Interfacial Engineering in All Solid-Sate Lithium Ion Batteries

Thesis (Ph.D.)--University of Washington, 2019All Solid-state Batteries (ASSBs), enabled by solid electrolytes (SEs) possessing lithium ion conductivities greater than those of the liquid electrolytes at room temperature, have the potential of achieving higher energy and power densities than the conventional liquid electrolyte-based battery systems by the integration of lithium metal anode. In addition, SEs are non-flammable which improves the safety and abuse tolerances of ASSBs. ASSBs, however, face challenges that include interfacial resistance towards the cathode and lithium metal anode, compatibility with high voltage cathode, chemical and mechanical stability against lithium dendrite growth, among others. This thesis focuses on the understanding, design, and optimization of the interface between the cathode and the SE in two ASSB systems. Thio-LISICON Li10GeP2S12 equivalent Li10SnP2S12 (LSPS) is comparable in ionic conductivity yet at a lower cost as an electrolyte for ASSBs. ASSBs with LSPS SE, lithium-indium alloy anode, and LiCoO2 (LCO) cathode were successfully fabricated and their electrochemical performance at 60 °C was examined. Atomic layer deposition (ALD) of Li3NbO4 on LCO was applied to improve the interfacial stability. The Li3NbO4 coating significantly improves the cycle stability of the ASSB, which retains about 85% of the initial capacity after 70 cycles at a current density of 0.13 mA/cm2, while the ASSB with uncoated LCO retains ~ 60% of the initial capacity after 70 cycles. Electrochemical impedance spectroscopy (EIS) tests indicate a rapid growth of charge transfer resistance upon cycling for the cell with the uncoated LCO, primarily due to the surface instability and build-up of a space charge layer between LSPS and LCO. However, the ASSBs with Li3NbO4 coated LCO show a more stable interface with a negligible impedance increase upon cycling, attributable to the ‘buffering’ and ‘passivating’ roles of the Li3NbO4 coating. The interfacial microstructure was analyzed to elucidate the underlying reasons for the impedance increase and the pivotal role of the Li3NbO4 coating. Our study indicates that surface coating significantly improves the cycle stability of the ASSBs with LSPS as the electrolyte mostly due to an improvement of the charge transfer mechanism. The coating reduced the interphase thickness and interfacial resistance Rsei to about a third of the uncoated one after 10 cycles. Using this knowledge in ASSBs, the previously unstudied Li2MCl4 (M=V, Cr, Fe, Mn, Co) were tested as an option for thick electrode/power batteries due to their high ionic conductivity (1.2 x 10-5 S/cm for Li2FeCl4 (LFC) vs. 5.0 x 10-5 S/cm for LiFePO4 at 25℃1), similar theoretical capacity (165 mAh/g for 1.2 Li+ out of Li2FeCl4 vs. 170 mAh/g for 1 Li+ from LiFePO42), and higher working voltage (3.7 V for LFC vs. 3.4 V for LiFePO41), and higher working voltage (3.7 V for LFC vs. 3.4 V for LiFePO4). These cathode materials were successfully synthesized by solid state reactions, as well as ball milling, using LiCl and MCl2 as the precursors. The crystal structure was determined by x-ray diffraction and confirmed with Rietveld refinements. Dissolution experiments were performed to find a suitable liquid electrolyte for these cathode materials, but the use of solid electrolytes seems necessary. Polymer electrolytes are similarly shown to have adverse reactions with the cathode material, making the use of ceramic solid electrolytes necessary. When using LSPS SE, LFC exhibited a reversible discharge capacity of 200-250 mAh/g, corresponding to 1.25-1.75 Li+ ions per formula unit, but it seems to correspond to a conversion reaction. Further experiments were performed to determine the reaction mechanisms. A judicious SE selection became imperative to establish the electrochemical properties of LFC and the rest of the Li2MCl4 family of materials. To that end, a SE in the halide family of electrolytes was synthesized and studied. The ionic conductivity of the recently discovered halide solid electrolyte Li3YCl6 (LYC) was studied as a function of its crystal structure. When the sample was quenched from 450 °C it exhibits a hexagonal phase, as opposed to the orthorhombic phase it presents when quenched from 350 °C. The high temperature hexagonal phase has an ionic conductivity of 0.05 mS/cm, while that for the lower temperature orthorhombic phase is twice as high at 0.10 mS/cm. Conversely, ball milling the sample, as reported by Asano et al. gives a room temperature ionic conductivity of 0.46 mS/cm, which is highly encouraging for the scalability of this cheap, soft, relatively safe SE. LFC is finally explored as a cathode material for ASSBs using LYC SE. This previously unstudied cathode material was cycled at a C/10 rate, or 0.195 mA/cm2 at 25 °C, obtaining a discharge capacity of about 118 mAh/g, and it reached 80% discharge capacity at the 180th cycle. LFC is able to deliver 112 mAh/g, 106 mAh/g, 81 mAh/g, 40 mAh/g of the initial specific discharge capacity when tested at a rate of C/5, C/3, 1C, and 2C, respectively. EIS studies showed that the biggest contributor to impedance was the solid electrolyte layer, followed by the interface between the cathode and the solid electrolyte. The interphase was confirmed through computational studies to be mainly composed of LiCl, FeCl2 and YCl3, which are either ionically (LiCl), or electronically conductive (FeCl2), and therefore, not detrimental to the impedance of the cell