In previous ageing studies in which the stack stress of commercially available lithium-ion (LCO/C) pouch cells is monitored during cycling [1], we observe that stack stress (Figs 1a and 2a) is linearly related to cell SOH (Figs 1b and 2b) as shown in Figs 1c and 2c [2]. The increase in stack stress is attributed to irreversible volumetric expansion of the anode which we assume to be related to SOH. This suggests the possibility of using stress to measure SOH, a method whose simplicity provides a distinct advantage to conventional computationally intense methods which rely on complex physical models of the underlying cells. Stack stress could be used as a standalone SOH monitoring system, or integrated into existing battery management systems to increase management fidelity. In this talk we investigate the origins of the stress-SOH relationship. We report the following data from ageing studies of cells cycled under different conditions: temporal stack stress evolution, capacity evolution, non-destructive differential voltage spectroscopy analysis, and destructive post-mortem analysis. All of the temporal stress and capacity data show a t 1/2 dependence suggestive of a diffusion limited mechanism such as SEI growth, as seen in Figs. 1d and 2d. This assertion is corroborated by the differential voltage spectroscopy measurements and destructive post mortem analysis, which also show evidence of a SEI growth mechanism. Stack stress is also observed to increase in cells held at 4.2V in the absence of cycling as shown in Fig. 2a, supporting the notion of a chemical mechanism. Based on the assumptions of an SEI growth mechanism, we present simple scaling arguments which predict a linear relationship between stack stress and SOH. We discuss the implications for this method and model as it applies to battery management systems and fundamental ageing studies. REFERENCES [1]. J. Cannarella, C. B. Arnold, “Stress evolution and capacity fade in constrained lithium-ion pouch cells,” J. Power Sources 245 (2014) 745-751. [2]. J. Cannarella, C. B. Arnold, "State of health and charge measurements in lithium-ion batteries using mechanical stress," submitted. FIGURE CAPTIONS Figure 1 . Data from a cell aged by cycling under a C/2 CCCV scheme showing (a) stack stress as a function of cycle, (b) C/10 capacity as a function of accumulated cycles, (c) stack stress as a function of SOH, and (d) t 1/2 dependence of the C/10 capacity data. Figure 2 . Data from a cell aged by holding at 4.2V at room temperature and periodic cycling at C/10 to measure capacity showing (a) stack stress as a function of time, (b) C/10 capacity as a function of time, (c) stack stress as a function of SOH, and (d) t 1/2 dependence of the C/10 capacity data. ACKNOWLEDGEMENTS Support was provided by the DoD through the NDSEG Program and by the Siebel Energy Challenge. J. C. also acknowledges the Rutgers-Princeton IGERT in Nanotechnology for Clean Energy.
The piezoelectrochemical coupling between mechanical stress and electrochemical potential is explored in the context of mechanical energy harvesting and shown to have promise in developing high-energy-density harvesters for low-frequency applications (e.g., human locomotion). This novel concept is demonstrated experimentally by cyclically compressing an off-the-shelf lithium-ion battery and measuring the generated electric power output.
Knowledge of how stack-level mechanical stress affects cell aging is important for the long term performance of practical li-ion cells which spend their entire service lives under compressive stress. This compressive stress is set by an initial manufacturing stack pressure, fluctuates with electrode expansion/contraction during charge/discharge, and increases over time due to irreversible electrode expansion. In a previous aging study in which we monitored the stack stress of constrained li-ion cells during cycling, we observed that higher levels of stack stress led to higher rates of capacity fade [1]. Through a post mortem analysis, we determined that the primary source of capacity fade was cycleable lithium loss, such that higher levels of stack stress led to an increase in parasitic side reactions. This degradation occurred in a spatially non-uniform manner such that localized regions of the cell’s graphite anode were covered with a surface film. We also observed similar distributions of localized pore closure in the separator leading us to hypothesize that stack stress and chemical degradation are coupled through separator deformation [2-3]. In this presentation we present a follow up study to the aforementioned aging study in which we present experimental evidence for the coupling of stack stress and chemical degradation through separator deformation. To isolate the effects of separator deformation on cell aging, coin cells are fabricated using conventional graphite and lithium cobalt oxide electrodes containing deformed separators. The deformed separators are deformed by applying high localized stresses to create controlled macroscopic patterns of pore closure as can be seen visually in Figure 1. Cells cycled with deformed separators show higher rates of capacity fade than cells cycled with pristine separators as seen from the capacity plot in Figure 2. Upon disassembly, localized visible surface films similar to those observed in [1] are found to be present on the graphite anode. We explain the observed surface film patterns by considering the relative rates of lateral transport that occur in the presence of separator pore closure which restricts normal transport through the separator membrane. Because the electrochemical potential of lithium in lithium cobalt oxide varies more strongly with concentration than it does in graphite, lateral ion transport is enhanced in the cathode. However, sluggish lateral transport in the anode results in high overpotentials which can result in temporary lithium plating and consequential chemical degradation. The proposed explanation is supported by three electrode measurements of a full cell in which the graphite electrode exhibits a negative potential vs. Li/Li+ for cells constructed with deformed separators as seen in Figure 3. We anticipate these issues to become increasingly important in next generation lithium-ion cells which are expected to make use of higher expansion electrode materials. REFERENCES [1]. J. Cannarella, C. B. Arnold, “Stress evolution and capacity fade in constrained lithium-ion pouch cells,” J. Power Sources 245 (2014) 745-751. [2]. J. Cannarella, C. B. Arnold, “Ion transport restriction in mechanically strained separator membranes,” J. Power Sources 226 ,(2013) 149-155. [3]. C. Peabody, C. B. Arnold, “The role of mechanically-induced separator creep in lithium-ion battery capacity fade,” J. Power Sources, 196 (2011) 8147-8153. ACKNOWLEDGEMENTS Support was provided by the DoD through the NDSEG Program and by the Siebel Energy Challenge. J. C. also acknowledges the Rutgers-Princeton IGERT in Nanotechnology for Clean Energy. FIGURE CAPTIONS Figure 1. Photograph of a separator that has been deformed locally in a ring-shaped pattern. Figure 2. Capacity evolution of a cells constructed with a deformed and pristine separator. Figure 3. Three electrode measurements of a full cell cycled with a deformed separator showing anode voltage dropping below 0V vs. Li/Li+.
During charge and discharge of a lithium battery, intercalation of lithium ions into the electrodes can cause their noticeable expansion, compressing the soft separator between them. To assess the role of these effects on the battery performance, it is necessary to know the response of the battery separator under compressive loading. Here we develop a model for predicting the elastic response of a commercial separator immersed in fluid to compression at different strain rates. We show that the response of the separator is determined by combination of viscoelastic behavior of the polymer skeleton and poroelastic behavior, due to the flow of the fluid in the pores. Poroelastic behavior causes effective stiffening of the separator, which increases with the strain rate. For a sample of ca. 1 cm in diameter these effects become pronounced at strain rates ≳ 10− 3 s− 1 and have to be taken into account in coupled mechano-electrochemical models for lithium-ion batteries.
Micro-power generation is an area developing to support autonomous and battery-free wireless sensor networks and miniature electronic devices. Electromagnetic power harvesting is one of the main techniques for micro-power generation and it uses the relative motion between wire coils and miniature magnets to convert mechanical energy to electricity according to Faraday's law of induction. Crucial for the design and analysis of these power systems is the electromechanical coupling factor K , which describes the coupling between the mechanical and electromagnetic energy domains. In current literature K is defined as NBl : the product between the number of turns in the coil ( N ), the average magnetic induction field ( B ), and the length of a single coil turn ( l ) . This paper examines the validity of the current K definition and presents two case studies involving cylindrical permanent magnets and circular coil geometries to demonstrate its limitations. The case studies employ a numerical method for calculating K which uses the toroidal harmonics technique to determine the magnetic induction field in the vicinity of the cylindrical magnet.
Processes causing battery aging can be both of electrochemical and mechanical nature. The studies of the latter are usually limited to the consideration of the processes in electrodes, particularly on the swelling of the electrodes due to lithium intercalation. However, it has been shown recently that the compression of the separator during the battery operation has significant impact on its performance [1, 2]. In order to understand the mechanical behavior of the separator during the battery operation, it is necessary to study its response to both static and dynamic loading when it is immersed in fluid. Recently Sheidaei et al. [3] and then Avdeev et al. [4] performed the tensile tests of commercial separators showing the difference in the mechanical properties for dry and wet conditions. However, the tensile testing does not represent the loading conditions in a battery. In the battery the loads on a separator are compressive both due to electrode swelling upon lithium intercalation or stacking loads [5]. In the current work we present the results of compressive tests of commercial macroporous polypropylene (PP) separator Celgard 3501 at different strain rates in dry and wet conditions. The experiments were carried out with three different fluids: water, methanol and dimethyl carbonate (DMC) – a common solvent in electrolyte for lithium-ion batteries. Here we focus on the region of small deformation (before yield), and calculate the effective (or ‘apparent’), Young’s modulus, which depends on the strain rate. The behavior of apparent Young’s modulus in our compression tests is qualitatively different from that of the tensile tests [3, 4]. Particularly we observed the noticeable stiffening of the wet sample at high strain rates (Figure 1). This effect cannot be explained by viscoelastic properties of the polymer, and should be attributed to poroelastic behavior – fluid drainage from the pores [6]. We performed simulations using Dynaflow finite element code [7] and calculated the apparent Young’s modulus corresponding to the experimental conditions. The calculations were done within the linear elastic model for the solid matrix, and gave qualitative agreement between the predicted and experimentally observed moduli (inset in Figure 1). Our current research is focused on including the viscoelasticity in the model in order to achieve the quantitative agreement. The difference between our results for the apparent Young’s modulus and results derived from the tensile tests [3, 4] shows that the mechanical properties from the tensile tests cannot be employed for modeling the separator in a battery, where the compressive loads take place. The results of our work will be further used in the development of predictive model for studying the coupled mechanics and electrochemistry of lithium-ion batteries. Such model can serve as a tool for optimization the materials and geometric parameters of lithium-ion cells to find a path towards extending their lifecycle. REFERENCES: [1] C. T. Love, J. Power Sources 196 (2011) 2905-2912 [2] C. Peabody, C. B. Arnold, J. Power Sources 196 (2011) 8147-8153 [3] A. Sheidaei et al. J. Power Sources 196 (2011) 8728-8734 [4] I. Avdeev et al. J. Mater. Eng. Perform. (2014) DOI: 10.1007/s11665-013-0743-4 [5] J. Cannarella, C. B. Arnold J. Power Sources 245 (2014) 745-751 [6] O. Coussy, Poromechanics, Wiley, 2004 [7] J. H. Prevost, Dynaflow, A Nonlinear Transient Finite Element Analysis Program, 1981 http://www.princeton.edu/~dynaflow/ ACKNOWLEDGEMENTS: G.G. would like to thank George Scherer for insightful discussions on poroelasticity. This work was supported by Addy Fund from the Andlinger Center for Energy and the Environment. Figure 1. Apparent Young’s modulus for the porous polypropylene separator as a function of strain rate (and corresponding charge rate). Inset shows the results of finite element modeling.
