Resumen de Alternative approaches for the fabrication of electrodes based on lifepo4 as cathodes in libsalternative approaches for the fabrication of electrodes based on lifepo4 as cathodes in libs
Lithium ion batteries are the power sources of choice for portable electronics, power tools and electric base transportation and they are ready to take over other niche and mainstream applications (from wearables to smart grid support to electric vehicles). Yet, this implies new requirements such as a combination of high energy and power densities, longer cycling life and, last but not least, substantially reduced costs.
The European Project SOlid MAterials for high power Li polymer BATteries (SOMABAT) aimed to develop more environmentally friendly, safer and better performing Li batteries. Most of the work presented in this thesis corresponds to our contribution to this project, consisting of the optimization of low-cost and ecofriendly methods for the preparation of LiFePO4 (LFP) and the development of nanostructured and hybrid composites based on this cathode material and conductive polymers or conductive nanocarbons like graphene.
Indeed, olivine-type LFP has been recognized as one of the most promising cathode materials for rechargeable Li batteries. Its advantages include high capacity, high stability, nontoxicity, and low cost. We used low-temperature methods for synthesizing nanocrystalline LixFePO4 with the olivine structure and different morphologies and microstructures are described. In order to overcome the well-known insulating nature of LFP, different methods, alternative to the conventional carbon-coating, were proposed and tested.
Chapter three describes the development of a low-cost hydrothermal (200°C) synthesis method. We were able to control the morphology using Poly-ethyleneimine (PEI) and characterize the corresponding Li+ diffusion coefficients for the various LFP samples. These materials exhibit a fractal morphology, high tap density and relatively high surface area. These characteristics could increase the point of contact between the LFP and the electrolyte. As a consequence of these properties the rate capability and discharge capacity of these samples were satisfactory, 164 mAh/g and a Li+ of 2.2 x 10-13 cm2/s.
The development of a solvothermal method (200°C, ethylene glycol), described in chapter four, allowed us to prepare LFP with a small particle size (ca. 60 nm) with only 5% of surfactant (pluronic acid). The performance of the corresponding carbon-coated material was optimal (168 mAh/g) and close to the theoretical capacity of LFP. Using this method, we synthesized LFP/RGO hybrids (also described in chapter four). The electrochemical performance of the optimal LFP/RGO hybrid (with small LFP particles of ca. 40 nm) was quite similar to that of the carbon-coated standard. In this chapter, we also explored the synthesis of hybrid LFP/PPy-Nanopipes, which showed reasonable electrochemical performance but only at low C-rate.
The studies described in chapter five deal with the use of a reflux method to synthesize LFP and LFP/RGO composites. Pure LFP crystallized with a nanoplatelet morphology but with the large crystal faces (parallel to bc plane) not optimal for Li+ diffusion and leading to a specific charge of 150 mAh/g. When grown on RGO the morphology of LFP changed and led to better electrochemical performance 165 mAh/g. The reflux approach not only turned out to be simple, but also highly reproducible and cost-effective.
As an alternative design of LFP electrodes, we developed electroactive LFP nanofluids based on the solid samples described before with the intention to use them as part of novel redox flow cells. The nanofluids were prepared by dispersing the solids into suitable electrolytic base fluids (aqueous Li2SO4) containing RGO stabilized in dispersion by addition of diaminobenzoic acid (DABA) as conducting component. Aqueous electrolytes (pH controlled) were selected for simplicity, high-conductivity and eco-friendly advantages. Nanofluids with LFP and RGO separately dispersed in the liquid and nanofluids with hybrid LFP/RGO nanoparticles were prepared and studied. The best electrochemical performance was found for the former, with a discharge capacity of ca. 130 mAh/g at 2C for a remarkable material in which RGO plays in the nanofluid the role that conventional carbon coatings play in solid electrolytes
