Controlled phase stability of highly Na-active triclinic structure in nanoscale high-voltage Na_2-2xCo_1+xP₂O₇ cathode for Na-ion batteries

With the increasing demand for high energy density in energy-storage systems, a high-voltage cathode is essential in rechargeable Li-ion and Na-ion batteries. The operating voltage of a triclinic-polymorph Na₂CoP₂O₇, also known as the rose form, is above 4.0 V (vs. Na/Na⁺), which is relatively high compared to that of other cathode materials. Thus, it can be employed as a potential high-voltage cathode material in Na-ion batteries. However, it is difficult to synthesize a pure rose phase because of its low phase stability, thus limiting its use in high-voltage applications. Herein, compositional-engineered, rose-phase Na_2-2xCo_1+xP₂O₇/C (x = 0, 0.1 and 0.2) nanopowder are prepared using a wet-chemical method. The Na_2-2xCo_1+xP₂O₇/C cathode shows high electrochemical reactivity with Na ions at 4.0 V, delivering high capacity and high energy density.