Beyond Ionic Conductivity

Decorative conceptual material artwork, not a microscopy image
ACSSI 2026
Beyond Ionic
Conductivity
Coordination-Transport-Interface Design Rules
for Durable Sodium Batteries
Assoc. Prof. Soorathep Kheawhom, PhD, FRSC
Department of Chemical Engineering, Faculty of Engineering
Chulalongkorn University
September 14, 2026
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The durability question

The durability question
A battery must
keep working,
cycle after cycle.
Three papers connect three scales
Electrolytes
Synthesis of Weldegebrieal et al., Santoso et al., and Wisan et al. (2026)
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Carbon pores
Full cells
J. Mater. Chem. A · Review
Materials Horizons · Opinion
Nanoscale Horizons · Focus

The CTI framework

The CTI framework
Three transparent colored spheres linked by separated three-dimensional arrows
C
Coordination
What surrounds
sodium?
T
Transport
Can ion supply
meet demand?
I
Interface
Does the interface
keep working?
The limiting process can change as the cell ages.
CTI framework. Weldegebrieal et al. 2026. DOI: 10.1039/D6TA01969B
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The local environment around sodium

The local environment around sodium
Simplified conceptual local coordination: solvent-separated ion pair, contact ion pair, and aggregate. Teal sodium, gold anion, pale-blue solvent.
Local neighbors can change the first reaction
Na⁺: teal Anion: gold Solvent: pale blue
Conceptual illustration. Outcome depends on chemistry and surface. Sources: Weldegebrieal et al.; Wisan et al. (2026)
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SSIP
Solvent-separated pair
CIP
Contact ion pair
AGG
Associated ion cluster

Ion supply under load

Ion supply under load
Can ion supply
meet the
electrode’s demand?
Association
Useful chemistry can come
with higher viscosity.
Temperature
Mobility can change
strongly in polymers.
Current density Areal capacity Temperature
Verify local structure in LHCEs. Follow polarization under the stated conditions.
Weldegebrieal et al. 2026. DOI: 10.1039/D6TA01969B
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Resistance through the life of a cell

Resistance through the life of a cell
Follow the same cell through time.
01
Formation
Establish the
initial state
02
Early cycling
Locate emerging
changes
03
Later cycling
Test whether
function persists
Separate resistive contributions. Match state of charge and temperature.
Weldegebrieal et al. 2026. DOI: 10.1039/D6TA01969B
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Different electrolytes, different limitations

Different electrolytes, different limitations
Electrolyte family Where to look first
Ionic liquids / HCE / LHCE Association and usable ion supply
Polymers Temperature, mobility, and contact
Oxides Grain boundaries, wetting, and contact
Sulfides Reactivity and mechanical behavior
Measurements establish what limits the actual cell.
Weldegebrieal et al., Fig. 3 and Table 2. DOI: 10.1039/D6TA01969B
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Halides and hydroborates

Halides and hydroborates
Halides
Sodium-metal side
Transport and compatibility
can limit operation.
Oxidative stability can support
cathode-side use.
Hydroborates
Sodium-metal side
Favorable passivation
in some systems.
Oxidative stability can limit
cathode-side use.
Hybrid designs can give different regions different jobs.
Weldegebrieal et al., Sections 4.4–4.7. DOI: 10.1039/D6TA01969B
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Inside a hard-carbon pore

Inside a hard-carbon pore
Conceptual hard-carbon cutaway showing open and closed pores, pore diameter, tortuosity, and surface chemistry
The electrolyte
we mix is only
the beginning.
Pore entrances and surfaces can
reshape the local environment.
Proposed conceptual framework
Concept based on Santoso et al., Fig. 1. DOI: 10.1039/D6MH00519E
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One electrolyte, two carbon structures

One electrolyte, two carbon structures
Initial Coulombic efficiency
Initial Coulombic efficiency: CMS-800 61 percent, CMS-1300 91 percent
Same electrolyte
1 M NaPF₆
EC:DEC (1:1)
Na-metal half cells
25 °C, 50 mA g⁻¹
~1 mg cm⁻², 40 µL
0.005–2.5 V
30 percentage points in first-cycle efficiency
Published half-cell data: Zhang et al., Nat. Commun. 2025, 16, 3634. DOI: 10.1038/s41467-025-59022-8
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Interphase growth and pore access

Interphase growth and pore access
Access preserved
Access restricted
Two possible pore morphologies: even interphase lining retains access; uneven growth narrows the entrance. No fixed outcome assigned to a coordination category.
The interphase must preserve pore access
Possible outcomes. Neither follows universally from SSIP, CIP, or AGG.
Conceptual illustration based on Santoso et al., Fig. 3. DOI: 10.1039/D6MH00519E
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Matched tests of carbon and electrolyte

Matched tests of carbon and electrolyte
Same carbon
Different electrolyte
Same electrolyte
Controlled carbons
Connect local chemistry with retained pore access.
Complements insertion, adsorption, and pore filling.
Proposed comparisons: Santoso et al. 2026. DOI: 10.1039/D6MH00519E
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An interphase in motion

An interphase in motion
Adsorption at an electrode surface versus absorption of solvent into a translucent organic-rich interphase, not into graphite.
Solvent can enter and change the interphase
Simplified conceptual illustration based on Wisan et al., Fig. 2. DOI: 10.1039/D6NH00245E
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Adsorption
AT a surface
Absorption
INTO the interphase

Two electrodes, one shared electrolyte

Two electrodes, one shared electrolyte
Conceptual migration of soluble species between hard-carbon anode and cathode through shared electrolyte. Paths are illustrative, not measured trajectories.
Each electrode can affect the other
Conceptual illustration based on Wisan et al., Fig. 1b–c. DOI: 10.1039/D6NH00245E
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Hard-carbon anode
Shared electrolyte
Cathode
Possible pathways require verification in sodium-ion cells.

The sodium budget

The sodium budget
A finite
sodium budget
Initial losses
Formation consumes
some sodium.
Later side reactions
Continued loss draws
from the same budget.
Electrode balance and electrolyte amount define the test.
Wisan et al., Sections 5.1–5.4. DOI: 10.1039/D6NH00245E
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Formation and operating history

Formation and operating history
01
Formation
Control the
early history
02
Operating stress
Specify voltage,
current, and time
03
Diagnostics
Compare resistance
and losses
A test should expose the failure we aim to prevent.
Weldegebrieal et al. 2026 and Wisan et al. 2026
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Evidence for an interphase mechanism

Evidence for an interphase mechanism
1
Performance
Does the cell perform better?
2
Composition
Which products are present, and where?
3
Evolution
How does the interface change with time?
4
Cause
Does a targeted change test the explanation?
A proposed guide to matching claims with evidence.
Proposed evidence guide: Wisan et al., Section 5.4. DOI: 10.1039/D6NH00245E
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Conditions that make results comparable

Conditions that make results comparable
The operating cell The measurement
Current density and areal capacity Impedance state of charge and temperature
Temperature and sodium inventory Formation and sample handling
Electrolyte amount Matched diagnostic protocol
Solid cells: pressure and architecture The same basis for comparison
Clear conditions make a comparison meaningful.
Weldegebrieal et al., Tables 1 and 3. Wisan et al., Table 2
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Design rules 1–3

Design rules 1–3
01
Connect coordination to interface behavior
Check the chemistry and follow resistance during use.
02
Test transport at the required ion flux
State current density, areal capacity, and temperature.
03
Separate resistance contributions
Identify the process that limits the actual cell.
Rules 1–3: Weldegebrieal et al., Section 2.4. DOI: 10.1039/D6TA01969B
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Design rules 4–6

Design rules 4–6
04
Follow resistance over time
Measure the change through formation and cycling.
05
Match the intervention to the failure
Give each coating, additive, or interlayer a clear purpose.
06
Compare under shared conditions
Make inventory, loading, pressure, and history explicit.
Rules 4–6: Weldegebrieal et al., Section 2.4. DOI: 10.1039/D6TA01969B
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When interface resistance grows

When interface resistance grows
A worked example: contact loss or chemical change?
Test Possible cause Intervention to test
Pressure response
and contact imaging		 Contact loss Compliant interface
or better contact
Chemical change
and resistance evolution		 Continued
decomposition		 Chemical passivation
Both processes can occur. Test the predicted response.
Selected diagnosis route: Weldegebrieal et al., Fig. 4. DOI: 10.1039/D6TA01969B
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Experiments that test the framework

Experiments that test the framework
Carbon pores
Does local chemistry preserve access?
Solid interfaces
Is the change physical or chemical?
Full cells
What happens under a limited inventory?
Proposed tests with controls for alternative explanations
Proposed experiments based on the three articles
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A working interface, cycle after cycle

Decorative conceptual material artwork, not a microscopy image
Preserve a
working interface,
cycle after cycle.
Beyond Ionic Conductivity
Synthesis of our recent Review, Opinion, and Focus articles
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Acknowledgments

Thank you
The people behind the science.
Laboratory members in a group photo
Our coauthors and
collaborators
Phonnapha Tangthuam
Jeremy Putra Wirjo Santoso
Getu Kassegn Weldegebrieal
Thanawat Wisan
I welcome
your questions.
Research support and Chulalongkorn University fellowships
TSRI Fund, Chulalongkorn University (IND_FF_69_151_2100_024)
NSRF via RCAD / PMU-B (B41G690088)
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