During the first lithiation of a SPAN cathode, irreversible chemical bond rearrangements increase electronic conductivity but consume part of the lithium inventory. Covalent C–S, N–S, and S–S bonds are broken and replaced by Li–S, Li–C, and ionic Li–N interactions. Lithium coordinated to nitrogen in the pyridine-like framework improves electron transfer and is associated with a reduction in the calculated HOMO–LUMO gap from approximately 1.92 eV to 0.5 eV; however, lithium that reacts with C=C and C=N bonds forms Li–C=C and Li–C–N configurations that are not fully reversed during the first charge, producing initial capacity fade.
The first cycle acts as both an electrochemical activation and an irreversible chemical conversion step: SPAN becomes more electronically conductive, but some lithium becomes trapped in chemical environments that cannot be completely delithiated.
Why SPAN Conductivity Increases During First Lithiation
Covalent bonds are converted into lithium-containing species
SPAN contains sulfur chemically integrated into a nitrogen-containing carbon framework rather than relying only on conventional crystalline sulfur. During the first discharge, applied electrochemical potential drives lithium into the material and breaks some C–S, N–S, and S–S bonds.
These reactions generate new Li–S, Li–C, and ionic Li–N species. The resulting structure has different bonding and electronic characteristics from the pristine SPAN material.
Lithium–nitrogen coordination improves electron transfer
A key contribution comes from the irreversible coordination of lithium with nitrogen sites, particularly nitrogen associated with the pyridine-like framework. This coordination changes the local electronic structure and facilitates electron transfer through the cathode.
The primary reference reports a reduction in the calculated HOMO–LUMO gap from 1.92 eV to 0.5 eV. A smaller gap generally indicates that electronic excitation and charge transport are more favorable, helping explain the observed increase in SPAN conductivity after initial lithiation.
The first cycle chemically activates the cathode
The conductivity increase should therefore be understood as electrochemical activation, not simply improved physical contact between particles. The first lithiation changes the bonding network itself and creates a more electronically favorable state.
This is why SPAN can show a substantial conductivity change even when the electrode formulation, current collector, and conductive additive remain unchanged.
Why Initial Capacity Fades
Some lithium reacts with carbon–carbon and carbon–nitrogen bonds
Not all inserted lithium participates in reactions that can be cleanly reversed. A portion reacts with C=C and C=N double bonds, producing Li–C=C and Li–C–N species.
These products are relatively irreversible under the subsequent delithiation conditions. The lithium remains chemically associated with the SPAN framework instead of returning fully to the electrolyte during the first charge.
Irreversible lithium consumption lowers first-cycle efficiency
The lithium bound in these configurations contributes to the discharge capacity but is not fully recovered on charge. Consequently, the first-cycle charge capacity is lower than the first-cycle discharge capacity, reducing initial Coulombic efficiency.
This is the central origin of the initial capacity fade described for SPAN: part of the apparent lithiation capacity is consumed by irreversible chemical conversion.
Conductivity improvement and capacity loss occur together
These two effects are not contradictory. The same first-cycle reactions that reorganize the bonding network and improve electron transport can also consume lithium irreversibly.
In practical terms, SPAN undergoes a trade: the cathode becomes more conductive and electrochemically accessible, but some of its first-cycle lithium uptake cannot be recovered.
How This Differs From Generic First-Cycle Loss
The dominant SPAN mechanism is cathode-framework chemistry
Initial irreversibility can have several origins in a full cell. For example, carbon anodes commonly consume lithium through solid-electrolyte interphase formation, while alloy anodes can lose capacity through irreversible structural changes.
For SPAN, the mechanism emphasized here is different: it is primarily the irreversible transformation of sulfur–carbon–nitrogen bonding and lithium coordination within the cathode framework.
Cell-level measurements may include multiple losses
A practical full cell can also exhibit electrolyte reduction, interfacial reactions, lithium inventory limitations, and electrode-contact effects. These processes may add to the measured first-cycle loss.
Therefore, battery-test data should not automatically attribute every difference between first discharge and first charge exclusively to SPAN chemistry. Half-cell testing, matched controls, and electrode-level characterization help separate intrinsic SPAN conversion from other cell contributions.
Understanding the Trade-offs
Higher conductivity does not guarantee higher reversible capacity
Improved electronic conductivity can enhance utilization of the active material, particularly when the pristine electrode is electronically limited. It does not, by itself, ensure that every lithium-containing species formed during discharge will be reversible.
The chemical reversibility of the new Li–S, Li–C, and Li–N environments remains equally important.
Irreversible activation can complicate material comparisons
Two SPAN materials may have different first-cycle capacities because they contain different distributions of sulfur linkages, nitrogen environments, and unsaturated carbon–nitrogen sites. Comparing only later-cycle conductivity or capacity can obscure differences in first-cycle chemical conversion.
Material comparisons should therefore report first-cycle discharge capacity, charge capacity, Coulombic efficiency, and subsequent capacity retention together.
Formation conditions influence the measured result
Current density, voltage limits, rest periods, electrolyte composition, electrode loading, and temperature can affect how completely the initial reactions proceed. Testing conditions can therefore change the apparent magnitude of both conductivity activation and initial capacity loss.
Consistent formation protocols are essential when evaluating synthesis changes or comparing SPAN formulations.
How to Apply This to SPAN Testing
The most useful approach is to treat the first cycle as a chemically distinct formation event rather than as a normal representative cycle.
- If your primary focus is conductivity activation: Track electrode resistance or impedance before and after the first lithiation, and relate the change to Li–N coordination and the reported reduction in the electronic energy gap.
- If your primary focus is initial Coulombic efficiency: Quantify the difference between first-cycle discharge and charge capacity, recognizing that Li–C=C and Li–C–N formation represents irreversible lithium consumption.
- If your primary focus is long-term capacity retention: Use controlled formation cycling before comparing later-cycle performance, while reporting the initial irreversible loss separately.
- If your primary focus is mechanism verification: Combine electrochemical data with structural and spectroscopic characterization to distinguish SPAN framework conversion from parasitic cell reactions.
- If your primary focus is materials optimization: Adjust SPAN synthesis and electrode formulation to balance electronically favorable bonding changes against excessive irreversible lithium binding.
Understanding this first-cycle chemistry allows researchers to interpret SPAN test results correctly and optimize the material for both conductivity and reversible capacity.
Summary Table:
| Factor | Conductivity Increase | Initial Capacity Fade |
|---|---|---|
| Bond Rearrangement | C-S, N-S, S-S bonds break, forming Li-S, Li-C, Li-N | Some Li reacts with C=C and C=N forming Li-C=C and Li-C-N |
| Lithium-Nitrogen Coordination | Reduces HOMO-LUMO gap from 1.92 eV to 0.5 eV | Not directly related |
| Reversibility | Irreversible but beneficial for electron transfer | Irreversible, reducing Coulombic efficiency |
| Mechanism | Electrochemical activation of the cathode framework | Irreversible lithium consumption |
| Impact on Performance | Enhanced electronic conductivity | Lower first-cycle charge capacity |
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