Ajax loop and history.js for SEO

There are some plugins that add the next/prev links in <head> and follow Google recomendations for paginated archives, which are similar to recomendations for infinite scroll. If you are interested in doing it at yor own, you can implement the ajax load-more in archive templates and do something like this (not fully tested, just an example):

// Print next and prev links for paged categories/archive/tags..etc
add_action( 'wp_head', 'cyb_next_prev_links_for_archives' );
function cyb_next_prev_links_for_archives() {

    if ( ! is_singular() ) {

        //No singular. We are in archive based pages

        global $paged;

        if( get_previous_posts_link() ){
            echo "<link rel="prev" href="" .get_pagenum_link( $paged - 1 ). "">\n";
        }

        if( get_next_posts_link() ){
            echo "<link rel="next" href="" .get_pagenum_link( $paged + 1 ). "">\n";
        }


    }

}

Also, it is worth to mention that rel=”next” and rel=”prev” links for archives have been proposed and approved for future releases.