How batteries work Chapter 15 Sodium and organic

Chapter 15

Sodium-ion and organic electrode materials

Can sodium, or molecules made of carbon, hydrogen and oxygen, do lithium's job?

Figure 15.1 A lithium ion fits in the gap of a close-packed oxide framework while a larger sodium ion does not; in a more open framework the sodium ion fits.
Li+Na+oxide ion
Sodium is bigger. Try this: compare the gap the two ions are offered in the left-hand framework.
SourceAfter Goodenough and Park 2013: Na+ needs a larger interstitial space than a close-packed oxide offers. The frameworks are schematic.

Beyond lithium

Every electrode in this module so far has stored lithium. Must it always be lithium? Lithium itself is a critical raw material, so its supply and sustainability matter too.1 A sodium-ion cell would rock sodium ions instead of lithium ions, on the principle of chapter 7. But Na+ is larger than Li+ and needs a larger interstitial space than many of the hosts that serve lithium offer, so sodium-ion cells need hosts of their own.2 The older sodium cells used molten electrodes instead: the sodium-sulfur battery, invented at Ford in 1967 after Kummer and Weber found fast Na+ conduction in a ceramic above 300 °C, runs at 300 to 350 °C.2,3 Sodium-ion is the first of the “beyond lithium” chemistries to become commercially viable, so far mainly for stationary storage, where energy density matters less.4

Negative electrodes: in use and coming
MaterialStatusCapacityvs LiTrade-off
Graphitethe standard372 mAh/gabout 0.15 Vstable in carbonate electrolytes1
SiOₓ or Si in graphitein commercial cells, in small amountsSi: 3579 mAh/gabout 0.25 VSi swells more than 280 %, SiO about 118 %; usually under 10 wt% SiOₓ or under 6 to 8 wt% Si1,5
Li4Ti5O12high-power niche175 mAh/gabout 1.55 Vzero strain, little SEI, fast and safe; low energy, high cost per kWh1,6
Niobium oxidesindustrial scale, high power200 to 350 mAh/g1 to 2 Vlittle volume change, very fast charging1
Lithium metalcoming, with solid electrolytes or anode-free cells3860 mAh/g0 Vdendrites and lithium lost every cycle (figures 12.5 and 12.6)1
Figure 15.2 Lithium, sodium, potassium and calcium compared: bars for the charge each metal stores per gram, a ladder of their standard reduction potentials against the standard hydrogen electrode, and their ions drawn to scale as shaded space-filling spheres on one baseline, with a dashed ring showing the size of Li⁺ inside each of the others and a 1 Å scale bar.
Four candidate charge carriers, compared as metals. Try this: tap each ion. Which one gives the most charge per gram, and which sits highest on the potential ladder?
SourceSpecific capacities, standard reduction potentials and ionic radii from Faruque et al. 2026, Table 14; the notes on sodium-ion and potassium-ion cells from their section 7.1. Ions drawn to scale with one another as space-filling spheres in the CPK (Jmol) colours that molecular models use for these elements; the bars and rungs use a deeper tone of the same colours. The shading and the soft halo are drawing style only.

Four candidate charge carriers

Why lithium, and what do its rivals offer? A 2026 review compares the metals themselves. Lithium stores 3861 mAh/g as metal and sits at −3.04 V against the standard hydrogen electrode; sodium stores 1165 mAh/g at −2.71 V, potassium 685 mAh/g at −2.93 V, and calcium, which gives two electrons per ion, 1337 mAh/g at −2.87 V. Their ionic radii are 0.76, 1.02, 1.38 and 1.00 Å.7 Lithium wins on charge per gram and on voltage; sodium is about 0.3 V less negative than lithium and its ion is larger, the price of its abundance.7

The review also says where sodium struggles. Na+ does not intercalate well into graphite, so hard carbon is the most promising negative electrode, while layered transition-metal oxides and polyanionic compounds serve as positive electrodes; fluoroethylene carbonate (FEC) helps stabilize the SEI. Potassium-ion cells offer high voltages and fast ionic diffusion, but the large K+ strains its hosts. Both are seen as complements to lithium-ion cells, not replacements, attractive where cost, resources and supply count for more than energy density.7

Figure 15.3 Three views. First, three columns showing how organic electrodes store charge, with oxidized states above and reduced states below: an n-type molecule reduced, taking an electron and a lithium ion; a p-type molecule oxidized, giving up an electron and taking an anion from the electrolyte; and a bipolar molecule doing both. Second, one real n-type molecule, Li2-PDCA, drawn as skeletal formulas through its two charging steps: the dianion with two lithium ions, the radical anion after the first electron and lithium ion leave at about 3.15 volts, and the neutral quinonediimine after the second at about 3.32 volts. Third, a ladder of average potentials against lithium for lithiated organic positive electrode materials in three groups, the M(Li2)-p-DHT salts with different spectator cations, other relatives of p-DHT, and the Wurster-type (p-phenylenediamine triflimides and cyanamides) and oximate materials, with a dashed line at 2.91 volts marking the air-stability threshold and a line at 3.43 volts for LiFePO4.
electronLi+anion from the electrolyteladder: above 2.91 Vbelow 2.91 V
Organic electrodes: how they store charge, and how high their voltage now goes. Try this: in the ladder, tap Li4-p-DHT and then Mg(Li2)-p-DHT; the organic ion is the same, only the spectator cation has changed.
SourceClassification after Guo et al. 2024. The two-step delithiation of Li2-PDCA, through a radical anion to the neutral quinonediimine form (Wurster-type redox of the p-phenylenediamine core), after Guo et al. 2024, section on the redox mechanism and Fig. 3A, with its solid-state potentials from their Table 1; the skeletal formulas draw one resonance form of each state. Potentials vs Li+/Li: the M(Li2)-p-DHT salts read from Jouhara et al. 2018, Fig. 5b (average potential against the ionic potential of M), Ba about 2.45, Li 2.55, Ca 2.85 and Mg 3.40 V, read by eye, ±0.03 V; Li4-o-DHT 0.3 V above Li4-p-DHT (same paper); Li4-p-DOBDA 3.35 V (Jouhara et al. 2025); Li2-BQDO about 3.0 V (Wang et al. 2023); Li2-PDCA 3.15 V (first plateau; 3.32 V second), Li2-PDFSA 3.55 V and Li2-DF-PDFSA 3.7 V (first plateau) in the solid (Guo et al. 2024, Table 1); air-stability threshold 2.91 V (Guo et al. 2024; Wang et al. 2023); LiFePO4 3.43 V (Safari and Delacourt 2011). The panels are schematic.

Organic electrodes

Every positive electrode so far stores its charge on a transition metal, which is mined and then processed at high temperature.8 Organic electrodes put the redox centre in a molecule or a polymer made of abundant light elements such as carbon, hydrogen, oxygen, nitrogen and sulfur; they can be made by environmentally friendly synthesis, and some could even be extracted from plants.9 Their potential can be tuned by molecular design, through the bonds or through space.9

They are sorted by the charge the molecule takes on (figure 15.3). An n-type molecule is reduced, and a cation such as Li+ balances it; a p-type molecule is oxidized, and an anion from the electrolyte balances it; a bipolar one does both, in different windows of potential.9 The p-type materials work at relatively high potential but store the anion, so they suit dual-ion or lithium-free cells rather than the rocking chair; n-type materials work with the lithium ion itself.8,9

The redox centre is a functional group: most often a conjugated carbonyl, which becomes an enolate when reduced, but also imines, organosulfur groups, nitroxide radicals and, recently, conjugated sulfonamides and oximates.8,9 Its potential follows the energy of the orbital that gives or takes the electron: groups that draw electron density away from the redox centre lower that orbital and raise the voltage. Most conventional n-type organic materials work below 3 V vs Li.9

A lithium-ion cell adds a condition. Its graphite arrives empty, so the positive electrode must bring the lithium, as LiCoO2 does. An n-type organic made in its oxidized form brings none, and needs lithium metal as the negative electrode; the answer is to make it in its reduced, lithiated form.8,9 But a lithiated organic working below about 2.91 V vs Li is thermodynamically prone to oxidation by air, so most cannot even be handled in a dry room.9

Small molecules also dissolve in the electrolyte, the main route of degradation of organic electrodes; making the material a polymer, building it into a framework, or replacing the liquid with a solid electrolyte are the usual remedies.9 And organic electrodes are usually mixed with a large share of conductive carbon, a quarter to 40 % of the electrode by mass in the studies cited here (a third in most of their half-cell tests).8,10

The p-DHT family

One family shows how far molecular design can go. Its parent is 2,5-dihydroxyterephthalic acid, written p-DHT: the p, for para, marks that the two hydroxy groups sit opposite each other on the ring. The fully lithiated salt, dilithium (2,5-dilithium-oxy)-terephthalate or Li4-p-DHT, carries two kinds of redox group on one benzene ring: two phenolates (–OLi) that react at about 2.6 V and two carboxylates (–CO2Li) that react at about 0.8 V, so the same salt can serve as a positive or a negative electrode. The carboxylates also make it nearly insoluble in carbonate electrolytes.10 It can also be derived entirely from renewable resources.10

Its 2.6 V is too low for a modern cell, and here a subtle change works: two of the four lithium ions are replaced by one magnesium ion. The magnesium stays in place while lithium comes and goes, so it is called a spectator cation.10,11 The salt, magnesium (2,5-dilithium-oxy)-terephthalate or Mg(Li2)-p-DHT, has a name that reflects its synthesis: magnesium hydroxide first neutralizes the two carboxylic acid groups of 2,5-dihydroxyterephthalic acid, and two equivalents of lithium hydroxide then neutralize the two phenol groups.10 The name might suggest that magnesium sits only at the carboxylates; the crystal structure, published in 2025, shows otherwise. Each Mg2+ sits in an octahedron of six oxygen atoms, four from phenolates and two from carboxylates. It is chelated by two p-DHT4− ions, each through a phenolate oxygen and the neighbouring carboxylate oxygen of that ion, closing two six-membered rings; two more phenolate oxygens complete the octahedron. Each phenolate oxygen, the redox centre, is bonded to one Li+ and two Mg2+ (see the structures below).11

Mg2+ is small and doubly charged: it has a high ionic potential, its charge divided by its radius. Its bonds to oxygen are therefore more covalent. At the phenolate oxygen, bonded to two Mg2+, they draw electron density from that oxygen and through it from the ring; together with its bonds to the carboxylates, they weaken the electron donation (the donor inductive effect) of the –O− and –CO2− groups into the ring. The phenolate becomes harder to oxidize, and the potential rises by about 0.8 V, to about 3.4 V, in the range of LiFePO4; a calculation from the solved structure and the predicted half-delithiated one gives 3.45 V, in line with the measurement.10,11 Across the barium, lithium, calcium and magnesium salts, measured in the same way, the average potential rises almost linearly with the ionic potential of the spectator cation.10

Once dehydrated, the magnesium salt is stable in air, and it cycles stably near 100 mAh/g; but, like bulk Li4-p-DHT, it gives only one of its two possible electrons, whereas exfoliated nanosheets of Li4-p-DHT have been reported to reach the full two-electron capacity.10,11 The 2025 study explains why. After one electron is removed, each ring carries a radical; the rings, stacked face to face, pair up into dimers whose spins cancel, a Peierls distortion, and electron spin resonance finds only about 1.2 % of one unpaired spin per formula unit. Calculations indicate that removing the second electron would weaken or even break the C=O bonds of the carboxylates; charging bulk material beyond 4.0 V instead starts an irreversible reaction, attributed to Kolbe electrolysis.11

Changing the organic ion instead also works. In the ortho isomer, dilithium (2,3-dilithium-oxy)-terephthalate or Li4-o-DHT, the two –OLi groups sit side by side, and the potential rises by about 0.3 V; like the para salt, it is oxidized by air.10 Carboxylates held off the ring by a methylene (CH2) spacer, in tetralithium 2,5-dioxido-1,4-benzenediacetate (Li4-p-DOBDA), are no longer conjugated with it; the potential still rises, to 3.35 V, which the authors attribute to a through-space charge modulation in the solid, with both electrons exchanged, 215 mAh/g, though with a large polarization on cycling.11

The structures in 3D: the p-DHT family, and where the cations sit
Figure 15.4 The p-DHT family in three dimensions. Ball-and-stick models, which can be turned, of three organic tetra-anions: p-DHT, o-DHT and p-DOBDA, with the two phenolate oxygen atoms, the redox centres, ringed. A fifth view shows schematically how magnesium and lithium ions bind to one p-DHT ion in the crystal of Mg(Li2)-p-DHT: each magnesium ion is chelated by a phenolate oxygen and the neighbouring carboxylate oxygen, and each phenolate oxygen is bonded to one lithium and two magnesium ions.
phenolate oxygen, the redox centreoxygencarbonmagnesiumlithium
The p-DHT family: the organic ions in 3D, and where the cations sit. Try this: turn p-DHT4− and then o-DHT4− and find the two ringed oxygens; then open the cations view and count the metal ions around one phenolate oxygen.
SourceThe three ions were computed for this page (GFN2-xTB with tblite, in an implicit water continuum, ALPB, that stands in for the surrounding cations); they are free ions, so the twist of the side groups can differ from the solid. The cations view is a schematic of the coordination reported for the solved crystal structure of Mg(Li2)-p-DHT (Jouhara et al. 2025, CCDC 2247807): bonding counts and polyhedra from the paper, positions not to scale.

From my own work. My doctorate took Mg(Li2)-p-DHT as its benchmark positive electrode and built it into soft solid-state cells against lithium metal, with a solid polymer electrolyte that limits the dissolution of the organic material; it also developed a route to recover the organic precursor from the cycled electrode material.

Beyond carbonyl chemistry: conjugated sulfonamides and oximates

Most lithiated organic positive electrodes, the p-DHT family included, rest on enolate and carbonyl chemistry.8 A newer line uses a different redox-active unit: a p-phenylenediamine core whose two nitrogen atoms each carry an electron-withdrawing group, methanesulfonyl (–SO2CH3) in the conjugated sulfonamides, trifluoromethanesulfonyl (–SO2CF3) in the conjugated triflimides and cyano (–C≡N) in the cyanamides. Conjugation spreads the negative charge of the lithiated form over the whole core and its withdrawing groups. On charge the core gives up two electrons in two steps, through a radical anion, to a neutral quinone diimine form: the Wurster-type redox of p-phenylenediamines.9 Because its charge is spread out, the anion is less nucleophilic, so the lithiated material resists water and oxygen in ambient air and still reacts reversibly at high potential.8,9

Trifluoromethanesulfonyl withdraws electrons more strongly than methanesulfonyl, and the swap raised the potential by 0.5 V: the conjugated triflimides (the authors’ name for the family) work at 3.26 to 3.8 V, the highest reported for n-type organics by 2024, and are stable in moist ambient air. The simplest cyanamide, Li2-PDCA, stores more, 320 mAh/g in theory, but its fully charged form dissolves in the electrolyte.9 A third class, the conjugated oximates, carries its charge on C=N–O− groups; one of them, Li2-BQDO, reaches about 350 mAh/g at about 3.0 V, close to 1 kWh/kg at the material level.8

Cycling stability remains a concern for most organic electrode materials, and it is too early to think of commercializing the newest of them.8 But their voltages now reach those of LiFePO4 and beyond, without cobalt or nickel.9,10

In an organic electrode the molecule is the host; its voltage is set by the groups around the redox centre, and even by the cation beside it.

You can now explain where each family of negative electrode sits on the map of capacity and potential and why extra capacity has diminishing returns; the three ways to store lithium; how graphite’s SEI forms and how graphite ages hot and cold; why fast charging risks plating and how titanate and niobium oxides avoid it; why silicon and conversion materials are hard to cycle; what limits lithium metal and anode-free cells, and the fixes; why the lithium/ceramic interface matters in solid-state cells; why sodium needs hosts of its own; the three families of positive electrode and what nickel, manganese and cobalt each bring; why nickel-rich NMC trades stability for capacity; and how organic electrodes store charge and how their voltage is tuned.

We have taken both electrodes from today’s materials towards tomorrow’s. Yet even a well-behaved cell wears out. Why? That is the question of chapter 16.

The mathematics of this chapter

Qpair = QposQneg/(Qpos + Qneg), E = Qpair(Vpos − Vneg)specific energy of the two active materials (figure 12.1); as Qneg → ∞, Qpair → Qpos

Li+ + e− + xC ⇌ LiCx; xLi+ + xe− + M ⇌ LixM; MxOy + 2yLi+ + 2ye− ⇌ yLi2O + xMintercalation, alloying and conversion (figure 7.3)

Qblend = (1 − w) × 372 + w × 3579 mAh/ga graphite/silicon blend with a mass fraction w of silicon (figure 12.3)

dafter/dbefore = (1 + ΔV/V)1/3how a volume change becomes a change of size: 3.81/3 = 1.56 for 280 %

ηE ≈ (Vpos − Vneg − h/2)/(Vpos − Vneg + h/2)round-trip energy efficiency with a hysteresis h on the negative electrode (figure 12.4)

Cn = CEn (no excess); R = N(1 − E)capacity of an anode-free cell after n cycles, and Brandt’s excess needed for N cycles (figure 12.6)

ΔV = Rint ivoltage lost across an interface of area-specific resistance Rint at current density i (figure 14.4)

Vcell = Vpos − Vneg (both vs Li/Li+)reading a cell voltage off figure 12.1, as on the ladder of chapter 3

Ematerial = Q × V̄energy per gram of active material: 350 mAh/g × 3.0 V = 1050 mWh/g, about 1 kWh/kg (an oximate, figure 15.3); a cell holds much less once everything else is counted

retention = Q300/Q5capacity kept between cycle 5 and cycle 300 (figure 13.1)

References for this chapter

Numbers in the text refer to this list. All references of the course, with the chapters that use each one.

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  2. 2 J. B. Goodenough, K.-S. Park, "The Li-Ion Rechargeable Battery: A Perspective", Journal of the American Chemical Society 135, 2013, 1167-1176. doi:10.1021/ja3091438
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  5. 5 G. G. Eshetu, H. Zhang, X. Judez, H. Adenusi, M. Armand, S. Passerini, E. Figgemeier, "Production of high-energy Li-ion batteries comprising silicon-containing anodes and insertion-type cathodes", Nature Communications 12, 2021, 5459. doi:10.1038/s41467-021-25334-8
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  7. 7 O. Faruque, M. J. Isalm, O. Faruq, M. M. Alam, "A comprehensive review of past and present developments of Li-ion batteries", Discover Sustainability 7, 2026, 881. doi:10.1007/s43621-026-02920-8
  8. 8 J. Wang, P. Apostol, D. Rambabu, X. Guo, X. Liu, K. Robeyns, M. Du, Y. Zhang, S. Pal, R. Markowski, F. Lucaccioni, A. E. Lakraychi, C. Morari, J.-F. Gohy, D. Gupta, A. Vlad, "Revealing the reversible solid-state electrochemistry of lithium-containing conjugated oximates for organic batteries", Science Advances 9, 2023, eadg6079. doi:10.1126/sciadv.adg6079
  9. 9 X. Guo, P. Apostol, X. Zhou, J. Wang, X. Lin, D. Rambabu, M. Du, S. Er, A. Vlad, "Towards the 4 V-class n-type organic lithium-ion positive electrode materials: the case of conjugated triflimides and cyanamides", Energy & Environmental Science 17, 2024, 173–182. doi:10.1039/d3ee02897f
  10. 10 A. Jouhara, N. Dupré, A.-C. Gaillot, D. Guyomard, F. Dolhem, P. Poizot, "Raising the redox potential in carboxyphenolate-based positive organic materials via cation substitution", Nature Communications 9, 2018, 4401. doi:10.1038/s41467-018-06708-x
  11. 11 A. Jouhara, E. Quarez, N. Gautier, F. Boucher, E. Janod, N. Dupré, J. Gaubicher, P. Poizot, "Unveiling the Peierls effect during electrochemical delithiation/lithiation process in magnesium (2,5-dilithium-oxy)-terephthalate using combined experimental and computational studies", Energy Storage Materials 81, 2025, 104456. doi:10.1016/j.ensm.2025.104456
  12. 12 M. Winter, R. J. Brodd, "What Are Batteries, Fuel Cells, and Supercapacitors?", Chemical Reviews 104, 2004, 4245-4269. doi:10.1021/cr020730k
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  14. 14 D. Shanmukaraj, P. Ranque, H. Ben Youcef, T. Rojo, P. Poizot, S. Grugeon, S. Laruelle, D. Guyomard, "Review—Towards Efficient Energy Storage Materials: Lithium Intercalation/Organic Electrodes to Polymer Electrolytes—A Road Map (Tribute to Michel Armand)", Journal of The Electrochemical Society 167, 2020, 070530. doi:10.1149/1945-7111/ab787a
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Cite this page

Using this page in teaching, a thesis or an article? Cite it as follows; the date is the last update of the course.

Shyma Sajeevan, A. (2026). Sodium-ion and organic electrode materials. How batteries work. https://asajeevan.github.io/batteries/sodium-organic/

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