I am an editor and editorial board member for a peer-reviewed medical journal indexed with the (US) National Library of Medicine.
I would expect and prefer your title without the article, but that depends on several assumptions:
Do you propose only one mechanism that explains how the process/phemomonon works?
Are you offering a hypothesis or proposed explanation in order to add to or stimulate a debate about other possible mechanisms?
I would think about the term "underlying"; are there other kinds of mechanisms related to coupled ion motion in lithium batteries, so that it's important to specify that this is about an "underlying" mechanism vs some other kind of mechanism? :)
What is your goal that leads you to ask this question? Are you writing it for a class? Are you planning to submit it to a journal? If so, which ones or kinds?
If I make my best guesses about these assumptions, my basic answer is that either choice is not necessarily right or wrong unless based on a particular standard or style guide (or, as is fairly often the case in the real world, the personal taste or preference of a teacher, editor, etc.).
I suspect that some, perhaps most, editors would accept either option and might not even pause to think about whether it should be there. Other editors or publications or style guides (for example IEEE format; or see, for example, this list might have standards or guidelines.
I suspect that, depending on several things, you might better explore by looking at published paper titles in related academic journals, or ask someone with specialized knowledge (email the question to a journal editor; ask questions on https://writing.stackexchange.com/, adding the tag "academic writing"; etc.). Looking for other resources might be a good idea because there could be other important issues that might influence a better title choice: Are you proposing a new hypothesis? Are you reviewing what has already been proposed or established by others? Did you perform any experiment? Etc.
This would also be a good question to ask a librarian to help you with, especially a school/university librarian.