Variation of Metal Precursor Salts and Biotemplates in the Synthesis and Morphological Control of Biotemplated LiNi0.8Mn0.1Co0.1O2 Lithium-Ion Cathodes
Authors: Emmett Ryan , Boston Rebecca , Nedoma Alisyn 
Published: December 2024

Lithium-ion cells are at the forefront of the fight to reduce the effects of climate change through their growing use in transport and energy storage, with electric vehicles being a focal point of their use. The public perception of current-gen electric vehicles, however, is that compared to current internal combustion engine vehicles their range is too low, emissions from production are too high, and they are too costly. In addition, current gen lithium-ion cells are produced using materials using slave labour such as cobalt, which can further reduce uptake of electric vehicles. Lithium-ion cells using high nickel NMC811 cathodes (LiNi0.8Mn0.1Co0.1O2) would provide higher capacities while reducing the reliance on cobalt, but significant capacity fade makes widespread adoption currently unfeasible.

The use of the novel wet chemical synthesis method “biotemplating”, a method that employs the use of an organic polysaccharide as a chelating agent, would allow for the increased adoption of NMC811 cathodes through the synthesis of novel morphologies that reduce capacity fade in NMC811 cathodes. By dissolving metal precursor salts alongside the Biotemplate, mixing until homogenous, and then preheating at up to 500 °C in air and calcining at up to 850 °C under oxygen, layered NMC811 is synthesised.

The addition of the biotemplate in solution ensures chelation of the metal ions onto the biotemplate will occur. The dissolution of the metal salts and biotemplate further improves the homogeneity of the solution, reducing the need for long periods of mixing or the formation of adverse reaction boundaries, while chelation of metal ions to the biotemplate reduces reaction pathways between the metal ions while reducing rates of recrystallisation of metal ions into intermediate products at lower temperatures. Calcination under oxygen is required for the layered NMC811 structure is formed, where lithium sites occupy one layer, and transition metals occupy the second layer. When not calcined under an oxygen atmosphere the structure approaches a rock salt structure, significantly reducing capacity.

This biotemplating method has already proved effective in the production of sodium-ion cathodes and yttrium-barium-copper-oxide superconductors and has great potential for lithium-ion cell manufacturing. The improved homogeneity, chelation of metal ions, and calcination under oxygen ensures lower temperatures and reaction times than NMC811 synthesised under solid state, reducing production costs, time, and carbon emissions resulting from synthesis while improving electrochemical performance.

In this talk, I will show how varying the biotemplates and metal precursor salts in the synthesis of NMC811 may produce differing particle morphologies and electrochemical characteristics. NMC811 cathodes were synthesised from a combination of pre-selected metal precursor salts, either acetates or nitrates, and a pre-chosen range of biotemplates including dextran, derived from sugar, sodium alginate from brown seaweed, and κ-carrageenan from red seaweed. The precursor salts and biotemplates were selected from characteristics such as ease of availability, and potential for novel morphologies and improvement in electrochemical performance referenced from previous works.

Differing precursor salts and biotemplates may lead to different morphological and electrochemical properties. While both acetate and nitrate salts produce nanoscale particles, nitrates produce particles with greater uniformity in shape and size, improving electrochemical stability, while acetates produce particles with a greater size and morphology distribution which increases overall capacity. Use of sodium alginate as a biotemplate has the potential to form nanowire morphologies of which allows for high ionic conductivity along the wire length, and so improved capacities. This results in high capacities when used in a lithium-ion coin cell, up to 240 mAhg-1 at 0.1 C (figure 1). Dextran meanwhile may produce plate morphologies with a large face, which gives a lower capacity but much improved electrochemical stability with capacity fade <1 mAhg-1 per cycle at 0.1 C. Κ-carrageenan on the other hand may produce elongated morphologies similar to sodium alginate and results in a cathode doped with lithium sulphate which has been shown to greatly improve cycling stability, but at the cost of lithiation sites.

Biotemplating as a method has great viability for commercialising NMC811 and unlocking the potential in other previously discounted and emerging electrochemistries for both lithium-ion and sodium-ion cathodes. The synthesis of NMC811 using biotemplating produces high capacities and electrochemical stability through morphological control, while reducing costs and emissions associated with production of tradition lithium-ion cathodes, as well as reducing the reliance on metals such as cobalt associated with lithium-ion cells which are monopolised by a small number of countries. This benefit can also be translated into chemistries currently under research, giving biotemplating a wide-ranging potential across electrochemistry and materials research as a whole with benefits to be passed onto consumers.

Figure 1