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Materials for high energy accumulators in traction and tools

Final Report Summary - MAHEATT (Materials for high energy accumulators in traction and tools)

The main objective of the MAHEATT project was the development of novel electrode materials with performances well beyond current state of the art, targeted for use in lithium-ion (Li-ion) batteries. Radical materials concepts were prioritised, primarily for the positive electrode. At the same time, resources were directed towards light weight compounds like borates that could deliver high capacities, as well as towards also new transition metal complex oxides. On the anode side, MAHEATT efforts were directed towards silicon (Si) nanoparticle based electrodes, with emphasis on flexible and conducting coatings that would allow for enhanced stability upon cycling. The synthesis efforts were fully integrated with advanced characterisation, including complete electrochemical testing and in situ studies during cycling, as well as theoretical modelling. A selected nanostructured cathode material was up-scaled, chemically modified via doping, surface modified by atomic layer deposition (ALD), electrochemically tested and benchmarked in larger test cells. MAHEATT generated considerably new insight on materials, aspects of nanostructuring and surface coatings, all being of high value in continued research and development (R&D) on optimised electrode materials for Li-ion batteries targeted towards traction and heavy duty tools.

The overall objective of the MAHEATT project was to develop a prototype, cost effective, Li-ion high energy battery technology with electrode performances well beyond the current state of the art, with automotive applications, such as hybrid vehicles and electric traction, and hand held tools as applications in mind. The work approached this goal via innovative synthesis and design of radically improved cathode materials and by optimising kinetics and stability through nanostructuring of electroactive materials.

The key objectives were to:

1. design synthesis routes for novel cathode materials;
2. design electroactive materials in nanoscopic form to provide good kinetics and stability;
3. develop coatings in order to improve stability and cycling behaviour;
4. benchmark and demonstrate capabilities of novel materials and concepts.

A cross-disciplinary approach was followed, with partnership of leading research institutions and leading European industry. The materials' challenges were addressed via feedback from advanced characterisation, theory and modelling.

MAHEATT had eight technical work packages (WPs). Their main objectives were the following:

1. WP1: Synthesis of new classes of cathode materials with very high energy densities
2. WP2: Improved synthesis of nano-silicon from intermetallics
3. WP3: Coating and assembly of nanosized electroactive materials
4. WP4: Computational modelling of stability and Li-diffusion in nanosized electroactive materials
5. WP5: Electrochemical and physicochemical characterisation, including in situ studies
6. WP6, upscaled production of specific cathode and anode materials
7. WP7: Benchmarking of novel materials and concepts and development of a demonstrator
8. WP8: Innovation based activities, securing novel intellectual property (IP) from the other seven WPs.

MAHEATT had the goal to develop a class of novel cathode materials. The positive cathode represented a limiting factor for the overall capacity of a Li-ion battery. A due focus was also put also on matching anode performance.

The need for high charge densities addressed light weight compounds with electroactive d-elements in a proper potential window. Jumps in oxidation states by more than one unit would be beneficial. The involved cations should be easily available, i.e. cheap and abundant, and environmentally benign. This put limitations on the search for novel systems. MAHEATT defined three categories of target materials; one being high risk with respect to likely success, the other two representing more safe approaches but having less impact on technology, i.e. they did not propose and radical changes in technology and did not drive ranges. As examples of the category of conventional compounds were transition metal borates and aluminates, e.g. LiFeBO3. The Li-ion transport properties in this category of insulating and semiconducting compounds based on oxyanions resembled those of LiFePO4. Therefore, we addressed understanding of transport properties in such types of materials more generally, inter alia via molecular dynamics modelling and studies of effects of various types of defects. Cation and anion doping and substitutions were attempted for a large number of candidate oxides. Up-scaling of promising electroactive borates and oxides was performed by means of nebulised spray pyrolysis and hydrothermal synthesis routes. MAHEATT focussed its activities on compounds in the high risk category.

MAHEATT benefitted from a number of synthesis tools, combined with strong insight in materials and nanochemistry. As guidance towards improved synthesis and selection of novel materials systems, physical and electrochemical characterisations along with density functional theory (DFT) and molecular dynamics modelling played important roles with respect to feedback and interactions between various WPs.

The key objective for MAHEATT target materials in relation to electric traction in the automotive sector was a battery that would provide at least 1.5 times, i.e. 240 mAh/g, larger charge density than the state of the art cathode materials.

The need for high energy densities in Li-ion batteries for the transportation sector naturally calls for light weight compounds with electroactive d-elements in a proper potential window. Jumps in oxidation states by more than one unit were desired. One category of target compounds of MAHEATT were transition metal borates, phosphates and oxides, with examples being LiFeBO3, Li2MP2O7 and LiCuFe2(VO4)3. MAHEATT succesfully synthesised and fully characterised such materials.

As guidance towards improved synthesis and selection of novel materials systems physical and electrochemical characterisations along with DFT and molecular dynamics modelling played important roles with respect to feedback and interactions between various work packages. ALD was developed as a tool towards screening of potential materials systems and for surface modification of developed nanoparticles and nanocomposites. The establishment of novel electroactive materials, coating technology, synthesis procedures, as well as Li based ALD processes led to patent applications.

The Li-ion transport properties in families of poorly conducting materials based on linker anions, such as borate, phosphate etc., resembled those in well known phosphates like LiFePO4; however, at lower weights and possibly higher potentials. Molecular dynamics modelling was used to shed light on their transport properties, including the effect of various types of defects.

Concerning anode materials, MAHEATT focussed on developing nano-silicon from Li-Si-M compounds. These nanometre sized particles and their assemblies represented very promising anode materials and MAHEATT addressed issues like packaging and interconnecting to solve problems with volume work and loss of electrical contacts, which were essential for ensuring stability during cycling. Several routes to mitigate such challenges were established and evaluated.

Advanced electrochemical tests, including in situ investigations during cycling and post mortem analysis, were undertaken to explore reaction mechanisms of the novel materials and the effect of doping and surface modification. This interacted with construction work on an optimised cell for in situ electrochemical studies by synchrotron radiation.

Up-scaling of such electroactive borates and oxides was been achieved via nebulised spray pyrolysis. One key parameter was the very control of the oxygen fugacity during the formation of, for example, the Fe(II) compound LiFeBO3. Up-scaling of high potent oxides was further done by hydrothermal methods.

MAHEATT developed novel cathode materials with stable cycling of 20 to 50 cycles in the range of 250 to 400 Ah/Kg. These successful searches for radically new cathode materials benefited from recent nanoscientific achievements, including coating and assembly.

The MAHEATT materials needed further improvements via focused R&D; however, they held a large potential for breakthroughs with respect to enhanced energy capacities for Li-ion batteries. Although this potential was large, as well as the potential for value creation and commercialisation by European industries, these materials were, by the time of the project completion, not yet in a state where further development could be expected to be done by the consortium industrial partners. MAHEATT therefore looked for opportunities for continued R&D within its materials' priorities. A number of patent applications were submitted in relation to synthesis approaches and to classes of electroactive materials. The dissemination was restricted to generic results in cases when advanced characterisation and modelling were used to gain fundamental insight into structure, stability, diffusion, electronic properties and electrochemistry, for confidentiality and patenting issues. MAHEATT produced publications, held 14 talks and posters and had 5 patent applications in process by the time of the project completion.

Further information on MAHEATT could be obtained at http://www.mn.uio.no/smn/english/research/projects/maheatt/index.html.

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