Impact of Non-Arrhenius Temperature Behavior on the Fast-Charging Capabilities of LiCoO2–Graphite Lithium-Ion Batteries

Abstract

Fast-charging lithium-ion batteries in 15 min or less is an important capability that may lead to greater electric vehicle adoption but remains challenging to implement. Heating to moderate temperature (40–50 °C) during the fast-charge step has been introduced as one method to mitigate the loss of lithium inventory by enhancing transport and kinetics. Unfortunately, this edge has two sides as even moderate temperature elevation will accelerate capacity fade due to interface and electrolyte degradation. While the thermal enhancement of transport and degradation is intuitive, the mechanistic effects of various temperature ranges on fast-charging capabilities are under-reported. The present work examines the balance between aging, temperature, and charge rate and describes cycling protocols in combination with high-temperature ranges that may enable fast-charging capabilities. A galvanostatic intermittent titration technique (GITT) analysis reveals non-Arrhenius diffusion behavior at the cell level. This shift is attributed to a mechanistic difference in graphite staging at temperatures above and below 40 °C. Coupled with differential capacity and voltage analysis, we indicate the specific phase transition that is kinetically sluggish at low temperatures relative to the other phase transitions but is comparable to the other phase transitions at high temperatures. This reduction of the transport bottleneck, in addition to the benefits of thermal activation of diffusion, further minimizes the likelihood of lithium plating that is triggered by particle scale transport challenges well before full lithiation of the graphite. This helps to explain recently described outsized successes of elevated temperature fast-charge protocols, but also questions the temperature at which fast-charge should take place, as the diffusivity gains for 55 vs 45 °C are less dramatic than 45 vs 35 °C, and side reactions may deter operation above 50 °C. These diffusivity studies are connected with long-term aging studies which indicate improved high-temperature aging at lower states-of-charge rather than higher states-of-charge. Taken together, we introduce a cycling protocol utilizing a constant current fast-charge at high temperature to take advantage of lower overpotentials, shorter duration at high states-of-charge, and improved cell diffusivity.