Although not replacing lithium-ion batteries completely, sodium-ion batteries are about to claim their share in the energy storage systems of tomorrow. The cathode material class of sodium layered oxides is similar to the widely employed lithium layered oxides in aspects of synthesis and processing. Nevertheless, there are challenges in adapting lab-scale as well as industrial processes to the relatively new battery technology.
Starting from a vast lab scale material screening, we explored the effect of Mg-doping in the promising NaxFe0.1MgyMn0.6-zNi0.3-y+zO2 layered oxide. XRD, SEM and ICP-OES characterisation together with electrochemical testing give a solid data base to identify the material composition Na0.81Fe0.10Mg0.12Mn0.56Ni0.22O2 as a suited composition for upscaling.
In a second step, we used a continuous stirred tank reactor (CSTR) as a first step in a scalable precursor synthesis route. The conditions for precipitation of a hydroxide precursor were carefully investigated. For the synthesis, different amounts of NH3 solution (chelating agent) and NaOH solution (precipitation agent) were tested in a systematic Design of Experiment. This led to the successful precipitation of spherical, dense hydroxide precursor particles. We were able to synthesize the Na0.81Fe0.10Mg0.12Mn0.56Ni0.22O2 in a kg-scale.
In electrochemical evaluation, the synthesized material showed an initial capacity of 121.6 mAh/g and a capacity retention of 82.8% after 100 cycles in coin cells against sodium metal. First full cell tests showed a capacity of 87 mAh/g and a retention of 98% over 100 cycles for the upscaled active material.
This investigation not only contributes to the understanding of new layered oxide compositions by a broad material screening, but also gives direct insights into the challenges of adapting a scalable precursor route. Investigating the precursor synthesis conditions in the CSTR is taking a first step from lab scale towards application. In this way we directly transfer our scientific findings to an industrial relevant process demonstrating the relevance of quick process integration.