Underlying Factors for Improved Performance of P2/O3 Biphasic Layered Oxides for Sodium Ion Battery

Sodium ion batteries (SIBs) are emerging as a viable alternative to lithium ion batteries (LIBs) due to comparable electrochemical properties. Recently, increased industrial interest in large-scale SIB production has bolstered their commercial prospects.However, the disparity in the performance of SIB when compared to LIBs is majorly due ion-size which makes insertion/desertion process kinetically less favorable. The ion insertion/desertion process during the charge-discharge cycle is accompanied by phase transitions on the cathode side. These transitions contribute to distinct challenges on the electrochemical performance, stability and cyclability. For instance, LiCoO₂ shows exceptional energy density and cycle life making it the most commercialized cathode material for LIBs. In contrast, NaCoO₂ faces issues due to multiple irreversible phase transitions during the cycling process[1]. Transition metal Layered oxides (TMO₂) have been regarded as the top choice for cathodes materials. They are classified into different phases, most represented are P2 and O3 depending on the stacking of close packed TMO₂ layers. The letters O and P denote octahedral and prismatic sites occupied by Li/Na ions while the numbers 2 and 3 denote the number of repeating TMO₂ layers[2].<br/>Empirically, P2 phase (Na_xTMO₂, 0.6&lt;x0.7) shows higher rate capabilities and lower capacity when compared to O3 phase (Na_xTMO₂, 0.8&lt;x&lt;1) attributable to difference in Na content and diffusion pathways[3, 4]. Composite biphasic cathode materials, harness synergistic benefits from both phases exhibiting reversible structural changes that outperform individual phase counterparts. This emerging focus on novel composite materials shows promise but lacks answers for fundamental questions on the changes in the chemical environment of the TMs with varying Na content and the reasons for observable reversible phase transitions in composite materials versus individual P2/O3 phases. In this work, we have studied Na_xMn_0.5Fe_0.25Ni_0.25O₂ (Na_xMFN)with increasing x value from 0.67 to 0.95. We have observed the transition from pure P2 phase (x=0.67) to biphasic P2/O3 (x= 0.75, 0.8 and 0.85) phase and pure O3 phase (x=0.95) from X-ray diffraction (XRD) analysis. The best performing composition is as expected, Na_0.85MFN with initial capacity values nearing 200 mAh/g with good cycling stability. Operando wide angle X-ray scattering (WAXS) technique showed the phase transitions which was compared with the results from ex-situ extended X-ray absorption fine structure (EXAFS) analysis showing changes in the chemical environment of the TMs. Interestingly, strain at the phase boundary of the intergrowth structure during the Na-insertion/desertion was observed. These results gave us deeper understanding on the phase transitions and improved electrochemical performance of biphasic composite material with optimal Na content. <br/>References<br/>[1] P.K. Nayak, L. Yang, W. Brehm, P. Adelhelm, From Lithium-Ion to Sodium-Ion Batteries: Advantages, Challenges, and Surprises, Angewandte Chemie - International Edition, 57 (2018) 102-120.<br/>[2] C. Delmas, C. Fouassier, P. Hagenmuller, Structural classification and properties of the layered oxides, Physica B+C, 99 (1980) 81-85.<br/>[3] Z. Cheng, X.-Y. Fan, L. Yu, W. Hua, Y.-J. Guo, Y.-H. Feng, F.-D. Ji, M. Liu, Y.-X. Yin, X. Han, Y.-G. Guo, P.-F. Wang, A Rational Biphasic Tailoring Strategy Enabling High-Performance Layered Cathodes for Sodium-Ion Batteries, Angewandte Chemie International Edition, 61 (2022) e202117728.<br/>[4] X. Hu, H. Guo, J. Gao, Z. Liu, X. Ma, Z. Li, Y. Ge, Z. Wen, X. Jiao, K. Sun, D. Chen, Component Regulatory Strategy for Advanced Biphasic Sodium Layered Oxide Cathodes, ACS Applied Energy Materials, 7 (2024) 460-468.