In-Situ Electrochemical Studies of Li-Ion Start-Stop Vehicle Batteries through a Novel Cell Design

Lithium ion batteries are now used extensively for vehicle traction applications. Because of their high specific power and long usage life, lithium ion batteries are very appealing for power assist vehicular applications. Implementation of Li-ion batteries for start-stop systems is one of the pragmatic approaches being considered for reducing vehicle fuel consumption. During start-stop operation, the engine automatically shuts down when the vehicle comes to stop and then quickly restarts when propulsion is desired by the operator. Such start-stop operations of the vehicle can effectively reduce the amount of engine idling time, leading to fuel savings.

One of the main challenges for start-stop applications is meeting the cold-cranking power requirements at -30^oC. Due to the kinetic limitations, pulse power performance of Li-ion batteries drop dramatically at low temperatures. Given the requirement for starting the engine at low temperature, it is typical that the cold cranking power capability determines the module size needed for start-stop systems; and therefore directly impacts the system weight and cost. It is essential to understand the source of the kinetic limitations (e.g. electrolyte resistance, electrode-electrolyte interfacial resistance, or diffusion resistance) and design a system that can dramatically improve the cold cranking power.

In this talk, we demonstrate the implementation of a new cell design to investigate the pulse power performance of LiMn₂O₄ (LMO) / Li₄Ti₅O₁₂ (LTO) batteries for start-stop applications. Figure 1 shows the schematic of the cell design; it is composed of four electrodes, with two LTO and two LMO electrodes adjacent to each other. The current collectors are porous, allowing the ions to pass through them. This type of design enables the in-situ investigation of LTO/LTO, LTO/LMO, LMO/LMO cells in a single cell while the neighboring electrodes are used for voltage monitoring. In this report, our focus is to investigate the kinetics of each individual electrode under low temperatures. Using EIS and linear voltage polarization studies, we provide insight on the source of resistance limiting the pulse power capability of the cell at low temperatures (-30^oC).  We found that electrolyte transport limitations control the cell performance at low temperatures, which directly impacts the cold cranking power.