Stabilization of 4.5 V Cr4+/Cr3+ redox reaction in NASICON-type Na3Cr2(PO4)3 by Ti substitution
Development of high-voltage cathode materials composed of abundant metals for rechargeable batteries is a crucial task to realize higher energy density in the large-scale electrical energy storages. Here we report a durable Cr4+/Cr3+ redox reaction at 4.5 V vs. Na/Na+ in NASICON-type Na2CrTi(PO4)3 (NCTP). An unstable Cr4+/Cr3+ redox in Na3Cr2(PO4)3 is successfully stabilized by the Ti substitution to Cr. Charge/discharge mechanism of NCTP was studied by powder X-ray diffraction and soft X-ray absorption spectroscopy.