Mechanistic Understanding of Aging Behaviors of Critical-Material-Free Li4Ti5O12//LiNi0.9Mn0.1O2 Cells with Fluorinated Carbonate-Based Electrolytes for Safe Energy Storage with Ultra-Long Life Span

Behind-the-meter storage (BTMS) systems—a viable method to minimize potential risk of blackout events and stabilize the grid—require a different type of cost-effective energy storage with excellent safety, ultra-long (>20 years) cycle life and reasonable energy density compared that of electric vehicles. To increase the energy density and reduce the cost of a long-term cyclable lithium-titanate-based cell, it is required to employ a critical-material-free high voltage cathode and an electrolyte with good electrochemical and transport properties. Here, the long-term electrochemical performance and behaviors of selected critical-material-free Li₄Ti₅O₁₂ (LTO)//LiNi_0.9Mn_0.1O₂ (LNMO) full cells for BTMS applications are evaluated and analyzed in the optimized voltage range of 1.4–2.7 V at 45 °C with different fluorinated carbonate-based electrolytes. The fluoroethylene carbonate (FEC)-based electrolyte cell shows the highest capacity retention of 57.9 % and Coulombic efficiency (CE) of 99.96 % after 1000 cycles, potentially attributed to a dense, homogenous and less resistive LiF-rich solid-electrolyte interphase (SEI) layer formed on the surface of LTO that may mitigate electrolyte decomposition and maintain relatively low cell impedance during cycling. The 3,3,3-fluoroethylmethyl carbonate (F-EMC)-based electrolyte cell, however, presents the worst performance with lower capacity and a sharp decrease of CE, due to unstable and non-uniform SEI formation and continuous oxidative electrolyte decomposition. This mechanistic understanding of cell aging behaviors and failure mechanisms with detailed analysis of surface chemistry and electrode morphology can guide design of new electrode chemistries and electrolyte formulations for the development of BTMS batteries.