Roles Of Binder And Separator Mechanical Behaviors In The Lithium-ion Battery Ageing And Safety

Because of their superior gravimetric and volumetric capacities, lithium-ion secondary batteries (Li-ion batteries) are increasingly employed in systems such as mobile electronics, alternative energy storage, and plug-in hybrid electric vehicles (PHEV)/all-electric vehicles (EV). Especially for PHEV and EV applications, much higher energy/power density, longer shelf/cycle life, and greater reliability are essential. To achieve such performance, intense research has been focused on the electrochemically active cell materials: positive and negative electrode active materials (AM) and electrolytes. In contrast, less attention has been paid to binders and separators, the electrochemically inactive materials, although for the former the electrochemical performance of batteries such as specific capacity and cycle life cannot be achieved if the adhesion strengths between electrode particles and between electrodes film and current collectors are insufficient to endure charge-discharge cycling, while for the latter any pore closure or rupture/penetration could lead to cell performance degradation or catastrophic consequences such as explosion and thermal runway. Therefore, in this thesis the mechanical behaviors of binders and separators were characterized to unveil the mechanical mechanisms that contribute to the ageing and reliability in Li-ion batteries.First, the role of binders in the mechanical integrity of electrodes for Li-ion batteries was studied. Based on the morphological analysis of the anode electrodes from aged Li-ion batteries, the strength of the binder was redefined as the micro-scale carbon particle/particle cohesion strength and macro-scale the electrode-film/copper-current-collector adhesion strength. Accordingly, a comprehensive evaluation method of the above micro-macro strength based on microscratch was proposed. This method coupled with microindentation and digital image correlation (DIC) techniques were used to study the mechanical properties of anode electrodes with different polyvinylidene fluoride (PVDF) binder loading. The dependences of microscratch coefficient of friction and the critical delamination load on the polyvinylidene fluoride (PVDF) binder content suggest that the strength of different interfaces is ranked as follows: Cu/PVDF<carbon-particle/PVDF<PVDF/PVDF. The electrolyte soaking-and-drying process leads to an increase in particle/particle cohesion but a decrease in electrode-film/Cu current-collector adhesion.The role of separator deformation in response to external mechanical stimuli in electrochemical performance of batteries was then studied. Based on the analysis of stress generated in separator due to lithium insertion/deinsertion induced electrode expansion and storage conditions, the process of pore closure in separators resulted from external stress was determined by Carroll-Holt pore-collapse relation model, which was then varificated by compression testing. Electrochemical characterization of mechanically stressed separators was also performed. We find the pore closure in the electrochemically inactive separator to be a cause of battery capacity fade.In addition to the external mechanical stress induced deformation, the mechanical behavior of polymeric separators in Li-ion batteries at elevated temperatures was also characterized by in-situ high temperature surface imaging using an atomic force microscope (AFM) coupled with power spectral density (PSD) analysis and DIC technique. The temperature dependent micro-scale morphology change of PP (polypropylene)-PE (polyethylene)-PP sandwiched separators (Celgard2325) was found. Both PSD and DIC analysis results show that the PP phase significantly closes its pores by means of dilation of the nanofibrils surrounding the pores in the transverse direction and shrinkage in the machine direction, when cycled at90°C, even below the separator’s shutdown temperature (120°C) and its own melting temperature (165°C).Besides the important role of separator in the battery ageing, the reliability of the separator is also crucial to the abuse tolerance of a battery. Therefore, deformation and fracture behaviors of five commercially available wet and dry processed polymer separators were investigated by conventional tensile testing coupled with in-situ tensile testing under an AFM. The evolution mechanism between the separator tensile properties and its microstructures was explained. For separators made by the dry process, material direction dictates the significant diversity of overall mechanical integrity of the separator, which is a result of the distinct deformation mechanism of the stacked lamellea in the separator. Moreover, in order to evaluate the fracture properties of these separators, the essential work of fracture (EWF) approach was adopted. The EWF results show that the fracture properties for the dry processed separators also present orientation dependence, which is then found to be results of different toughening mechanisms.The remainder of this dissertation focuses on the effects of tensile-penetration coupled stress on the penetration strength of lithium-ion battery separators. Because the pre-tensile stress has the potential to dramatically decrease the penetration strength of the separator, we thus proposed a new method that is able to measure the penetration strength of a battery separator under the pre-stressed condition. Moreover, the process and mechanism of the penetration into the separator under coupled stress were also elucidated.