Enhancement in rate performance and high voltage structural stability of P3/O3 Na0.9Fe0.5Mn0.45Ni0.05O2 layered oxide cathode

Abstract

We report synthesis and sodium storage performances of two biphasic P3/O3 materials, Na0.9Fe0.5Mn0.5O2 (P3/O3 NFM) and Na0.9Fe0.5Mn0.45Ni0.05O2 (P3/O3 NFNM). The choice of composition and Ni doping context have been justified. On replacing 5 % Mn by Ni, we found improvements in (de)sodiation capacity and rate performance. The delivered storage capacities of P3/O3 NFM and P3/O3 NFNM in the first cycle between 4.4 and 2.0 V are 118 mAh g−1 and 148 mAh g−1 at C/10 (0.015Ag−1) rate, respectively. The subsequent cycles have a narrow voltage window of 4.2 to 1.9 V for optimized cycling performance. In this voltage window, P3/O3 NFNM demonstrates more than twice the discharge capacity of P3/O3 NFM at 3C rate (0.45Ag−1); further it retains 74 % of its capacity after 200 cycles at 1C rate (0.15Ag−1) as compared to 57 % in P3/O3 NFM. X-ray diffraction studies confirm that Ni doping helps in retaining the P3 phase throughout the charge cycle in P3/O3 NFNM which helps in achieving a higher discharge capacity and better cycling. A greater reduction in the internal resistance, specifically the charge transfer resistance along with enhanced hole conductivity is observed from impedance studies of fully desodiated P3/O3 NFNM due to the maintenance of the P3 phase. XANES studies reveal that the charge compensation mechanism of Fe and Mn changes upon Ni doping which is attributed to the structural stability of P3/O3 NFNM at higher voltages.