protected function getPrevArticleId($id)
  {
    return Article::where('id', '<', $id)->max('id');
  }

protected function getNextArticleId($id)
  {
    return Article::where('id', '>', $id)->min('id');
  }

 public function scopePublished($query)
  {
    $query->where('published_at','<=',Carbon::now());
  }

$articles = Article::latest('published_at')->published()...

<li class="previous">
@if($prev_article)
<a href="/post/{{ $prev_article->slug }}" rel="prev"><i class="fa fa-chevron-left"></i><strong>上一篇</strong><span> {{ $prev_article->title }}</span> </a>
@endif
</li>
<li class="next">
@if($next_article && $next_article->published_at < Carbon\Carbon::now())

<a href="/post/{{ $next_article->slug }}" rel="next"><i class="fa fa-chevron-right"></i><strong>下一篇</strong> <span> {{ $next_article->title }}</span></a>
@endif
</li>