Defect‐Controlled Formation of Triclinic Na2CoP2O7 for 4 V Sodium‐Ion Batteries

Simple defects such as sodium deficiencies can induce the selective synthesis of triclinic Na₂CoP₂O₇, 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.