Solid-State NMR Study of the Paramagnetic P2-Na0.67Mn0.95Mg0.05O2 Na-Ion Battery Cathode

Abstract

Lithium-ion batteries have been preferred to their sodium-ion counterparts for their higher energy density and operating voltages, but concerns about lithium shortage and rising costs have encouraged the scientific community to turn its attention to the more sustainable sodium-ion batteries (NIBs). Layered sodium transition metal oxides, such as P2-Na0.67Mn1-xMgxO2 (0≤x≤1)1, have a high theoretical capacity and are promising NIB cathodes. The structural stability required for long-term cycling remains a major challenge due to the ionic size of Na+ (70% larger than Li+), whilst intermediate phases with particular Na+/vacancy ordering patterns tend to form upon cycling. An understanding of the underlying structural and electronic processes occurring on charge and discharge is a necessary step towards devising higher performance electrode materials2. Solid-state Nuclear Magnetic Resonance (ssNMR) allows the local distortions and variations in the electronic structure and in oxidation states upon cycling to be investigated3. We present both ex situ and in situ 23Na NMR results on the P2-Na0.67Mn0.95Mg0.05O2 cathode. Paramagnetic interactions in P2-Na0.67Mn0.95Mg0.05O2 lead to large NMR shifts and very broad resonances. Fast magic angle spinning (MAS) ex situ NMR was performed to enhance the resolution and allow the signals from different local environments (with different chemical and hyperfine shifts) to be distinguished4. In order to follow the electrochemical processes in real time, and to gain insight into the underlying Na (de)intercalation mechanisms, we performed in situ ssNMR experiments on a P2-Na0.67Mn0.95Mg0.05O2//Na(s) half-cell. A schematic of the in situ bag cell is shown in Figure 1a5,6,7. In order to excite the 100 kHz wide static 23Na NMR signal observed for the pristine cathode film, we made use of the recently developed Automatic Tuning Matching Cycler (ATMC) in situ NMR approach8. The low intensity, broad cathode signal is difficult to observe due to complete overlap with the more intense signals arising from the electrolyte (1M NaTFSI in PC) and the Na metal anode2. We carried out a so-called T1 filter NMR experiment which takes advantage of the different relaxation behaviors of the various components of the bag cell to discriminate between the overlapping 23Na NMR signals. This lead to significant enhancement of the broad feature arising from the cathode. 23Na in situ NMR data measured upon initial charge at a rate of C/50 reveals an expected increase of the Na metal peak intensity due to the formation of Na-metal dendrites, and the disappearance of the broad cathode feature (see Figure 1b). These encouraging results show that in situ NMR can be applied to even extremely difficult systems - from an NMR perspective - such as paramagnetic materials2. Ongoing work focuses on the optimization of the electrochemical performances of the in situ cell to allow for multiple cycles and higher rates to be investigated. The conventional bag cell is replaced by a new cell design with increased pressure and sample size. We expect the techniques developed in this work to enable us to investigate a greater range of electrode materials using in situ NMR.