Coupled Mechanical and Electrochemical Phenomena in Lithium-Ion Batteries

Lithium-ion batteries are complex electro-chemo-mechanical systems owing to a number of coupled mechanical and electrochemical phenomena that occur during operation. In this thesis we explore these phenomena in the context of battery degradation, monitoring/diagnostics, and their application to novel energy systems. We begin by establishing the importance of bulk stress in lithium-ion batteries through the presentation of a two-year exploratory aging study which shows that bulk mechanical stress can significantly accelerate capacity fade. We then investigate the origins of this coupling between stress and performance by investigating the effects of stress in idealized systems. Mechanical stress is found to increase internal battery resistance through separator deformation, which we model by considering how deformation affects certain transport properties. When this deformation occurs in a spatially heterogeneous manner, local hot spots form, which accelerate aging and in some cases lead to local lithium plating. Because of the importance of separator deformation with respect to mechanically-coupled aging, we characterize the mechanical properties of battery separators in detail. We also demonstrate that the stress state of a lithium-ion battery cell can be used to measure the cell's state of health (SOH) and state of charge (SOC)--important operating parameters that are traditionally difficult to measure outside of a laboratory setting. The SOH is shown to be related to irreversible expansion that occurs with degradation and the SOC to the reversible strains characteristic of the cell's electrode materials. The expansion characteristics and mechanical properties of the constituent cell materials are characterized, and a phenomenological model for the relationship between stress and SOH/SOC is developed. This work forms the basis for the development of on-board monitoring of SOH/SOC based on mechanical measurements. Finally we study the coupling between mechanical stress and voltage in lithium-ion batteries. While the voltage changes at typical levels of stress are relatively insignificant from the standpoint of battery performance, we show that this piezoelectrochemical phenomenon is well-suited for certain mechanical energy harvesting applications. We demonstrate the working principle for mechanical energy harvesting and explore the potential of this technology

Ion transport restriction in mechanically strained separator membranes

h i g h l i g h t s < We model and measure the resistance increase associated with separator deformation. < We show a Bruggeman relation can model tortuosity changes in deformed separators. < We measure the a and g Bruggeman parameters for monolayer separator membranes. < We measure in situ the impedance changes in a pouch cell under applied compression. a r t i c l e i n f o t r a c t We use AC impedance methods to investigate the effect of mechanical deformation on ion transport in commercial separator membranes and lithium-ion cells as a whole. A Bruggeman type power law relationship is found to provide an accurate correlation between porosity and tortuosity of deformed separators, which allows the impedance of a separator membrane to be predicted as a function of deformation. By using mechanical compression to vary the porosity of the separator membranes during impedance measurements it is possible to determine both the a and g parameters from the modified Bruggeman relation for individual separator membranes. From impedance testing of compressed pouch cells it is found that separator deformation accounts for the majority of the transport restrictions arising from compressive stress in a lithium-ion cell. Finally, a charge state dependent increase in the impedance associated with charge transfer is observed with increasing cell compression

Strain Derivatives for Practical Charge Rate Characterization of Lithium Ion Electrodes

During battery use, electrode materials are known to expand and contract in repeatable patterns, and this strain has been previously correlated with battery properties such as state of charge and state of health. In this study, we show that the second derivative of strain, d2ε/dQ2, is mathematically proportional to dV/dQ within an electrode stage. We also experimentally quantify peaks in the strain curves for electrode stage transitions at practical charge rates of up to C/2 and confirm that transitions are visible in the practical scenario of discharging at the higher rate of 1C. Moreover, the location of the transition measured by d2ε/dQ2 changes by less than 10% from 0.05 C to 0.5 C, but the transition measured with dV/dQ decreases by more than 15% from 0.05 C to 0.3 C, demonstrating the reliability of strain to measure electrode transitions at moderate charge rates. We also note that d2ε/dQ2 exhibits similar peak shifts as those expected in dV/dQ as the cell ages. Our derivations for the model system of graphite and lithium cobalt oxide can be generalized to other battery systems and used to characterize materials at practical charge rates impossible with only voltage