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Bulk-type all-solid-state lithium-ion batteries: remarkable performances of carbon nanofiber supported MgH composite electrode 2

Liang Zeng, Takayuki Ichikawa, Koji Kawahito, Hiroki Miyaoka, and Yoshitsugu Kojima ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11314 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Applied Materials & Interfaces

Bulk-Type All-Solid-State Lithium-Ion Batteries: Remarkable Performances of Carbon Nanofiber Supported MgH2 Composite Electrode Liang Zeng,† Takayuki Ichikawa,*,†,‡ Koji Kawahito,§ Hiroki Miyaoka† and Yoshitsugu Kojima†,§

†Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, HigashiHiroshima 739-8530, Japan ‡Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan. E-mail: [email protected]; Tel: +81 82 424 5744 §Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan KEYWORDS: Metal hydrides, Negative electrode, Conversion type electrode, All-solid-state, Lithium-ion batteries

ACS Paragon Plus Environment

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ABSTRACT: Magnesium hydride MgH2, a recently developed compound for lithium-ion batteries, is considered to be a promising conversion-type negative electrode material, due to its high theoretical lithium storage capacity of over 2000 mA h g-1, suitable working potential and relatively small volume expansion. Nevertheless, it suffers from unsatisfactory cyclability, poor reversibility and slow kinetics in conventional non-aqueous electrolyte systems, which greatly limit the practical application of MgH2. In this work, a vapor grown carbon nanofiber was used to enhance the electrical conductivity of MgH2, with using LiBH4 as the solid-state electrolyte. It shows that a reversible capacity of over 1200 mA h g-1 with an average voltage of 0.5 V (vs Li/Li+) can be obtained after 50 cycles at a current density of 1000 mA g-1. In addition, the capacity of MgH2 retains over 1100 mA h g-1 at a high current density of 8000 mA g-1, which indicates the possibility of using MgH2 as negative electrode material for high power and high capacity lithium-ion batteries in future practical applications. Besides, the widely studied sulfidebased solid electrolyte was also used to assemble battery cells with MgH2 electrode in the same system, and the electrochemical performance was as good as that of using LiBH4 electrolyte.

1. INTRODUCTION Global warming is one of the most pertinent problems the world faces today. Combustion of fossil fuels is the leading cause of global warming,1-2 and therefore alternative clean energies need to be developed as soon as possible to reduce our reliance on fossil fuels. Lithium-ion batteries (LIBs) with zero emission, used for laptops, smart phones, electric vehicles (EVs), and even some aircrafts, are regarded as one of the most important energy carriers in our daily lives.3-


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However, due to the sluggish development of LIB technology, it has become increasingly

difficult for LIB products to keep up with the pace of growing demand for clean energy. The conversion-type electrode materials with high lithium storage capacity have been extensively studied in recent years in order to develop high energy density LIBs.4-6 Among these materials, metal hydride (MH), a recently developed compound as negative electrode for LIBs, is considered to be a promising candidate. The electrode properties of MH were first reported by Oumellal et al. in 20087-8 and were followed by several research groups.9-24 The lithiation and de-lithiation processes of MH can be written as follows, which is a hydride conversion reaction: MHx + x Li+ + x e- ⇌ M + x LiH

(1)

MgH2, in particular, as the most promising negative electrode material for LIBs in all metal hydrides, possesses a high theoretical Li storage capacity of 2038 mA h g-1 with very small polarization.7, 11, 15 The working voltage of MgH2 is around 0.5 V (vs. Li/Li+), which is higher than the lithium plating voltage but much lower than many other conversion-type electrode materials,5 enables it to obtain higher voltage in a full cell. Although it has numerous advantages, the research on MgH2 is rather limited as it suffers from unsatisfactory cyclability, poor reversibility and slow kinetics in conventional non-aqueous electrolyte systems, which greatly limit the practical application of MgH2.7-9, 11, 16, 20 For instance, even if the MgH2 electrode was nano-confined, the reversible capacity reduced to less than 500 mA h g-1 after 20 cycles at a rate of 0.05 C in 1 M LiPF6 electrolyte (EC:DMC = 1:1), and the initial coulombic efficiency is less than 50%.16 According to these facts, it can be inferred that conventional non-aqueous electrolyte didn’t allow a significant capacity retention enhancement for these kinds of materials to date. In our previous study, a novel solid-state Li-ion conductor LiBH425-29 was used as electrolyte together with an MgH2-LiBH4 composite working electrode and a lithium metal counter


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electrode for an all-solid-state battery cell.23 Although high Li-ion conductivity (~10-3 S cm-1) is only shown in its high temperature phase, the LiBH4 electrolyte not only acted as Li-ion conductor but also promoted the H- conductivity, owing to the hydrogen exchange effect between LiBH4 and MgH2,30 which leads to a better reversibility for the above mentioned hydride conversion reaction (eqn. 1). Moreover, the kinetics was also enhanced by the high temperature of 120 °C. As a result, a reversible capacity of 1586 mA h g-1 was obtained at a relatively high current density of 800 mA g-1 (ca. 0.4 C), and the capacity retained 924 mA h g-1 after cycling for 50 times.23 Due to MgH2 being an insulator,31 enhancing the electrical conductivity of MgH2 electrode is key to further improving its electrochemical performances. In the present work, a vapor grown carbon nanofiber (VGCF) composed of fibers of ca. 150 nm in average diameter and ca. 10 µm in average length32 was employed as conductive agent. It was reported that VGCF is suitable to form continuous electron-conducting paths in the composite electrodes thanks to its fibrous morphology,33-34 which may lead to enhancing the electrical conductivity of the MgH2 electrode. In the meantime, LiBH4 was selected as the solid-state electrolyte to serve the MgH2 electrode, which is in consistence with our previous study. Additionally, a common sulfide-based solidstate electrolyte material 80Li2S·20P2S5 glass33-35 was also used in this work to assemble battery cells with MgH2 electrode, and the electrochemical performances of all the above samples will be discussed.

2. EXPERIMENTAL SECTION 2.1 Materials synthesis and battery assembly. Prior to being used as solid-state electrolyte, the commercial LiBH4 (≧95%, Sigma Aldrich) sample was heated at 200 °C under vacuum for


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overnight in order to remove ab- and adsorbed water. The sulfide-based electrolyte was synthesized by continuous ball-milling (20 h) for the mixture of Li2S (99.9%,) and P2S5 (99%, Sigma Aldrich) in a molar ratio of 80:20 using a planetary ball-mill apparatus (Fritsch P7) to obtain the 80Li2S·20P2S5 glass (named LPS hereafter).33, 35 The active material MgH2 used in this study was doped with 1 mol% Nb2O5 (99.5%, Sigma Aldrich) by 20 h ball-milling, in order to improve the kinetics of the hydride conversion reaction, which was shown in our previous studies.15, 23 The VGCF supported MgH2 composite electrode was prepared by mixing MgH2, VGCF (Showa Denko) and LiBH4 at a weight ratio of 50 : 25 : 25 via hand-milling using agate mortar for 5 minutes, followed by ball-milling for 2-15 hours. For the samples using LPS as solid-state electrolyte, the working electrode materials were prepared by ball-milling MgH2, LPS (or LiBH4), and VGCF at a weight ratio of 40 : 40 : 20 for 2 h, respectively. To prepare the triple-layer battery pellet, LiBH4 electrolyte (80 mg) was firstly loaded into a SUS die (15 mm in diameter) and pressed at 40 MPa, then the working electrode powder (5 mg) was sprinkled uniformly on the top of the electrolyte layer and pressed at 160 MPa to form a double-layer pellet. Next, the pellet was removed from the die set and a lithium foil was placed on another side of LiBH4 electrolyte as the counter electrode. Finally, the triple-layer pellet was put into a two-electrode coin-type cell (R2032 type) sealed with PFA O-ring for battery testing. All the procedures were done in the Ar-filled glove box. (The illustration of the whole battery structure is shown in Figure S1.) 2.2 Characterization and electrochemical measurements. The phase identification of composite electrode samples was conducted by powder X-ray diffraction (XRD, RINT2000, Rigaku) measurement using Cu Kα radiation at room temperature. XRD data were collected


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between the 2θ range of 20° to 60° in increments of 0.02°. The samples for XRD were protected with polyimide film to avoid the oxidation and the adsorption of water during measurement. The discharge-charge properties of the batteries were performed with galvanostat (HJ-SD8, Hokuto Denko Co.) between 0.3 and 1.0 V (vs. Li/Li+) at the current densities of 200 ~ 8000 mA g-1 (0.1 ~ 4 C). The capacity was calculated based on the weight of MgH2 and the electrolyte and carbon contributions were not considered. All the electrochemical measurements were conducted at 120 °C to obtain the fast Li-ion conduction. The morphology of the electrode samples before and after discharge-charge processes was observed by a scanning electron microscopy (SEM, JEOL JSM-6300) equipped with an energy dispersive X-ray spectrometer (EDS, JEOL EX-54175JMU).

3. RESULTS AND DISCUSSION 3.1 XRD characterization and SEM morphology observation of VGCF supported MgH2 composite. Figure 1 shows the XRD patterns of VGCF supported MgH2 composites ball-milled for 0–15 h. It can be seen that the diffraction peaks of MgH2, LiBH4, VGCF, and Nb2O5 phases were found on each pattern without showing new phases, indicating that no mechanochemical reaction occurred during the ball-milling process. It is noteworthy that the Nb2O5 phase was hardly to be detected by XRD when ball-milling with MgH2 in our previous studies,15,

23, 36

however, it

became detectable in this study (Figure S2) which might be due to commercial samples of Nb2O5 from different batches. With increasing ball-milling time, the peak intensities of LiBH4 and Nb2O5 were further reduced although the XRD patterns remained almost unchanged, indicating the particle size was decreased. The MgH2 peaks almost remained the same in each XRD patterns as it was pre-milled for 20 h before mixing with other phases, thus the particle size


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cannot be further reduced. The most significant change on the XRD patterns came from VGCF, which showed a very strong peak at around 26° reflects to the (002) plane (Figure S3). This peak almost disappeared on the curve of 15 h milled sample, revealing that the crystal structure of VGCF might be intensively destroyed by ball-milling effect. LiBH4 g

♦

•

g

• MgH ♦VGCF 2

•

•

♦

•

•

♦

♦

•

BM5h

•

BM2h

• •

20

•

• •

BM15h

g

•

g

•

♦

g Nb2O5

•

♦

Intensity (a.u.)

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30

♦

40 2θ (degree)

•

50

BM0h

60

Figure 1. XRD patterns of VGCF supported MgH2 composite electrode planetary ball-milled for 0-15 h.

Figure 2 displays the morphology of VGCF supported MgH2 composite samples ball-milled for 0–15 h. As shown in Figure 2a, non-uniform particles in the range of 0.5–5 µm was found in the non-milled sample and a large number of nanofibers (VGCF) distributed throughout the particles. When ball-milling treatment was applied on the samples for 2-15 h (Figure 2b, c and d),


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the size distribution became more and more uniform with a reduction in the particle size. In the meantime, the number of the nanofibers gradually decreased along with increasing in the milling time. After 15 h of ball-milling (Figure 2d), most of the particles became smaller than 2 µm. Furthermore, it was hardly to find any nanofibers on this SEM image, suggesting that almost all the VGCF was destroyed by ball-milling effect and decomposed into small pieces throughout the sample, which is in consistence with the XRD result shown by Figure 1.

Figure 2. SEM images of VGCF supported MgH2 composite electrode planetary ball-milled for (a) 0 h, (b) 2 h, (c) 5h, and (d) 15 h.

3.2 Electrochemical properties of VGCF supported MgH2 composite electrode working with LiBH4 electrolyte.


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The as-prepared VGCF supported MgH2 composite material was used as working electrode with Li metal counter electrode to assemble half-cells with an MgH2|LiBH4|Li configuration. The half-cells were cycled at a constant current density of 1000 mA g-1 (0.5 C) at 120 °C. The cut-off voltage was set to be 0.3-1.0 V (vs Li/Li+), in order to avoid the Li-Mg alloying reaction at around 0.1 V and the Mg(BH4)2 formation at above 1.0 V.23

Figure 3. (a) Initial galvanostatic discharge–charge curves and (b) Cycling performance of VGCF supported MgH2 composite electrodes at a current density of 1000 mA g-1 in the voltage range of 0.3–1.0 V at 120 °C (c) Initial capacity retention and coulombic efficiency as a function of ball-milling time. (d) Rate capability of the 2 h milled VGCF supported MgH2 composite electrode.


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Figure 3a shows initial galvanostatic discharge-charge profiles of the VGCF supported MgH2 composite milled for different durations. It is observed that the non-milled sample showed a discharge capacity (Li+ incorporation into MgH2) of 1382 mAh g-1 and charge capacity of 840 mAh g-1, corresponding to a coulombic efficiency of 60.8%. However, when ball-milling treatment was applied to the samples, the initial discharge capacity was significantly increased to 1731, 1702 and 1692 mAh g-1 for the 2, 5 and 15 h milled sample, respectively, with a calculated coulombic efficiency of, respectively, 96.7%, 96.3% and 95.5% (Figure 3c). It can also be seen that the discharge-charge polarization of the ball-milled MgH2 composite electrodes became slightly larger with increasing milling time, in other words, the electrical conductivity was weakened by long time ball-milling effect, which can be explained by SEM observation. As shown in Figure 2, most of the carbon nanofibers were retained in the 2 h milled sample, while it was totally disappeared in the 15 h milled sample. The fibrous morphology of VGCF is useful to form continuous electron-conducting paths,33-34 which leads to enhancement of the electrical conductivity of the composite electrode. As a result, the 2 h milled sample showed the lowest polarization and the highest reversibility in the initial discharge-charge cycle. In addition to these results, we had prepared 3 samples with different combinations of MgH2, LiBH4, and VGCF to obtain the optimal weight ratio of the solid electrolyte and VGCF. The results showed that the 10 wt% VGCF sample showed very poor reversibility in the initial discharge-charge cycle (Figure S4), while the 50MgH2-25LiBH4-25VGCF sample showed highest reversible capacity and highest coulombic efficiency. Figure 3b presents the cyclic performances of the VGCF supported MgH2 composite electrodes milled for different durations. It shows that the capacity of the non-milled sample rapidly faded to 202 mAh g-1 after only 10 cycles. Nevertheless, it can be observed that ball-


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milling treatment effectively not only increased the reversibility but improved the cycling durability of VGCF supported MgH2 composite electrode. After 50 cycles, the discharge capacity remained at 886, 1100, and 1221 mAh g-1 for the samples milled for 2, 5 and 15 h, respectively, and the corresponding capacity retentions were calculated to be 52.9%, 67.1% and 75.6% shown in Figure 3c. The 15 h milled sample showed the best cyclability, which was closely related to the particle size. As demonstrated in Figure 2, the 15 h milled sample exhibited the most uniform size distribution and smallest particle size, resulted in an increased specific surface area, which should be the major reason for its preferable cycling performance. This result was very similar to a previous report using ball-milled Li2MgSi as anode material for LIBs.37 It is believed that the composite electrode with carbon species forms aggregates, which may occlude the surface of the active materials and the solid electrolyte particles.38-39 In this study, 25 wt% of carbon additive was used to increase the electrical conductivity of MgH2, as a result, such a large number of carbon species could easily form agglomeration during continuously discharge-charge processes especially at such a high temperature of 120 °C, which caused the capacity fading after long time cycling. In order to confirm this presumption, a battery cell after 50 cycles was opened, and the working electrode material was carefully scratched from the pellet to measure SEM shown in Figure S5. It can be seen that the particles size increased to 10~20 µm, which was 5~10 times larger than the as-milled sample, proved that the agglomeration of particles was responsible for the capacity fading. The rate performance of VGCF supported MgH2 composite electrode was investigated shown in Figure 3d. The discharge capacity was 1796, 1679, 1603, 1502 and 1177 mA h g-1 at the current density of 200, 1000, 2000, 4000 and 8000 mA g-1, respectively, revealing an excellent rate capability which is contributed by the effect of VGCF as conductive agent. It is noteworthy


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that even at a high current density of 8000 mA g-1 (4C), the reversible capacity was still over 1100 mA h g-1, almost 3 times of the theoretical Li storage capacity of graphite. Moreover, the polarization between discharge and charge was only about 0.25 V, which means the energy loss during discharge and charge processes could be very small, indicating the possibility of using MgH2 as negative electrode material for high power and high capacity LIBs in future practical applications.

3.3 Electrochemical properties of the VGCF supported MgH2 composite electrode working with LPS electrolyte. It had been shown that MgH2 electrode worked well with LiBH4 electrolyte for a half all-solidstate lithium-ion battery cell, however, to make a full battery cell it’s necessary to find a suitable positive electrode material in this system. As we know, most of the positive electrodes are oxidizing materials while LiBH4 is known as a strong reducing agent, and therefore direct contact of LiBH4 with conventional positive electrodes (e.g. LiCoO2) may lead to the reduction of the oxide phases. There are few reports on the positive electrode performance with using LiBH4 as solid electrolyte. Takahashi et al. built a thin intermediate layer between LiBH4 and LiCoO2 by coating LiCoO2 with Li3PO4 to overcome the interfacial reaction, their result showed the interfacial resistance was reduced by three orders of magnitude compared to the non-coated sample, and the discharge capacity retained about 85 mA h g-1 at a current density of 0.05 mA cm-2 after 30 cycles.40 Even though their result showed some possibility of using LiBH4 electrolyte for a full battery cell, the reduced capacity and slow kinetics are still big issues to be overcome.


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As an alternative, the sulfide-based solid-state electrolyte material could be an ideal substitution to LiBH4 if it can well cooperate with MgH2 electrode, since it had shown impressive electrochemical performances with many conventional positive electrodes for bulktype all-solid-state LIBs.33-35,

41-42

In this part, the 80Li2S·20P2S5 glass (LPS) material was

synthesized as solid-state electrolyte and Li metal was also used as counter electrode. For the MgH2 working electrode, two different constitutions were prepared shown by Figure 4, one consisted of MgH2, LPS and VGCF (sample A), another consisted of MgH2, LiBH4 and VGCF (sample B). Both of these two samples were prepared by ball-milling for 2 h at a weight ratio of 40 : 40 : 20.

Figure 4. Battery configurations of MgH2-LPS-VGCF electrode (left) and MgH2-LiBH4-VGCF electrode (right) working with 80Li2S·20P2S5 solid-state electrolyte.

Figure 5a and b show the galvanostatic discharge-charge curves and cyclic property of sample A. It can be seen that MgH2-LPS-VGCF electrode delivered a discharge capacity of 1214 mA h g-1 in the first cycle, but the charge capacity was only 575 mA h g-1, corresponding to a coulombic efficiency of 47.3%. As increasing the cycle number, the discharge capacity rapidly fades to under 100 mA h g-1 after 15 cycles. Interestingly, the MgH2-LiBH4-VGCF electrode (sample B) showed a discharge capacity of 1728 mA h g-1 with 93.6% coulombic efficiency for the initial cycle (Figure 5c), and this capacity reached its maximum of 1889 mA h g-1 in the third


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cycle, then gradually dropped along with the increased cycle number. It can be seen that there is an extra voltage plateau at around 0.7 V on the charge curves, which was not found when using LiBH4 electrolyte. This result indicated other delithiation processes might be occurred, such as Li extraction from the carbon additive. After 50 cycles (Figure 5d), the discharge capacity remained at 1017 mA h g-1 with over 99.5% coulombic efficiency, and the corresponding capacity retention was calculated to be 58.8%, indicating that MgH2 electrode can also cooperate well with the LPS electrolyte if LiBH4 was included in the working electrode.


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Figure 5. (a) Galvanostatic discharge-charge curves and (b) cyclic property in the voltage range of 0.3–2.0 V for sample A; (c) galvanostatic discharge-charge curves and (d) cyclic property in the voltage range of 0.3–1.0 V for sample B. The battery cells run at a current density of 1000 mA g-1 at 120 °C.

In terms of Li ionic conductivity, LiBH4 and LPS are in the same order in this system. However, due to the fact that the initial coulombic efficiency of sample B was much higher than sample A, it can be deduced that the existence of LiBH4 in the working electrode not only provides the conductive paths for Li-ion, but also improves the electrochemical reversibility. According to our earlier findings,23, 30 the hydrogen exchange effect between MgH2 and LiBH4 could strongly enhanced the H- mobility of MgH2, which might lead to accelerating the hydride conversion reaction. That is to say, LiBH4 played a very important role in promoting the reversibility of the conversion reaction. Even if LPS was used as solid electrolyte, the LiBH4 species was needed to be included in the MgH2 electrode as well for better discharge-charge properties. However, the deeper reasons of how LiBH4 promotes the hydride conversion reaction is still not very clear, which reamins a challenge for futher investigations. Taking account into these factors, the H- mobility in MgH2 greatly affects its battery performances. When designing batteries with other metal hydrides, it’s better to make sure that the H- mobility is enough to complete the reversible hydride conversion reaction.

4. CONCLUSIONS

In this study, we have demonstrated the electrochemical performances of VGCF supported MgH2 composite electrodes with using LiBH4 and LPS solid electrolytes, respectively. The


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existence of VGCF significantly enhanced the electrical conductivity of MgH2, while long time ball-milling treatment provided small particle size and uniform size distribution, resulting in durable cyclability and excellent rate capability of MgH2 electrode with LiBH4 electrolyte. It is worth noting that LPS electrolyte could also show favourable electrochemical performances with MgH2 electrode, which indicates the possibility of making a bulk-type all-solid-state full battery cell consisting of lithium metal oxide (LMO) positive electrode, LPS solid electrolyte and MH negative electrode, since LPS had already shown its capability with many conventional positive electrodes (e.g. LiCoO2). It can be expected that high energy density and high power all-solidstate LIBs will be fabricated with the contribution from metal hydrides in the near future.

ASSOCIATED CONTENT Supporting Information. Schematic illustration of the all-solid-state coin cell, XRD profile of 1 mol% Nb2O5-doped MgH2, XRD profile of VGCF, Weight ratio optimization for the working electrode, SEM images after electrochemical cycles were included in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Takayuki Ichikawa. Email: [email protected]. Fax: +81-82-424-5744. Notes The authors declare no competing financial interest.


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Unravelling Small-Polaron Transport in Metal ... - ACS Publications

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