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  5. Zinc-nickel rechargeable batteries - promising as electricity storage devices

Zinc-nickel rechargeable batteries - promising as electricity storage devices

Author: Prof. Dr. Jochen Fricke, Energy Technology Cluster (as of: October 2017)

Lithium-ion batteries dominate the power storage market today. F&E activities in recent years have increased the performance and Fig. 1. schematic ZnNi battery (Fig. J. Fricke)energy density,  the cycle stability and safety of these batteries significantly improved. However, spectacular fires, expensive components such as lithium and cobalt, the energy density, which is unlikely to be decisively increased, as well as questions of recyclability  stimulate the search for even better storage.

As promising candidates are  zinc-based batteries with zinc anode and cathodes preferably made of nickel oxyhydroxide (Fig. 1). Zinc is available everywhere, inexpensive, has 2 electrons compared to lithium with only over 1 electron. As aqueous systems, Zn batteries impress with their non-flammability.

First, the basics: the reaction equation at the positive electrode (cathode), which provides about 0.49 V (discharging to the right), is:

2 NiOOH + 2 H2O + 2 e-  <---> 2 Ni(OH)2 + 2 OH- 2 N i O O H + 2 H 2 O + 2 e - ⇌ 2 N i ( O H ) 2 + 2 O H - {displaystyle mathrm {2NiOOH+2H_{2}O+2e^{-}rightleftharpoons 2Ni(OH)_{2}+2OH^{-}} }

At the negative electrode (anode), which supplies 1.24 V, :

Zn + 2(OH)-  <---> Zn(OH)2 + 2 e-  Z n + 2 O H - ⇌ Z n ( O H ) 2 + 2 e - {displaystyle mathrm {Zn+2OH^{-}rightleftharpoons Zn(OH)_{2}+2e^{-}} }

The total reaction results in:

2 H2O + Zn + 2 NiOOH <---> Zn(OH)2 + 2 Ni(OH)2 2 H 2 O + Z n + 2 N i O O H ⇌ Z n ( O H ) 2 + 2 N i ( O H ) 2 {displaystyle mathrm {2H_{2}O+Zn+2NiOOHrightleftharpoons Zn(OH)_{2}+2Ni(OH)_{2}} }

The operating voltage is 1.6 to 1.8 V, and the theoretical mass-specific energy density is 370 Wh/kg.

Zinc-nickel batteries have been known for a long time

Zinc-nickel batteries have been known since 1901. At that time Thomas Alva Edison already received a patent on this system. Zn-Ni batteries were further developed by the Irish chemist J.J. Drumm, who used them as power storage in the Dublin-Bray Railway Line between 1932 and 1948 [1].  However, the life of these batteries was very limited because the zinc anode did not fully recede during frequent recharging and dendrites formed, piercing the membrane and causing short circuits.

Decisive progress was made in the 1990s by Polish-born Morris Eisenberg. A chemist at Stanford University, he optimized the electrolyte, which led to a significant increase in charge-discharge cycles. However, his company Next Century Power  failed to make the process commercially viable - especially since Eisenberg died. In 2003, the undisclosed know-how flowed into the newly founded company PowerGenix, which developed the Zn-Ni battery to market maturity by 2008. PowerGenix has since been acquired by ZincFive, Inc, a specialist in efficient battery management systems [2].

Because Zn-Ni batteries, unlike Li-ion batteries, do not require temperature control, less sophisticated electronic charge/discharge control, or special structural protection against accidents, they have a significant weight advantage.

Zn-Ni batteries are on the market today in cylindrical form as AAA and AA types with about 1 and 3 Wh, respectively, and as rectangular power blocks with up to about 1 kWh  . Thereby, mass-specific energy densities of  70 Wh/kg have been realized with several hundred cycles [3]. Particularly large mass-specific power densities of 3 kW/kg are achieved mainly because of the high electrical conductivity of the zinc anode. The efficiency is comparatively high at 85 to 90%. The batteries function without problems even at very low temperatures. Their self-discharge is low, and the residual energy is still 80% after 3 months [4].

Research successes  particularly with regard to even higher cycle strength are being achieved by the Naval Research Laboratory in Fig.2. conventional Zn powder anode (left) and porous monolithic 3D Zn electrode (Fig. J. Fricke, after [5]). Washington, DC reported. The technical implementation is carried out in the company EnZink, Inc. in California [5]. To avoid dendrite formation, a monolithic porous 3D anode is used instead of a  powder electrode (Fig.2), which is shown to be structurally stable even under many deep discharges. The electrolyte was also optimized to contain 1 molar LiOH in addition to 6 molar KOH; the zinc electrode was impregnated with Ca(OH)2.

Fig. 2. conventional Zn powder anode (left) and porous monolithic 3D Zn electrode (Fig. J. Fricke, after [5]).    

ZnNi batteries are superior to the long-available ZnMH batteries in many application fields mainly because of their higher voltage and  higher efficiency. The extent to which ZnNi batteries  with the new monolithic zinc sponge anodes  can succeed in the automotive and grid stabilization sectors remains to be seen. For safety reasons and in terms of recyclability, it would be highly desirable.

[1] https://en.wikipedia.org/wiki/Nickel-zink_battery

[2] www.elektroniknet.de/elektronik/power/der-nickel-zink-akku

[3] www.zincfive.com/nickelzinc-batteries

[4] www.pocketnavigation 2/17/2017 by Tobias, with lots of test data.

[5] J. F. Parker et al. "Rechargeable nickel-3D zinc batteries, Science356, 415(2017)