Optimising P-type Na0.67Mn0.9Mg0.1O2 cathodes via synthesis method, doping, and phase composition

The shift to renewable, low-carbon energy generation creates intermittency in supply. To reduce reliance on fossil fuels, large-scale energy storage is required to store energy when it is in abundance and supply it when scarce. Sodium-ions batteries (NIBs) can enable this transition by using low-cost, sustainable materials. The P3 and P2 phases of Na0.67Mn0.9Mg0.1O2 (NMMO)are presented here are candidates for large-scale storage.
In Chapter 3, a biotemplating synthesis using naturally occurring polysaccharide dextran successfully synthesised these materials without impurities. Conventional solid state methods could not produce single phase P3-NMMO, and its initial capacity was 95 mAh g-1, compared to 142 mAh g-1 for the biotemplated P3 phase. Biotemplating produced sharply faceted plates of P2-NMMO, with a higher initial capacity than the P2 synthesised via solid state methods, which has rounded plates. The biotemplated materials exhibited better rate capability than the solid state synthesised materials. These differences manifested despite synthesis methods for each phase using identical 20 h calcination regimes.
In Chapter 4, P-type NMMO was produced using only a biotemplating synthesis, with a calcination time of 2 h. This led to an increase in capacity retention for the P3 phase from 73% to 82%, but a decrease for P2 from 73% to 63%. P-type NMMO was doped with 1% and 2% Ca (NCMM) to improve the capacity retention and rate capability. This did not work in P3-NMMO, but 1% and 2% Ca doping increased the capacity retention of P2-NCMM from 63% to 73%. Initial capacity for all materials showed no significant change. The rate capability of 1% Ca P2-NCMM was better than both 0% and 2% P2-NCMM. These effects may be contributed to by the large increase in particle size of 1% Ca-P2-NCMM compared to all other samples in this chapter.
The other method of improving capacity retention and rate capability in P-type NMMO was to combine the two phases in Chapter 5. A range of biphasic samples with varying P3/P2 ratios were generated by altering the calcination temperature and compared against the same P3/P2 ratios with the P3 and P2 phases calcined separately and mixed post-synthesis. Of all the samples in this chapter, none had higher capacity or retention than the biotemplated P3-NMMO. For both preparation methods, increasing the phase fraction of P2-NMMO decreased the capacity retention. The mixed P3/P2-NMMO displayed higher capacity retention than the biphasic samples at each phase ratio. This may be because the biphases showed increased electrochemical activity of the P2 phase, leading to structural changes and more rapid degradation.
Using biotemplating to produce NIBs can significantly reduce the energy cost of production, while improving upon the performance characteristics of the material when generated via solid state methods. NMMO is a high capacity, low-cost cathode material that can be further optimised with further exploration of the strategies identified herein.