Hybrid Sn3O2(OH)2/Graphene Nanomaterials: From Solid Li-Ion Battery Anode to Flowable Nanoelectrofuel

Lithium ion batteries (LIBs) are the dominant rechargeable power source for portable and small scale electronic applications. In order for LIBs to be realistically and practically applied to large scale systems, i.e. electric vehicles, significant improvement in overall capacity and energy density need to be achieved. This can be done through altering cell geometries, electrolyte optimization, or most importantly, improvements to the materials used within the battery. Use of tin as an anode material can potentially improve current carbon-based anode capacities from a theoretical maximum 372 mAh/g for carbon to 994 mAh/g for metallic tin. However, structural stresses during lithiation/delithiation create irreversible destruction of metallic tin anodes which quickly degrades the overall capacity and performance of the battery. Two of the most popular techniques to mitigate this effect are (1) using smaller particle sizes where the stress on the crystal is reduced and (2) addition of lithium-inactive elements to act as a buffer during volumetric changes. Oxygen from oxides will irreversibly react with lithium forming Li₂O, which acts as the volumetric stabilizer. Along with material optimization, the use of a flow-cell geometry can dramatically increase the system level energy density due to reduced packing, while also solving many of the engineering obstacles in creating a cost-effective system. Although flow-cell geometries provide their own difficulties (suspension stability, limited electrical contact, etc.), the use of a nanoparsticle suspensions (nanoelectrofuels) has the potential to transform the outlook of battery systems.

This study focuses on comparative characterization of solid casted and nanoelectrofuel anodes from the same type of hybrid nanomaterial composed of Sn₃O₂(OH)₂nanoparticles that were deposited directly onto graphene nano-platelets (GnP). By using this hybrid particle morphology, we achieved good electrical conductivity to an ensemble of individual anode nanoparticles and kept the size of individual particles under the self-healing threshold to address degradation of Sn nanoparticles due to the well-known volume expansion mechanism.

Electrochemical behavior (galvanostatic charge/discharge curves) were measured in pouch and coin cells for solid-casted hybrid anodes and for the same nanomaterials formulated into nanoelectrofuels the  tests were conducted in a custom nanoelectrofuel test cell simulating flow with magnetic stirrer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for characterization of starting nanomaterials as well as current collectors and nanomaterials after electrochemical tests.

In situ x-ray absorption spectroscopy, specifically extended x-ray absorption fine structure (EXAFS) spectroscopy measurements were made to determine local structural changes around Sn atoms during several charge/discharge cycles of a solid casted electrode in a pouch cell. A new degradation mechanism for these materials different from well-known volume expansion is proposed based on EXAFS modeling, and correlating observed structural changes to electrochemical performance.

Although nanoelectrofuel electrodes are very complex systems that require significant efforts in formulation as well as cell engineering, we were able to achieve a respectable reversible capacity compared to solid casted electrodes. Initial electrochemical and material characterization will be presented along with plans for future improvement.