On the Use of LiCoO2 Nanoparticles As Storage Material for Redox Flow Battery
Authors: Rano Simon , Laberty-Robert Christel , Ngo Kieu , Sanchez-Sanchez Carlos , Vivier Vincent 
Published: February 2020

Redoxflow storage technologies soared over the past decades, but still remains limited by the solubility of redox species. A promising alternative could be the semi-solid redox flow batteries (SSRFB) that recently came out and which consists in using insertion material particles in suspension as storage vector [1]. Such a SSRFB leads theoretically to both higher efficiency and capacity, along with reduced species crossover. Nonetheless, it carries at least two challenges: studying the electrochemical behavior of dispersed nanoparticles and solving ion conductive membrane issues usually used for such systems.
The use of ultramicroelectrodes (UME) under certain conditions enables the detection and characterization of electroactive nanoparticles in suspension trough the current monitoring [2]. Indeed, if the UME is polarized above the oxidation potential of the particle, the collisions will generate a current transient depending on the size of the particles and its oxidation kinetic [3]. In this work, we investigate the behavior of aqueous LiCoO2 nanoparticles (LCO-NP) suspension used as SSRFB catholyte. The electrochemical response shows oxidative transients with an amplitude ranging from 5pA to 500 pA depending on the aggregate size and they are ascribed to the Li+ desinsertion reaction. While the transient area gives an accurate count of the LCO-NP forming the related aggregate, experiments also revealed a direct relation between the collision rate and the concentration. It thus allows a monitoring of aggregation as a function of time. More interestingly, similar size aggregates have been observed by Scanning Electronic Microscopy imaging of used UMEs after single NPs collision experiments. Furthermore, a detailed analysis of the electrochemical response also shows that all transients present a similar current decay. Diffusion calculations forecast an exponential decay of the current, driven by Li+ diffusing trough the LCO-NP lattice agree with particle size and Li+ diffusion coefficient. These experiments prove the possibility to charge an LCO-NP based SSRFB catholyte. Cyclic voltammetry measurements performed in presence of LCO-NPs revealed that if an over-potential is necessary to oxidize dispersed particles, the remaining adsorbed particles still behaves as expected for a common LCO cathode and can be easily removed. These results show the possibility of running a flow-cell based on a Li-ion material suspension.
Additionally, the present work also aims at increasing the cost efficiency and reliability of eventual SSRFBs by the mean of microfluidics technics since some studies prove the opportunity to remove the cell separator under laminar flow conditions [4].
Micro flow-cells were made using typical microfabrication process and 3-D printing. The cell design consisted in a single PMDS microchannel with two inputs and outputs. The electrodes were made out of carbon felt for high porosity and contact surface. Preliminary results show that the use of such system provide a powerful alternative to conventional redox flow system.
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[2] Xiao, X.; Fan, F. F.; Zhou, J.; Bard, A. J.; Uni, V.; Station, A. Current Transients in Single Nanoparticle Collision Events. 2008, No. 22, 16669–16677.
[3]Cheng, W.; Compton, R. G. Electrochemical Detection of Nanoparticles by “Nano-Impact” Methods. TrAC - Trends Anal. Chem.
2014, 58, 79–89.

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