10
infiltrated with aqueous 2 M ZnSO 4 electrolyte. The success of Pan et al. was due to
additive manganese sulfate (0.1 M MnSO 4 ) in the electrolyte, which prevented loss
of Mn from the electrode during the energy storage reactions. This same chemical
method and experimental cyclability was reported by a group of Korean researchers
in 2001 (US Patent US6187475 B1). Unfortunately, this acidic-MnO 2 cathode
worked only with a small loading of active material (1.4 mAh/cm
2
), which will keep
overall battery costs high. It is promising that Pan et al. pair the α-MnO 2 cathode
with a Zn foil to claim a new battery with ~170 Wh kg
−1
(where the mass in their
calculation ignores the non-cycling Zn foil). Another recent breakthrough succeeded to cycle ~2 electrons per Mn (617 mAh/g) for 6000 full cycles with practical
loading of active materials (30 mAh/cm
2
) by adding bismuth and copper to EMD or
birnessite δ-MnO 2 ([14, 15], 2018). Only previous literature from Wroblowa’s
group [42, 43] cycled the complete 617 mAh/g Mn capacity reversibly, but they
were unable to cycle even half that capacity galvanostatically. Yadav et al. [15, 44,
45] showed full-cell cycling of a Zn-Mn battery at 2-electrons per Mn and high Zn
utilization, with chemistry as shown in Fig. 7, reaching several 100 cycles and
energy density above 100 Wh/L based transparently upon full cell specifications.
This new Mn-Zn technology is appropriate for grid-scale energy storage as shown
in Fig. 1, and perhaps even for transportation uses (Fig. 7).
Table 1 shows a survey of performance for recent work on manganese electrodes.
The key metrics for comparing the practicality of the Mn cathodes are cycled capacity per area (mAh cm
−2
), cycled capacity per total cathode volume (mAh cm
−3
),
capacity per total solids mass (mAh g
−1
), and cycle life. Combining the Cu-Bibirnessite cathodes with a zinc anode creates an impressive cell with the best performance shown in Fig. 1. Unfortunately the zinc ions have a poisoning effect on the
birnessite-MnO 2 [14, 15, 43] by causing the formation of haeterolite ZnMn 2 O 4 [15].
Most common separators allow the zinc to cross from the anode to the cathode
material, thus zinc-blocking separators are the focus of Sect. 3.4. Further, the zinc
electrode suffers from poor cycle life when their percent utilization is pushed above
~5%, thus zinc cyclability is the focus of Sect. 3.3. Nevertheless, [15] showed separators of Ca(OH) 2 could temporarily block zinc crossover and allow the Mn-Zn cell
to cycle for 1000 cycles with an energy density near 160 Wh L
−1
based transparently
upon all materials used. The relationship between cell energy density, percent loading of Mn active material (for a 0.5 mm cathode with ~0.1 g cm
−2
of total mass), and
zinc utilization is shown in Fig. 8. Cost of energy storage ($/kWh) and ($/kWhthroughput) is very sensitive to the metrics in Table 1. Technologies from [14, 15]
and [14, 15] achieve costs under $150/kWh and $50/kWh.
3.2 Current Research on Rechargeable Zinc Anodes
As mentioned in Sect. 2.1, 2.2, and 3.2, the zinc electrode is the least reliable component of rechargeable Mn-Zn or Ni-Zn batteries, and usually leaves ~80% of its
theoretical capacity un-utilized. This produces great opportunity for improvement,
and is perhaps why research on zinc electrodes is still very active. A literature survey
D. E. Turney et al.
infiltrated with aqueous 2 M ZnSO 4 electrolyte. The success of Pan et al. was due to
additive manganese sulfate (0.1 M MnSO 4 ) in the electrolyte, which prevented loss
of Mn from the electrode during the energy storage reactions. This same chemical
method and experimental cyclability was reported by a group of Korean researchers
in 2001 (US Patent US6187475 B1). Unfortunately, this acidic-MnO 2 cathode
worked only with a small loading of active material (1.4 mAh/cm
2
), which will keep
overall battery costs high. It is promising that Pan et al. pair the α-MnO 2 cathode
with a Zn foil to claim a new battery with ~170 Wh kg
−1
(where the mass in their
calculation ignores the non-cycling Zn foil). Another recent breakthrough succeeded to cycle ~2 electrons per Mn (617 mAh/g) for 6000 full cycles with practical
loading of active materials (30 mAh/cm
2
) by adding bismuth and copper to EMD or
birnessite δ-MnO 2 ([14, 15], 2018). Only previous literature from Wroblowa’s
group [42, 43] cycled the complete 617 mAh/g Mn capacity reversibly, but they
were unable to cycle even half that capacity galvanostatically. Yadav et al. [15, 44,
45] showed full-cell cycling of a Zn-Mn battery at 2-electrons per Mn and high Zn
utilization, with chemistry as shown in Fig. 7, reaching several 100 cycles and
energy density above 100 Wh/L based transparently upon full cell specifications.
This new Mn-Zn technology is appropriate for grid-scale energy storage as shown
in Fig. 1, and perhaps even for transportation uses (Fig. 7).
Table 1 shows a survey of performance for recent work on manganese electrodes.
The key metrics for comparing the practicality of the Mn cathodes are cycled capacity per area (mAh cm
−2
), cycled capacity per total cathode volume (mAh cm
−3
),
capacity per total solids mass (mAh g
−1
), and cycle life. Combining the Cu-Bibirnessite cathodes with a zinc anode creates an impressive cell with the best performance shown in Fig. 1. Unfortunately the zinc ions have a poisoning effect on the
birnessite-MnO 2 [14, 15, 43] by causing the formation of haeterolite ZnMn 2 O 4 [15].
Most common separators allow the zinc to cross from the anode to the cathode
material, thus zinc-blocking separators are the focus of Sect. 3.4. Further, the zinc
electrode suffers from poor cycle life when their percent utilization is pushed above
~5%, thus zinc cyclability is the focus of Sect. 3.3. Nevertheless, [15] showed separators of Ca(OH) 2 could temporarily block zinc crossover and allow the Mn-Zn cell
to cycle for 1000 cycles with an energy density near 160 Wh L
−1
based transparently
upon all materials used. The relationship between cell energy density, percent loading of Mn active material (for a 0.5 mm cathode with ~0.1 g cm
−2
of total mass), and
zinc utilization is shown in Fig. 8. Cost of energy storage ($/kWh) and ($/kWhthroughput) is very sensitive to the metrics in Table 1. Technologies from [14, 15]
and [14, 15] achieve costs under $150/kWh and $50/kWh.
3.2 Current Research on Rechargeable Zinc Anodes
As mentioned in Sect. 2.1, 2.2, and 3.2, the zinc electrode is the least reliable component of rechargeable Mn-Zn or Ni-Zn batteries, and usually leaves ~80% of its
theoretical capacity un-utilized. This produces great opportunity for improvement,
and is perhaps why research on zinc electrodes is still very active. A literature survey
D. E. Turney et al.