Mechanical Testing of Battery Electrodes for Applications in Flexible Lithium Ion Batteries
Authors: Gaikwad Abhinav Machhindra , Arias Ana Claudia 
Published: February 2020

Over the past couple of years there has been a tremendous interest in flexible sensors for the purpose of activity tracking and health monitoring purposes. With the helps of novel designs, these devices can conform to the curvature of the human body, which helps improve the signal to noise ratio, and the comfort of the user. To power such devices, the battery should be flexible, and retain its capacity after multiple flexing cycling. Batteries based on lithium ion chemistry are an ideal choice for flexible batteries due to its high energy density and stable electrochemical performance.

Flexible battery consists of an anode and cathode, which are separated by a thin polymeric separator. Repeated bending can lead to swelling and delamination of the active layers. Currently, there are no standard protocols to test the mechanical strength of battery electrodes. To prevent sudden failures in flexible batteries, the thickness of the electrodes are limited to 20-40 micron to reduce the maximum stresses the electrode would experience during flexing. Reducing the electrode thickness decrease the energy density of the battery.

In flexible lithium ion batteries, the relation between the electrode composition and its mechanical flexibility is poorly understood. Factors such as shape and size of the particles, and choice of binder plays an important role in defining the mechanical characteristics of the electrodes. By understanding the link between different variables in the ink composition to the mechanical property of the electrodes, mechanically resilient electrodes can be designed.

In this talk, I will present two mechanical testing methodologies to quantify the strength of battery electrodes. The first method is based on combining conventional tape test with an INSTRON. Based on the force required to peel the active layers, and the resulting failure mode, the mechanical strength of electrodes with different compositions can be compared and the mode of failure during flexing can be predicted. The second testing procedure is based on dragging a micro-tip through the active layers. By measuring the force required to remove the active layer and the mode of cracking, the mechanical strength of electrodes can be compared. We study the effect of tip size, drag speed and tip height on the force required to drag a tip through the electrode.

We study the effects of conductive additives, particle size and binder choice using the tape and drag test. The mechanical strength of cathode increases by a factor of two using a combination of graphite and anode as the conductive additive. The larger size of graphite particles improves the mechanical strength of the electrode and the carbon particle occupies the void spaces between the graphite particles, which improve the electronic conductivity of the electrode. The electrodes with small size particles have poor cohesion strength due to larger exposed surface area, which reduces the effective thickness of the binder on the particles. The adhesion between the graphite anode and the foil increase when the PVDF binder is replaced with PSBR binder. Based on the result of the mechanical testing we were able to modify the ink composition and fabricate flexible lithium ion battery with improved electro-mechanical performance.