Inhibiting Li-Ion Battery Operation at High Temperature Using Responsive Polymers

In recent years, remarkable progress in the development of Li-ion battery technology has led to devices that deliver records in power and energy density. The thermal instability in such batteries, however, creates a major roadblock to implementing these devices in large-format systems to support emerging applications in transportation and energy storage from intermittent sources, such as wind and solar. Efforts to mitigate safety issues pertaining to flammability, reactivity, and thermal runaway typically involve the implementation of low conductivity solid-state materials or irreversible and destructive safety devices, which prove problematic for high-power, large-scale systems.

In this presentation, we will discuss a new approach to achieve thermal stability in Li-ion batteries using an electrolyte system that contains a polymer that phase separates from solution above a specific temperature. The phase separation causes the solid polymer to coat the electrode and separator leading to an increase in internal resistance, which prevents the flow of current within a specified voltage range. This approach is advantageous over existing methods because the polymer phase separation is a local process, that is, if hot spots form within the battery, the polymer within that area will phase separate and coat the electrode while the rest of the device continues to operate.

Two responsive electrolyte systems will be described: poly(ethylene oxide) (PEO) in ionic liquids (ILs) and poly(benzyl methacrylate) PBMA in ILs. Both systems exhibit Li-ion concentration dependent phase behavior, where ion concentration affects the temperature at which the phase transition occurs and the reversibility of the phase transition. Two distinct differences exist between these electrolyte systems. The PEO-IL system exhibits a change in solution conductivity and charge transfer resistance above the phase transition temperature, while the PBMA-IL system only exhibits an increase in charge transfer resistance, but to a much greater extent. Solution and ion transport resistances are measured using electrochemical impedance spectroscopy (EIS) on stainless steel electrodes to determine changes in solution resistance with increasing temperature, and on porous carbons to measure ion-transport resistances. The influence of ion and polymer concentration on the phase transition temperature will be discussed, along with the extent to which these properties can changed with temperature.

Li-ion batteries were fabricated using two electrode configurations, each including a high temperature stable separator (Dreamweaver, International) soaked in the responsive electrolytes. In one setup, lithium titanate (LTO) anodes were used with lithium iron phosphate (LFP) cathodes due to the stability of each material in various ionic liquids. With this cell, we investigated key battery characteristics as a function of temperature using EIS and constant current charge-discharge measurements. With properly formulated electrolytes, we showed that battery operation was inhibited above the phase transition temperature of the electrolyte, which fell in the range of 120-150 °C. Figure 1 shows the discharge characteristics of the LTO-LFP cell at a 4C discharge rate at 60 °C and at 150 °C. Above the phase separation temperature, the internal resistance of the cell increases leading to a rapid decrease in potential as the cell tries to deliver the specified current. Open circuit voltage measurements confirm the change in internal resistance with temperature.

In another cell configuration, we investigate the performance of responsive electrolytes with lithium cobalt oxide (LiCoO₂) cathodes and graphitic carbon anodes. These batteries represent state-of-the-art cells with high operating voltages and high power capabilities. The performance and stability of responsive electrolytes in these systems will be discussed and compared to LTO-LFP systems, and conventional batteries comprising electrolytes with lithium salt in propylene carbonate.

Electrolytes with temperature-responsive polymers provide a new approach to achieve thermal stability in Li-ion batteries. Results presented in this work will provide the foundation for the development of advanced materials that enable large-format and safe lithium batteries capable of delivering high-power.