How batteries work Chapter 14 Solid-state cells

Chapter 14

Solid electrolytes and solid-state cells: polymer and inorganic ion conductors

Why replace the liquid with a solid, and why is it so hard?

Figure 14.1 Three panels. Liquid: a lithium ion moves inside a shell of solvent molecules and sometimes pairs with an anion. Dry polymer: the ion moves only while the chain segments around it move; cooling the polymer freezes both. Ceramic: the ion moves through a rigid framework.
Li+anionsolvent moleculepolymer chainliquidpolymerceramic, with its framework in grey
Three ways to carry a lithium ion. Try this: cool the polymer and watch which of the three ions stops.
SourceLiquid: organic electrolytes have a lower solvating power than water and show ion pairing (Winter and Brodd 2004, section 2.2); LiPF6 in carbonates (their Table 2). Dry polymer: Li+ hopping between coordinating sites helped by the segmental motion of the chains (Shanmukaraj et al. 2020; Tarascon and Armand 2001, Figure 3 and text). Ceramic: a garnet, after Goodenough and Park 2013; the hops are a drawing device, not a mechanism from the sources. Schematic, not to scale.

So far our ion has travelled through a liquid. Two other kinds of electrolyte carry lithium differently: a polymer, where there is no solvent at all, and a ceramic, where the ion moves through a solid framework. Let us take the polymer first.

A dry polymer such as poly(ethylene oxide), PEO, dissolves a lithium salt with no solvent at all. Wright and co-workers first saw ionic conduction in PEO complexes of sodium and potassium salts, and in 1978 Armand proposed the first dry PEO-lithium electrolyte for lithium batteries, conducting about 10−4 S/cm at 40 to 60 °C.1

The Li+ ion is held by the oxygen atoms of the chain and hops from one coordinating site to the next, helped by the segmental motion of the chain; the motion happens in the amorphous, not the crystalline, regions of the polymer.1,2 That is the weakness of the idea: no chain motion, no conduction.

An electrolyte must carry ions and block electrons; how it carries them decides how fast, and at what temperature.

Figure 14.2 Logarithm of conductivity against temperature for five electrolytes, high temperature on the left: PEO with lithium triflate lowest, PEO with LiTFSI ten times higher, plasticized PEO-LiTFSI ten times higher again, and a gel polymer 2 to 5 times below the organic liquid at the top; a dashed line marks the 80 degrees at which dry-polymer lithium cells run.
Conductivity against temperature: the gaps follow the cited ratios, the heights are schematic. Try this: switch off everything except the two dry-polymer curves and read the gap between them at 40 °C.
SourceShape after Tarascon and Armand 2001, Figure 8, schematic. Only the ratios stated in the text of that article are encoded: about one order of magnitude from LiCF3SO3 to LiTFSI in PEO, about one more with 10 to 25 % plasticizer, and gels 2 to 5 times below the liquid; the 80 °C operating temperature of the dry-polymer lithium cell is theirs too. The curves are illustrative curved forms; absolute values, and the height of the liquid-and-gel pair relative to the polymer group, are not taken from any source.

How close can a polymer come to the liquid? The polymer route improved by steps. Changing the salt in PEO from lithium triflate, LiCF3SO3, to LiTFSI, a salt Armand introduced, gains about an order of magnitude, its bulky anion acting as a plasticizer;1 adding 10 to 25 % of a plasticizer gains another, and such lightly plasticized polymers still suit a lithium-metal electrode. Gels, 60 to 95 % liquid held in a polymer, come within 2 to 5 times of the liquid’s conductivity but need a lithium-ion configuration. A dry-polymer lithium cell runs at up to 80 °C.2

LiTFSI itself is a good salt, conductive and safe, with one catch: above about 4 V (3.7 V in PC:DME, as we saw in figure 8.3) it corrodes the aluminium current collector unless a passivating salt is added.2,3 Liquid or polymer, a nonaqueous electrolyte is thermodynamically stable over a window of about 3.5 V but is used, kinetically, up to 5.5 V.2

That window depends on more than the solvent alone, because the neighbours matter: a single EC molecule is computed to resist oxidation to about 6.9 V vs Li/Li+; next to a PF6− anion, which can take a hydrogen from it as HF, the limit falls, and when the hydrogen passes to a neighbouring EC it falls to about 5 to 5.2 V, almost 2 V lower.4 In the usual mixture, the cyclic EC dissolves the salt and builds the SEI, while a linear carbonate such as DMC thins the liquid so it conducts better in the cold.4

Worked example. Let us see what the conductivity costs, for a 25 μm electrolyte layer (an illustrative thickness) of 1 cm2, using R = l/(κA) from chapter 0. liquid, κ = 10−2 S/cm: R = 0.0025 cm ÷ (10−2 S/cm × 1 cm2) = 0.25 Ω a solid at κ = 10−4 S/cm: 25 Ω, a hundred times more at 1 mA: ohmic drop 0.25 mV against 25 mV This is why 10−4 S/cm is the bar a solid electrolyte has to clear, and why it has to be thin.5–7

For a sense of scale, a 2026 review puts solid polymer electrolytes at 10−5 to 10−4 S/cm at room temperature, against 10−3 to 10−2 S/cm for liquid electrolytes.8

Figure 14.3 Left: the four requirements for a solid electrolyte, with the garnet ceramic and the suggestion of ceramic-polymer composites. Right: an electrode particle swelling and shrinking inside a rigid solid electrolyte; when it shrinks back the electrolyte does not follow, a gap opens and the ion path is broken.
electrode particlesolid electrolyteion path in contactbroken path
Why solid electrolytes are hard: four requirements, and a contact that has to survive breathing. Try this: press Play and watch for the moment the ion paths turn red.
SourceRequirements, the garnet conductivity and the composite suggestion after Goodenough and Park 2013 (a 2013 assessment); contact loss of inorganic solid electrolytes against electrodes that change volume, also theirs. The size of the volume change and the way the gap opens are schematic.

That leaves the ceramic, and the question of why the liquid has not simply been replaced by a solid. A solid separator that blocks dendrites would make a lithium-metal negative electrode possible (chapter 12). To do so, a solid electrolyte must conduct Li+ at above 10−4 S/cm, block dendrites without being reduced by lithium, stay chemically stable, and form a robust, flexible thin membrane. The garnet Li7−ₓLa3Zr2−ₓTaₓO12 conducts above 10−3 S/cm and is stable against lithium; a 2013 assessment suggested that meeting all four requirements at once points to ceramic-polymer composites. A polymer above 10−4 S/cm with good electrode contact had, in 2013, yet to be demonstrated.7

Even a good solid conductor has to keep touching the electrodes. Electrode particles swell and shrink as lithium goes in and out, and against an inorganic solid electrolyte the contact is lost.7 In a liquid, the electrolyte simply flows after them.

Figure 14.4 Left: lithium against a ceramic electrolyte, touching only at points with voids between, or wetting it through a thin interlayer. Right: bars on a log scale of the interface resistance before and after three interlayers, a lithium-graphite composite, an aluminium layer and atomic-layer-deposited alumina. A slider sets the current density and the readout gives the voltage lost at each interface.
bare interfacewith an interlayerlithiumceramic
Solid-state cells: the trouble is where the lithium meets the ceramic. Try this: at 1 mA/cm2, read the voltage lost at the bare ALD interface; then add the interlayer.
SourcePoor wetting, unstable SEI and dendrites at the lithium/solid-electrolyte interface, and the interface resistances before and after a Li–C composite (381 to 11 Ω cm2), an aluminium layer (950 to 75 Ω cm2) and ALD Al2O3 (1710 to 1 Ω cm2) on garnet, after Rahman et al. 2025, sections 6.1 and 7.1 and Fig. 9. The voltage lost, R × i, is this page’s arithmetic; the contact drawing is schematic.

Negative electrodes for solid-state cells

So far our lithium metal has faced a liquid. The alternative is to put a solid electrolyte against it, and the main reason for moving to solid electrolytes, which we met in chapter 14, is to use lithium metal. Graphite and titanate work in solid-state cells but store too little to be worth it; silicon is attractive but its swelling is hard on a rigid electrolyte.9 A solid electrolyte should be stiffer than the electrode to hold back dendrites, and lithium is soft; yet dendrites still grow through ceramic electrolytes, probably along their grain boundaries, and this is not yet understood.9

The lithium/ceramic interface has three problems: poor wetting, an unstable SEI and dendrites. Interlayers of Al2O3, LiNbO3 or ZrO2, often made by atomic layer deposition, a polymer-gel layer, or an additive such as dimethyl sulfate that forms a stiff Li2S/Li2O interphase, all improve it.9 Two other designs avoid the problem: columnar silicon, whose columns change volume anisotropically and so build up less stress, kept 92 % of its capacity after 50 cycles with a sulfide electrolyte against 50.6 % in a liquid one, at a Coulombic efficiency of about 99.95 %; and Samsung’s silver–carbon layer, which needs no excess lithium and gave a prototype pouch cell of more than 900 Wh/L that lasted 1000 cycles.9

Worked example. An interface of 1710 Ω cm2 at a current density of 1 mA/cm2: ΔV=Ri=1710 Ωcm2×0.001 A/cm2=1.71 V with the Al2O3 interlayer, 1 Ω cm2: 1 mV Without the interlayer the interface alone would eat most of a lithium-metal cell’s voltage.

References for this chapter

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

  1. 1 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
  2. 2 J.-M. Tarascon, M. Armand, "Issues and challenges facing rechargeable lithium batteries", Nature 414, 2001, 359-367. doi:10.1038/35104644
  3. 3 W. A. Henderson, "Nonaqueous Electrolytes: Advances in Lithium Salts", in T. R. Jow, K. Xu, O. Borodin, M. Ue (eds.), Electrolytes for Lithium and Lithium-Ion Batteries, Modern Aspects of Electrochemistry 58, Springer, New York, 2014, 1-92. doi:10.1007/978-1-4939-0302-3_1
  4. 4 O. Borodin, X. Ren, J. Vatamanu, A. von Wald Cresce, J. Knap, K. Xu, "Modeling Insight into Battery Electrolyte Electrochemical Stability and Interfacial Structure", Accounts of Chemical Research 50, 2017, 2886-2894. doi:10.1021/acs.accounts.7b00486
  5. 5 A. J. Bard, L. R. Faulkner, H. S. White, "Electrochemical Methods: Fundamentals and Applications", Wiley, 3rd edition, 2022.
  6. 6 M. Winter, R. J. Brodd, "What Are Batteries, Fuel Cells, and Supercapacitors?", Chemical Reviews 104, 2004, 4245-4269. doi:10.1021/cr020730k
  7. 7 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
  8. 8 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
  9. 9 M. M. Rahman, U. Nisar, A. Abouimrane, I. Belharouak, R. Amin, "Valuation of Anode Materials for High-Performance Lithium Batteries: From Graphite to Lithium Metal and Beyond", Electrochemical Energy Reviews 8, 2025, 14. doi:10.1007/s41918-025-00249-w

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). Solid electrolytes and solid-state cells: polymer and inorganic ion conductors. How batteries work. https://asajeevan.github.io/batteries/solid-state/

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