Zinc air batteries are safe and inexpensive aqueous systems with a high theoretical energy density of 1350 Wh/kg (based on zinc mass). The primary zinc air cells have been long used as power sources for hearing aid. Attemps have been made for more than 40 years to use them as electrically rechargeable (secondary) systems, however, the insufficient lifetime of both zinc and air electrodes are still unsolved issues. Recent progress on the air electrode shows promise for better rechargeability. This study focuses on the improvement of the zinc electrode cyclability to show the possibility of “beyond lithium ion batteries”.
2. Results and Discussion
Morphology changes such as dendrite formation and shape changes have been indicated as the primary origin of zinc electrode deterioration [1]. Here we apply synchrotron-beam assisted analytical methods, which are currently used for lithium ion systems, to the zinc electrode system to clarify the underlying mechanism.
Operando X-ray diffraction [2] and operando X-ray fluorescence imaging [3] suggests that the soluble zincate is the origin of the morphology changes. Namely, zincate anions Zn(OH)42-formed by the electrochemical reaction (1) diffuse away from the electrode during discharging before they are decomposed by the following chemical reaction (2) to form the final product zinc oxide ZnO.
Zn + 4OH- = Zn(OH)42-+ 2e- (1)
Zn(OH)42- = ZnO + H2O+ 2OH- (2)
To restrict the zinc solubility by moving the equilibrium of reaction (2) to the right hand side, H2O activity control was applied by adding organic solvents in alkaline electrolytes [4]. The effect of propylene glycol (PG) was intensively examined as follows. Figures 1(a) and 1(b) show that the addition of PG effectively limits the thermodynamic solubility of ZnO and also kinetically assists the decomposition of supersaturated zinc species in the alkaline solution. The zinc electrode with PG mixed solvents exhibited long cycle life as shown in Fig. 1(c) at a high zinc utilization rate of 75%, revealing the possibility of attaining both long cycle life and high energy density. The vulnerability of PG to oxidation at the air electrode can be evaded by using organic solvents with higher molecular weights. In addition, surface coating of the zinc particles are also effective in enhancing the rechargeability, owing to the zinc oxidation/reduction processes inside the shell of the coat.
Acknowledgment
This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project of NEDO (Japan).
References
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[4] A. Nakata, H. Arai, T. Yamane, T. Hirai and Z. Ogumi, J. Electrochem. Soc., 163, A50 (2016).
[5] A. Nakata, T. Kakeya, M. Ono, H. Arai and Z. Ogumi, The 228th Electrochemical Society Meeting, Abstract No. 208 (2015).
[6] T. Kakeya, A. Nakata, H. Arai and Z. Ogumi, 56thBattery Symposium in Japan, 1G16 (2015).
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