18
found little effect from zincate in the electrolyte. Gong et al. [76] use an impractically small amount NiAlCo layered double hydroxide in a 2 mm thick nickel foam
to claim ~300 Wh kg
−1
, but they miscalculate this metric because they ignore the
mass of their carbon conductive matrix and the mass of excess electrolyte needed
for their electrode. Lee et al. [89] make a nickel cathode (with a miniscule coverage
of 2 mg active material per cm
2
) integrated with an air electrode and report 980 Wh
per kg of nickel active material, but they do not include inactive zinc mass, or inactive cathode materials, or electrolyte mass or air-electrode mass, therefore their performance number has limited meaning in practical terms (see Table 1 for best
practices). Wang et al. [90] cycle an electrode with CNTs and nanosheets of NiO,
reporting 155 mAh capacity per g of the NiO-CNT positive electrode and a cell
voltage of 1.75 V, then claim an energy capacity of 228 Wh per gram of total zinc
plus NiO-CNT mass. Their calculation suggests they only counted the zinc mass
that was actively cycling, which makes their calculation of energy capacity misleading and difficult to interpret, an example of the too-frequent failure of peer-review
to produce a transparent battery publication.
Raman, XPS, XRD, FTIR, and electrochemical measurements provide insight
on the structural properties of nickel hydroxides, as seen in Hall et al. [33, 91, 92].
A comprehensive review of methods of synthesis for each type of nickel hydroxide
is found in Hall et al. [93]. Much room for progress still exists in nickel hydroxide
technologies if the higher oxidation states of nickel can be accessed. Some evidence
for these higher oxidation states already exists [34], but unfortunately the electrolysis of water interferes with the stability of these compounds. Alternatively, there is
advantage to use of the α and γ phases of NiOOH and Ni(OH) 2 , respectively, instead
of limiting the reactions to the β phases of these materials [94]. Some recent progress has been made in this direction [95].
Due to the thousands of tons of nickel electrode in consumer products, recycling
and sustainability is a major issue. Nickel electrodes include valuable or toxic metals,
including nickel, cadmium, cobalt, copper, cerium, lanthanum, praseodymium, and
neodymium, therefore recycling is important for sustainability. Research on recycling
of Ni-MH and Ni-Cd now comprises a significant fraction of the current literature on
nickel battery electrodes. Most recycling methods use hydrometallurgical leaching of
the metals, then subsequent processes to separate the dissolved metals. Work by
Reddy et al. [96] and Rodrigues and Mansur [97] are salient examples of this recycling process, and both use solvent-solvent extraction with well-known solvents like
Cyanex 932 and 272. Development of new and more efficient ionic liquid solvents is
notably undertaken in [98]. One can imagine Ni-Zn batteries can substantially simplify the life cycle, since toxic and rare metals can hypothetically be avoided altogether.
3.4 Next-Generation Separators for Alkaline Batteries
The traditional job of a separator is to prevent electron flow while permitting ionic
passage between the electrodes. In rechargeable Mn-Zn chemistry, novel separators
are sought to block the transfer of all zinc species between the electrodes, because
D. E. Turney et al.
found little effect from zincate in the electrolyte. Gong et al. [76] use an impractically small amount NiAlCo layered double hydroxide in a 2 mm thick nickel foam
to claim ~300 Wh kg
−1
, but they miscalculate this metric because they ignore the
mass of their carbon conductive matrix and the mass of excess electrolyte needed
for their electrode. Lee et al. [89] make a nickel cathode (with a miniscule coverage
of 2 mg active material per cm
2
) integrated with an air electrode and report 980 Wh
per kg of nickel active material, but they do not include inactive zinc mass, or inactive cathode materials, or electrolyte mass or air-electrode mass, therefore their performance number has limited meaning in practical terms (see Table 1 for best
practices). Wang et al. [90] cycle an electrode with CNTs and nanosheets of NiO,
reporting 155 mAh capacity per g of the NiO-CNT positive electrode and a cell
voltage of 1.75 V, then claim an energy capacity of 228 Wh per gram of total zinc
plus NiO-CNT mass. Their calculation suggests they only counted the zinc mass
that was actively cycling, which makes their calculation of energy capacity misleading and difficult to interpret, an example of the too-frequent failure of peer-review
to produce a transparent battery publication.
Raman, XPS, XRD, FTIR, and electrochemical measurements provide insight
on the structural properties of nickel hydroxides, as seen in Hall et al. [33, 91, 92].
A comprehensive review of methods of synthesis for each type of nickel hydroxide
is found in Hall et al. [93]. Much room for progress still exists in nickel hydroxide
technologies if the higher oxidation states of nickel can be accessed. Some evidence
for these higher oxidation states already exists [34], but unfortunately the electrolysis of water interferes with the stability of these compounds. Alternatively, there is
advantage to use of the α and γ phases of NiOOH and Ni(OH) 2 , respectively, instead
of limiting the reactions to the β phases of these materials [94]. Some recent progress has been made in this direction [95].
Due to the thousands of tons of nickel electrode in consumer products, recycling
and sustainability is a major issue. Nickel electrodes include valuable or toxic metals,
including nickel, cadmium, cobalt, copper, cerium, lanthanum, praseodymium, and
neodymium, therefore recycling is important for sustainability. Research on recycling
of Ni-MH and Ni-Cd now comprises a significant fraction of the current literature on
nickel battery electrodes. Most recycling methods use hydrometallurgical leaching of
the metals, then subsequent processes to separate the dissolved metals. Work by
Reddy et al. [96] and Rodrigues and Mansur [97] are salient examples of this recycling process, and both use solvent-solvent extraction with well-known solvents like
Cyanex 932 and 272. Development of new and more efficient ionic liquid solvents is
notably undertaken in [98]. One can imagine Ni-Zn batteries can substantially simplify the life cycle, since toxic and rare metals can hypothetically be avoided altogether.
3.4 Next-Generation Separators for Alkaline Batteries
The traditional job of a separator is to prevent electron flow while permitting ionic
passage between the electrodes. In rechargeable Mn-Zn chemistry, novel separators
are sought to block the transfer of all zinc species between the electrodes, because
D. E. Turney et al.