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Sodiation-Desodiation Reactions of Various Binary Phosphides as Novel Anode Materials of Na-Ion Battery Hiroyuki Usui, Yasuhiro Domi, Ryota Yamagami, Kohei Fujiwara, Haruka Nishida, and Hiroki Sakaguchi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00241 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Energy Materials

Sodiation−Desodiation Reactions of Various Binary Phosphides as Novel Anode Materials of Na-Ion Battery Hiroyuki Usui†,‡, Yasuhiro Domi†,‡, Ryota Yamagami§,‡, Kohei Fujiwara†,‡, Haruka Nishida†,‡, and Hiroki Sakaguchi*,†,‡ †

Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Minami, Koyama-cho, Tottori 680-8552, Japan

§

Course of Chemistry and Biotechnology, Department of Engineering, Graduate School of Sustainability Science, Tottori University, 4-101 Minami, Koyama-cho, Tottori 680-8552, Japan

‡

Center for Research on Green Sustainable Chemistry, Tottori University, 4-101 Minami, Koyama-cho, Tottori 680-8552, Japan

ABSTRACT

Sodiation−desodiation behaviors were investigated for electrodes comprised of various binary phosphides, InP, CuP2, GeP, SiP, and LaP as anode materials of Na-ion battery. Although LaP electrode did not react with Na, the other electrodes showed reversible sodiation−desodiation reactions in the initial cycles. Rapid capacity decays were observed for CuP2, GeP, and SiP electrodes. In contrast, a better cyclability was attained for InP electrode. These results indicate that binary phosphides (M−P) require four properties for improving cyclability: i) low thermodynamic stability of M−P, ii) high electronic conductivity of M, iii) low hardness of M, and iv) reactivity of M with Na.

KEYWORDS. Binary phosphide compounds; Indium phosphide; Na-ion battery; Anode material; Nanostructure

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An application of Na-ion batteries (NIBs) is seriously desired for large-scale stationary batteries because sodium resource has great advantages compared to lithium resource. Na is widely available from many sources including seawater, while a large portion of Li resource is localized in a few countries of South America. Its abundance of 23000 ppm in the crust is much higher than lithium’s one of 20 ppm. In addition to these, Na resource as carbonates has a much lower cost of 150 $ t−1 than Li carbonate resource (5000 $ t−1)1. Researchers have recently developed some electrode materials for NIB exhibiting good performances comparable to those of Li-ion batteries (LIB), which is, however, still insufficient to promote commercialization of NIB. To realize a practical application of NIB, the electrode materials are currently required to show performances exceeding LIB electrode materials. As the most promising anode material, hard carbons have been intensively studied. Komaba et al. been reported that Na ions can be reversibly inserted into disorder layered structure and nanopores of hard carbon, resulting in stable cyclabilities and reversible capacities of 220−260 mA h g−1 for 100 charge−discharge cycles2,3. In the most recently, the hard carbon electrode maintained the capacity of as high as 320 mA h g−1 for 100 cycles4. The authors have recently succeeded to develop many promising materials: Nb-doped rutile TiO2 showing insertion reaction5-7, and alloying/dealloying reaction-based materials using tin (Sn)8-11, phosphorus (P)10-12, and silicon (Si)13. In particular, P and Sn are very attractive elements because those have high theoretical capacities (Na3P: 2596 mA h g−1 and Na15Sn4: 847 mA h g−1). Si is also attractive in the light of natural abundance. We have firstly confirmed reversible charge−discharge reactions for cluster-sized Si dispersed in SiO2 matrix13. When the fully-sodiated composition is Na0.76Si14, its theoretical capacity corresponds to 725 mA h g−1, which is higher than other elements such as germanium and indium (NaGe: 369 mA h g−1 and NaIn: 234 mA h g−1). In contrast, the elements of Si, P, and Sn have critical disadvantages: bulk Si is well known to be Na-inactive in crystalline phase13,15. In its amorphous phase, there is still some debate over whether Na-active14,16 or Na-inactive13. In exchange of their high capacities, P and Sn show significant volume expansions of 490% and 525% when those form Na3P and Na15Sn4 by sodiation, which causes disintegration of electrode and formation of void spaces in active material layer. In addition, Sn particles aggregate during charge−discharge reactions to enhance the disintegration, while Na3P has a poor electronic conductivity owing to its energy band gap of about 2 eV17. These phenomena lead to an electrical isolation of the active material layer in early cycles. In general, electrodes of P and Sn show a rapid capacity decay and a short cycle life. From the viewpoint of NIB application, it is important to develop a high-capacity active material. To meet the demand, we have newly developed a tin phosphide (Sn4P3) for NIB anode9,10. Although the cycle life of Sn4P3 electrode was not outstanding in a conventional organic electrolyte, the authors have firstly revealed that it exhibited an excellent cycling performance by choosing a suitable electrolyte based on a


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pyrrolidinium-based ionic liquid10. The Sn4P3 anode attained a remarkably high capacity of 750 mA h g−1 even at the 200th cycle. For other electrodes of P12 and LaSn311 also, the performances were drastically improved by this electrolyte. The authors have proposed that ionic liquid electrolyte has an important role to effectively extract a potential performance of these high-capacity electrodes. The role is to suppress a continuous decomposition of electrolyte on electrode surface by its high electrochemical stability9-12 and by forming a chemically stable surface layer including NaF and NaSO2F derived from decomposition of FSA anions18. For organic solvent, a surface layer containing NaF is similarly formed by adding fluoroethylene carbonate8. A possible mechanism of the Sn4P3 electrode can be explained by a complementary effect of Sn and P: an elemental Sn acts as a conducting pathway to activate reversible desodiation of nonconductive Na3P, while the Na3P phase provides a shield matrix preventing Sn aggregation9,19,20. Our microscopic observation and elemental analysis10 have clearly demonstrated that the first desodiation of Sn4P3 forms nanostructured domains in which crystalline Sn nanoparticles are dispersed in the amorphous-like P (a-P) matrix. Okamoto et al. also have revealed that nanostructured Sn can effectively improve the cyclability21. These experimental results support the possible mechanism of Sn4P3 anode.

Format ty with ion

Na

Elem energy (Theore ent of M−P

tical

/ kJ capacity mol

−1

/ mA h

ical Electroni propert Perform y to

c

conducti relax

/ S cm

−1

Sn

（−10 ）〇

In

（−54 ）

（847）

〇（234）

〇 Cu （−19 0）

−2

〇（9.1×10 4

）

〇（1.2×10 5

）

〇 ×

(Mohs

e

ss)

2596 6.7×10

〇

M−P

hardne

−1

〇

ance of

stress electrod

vity

g ) P

Table 1.

Mechan

Reactivi

（5.9×10 5

）

0.5

〇（1.5）

〇（1.2）

〇

〇

〇（3.0）

×


3


〇 Ge （−16

△ （369）

）

△ （2.2×10 −2

〇 Si

（−38

（725）

0）

（6.0）

（1.0×10

×

−5

）

× （7.0）

〇

× （−36

×

×

△

）

La

）

Page 4 of 14

×

（1.6×10 4

）

×

〇（2.5）

×

This mechanism was, however, deduced only from the results of Sn4P3 and SnP310. In binary phosphide (M−P), there is still a controversial matter about the properties required for element (M) to enhance phosphide’s anode performance. To clarify the required properties, it is necessary to study sodiation−desodiation behaviors of various phosphide electrodes and to discuss these electrode reactions. In this study, we therefore chose some elements of In, Cu, Ge, Si, and La as elements to be composed with P, and investigated anode properties of their phosphides (M−P). Among them, InP, GeP, SiP, and LaP were evaluated for the first time as NIB anodes. As listed in Table 1 and Table S1, these elements and their phosphides have different physical, electrochemical, and thermodynamic properties each other, which is very useful to discuss the required properties and develop new anode materials consisting of phosphide-based compounds. Active material syntheses of various phosphorus compounds (InP, CuP2, GeP, SiP, and LaP) were performed by a mechanical alloying (MA) method using P and other element (M: In, Cu, Ge, Si, and La). The procedures were described in Fig. S1. It was confirmed that the MA treatments successfully produced active material powders of InP, CuP2, GeP, SiP, and LaP (Fig. S2−S5). Electrodes were evaluated in an ionic liquid electrolyte comprised of sodium bis(fluorosulfonyl)amide (NaFSA)-dissolved in N-methyl-Npropylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA), as shown in Fig. S6.


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Figure 1 shows sodiation(charge)−desodiation(discharge) curves of phosphide electrodes at the initial two cycles. In case of the electrode consisted of P alone (Fig. S7), sloping plateaus were observed for charge and discharge curves at 0.3−0.5 V and 0.5−0.8 V versus Na+/Na, indicating the phase transformations between P and Na3P22-24. The potential hysteresis of P electrode is attributed to a difference in intermediate phases between sodiation and desodiation24. All the phosphide electrodes except for LaP exhibited sloping plateaus similar to those of the P electrode. In the initial five cycles, the electrodes of InP and CuP2 maintained their initial capacity (Fig. S8 and Fig. S9). By contrast, capacity degradations were observed for the GeP electrode by the fifth cycle (Fig. S10). An electrode of P-rich phase (GeP5) delivered a more pronounced degradation (Fig. S11), which is probably attributed to the

2

(a)

Potential / V vs. Na+/Na

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1st cycle InP

SiP

GeP

LaP

1

CuP2

2 2nd cycle

(b) SiP

GeP

CuP2

LaP

1

0

InP

0

200

400

600

800

1000

−1

Capacity / mA h g Figure 1.

larger mole fraction of P producing electrode disintegration. The SiP electrode showed a very rapid capacity decay: the discharge capacity reduced by half within the initial five cycles (Fig. S12). The four kinds of phosphide electrodes demonstrated sodiation−desodiation in the initial cycles though there are differences in the capacities. Contrary to this, no reactivity was confirmed for the LaP electrode: the capacity was less than 13 mA h g−1 (Fig. S13), which is only 2.7% of the theoretical capacity (Table S1). La is Na-inactive element. Nevertheless, we should note that the theoretical capacity of LaP is as high as 473 mA h g−1 if P reacts with Na. The extremely low capacities indicate that LaP is thermodynamically stable against sodiation reaction. As shown in Table 1, LaP has a larger negative value of the Gibbs


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energy of formation compared with other phosphide25. An XRD analysis (Fig. S14) demonstrated that LaP does not show phase separation by sodiation−desodiation. In contrast, a disproportion is suggested occur for other phosphides. We consider that a lower thermodynamic stability is required for phosphide anode materials based on alloying/dealloying reactions. Figure 2 represents cycling performances of these phosphides and Sn4P310. The SiP electrode showed a steep capacity decay in the initial ten cycles, resulting in a very poor cyclability similarly to the P electrode. Although the performance was slightly improved for the GeP electrode, the discharge capacity quickly degraded by the 40th cycle. The capacity of the CuP2 electrode was also rapidly decreased during the initial 40 cycles. In contrast, the InP electrode exhibited much better cyclability: the discharge capacity of 500 mA h g−1 maintained for 100 cycles though the capacity gradually reduced in the subsequent cycles. The capacity of InP was about 300 mA h g−1 lower than that of Sn4P3, which probably originating from the difference in the theoretical capacities of In (234 mA h g−1) and Sn (847 mA h g−1). The theoretical capacities of InP and SnP can be estimated to be 730 and 1130 mA h g−1 because the

Discharge capacity / mA h g −1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800

Sn4P3 CuP2

600

InP

400

P

200

GeP

SiP

LaP 0 0

20

40

60

Cycle number

80

100

120

Figure 2.

elemental weight ratios in these phosphides are In:P=79:21 and Sn:P=84:16 wt.%. The achievement rate of theoretical capacity for InP is as high as 76%, which is comparable to that of 72% for Sn4P3 (Table S1). These active materials could successfully suppress the electrode disintegration in contrast to the other phosphides.


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To reveal the origin of the better performance for InP, we focused its phase changes during charge−discharge reactions. From XRD measurements (Fig. S15), it was indicated that InP phase disappeared in the sodiation and desodiation states, and that any crystalline phase was not clearly detected. Figure 3 compares CV profiles for the electrodes of InP, In, and P. In terms of phosphorus, it has been reported that stepwise sodiation and desodiation cause cathodic peaks at 0.1 and 0.25 V and anodic peaks at 0.53, 0.66, and 0.75 V vs. Na+/Na19,23. In this study, the CV profile of the P electrode coincided with it. The In electrode showed a reduction current of electrolyte decomposition reaction in 0.6−0.9 V, which appears to be more pronounced compared to the P’s CV profile because the In’s CV profile is enlarged

owing to its smaller Na-storage amount. In addition to this, we observed reduction and oxidation peaks based on In’s alloying/dealloying reactions at approximately 0.3 and 0.45 V. These potentials corresponds to the values reported by other researcher26. Two kinds of composition, NaIn and Na2In, are known as a


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Na−In alloy. In this study, NaIn phase is expected to form by alloying of In with Na because the first sodiation capacity of 162 mA h g−1 (Fig. S16) was closer to theoretical capacity of NaIn formation (234 mA h g−1) than that of Na2In formation (467 mA h g−1). In regard to the InP electrode, both electrode reactions of In and P were recognized. This result offers an important knowledge for its reaction mechanism: InP shows a phase separation at the first sodiation to form elemental In and P, and then In and P individually react with Na. CV results for other phosphide electrodes are summarized in Fig. S17. In the CuP2 electrode, only P reacted with Na because Cu is Na-inactive element. The GeP electrode exhibited broad peaks at 0.5 V at sodiation and 0.85 V at desodiation in addition to the phosphorus’ peaks. The broad peaks are possibly attributed sodiation/desodiation of amorphous-like Ge (a-Ge)27, indicating that disproportion of GeP occurred to form a-Ge and P phases. Kajita et al. have reported that a-Ge in GeO show similar sodiation/desodiation reactions28. The SiP electrode showed cathodic peaks of P’s sodiation and no anodic peak. The reason is presumably that severe volume expansion of P during the sodiation progressed the electrode disintegration even at the first cycle in the CV measurement. Figure 4(a) displays TEM observation result for InP active material after the first desodiation. This image represents that crystalline nanoparticles with dark contrast were dispersed, and that their sizes were 9.3±1.7 nm. In the corresponding selected area electron diffraction (SAED), we observed spare diffraction spots rather than Scherrer ring, indicating the existence of nanocrystalline material. An SAED pattern analysis proved the formation of an elemental In phase (Fig. S18). Figure 4(b) shows its high magnification TEM image. Lattice fringes were confirmed in the dark-contrast nanoparticles. The two kinds of fringe spacings (0.269 and 0.222 nm) agreed with indium’s lattice spacings of d101 (0.2715 nm) and d110 (0.2298 nm). On the other hand, any fringes of InP and P were never found, suggesting that

amorphous-like P (a-P) phase exists in white contrast region. These results revealed that InP undergoes phase separation in the initial sodiation to form In and P, and that their sodiation−desodiation produce a nanostructure in which crystalline In nanoparticles are dispersed in a-P matrix. A similar nanostructure


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has been confirmed for Sn4P3 electrode exhibiting excellent performance10. A numerical study has also predicted that Na15Sn4 nanoparticles are embedded in a Na3P matrix at sodiation state29. As summarized in Table 1, In and Sn show low Mohs hardness and high ductility. Thus, In and Sn phases could relax a strain induced from the P’s volume changes. In addition, these elements are Na-active materials with a high electronic conductivity which indicating that In phase and Sn phase can change their morphologies by the sodiation−desodiation of themselves in response to the P’s volume changes Consequently, the void spaces could be filled by their morphological changes, preventing the electrical isolation of the active material layer. Furthermore, their high conductivities compensate a poor electronic conductivity of Na3P phase created by P’s sodiation. Although a reversible conversion reaction of CuP2 has been reported30, our XRD measurements did not show crystalline CuP2 phase after the desodiation (Fig. S19), indicating that CuP2 in this study showed the phase separation to form nanocrystalline Cu and a-P phases. Even if CuP2 forms the similar nanostructure by the desodiation, the morphological change of Cu phase can not sufficiently response the P’s volume changes because Na-inactive Cu does not show its volume change, leading to a partial electrical isolation and the resulting capacity decay. On the other hand, Ge and Si have lower reactivity with Na, smaller electronic conductivity, and much higher hardness compared with Sn and In (Table 1). It is suggested that these properties are unfavorable for improving the electrode disintegration. From these results, we conclude that binary phosphides (M−P) require four properties to exhibit better performance: i) M−P should have a low thermodynamic stability to cause its phase separation. As the result, elemental P shows sodiation−desodiation while M is needed to play the role to compensate the P’s disadvantages, large volume changes and low electronic conductivity of Na3P. ii) A high electronic conductivity is, accordingly, demanded for M. iii) A low hardness is required for M so that M can relax strain from P induced by its volume changes. iv) M is preferable to show reactivity with Na because the electrical isolation of active material layer can be effectively suppressed by morphological change of M during its sodiation−desodiation. In summary, we evaluated the sodiation−desodiation behaviors for electrodes comprised of various binary phosphides in ionic liquid electrolyte. The reversible sodiation−desodiation reactions were confirmed for the electrodes except for LaP with the high thermodynamic stability. We investigated for the first time the reaction mechanisms of InP, GeP, and SiP. By cyclic voltammetry and TEM observation for InP, it was demonstrated that phase separation of InP occurs in the initial sodiation−desodiation to form a nanostructure in which In nanoparticles are dispersed in the a-P matrix. The nanostructure probably contributes to enhance the anode performance, similarly to Sn4P3 electrode showing such nanostructure after desodiation. In contrast, rapid capacity decays were found for the electrodes of CuP2, GeP, and SiP. To exhibit better performance, binary phosphides (M−P) require four properties: i) a low


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thermodynamic stability of M−P, ii) a high electronic conductivity of M, iii) a low hardness of M, and iv) a reactivity of M with Na. The knowledge is very noticeable for developing novel high-capacity anode materials consisting not only of binary phosphides but also of ternary and quaternary phosphides.

ACKNOWLEDGMENT This study was partially supported by Advanced Low Carbon Technology Research and Development Program (ALCA, 16200610802), Joint Usage/Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE29A-14), and Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Number 17H03128, 17K17888, 16K05954). A part of this work was supported by “Advanced Characterization Nanotechnology Platform, Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan” at the Research Center for Ultra-High Voltage Electron Microscopy in Osaka University (A-17-OS-0020). The authors thank Prof. H. Yasuda and Dr. T. Sakata for their helpful assistances in HR-TEM observations.

ASSOCIATED CONTENT Supporting Information Theoretical capacity list, Active material synthesis, Preparation and evaluation of electrodes, XRD patterns, Charge–discharge curves, CV profiles, SAED pattern.

AUTHOR INFORMATION Corresponding Author: Hiroki Sakaguchi * Tel./Fax: +81-857-31-5265, E-mail: [email protected]


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Table caption Table 1. Physical and thermodynamic properties of various elements (M) and their phosphides (M−P). The electronic conductivity of P shows value of black phosphorus. Cu and La are Na-inactive elements. As a degree of ability to suppress an electrode disintegration induced by P’s volume changes, mechanical property (Mohs hardness) is described. Formation energy describes Gibbs energy of formation for M−P (Sn4P3, InP, CuP2, GeP, SiP, and LaP)25 at 300 K.

Figure captions Figure 1. Charge−discharge curves of various phosphide electrodes (a) at the first cycle and (b) at the second cycle under the current density of 50 mA g−1 in the ionic liquid electrolyte (NaFSA/Py13-FSA).

Figure 2. Anode performances of various phosphide electrodes cycled under the current density of 50 mA g−1 in the ionic liquid electrolyte (NaFSA/Py13-FSA). For comparison, the result was shown for the electrode of P alone.

Figure 3. CV profile of InP electrode at the first cycle. For comparison, the figure shows the results of In and P electrodes.

Figure 4. (a) TEM image of InP after the first desodiation. The particle size was 9.3±1.7 nm. (b) Highmagnification TEM image showed that crystalline In nanoparticles are dispersed in amorphous-like P matrix. (c) Phase changes of InP by charge/discharge reactions.

Table of Contents Graphic

Reversible capacity / mA h g −1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800

CuP2 600

InP

400

P 200

GeP

SiP

LaP 0 0

20

40

60

80

Charge−discharge cycle number

100

120


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