How Do Lithium Ions Actually Move Through a Solid Electrolyte?

Solids don't flow, so how does lithium still get through them? Here's what happens at the atomic scale — and why the answer changes how you should read a conductivity number.

Ask a battery researcher why a solid-state cell needs a solid electrolyte, and the answer is easy. Ask how that electrolyte actually conducts lithium when the crystal lattice itself isn't moving, and it gets more interesting.

The short version: lithium ions don't travel with the lattice. They relay through it, one atomic-scale hop at a time, using defects the lattice already contains. This post walks through that relay — the three hopping mechanisms involved, the energy barrier each hop has to clear, and why a high conductivity number on a datasheet doesn't automatically translate into a low-resistance cell.

A crystal lattice is never perfectly packed

Zoom into an inorganic solid electrolyte at the atomic scale and you'll find a framework of anions — sulfur, oxygen, phosphorus, chlorine — with lithium ions occupying the sites that framework leaves open, arranged in a repeating periodic structure. But real crystals are almost never perfectly ordered. Two kinds of point defects matter here:

Vacancies — sites that should hold a lithium ion but don't, like an empty space in a parking garage.
Interstitial ions — lithium ions squeezed into gaps outside the normal lattice sites.

Doping, non-stoichiometric compositions and intrinsic structural disorder all continuously generate these vacancies and off-site positions. It's these "imperfections" that make transport possible in the first place: the lattice framework itself doesn't move, but lithium ions can shift position by using the openings it leaves behind.

Three ways to hop

Long-range lithium migration through a solid always comes down to swapping places — but there are three distinct ways it happens.

Vacancy hopping. A lithium ion at one site jumps into a neighboring vacancy; the site it just left becomes the new vacancy for the next ion. Each individual jump only covers one lattice spacing, but enough of these in sequence add up to migration across the whole material.

Interstitial migration. A lithium ion in an interstitial position jumps directly to the next interstitial site, and sometimes knocks a lithium ion out of a normal lattice site in the process — a domino effect that passes displacement down the line.

Concerted migration. In some fast ion conductors, several neighboring lithium ions move almost simultaneously rather than hopping one at a time. This collective motion can lower the energy cost of certain pathways, but it isn't a mechanism every solid electrolyte relies on — which pathway dominates depends on the specific crystal structure.

Without an applied field, lithium ions hop in every direction with roughly equal probability, so there's no measurable net current. During charge and discharge, the electric field and the difference in lithium chemical potential across the cell give each hop a slight directional bias. Only after billions of these hops accumulate does that bias show up as the ionic current an instrument can measure.

Every hop has to clear an energy barrier

Moving from one stable site to the next, a lithium ion has to pass through a narrower, higher-energy transition region — the migration barrier. Picture two stable sites as adjacent valleys: the ion has to climb over the pass between them. A lower pass means a higher chance of getting through; too high a pass leaves most ions simply vibrating in place, with the hop rate falling off exponentially with temperature. That temperature dependence is what the activation energy term in the Arrhenius relation captures.

Two related terms are worth separating clearly, since they're often conflated:

Diffusion coefficient — how fast individual lithium ions move.
Ionic conductivity — the material's overall ability to transport charge.

Fast-moving ions don't help much if too few of them are mobile; plenty of mobile ions don't help either if every hop is blocked by a high barrier. Designing a fast ion conductor comes down to optimizing both at once — how many lithium ions can move, and how easily they move.

What kind of structure conducts fastest

Whether a solid electrolyte turns out to be fast generally comes down to three design levers:

Design lever	Why it matters
Site occupancy	Too few available sites and there aren't enough mobile ions to carry the relay; every site filled and there's no vacancy left to use.
Pathway connectivity	A one-dimensional channel gets blocked easily by a single local defect; two- or three-dimensional networks generally offer detours around obstructions.
Framework polarizability	A softer, more polarizable anion framework presents less resistance to a passing lithium ion.

That last point is one reason many sulfide electrolytes show higher conductivity — sulfur ions are larger and their electron clouds polarize more easily. But it isn't an absolute rule: oxide systems like LLZO reach very high room-temperature conductivity too, through the right crystal structure and defect design.

High conductivity doesn't automatically mean a good battery

A room-temperature ionic conductivity of 1 mS/cm is often treated as a practical threshold for inorganic solid electrolytes. Some sulfide superionic conductors have been reported reaching roughly 25 mS/cm at room temperature — clear evidence that "solids conduct lithium slowly" was never a reliable intuition.

But that's still just the intrinsic performance of the material. Once it's built into a real cell, lithium ions also have to cross the composite electrode layer, traverse the electrode/electrolyte interface, and navigate around grain boundaries and pores. A fast-conducting material doesn't help if interfacial contact is poor or stack pressure is uneven — the full-cell impedance will still come out high regardless of what the powder's own conductivity suggests.

That's exactly why the pressure applied across a battery stack has to be controlled and reproducible when characterizing solid electrolytes or solid-state cells. Too little pressure and particle-to-particle contact is incomplete, inflating measured impedance and making a good material look worse than it is. Uneven pressure makes it nearly impossible to compare conductivity data across batches or labs.

Best suited for: anyone running EIS on pressed pellets, composite electrolytes or full solid-state cells where stack pressure needs to stay known and repeatable between runs.
Good to know: a self-leveling, three-column test frame with a built-in pressure sensor lets you separate "the material is slow" from "the test setup added resistance" — which is exactly the distinction a raw conductivity number can't make on its own.

View the PB001 solid-state battery test fixture →

The takeaway

Lithium ions don't move along with the solid framework — they relay through the vacancies and interstitials the lattice makes available. Vacancies give them somewhere to land, connected pathway networks decide which direction they can go, and the migration barrier decides how fast they get there. Next in this series: the variable that's easiest to overlook when a solid electrolyte moves from the lab bench to a production line — grain boundaries.

Frequently asked questions

What ionic conductivity can solid electrolytes typically reach?
A room-temperature ionic conductivity of 1 mS/cm is often treated as a practical threshold for inorganic solid electrolytes. Some sulfide superionic conductors have been reported reaching around 25 mS/cm at room temperature — the same order of magnitude as mainstream liquid electrolytes.

What mechanisms let lithium ions migrate through a solid electrolyte?
Three main mechanisms: vacancy hopping, interstitial migration, and concerted migration. Which one dominates depends on the material's crystal structure.

Why do sulfide solid electrolytes usually have higher conductivity than oxides?
Sulfur ions are larger and polarize more easily, lowering resistance to lithium hopping. But this isn't an absolute rule — oxide systems like LLZO can also reach very high room-temperature ionic conductivity.

If a material has high intrinsic conductivity, is the resulting battery guaranteed to perform well?
Not necessarily. Lithium ions must also cross the composite electrode layer, the electrode/electrolyte interface, and navigate grain boundaries and pores inside a real cell, so interfacial contact and stack pressure also determine actual battery performance.

Why does testing solid electrolyte ionic conductivity require controlled stack pressure?
Insufficient pressure causes incomplete contact and inflated impedance, while uneven pressure makes data hard to compare across batches or labs — so EIS testing needs to run under controlled, reproducible stack pressure.

References

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Bachman, J. C. et al. Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction. Chemical Reviews 116, 140–162 (2016). DOI: 10.1021/acs.chemrev.5b00563
Wang, Y. et al. Design principles for solid-state lithium superionic conductors. Nature Materials 14, 1026–1031 (2015). DOI: 10.1038/nmat4369
Kato, Y. et al. High-power all-solid-state batteries using sulfide superionic conductors. Nature Energy 1, 16030 (2016). DOI: 10.1038/nenergy.2016.30
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Design of Solid Electrolytes with Fast Ion Transport: Computation-Driven and Practical Approaches. Energy Material Advances (2023). DOI: 10.34133/energymatadv.0015
Unraveling the Underlying Mechanism of the Li⁺ Migration Inside Halide Solid-State Electrolytes: Structural Tuning and Defect Manipulation. Crystals (2026). DOI: 10.3390/cryst16050335
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Zhang, J. et al. Challenges and Strategies of Low-Pressure All-Solid-State Batteries. Advanced Materials (2025). DOI: 10.1002/adma.202413499