A critical discussion of the current availability of lithium and zinc for use in batteries

In the literature on zinc-based batteries, it is often highlighted that zinc offers significant advantages over lithium due to its abundance, affordability, and accessibility. Additionally, aqueous rechargeable zinc batteries are promoted as a sustainable and cost-effective alternative to lithium-ion batteries, especially for renewable energy storage. The aim of this Comment is to provide a perspective on these statements, elucidating their foundations and implications and giving a quick but comprehensive background to authors and readers that deal with this topic, focusing specifically on batteries with zinc ions shuttling reversibly between the metallic negative electrode and the insertion-type positive electrode.

The salts for the zinc-ion batteries mentioned in the text which are also bulk chemicals are zinc chloride (ZnCl2), zinc sulfate (ZnSO4) and zinc acetate (ZnAc).2][3] Since ZnSO4 and ZnAc are usually sold respectively in the heptahydrate and dihydrate forms, their price was corrected take into account only the dry fraction.We calculated then the aqueous electrolytes density and cost, assuming a bulk price of deionized water (conductivity <0.5 µS•cm -1 ) of 0.1 $•kg -1 and considering both a 2 m solution and a solution with molality equal to the solubility limit of these salts in water at room temperature (see Table S1).We chose to perform the simulations with the cheapest electrolyte, i.e., the 2 m ZnCl2 one, reducing further the price to 0.5 $•L -1 .

Stainless-steel foil cost
The cost of the stainless-steel foils was calculated with the formula where   is the current collector cost per square meter,   is the stainless steel cost per kg (assumed to be 2 $•kg -1 , according to the latest bulk prices), 5   is the processing cost to produce a foil per kg of material,   is the stainless steel density (8 g•cm -3 ), and   is the thickness of the stainless steel foil.
Currently, the processing costs to obtain 15 µm and 8 µm thick aluminium and copper foils are respectively 1.95 $•kg -1 and 2.6 $•kg -1 , 6,7 but no information were found on the processing costs of steel foils.Since we expect that the processing of steel is more difficult due to the lower ductility and malleability than these other two metals, we assumed a cost of 3 $•kg -1 for a 20 µm foil.Hence, according to the reported formula, we obtain a steel foil price of 0.

Simulations of battery cost and energy density
The assessment of energy density and cost involved the utilization of the open source BatPaC 5.0 software.A detailed explanation of the BatPaC model can be found in the corresponding report released by the Argonne National Laboratory. 8This model is specifically designed for simulating battery packs with specified energy and power ratings, taking into account the costs associated with various components such as active materials, conductive carbon, binders, separators, electrolytes, current collectors, casings, pack current collectors, cooling systems, labor, and overheads.Additionally, it factors in the investment costs related to the production site.
The simulated battery pack is intended for domestic energy storage, featuring a power rating of 7 kW and an energy rating of 11.5 kWh.It consists of 36 cells in each module, with a total of 2 modules in a row.Overall, the battery pack comprises 72 cells.The production volume is set at 25,000 packs per year.
The LFP and NMC 622 battery packs were simulated using the default values found in the BatPaC 5.0 model (high-energy configuration) and updating the costs of the main components according to the current prices. 9The Zn||MnO2 battery was simulated using the voltage vs.
specific capacity discharge curve of Figure 2.a from the work of Pan et al. as input for the OCV points. 10As remarked in the main text, we took care of eliminating the costs associated to the dry room and the negative electrode production for the rechargeable zinc battery.In fact, the aqueous electrolyte removes the need of a dry environment for the materials and battery processing.Additionally, as the negative electrode consists of a zinc metal foil, there are no coating and drying processes required to produce it.All the main parameters of the simulations are reported in Table S2, and the main results in Table S3.

8 $•m - 2 .
The value of 0.4 $•m -2 reported in Figure 2.b of the main text was obtained by setting a stainless steel foil thickness of 10 µm and keeping the processing costs constant.

Table S1 -
Details on the zinc salts and the related aqueous electrolytes for zinc batteries

Table S2 -
Main parameters used in the simulation of the battery packs Ratio between the capacity of the anode and the capacity of the cathode a