UCLA

Novel battery technology must be capable of providing both increased energy density and power density to keep up with global energy demand. This dissertation addresses a variety of structural challenges present in energy storage materials for both positive and negative electrode applications. Several materials (LiMn2O4, MoS2, and SbSn) have been synthesized as nanoporous architectures to reduce ion diffusion length, mitigate volume changes, and enhance structural stability. The first part of this work describes a facile synthesis for nanoporous LiMn2O4 and how the self-discharge problem is mitigated by doping with Al3+. Through x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and electrochemical rate cycling, we show that the Al3+ dopant preferentially substitutes Mn3+ at the surface, stabilizes the spinel structure at lower temperature (200˚C), mitigates manganese dissolution, while still maintaining good rate cycling up to current densities of 20C. The second and third chapters of this dissertation focus on de-convoluting the effects of reducing crystal size and lattice disorder in mesoporous MoS2 pseudocapacitors. A series of mesoporous MoS2 powders with different sizes and degrees of crystallinity are synthesized through a sulfurization reaction. Crystal size is controlled by the size of mesoporous MoO2 precursor used, and disorder is controlled by higher annealing temperatures. Ambient synchrotron x-ray total scattering / pair distribution function (PDF) analysis techniques enable us to quantify the extent of layer shifting compared to layer expansion disorder in the matrix of samples. Operando x-ray diffraction (XRD) demonstrates that both size and disorder effects cause the suppression Li-intercalation induced phase transitions in MoS2. The reduced size and increased disorder are correlated to enhanced fast-charging performance up to rates of 100C. Kinetic analyses show size and disorder effects increase the fraction of capacitive current in mesoporous MoS2, demonstrating that reducing crystal size and increasing lattice disorder are both effective methods for introducing pseudocapacitive charge storage. Using operando PDF to study our matrix of size and disordered MoS2, further understanding into the dynamic evolution of disorder and local structure is achieved. The final chapter elucidates the formation of both crystalline and amorphous phases evolved during the cycling of nanoporous SbSn alloying anode through both operando XRD and PDF. The nanoporous architecture of SbSn, intermetallic phase, and ductile amorphous intermediates all provide means of buffering the volume expansion during cycling, resulting in improved long-term cycling stability.