Rational design of an optimal Al-substituted layered oxide cathode for Na-ion batteries.

• Materials studied lie in the Na 3/4 (Mn-Al-Ni)O 2 pseudo-ternary system. • P3/P2 phase fraction was manipulated by adjusting the calcination temperature. • Superior electrochemical performance of P3/P2 biphasic cathodes. • Absence of the P3 to O3 phase in the Al-substituted P3-type compounds. Layered oxide cathodes, a promising avenue for Na-ion batteries, hold the highest potential for commercialization. Herein, we delve into the structural and electrochemical properties of Al-substituted layered oxides in our quest to pinpoint the optimal cathode composition in the Na 3/4 (Mn-Al-Ni)O 2 pseudo-ternary system. The cathode materials investigated were synthesized in three distinct phase configurations, which include two monophasic configurations with P3 and P2-type structures and a third biphasic cathode with equal proportions of P3 and P2 phases. The fractions of the P3 and P2 type phases in the cathode materials were manipulated by adjusting the calcination temperature. The varying concentration of Mn3+ and Mn4+, confirmed by X-ray photoelectron spectroscopy, was found to impact the cyclic stability of these materials significantly. During electrochemical testing, the P3 cathodes showed impressive rate performance and exhibited an excellent specific capacity of 195 mAh g−1 at 0.1C. Regarding cyclic performance, the biphasic cathodes consistently outperformed their monophasic counterparts, with P3/P2-Na 0.75 Mn 0.50 Ni 0.25 Al 0.25 O 2 exhibiting 82 % capacity retention after 300 cycles. Analysis of operando Synchrotron XRD data revealed an absence of P3 to O3 type phase transition in the cathodes even at low voltages where large structural variations to the unit cell structure were observed. The absence of P3 to O3 transformations and the superior electrochemical performance of Na 0.75 Mn 0.50 Ni 0.25 Al 0.25 O 2 underlines the importance of Al substitution and P3/P2 biphasic structure in enhancing the electrochemical performance of layered oxides. [ABSTRACT FROM AUTHOR]