All you can do if the surface becomes slippery in winter, when it is icy, is to treat it as you would with any other areas of paving. Apply some salt to melt the ice, or try using some sharp sand to improve the grip. In fact, you may find your sealed surface is less slippery in frosty conditions than unsealed surfaces. This is because the sealing prevents moisture entering the paving overnight. It is this moisture which is within the paving that, as it expands in frosty conditions, lifts up through the paving and forms the frosty surface that you notice first thing in the morning. With a sealed surface this is far less likely to happen.