Players are trying to steal magical items from Powerful Elemental Beings. They are not able to fight with them, so they need to somehow find a way to outrun them, preferably using environmental barriers.

By barrier I mean something the party can pass by on foot, but the elemental cannot (and the elemental cannot fly). For example, a Fire elemental would not be able to cross a river, and a Water elemental would not be able to cross a line made of salt.

The barrier should be natural, so players can encounter it in the wild (or it can be created by some druid spells).

The Lightning Elemental

One of the elementals will be a Lightning Elemental. To create this, I'm using the Air Elemental but removing the fly speed and giving them a 90' normal walk speed and changing the damage type of the Slam and Whirlwind to Lightning from bludgeoning.

I'm looking for a rules or common sense-based barrier that would slow this lightning elemental down.

The Party

The party is made of 11th level characters consisting of a druid, ranger, rogue, and fighter.

Chasms

Given that the creature does not have a fly speed, your best bet is to either have a chasm or let the players create a chasm between the elemental and their escape route.

You could either have it spanned by a bridge or unspanned and have your players figure out a way across, or let the players figure out a way to create one.

Once running, they need to come up with a way to destroy the bridge (if the chasm is real) so that the elemental can't follow them over the chasm.

If you want to make this 'thematic' electricity/lightning can't cross a gap without something to for it to finish the circuit.

But gotta slow it down first

The main issue is the speed of the Elemental. It's going to outrun your players even if they're dashing. A rogue using a Dash as an action and Bonus Action can outpace if it's slowed down, but the other players can't. Slowing it down with spells is your best bet here, but the players will be banking on it failing the save.

Some druid options to create difficult terrain for that are spike growth, erupting earth, plant growth, control winds, and maybe hallucinatory terrain to trick it into thinking there's a chasm.

The Wall spells may also work enough to slow it down, but do note when looking at options that the creature is immune to the following conditions: Exhaustion, Grappled, Paralyzed, Petrified, Poisoned, Prone, Restrained, Unconscious.

Let your players get creative

Rather than trying to come up with solutions for your players, let them come up with ways they want to do things and then adjudicate whether or not they'll work.

There is an evocative line in the description of the Fire Elemental:

Water can halt its destructive progress, causing the fire elemental to shrink back, hissing and smoking in pain and rage.

... but there's nothing about Water Elementals and lines of salt. Although that's nice and seems like reasonable lore (fits with a highly-elemental setting like the Codex Alera series), the actual thing in the rules is that freeze damage slows the elemental (but not very much, really). Likewise, the Earth Elemental doesn't have an opposed-element weakness, but rather is vulnerable to sonic damage — thunder. And Air Elementals aren't given any special vulnerabilities at all!

Overall, the 5E Elementals don't particularly have a pattern of counter-forces of the opposing element, or any sort of "rock-paper-scissors" thing where one beats the other. That might be a missed opportunity, but, eh, that's what it is.

Therefore, I don't think there's really a canonical, rules- or official-lore-based answer, because there's not a pattern to slot the homebrew monster into. It comes down to making up something which feels satisfying, and that's really up to you and your imagination.

D&D Beyond has a Homebrew Lightning Elemental, which suggests:

They, like true lightning, are drawn to objects made of metal.

... and you could take that as inspiration. This may be easier in an urban or steampunk setting than in a traditional high-fantasy one — you might need to set up something with a dwarven mining operation, ancient ruins, or a-wizard-did-it chunk of metal.

Or, for an alternate approach — in some fantasy settings, the counter or opposite to lightning/electricity is something nature or plant-based. In D&D, one particular monster that comes to mind is the Shambling Mound, which has:

Lightning Absorption. Whenever the shambling mound is subjected to lightning damage, it takes no damage and regains a number of hit points equal to the lightning damage dealt.

If you want a natural, wilderness setting, perhaps something could be done with that — although, this may be a case of trading one serious problem for another.

No matter what you pick, I think this will go better — and give your players more feeling of agency and involvement — if you set up the idea of opposing elements beforehand. Work out how the elements in your setting are related. If there is a "lightning elemental", is lightning itself an element? Is it part of air? Or are there energies associated with various elements? Maybe lightning is between water and air, and therefore opposed to lava (which is of course between earth and fire). Are wood and metal elements? And so on. Work all of this out, and then provide plenty of opportunities for your players to learn all of this beforehand, and perhaps see it in action (smaller elemental encounters before this big scene). In my experience, the general instinct of D&D players is to make a heroic battle against all odds, so if there's supposed to be an alternate solution, it's helpful to get everyone thinking in that way beforehand. (You don't need to lead them to the specific solution: just lay out that opposing elements may be a useful tool, and let them pick up the specifics.) I ran a game several years ago centered around elemental cults, and these details made it more fun for everyone.

Let's go the common sense route. How does one protect oneself from lightning normally? A lightning rod! The lightning rod will guide the lightning through a lightning protection system into the ground where it can do no harm. So let's see if we can come up with some lightning protection systems of our own.

A metal fence

Conducts electricity, is anchored in the ground and is not easy to get around. An ideal lightning elemental stopper. Even a chain-link fence would do. Only downside is a metal fence is not a naturally occurring phenomenon. Though it is not inconceivable to encounter an old rusted metal fence in the woods (maybe something interesting is nearby).

Metallic shrubberies

It's not inconceivable that there's a plant that contains a high dose of metals in itself or has iron-plated stems. Or maybe the plant conducts electricity in another way. An abundance of undergrowth of these plants would make a natural lightning protection screen and stop the elemental.

A metal gate

Like a fence it conducts electricity, is anchored in the ground and is not easy to get around. But again, not a natural occurrence. If your party is near a castle or town, this might be a logical barrier though.

Water

Conducts electricity, is anchored in the ground (well sorta) an is definitely not easy to get around. Even if the electricity is not actually conveyed into the earth, but I think the absorption capacity of a river lake, or sea is far bigger than even a very powerful elemental could create.

A Faraday cage

This would protect the players from the elemental, but also trap them. Maybe you could come up with a cave or special dwelling that could function as a Faraday cage (I'll admit this is far fetched though).

A metal spear

Two metal rods connected by a metal chain could be a powerful weapon against a lightning elemental. You just stick one end in the ground, and throw the other one (or poke the baddie if you wrapped a piece of insulation around the stick). A long metal rod or spear that you use to 'nail' the elemental to the ground could also function as a good weapon.

I can't talk about rules, but you said:

I'm looking for a rules or common sense-based barrier that would slow this lightning elemental down.

So I will use common sense-based.

Think about a lightning elemental, that thing doesn't exist in real life, so the closest thing we have is a statically charged animated stormy cloud which hates PC. So, it doesn't differ much from a normal cloud... so, it could be discharged or repelled.

Metal - Lightning rod

Any metal barrier which connects to the ground can stop the lightning elemental. Basically, you must turn the metal barrier into a Lightning rod. If the elemental tries to touch it it will lose his electrical charge (HP? Dmg? Speed?) and discharge.

A conductive floor should dissipate, but be aware that if any PC touches that metal it will get hurt.

A Forge Cleric could use its Artisan's Blessing. Not sure about a druid.

Water

If you don't have metal, you can use water, like tidal wave or another water spell. Water is a conductor, and also its ability to store energy. The first batteries were Salt-Water batteries, so a body of water is able to diffuse and store the electrical charge of a lightning elemental, slowing down them.

Non-Conductive

If you want to imprison him just make a full metal cage, like a box of glass, plastic, ceramic, etc. If the box is fully closed the creature won't be able to move through it since it will be isolated from the rest.

I don't know about druids, but I am quite sure there must be a spell to mold or shape earth.

Faraday Cage

The opposite will be a Faraday cage were the PCs can get into. A cage made of conductive material, even if it isn't completely sealed will stop any kind of electricity or magnetic field to get into. Your characters will be safe to do a long rest.
A Forge Cleric could use its Artisan's Blessing. Not sure about a druid.

Magnetism

I can't find any citation but I have heard that magnets can repel or deviate electricity. Lodestone, a stone used to cast the cantrip mending, also found in a component touch has a magnetic field, you might use that.

Also, a Forge Cleric could use its Artisan's Blessing since it's metal.

Force?

It isn't explained much what is the force elemental damage in D&D, but if you like you could think it as a strong magnetic field. The spell, wall of force may help.

Also, I found the spells Magnetic Field and Magnetic Orb, but they are homebrew.