September 17, 2026 — Southwest Research Institute (SwRI) has developed a new method to determine when sodium-ion (Na-ion) and lithium-ion (Li-ion) battery technologies can provide comparable electrical performance for defined applications.
Li-ion batteries can provide more energy storage at a lighter weight than most alternatives, making them the standard for many applications. Though lithium is not exactly a rare-earth mineral, it can be expensive to mine and source. Na-ion cells use cheaper, more abundant salts and lower-cost current collectors. By contrast, these cells typically store less energy overall and can be both heavier and bulkier in form.
As part of an internally funded project, SwRI researchers set out to discover operating conditions under which sodium-ion battery cells could provide comparable electrical performance to lithium-ion cells for the same duty cycle. The research team focused on substituting for 18650-format cell batteries, which are cylindrical batteries 18 mm in diameter and 65 mm long. These batteries are commonly used to power anything from laptops to power tools and electric vehicles.
Lithium demand is increasing, which challenges production capacities and has driven industry interest in sodium-ion technologies. Substituting cheaper Na-ion batteries in existing technologies that use Li-ion cells could alleviate supply chain and affordability issues. Existing battery-comparison approaches typically evaluate parameters like capacity, voltage, impedance, efficiency or power capability individually. They do not, however, quantify the operational regions where different battery technologies can perform the same application duties.
SwRI categorized Na-ion and Li-ion cell performance over a broad range of operating conditions. From this, they developed an application-aware Electrical Interchangeability Index (EII), which is a numerical measure of how closely two battery technologies can satisfy the same application-specific electrical duty. This EII framework can help engineers compare whether two battery technologies can deliver and absorb the same application-specific power within their usage constraints.
“We identified ‘high-EII’ operating regions, where the two cell types demonstrated strong functional equivalence with negligible differences in surface temperature rise, available capacity and power capability, and ‘low-EII’ regions, where substitution would be unacceptable,” said Shuvodeep Bhattacharjya, the project lead and a senior research engineer in SwRI’s Powertrain Engineering Division. “Our application-aware Electrical Interchangeability Index provides equipment manufacturers and designers with a quantitative measure of whether two batteries can execute the same duty cycle or whether a substitution could compromise performance or require changes to the battery system.”
The project also helped SwRI establish metrics to support interchangeability assessments, such as achieved charge and discharge rates, voltage-limit measurements and power-tracking measures, among others. High-EII regions demonstrated repeatable functional interchangeability, while low-EII regions exposed limitations that prevented equivalent application performance.
“This initial work provides a structured, verifiable method to evaluate where new chemistries like sodium-ion can plug into existing designs and where redesign is recommended or even mandatory,” said Scott Sjovall, manager of SwRI’s Battery System Research and Innovation Section. “From a business perspective, SwRI’s EII can support chemistry down-selection, second-source qualification and assessment of alternative or hybrid battery technologies while reducing unnecessary system redevelopment.”
The team plans to expand the EII framework to additional battery chemistries, cell formats and application domains. They are also evaluating future extensions related to aging, degradation and thermal-management effects.
The project was funded through SwRI’s Internal Research and Development program, which provides resources for future-focused, unproven concepts to advance technology for government and industry clients. In fiscal year 2025, SwRI invested more than $13 million to fund IR&D projects to pioneer new technologies, expand institutional knowledge and capabilities and encourage the professional growth of its staff. Learn more.
For more information, visit Battery Testing & Research or contact Jesús Chávez at +1 210 522 2258, Communications Department, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166.