Other coatings like SiO2 and TiO2 nanoparticles have also been explored. For NCA, potassium doping has been seen to protect the structure from side reactions with the electrolyte . Dopants like copper and iron decrease the capacity but improve cyclability and rate capability . Chromium doping has been seen to enhance NCA capacity. Zr-doped NCA has been seen to improve capacity retention .