All-Solid-State Lithium Battery Using Mg(BH4)2 As an Electrode
Authors: Sato Hiroki , Inoishi Atsushi , Sakamoto Ryo , Minami Hironari , Izumi Hiroaki , Okada Shigeto 
Published: December 2020

All-solid-state batteries are required to show the large capacity and reduce of the interfacial resistance between the solid electrolyte/electrode interface. To address these issues, “complex hydrides” have recently received attention as solid electrolytes for all-solid-state lithium batteries. In this study, we focus on Mg(BH4)2 as conversion-type anode active material for all-solid-state rechargeable lithium batteries. Mg(BH4)2 has been reported as an anode active material for Li-ion battery with 1M LiPF6/EC+DMC(1:1) [1]. However, the reaction mechanism of the Mg(BH4)2 anode is still unclear. There are following 4 types of possible conversion reactions, and the reaction formula (1) and (2) were assumed in Ref. 1.

Mg(BH4)2 + 8Li → 8LiH + 2B + Mg (1) ; The theoretical capacity: 3970 mAh/g

Mg(BH4)2 + 6Li → 6LiH + 2B + MgH2 (2) ; The theoretical capacity: 2978 mAh/g

Mg(BH4)2 + 4Li → LiBH4 + 3LiH + B + 0.5Mg + 0.5MgH2 (3) ; The theoretical capacity: 1985 mAh/g

Mg(BH4)2 + 2Li → 2LiBH4 + Mg (4) ; The theoretical capacity: 992 mAh/g

However, since its initial discharge capacity was less than 700 mAh/g, we inferred that the discharge reaction formula should be formula (4) with the smallest theoretical capacity. In the case of (4), we can expect the formation of LiBH4, which is a Li ion conductor, as a discharge product [2]. In solid-state batteries, it is necessary to include a solid electrolyte of almost the same weight in addition to the active material in the electrodes, which is a major hindrance to the improvement of the energy density of the solid-state battery. However, if Mg(BH4)2 self-generates the ionic conductor LiBH4 according to the reaction formula (4), Mg(BH4)2 anode should be freed from the handicap. In addition, another discharge product, Mg metal, should be useful to reduce the interfacial resistance. To confirm the self-generation effect of LiBH4, we compare the anode performance of Mg(BH4)2 with/without LiBH4;

Mg(BH4)2 + LiBH4 + acetylene black (AB) = 40 : 30 : 30 (weight ratio)

Mg(BH4)2 + AB = 70 : 30 (weight ratio)

The electrodes were mixed by mechanical milling of a mixture of Mg(BH4)2 : AB = 70 : 30 (weight ratio) and / or Mg(BH4)2 : LiBH4 : AB = 40 : 30 : 30 (weight ratio) at 400 rpm under argon atmosphere. Milling was performed using a planetary-type mill (FRITSCH Pulverisette 6). The solid electrolyte LiBH4 and the anode mixture were uniaxially pressed at 10 kN and 30 kN to obtain two-layer pellets, respectively (diameter; 10 mm). The two-layer pellets were pressed against Li metal to form half cells, and discharge/charge measurements were performed using HS cells (Hohsen Corp.).

Figure 1 shows discharge/charge curves of anode mixture with/without LiBH4 at 120 oC. From the discharge/charge curves of Figs. 1 (a) and (b), it can be seen that the reversible reaction proceeded even without LiBH4 in the anode electrode mixture. Furthermore, in comparison of the specific capacity based on the total weight of the composite anode including AB, the initial reversible capacity of anode without LiBH4 in Fig. 1 (a) was much larger than that of anode with LiBH4 in Fig. 1 (b). Although there is room for improvement in the utilization of anode without LiBH4, a flat voltage plateau was also observed around 0.8 V with only 0.1 V overvoltage even at 0.5 mA/cm2 high rate condition. These results suggest that the anode self-assembles LiBH4 with high ionic conductivity during the discharge process according to the formula (4), as initially expected. In addition, the cycling performance of the anode without LiBH4 was almost similar to that of the anode with LiBH4 as shown in Figs. 1 (c) and (d). From the result of the ex-situ XRD, it was confirmed that Mg metal was formed in the discharged sample, although the diffraction peak of LiBH4 could not be detected. We will discuss the detail reaction mechanism during the cycling at the presentation.

References

[1] D. Meggiolaro, L. Farina, L. Silvestri, S. Panero, S. Brutti, P. Reale, Energies, 9(4) (2016) 238.

[2] H. Maekawa, M. Matsuo, H. Takamura, M. Ando, Y. Noda, T. Karahashi, S. Orimo, J. Am. Chem. Soc.,
131 (2009) 894.

Figure 1