Electrochemical performance of metal hydrides in conventional organic liquid electrolyte (1M LiPF6 (DMC:EC=1:1).
The recently developed metal hydride (MH)-based material is considered to be a potential negative material for lithium-ion batteries, owing to its high theoretical Li storage capacity, relatively low volume expansion, and suitable working potential with very small polarization. However, it suffers from the slow kinetics, poor reversibility, and unfavourable cyclability in conventional organic liquid electrolyte systems, which enormously limit its practical application. In this chapter, we describe an all-solid-state battery system consisting of MH working electrode, LiBH4 solid electrolyte, and Li metal counter electrode. The electrochemical properties of MgH2 and TiH2 composites are investigated, which showed much better performances using LiBH4 as solid-state electrolyte than using conventional organic liquid electrolyte.
- Metal hydride
- solid state
- conversion electrode
Currently, lithium-ion batteries (LIBs) with zero emission are considered to be the most important energy carrier in our daily life, which are absolutely imperative devices to power mobile phones, laptops, electric vehicles (EVs), and so on [1, 2]. Developing high-capacity materials for LIBs is necessary to satisfy the continuously rising need for energy. In the past tens of years, graphite is always used as negative electrode for commercial LIBs with an insufficient specific capacity (370 Ah kg−1, 840 Ah L−1) . To overcome these restrictions, new concepts for the negative electrode must be developed. In other words, seeking suitable high-performance anode materials is essential and urgent.
In order to develop LIBs with high energy density, the conversion-type electrode materials had been intensively studied in recent years owing to their high Li storage capacity [2, 4]. In 2008, metal hydride (MH) was firstly reported as negative electrode for this purpose . Compared to other conversion-type electrode materials, MH exhibits not only high theoretical Li storage capacities, but also relatively low volume expansions and suitable working potentials with very small polarization between charge and discharge . The Li+ insertion/extraction reactions for MHs can be written by the following hydride conversion reaction:
The theoretical gravimetric and volumetric capacities of some potential MHs as negative electrode for LIBs are shown in Figure 1. It can be seen that the capacities of all hydrides presented in this figure are larger than that of graphite. MgH2 possesses the highest gravimetric capacity of 2038 Ah kg−1, while Mg2FeH6 possesses the highest volumetric capacity of 3995 Ah L−1. These large capacities lead MHs to a good candidate material for negative electrodes in LIBs for stationary as well as mobile applications for which the volumetric capacity plays a key role .
2. Metal hydride electrodes working in conventional organic liquid electrolyte systems
Among all of the MHs, MgH2 is considered as the most promising candidate owing to its high theoretical Li storage capacity of 2038 mAh g−1, a suitable working potential of around 0.5 V (vs Li+/Li), a relatively small volume change of 83%, and so on [5, 7]. The Li+ insertion/extraction reaction of MgH2 can be written by the following hydride conversion:
Further lithiation results in Li–Mg alloying reaction, which is partially irreversible.
For instance, the MgH2 electrode working in 1 M LiPF6 (DMC:EC=1:1) showed a reversible capacity of 1480 mAh g−1 in the first cycle; however, the capacity fades rapidly and reduces to less than 200 mAh g−1 after only 10 cycles. Sustained capacities could be achieved only by using a special copper foam current collector for particle confinement and limiting the reaction to 520 mAh g−1 . A more recent report shows that a reversible capacity of about 500 mAh g−1 can be obtained after 20 discharge–charge cycles at a rate of 0.05 C with using nanoconfinement technique for MgH2, but the coulombic efficiency of the first cycle is less than 50% .
After the discovery of MH as negative material for LIBs by Oumellal et al, some other groups followed their work and extended the research to many other hydrides, that is, TiH2, TiNiH, Mg2FeH6, AlH3, LiAlH4, NaAlH4, and so on [6, 8–24]. The electrochemical performances of these materials are summarized in Table 1.
|Sample||Theoretical capacity (mAh g−1)||Reversible capacity of 1st cycle (mAh g−1)||Current density (C)||Cycling performance||Composition of electrode||Voltage range (V vs Li/Li+)||Reference|
|MgH2||2038||1480||1/20||520 mAh g−1|
after 50 cycles*1
|MgH2:Carbon SP = 85:15||0.15–3.0|||
50 mAh g−1 after
carbon = 7:1:2
|MgH2||2038||1900||1/20||542 mAh g−1|
after 40 cycles
carbon = 1:1:1
|TiH2||1074||1072*2||1/200||N/A||TiH2:C150,480 carbon = 10:1||0.005–3.0|||
carbon = 10:1
carbon = 10:1
carbon = 10:1
carbon = 10:1
|MgH2||2038||950||1/20||500 mAh g−1|
after 20 cycles
carbon = 80:20
carbon = 5:2:3
carbon = 5:2:3
carbon = 5:2:3
|Mg0.7Ti0.3H2||1748||1540||1/20||530 mAh g−1|
after 7 cycles
|MgH2:TiH2:Ct,x = 7:3:1||0.005–3.0|||
|AlH3||2678||1100||1/30||393 mAh g−1|
|AlH3:AB:CMC-f = 1:1:1||0.005–3.0|||
|NaAlH4||1984||770||1/30||250 mAh g−1|
after 10 cycles
In spite of numerous advantages of MH mentioned above, the research on MH as negative electrode materials did not get enough attention since it suffers from the slow kinetics, poor reversibility, and unfavourable cyclability in conventional organic liquid electrolyte systems, which enormously limits its practical application.
3. MgH2 composite electrode working with LiBH4 solid-state electrolyte
From the above results, it can be inferred that the conventional organic liquid electrolyte systems might not be suitable for MH electrodes, thus an idea of using solid-state Li-ion conductor as electrolyte had come out.
There are many types of solid-state Li-ion conductors, including LISICON-type, Garnet-type, Perovskite-type, NASICON-type lithium-ion conductors, and glassy and glass-ceramic systems made of oxides and sulphides [25, 26]. When seeking a suitable medium to achieve a reversible conversion reaction by MH shown by Eq. (2), the Li+ conductivity is one side to be considered, and another important factor is the H− conductivity. As we know, the MgH2–LiBH4 composite is a well-known material in hydrogen storage research field. In this system, the mobility of H− in MgH2 can be strongly enhanced by LiBH4, due to a superior hydrogen exchange effect between MgH2 and LiBH4 . That is to say, this effect may lead to a significant improvement of H− conductivity when using this material as negative electrode for LIB. In the meantime, it was reported that LiBH4 showed remarkable high Li-ion conductivity to the order of 10−3 S cm−1 at its HT phase (>115°C) with a very stable operating window between 0 and 5 V vs Li+/Li, which can be used as solid-state electrolyte for LIBs [26, 28–30]. Considering the above two effects, when using MgH2–LiBH4 as negative electrode and LiBH4 as solid-state electrolyte, the hydride conversion reaction of Eq. (2) could be dramatically accelerated, which may lead to much better battery performance for reversibility and cyclability.
In this part, a battery cell constructed of MgH2–LiBH4|LiBH4|Li (illustrated in Figure 2) had been fabricated for electrochemical tests. 1 mol% Nb2O5-doped MgH2 (named c-MgH2 hereafter) was also used in this study for comparison, which had been proved to have positive effect in battery performance [20, 21, 23]. The working electrode was prepared by simply ball-milling MgH2/c-MgH2, LiBH4, and conductive carbon (acetylene black) with no further optimization. The thermal stability of MgH2–LiBH4 composite was measured by thermogravimetry with differential thermal analysis (TG–DTA), which shows a weight loss of less than 1 wt% in the temperature of RT–275°C (Figure 3). To obtain the fast Li+ conduction from LiBH4, all electrochemical measurements in this study were conducted at 120°C. The electrode evolution upon discharge–charge process at different stages was investigated by means of powder X-ray diffraction (XRD).
Figure 4 shows the first galvanostatic discharge–charge curves for MgH2–LiBH4 and c-MgH2–LiBH4 composite electrodes using LiBH4 as solid-state electrolyte in the voltage range of 0.3–1.0 V vs Li+/Li at a current density of 100 mA g−1. It can be seen that on the discharge curve (Li+ incorporation into MgH2), the potential gradually dropped to near 0.5 V, followed by showing a long flat plateau for c-MgH2 electrode and a slope for MgH2 electrode, corresponding to the hydride conversion reaction shown by Eq. (2). The discharge process was cut at 0.3 V to avoid the Li–Mg alloying reaction: Mg + x Li+ + x e− ↔ MgLix, which is a partial irreversible reaction and would weaken the cyclability of MgH2. On the charge curve (Li+ extraction), a plateau slightly above 0.5 V was observed for both samples. The polarization between Li insertion and extraction was 0.1 V for MgH2 electrode and 0.05 V for c-MgH2 electrode, which are much smaller than that of the MgH2 electrodes working with organic liquid-based electrolyte in some previous reports [5, 9]. Such a small polarization could be owing to the high working temperature of 120°C, which brings better kinetic properties to the reaction. The initial reversible capacity and coulombic efficiency were 1650 mAh g−1 and 94.7% for c-MgH2 electrode and 1572 mAh g−1 and 93.7% for MgH2 electrode. The above results indicated that the existence of LiBH4 significantly promoted the hydride conversion reaction due to its dual effects on Li+ conductivity and H− conductivity. In addition, the Nb2O5 doping to MgH2 further improves the reaction kinetics, resulting in better reversibility and higher initial coulombic efficiency.
The ex situ XRD measurement had been carried out at different stages upon the initial discharge–charge process shown by Figure 5. The LiBH4 phase appeared in all the stages since it was included in the working electrode. At open circuit voltage stage (i), a small amount of Mg phase was detected in the XRD pattern as an impurity, which came from the as-received commercial MgH2 sample. As Li reacted with MgH2, the LiH and Mg peaks appeared while MgH2 peaks disappeared at 0.3 V (ii), indicating the hydride conversion reaction shown by Eq. (2) had occurred. When recharging the same battery cell, clear MgH2 peaks reappeared with weakened LiH and Mg peaks at the intermediate stage (iii). Finally, the LiH phase totally disappeared with a very small amount of Mg at 1.0 V (iv), which is equivalent to stage (i). The above results verified the good reversibility of the hydride conversion reaction. Interestingly, a new phase of Mg(BH4)2 was found (Figure 6) when further charging the battery cell to 2.0 V, which implies a Mg(BH4)2 generation reaction might have occurred between 1.0 and 2.0 V shown by Eq. (4).
To avoid this undesirable reaction, the galvanostatic discharge–charge cycling tests were performed between 0.3 and 1.0 V at a current density of 800 mA g−1 as shown in Figure 7. It shows that the MgH2–LiBH4 electrode delivered a highly reversible capacity of 1488 mAh g−1 in the first cycle; however, only 270 mAh g−1 remained after 93 cycles. In the meantime, a reversible capacity of 1586 mAh g−1 is shown in the first cycle for the Nb2O5-doped electrode, which retained at 700 mAh g−1 after cycling for 100 times. This result suggests the cyclability of the MgH2–LiBH4 electrode is also improved by the Nb2O5-doping effect. The galvanostatic discharge–charge curves of Nb2O5-doped electrode are shown by Figure 8. It can be observed that the plateau voltage of the discharge and charge profiles almost has no changes during 100 cycles with coulombic efficiency over 99.5%, which indicated that the working electrode operated properly between 0.3 and 1.0 V, namely, there was no side reaction during the discharge–charge cycles. Moreover, the suitable Li insertion/extraction voltage of around 0.5 V (vs Li+/Li) and the small polarization are particularly remarkable, making it a promising candidate as negative electrode material for LIBs.
The rate capability of the Nb2O5-doped electrode (c-MgH2–LiBH4) was also investigated as shown by Figure 9. It can be seen that the discharge capacity was 1510, 1600, 1659, 1704, and 1742 mAh g−1 at the current density of 3200, 1600, 800, 400, and 100 mA g−1, respectively, indicating that this sample possessed excellent rate capability. The fast kinetics of this material could be ascribed to rapid Li+ and H− diffusion in the working electrode and on the interface between electrode and electrolyte. It is worth noting that the plateau voltage gradually decreased along with increase in the current density, which was caused by the insufficiency of electrical conductivity at high discharging rate. As we know, the electrical conductivity of electrode material can be remarkably improved via surface modification technique, for example, carbon coating , which can be also applied in this system for future working direction in order to obtain better high rate performance.
4. TiH2 electrode working with LiBH4 solid-state electrolyte
Another promising candidate of MHs as negative electrode is titanium hydride (TiH2), which has a theoretical capacity of 1074 mAh g−1 with a low working potential of 0.163 V vs Li+/Li by the following reaction:
In the viewpoint of making high-power battery, TiH2 is preferable to MgH2 as negative electrode since its theoretical working potential is much lower than MgH2, which leads to a higher voltage for a full battery cell. The electrochemical Li insertion/extraction mechanism of TiH2 was reported by Oumellal et al. . However, the cyclic properties have not been reported yet due to the poor reversibility in conventional organic liquid-based electrolyte.
In this part, we had prepared the TiH2–LiBH4 composite as the negative electrode together with LiBH4 as the solid-state electrolyte for an all-solid-state battery cell, in order to obtain good cyclic performance for TiH2. This system is similar to the above content for MgH2 electrode, which shows favourable electrochemical performances. The phase evolution of TiH2 electrode upon charge–discharge in the solid-state cell, cyclic properties, and rate performance are investigated.
Figure 10 shows the initial charge–discharge curves of the TiH2 electrode between 1.0 and 0.05 V using LiBH4 solid electrolyte under the current density of 400 mA g−1 at 120°C. The Li insertion curve shows slope and plateau regions: slope region (0.4–0.15 V) and plateau region (0.15 V). From some previously reported results , it was considered that the slope region indicated fcc-TiH2 reacted with lithium to form LiH and partially transformed into a distorted orthorhombic phase fco-TiH2-x, and slope region indicated that both of fcc-TiH2 and fco-TiH2-x react with lithium to form Ti and LiH (TiH2 + 2Li+ + 2e− → Ti + 2LiH). Unlike the case of MgH2, no plateau could be observed below 0.15V, indicating that Ti did not electrochemically react with Li to form Li–Ti alloy. During the initial Li insertion, the capacity of 1225 mAh g−1 was obtained, while the theoretical capacity was 1074 mAh g−1. This overcapacity could be due to the existence of acetylene black in the electrode, which contributes on the Li storage capacity as an anode.
On the Li extraction curve, the plateau corresponding to de-lithiation process of TiH2 conversion reaction was observed at higher voltages than 0.15 V, indicating the higher polarization of electrode compared with the case of MgH2 system . Furthermore, a short plateau at 0.75 V was observed and the whole Li extraction curve showed 1052 mAh g−1 corresponding to 86% coulombic efficiency.
In order to identify the electrochemical reaction, ex situ XRD measurements were performed at the different states of charges as shown in Figure 11. It is noteworthy that all the profiles shown in Figure 11 contain peaks corresponding to LiBH4, which should always be included in the electrodes. At the initial state (i), the broad peaks corresponding to TiH2 were observed due to the ball-milling effect. After the Li insertion (ii), the broad peaks slightly shifted to higher angle, indicating that the TiH2 structure should be slightly shrunk due to partial extraction of hydrogen. And a small peak corresponding to fco-TiH2-x and Ti were observed. With further lithiation (iii), it was clearly observed that the TiH2 changed to Ti and LiH. For the Li extraction process, H atoms were transferred from LiH to Ti. Then, as shown in Figure 10 (iv, v), peaks corresponding to TiH2 reappeared, then the Ti peak was strongly reduced. These facts indicate that TiH2 electrode reveals high electrochemical performance by using the LiBH4 solid-state electrolyte. The combination of hydride and borohydride for anode and electrolyte, respectively, could lead to a better performance.
Unfortunately, the reaction at 0.75 V as shown in Figure 10 was not clarified, yet even though we have performed the XRD measurement at various states of charge. As one of the possible reasons, it could be caused by the Li extraction process from acetylene black, which cannot be detected by XRD due to its amorphous feature.
Figure 12 shows the cyclic performance of TiH2–LiBH4 composite electrode in the voltage range of 1.0–0.05 V at a current density of 400 mA g−1 at 120°C. Almost the same profile as the initial cycle was observed in later cycles. The Li insertion capacities of the 2nd, 10th, and 50th cycles were 1094, 1035, and 878 mAh g−1, respectively, which corresponding to a capacity retention of about 80% for 50 charge–discharge cycles. This result revealed for the first time that the charge–discharge process of TiH2 electrode is recyclable, which makes it a promising candidate for LIBs.
The rate capability of the TiH2 composite electrode was also investigated as shown in Figure 13. All the discharge profiles in this figure were created by the initial discharge cycle of independent cells at different current densities of 400, 800, and 1600 mA g−1, which are corresponding to about 1/3 C, 2/3 C, and 4/3 C, respectively. It can be seen that discharge capacity was 1225, 1165, and 1007 mAh g−1 at the current density of 400, 800, and 1600 mA g−1, respectively, indicating a very good rate capability of TiH2 electrode in this system. For comparison, the charge–discharge for TiH2 in a conventional organic electrolyte could be performed only at 10 mA g−1 . Like in the MgH2 system, the fast kinetics of TiH2 could also be ascribed to rapid Li+ and H− diffusion in the electrode material and on the electrode–electrolyte interface at a high working temperature of 120°C.
In summary, we have demonstrated an all-solid-state battery system consisting of MH working electrode, LiBH4 solid electrolyte, and Li metal counter electrode. The MgH2–LiBH4 and TiH2–LiBH4 composite electrodes showed much better performances using LiBH4 as solid-state electrolyte than using conventional organic liquid electrolyte in some previous reports. LiBH4 here acts as multifunctional ion conductor, promotes not only Li+ but also H− conductivity, resulting in much higher reversibility for the hydride conversion reaction. In addition, Nb2O5 doping shows significant improvement on cyclability of MgH2. The suitable operating voltage, large reversible capacity, and the very small polarization are particularly remarkable to make MH a promising negative electrode material for all-solid-state LIBs. Besides, not only LiBH4 but also a large number of hydride-based solid-state alternatives for example, Lim(BH4)nX (X = Cl, Br, I) [29, 32], LiBH4–LiNH2 [29, 33], and LiAlH4/Li3AlH6  can also be considered for this system. The working temperature can be altered while using these various substitutions, some of which even show high Li-ion conductivity at room temperature. This work shows a new direction to search for potential high-performance negative electrode materials for LIBs. The electrochemical performance of numerous MH-based materials will be investigated in a similar system as in this article in the future.