You can use {{ 'wave' + i }} instead of wave{{i}}. Make it as Angular Experession. This is the full ngFor:
<li *ngFor="let episode of episodes; let i = index">
<div id="{{ 'wave' + i }}"></div>
</li>
It works and creates the div with the correct id
document.getElementById("wave1")
Output will be like this:
<div id="wave1">wave1</div>
but remember your code will start from wave0 as i = index starts from 0.