Kevin James Rhodes

Visualizing the chemistry and structure dynamics in lithium-ion batteries by in-situ neutron diffraction

We report an in-situ neutron diffraction study of a large format pouch battery cell. The succession of Li-Graphite intercalation phases was fully captured under an 1C charge-discharge condition (i.e., charge to full capacity in 1 hour). However, the lithiation and dilithiation pathways are distinctively different and, unlike in slowing charging experiments with which the Li-Graphite phase diagram was established, no LiC24 phase was found during charge at 1C rate. Approximately 75 mol. % of the graphite converts to LiC6 at full charge, and a lattice dilation as large as 4% was observed during a charge-discharge cycle. Our work demonstrates the potential of in-situ, time and spatially resolved neutron diffraction study of the dynamic chemical and structural changes in “real-world” batteries under realistic cycling conditions, which should provide microscopic insights on degradation and the important role of diffusion kinetics in energy storage materials.

Understanding the Degradation of Silicon Electrodes for Lithium-Ion Batteries Using Acoustic Emission

Silicon is a promising anode material for lithium ion battery application due to its high specific capacity, low cost, and abundance. However, when silicon is lithiated at room temperature it can undergo a volume expansion in excess of 280% which leads to extensive fracturing. This is thought to be a primary cause of the rapid decay in cell capacity routinely observed. Acoustic emission (AE) was employed to monitor activity in composite silicon electrodes while cycling in lithium ion half-cells using a constant current-constant voltage procedure. The major source of AE was identified as the brittle fracture of silicon particles resulting from the alloying reaction that gives rise to LixSi phases. The largest number of emissions occurred on the first lithiation corresponding to surface fracture of the silicon particles, followed by distinct emission bursts on subsequent charge and discharge steps. Furthermore, a difference in the average parameters describing emission during charge and discharge steps was observed. Potential diagnostic and materials development applications of the presented AE techniques are discussed.

Structural transformation in a Li1.2Co0.1Mn0.55Ni0.15O2 lithium-ion battery cathode during high-voltage hold

A decrease in the c-lattice parameter was observed in Li1.2Co0.1Mn0.55Ni0.15O2 during constant voltage holding at 4.5 V by in situ X-ray diffraction. Comparison of magnetic susceptibility data before and after high-voltage hold reveals the change in average oxidation states of transition metal ions during high-voltage holding process. Transmission electron microscopy studies show the spinel reflections with fundamental trigonal spots from the particles after high-voltage hold indicating substantial structural modification. The structural transformation was believed to occur due to the oxygen release and/or the migration of transition metal cations to lithium layer during constant voltage holding.

Novel cell design for combined in situ acoustic emission and x-ray diffraction study during electrochemical cycling of batteries

An in situ acoustic emission (AE) and x-ray diffraction cell for use in the study of battery electrode materials has been designed and tested. This cell uses commercially available coin cell hardware retrofitted with a metalized polyethylene terephthalate (PET) disk, which acts as both an x-ray window and a current collector. In this manner, the use of beryllium and its associated cost and hazards is avoided. An AE sensor may be affixed to the cell face opposite the PET window in order to monitor degradation effects, such as particle fracture, during cell cycling. Silicon particles, which were previously studied by the AE technique, were tested in this cell as a model material. The performance of these cells compared well with unmodified coin cells, while providing information about structural changes in the active material as the cell is repeatedly charged and discharged.

Electrically conductive nano graphite-filled bacterial cellulose composites

A unique three dimensional (3D) porous structured bacterial cellulose (BC) can act as a supporting material to deposit the nanofillers in order to create advanced BC-based functional nanomaterials for various technological applications. In this study, novel nanocomposites comprised of BC with exfoliated graphite nanoplatelets (xGnP) incorporated into the BC matrix were prepared using a simple particle impregnation strategy to enhance the thermal properties and electrical conductivity of the BC. The flake-shaped xGnP particles were well dispersed and formed a continuous network throughout the BC matrix. The temperature at 10% weight loss, thermal stability and residual ash content of the nanocomposites increased at higher xGnP loadings. The electrical conductivity of the composites increased with increasing xGnP loading (attaining values 0.75 S/cm with the addition of 2 wt.% of xGnP). The enhanced conductive and thermal properties of the BC-xGnP nanocomposites will broaden applications (biosensors, tissue engineering, etc.) of BC and xGnP.

Effects of ultramicroelectrode dimensions on the electropolymerization of polypyrrole

Anode geometry can significantly affect the electrochemical synthesis of conductive polymers. Here, the effects of anode dimensions on the electropolymerization of pyrrole are investigated. Band microelectrodes were prepared with widths ranging from 2 to 500 mum. The anode dimension has a significant effect on the resulting thickness of polymer film. The electropolymerization process deviates significantly from that predicted by simple mass transfer considerations when electrode dimensions are less than approximately 20 mum. Polymer film thickness is thinner than expected when electrode dimensions become less than approximately 10 mum. A simple mathematical model was derived to explain the observed effects of anode dimensions on the polymerization process. Simulation results confirm that diffusive loss of reaction intermediates accounts for the observed experimental trends. The described simulation facilitates understanding of the electropolymerization processes and approaches to the controlled deposition of polypyrrole, particularly at the submicron scale, for microelectromechanical systems and biomedical applications.