High performance battery (Researchers at Japan’s AIST)

Key components, cell voltage, and cell capacity of Li-ion battery (a), Ni-MH battery (b), and the proposed Ni-Li battery (c). Credit: ACS, Li et al. Click to enlarge.
Researchers at Japan’s National Institute of Advanced Industrial Science and Technology (AIST) have developed a prototype of a battery that can simultaneously offer the high cell voltage of Li-ion cells and the large cell capacity of Ni-MH cells: a rechargeable nickel (cathode) / lithium metal (anode) battery using a hybrid aqueous and organic electrolyte separated by a superionic conductor glass ceramic

The proposed Ni-Li battery offers both a high cell voltage (3.49 V) and a large cell capacity (268 mAh/kg), which together create an ultrahigh energy density. The theoretical energy density calculated using only the active electrodes and cell voltage for the Ni-Li battery is 935 Wh/kg. With the same calculations, NiMh offers 214 Wh/kg, and cobalt oxide Li-ion cells offer 414 Wh/kg. A paper on the proposed Ni-Li system was published 5 October in the Journal of the American Chemical Society.

“The amount of electrical energy E (Wh/kg) that a battery is able to deliver is a function of the cell voltage U (V) and capacity Q (Ah/kg), both of which are linked directly to the chemistry of the system.”
—Li et al.
Current prominent battery systems such as Li-ion and NiMH demonstrate “huge gaps” between expected and practical performances, the researchers note. Li-ion cells are hobbled by the limited inherent capacity of their cathode materials; by their low power densities which are restricted by the slow electrode kinetics relating to Li intercalation/deintercalation from the host materials; and safety issues.

As for NiMH, although both the cathode and anode material can deliver a large capacity, the cell voltage is only 1.32 V due to the limitation of aqueous electrolyte.

One radical exploration is to break the routine of classical batteries which involves a single electrolyte. If an aqueous electrolyte and organic electrolyte can be smartly integrated in one battery, it would enable state-of-the-art combination choices for the existing battery chemistry. Recently, a superionic conductor glass ceramic film (LISICON) with stability in aqueous solution and its application in a Li-air battery has been reported. [Earlier post.]

Here, we proposed integrating a nickel hydroxide electrode working in an aqueous solution as the cathode and a Li metal working in an organic electrolyte as the anode by a LISICON film to fabricate a rechargeable Ni-Li battery.

Li is the most negative metal while at the same time possessing an ultrahigh capacity of 3, 860 mAh/g, thus facilitating the design for high energy density. However, the uneven plating of Li in the form of dendrites during discharge-recharge cycles may puncture the polyolefin thin separator, leading to short circuit hazards. In the Ni-Li battery, the rigid ceramic LISICON film is hardly punctured by Li dendrites thus enabling the utilization of Li metal.

As for a cathode electrode, nickel hydroxide, with a less positive potential and an aqueous solution as the electrolyte, is inherently safer than the case of the cathode in the Li-ion battery.

—Li et al.
Work on the Ni-Li battery is in very early stages, the researchers said. Although the power ability of the Ni-Li battery is expected to be superior to that of the Li-ion battery regarding the electrode kinetics, the current data “are not satisfying” due to the low conductivity of the LISICON film.

Although assembly of such a battery seems “somewhat complicated”, they wrote, the implementation of a hybrid electrolyte can provide a variety of choices for electrode materials.

In summary, we propose a rechargeable battery system by integrating two reversible electrode processes associated with an aqueous and a nonaqueous electrolyte, respectively. The prototype Ni-Li battery promised an ultrahigh theoretical energy density as well as a high power potential, which reinforced the view that it is an important avenue to fulfill the best-performing combination for an electrode/electrolyte/electrode system.