Standard Li-ion battery cells contain a flammable electrolyte that poses intrinsic risk of thermal runaway; this risk limits Li-ion battery applications within safety-critical environments. The objective for solid-state Lithium-ion (Li-ion) battery development is to design inherently safer Li-ion cells, while maintaining the operational performance requirements of the end user. Polymer electrolytes often exhibit high impedance in comparison to standard liquid electrolytes, resulting in high Ohmic and concentration polarization (waste heat) during cell charging/discharging. A limiting current density (i/cm2) exists for the process of Li-ion insertion/de-insertion during such cycling, which is inversely proportional to cell polarization and serves as boundary for maximum rate capability of the cell. During charging, the presence of polarization limits the rate at which Li-ions can flow in the desired manner; increased voltage is required to maintain this transport. This causes the cell operating voltage to raise above that of equilibrium voltage, referred to as an over-potential. Similarly, during discharge, the presence of polarization will cause energy within the cell to be wasted as heat without opportunity to add energy into the system, causing the cell operating voltage to drop below that of equilibrium voltage, referred to as an under-potential. The under-potential experienced during discharge may result in a cell reaching its lower cutoff voltage prior to achieving full capacity delivery. By taking energy as the integral of voltage with respect to change in capacity during charging or discharging, it is clear that this polarization will limit energy utilization on discharge, and energy efficiency during cell cycling. This work demonstrates a hybrid polymer electrolyte design, containing low molecular weight polymer electrolyte in the pores of each electrode and high molecular weight polymer electrolyte separating each electrode. Decreased cell impedance, as well as increased energy utilization and efficiency during cycling has been demonstrated via multi-phase tuning of polymer molecular weight and Li-salt concentration, with respect to conventional polymer electrolyte systems. For glimpse to the future, this work illustrates developmental progress on merging this novel electrolyte system with bi-polar cell stacks, to enable voltage stability within each electrode pair, while achieving higher cell level operating voltage, with respect to conventional polymer electrolyte based cell limitations.
