Defect-Controlled Formation of Triclinic Na2 CoP2 O7 for 4 V Sodium-Ion Batteries.

Simple defects such as sodium deficiencies can induce the selective synthesis of triclinic Na2 CoP2 O7 , providing an increase in energy density of more than 40 % compared to the stoichiometric polymorph that is preferentially formed under the commonly used synthesis conditions. Such a significant improvement, which was achieved just by changing the crystal structure, suggests that controlling the polymorphism could be an effective and facile method for developing high-performance electrode materials and that defects can play a remarkable role in this process.