Nanostructured lithium-ion batteries

Li-ion batteries, despite their technological importance are low-tech devices. Electrodes are typically fabricated by grinding down constituent materials and processing them to a paste which is then sintered to provide a porous layer. This approach does not allow fine-tuning of the porosity to optimise electrochemical performance. Since ion intercalation is limited to the nanometre scale, nanoporous electrodes are in principle required to maximise the energy density of the battery. Extended nanoporosity in battery material however leads to diffusion, limited transport of the Li-ions to the electrode surfaces and, typically, to the mechanical instability of the material upon cycling. Both significantly reduce the utility of nanostructured materials in Li-ion batteries.

Given the current battery manufacture processes, and the considerations of the “background” section, improved materials for electrode manufacture must fulfil several requirements. They should:

1. be compatible with current manufacturing processes,
2. be low-cost, but
3. allow the engineering of the pore-size down to the nanometre length scale.

Hence, the aim of the project was to produce particles with an overall size of the granular material typically used in commercial battery manufacture, but with an internal porosity that can be fine-tuned during the synthesis of these particles. Such a system, if mass produced, could then be employed in standard battery manufacture, yielding Li-ion batteries with higher capacity, rate performance (capacity upon quick charge and discharge) and cyclability (low capacity fading upon many charge and discharge cycles).

The general approach to achieving the aims above consisted in the combination of sol-gel synthesis of inorganic materials with block-copolymer self-assembly, as previously demonstrated by the Steiner group for other material functionalities (e.g. photovoltaics, optics). The initial approach therefore made use of the co-assembly of block copolymers with a sol-gel chemistry approach to produce nanostructured anatase titania spheres of several micrometres in diameter. This is a proof of principle that battery relevant materials can be easily synthesised with highly controlled hierarchical morphologies, extending from the nano- to the microscale. This was extended by a second complementary approach, in collaboration with the University of Nottingham, where similar nano-structured self-assembled polymer spheres were synthesised in supercritical CO2. Both of these approaches serve as a generic platform for the manufacture of a range of inorganic battery electrode materials. The usefulness of this approach was demonstrated through the manufacture of hierarchical mesoporous lithium iron phosphate, a commonly used battery cathode material. In a half-cell against Li-metal, mesoporous Li-iron phosphate electrodes that were deposited using industry standard processes exhibited outstanding cyclability and excellent cycle life. As an often-used corresponding anode lithium titatanate was similarly synthesised as mesoporous spheres. Similar to the Li-iron phosphate, excellent mechanical and electrochemical properties were found. Overall, this project has produced electrode materials with well-defined porosities that considerably improve battery performance, while remaining compatible with standard battery manufacture protocols.

Implications for research

These results significantly advance the understanding of the role of electrode structure in the performance of these materials in batteries. They provide a strong foundation for the extension of this approach to a wide range of other electrode materials, which would in turn trigger further innovative research. The results were published in high-impact scientific journals. The work also improved the visibility of Swiss battery research internationally.

Implication for practice

The results of this project have the potential for industrial implementation, but a number of market constraints stand in the way of their rapid implementation.

Hierarchically structured materials for super-capacitors and batteries