SwRI develops index to test sodium-ion battery interchangeability

September 20, 2026 at 09:41 PM2 min

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Southwest Research Institute (SwRI) has developed an Electrical Interchangeability Index (EII) to determine where sodium-ion batteries can replace lithium-ion cells without major system redesign. The index was validated by testing sodium-ion cells against 18650 lithium-ion batteries across a wide range of operating conditions. The method identifies both high-EII regimes with near-identical performance and low-EII regimes where direct replacement fails.

Key Facts

SwRI researchers introduced the Electrical Interchangeability Index (EII) to assess functional interchangeability between battery chemistries.
The EII was tested using sodium-ion cells and 18650 lithium-ion batteries, which are 18 mm in diameter and 65 mm long.
High EII values indicate that switching to a different chemistry is possible without significantly affecting device operation.
Low EII values signal that the battery system must be redesigned or that replacement is impossible in that specific use case.
The tests covered charge and discharge rates, voltage limits, available capacity, heating, and power-profile tracking.

The EII Metric

The Electrical Interchangeability Index (EII) was created by Southwest Research Institute (SwRI) to measure how functionally interchangeable two battery technologies are in a given application. A high EII value means a switch to another chemistry can be made without substantially affecting device operation. A low EII value indicates that the battery system must be redesigned or that replacement is impossible in that specific case. The index accounts for real requirements of a particular operating scenario rather than relying on generic comparisons of capacity, voltage, internal resistance, efficiency, or power.

Validation Testing

Researchers compared sodium-ion cells with common 18650 lithium-ion batteries, cylindrical cells measuring 18 mm in diameter and 65 mm in length. The batteries were tested across a wide range of regimes, including charge and discharge rates, operating voltage limits, available capacity, heating, and ability to follow a specified power profile. Tests identified high-EII regimes where sodium-ion and lithium-ion cells showed nearly identical heating, available capacity, and power. Low-EII regimes were also found where direct replacement of one battery type with another is impossible without system changes.

Sodium-Ion Limitations

Lithium-ion batteries dominate portable electronics, electric transport, and energy storage because of their high specific energy density. Sodium is significantly more abundant and allows the use of cheaper materials, but sodium-ion cells typically have lower energy density, making battery systems heavier and larger. SwRI developers stress that the EII does not make sodium-ion batteries a universal replacement for lithium-ion batteries.