Driving Zn-MnO2 grid-scale batteries: A roadmap to cost-effective energy storage

Zn-MnO2 batteries promise safe, reliable energy storage, and this roadmap outlines a combination of manufacturing strategies and technical innovations that could make this goal achievable. Approaches such as improved efficiency of manufacturing and increasing active material utilization will be important to getting costs as low as $100/kWh, but key materials innovations that facilitate the full 2-electron capacity utilization of MnO2, the use of high energy density 3D electrodes, and the promise of a separator-free battery with greater than 2V potential offer a route to batteries at $50/kWh or less. Large-scale energy storage is certain to play a significant, enabling role in the evolution of the emerging electrical grid. Battery-based storage, while not a dominant form of storage today, has opportunity to expand its utility through safe, reliable, and cost-effective technologies. Here, secondary Zn–MnO2 batteries are highlighted as a promising extension of ubiquitous primary alkaline batteries, offering a safe, environmentally friendly chemistry in a scalable and practical energy dense technology. Importantly, there is a very realistic pathway to also making such batteries cost-effective at price points of $50/kWh or lower. By examining manufacturing examples at the Zn–MnO2 battery manufacturer Urban Electric Power, a roadmap has been created to realize such low-cost systems. By focusing on manufacturing optimization through reduced materials waste, scalable manufacturing, and effective materials selection, costs can be significantly reduced. Ultimately, though, coupling these approaches with emerging research and development advances to enable full capacity active materials utilization and battery voltages greater than 2V are likely needed to drive costs below a target of $50/kWh. Reaching this commercially important goal, especially with a chemistry that is safe, well-known, and reliably effective stands to inject Zn–MnO2 batteries in the storage landscape at a critical time in energy storage development and deployment.


Introduction
Large-scale battery-based energy storage is a key enabler in grid modernization for integration of intermittent renewable energy resources like wind and solar photovoltaics, for efficient grid decarbonization, for improving the resiliency of the grid infrastructure, and for providing grid operators with a flexible resource that can offer multiple grid services in the power and energy markets. At residential, commercial, and grid scales, low-cost and efficient battery energy storage systems can reduce the need for costly carbon-based electricity generation and defer expensive upgrades to transmission and distribution infrastructure. Storage stands to revolutionize electricity retail and wholesale market dynamics with arbitrage, time shifting, demand charge reductions, and other mechanisms.1,2 Installation of battery energy storage systems (BESSs) for residential and commercial use and at grid scale have been increasing rapidly in the last few years for a range of applications, primarily in the power markets and in markets with high energy prices. [3][4][5] The amount of installed energy storage capacity is still miniscule, however. The United States has a grid with nearly a 1 TW base load capacity, but the total amount of battery energy storage on the grid is limited to approximately 0.1% of that load capacity! The vast majority of storage capacity in the USA comes from pumped hydroelectric storage, but in all, existing storage still only represents about 15 min of ride-through energy. Clearly, there is a need for more storage, including battery storage.
Although the ultimate solution to grid-scale energy storage will no doubt include a host of complementary technologies, the limited amount of currently implemented storage currently reveals tremendous opportunity for under-represented electrochemical (battery) energy storage to grow if key issues around battery safety, long-term performance, and cost-effective production can be resolved. Here we describe a vision for an emerging Zn-MnO 2 battery technology with the potential to change the face of large-scale energy storage as a safe, reliable, and low-cost technology. Recognizing the value of batteries has motivated the use of other battery chemistries, such as lithiumion, lead-acid, sodium metal, and flow batteries, but these technologies continue to confront roadblocks to widespread utilization. Long-term reliability remains an issue for some systems, and costs generally exceed $200/kWh delivered, making them too expensive for widespread adoption at either residential or grid scales.6 Fire safety is also a concern for some technolo-gies7,8 as are supply chain issues9 and social and environmental impacts. 10 Zinc-manganese oxide (Zn-MnO 2 ) batteries have the potential to overcome these obstacles. 11 The basic constituents of these batteries are already ubiquitous in the form of the commonly used disposable alkaline batteries. Both zinc and manganese are geologically abundant, supply chains and manufacturing processes are well established, operating temperatures for the Zn-MnO 2 system are low, and fire danger is essentially non-existent. Moreover, neither zinc nor manganese, as waste products, present significant environmental problems.10,12-14 The traditional Zn-MnO 2 battery, however, is a primary battery, one that is not rechargeable. Grid-scale energy storage requires a secondary battery, reliably rechargeable thousands of times. In the past 10 years, significant research and development efforts have produced functional secondary Zn-MnO 2 batteries utilizing Zn anodes and carefully engineered MnO 2 cathode chemistries.13,15-18 These batteries have advanced to early stages of commercial development and implementation, but challenges remain to increase the system value by improving power capacity, increasing the functional lifetime, and critically, increasing active material utilization while maintaining rechargeability. These prior efforts have aimed to decrease battery cost to below $100/kWh, making them much more accessible as deployable storage solutions. This paper, however, will identify key challenges and outline a research and development roadmap to develop a secondary Zn-MnO 2 battery cell manufacturable at an enabling cost of $50/kWh, with performance characteristics making it competitive with lead-acid and lithium-ion batteries in all markets. To provide a baseline model for manufacture of these batteries, we will explore the pilot-scale manufacturing currently utilized by Urban Electric Power (UEP, Pearl River, NY), which produces up to 500 300Ah, 1.3 V cells per day for a current cost of approximately $300/kWh.

An overview of Zn-MnO 2 battery chemistry
Alkaline Zn-MnO 2 batteries typically comprise a Zn anode and an MnO 2 cathode, separated by a porous polymer membrane (separator) and an aqueous alkaline electrolyte comprised of potassium hydroxide in water. In the simplest terms, when the charged battery discharges, the Zn anode is oxidized, leading to the formation of soluble ionic zinc species, such as Zn(OH) 4 2− . Meanwhile manganese in the MnO 2 is reduced to form MnOOH or Mn(OH) 2 species, depending on whether the Mn 4+ is reduced to Mn 3+ or Mn 2+ , respectively. In conventional primary cells, the formation of insulating, electrochemically inactive spinel compounds in the cathode occurs during the second-electron process, limiting cathode capacity to essentially the first-electron of MnO 2 (308 mAh/g). These spinel compounds are hausmannite (Mn 3 O 4 ) and hetaerolite (ZnMn 2 O 4 ), the latter of which forms in the presence of zincate [Zn(OH) 4 ] 2− , which is the soluble discharge product of the Zn anode in alkaline electrolyte and can pass through the separator to the cathode.11,19 Hydroxide ions in the electrolyte shuttle through the separator to balance charge in the system during these reactions. Understanding and engineering the specific chemistries behind these reactions are among the keys to making this battery chemistry rechargeable. In a recent review, Lim et al. have provided a more detailed discussion of Zn-MnO 2 battery chemistry and a comparative summary of properties and performance metrics from research and development efforts around Zn and MnO 2 electrodes published in the literature.11 As a point of clarification, the batteries discussed here are alkaline batteries, distinct from Zn-ion batteries,20 which rely on divalent Zn 2+ transport and intercalation, or non-alkaline (the so-called "mild") aqueous Zn-MnO 2 batteries that do not use the highly alkaline aqueous electrolytes described here. 16 These alternative Zn-MnO 2 batteries are less commercially mature and are not the focus of the present, practical roadmap to low-cost battery manufacturing (Fig. 1). The theoretical energy density of the Zn anode (820 mAh/g) is higher than the MnO 2 in the cathode, which means that the energy storing capacity of the cell is effectively limited by the MnO 2 cathode. Because the functional cost of a battery is typically described in terms of the dollars per kWh of storable energy, the energy density (the product of capacity and voltage, normalized by mass or volume) of these systems is important, and that makes the capacity of the MnO 2 important. When the manganese in the MnO 2 cathode is reduced from Mn 4+ to Mn 3+ and only one electron-or the "first" electron-is used, the theoretical capacity of this system is 308 mAh/g of MnO 2 . In contrast, if the Mn 4+ can be reduced to Mn 2+ , accessing the "second" electron, then the capacity increases to a tantalizing 616 mAh/g. Here, we will outline the challenges to realizing cost-effective utilization of the first-and secondelectron chemistries.

Current state of the art
The current state of the art in the development of a rechargeable Zn-MnO 2 cell utilizes the first-electron capacity (1e − , theoretical energy density of 308 mAh/g) of MnO 2 . This chemistry utilizes a relatively well-behaved proton insertion chemistry that has proven highly reversible-one of the key innovations for this rechargeable technology. This approach works, so long as the battery only accesses approximately 20% of the 1e − MnO 2 capacity (referred to as 20% depth of discharge) and only approximately 9% of the total Zn capacity.15,21 These inefficiencies, however, result in added weight, volume, and cost from electrochemically inactive electrode materials with no contribution to the useful storage capacity. In the UEP system, the current bill of materials (BOM) using pilot-scale manufacturing is approximately $300/kWh for a 300 Ah, 1.3 V, 390 Wh battery. This battery can achieve 300 cycles at 20% utilization of the first-electron capacity. This system is already competitive with lead-acid cells for certain applications;22 however, reductions to this manufacturing costs are required for the battery to be competitive in broader markets.

Roadmap to $50/kWh
To be commercially competitive with incumbent technologies, cell manufacturing costs need to be reduced from $300/ kWh to atmost $100/kWh. We argue in this paper that rechargeable Zn-MnO 2 cells can be produced in the near future at $50/ kWh. At a high level, this can be achieved through a combination of reduced production costs and increased cell capacity (energy density). The product development roadmap described in this paper provides a prospective path to improving the costeffectiveness of cell manufacturing both by reducing costs and increasing capacity through a variety of enhancements to material specifications, manufacturing processes, and electrochemical material interactions. Such changes are likely to be incremental and cooperative; no single change to the technology or the manufacturing is likely to immediately drive down battery cost.
UEP's continuous manufacturing process (Fig. 2) employs paste coating of both anodes and cathodes, which are cut to length and width as specified for the desired form factor. Tabs are welded onto the dried electrodes, which are then wound into a jellyroll format before being assembled into the cell container. As an emerging technology, current cells are being produced at pilot scale, which inherently contains inefficiencies that generate waste and increase costs. Estimates from 2019 indicate material waste rates of up to 20% using the current pilot-scale equipment and manufacturing processes.
Due to the innovative nature of the secondary (rechargeable) Zn-MnO 2 cell manufacturing, fully automated manufacturing equipment is not currently available. Existing pilot facilities utilize equipment designed for other battery chemistries and rely heavily on manual labor. Further, pilot plants are limited in production to 500 cells per day, only a small fraction of current lead-acid or lithium-ion battery production. This small-scale production model does not fully optimize labor or capital infrastructure utilization. Utilizing a Lean Six Sigma process review, basic improvements to the pilot production process have been identified, including more precise electrode cutting and slitting, paste recycling, and equipment repositioning. Such improvements would be expected to reduce material waste rates at the pilot scale from 20% to around 5%.
The electrode coating process is vital in determining a battery's functional energy density, which is directly related to battery cost. Both the Zn anode and the MnO 2 cathode are coated as slurries on copper and nickel current collectors, respectively. These active materials, along with mixing additives that help facilitate the coating process, represent nearly 30% of the bill of Figure 1. Schematic depiction of a Zn-MnO 2 battery showing theoretical energy densities of the Zn anode (820 mAh/g) and MnO 2 cathode for both one-electron (308 mAh/g) and two-electron (616 mAh/g) access.
material (BOM) cost, making efficiencies in the coating process necessary to achieve competitive production costs. Cell capacity is dictated, in part, by the amount of accessible active material present on the current collector. Greater Zn and MnO 2 availability, while in balance with one another, creates greater cell capacity. Coating uniformity, active material dispersion, and final areal density (possibly improved through subsequent compaction or calendaring) of the coated active material all impact the electrode performance.
Finally, scaling of the optimized production line will dramatically reduce the cost per unit of energy by more effectively utilizing labor and capital costs. For example, increasing production to 12,000 cells/day is expected to enable volume material purchasing and provide the economy of scale that could reduce overall BOM by up to 30%.
To successfully reach the $50/kWh target, it is necessary to integrate lower cost materials that maintain or enhance cell performance. One key target in this effort is to substitute lower cost non-active materials, specifically current collectors and separators, which are both costly items in the BOM. Current collectors generally represent a significant portion of material costs associated with battery cell construction, as they must meet high standards of conductivity, strength, flexibility, and active material coatability. As an example, UEP employs cathodic nickel (Ni) and anodic copper (Cu) current collectors in the Zn-MnO 2 cell. While effective, both the Ni and Cu current collectors, which are in an expanded mesh form, are high-cost materials that represent approximately 28% of the cylindrical cell's bill of materials. Recent progress has shown that replacement of costly current collectors with materials such as cold-rolled steel provides excellent performance at a much lower cost.
Separators can also be high-cost items as they must allow for ionic transfer while still providing a strong and flexible physical barrier between electrodes. UEP utilizes regenerated cellulose (i.e., cellophane) in the rechargeable Zn-MnO 2 cell. Cellophane is a relatively costly material, and it is still quite susceptible to cell failure. The early failure rate of cells using only cellophane separators can be as high as 20-30%, with more than two-thirds of these failures attributable to short circuits between the anode and cathode. Identifying effective alternatives to both current collectors and separators that can reduce cost while maintaining or even enhancing performance is a significant research focus and is estimated to reduce the overall BOM by 20%.
By reducing waste, making inactive material substitutions, and producing cells on a larger scale, it is reasonable to expect the battery cost could be reduced to below $200/kWh. To make the changes needed to drive the cost toward the game-changing $50/kWh, however, the accessible capacity of the active material must be increased. As mentioned earlier, the current rechargeable Zn-MnO 2 cell accesses approximately 20% of the first-electron capacity (1e − ) of MnO 2 and only approximately 9% of the total Zn capacity. If Zn utilization was doubled to 18% and MnO 2 utilization was doubled to 40%, cell capacity would double, driving down the cost of storage to less than $100/kWh. Even lower values are possible with significant reductions in current collector and inactive material costs outlined above. To date, however, increasing utilization of current materials reduces cycle life. For example, utilizing 40% of the MnO 2 currently reduces cycle life from hundreds to tens of cycles. Alternative materials solutions are necessary to improve this important metric.
To address these issues, innovations on both the Zn anode and the MnO 2 cathode are needed. Consider first increasing Zn utilization from 9 to 18%. Accessing high utilizations usually results in rapid deterioration in capacity of Zn anodes because of loss of active material during discharge, uneven dendritic formation during charging that results in battery shorts, redistribution of active material (shape change) that results in loss of material for subsequent charge and discharge, and formation of passivated ZnO around Zn, which results in loss of the needed electronic network on the anode.
The UEP R&D team has discovered that charging the Zn anode is the most critical step in ensuring repeatable highcapacity utilization. In a recently published paper,23 the team developed a method of nesting the Zn particles in a conductive carbon framework which allows for efficient plating on charging and dissolution on discharge. The newly developed anodes are able to cycle at 810 mAh/g, very near the theoretical Zn capacity of 820 mAh/g. Importantly, this development can be implemented using existing production equipment. In addition to the Zn anode, higher capacity must be available from the MnO 2 cathode, and the most direct route to this goal is by accessing the "second electron" of manganese. Recent demonstrations have shown that modified MnO 2 electrodes cycle near theoretical second-electron capacity with the strategic addition of bismuth oxide and copper.13,18 The bismuth oxide acts as a complexing additive to the cathode composition, which, through its dissolved ions in the electrolyte, can prevent dissolved manganese ions from reacting to form inactive spinel phases. The addition of copper to the cathode reduces the charge transfer resistance between the MnO 2 particles and the electrolyte, which is usually associated with metal oxide electrodes on account of their poor electronic properties. The cathodes developed by UEP are able to cycle 80-100% of the theoretical capacity over 3000 cycles. Implementation of these promising electrodes in scalable battery assemblies offers a route toward the high capacity needed for low-cost storage implementation.
Another promising approach to improving active materials utilization efficiency and overall capacity in both electrodes is the application of three-dimensional (3D) electrode structures. Carefully designed 3D frameworks allow for efficient ionic transport and charge transfer. Prototype 3D Zn electrodes have shown discharge and charge performance near theoretical capacity, and deleterious dendrites have not been observed in this new design because of the unique pore network created during the manufacturing of these 3D anodes.23,24 For 3D MnO 2 electrodes, 3D cathodes can be embedded with ultrahigh loadings of optimized MnO 2 to increase cathodic energy density. Numerous 3D printing methods are in development that may provide avenues to cost-effective manufacture of the desired high-energy-density materials.
One forward-looking direction that holds tremendous promise toward low-cost battery production is increasing the voltage of the battery. The energy density of the battery is directly tied to this voltage, which means that if cost-effective strategies could increase the battery voltage from 1.3 to over 2 V, the effective energy cost of the battery could be dramatically reduced. A recent paper by Yadav et al. describes breaking this "2 V" barrier,24 by using a polymer gel electrolyte containing potassium hydroxide (KOH) (coupled with an acidic cathode electrolyte) that also allowed for elimination of costly separators and stabilized the Zn anode. Meanwhile, this system also decreased zincate crossover that can limit battery lifetimes (Fig. 3).
Along the road to low-cost storage, reduced materials waste, large-scale manufacturing, and inactive materials, substitutions are likely to reduce manufacturing costs from current values near $300/kWh to approximately $200/kWh. Doubling the usable capacity of the Zn anode and MnO 2 cathode with current manufacturing is likely to cut that value to closer to $100/kWh. The key to reaching or exceeding the goal of $50/kWh storage, however, involves changes to access the full 2-electron capacity of the MnO 2 , use of high-energy-density 3D electrodes, and the promise of a separator-free battery with greater than 2 V potential. Based on ongoing research, these are accessible targets that could be reached within a few years, and they could be integrated into existing manufacturing lines, making large-scale implementation a tantalizing possibility. Few technologies can offer the potential of large-scale manufacturing of batteries that can offer storage at costs as low as $50/kWh. Zn-MnO 2 batteries have the potential to realize this goal in a safe, environmentally friendly, and reliable package that could revolutionize large-scale energy storage at a time when it is needed most.