Bio-Assisted Synthesis of Fe-Based Electrode Materials

Presently, the production of high-energy Li-ion based storage systems comes with large energy consumption and environmental impact, the most being the production of the electrode materials because of the high temperatures generally required and of the chemical nature of their components (transition metals). The main approaches explored to tackle this issue are i) finding new materials, enlisting alternative redox centers such as organics ii) exploring new synthetic ways using much lower temperatures iii)looking for efficient recycling processes. Living (i.e. aquatic) beings, owing to specific enzymatic-based metabolisms, are able to spontaneously trap, concentrate and transform soluble species, leading to precipitates (e.g. oxides, phosphates, carbonates …) with specific size, texture, morphology, at ambient temperature. In this study, we will exemplify the benefit of using bacteria to produce highly textured Fe-oxides and Fe-phosphates providing improved cyclability and power capability when reacted with lithium.

Firstly, we report a method based on bacterial iron biomineralization for the synthesis of a-Fe₂O₃ as conversion electrode material. This high-yield (> 80%) synthesis approach enlists (1) the room temperature formation of g-FeOOH via the use of the anaerobic Fe(II)-oxidizing bacteria Acidovorax sp. strain BoFeN1, and (2) the transformation of these BoFeN1 / g-FeOOH assemblies into an alveolar bacteria-free a-Fe₂O₃ material by a short heat treatment under air. As the g-FeOOH precursor particles are preferentially precipitated between the two membranes of the bacteria cell wall (40-nm thick space), the final material consists of highly monodisperse nanometric (~ 40 x 15 nm) and oriented hematite crystals, assembled to form a hollow shell having the same size and shape as the initial bacteria (bacteriomorph) (Figure 1).

Besides X-Ray and electron diffraction studies, electrochemical galvonastatic signatures vs. Li (down to 0 Volt) confirm the formation of hematite (Figure 2, left). However, the capacity retention of the bacteriomorph samples is found to be largely improved in comparison with samples which organization was prior destroyed by hand-milling while not altering the size/structure of the primary oxide grains (Figure 2, center). In addition, long cycling tests enlightened a progressive in-situ loss of the bacteriomorph organization that correlates the observed long-term capacity decay.

This original porous organization induced by the bacteria also positively impacts the power capability of the textured powder. Indeed, only 30% of the capacity is lost when cycling is performed from 1Li / 100 h to 1 Li / 6 min while the ground untextured powder is losing 85% of its initial and still low charge capacity (Figure 2, right). Thanks to the dual textural control enabled by the bacteria (micrometric-level organization of nanometric primary grains), correlated enhanced reversibility and impressive high power capability could thus be achieved (J.Miot et al, Energy & Environmental Science 7(1), 451, 2014).

Secondly, bacterially-induced eco-efficient and scalable room-temperature synthesis method can be applied to other systems, provided the bacteria strain is properly selected and adapted to the targeted material. For the room-temperature precipitation of Fe-based phosphates, Bacillus pasteurii was selected as its metabolism enlists urease enzyme controlling the decomposition of urea, thus the release ammonia in the medium, so finally its local pH. This allows the formation of nanometric and organized active grains (Figure 3) with specific assets towards Li-reactivity that will be also detailed.