Use this code bellow:
<?php if ( is_user_logged_in() ) : ?>
<li> <a href="https://wordpress.stackexchange.com/questions/269073/<?php echo wp_logout_url( get_permalink() ); ?>">Logout</a></li>
<?php else : ?>
<li> <a href="http://your-site-name/login/">Login</a></li>
<?php endif;?>
UPDATE
<?php if ( is_user_logged_in() ) :
$current_user = wp_get_current_user();
?>
<li> <a href="https://wordpress.stackexchange.com/questions/269073/<?php echo wp_logout_url( get_permalink() ); ?>">Logout</a></li>
<li>Username: <?php echo $current_user->user_login; ?></li>
<?php else : ?>
<li> <a href="http://your-site-name/login/">Login</a></li>
<?php endif;?>