An electroactive material for use in an electrochemical cell, like a lithium ion battery, is provided. The electroactive material comprises silicon or tin and undergoes substantial expansion during operation of a lithium ion battery. A polymeric ultrathin conformal coating is formed over a surface of the electroactive material. The coating is flexible and is capable of reversibly elongating by at least 250% from a contracted state to an expanded state in at least one direction to minimize or prevent fracturing of the negative electrode material during lithium ion cycling. The coating may be applied by vapor precursors reacting in atomic layer deposition (ALD) to form conformal ultrathin layers over the electroactive materials. Methods for making such materials and using such materials in electrochemical cells are likewise provided.