(Invited) Ionic Conduction in Metal Borohydrides and Their Application to All-Sold-State Batteries

Abstract

Metal borohydrides such as LiBH4 are widely used as a reducing agent in the synthesis of organic chemistry and a hydrogen storage medium. Meanwhile, ionic conduction as a new function of the metal borohydrides has been the focus of much attention since a high lithium ion conductivity of approximately 10-3 S/cm for LiBH4 was reported in 2007 [1]. To date, a number of metal borohydride-based ionic conductors such as Li3K3La2(BH4)12 and Na2B12H12 have been reported [2, 3]. In this study, the recent progress of metal borohydrides as an ionic conductor will be briefly overviewed. The advantages of the metal borohydrides as a solid electrolyte for secondary batteries follow: 1) good chemical compatibility with metallic anode such as Li, 2) good formability, 3) negligible grain boundary resistance, and 4) light weight. Meanwhile, because of its nature as a strong reducing agent, it comes with the risk of decomposing by reacting with oxidative cathode materials such as LiCoO2. The solution would be to use an intermediate layer in between the solid electrolyte and the cathode. In this study, all-solid-state batteries with the LiBH4 solid electrolyte were constructed and their cell performance was determined, especially focusing on the effect of the intermediate layer [4, 5]. In order to investigate the effects of an intermediate layer on electrochemical performance in all-solid-state cells, various materials such as TiO2 and Li3PO4 were deposited on LiCoO2 thin films. The thin films were grown by pulsed laser deposition (PLD) and atomic layer deposition (ALD). AC impedance analysis and galvanostatic charge-discharge measurements were carried out at 120°C, at which LiBH4 shows a high ionic conductivity of ≈10-3 S/cm. A cell using a bare LiCoO2 without an intermediate layer gave a large interfacial impedance more than 104 Ω. For the LiCoO2 cell with a 25 nm-thick Li3PO4 coating, the impedance reduced to around 20 Ω, and almost no change in resistance was observed even after 30 cycles of charge-discharge. The use of the intermediate layer resulted in significant improvements in both the capacity and cycle performance. The initial discharge capacity of the cell was estimated at 89 mAh/g. It also showed high capacity retention upon cycling: the discharge capacity after 30 cycles was more than 97% of the initial discharge capacity. Furthermore, a high-capacity composite anode comprising of the LiBH4 solid electrolyte and carbon materials was developed. In the composite anode, a massive aggregated graphite (MAG) and acetylene black (AB) were used as the carbon material. A ball-milled composite anode of AB and LiBH4 showed the first charge capacity of 883 mAh/g; however, the capacity fade was significant. 7Li-MAS NMR analysis revealed that the state of Li in the composite was not Li metal nor LiC6 but ionic Li on the surface and defects of AB. In addition, the peak of irreversible component suggests that Li inserted in the early stage is difficult to remove. This indicates that the irreversibility for the composite anode is caused by the poor contact between AB and LiBH4 at the discharge process. To overcome the problem, the use of intermediate layers was effective. The optimized composite anode showed the reversible capacity of higher than 500 mAh/g. The tasks and prospects for metal borohydride-based ionic conductor will be also mentioned. [1] M. Matsuo et al., Appl. Phys. Lett., 91 (2007) 224103. [2] M. Brighi et al., J. Alloys Comp., 662 (2016) 388. [3] T. J. Udovic et al., Chem. Comm., 50 (2014) 3750. [4] K. Takahashi et al., J. Power Sources, 226 (2013) 61. [5] K. Takahashi et al., Solid State Ionics, 262 (2014) 179.