In-Situ Stress Evolution in Thin Film Electrodes during Electrochemical Cycling in Li-Batteries
Authors: Karan Naba , Sheth Jay , Abraham Daniel P , Sheldon Brian W. , Guduru Pradeep R. 
Published: February 2020

Electrochemical cycling induced mechanical damage of electrode materials actively contributes towards the performance degradation of lithium-ion batteries. The correlation between mechanical damage and performance degradation in anode materials that show large volume changes, such as silicon, graphite and tin, has been amply demonstrated. On the hand, typical oxide electrodes undergo only a few % volume changes, and the non-reversible nature of the crystal structure evolution as a function of lithium concentration in such electrodes is, in general, believed to be the limiting factor for performance degradation. For example, cycling in LixCoO2 is generally limited to 0≤x≤0.5, mainly, due to the irreversibility associated with the crystal structure changes beyond further Li extraction/re-insertion. However, due to their brittle nature only a few % volume changes can have significant implication on the mechanical damage leading to performance degradation for such ceramic oxide electrodes. Thus, the issue of electrochemical cycling induced mechanical degradation in oxide electrodes is being actively explored in recent years [1-2].
In this work, we present in-situ stress evolution of i) two canonical cathode systems, namely layered LiCoO2 and spinel LiMn2O4 and ii) one conversion electrode system, Co3O4 in thin film configuration to quantify the driving force leading to the mechanical degradation. In-situ stress evolution in thin film electrodes was measured by monitoring the change in the elastic substrate curvature during electrochemical cycling in a suitably designed beaker cell using multiple-beam optical sensing (MOS) method. Thin films of the electrodes were prepared using solution deposition technique. Structural characterizations using XRD and Raman spectroscopy showed predominant presence of desired (poly)crystalline phases in the as prepared samples. In addition, SEM images also revealed the presence of dense microstructural features in the as prepared films. During Li-extraction from layered LixCoO2, there was almost linear increase in compressive stress up to ~50% Li removal, which is consistent with its lattice parameter evolution during Li removal [3], and a maximum compressive stress of ~0.35 GPa was observed for x~0.5. Upon lithiation there was almost reversible stress evolution in LixCoO2. Similar behavior was also observed for subsequent cycles as well, while limiting the upper charging cut-off voltage to 4.3V. On the other hand, initial delithiation from spinel LixMn2O4 induces tensile stress up to ~4.1V, beyond which the induced stress reverses direction (termed as “compressive drop”) with further delithiation (up to 4.3 V). This reversal of stress evolution in the later stages of delithiation from spinel LixMn2O4 is in apparent contradiction with the lattice parameter evolution of spinel LixMn2O4 during lithium extraction [4]. Upon lithium re-insertion (up to 3.5V), induced compressive stress increases linearly. The subsequent cycles (in the 4V region), however, did not show any “compressive drop” during later stages of delithiation and the induced stress evolved reversibly during delithiation-lithiation. The origin of this first cycle “compressive drop” in spinel LiMn2O4 is not known at present. In an attempt to establish the origin of the observed first cycle “compressive drop” in spinel LiMn2O4 thin films, stress measurement data varying multiple parameters including cathode film thickness, reannealing a cycled electrode will be presented. The effect of stress evolution in these thin film electrodes during cycling as a function of cycling voltage window and current density will also be presented and discussed in the light of their crystal structural changes.

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