Effect of Mechanical Stress on Electrochemical Properties of Li Ion Batteries Cathodes

1. Introduction

Recently, all-solid state Li ion batteries are widely investigated to establish more reliable and flexible battery systems [1, 2]. Compared to conventional batteries with liquid electrolytes, all-solid-state batteries can be suffered from undesired strains induced at hetrointerfaces between solids due to differences of crystal structures and chemical/thermal expansion coefficients. Such a strain may cause performance improvement/deterioration or failure of all-solid-state batteries. For example, it was reported that charge and discharge properties of an Al electrode is affected by the strain [3]. Therefore, it is important to understand the effect of strain on the electrochemical properties for the design and development of all-solid-state batteries. The purpose of this study is to reveal the effect of strain on electrochemical behaviors of cathode materials for all-solid-state Li batteries. For this aim, dense thin film LiCoO₂ (LCO) electrodes were fabricated on an Li ion conducting glass ceramics (LICGC) by the PLD method, and the variation of the chemical potential of Li in the electrode due to external strain was evaluated by measuring electromotive force (EMF) between compressively and tensely strained LCO electrodes.

2. Experimental

   A schematic illustration of the specimen is shown in Fig.1. Dense thin film LCO electrodes were fabricated on both surfaces of an Li ion conducting glass ceramics LICGC (OHARA INC.) by the PLD method at 873 K with P(O₂) of 20 Pa. Then, Al films as counter electrodes were also deposited next to the LCO electrodes by Ar ion sputtering at room temperature in Ar atmosphere of about 0.67 Pa. The Li contents of the LCO dense films were controlled by the electrochemical de-lithiation and estimated from the amount of the electricity passed through the cell. After the de-lithiation process, the Al electrode side was cut down. Two LCO electrodes were short circuited for about 24 hours to equilibrate the Li chemical potential. After the equilibration, the EMF between two LCO electrodes was measured with applying compressive and tensile strain to the upper and the lower LCO electrodes, respectively, by the four points bending method as shown in Fig.1.

3. Results and Discussion

   The obtained LCO electrode was oriented in the direction of (003) along the out of plane direction, i.e., the Li conductive plane was almost parallel to the substrate. Fig.2 shows the result of OCV measurements under different strain states. The EMF was generated when the strain was applied to the specimen. The compressively strained LCO electrode was negatively charged while the tensely strained LCO electrode was positively charged. The EMF emerged immediately after the application of the strain, was kept at an almost constant value while keeping the constant strain state, and became nearly zero after the release of the strain. The observed EMF value increased as the applied strain increased. Since the generation of EMF represents the difference of Li chemical potential between the compressively and tensely strained LCO electrodes, it is considered that the structural deformation due to the external strain induces the Li chemical potential variation. In the presentation, we will discuss the origin of the EMF from the thermodynamical points of views.

References

[1] V.A.Sethuraman, et al., J. Power Sources, 206 (2012) 334-342.

[2] K.H. Kim, et al., J. Power Sources, 196 (2011) 764-767.

[3] T. Ichitsubo, et al., J. Electrochem. Soc, 159(1) (2012) A14-A17.