Invited Presentation: Protected Lithium Electrode for Aqueous Lithium-Air Rechargeable Batteries

Protected Lithium Electrode for Aqueous Lithium-Air Rechargeable Batteries

Nobuyuki Imanishi

Department of Chemistry, Mie University, Tsu, Japan

The novelty of aqueous lithium air battery is the introduction of a protected lithium electrode using an impermeable lithium ion-conducting glass ceramics, Li_1+x+y(Ti,Ge)_1-xAl_xP_3-ySi_yO₁₂ (LTAP), which enables lithium metal anode feasible in the aqueous electrolyte with a narrow electrochemical window and boosts the specific energy density of this system coupling with a breathing air cathode [1]. The inherent drawback of LTAP is the high reduction tendency of its constituent, Ti⁴⁺ or Ge⁴⁺, in contact with lithium metal, which results in the generation of insulated reaction products. Several lithium-ion conducting interlayers have been proposed to address this issue, such as lithium nitride, lithium phosphorus oxynitride, polymer electrolytes as well as accustomed organic electrolytes. The two former materials are generally prepared by vapor deposition, which involves considerable cost and makes the preparation of large-sized cells difficult. In contrast, large-sized sheets of polymer electrolytes can be easily fabricated along with little concerns over lithium dendrites formation unlike the case for organic electrolytes.

Protected lithium electrodes consisting of lithium metal anode, polymer electrolytes-based interlayers and LTAP are our proposed solution for the practical development of aqueous lithium air battery. However, two contradictory issues emerge; those are the lithium dendrite formation at the interface between lithium metal and polymer electrolyte interlayers and the rate capability of protected lithium electrodes.

In our previous studies, the interfacial resistance between lithium metal and polymer electrolytes has been demonstrated a key factor in initiating lithium dendrite formation by addition of ionic liquids, oligomer ethers and/or nanofillers. All these additives can reduce the interfacial resistance substantially and prolong the onset time of lithium dendrite initiation. A prospective additive of tetraethylene glycol dimethyl ether was revealed to exhibit the onset time of ca. 13 h for protected lithium electrodes at 1.0 mA·cm^-2. The anion of lithium salts complexed with polymer matrix was also identified to influence the dendrite initiation in our recent study, where bis (fluorosulfonyl) imide-FSI^- exhibited the longest short-circuit time of symmetrical lithium pouch cell compared to other larger analogues.

At high current densities, the onset time of lithium dendrite initiation is shortened and thus the capacity of protected lithium electrode that can deliver is limited. A protected lithium electrode capable of delivering a high areal capacity of 12 mAh·cm^-2 or more at no less than 1.0 mA·cm^-2 is critical to develop aqueous lithium air batteries for electric vehicle applications. However, the binary polymer electrolytes we are developing usually exhibit a low limiting diffusion current density and are prone to initiate lithium dendrite formation when the operating current density exceeds its inherent limiting diffusion current density due to the quick depletion of lithium cations and the accompanying instability of interface. Moreover, the low exchange current density for polymer electrolytes at the interface also limits the rate capability of protected lithium electrodes.

Based on our previous findings, we are now envisaging a new concept for the lithium-ion conducting interlayer, such as solvent-in-salt systems and single-ion polymer electrolytes, to diminish the trade-off between the two aforementioned issues for protected lithium electrodes in the development of aqueous lithium air battery.

[1] N. Imanishi and O. Yamamoto, Rechargeable lithium–air batteries: characteristics and prospects, Mater. Today, 17, 24 (2014).