 70 wt.% carbon coated Mg0.5Ti2(PO4)3, 20 wt.% super P active carbon, and 10 wt.% poly-vinylidenefluoride binder, which was pasted on a copper current collector using N-methyl-2-pyrrolidone. The negative electrode used in the coin cells was metallic Li (for LIB) or metallic Na (for SIB). A piece of Celgard 2320 membrane (for LIB) and Whatman GF/C glass fiber filter (for SIB) was used as the separator. For the LIB tests, the electrolyte was 1.0 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate (EC), dimethylcarbonate (DMC), and ethyl methyl carbonate (EMC) (1:1:8 by v/v ratio). For the SIB tests, the electrolyte was 1.0 M sodium perchlorate (NaClO4) dissolved in ethylene carbonate (EC) and propylene carbonate (PC) (1:1 by v/v ratio). Galvonostatic charge-discharge cycling was performed on a LAND-2010 automatic battery tester. Cyclic voltammetry (CV) was performed on a Bio-Logic VSP multichannel electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was performed on the VSP instrument using an ac voltage of 5 mV in the frequency range of 1 MHz to 1 mHz. For ex-situ XRD, TEM, and XPS experiments, the LIB and SIB cells were disassembled after charging or discharging the electrodes at certain voltages. The electrode composite was scraped off the electrode and carefully washed by DMC to remove any 6


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absorbance. RESULTS AND DISCUSSION Structure and morphology The crystal structure of the material was studied by X-ray diffraction, as shown in Figure 1. Rietveld refinement of the Mg0.5Ti2(PO4)3 phase was performed based on the R-3c space group, as suggested by Barth et al.20 and the results are provided in Table S1 (Supporting Information). The inset of Figure 1 shows a schematic diagram of the crystal structure of Mg0.5Ti2(PO4)3. It shows that the M1 site of Mg0.5Ti2(PO4)3 is the 6b (0, 0, 0) Wyckoff site, and is half occupied by Mg2+ ions. The M2 18e-site is fixed at (0.645, 0, 0.25) and is not occupied. The lattice parameters of Mg0.5Ti2(PO4)3 were refined as a = 8.5163(2)Å and c = 20.9413(8) Å. In addition, the strong peaks at ~ 23o and ~ 27o indicate a significant amount of TiP2O7 impurity, determined as 14.4 wt.% by the Rietveld refinement. According to Barth et al20. formation of TiP2O7 was a result of enrichment in Mg forming a small portion of Mg0.5+xMgxTi2x(PO4)3 with simultaneous separation of TiP2O7 from Mg0.5Ti2(PO4)3. The enrichment in Mg could be resulted from the inhomogeneous distribution of Mg in the precursor since the weight percentage of Mg(CH3COO)2 in the precursor was only 7 %. The large amount of TiP2O7 impurity in the present Mg0.5Ti2(PO4)3 product could seriously affect the electrochemical properties of the material. Future work should be done to improve the phase purity of Mg0.5Ti2(PO4)3.

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Figure 1. Rietveld refinement of the Mg0.5Ti2(PO4)3 material. The inset shows a schematic diagram of the crystal structure of Mg0.5Ti2(PO4)3. Scanning electron microscopy showed that the carbon-coated Mg0.5Ti2(PO4)3 was composed of aggregated particles of irregular shape (Figure 2a). The particles were not uniform in size, but most of them were in the range of 100 ~ 300 nm. Next, the microstructure of the material was studied using a high-resolution transmission electron microscope (HRTEM) as shown in Figure 2b. The fast Fourier transform (FFT) analysis of HRTEM imaging confirmed the (113), (226) and (146) planes of hexagonal Mg0.5Ti2(PO4)3, which are in agreement with the XRD results. The ca. 5 nm thickness of the surface layer was due to the surface carbon coating, which was further confirmed by Raman scattering. Figure S1 (Supporting Information) shows that two strong Raman bands were observed in the high wavenumber region, assigned to the D-band (1331.5 cm-1, disordered sp3 mode) and the G-band (1571.8 cm-1, graphitic sp2 mode) of carbonaceous materials. The intensity ratio of the D to G bands (ID/IG) was about 1.02, indicating a low graphitic degree of the surface carbon. In 8


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addition, the Raman bands observed at 900 - 1100 cm-1 resulted from the PO4 groups of Mg0.5Ti2(PO4)3. The thermal gravimetric (TG) analysis shows a continuous weight loss in the temperature range of 100 - 600 oC (Figure S2, Supporting Information). This indicates that the content of carbon in the material was 3.3 wt.%. Next, the uniformity of different elements of the carbon coated Mg0.5Ti2(PO4)3 was studied by EDX analysis. Figure S3 (Supporting Information) shows the STEM image and elemental mappings of the material, which confirms that the Mg, Ti, P, O and C elements were evenly distributed throughout the material.

Figure 2. SEM (a) and HRTEM (b) images of the Mg0.5Ti2(PO4)3 material. Charge-discharge performance in 0.01 - 3.0 V vs. Na+/Na0 and Li+/Li0 The charge-discharge performance of the carbon-coated Mg0.5Ti2(PO4)3 as the negative electrode for SIBs and LIBs was studied in the voltage window of 0.01 - 3.0 V vs. Na+/Na0 and Li+/Li0. Assuming that the vacant M1 and M2 voids of Mg0.5Ti2(PO4)3 became occupied by inserted ions, 3.5 Na+ ions per formula unit could intercalate into the material producing a theoretical capacity of 238.8 mAh g-1. Figure 3a shows the charge-discharge profiles of Mg0.5Ti2(PO4)3 in the SIB cells at the 20 mA g-1 current density. The discharge capacity of the second cycle was 268.6 mAh g-1, about 11.1 % of which was attributed to Na+ ion storage in the super P conductive 9



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additive (Figure S4, Supporting Information). The excessive capacity of the first discharge was attributed to the formation of SEI film (confirmed below). All the charge-discharge profiles are analogous starting at the second cycle, indicative of excellent reversibility during repeated Na+ insertion and extraction. In addition, the material showed good cycle stability, as 84.7 % of its second discharge was retained after 50 cycles (Figure 3b). The capacity retention was even better at higher current rates. As shown in Figure 3c, the material could deliver 159.8 mAh g-1 at the 1 A g-1 current density, and 99.1 % of the capacity was retained after 300 cycles. Further, the rate capability of Mg0.5Ti2(PO4)3 was studied with progressively increased current densities. As shown in Figure 3d, the material showed reversible capacities of 203.4, 192.6, 173.2, 151.3, 125.2, and 94.4 mAh g-1 at the current densities of 100, 200, 500, 1000, 2000, and 5000 mA g-1, respectively. When the applied current returned to 100 mA g-1, the specific capacity returned to 207.8 mAh g-1, demonstrating the excellent rate capability of the SIB cell. The other feature of interest is that the average Na storage voltage of Mg0.5Ti2(PO4)3 was ca. 0.8 V, higher than that of hard carbon (~ 0.1 V)26 and Na2Ti3O7 (~ 0.3 V)27. Hence, using Mg0.5Ti2(PO4)3 as a negative electrode could effectively avoid the risk of sodium plating. We constructed a demonstrating full Na-ion battery using Na3V2(PO4)2F3 as the positive electrode and Mg0.5Ti2(PO4)3 as the negative electrode. Previous work showed that Na3V2(PO4)2F3 is a promising positive electrode material for Na-ion batteries exhibiting a high average voltage of 3.6V28 . Preliminary results showed that the battery produced an average discharge voltage of ~ 2.4 V and a discharge capacity of 101.9 mAh g-1 at 0.5 C rate with reasonable cycle stability (Figure S5, Supporting Information). These result were comparable to those of the Na0.66[Li0.22Ti0.78]O2//Na3V2(PO4)3 full cell reported by Hu et al. which showed an average discharge voltage of ~ 2.5 V and a 10


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specific capacity of 85 mAh g-1 29.

Figure 3. Charge-discharge properties of Mg0.5Ti2(PO4)3 in SIB cells: (a) voltage profiles, (b) cycling performance at 20 mA g-1, (c) cycling performance at 1 A g-1, and (d) rate-dependent cycling performance.

Figure 4a shows the charge-discharge profiles of Mg0.5Ti2(PO4)3 in LIB cells at the 100 mA g-1 current density. The first discharge profile was different from the subsequent ones, indicating that the material underwent irreversible structural changes during the initial electrochemical process. In addition, a specific capacity of 1237.1 mAh g-1 was obtained for the first discharge, much larger than the assumed theoretical capacity based on the insertion of 3.5 Li+ ions per formula unit of Mg0.5Ti2(PO4)3. Thus, it is likely that Mg0.5Ti2(PO4)3 exhibited alternate electrochemical mechanisms instead of the Li+ intercalation reaction when used as a negative electrode for LIB. The material showed irreversible capacity loss during the first charge, resulting in a Coulombic efficiency of 50.6 %. A discharge capacity of 629.2 mAhg-1 was obtained in the second cycle, and was well maintained in subsequent cycles (Figure 4b). About 11



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6.3 % of the capacity was attributed to Li+ ion storage in the super P conductive additive (Figure S6, Supporting Information). The rate capability test showed that the material maintained a large initial capacity of 399 mAh g-1 at the 1 A g-1 current density (Figure 4c). However, the material showed relatively fast capacity fading at this high current rate. The capacity retention was only 45.2 % after 300 cycles. In addition, the rate capability of Mg0.5Ti2(PO4)3 in LIBs was worse than that in SIBs (Figure 4d). For example, as the current density increased to 5 A g-1, only 80.4 mAh g-1 was obtained for the LIB cell, but 94.4 mAh g-1 was obtained for the SIB cell at the same current density.

Figure 4. Charge-discharge properties of Mg0.5Ti2(PO4)3 in LIB cells: (a) voltage profiles, (b) cycling performance at 100 mA g-1, (c) cycling performance at 1 A g-1, and (d) rate-dependent cycling performance. Reaction mechanism of the SIB cells The significant differences between the cycling performance of Mg0.5Ti2(PO4)3 in SIB and LIB suggest that the material may use different electrochemical mechanisms in the two battery systems. We first studied the reaction mechanism of 12


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Mg0.5Ti2(PO4)3 in SIB. Figure 5 shows the CV curves of the material in the initial three cycles recorded at a scan rate of 0.05 mV s-1. The broad current peak at 1.1 V and the small one at 0.45 V were absent in subsequent cycles. These current peaks were attributed to the formation of SEI film, as further confirmed by EIS analysis. Figure 6 shows a series of Nyquist plots of Mg0.5Ti2(PO4)3 during the first cycle. Only one semicircle was observed at the discharge state of 1.5 V, which was attributed to the charge transfer process. When the electrode was discharged to 1.0 V, a new semicircle was observed in the high-to-medium frequency region. Appearance of this semicircle demonstrates the formation of SEI.

Figure 5. CV curves of Mg0.5Ti2(PO4)3 in SIB cells recorded at a scan rate of 0.05 mV s-1.

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Figure 6. Nyquist plots of Mg0.5Ti2(PO4)3 in SIB cells during the first cycle: (a) discharge to 1.5 V, (b) discharge to 1.0 V, (c) charge to 1.5 V, (d) charge to 2.5 V.

One can see two reversible redox couples from the CV curves. The one observed at 2.0/2.2 V was due to Ti4+/Ti3+ redox, which obeyed the following reaction: (Mg0.5)M1Ti2(PO4)3 + 2.0 Na+ + 2.0 e- ⇌ (Mg0.5)M1(Na2)M2Ti2(PO4)3

(1)

The other one, at 0.3/0.5 V, was assigned to further sodiation of Na2Mg0.5Ti2(PO4)3 with partial reduction of Ti3+: (Mg0.5)M1(Na2)M2Ti2(PO4)3 + 1.5 Na+ + 1.5 e- ⇌ (Na0.5Mg0.5)M1(Na3)M2Ti2(PO4)3 (2) The 1.7 V difference between the Ti4+/Ti3+ and Ti3+/Ti2+ redox couples was in good agreement with observed for NaTi2(PO4)3 with a similar NASICON structure26. In addition, two small peaks were observed near 0.08/0.1 V, and were attributed to Na+ insertion/extraction in the super P conductive additive27. The CV profiles were repeated in subsequent cycles, indicating excellent structure and electrochemical reversibility of the SIB cell. Figure S7 (Supporting Information) shows a series of ex-situ XRD patterns of the electrode during the first cycle. Even though all attempts to perform Rietveld refinements failed due to the poor quality of the XRD data, a 14


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crystal structure similar to that of Mg0.5Ti2(PO4)3 was basically retained during the charge-discharge process. Figure 7 shows the HRTEM image of the fully discharged electrode. The clear lattice fringes could be assigned to the (113) and (012) reflections of hexagonal Mg0.5Ti2(PO4)3, which confirms the structural stability of Mg0.5Ti2(PO4)3 during Na+ intercalation. Moreover, ex-situ XRD was performed for the Mg0.5Ti2(PO4)3 electrode after 100 cycles (Figure S8, Supporting Information). The XRD pattern still showed the major diffraction peaks of Mg0.5Ti2(PO4)3. The decreased peak intensities indicate lowed crystallinity of the electrode after long time cycling.

Figure 7. HRTEM image of the fully discharged Mg0.5Ti2(PO4)3 electrode in SIB cells.

Reaction mechanism of the LIB cells Figure 8 shows the initial three CV curves of Mg0.5Ti2(PO4)3 in LIB cells. There were four current peaks in the voltage window of 2.0-2.9 V. The one located at 2.58 V was attributed to Li+ intercalation into the TiP2O7 impurity30-31. This impurity likely did not show any CV reflections in the SIB cell because it is not active for Na+ 15



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insertion, as suggested by Liu et al28. The other three peaks, at 2.85 V, 2.5 V, and 2.2 V, are due to Li+ intercalation into Mg0.5Ti2(PO4)3, accounting for the Ti4+/Ti3+ redox and the Mg0.5Ti2(PO4)3 to Li2Mg0.5Ti2(PO4)3 phase transformation. Separation of these peaks was attributed to Li ordering at the M1 and M2 sites. More specifically, the first one corresponded to Li ordering at the M1 site, and the other two were due to Li ordering at the M2 site, consistent with that previously observed for Li3Ti2(PO4)332. These ordered phase transformations were not observed for the SIB cells, possibly due to the large size of the Na+ ions.

Figure 8. CV curves of Mg0.5Ti2(PO4)3 in LIB cells recorded at a scan rate of 0.05 mV s-1.

There should be 1.5 mol vacancies remaining in the Li2Mg0.5Ti2(PO4)3 structure after completion of the Ti4+/Ti3+ redox reaction. Filling these vacancies should occur near 1.0 V because the Ti3+/Ti2+ redox potential is about 1.7 V lower than that of Ti4+/Ti3+. However, no well-defined current peak was observed in this range, and instead there was a slow decrease between 1.2 V and 0.5 V. This indicates some simultaneous side reactions, such as the decomposition of the electrolyte or the 16


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formation of SEI film occurring in the LIB cell along with the phase transformation of Li2Mg0.5Ti2(PO4)3 to Li3.5Mg0.5Ti2(PO4)3. Indeed, from the Nyquist plots, one can see a new semicircle corresponding to the SEI film when discharging the electrode to 1.0 V vs. Li+/Li (Figure S9, Supporting Information). The CV curve showed a strong reduction peak at 0.4 V. Since all the M1 and M2 voids of Li3.5Mg0.5Ti2(PO4)3 were occupied by Li+ and Mg2+, further intercalation of Li+ would destroy the crystal structure of the material. As a result, the decomposition reaction would occur, forming metallic Mg and Ti, as well as a Li3PO4 phase, according to the following reaction: Li3.5Mg0.5Ti2(PO4)3 + 5.5 Li+ + 5.5 e- → 0.5 Mg + 2 Ti + 3 Li3PO4

(3)

This reaction absorbed 5.5 mol Li+ ions from the electrolyte, consistent with the observed large current peak in the CV curve. According to the above analysis, a complete reaction of Mg0.5Ti2(PO4)3 with Li+ involved 9.0 Li+ ions storage in the electrode, resulting in a theoretical capacity of 614.1 mAh g-1. The decomposition reaction was further confirmed by X-ray diffraction. Figure S10 (Supporting Information) shows the ex-situ XRD patterns of the electrode during the first cycle, showing that a long-range ordered crystal structure was maintained until 0.5 V. When the electrode was discharged to 0.25 V, all diffraction peaks quickly disappeared except those of the Cu current collector, indicating that the material lost its long-range ordering when more than 3.5 mol Li+ ions were intercalated. However, neither metallic Mg or Ti, nor Li3PO4 was detected by XRD, probably because of their small crystallite size or amorphous nature. The amorphous state of the electrode in LIB was also confirmed by the ex-situ XRD pattern collected after 100 charge-discharge cycles (Figure S11, Supporting Information). To investigate this further, the microstructure of the fully discharged material was studied by HRTEM. The large Mg0.5Ti2(PO4)3 particle was completely pulverized at the end of the first discharge (Figure 9a). The 17



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amorphous matrix observed in HRTEM was composed of the SEI film and Li3PO4 as indicated by the diffuse diffraction rings in the selected area electron diffraction (SAED) pattern (Figure 9c). Under larger magnification, small crystallites with particle size ca. 5 nm that were encapsulated in the amorphous matrix were observed. A close examination of the lattice fringes (Figure 9b) and the bright spots in the SAED pattern (Figure 9d) showed that these small crystallites are metallic Mg and Ti.

Figure 9. TEM (a) and HRTEM (b) images, and corresponding SAED patterns (c, d) of the fully discharged Mg0.5Ti2(PO4)3 electrode in LIB cells.

With further reduction, a new current peak started at 0.2 V and remained until 0.01 V. Decomposition of Li3.5Mg0.5Ti2(PO4)3 was excluded since ex-situ XRD confirmed that this reaction was already completed at 0.25 V. As reported, the alloy/de-alloy reaction of Li3Mg should occur at ~ 0.1 V, contributing a capacity of 102.3 mAh g-1 to the LIB cell. (Ti is a non-alloy metal for Li)33. However, the

18


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charge-discharge experiment showed negligible capacity at 0.01-0.1 V (Figure 4a). This indicates that this alloy reaction was unfavorable in the current LIB cell. Because Li3PO4 cannot store Li ions via the intercalation reaction, the electrochemical process below 0.2 V was most likely due to interfacial Li+ storage in the electrode. Similar behavior has been reported for some transition metal fluorides such as TiF3 and FeF2 and some phosphate polyanion materials such as LiFePO46, 34. The CV curves were repeatable in the following cycles, indicating good reversibility of the electrode reactions. During oxidation, the CV curve showed a constant current between 0.01 V and 1.7 V due to the de-adsorption of Li+ from the electrode. Next, a broad current peak appeared in the region of 1.7-3.0 V. The reactions occurring in this region were partly attributed to decomposition of the SEI film under the catalytic effects of Ti nanoparticles and the de-adsorption of Li+ ions. Wang et al. also observed a similar effect for their Fe/Li3PO4 electrode7. DISCUSSION The

above

analysis

showed

that

Mg0.5Ti2(PO4)3

exhibited

different

electrochemical properties in LIB and SIB. When the material was discharged to 0.01 V vs. Na+/Na0, a fast Na+ intercalation reaction was observed, resulting in a discharge capacity of 268.6 mAh g-1. This was much larger than the 208 mAh g-1 reported for NaTi2(PO4)3, a well-known negative electrode for SIB35. This could be attributed to the greater vacancies in the structure because only half of the M1 void of Mg0.5Ti2(PO4)3 was occupied by Mg, in contrast, all of the M1 void of NaTi2(PO4)3 was occupied by Na. Thus, the large capacity could be attributed to activation of the Ti3+/Ti2+ redox. It is known that the capacities of titanium oxides, such as TiO2, Li4Ti5O12 and Na2Ti6O13, are usually limited by the Ti4+/Ti3+ redox36-38. In the case of Mg0.5Ti2(PO4)3, the strong inductive effect of PO4 could effectively control the ionic 19



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and covalent behavior of the transition metal-oxygen bonds to activate the Ti3+/Ti2+ redox, just like that observed for Na3Ti2(PO4)3 and NaTi2(PO4)319, 39. In order to test this, XPS analysis was performed to validate the changes in the valence state of Ti. The Ti 2p3/2 XPS of the pristine Mg0.5Ti2(PO4)3 was observed at 459.4 eV, consistent with that of Ti4+37 (Figure 10a). When the electrode was discharged to 0.01 V, the Ti 2p3/2 XPS could be fitted by two subcomponents of Ti3+ (458.6 eV) and Ti2+ (457.1 eV)38-39, which confirmed the deep reduction of Ti4+ (Figure 10b). The oxidation state of Ti was recovered to Ti4+ after the first charge (Figure 10c). In the meanwhile, the Mg ions were maintained as Mg2+ during the whole charge-discharge cycling (Figure 10d-f). This indicates that only Ti ions participated in the electrochemical reactions of the SIB cell.

Figure 10. Ti 2p (a-c) and Mg 1s (d-f) XPS patterns of the pristine Mg0.5Ti2(PO4)3 electrode and the electrodes after first discharge-charge cycles in SIB. In contrast to the Na+ intercalation reaction that occurred in SIB, Mg0.5Ti2(PO4)3 showed an interfacial Li+ storage mechanism in LIB. Moreover, the formation and 20


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decomposition of the SEI film also contributed to capacity. We used EIS to study the electrochemical mechanism of Mg0.5Ti2(PO4)3 in LIB. Figure 11 shows the Nyquist plots of the LIB cell during the first cycle. Only one semicircle corresponding to the charge transfer process was observed upon the electrode discharging to 1.5 V. With further discharging to 0.01 V, the other semicircle that appeared in the high-to-medium frequency region was likely due to the SEI film. Therefore, the Nyquist plots could be simulated according to the equivalent circuit that is shown as an inset in Figure 11b, where Re, Rsf, and Rct represent the ohmic resistance, SEI resistance, and charge-transfer resistance, respectively. W is related to Li+ diffusion in the electrode bulk. The constant-phase element (CPE) is defined as Z = 1/B (jω)n where j = √−1 and ω is the angular frequency. The value of n reflects the distortion of the semicircle, and for values of n closer to 1.0, the electrode behaves more like an ideal capacitor40. The interfacial Li+ capacity that was correlated with the double layer capacitance (Cdl) of the charge-transfer process was determined using the corresponding CPE. The simulated Rsf, Rct, Cdl, and n parameters are listed in Table 1. The material showed an Rsf value of 204.2 Ω at the end of the first discharge. This parameter slightly decreased to 199.1 Ω when the electrode was charged to 1.5 V, and then decreased to ~ 0 Ω with further charging to 2.5 V. Evolution of the Rsf values indicates that most of the SEI film decomposed above 1.5 V vs. Li+/Li0 during the charge process. The Cdl parameter promptly increased from 5.6 µF to 283.9 µF when the electrode was discharged to 0.01 V. At the same time, the n parameter increased from 0.77 to 0.85. Together these changes indicate that the electrode improved its ability to act as an electrochemical capacitor at the end of the discharge process. During the following charge process, the Cdl and n parameters decreased to 2.8 µF and 0.78, respectively, consistent with Li+ de-adsorption. 21



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Figure 11. Nyquist plots of Mg0.5Ti2(PO4)3 in LIB cells during the first cycle: (a) discharge to 1.5 V, (b) discharge to 0.01 V, (c) charge to 1.5 V, and (d) charge to 2.5 V. Table 1. Simulated electrochemical kinetic parameters of Mg0.5Ti2(PO4)3 in LIB cells during the first cycle. State

Rsf (Ω)

Rct (Ω)

Cdl (µF)

n

Discharge to 1.5 V

/

135.1

5.63

0.77

Discharge to 0.01 V

204.2

1473

283.9

0.85

Charge to 1.5 V

199.1

412

349.8

0.82

Charge to 2.5 V

/

212.7

2.8

0.78

The decomposition of Li3.5Mg0.5Ti2(PO4)3 occurred during the first discharge was essential for the interfacial Li+ storage. First-principle calculations of the equation,

∆ = 3 0.5 2 − 5.5 − . .

(4)

showed that the formation energy of Ti, Mg, and Li3PO4 from Li3.5Mg0.5Ti2(PO4)3 was -9.964 eV/f.u. In comparison, the formation energy of Ti, Mg, and Na3PO4 from Na3.5Mg0.5Ti2(PO4)3 was -5.892 eV/f.u. This indicates that the decomposition of Li3.5Mg0.5Ti2(PO4)3 was easier than that of Na3.5Mg0.5Ti2(PO4)3. Decomposition of 22


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Li3.5Mg0.5Ti2(PO4)3 with deep lithiation was validated by XPS analysis. The ex-situ XPS of the fully discharged Mg0.5Ti2(PO4)3 showed the presence of Li, C, O, and P which are essential elements of the SEI film and Li3PO4 (Figure S12, Supporting Information). However, the XPS signals of Mg and Ti were hardly detected, indicating that these elements were not on the surface of the electrode. However, intensive Mg 2p and Ti 2p peaks were observed after etching the electrode with Ar ions (Figure 12). This indicates that the Mg and Ti nanocrystals were formed inside the SEI and Li3PO4 matrix. Note that the observed higher Mg 2p (51.1 eV) and Ti 2p (459.4 eV) binding energies relative to those of Mg0 (50.0 eV) and Ti0 (454.2eV) were an unavoidable result of Ar etching.

Figure 12. Mg 2p (a) and Ti 2p (b) XPS of the Mg0.5Ti2(PO4)3 electrode after the first discharge in LIB.

The nucleation and growth of Ti, Mg, and Li3PO4 from Li3.5Mg0.5Ti2(PO4)3 involved complex charge and mass transportation via a solid-state reaction. Based on the above analysis, it is reasonable to suggest a “Pitaya” structure for the decomposed electrode. Migrations of the cations (including Li, Mg and Ti) and anions (PO4) are considered in describing the formation of the “Pitaya” structure, as illustrated in Figure 13. First, the SEI film persisted at the surface of the particle forming the outside of the “Pitaya.” When the electrode was discharged to 0.5 V, the intercalated 23



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Li+ ions switched on the decomposition of Li3.5Mg0.5Ti2(PO4)3. The Mg and Ti ions are much less mobile than Li and PO434 and, therefore, they would be reduced to metallic Mg and Ti near their initial atomic sites. Therefore, the Mg and Ti particles would take the form of the skeleton occupying almost the same space as the Li3.5Mg0.5Ti2(PO4)3 framework. Concurrently, the PO4 anion, with its higher diffusivity, should more readily migrate out from the skeleton to form Li3PO4 upon reaction with Li+ from the electrolyte. However, the PO4 anions do not move very far from their original sites due to the relatively faster diffusion of Li+.

Figure 13. Schematic diagram of the “Pitaya” structure for the decomposed Mg0.5Ti2(PO4)3 electrode in LIB cells. Next, we considered a discharge process to describe the interfacial Li+ storage in the LIB cells. During the discharge process, the electrode adsorbed Li+ ions from the electrolyte. At the same time, electrons were provided from the external circuit to maintain the neutral charge of the electrode. Because metallic Ti and Mg and Li3PO4 are not reactive with Li+ ions, charge separation occurred at the interfacial region of the metal particles and the Li3PO4 matrix5. As a result, electrons accumulated in the metal particles and Li+ ions stored in the Li3PO4 matrix until the electrode potential became equal to the chemical potential of Li0, as shown in Figure 13. However, the movement of electrons towards the Ti/Mg particles was hindered by the insulating 24


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SEI film and Li3PO4, resulting in the poor rate performance of Mg0.5Ti2(PO4)3. Moreover, poor capacity retention, especially at high cycle rate, remains a problem for this interfacial Li+ storage material. This could be attributed to the “electrochemical Ostwald ripening” of the nanocomposite electrode, in which the nanocrystals tend to agglomerate together with cycling to reduce their surface energy40. This will decrease the surface area of the active material resulting in the loss of electric contact between the electrode film and the current collector, resulting in a subsequent decrease in capacity.

CONCLUSIONS We prepared carbon-coated Mg0.5Ti2(PO4)3 using the sol-gel method. The material showed a specific capacity of 268.6 mAh g-1 when cycled at 0.01-3.0 V vs. Na+/Na0, following a reversible Na+ intercalation reaction. With this energy-storage mechanism, Na+ ions could migrate in the NASICON framework with low activation energy resulting in excellent rate capability. This material also showed large discharge capacity and excellent cycle stability, demonstrating that Mg0.5Ti2(PO4)3 may be a promising negative electrode for SIBs. In comparison, Mg0.5Ti2(PO4)3 exhibited a capacity of 629.2 mAh g-1 in the voltage window of 0.01-3.0 V vs. Li+/Li0. This large capacity was attributed to the interfacial Li+ storage and formation/decomposition of the SEI film. The pristine Mg0.5Ti2(PO4)3 particles decomposed to metallic Ti and Mg nanocrystals after the first discharge, and these particles became encapsulated in the SEI and Li3PO4 matrix. In subsequent cycles, electrons accumulated in the Ti/Mg particles and Li+ ions stored in the Li3PO4 matrix until the electrode potential equaled the chemical potential of Li0. However, the interfacial Li+ storage showed sluggish electrochemical kinetics because the electrons must cross the insulated SEI film and the Li3PO4 matrix prior to interaction with the Ti and Mg nanoparticles. This resulted 25



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in the poor rate capability and cycle stability in LIBs. The ability of interfacial Li+ storage is closely related with the surface area of the material, and the larger the surface area, the larger the specific capacity. In this work, the sub-micron sized Mg0.5Ti2(PO4)3 electrode showed a Li+ capacity of 600 mAh g-1. This capacity was much larger than those of graphite41 and TiO242 based on the intercalation reaction, and was comparable to some other ones based on the conversion reaction such as MoS243 and NiO44. We expect that the Li+ capacity of Mg0.5Ti2(PO4)3 could be effectively improved by decreasing the material particles to nanometer scale. However, it should be pointed out that the inclined voltage profiles of such materials bring some difficulties in design of full batteries. The full battery performance can be optimized by adjusting the working voltage window and balancing the weight ratio of the positive and negative electrodes. Finally, even though a negative formation energy (-5.892 eV/f.u.) was obtained from

first-principle

calculations,

the

electrochemical

decomposition

of

Na3.5Mg0.5Ti2(PO4)3 was not observed in this work. This is probably due to the large electrode polarization of the SIB cell. Some strategies, such as the preparation of nano Mg0.5Ti2(PO4)3 or making composite materials with carbon additives may allow the decomposition of Na3.5Mg0.5Ti2(PO4)3. If such a strategy can be identified, we can obtain a large Na+ capacity by taking advantage of the interfacial Na+ storage mechanism.

ASSOCIATED CONTENT Supporting Information

26


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The Supporting Information is available free of charge on the ACS Publications website at DOI 10.1201. The provided information includes the Raman and TG curves, elemental mappings of the carbon coated Mg0.5Ti2(PO4)3, charge-discharge performance of super P in SIB and LIB, ex-situ XRD patterns of Mg0.5Ti2(PO4)3 during the first cycle in SIB and LIB, Nyquist plots of Mg0.5Ti2(PO4)3 during the first cycle in LIB, and Rietveld refinement results of the Mg0.5Ti2(PO4)3 material.

AUTHOR INFORMATION Corresponding author *Email: [email protected] (Y. J. Wei);[email protected] (F. Du); Tel& Fax: 86-431-85155126 Notes The authors declare no competing financial interest. ACKNOWEDGEMENTS This work was supported by the Ministry of Science and Technology of China (No. 2015CB251103) and the National Natural Science Foundation of China (No. 51472104, 21473075, 51272088). Q.F. acknowledges a research scholarship from the Ministry of Science, Research, and the Arts of Baden-Württemberg (MWK) in the frame of the competence network “Functional nanostructures.” The authors thank Dr. Daria Mikhailova from IFW Dresden for a useful FullProf recommendation.

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NASICON-Type Mg0.5Ti2

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