Alternating Current Impedance Electrochemical Modeling of Lithium-Ion Positive Electrodes

521

449

Increasing the areal capacity or electrode thickness in lithium ion batteries is one possible means to increase pack level energy density while simultaneously lowering cost. The physics that limit use of high areal capacity as a function of battery power to energy ratio are poorly understood and thus most currently produced automotive lithium ion cells utilize modest loadings to ensure long life over the vehicle battery operation. Here we show electrolyte transport limits the utilization of the positive electrode at critical C-rates during discharge; whereas, a combination of electrolyte transport and polarization lead to lithium plating in the graphite electrode during charge. Experimental measurements are compared with theoretical predictions based on concentrated solution and porous electrode theories. An analytical expression is derived to provide design criteria for long lived operation based on the physical properties of the electrode and electrolyte. Finally, a guideline is proposed that graphite cells should avoid charge current densities near or above 4 mA/cm 2 unless additional precautions have been made to avoid deleterious side reaction. Lithium-ion (Li-ion) batteries are currently being used as the primary energy storage device in hybrid, plug-in, and all electric vehicles. This commercialization has been possible only by leveraging decades of previous scientific and engineering advances on materials, electrodes, and cell development. However, interactions in this complex system are still not fully understood. Automotive grade battery cells are required to fulfill a variety of optimization criteria in order to meet customer expectations and enable highly functional, robust and competitive products. In Fig. 1, key cell level criteria are shown for available technology as well as future development goals. Many of these values are highly influential on each other. In order to optimize one of the criteria it is always necessary to critically evaluate the impact on others. Key goals are to increase vehicle range and decrease cost at the same time. Minimizing the fraction of non-active material is an intuitive path to achieve these goals; however, the cell power and rate capability must simultaneously be maintained. [1][2][3] To derive clear development goals, the high level targets can be broken down to specific component requirements on the different levels of a storage system. 1,4 In Fig. 2, an analysis is shown for a state of the art prismatic hard-case automotive cell format. A battery level specific energy of ∼225 Wh/kg is widely accepted to be a critical value for sustainable implementation of long range electric vehicles. It represents a useful ratio between vehicle weight and range. In order to achieve this high specific energy, all subcomponents of the storage system have to meet demanding requirements as well. On the cell level, large format cells are favorable as they reduce the amount of cell housing needed per cell volume. Prismatic cell formats have a positive influence on the packing densi...

Section: Theoretical Frameworkmentioning

confidence: 99%

Section: Theoretical Frameworkmentioning

confidence: 99%

Optimizing Areal Capacities through Understanding the Limitations of Lithium-Ion Electrodes

Gallagher

Trask

Bauer

et al. 2015

521

449

“…The effect of various parameters in the model, such as exchange current density, double-layer capacitance, and solid and solution phase diffusion coefficients, are discussed for similar systems by Doyle et al 5 and Dees et al 7 and will not be reiterated here. However, previous works have paid minimal attention to identifying the relative contribution of solution and solid phase processes toward the overall impedance spectrum.…”

Section: Effects Of Solid and Solution Phase Diffusion Limitations Inmentioning

confidence: 99%

“…[5][6][7] Closed-form analytical or symbolic solutions are also presented for such systems but under certain limiting operational and design conditions. Meyers et al 8 presented an analytical solution for the impedance response of a porous intercalation electrode in the absence of solution phase diffusion limitations.…”

mentioning

confidence: 99%

Analytical Expression for the Impedance Response for a Lithium-Ion Cell

Sikha

White

2008

An analytical expression to predict the impedance response of a dual insertion electrode cell (insertion electrodes separated by an ionically conducting membrane) is presented. The expression accounts for the reaction kinetics and double-layer adsorption processes at the electrode-electrolyte interface, transport of electroactive species in the electrolyte phase, and insertion of species in the solid phase of the insertion electrodes. The accuracy of the analytical expression is validated by comparing the impedance response predicted by the expression to the corresponding numerical solution. The analytical expression is used to predict the impedance response of a lithium-ion cell consisting of a porous LiConormalO2 cathode and mesocarbon microbead anode. A qualitative graphical method to identify the co-existence of solid and solution phase transport limitations in the impedance spectra of insertion electrodes is also discussed in the paper.

“…To gain more understanding of the physical processes, macrohomogenous models for porous electrodes have been used by some researchers. [12][13][14][15][16][17][18] These models primarily use porous electrode theory 19,20 to describe the porous nature of the electrode/separator and concentration solution theory to treat the transport processes in the electrolyte phase. The thermodynamics and kinetics of the reactions at the electrode/electrolyte interface are also described in these models in detail.…”

Section: -7mentioning

confidence: 99%

Analytical Expression for the Impedance Response of an Insertion Electrode Cell

Sikha

White

2007

An analytical expression for the impedance response of an insertion cathode/separator/foil anode cell sandwich is presented. The analytical expression includes the impedance contributions from interfacial kinetics, double-layer adsorption, and solution-phase and solid-phase diffusion processes. The validity of the analytical solution is ascertained by comparison with the numerical solution obtained for a LiCoO 2 /polypropylene/lithium metal cell. The flexibility of the analytical solution is utilized to analyze various limiting conditions. An expression to estimate solid-phase diffusion coefficient of insertion species in a porous electrode influenced by the solution-phase diffusion process is also derived. Electrochemical impedance spectroscopy ͑EIS͒ technique has been extensively used in the analysis of lithium battery systems, especially to determine kinetic and transport parameters, 1-3 understand reaction mechanisms, 4 and to study degradation effects. [5][6][7] However, the mathematical interpretation of the impedance response of electrochemical systems is complicated by the processes occurring in the system. This drives researchers to adopt lumped circuit models [8][9][10] or finite transmission line models 11 to interpret impedance data. However, these types of models provide little information on the fundamental physical processes occurring in the cell. To gain more understanding of the physical processes, macrohomogenous models for porous electrodes have been used by some researchers. [12][13][14][15][16][17][18] These models primarily use porous electrode theory 19,20 to describe the porous nature of the electrode/separator and concentration solution theory to treat the transport processes in the electrolyte phase. The thermodynamics and kinetics of the reactions at the electrode/electrolyte interface are also described in these models in detail. Most of these models also account for the solidphase diffusion of the active species into the bulk. While such detailed models throw light on the impedance behavior of systems when complicated by transport and kinetic processes, the mathematical interpretation is not straightforward.In the list of comprehensive models developed to simulate the impedance behavior of lithium batteries, Doyle et al. 13 simulated the impedance response for a metal anode/separator/porous cathode system with all the above-mentioned details. Guo et al. 15 used a similar model to estimate the diffusion coefficient of lithium in carbon. Later, Dees et al. 18 included an electronically insulating oxide layer at the electrode/electrolyte interface to model a LiNi 0.8 Co 0.15 Al 0.05 O 2 cathode. However, these models use a numerical scheme to solve for the variables to obtain the frequency domain impedance spectrum. There are also some analytical models 14,16,17 available in the literature, but they are not as comprehensive as the numerical models. Meyers et al.14 presented an analytical solution to the impedance response of a porous electrode in the absence of solution-phase concentr...