Articles and methods involving protective membranes for electrochemical cells are generally provided. In some embodiments, a composite protective layer comprising particles and a polymeric binder may be disposed on an electroactive material. The particles may be reactive with lithium, may capable of intercalating lithium, and/or may comprise intercalated lithium. In some embodiments, the electroactive material may be in the form of a first electroactive layer, and a second electroactive layer may be disposed on the composite protective layer. Certain embodiments relate to activating a composite protective layer by intercalating lithium into particles within the layer and/or by reacting the particles with lithium metal.