我想数据库动态添加字段，用了execute（）方法，可是一直报错：

代码：$sql = "ALTER TABLE ".C('DB_PREFIX')."nav ADD 'zh-cn' VARCHAR(100) CHARACTER SET utf8;";
$re=M()->execute($sql);

错误是：1064:You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''zh-cn' VARCHAR(100) CHARACTER SET utf8' at line 1 [ SQL语句 ] : ALTER TABLE boz_nav ADD 'zh-cn' VARCHAR(100) CHARACTER SET utf8;

查了百度都说语法错误，可是找不到语法错误了，请大神们指示