Design of a multi-layer electrolyte cell using Li1.3Ti1.7Al0.3(PO4)3 ceramic electrolyte material

Abstract

Energy storage systems have been developed to satisfy the demand of energy density for electric vehicles and large-scale ESS plants. The electrochemical stability window of commercial carbonate-base electrolytes is from cathodic limit 0.8V to anodic limit 4.3V vs. Li/Li+. The oxidation decomposition of carbonate electrolytes prevents realization of 5.0V-class high voltage of Li-ion battery (LIB) because of problems such as capacity fading by decomposition of electrolytes and the Mn-dissolution of cathodes. In this thesis, Multi-layer electrolyte cell (MEC) has improved the electrochemical high voltage stability and thermal stability by introducing an ionic liquid electrolyte as the cathode-side electrolyte. The ionic liquid electrolyte has high electrochemical stability for high potential oxidative reaction compared to a carbonate-base electrolyte and thermal stability. The first MEC model has improved the stability of high potential and elevated temperature, but it has low reversible capacity compared to commercial electrolyte coin-cell. This is caused by the polarization losses: 1) activation polarization, 2) concentration polarization and 3) Ohmic polarization. The Ohmic polarization is caused by the ionic conductivity of the electrolyte, electronic resistance of the active materials and current collectors and the interfacial resistance between them. MEC has high internal resistance of multi-layer electrolyte because the ionic liquid electrolyte and ceramic electrolyte pellet have interfacial resistance between them. We expanded the contact surface with porous LTAP pellet and ionic liquid to reduce the internal resistance of MEC. The porous type MEC has enhanced reversible capacity and good stability for high potential and temperature at low C-rate. MEC designs have intrinsic high internal resistance compared to commercial LIB, so the composition electrolyte of MEC should be develop their ionic conductivity and good interfacial resistance. The graphite anode forms the SEI layer (Solid electrolyte interphase) consuming the reversible lithium ion in LIB because the electrochemical stability window of the electrolyte is higher than the redox potential of graphite. The ceramic electrolyte Li1.3Ti1.7Al0.3(PO4)3 (LTAP) materials could provide an additional lithium source to recover the capacity of LIB full-cell. They have NASICON – type structure of two Li sites M1, M2. The M1 site is occupied by existing Li+ ion, additional Li+ ions can be inserted into the vacant M2 site. After the first charge cycle, LTAP has ionic conductivity 10-4 S/cm, so it helps lithium ion conduct inside the cell. LTAP materials provide an additional lithium source about 110 mAh/g. The contribution of lithium source material is confirmed by applying to graphite / LiCoO2 full-cell. To achieve higher capacity, ceramic electrolyte materials that have higher capacity could be applied.