The long life of lithium ion batteries makes them the rechargeable of choice for everything from implantable medical devices to wearable consumer electronics. But lithium ion batteries rely on liquid chemistries involving lithium salts dissolved in organic solvents, creating flame risks that would be avoided if the cells were completely solid-state. Now a team of researchers at Tohoku University in Japan has created a new type of lithium ion conductor for future batteries that could be the basis for a whole new generation of solid-state batteries. It uses rock salt Lithium Borohydride (LiBH4), a well-known agent in organic chemistry laboratories that has been considered for batteries before, but up to now has only worked at high temperatures or pressures.
In the journal APL Materials, the researchers describe how they doped a cubic lattice of KI molecules with the LiBH4. This allowed them to stabilize the high-pressure form of Lithium borohydride and make a solid solution at normal atmospheric pressure that was stable at room temperature.
In making the new technology, the team made the peculiar discovery that the Li+ ions functioned like pure Li+ ion conductors, even though they were just doping the KI lattices. This is the reverse of the normal doping technique, in which a small amount of stabilizing element would be added to an ionic conductor abundant in Lithium.
“In other words, LiBH4 is a sort of ‘parasite’ but not a host material,” said Hitoshi Takamura who led the research at Tohoku University. He and his colleagues have called this mechanism “parasitic conduction” and have suggested that it could be broadly applied in the search for new batteries — anywhere that small amounts of Li+ ions could be used to dope an oxide, sulfide, halide or nitride host material.
“This work suggests the potential of this mechanism in the ongoing search for the perfect material for use in solid state batteries,” added Takamura. “The urgency of this quest has been abundantly clear after the grounding of so many aircraft in recent months.”
"In our previous research, high lithium-ion conductivity of LiBH4 has been clarified and an all-solid-state LIB using LiBH4 developed. However, the pure LiBH4 seems to have anisotropy for lithium-ion conduction; that may show lower lithium-ion conductivity in some direction. To overcome the problem, in this current work, we focused on the different crystal structure of LiBH4, that is to say, the rock-salt-type (NaCl-type) structure. The rock-salt-type LiBH4 is cubic and isotropic; however, the structure is only stable above 200 degrees C and under extremely high pressure of above 40,000 bar!" Takamura observed.
"So, the issue is how to obtain the rock-salt-type LiBH4 at room temperature and under atmospheric pressure. In general, to stabilize an unstable structure, a technique taken is to dope some elements (dopants) into the objective compound in this case LiBH4. However, in this research, KI (potassium iodide) with the rock-salt-type structure is selected as a host and then LiBH4 is doped into KI. As a result, so-called solid solution of KI and LiBH4 was successfully prepared. In addition, the solid solution of KI-LiBH4 shows a certain level of lithium-ion conductivity even for small amount of LiBH4, for example 25%LiBH4. This means that lithium ion plays a major role for ionic conduction even though major constituent cation is potassium ion but not lithium. In other words, lithium ion is a sort of “parasite”. So, we call the way lithium ions move in the KI-LiBH4 solid solution as “parasitic conduction mechanism," Takamura continued.
If the Parasitic Conduction Mechanism occurs within a given Li + ion conductor, a light amount of Li doping would be sufficient for material design, which means a new ion conductor could be fabricated without the limits imposed by the solubility limit of a given system. This relatively uncommon method of material synthesis involving the Li compound doping of the existing crystal lattice has the potential to become a standard procedure in the quest for a new ion conductor and to provide the stabilization of a given phase. This research on the so-called Parasitic Conduction Mechanism is expected to contribute to the further progress of solid state ionics.
In conclusion, the researchers synthesized the H.P. form of LiBH 4 under ambient pressure by doping LiBH 4 with the KI lattice by sintering. The formation of a KI – LiBH 4 solid solution was confirmed both macroscopically and microscopically. That is, the H.P. form was clearly stabilized by chemical modification. The obtained sample was shown to be a pure Li + conductor despite its small Li + content. This conduction mechanism, where the light doping cation played a major role in ion conduction, was termed the "Parasitic Conduction Mechanism." This mechanism made it possible to synthesize a new ion conductor and is expected to have enormous potential in the search for new battery materials.
"The importance of our most-recent research is to propose a new way of material design for developing new lithium-ion conductors. As mentioned above, for most cases, a small amount of stabilizing element is added to an unstable ionic conductor. Meanwhile, in this study, LiBH4 is “doped” to the cubic KI host lattice to stabilize its (LiBH4) unstable rock-salt form. This also implies that heavy doping of lithium-ion is not necessary to make a pure lithium-ion conductor. This parasitic conduction mechanism has a possibility to be applied in any lithium-ion conductors. This mechanism can take place, if a small amount of lithium ion can be doped to any oxides, sulfides, halides, and nitrides to be a host framework. Based on this idea, one can search a new system without a conventional prejudice that lithium-ion conductor should have considerable amount of lithium ion," Takamura concluded.