Rechargeable Battery Electrodes having Optimized Particle Morphology

A battery, comprising an anode and a cathode, the cathode comprising particles having an aspect ratio between 0.09 and 0.5. In certain embodiments, the particles are ellipsoidal. The battery may comprise an electro chemical lithium-ion battery structure. Optimal particle microstructure morphologies are identified to deliver high power or energy densities.

TECHNICAL FIELD

The present application relates to electrochemical batteries, and more specifically, to electrodes for rechargeable electrochemical batteries.

BACKGROUND

In the pursuit of optimizing the reliability and performance in porous rechargeable batteries, it is desirable to manufacture slurries that possess a large reactive area density while simultaneously providing as little ohmic resistance as possible to passing charge-carrying ions. Traditionally, higher energy densities are achieved through the maximization of the amount (or volume fraction) of active material that the electrode can store—a property that is specified by compacting the fabricated electrode layer—leading to an in-plane alignment of the particles, and thus specifying the microstructural properties of the battery. Resultant microstructural properties of the battery include the reactive area density of the particles, which specifies the local power density at a given instant. Conversely, the delivered power density per unit mass is limited by the polarization losses imposed by the tortuosity of the electrode. An increasingly tortuous microstructure suppresses lithium diffusion and resists the access of useful charge to the active material. In both cases, the starting particle morphology, its volume fraction, alignment, and spatial distribution during battery fabrication, specifies the performance and lifetime reliability.

The attached drawings are for purposes of illustration and are not necessarily to scale.

DETAILED DESCRIPTION

In the following description, some aspects will be described in terms that would ordinarily be implemented as software programs. Those skilled in the art will readily recognize that the equivalent of such software can also be constructed in hardware, firmware, or micro-code. Because data-manipulation algorithms and systems are well known, the present description will be directed in particular to algorithms and systems forming part of, or cooperating more directly with, systems and methods described herein. Other aspects of such algorithms and systems, and hardware or software for producing and otherwise processing the signals involved therewith, not specifically shown or described herein, are selected from such systems, algorithms, components, and elements known in the art. Given the systems and methods as described herein, software not specifically shown, suggested, or described herein that is useful for implementation of any aspect is conventional and within the ordinary skill in such arts.

The dependence of reactive area density and tortuosity on particle morphology is qualitatively reflected on the dependence of these parameters on the average particle aspect ratio: more oblate particles are intuitively expected to exhibit a higher reactive area density and a higher tortuosity, while more needle-like particles possess a smaller reactive area density and impart less tortuous conditions for diffusion. This qualitatively suggests that an optimal particle aspect ratio and battery microstructure exists such that the mean reactive area density is maximized while the through thickness tortuosity is minimized, resulting in a maximized energy density. In the present disclosure, the optimal particle microstructure morphologies are identified to deliver high power or energy densities. The underlying mechanisms that control the battery performance are identified and rationalized as a stepping stone to propose optimal cathode particle aspect ratios.

The reactive area per unit volume, denoted herein as ρA, is a direct measure of the power density of an electrode layer. Historically, it is defined as:

for a distribution of perfectly spherical particles. Here, rpis the microstructurally averaged particle radius and ! is the electrode porosity. In general, the average area per until volume is a direct function of the particle morphology, and the polydispersity of the characteristic size, leading to expressions of the form:

where A is the shape factor, a quantity that captures the effects of the representative particle geometry, such that A->3 in the limit of perfectly spherical, smooth particles.

For perfectly aligned ellipsoidal particles with principal axes a=b≠c, the shape factor may be expressed in terms of particle aspect ratio, ra, by the following set of equations:

In the limit of ra=1 for spherical particles, both components of Eq. 4 approach A=3. In contrast, the through-thickness tortuosity of a porous electrode layer depends on the particle morphology through the Bruggeman relation:

Where α is is the generalized Bruggeman exponent and τ is the ratio of diffusivity in free space to the diffusivity in perfectly homogenous, spatially averaged porous media. In this context, recent work by Garcia and coworkers has analytically defined the effects of microstructure morphology on the average electrode properties in porous rechargeable batteries. Specifically, morphological anisotropy is expressed in terms of an average aspect ratio. For perfectly aligned monodispersed particles, the Bruggeman exponent is specified in terms of the depolarization factor, L:

which is a measure of the effect of particle morphology on the internal electric field of a statistically representative particle and dictates the charge flow in the particle vicinity. The depolarization factor is defined as:

In one example, an analysis was performed in the Open Source software, dualfoil.py [5-6], a python-based interface that enables the intuitive and hierarchal control of the dualfoil legacy code, as made public by John Newman and coworkers. The resultant output was post-processed by a set of Matplotlib-based visualization modules that provide the user with the ability to rapidly set up complex, multiscale simulations. Further, the dualfoil legacy code was extended to consider arbitrary electrode Bruggeman exponents, α, and shape factors, A, as design adjustable parameters, to capture their individual contributions, and simultaneously assess their impact as determined by their representative aspect ratio and degree of alignment. Specifically, the LiC6|LiMn2O4system was analyzed. Design adjustable and material parameters are displayed in Table 1 below

Material parameters used hereon are identical to those widely used and validated by Doyle and coworkers, and widely used in the literature by authors such as Arora et al., which uses equivalent design adjustable parameters, against a wide range of equally validated battery systems. For the individual Bruggeman exponent and shape factor parameters analysis, simulated cells were discharged at 22.5 A/m2, in order to assess the effects varying Bruggeman exponents and shape factors. The Bruggeman exponent analysis comprised 24 simulations that varied the cathode particle Bruggeman exponent, α, from 0.5 (spherical) to 12 (oblate) by at intervals of 0.5 (spherical). Each simulation took on the order of 4 minutes of wall time. The shape factor analysis comprised 40 simulations that varied the cathode particle shape factor from 1 (prolate) to 40 (oblate). The effect of particle morphology aspect ratio (herein denoted as the minor axis to major axis ratio, c/a) was quantified by numerically implementing Equations 4 through 6, which directly relate the c/a-ratio to the Bruggeman exponent and the particle area density. The cathode particle aspect ratio was varied from 0.075 to 1 (spherical) at intervals of 0.001. The discretization of the analyzed range of parameter values was further refined in those aspect ratio regimes that exhibits improved cell performance.

The effect of the Bruggeman exponent alone is summarized inFIG. 1for a current density of 22.5 A/m2. In this section, A=3. Here, increasingly oblate particles display a sharp drop in energy density, indicating that a critical α˜6 exists above which tortuosity-induced losses dominates the system performance. At the microstructural levelFIGS. 2a-2dshow that as α increases the tortuosity-induced polarization losses increase, leading to an voltage drop across the thickness of the cathode, asymptotically reaching a maximum near α=6.5. In the anode layer, the voltage deviations from equilibrium decrease as a result of the transport limitations that develop in the cathode.

Calculations demonstrate that a decrease in the aspect ratio of the particles induces a shift in the local intercalation dynamics (seeFIGS. 3a-3d). Here, the spatial distribution of Butler-Volmer interfacial current density in the cathode progressively shifts from developing the hallow and broad reaction zone that propagates from the separator-cathode interface to the back contact, to first becoming more localized and Gaussian-like in shape for intermediate α-values, to propagating in a retrograde fashion: from the back contact to the separator-cathode interface. In contrast, the intercalation dynamics in the anode get progressively suppressed, and a secondary reaction zone develops at the anode-back contact interface.

As a result of increasing the microstructural transport limitations in the cathode layer and the corresponding shift in intercalation dynamics, the through-thickness spatial distribution of intercalated material shifts from having a uniform intercalation with a localized front that propagates from the separator-cathode interface towards the cathode back contact to intercalating from the back contact towards the front of the electrode layer,FIG. 5. Moreover, in the limit of very oblate-shaped particles or α-values greater than the identified critical exponent, intercalation will be localized in the cathode back contact region. Mass deintercalation is balanced in the anode by leading to primarily utilize active material in the anode-separator region.

Similarly,FIGS. 5a-5dshow that the lithium-ion transport limitations induced in the electrolyte phase by the increase in tortuosity leads to a progressively localized lithium depletion in the cathode layer. In the limit of very high α-values, lithium depletion primarily occurs in the back of the cathode layer.

The effect of active material particle morphology anisotropy alone is analyzed by quantifying the effect of the shape factor as described by Equations 2 and 3 on the energy density, seeFIG. 6, for a current density of 22.5 A/m2. In this section, α=½. As intuitively expected, results demonstrate that there is a linear increase in the energy density as the area per unit volume increases up to a shape factor of A˜23 (or aspect ratio of (c/a)˜0.05), beyond which it levels off to a constant energy density of ˜215 Wh/kg. Leveling off occurs because the porous battery becomes diffusion limited further supporting the existence of a microstructureparticle morphology combination that maximizes its macroscopic, delivered energy density.

Inside the cathode layer,FIGS. 7a-7dshow that as the shape factor increases, the voltage drop across the cathode layer decreases, shifting the overpotential drops to the anode layer. As the cathode particles become more reactive, the anode becomes the limiting component for it is unable to keep up with the intercalation kinetics of the cathode.FIGS. 8a-8dsupport this statement, as the reaction rate becomes uniform in the cathode and the anode develops a secondary peak that moves through the anode as lithium flows to the front of the anode in order to maintain the reaction in the cathode. The main reaction zone peak at the anode-separator interface remains unaffected in magnitude as the shape factor increases.

The spatial extent of intercalated lithium in the cathode remains qualitatively unchanged as the area density increases (seeFIGS. 9a-9d); however, the discharge time becomes anode limited because the two reaction zones that develop in the negative porous layers.FIGS. 10a-10dshow that the greater degree of deintercalation in the anode with increasing shape factor leads to greater lithium accumulation in the back of the anode and sharper concentration gradients, thus favoring the formation of SEI and dendrite growth.

The combined effect of reactive area density and tortuosity on the cell's energy density is shown inFIG. 11for a current density of 22.5 A/m2. Results demonstrate that as the particles become increasingly oblate, the macroscopic energy density increases as a result of the increase in area density, until it reaches a maximum at (c/a)˜0.107, to deliver an optimal ˜93 Wh/kg. Oblate particles with a smaller aspect ratio. (e.g., (c/a)˜ 1/20), the throughthickness polarization losses due to tortuosity are higher than the benefits provided by the increase in area density. . This energy density peak occurs well below the regime where the lithium transport properties of the electrolyte become limiting (seeFIGS. 6 and 11). The effect of microstructure on the spatial; overpotential distribution is summarized inFIGS. 12a-12dwhere the onset of large voltage gradients induced by the increase of tortuosity in the cathode for oblate particles is delayed as a result of the increase of reactive area density. Moreover, for the optimal aspect ratio of c/a)=0.107, the overpotential in the electrolyte approaches its equilibrium (zero) value due to the high reactivity of the cathode particles, which is driving intercalation in combination with minimal losses. Oblate particles with an aspect ratio below the optimal (c/a) value further results in a dramatic increase of tortuosity induced polarization losses, which in turn completely suppress the delivery of any useful charge.

The effect of morphological anisotropy on the interfacial current density is summarized inFIGS. 13a-13d. Here, as perfectly aligned platelets become increasingly oblate, the reaction zone is qualitatively influenced by the increasing tortuosity of the porous microstructure, but the onset of the retrograde propagation of the reaction front (from the back contact to the separator-cathode interface),is delayed by the local increase of the reactive area density. The magnitude of the peak of the reaction zone is also lower. At the optimal aspect ratio ((c/a)˜0.107), the reaction zone in the both cathode and anode spread out across the layers, and while it is not uniform, it shows that it maximizes the utilization of active material. As particles become increasingly prolate, the volume fraction distribution of utilized cathode material changes from being uniformly distributed to intercalating from the cathode back contact to the front of the electrode layer (seeFIG. 14athrough 14c). For (c/a)-ratios smaller than the optimal value, lithium only intercalates in the vicinity of the cathode-back contact interface Similarly, the depth of deintercalation in the anode its the most uniform when it reaches the optimal (c/a) value. Away from the optimum, lithium deintercalates preferentially in the vicinity of the anodeseparator boundary. In the electrolyte phase, lithium depletion in the cathode layer is favored for spherical and very prolate-shaped particles,FIGS. 15a-15d. However, at the ideal (c/a)-ratio, deviations from its initial equilibrium value are small because transport and intercalation are in dynamic equilibrium. In the anode layer, lithium gradients in the electrolyte phase are minimal at the optimum particle morphology, suggesting that salt precipitation can be avoided at the optimum battery microstructure.

Overall, the analysis above demonstrates that, for fixed current density, at least one optimal microstructure morphology exists that maximizes performance.FIG. 16amacroscopic delivered power density as a function of energy density, for fixed microstructure. In general, (c/a)<0.09 will not deliver improvements neither in energy nor lower density because the aspect ratio is so low that the polarization losses due to tortuosity overshadow any possible area density enhancements. Qualitatively, this is in agreement with what has been observed experimentally in the literature. In contrast, for (c/a)>0.09, the resultant set of Ragone plots illustrate three primary types of response:

Alternatively,FIG. 16bshows that high aspect ratios that exhibit an optimal energy density response will drop significantly its capacity as the current density increases. Moreover, while an optimal microstructure exists for each current density, two types of microstructures can deliver the same (nonoptimal) energy density for the same power. The difference becomes more pronounced as particles become increasingly prolate. Moreover, for a specified current density, a worst microstructure exists that delivers the same power, but the lowest energy density.

The invention is inclusive of combinations of the aspects described herein. References to “a particular aspect” and the like refer to features that are present in at least one aspect of the invention. Separate references to “an aspect” (or “embodiment”) or “particular aspects” or the like do not necessarily refer to the same aspect or aspects; however, such aspects are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to “method” or “methods” and the like is not limiting. The word “or” is used in this disclosure in a non-exclusive sense, unless otherwise explicitly noted.

The invention has been described in detail with particular reference to certain preferred aspects thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention.