Как сделать анимированную кнопку на css

.btn { 
    display: inline-block; 
    padding: 14px 22px 14px 22px; 
    line-height: 1; 
    font-weight: bold; 
    font-size: 11px; 
    border-radius: 4px; 
    margin: 20px auto; 
    color: #fff; 
    border: 2px solid #292929; 
    background: #292929; 
    position: relative; 
    letter-spacing: 1px; 
    text-decoration: none; 
    overflow: hidden; 
} 
 
.btn i { 
    top: -100%; 
    position: absolute; 
    left: 0; 
    width: 100%; 
    height: 100%; 
    line-height: 40px; 
    text-align: center !important; 
    font-size: 16px; 
    transition: all 300ms linear; 
} 
.btn span { 
    top: 0; 
    position: relative; 
    transition: all 300ms linear; 
} 
 
.btn:hover i{ 
  top: 0px; 
} 
 
.btn:hover span{ 
  top: 100px; 
}

  <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css" /> 
   
   
<a href="#" class="btn"> 
  <i class="fa fa-eye" aria-hidden="true"></i> 
  <span>Portfolio</span> 
</a>