Understanding and Optimizing Inactive Doping in P2-Type Sodium Layered Oxides for Sodium Ion Batteries
Authors: Xu Mingfeng , Rapp Philip , Gammaitoni Giovanni , Gasteiger Hubert Andreas , Bianchini Matteo 
Published: December 2024

Sodium ion batteries are promising alternatives to complement lithium ion batteries in the modern energy storge market. Among sodium-based cathodes, layered oxides (NaxTMO2, TM = transition metal) are one of the most attractive candidates. P2 and O3 are two of the most studied structures in the NaxTMO2 system. Regarding Na+ conductivity, P2 is believed to be preferable over O3 as it provides open prismatic pathways with a lower energy barrier for Na diffusion.[1] However, a drawback for P2 materials is that they are typically synthesized with low Na content, often close to x = 2/3, and can rarely reach beyond 3/4, which is not ideal when building full cells.[2] The other major issue of P2 is the P2–O2 phase transition taking place at high voltage during electrochemical cycling, which is not well reversible and limits its cycling stability and capacity retention.[3,4] Our work aims to improve the Na inventory by doping inactive elements in pristine cathode materials while maintaining their cycling stability.
In this presentation, we would first focus on a family of Li doped P2 materials with nominal compositions of Na5/6LiyNi5/12-3y/2Mn7/12+y/2O2 (y = 2/18, 3/18, 4/18, 5/18). We report on the characterizationsof the crystal structures of these materials, as well as the temperature-resolved in situ XRD during their solid-state synthesis. These studies provide guidance for further synthesis of desired phases and compositions of the P2 materials.
Electrochemically, this family of materials show an increasing trend of polarized hysteresis in the first charge and discharge cycle, which suggests the capacity contribution from oxygen redox. We coupled semi-simultaneous operando XRD and x-ray absorption near edge structure (XANES) to understand the structural evolution and redox behavior during this process. In particular, we observed that Li in the TM site mitigates phase transitions at high voltage, with a threshold likely between 0.15 and 0.18 Li per formula unit (f.u.). An increasing Li amount facilitates the activation of the O-redox starting at lower voltages (in attached Figure). To further understand the contribution by TM- and O-redox, we studied the gassing behavior using online electrochemical mass spectrometry (OEMS) to detect the amount and onset of oxygen release depending on the materials composition.
We studied rate performance and cycling stability of these materials in half cells against sodium metal reference. The 0.15 and 0.18 f.u. Li stoichiometric compositions prove to deliver the best balance of cycling stability and capacity. We further applied these two compositions against hard carbon in exploration of their possibility in full cells. We also compared the electrochemical characteristics of similar compositions with distinct morphology. Finally, we would share the ongoing progress of other inactive element doping beyond TM layer, e.g., Ca in the alkali layers in the P2 materials.
References
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2020, 19, 1088–1095.
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2001, 148, A1225.
[4] L. Yu, H. Dong, Y.-X. Chang, Z. Cheng, K. Xu, Y.-H. Feng, D. Si, X. Zhu, M. Liu, B. Xiao, P.-F. Wang, S. Xu, Sci. China Chem.
2022, 65, 2005–2014.

Figure 1