I wasn't aware of the others, but I have read through James Cox's article
before.  The part you're looking for, specifically, is on page 175:

Cox suggests that Jim "… does not elaborate a lie about it; he simply
conceals his knowledge from Huck." Cox continues by suggesting that one of
Jim's motives might be "his tender wish to spare Huck the knowledge of being
an orphan," but also states that "Jim has good reason to suspect that a Huck
free of his Pap might leave him high and dry."

My personal opinion is that Jim is unselfishly motivated, and as others have
pointed out, really the moral center of the book.

Michael MacBride
Southern Illinois University, Carbondale