NaxMnyNi1-YO2 Cathode Materials for Sodium-Ion Batteries: Structure, Synthesis, Electrochemistry and Influence of Ambient Storage
Authors: Pfeiffer Lukas , Axmann Peter , Wohlfahrt-Mehrens Margret 
Published: November 2022

Expanding energy generation from renewables is inevitable to reduce the impact of man-made climate change. With that, the need for intermediate energy storage is gaining in significance. Today, lithium-ion batteries (LIBs) are dominating mobile drive-trains and also play a key role in stationary energy storage. LIBs are incorporating critical raw materials in view of availability and economic importance, such as cobalt, lithium, copper and graphite.
Sodium-based batteries, in which sodium replaces lithium as ionic charge-carrier, utilize the same working principles but substitute critical raw materials for abundant and cost-effective alternatives. Hard carbon replaces graphite as anode active material, copper foils are substituted by aluminum current collectors and manganese-based cobalt-free layered host lattices offer promising performance as cathode active material 1,2. By applying established production processes, investment costs are reduced and a rapid scale-up is enabled (Drop-In technology) 3, making SIBs a sustainable, efficient and cost-effective complementary technology to LIBs 4.
Among various known cathode active materials for SIBs, the family of layered sodium transition metal oxides (NaxMO2, 1>x>0) offers promising electrochemical performance 2,5–7. These compounds show a wide structural variety (O3, P3, P2) due to the ionic radius of sodium and the tendency for Na+/vacancy ordering 8.
The scope of our presentation will be low-cost manganese-based, cobalt-free layered NaxMnyNi1-yO2 cathode active materials for SIBs. We will discuss the influence of the transition metal stoichiometry y on the structure based on Neutron and X-ray diffraction experiments. Using advanced electrochemical methods and diffraction experiments, these structural models are then correlated with physical and electrochemical properties such as Na+/vacancy orderings, solid diffusion coefficients and potential profiles. For y = 3/4, a synthesis phase diagram will be presented covering a broad range of sodium content x and calcination temperature. For phase-pure P2-NaxMn3/4Ni1/4O2, we will present the influence of the calcination process on the structure and discuss the electrochemical properties in half-cells in-depth. For optimized materials, attractive initial specific discharge capacities beyond 220 mAh g-1 are obtained in sodium half-cells between 1.5 – 4.3 V. A capacity decay occurs during electrochemical cycling within this full voltage window. The origin of the capacity decay will be discussed based on electrochemical studies and ex-situ investigations of the morphology with SEM and local structure with HRTEM. Finally, we will present the influence of storage in ambient air to gain insights on the large-scale processability of the materials.
The chosen synthesis route adapts industrially established processes for NCM production for SIB cathode materials, enables to tune powder properties to technical specifications and is highly scalable. The broad scope of this work addresses raw material questions, fundamental investigations and industrially relevant production processes.
ACKNOWLEDMENTS:
The German Federal Ministry of Education and Research (BMBF) supported this work within the project TRANSITION (03XP0186C) and ExcellBattMat (03XP0257A and 03XP0257C).
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