Deciphering the role of optimal P2/O3 phase fraction in enhanced cyclability and specific capacity of layered oxide cathodes

Poor cyclability and rate performance of layered oxide cathodes have impeded their deployment in Na+ batteries. Here, Mn-rich biphasic cathodes with tuned P2/O3 phase fractions were synthesized by varying Na in NaxMn0.60Ni0.30Cu0.10O2 (NMNC-x) (0.80 ≤ x ≤ 1.00) and the P2/O3 phases were confirmed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure spectroscopy. The Rietveld refinement of XRD data showed that the O3 phase fraction increased from 13 % to 89 % with increasing x from 0.80 to 1.00. The NMNC-0.90 sample (with 54 % O3 phase) demonstrated excellent rate performance (119 and 100 mAh/g at 1C and 3C, respectively) but poor cyclic stability (64 % capacity retention at 1C after 200 cycles). P2 dominant NMNC-0.80 showed exceptional rate performance (147 & 103 mAh/g at 0.1C & 3C, respectively) and O3 dominant NMNC-1.00 exhibited a high specific capacity (187 mAh/g at 0.1C). These cathodes showed excellent capacity retention of around 91 % at 1C after 200 cycles. Operando Synchrotron XRD results confirmed a reversible O3 ↔ P3 phase transformation at ∼3.4 V. Further, a significant mismatch in the levels of the strain developed in the P2 and O3 phases was observed in NMNC-0.90 sample during charge/discharge, explaining its poor cyclability. Na+ diffusion coefficients calculated from the galvanostatic intermittent titration technique were in the 10−12 − 10−10 cm2 s−1 range. This investigation shed light on the optimum P2/O3 phase fraction for designing biphasic cathodes with high specific capacity and cyclability.