Is There Any Benefit of Coating Si Particles for a Negative Electrode Material for Lithium-Ion Batteries with Metal-Organic Frameworks? The Case of Aluminum Fumarate

Thanks to its high gravimetric andvolumetric capacities, silicon(Si) is one of the most promising alternatives to graphite for negativeelectrodes for lithium-ion batteries. Its practical use is neverthelesshampered by its low capacity retention, resulting from its high volumevariation upon cycling driving the formation of an unstable solidelectrolyte interphase (SEI). Coatings of Si particles with metal-organicframeworks (MOFs) acting as artificial SEIs were recently reportedand found to lead to improved electrochemical performances in a fewcases. We here developed a room temperature route to coat Si particleswith the aluminum fumarate MOF (Al-fum), in conditions compatiblewith the aqueous formulation of state-of-the-art Si electrodes. Thanksto a variety of characterization techniques, including IR and solid-stateNMR spectroscopies, powder X-ray diffraction, and scanning transmissionelectron microscopy coupled with energy-dispersive X-ray analysis(STEM-EDX), we show that a layer of ca. 20 nm of MOF is grown at thesurface of the Si particles. Nevertheless, such a coating does notranslate into any major modification of the electrochemical performancewhen the Si particles are integrated in electrodes with a loadingof practical interest (& SIM;2 mg(Si) cm(-2)). Postmortem characterizations revealed that Al-fum, although beinghighly stable toward water, evolves in the standard LP30 electrolytethrough a reaction with the PF6 (-) anions.The MOF further reacts during the first electrochemical reduction,ultimately leading to lithium aluminate phases, still located at thesurface of the Si particles. Considering the growing interest of MOFsin the field of electrochemical energy storage, this let us concludethat there is probably a general need to more deeply and systematicallyevaluate the stability of MOFs toward battery electrolytes and electrochemicalprocesses.