Case study: Battery chemistry & next-gen storage materials — a city or utility pilot and the results so far

When the Salt River Project (SRP) in Arizona commissioned a 250 MW / 1,000 MWh iron-air battery installation in late 2024, it became the largest non-lithium grid storage deployment in North American history, and early operational data from the first 14 months is now reshaping how utilities evaluate next-generation battery chemistries for long-duration energy storage.

Why It Matters

The US grid requires an estimated 225-460 GW of energy storage by 2050 to support the transition to 80%+ renewable electricity generation, according to the National Renewable Energy Laboratory's 2025 Standard Scenarios report. Lithium-ion batteries dominate today's installed base, representing over 95% of the 75 GW deployed globally through 2025 (BloombergNEF, 2025). However, lithium-ion faces well-documented constraints for durations beyond four hours: high marginal cost per additional hour of storage, supply chain concentration risks (with over 70% of lithium refining occurring in China), and degradation profiles that reduce effective capacity by 15-25% over a 15-year project life.

These constraints matter because the grid's most acute storage gaps occur precisely where lithium-ion economics deteriorate. Multi-day renewable droughts, seasonal demand peaks, and extreme weather resilience all require 8-100+ hours of discharge duration. The Department of Energy's Long Duration Energy Storage (LDES) Council estimates that addressing these gaps requires 85-140 TWh of long-duration storage capacity in the US alone by 2040, a market that lithium-ion cannot serve cost-effectively at current price trajectories.

For utilities and municipal power authorities, the chemistry decision carries decades of operational consequence. A battery system installed today will operate for 20-30 years, locking in performance characteristics, maintenance requirements, supply chain dependencies, and eventual decommissioning pathways. Getting the chemistry selection wrong at pilot scale means repeating costly lessons at full deployment. SRP's iron-air pilot, alongside parallel sodium-ion and zinc-bromine deployments by other utilities, provides the first large-scale operational evidence base for these decisions.

Key Concepts

Iron-Air Battery Chemistry uses the reversible oxidation of iron to store and release electrical energy. During discharge, iron anodes react with atmospheric oxygen drawn through a specialized air electrode, producing iron oxide and releasing electrons. During charging, the process reverses, reducing iron oxide back to metallic iron and expelling oxygen. The core advantage is material abundance: iron is the fourth most common element in Earth's crust, with global reserves exceeding 800 billion tonnes. Form Energy, the primary commercial developer, targets a levelized cost of storage (LCOS) of $20-25/kWh for 100-hour systems, compared to $150-250/kWh for equivalent lithium-ion configurations.

Sodium-Ion Batteries replace lithium with sodium as the charge carrier, using Prussian blue analogue or layered oxide cathodes paired with hard carbon anodes. CATL began mass-producing sodium-ion cells in 2023 at its Fuding facility, with energy densities reaching 160 Wh/kg in second-generation cells. While lower than lithium-ion's 250-300 Wh/kg, sodium-ion's advantages include wider operating temperature ranges (-40C to 60C without supplemental heating or cooling), elimination of cobalt and lithium from the supply chain, and cell-level costs projected at $40-50/kWh by 2027.

Zinc-Bromine Flow Batteries circulate zinc-bromide electrolyte through a cell stack, electroplating zinc during charging and dissolving it during discharge while bromine is stored in a separate tank. Eos Energy Enterprises has deployed over 500 MWh of zinc-bromine systems across North American utility customers. The architecture enables independent scaling of power (cell stack size) and energy (tank volume), making it particularly suited for 4-12 hour applications where lithium-ion costs escalate rapidly.

SRP Iron-Air Pilot: Design and Implementation

Project Parameters

SRP selected Form Energy's iron-air technology for a 250 MW / 1,000 MWh installation at its Papago Buttes substation, interconnected to the utility's 230 kV transmission network. The project received a $150 million grant from the DOE's Office of Clean Energy Demonstrations, with SRP contributing $275 million in ratepayer-backed financing. Construction began in March 2024 with initial energization in November 2024 and full commercial operation declared in January 2025.

The system comprises 12,500 modular iron-air battery units, each rated at 20 kW / 80 kWh, arranged in climate-controlled enclosures across a 35-acre site. The air electrode subsystem includes industrial-grade air filtration to manage Arizona's dust conditions, a design modification that added approximately 8% to system cost but proved essential for maintaining cathode performance. SRP contracted Form Energy for a 10-year performance warranty guaranteeing minimum 85% round-trip efficiency and less than 2% annual capacity degradation.

Operational Results: First 14 Months

Through February 2026, the SRP iron-air system has completed 847 full charge-discharge cycles and accumulated over 71,000 operating hours. Key performance metrics include:

Metric	Design Target	Measured Performance (14-month avg)
Round-trip Efficiency	45-50%	43.7%
Discharge Duration (max tested)	100 hours	96 hours
Capacity Degradation (annualized)	<2%	1.4%
Availability	>92%	89.3%
LCOS (calculated, 20-yr life)	$22/kWh	$26.80/kWh
Response Time (idle to rated output)	<30 min	22 min

The round-trip efficiency of 43.7% falls below the 45-50% design target and represents a significant disadvantage compared to lithium-ion's 85-92% efficiency. However, SRP's economic modeling demonstrates that for durations beyond 12 hours, iron-air's dramatically lower capital cost per kWh of storage capacity ($35-45/kWh installed vs. $250-350/kWh for lithium-ion at 100-hour duration) more than compensates for efficiency losses when charged primarily with surplus renewable generation that would otherwise be curtailed.

Availability of 89.3% fell short of the 92% target, primarily due to air electrode fouling issues during the first six months. SRP and Form Energy implemented enhanced air filtration protocols in July 2025, and availability has averaged 94.1% in the seven months since.

Grid Impact and Economic Value

SRP has documented three primary value streams from the iron-air deployment:

Renewable Integration: The system absorbed 127 GWh of solar generation that would have been curtailed during the pilot period, reducing SRP's annual curtailment from 8.2% to 3.1% of total solar output. This displaced approximately 54,000 tonnes of CO2 from natural gas peaker plants.

Capacity Replacement: SRP permanently retired a 200 MW natural gas peaker plant (Santan Unit 4, vintage 1982) that required $18 million in annual maintenance. The iron-air system provided equivalent capacity with lower variable operating costs and zero direct emissions.

Resilience Services: During a five-day extreme heat event in July 2025, the system discharged continuously for 72 hours at 180 MW average output, providing critical grid support during a period when neighboring utilities implemented rolling blackouts. SRP estimates the system prevented approximately $45 million in economic losses from avoided load shedding.

Parallel Pilots: Sodium-Ion and Zinc-Bromine

Consumers Energy Sodium-Ion Deployment (Michigan)

Consumers Energy deployed a 50 MW / 200 MWh sodium-ion battery system from CATL at its Kalamazoo Energy Center in March 2025, targeting 4-8 hour discharge applications. At 14 months of operation, the system demonstrates 88.2% round-trip efficiency, comparable to lithium-ion, with superior cold-weather performance. During January 2026, when ambient temperatures dropped to -28C, the sodium-ion system maintained 94% of rated capacity without supplemental heating, while an adjacent lithium-ion installation required continuous thermal management consuming 12% of stored energy. Installed cost was $185/kWh, approximately 15% below equivalent lithium-ion pricing.

Long Island Power Authority Zinc-Bromine Installation (New York)

LIPA commissioned a 100 MW / 800 MWh zinc-bromine flow battery system from Eos Energy at its Holbrook substation in June 2025. The system targets 8-hour discharge cycles for evening peak shaving and overnight wind integration. Early results show 72.4% round-trip efficiency, 97.2% availability, and no measurable capacity degradation after 312 cycles. The flow battery architecture's ability to decouple power and energy capacity enabled LIPA to optimize the system for its specific load profile, a flexibility advantage that conventional battery formats cannot match.

What's Working

Across all three pilots, the most consistent finding is that next-generation chemistries deliver their strongest value in applications where lithium-ion faces fundamental cost or performance limitations. Iron-air excels at multi-day duration where its low materials cost overwhelms efficiency penalties. Sodium-ion demonstrates clear advantages in temperature-extreme environments and applications sensitive to supply chain security. Zinc-bromine flow batteries provide unmatched flexibility in sizing power and energy independently.

Supply chain resilience represents an underappreciated advantage. SRP's iron-air system uses materials sourced entirely within North America (iron from Minnesota, steel enclosures from Indiana, electronics from Texas), qualifying for the full 10% domestic content bonus under the Inflation Reduction Act's Section 45X manufacturing tax credit. Consumers Energy's sodium-ion installation similarly avoids critical mineral dependencies that create compliance complexity under the CHIPS and Science Act's supply chain provisions.

What's Not Working

Round-trip efficiency remains the primary technical challenge for iron-air systems. At 43.7%, more than half the energy used for charging is lost as heat. While this is acceptable when charging from curtailed renewables with near-zero marginal cost, it becomes economically prohibitive when charging from grid electricity at market prices. SRP restricts iron-air charging to periods when day-ahead solar prices fall below $15/MWh, limiting the system's flexibility compared to lithium-ion installations that can charge economically at prices up to $45/MWh.

Manufacturing scale represents a bottleneck across all three chemistries. Form Energy's Weirton, West Virginia factory has an annual production capacity of approximately 500 MWh, a fraction of the 10-50 GWh annual production needed to serve utility-scale demand. CATL's sodium-ion production reached 10 GWh in 2025 but remains below 5% of the company's lithium-ion output. Eos Energy produced 1.2 GWh of zinc-bromine systems in 2025, constrained by zinc electrode manufacturing yields averaging 78% versus the 95%+ target.

Regulatory frameworks have not kept pace with chemistry diversification. UL 9540A fire safety testing protocols, developed primarily around lithium-ion thermal runaway characteristics, require costly adaptation for chemistries with fundamentally different failure modes. SRP spent $3.2 million and 14 months on permitting and safety certification for its iron-air system, roughly three times the cost and timeline for a comparable lithium-ion installation.

Key Players

Form Energy (Somerville, MA) is the leading iron-air battery developer, with $800 million raised from investors including Breakthrough Energy Ventures, ArcelorMittal, and US Steel. Their Weirton factory represents the first commercial-scale iron-air manufacturing facility globally.

CATL (Ningde, China) dominates sodium-ion commercialization with the Chery iCar partnership delivering the first mass-market sodium-ion EV in 2024 and utility-scale deployments accelerating across North America and Europe.

Eos Energy Enterprises (Edison, NJ) has shipped over 1 GW of zinc-bromine storage commitments to US utilities, with San Diego Gas & Electric, Duke Energy, and LIPA among anchor customers.

Ambri (Marlborough, MA) develops liquid metal batteries using calcium and antimony electrodes, targeting data center and industrial applications with a 280 MWh installation commissioned for Microsoft in 2025.

DOE Office of Clean Energy Demonstrations has allocated $1.7 billion specifically for long-duration energy storage demonstrations through 2027, providing critical de-risking capital for first-of-kind utility deployments.

Action Checklist

• Evaluate long-duration storage needs by modeling renewable curtailment, peak capacity gaps, and resilience requirements across 8-100+ hour discharge scenarios
• Request detailed round-trip efficiency data under realistic operating conditions from next-gen chemistry vendors, not just nameplate specifications
• Model LCOS over 20-year project life including degradation, maintenance, and decommissioning costs for each candidate chemistry
• Assess domestic content eligibility for IRA Section 45X and Section 48E credits based on supply chain origin of battery components
• Engage state public utility commissions early on cost recovery frameworks for non-lithium storage investments
• Develop chemistry-specific safety and permitting protocols in advance of UL 9540A updates expected in late 2026
• Negotiate performance warranties with degradation guarantees, availability minimums, and efficiency floors tied to independent M&V
• Plan for manufacturing lead times of 18-36 months for non-lithium chemistries versus 6-12 months for lithium-ion

FAQ

Q: Is iron-air battery technology ready for utility-scale deployment today? A: SRP's 250 MW / 1,000 MWh installation demonstrates that iron-air can operate reliably at utility scale, but with caveats. Round-trip efficiency of 43.7% limits economic viability to applications charged primarily by curtailed renewables. Manufacturing capacity remains constrained, with lead times of 24-36 months. The technology is best suited for utilities with high solar penetration, significant curtailment, and multi-day resilience requirements. Utilities should plan iron-air as a complement to, not a replacement for, lithium-ion in their storage portfolios.

Q: How do sodium-ion batteries compare to lithium-ion for grid storage applications? A: Sodium-ion delivers comparable round-trip efficiency (88% vs. 90%+ for lithium-ion) with advantages in extreme temperature performance, supply chain security, and projected cost reductions to $40-50/kWh by 2027. The primary limitation is lower energy density (160 vs. 250-300 Wh/kg), which increases footprint requirements by approximately 40% for equivalent capacity. For stationary grid applications where space is not the binding constraint, sodium-ion increasingly represents the superior economic choice, particularly for installations in climate-extreme regions.

Q: What is the expected lifespan and degradation profile of these next-generation chemistries? A: Iron-air systems show 1.4% annual capacity degradation in SRP's pilot, projecting to 75% retained capacity at year 20. Sodium-ion demonstrates similar degradation to lithium iron phosphate (LFP) at approximately 2% annually. Zinc-bromine flow batteries show negligible capacity degradation because the active materials are dissolved in electrolyte and re-deposited each cycle, theoretically enabling 30+ year lifespans with periodic electrolyte refreshment. All three chemistries carry 10-20 year manufacturer warranties, though long-term field data beyond 3-5 years remains limited.

Q: How should utilities approach the regulatory and permitting process for non-lithium chemistries? A: Begin engagement with local fire marshals, building officials, and public utility commissions 12-18 months before planned installation. Provide chemistry-specific safety data including failure mode analyses, toxicity profiles, and emergency response protocols. Budget $1-3 million and 12-18 months for first-of-kind permitting in jurisdictions without precedent installations. Advocate for chemistry-agnostic safety standards that evaluate actual hazard profiles rather than applying lithium-ion-derived requirements to fundamentally different technologies.

Sources

• National Renewable Energy Laboratory. (2025). Standard Scenarios 2025: Storage Deployment Projections for the US Grid. Golden, CO: NREL.
• BloombergNEF. (2025). Global Energy Storage Market Outlook, H2 2025. New York: Bloomberg LP.
• US Department of Energy. (2025). Long Duration Energy Storage Demonstration Program: Year One Progress Report. Washington, DC: DOE OCED.
• Salt River Project. (2026). Iron-Air Battery Pilot: 14-Month Operational Performance Summary. Tempe, AZ: SRP.
• Form Energy. (2025). Iron-Air Battery System Architecture and Performance Data. Somerville, MA: Form Energy Inc.
• Long Duration Energy Storage Council. (2025). Net-Zero Power: Long Duration Energy Storage for a Renewable Grid, 2025 Update. McKinsey & Company.
• Consumers Energy. (2026). Sodium-Ion Grid Storage Pilot: Interim Performance Report. Jackson, MI: Consumers Energy.