You can find this expansion in the book Lorentzen L. & Waadeland H. Continued fractions with applications North-Holland Publishing Co., 1992 (formula (4.3.10)). As I understand, this document is a more recent version of the appendix of this book. Here the desired formula has the number (3.3.10). However, the validity of the expansion for $\operatorname{li}(x)$ does not include the domain $x > 1$ that you seek.
The expansions in the references are not known to be valid on the domain sought in the original post.
Jesse Elliott
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