Mechanics of Battery Cells and Materials

Xiangchun Zhang, Myoangdo Chung, HyonCheol Kim, Chia-Wei Wang, and Ann Marie Sastry 

Mechanical Failure Analysis of Battery Cells and Materials Significance and Challenges 

The combination of the multiple pertinent physics in the battery cell, incorporating the interplay of manufacturing- and service-induced loads and failure mechanisms, will surely inform advanced modeling. Of the commonly recognized battery performance degradation mechanisms, namely electrolyte decomposition, active material dissolution, passive layer formation, lithium deposition, and mechanical failure of battery materials, mechanical failure is the least understood at present, although there is ample evidence of the importance of these effects, and they are the subject of this chapter. 

Mechanical failure of battery materials has been putatively linked to battery performance degradation over time [1, 2]. It has been postulated that mechanical failure of battery active materials (i) increases surface area of active materials subjected to side reactions and consumes active material (which causes capacity loss), (ii) results in loss of electric contact between the active material particles and between active materials and current collectors [3, 4], increasing the internal impedance for charge transfer, and (iii) potentially redistributes active material particles and decreases volume fraction of electrolyte, making the hthium ion (Li-ion) transport become electrolyte phase limited [5-9]. Mechanical failure of 

Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and Jurgen O. Besenhard. 

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Research efforts in li-ion batteries have thus been widely distributed over the multiple intrinsic length scales, as illustrated in Figure 26.3. Although new materials syntheses and battery performance tests have been successively employed over the last three decades, recent important findings have come from particle-/micro-scale modeling and experimental studies, which investigate how the mechanics of battery cells and materials affect cell performance during the cycle Hfe, as described in Section 26.2.2. 

Multiple length scales and time scales involved in electrochemical and mechanical phenomena seriously complicate the analysis of battery cells and materials. Special care has to be taken to devise a framework to incorporate the disparate length and time scales in the modeUng of Li-ion batteries. 

Recovery of metals such as copper, the operation of batteries (cells) in portable electronic equipment, the reprocessing of fission products in the nuclear power industry and a very wide range of gas-phase processes catalysed by condensed phase materials are applied chemical processes, other than PTC, in which chemical reactions are coupled to mass transport within phases, or across phase boundaries. Their mechanistic investigation requires special techniques, instrumentation and skills covered here in Chapter 5, but not usually encountered in undergraduate chemistry degrees. Electrochemistry generally involves reactions at phase boundaries, so there are connections here between Chapter 5 (Reaction kinetics in multiphase systems) and Chapter 6 (Electrochemical methods of investigating reaction mechanisms). 

Solid state materials that can conduct electricity, are electrochemically of interest with a view to (a) the conduction mechanism, (b) the properties of the electrical double layer inside a solid electrolyte or semiconductor, adjacent to an interface with a metallic conductor or a liquid electrolyte, (c) charge-transfer processes at such interfaces, (d) their possible application in systems of practical interest, e.g. batteries, fuel cells, electrolysis cells, and (e) improvement of their operation in these applications by modifications of the electrode surface, etc. 

The proposition of a one-electron mechanism for the electron-transfer reduction of dioxygen and the associated conclusions present significant ramifications relative to the development of improved fuel cells and metal-air batteries. To date the practical forms of such systems have used strongly acidic or basic electrolytes. Such solution conditions normally cause atom transfer to be the dominant reduction process for molecular oxygen at metal electrodes. Hence, the search for effective catalytic materials should be in this context rather than in terms of a one-electron-transfer process. 

Current collector — In the battery discipline, a good electron conductor support designed to transfer electrons from the external circuit to the active materials of the cell. Current collectors are usually metal foils or nets that are inert under the operational chemical and electrochemical conditions. In some cases carbon cloth is also used. In secondary - lead-acid batteries the chemical nature of the current collectors (plates, grids) is particularly imperative, as it influences the self-discharge and the performance under overcharge and discharge conditions. Frequently, current collectors have also the important role of imparting mechanical stability to the electrodes. 

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