No abstract

Achieving high energy density in all-solid-state lithium batteries will require the design of thick cathodes, and these will need to operate reversibly under normal use conditions. We use high-energy depth-profiling X-ray diffraction to measure the localized lithium content of Li1–x Ni1/3Mn1/3Co1/3O2 (NMC111) through the thickness of 110 μm thick composite cathodes. The composite cathodes consisted of NMC111 of varying mass loadings mixed with argyrodite solid electrolyte Li6PS5Cl (LPSC). During cycling at C/10, substantial lithiation gradients developed, and varying the NMC111 loading altered the nature of these gradients. Microstructural analysis and cathode modeling showed this was due to high tortuosities in the cathodes. This was particularly true in the solid electrolyte phase, which experienced a marked increase in tortuosity factor during the initial charge. Our results demonstrate that current distributions are observed in sulfide-based composites and that these will be an important consideration for practical design of all-solid-state batteries.

High-resolution X-ray computed tomography (CT) has become an invaluable tool in battery research for its ability to probe phase distributions in sealed samples. The Cartesian coordinates used in describing the CT image stack are not appropriate for understanding radial dependencies, like that seen in bobbin-type batteries. The most prominent of these bobbin-type batteries is alkaline Zn–MnO2, which dominates the primary battery market. To understand material radial dependencies within these batteries, a method is presented to approximate the Cartesian coordinates of CT data into pseudo-cylindrical coordinates. This is important because radial volume fractions are the output of computational battery models, and this will allow the correlation of a battery model to CT data. A selection of 10 anodes inside Zn–MnO2 AA batteries are used to demonstrate the method. For these, the pseudo-radius is defined as the relative distance in the anode between the central current collecting pin and the separator. Using these anodes, we validate that this method results in averaged one-dimensional material profiles that, when compared to other methods, show a better quantitative match to individual local slices of the anodes in the polar θ-direction. The other methods tested are methods that average to an absolute center point based on either the pin or the separator. The pseudo-cylindrical method also corrects for slight asymmetries observed in bobbin-type batteries because the pin is often slightly off-center and the separator often has a noncircular shape.

The discharge of an alkaline Zn anode involves the conversion of active material to aqueous zincate ions and solid ZnO. With the mobility of zincate in the electrolyte, complex ZnO morphologies and radial distributions have been observed to occur when using different discharge protocols. An apparent continuum of ZnO densities can be seen in an intermittently discharged anode, and can be appropriately binned into two forms, namely type I and type II, which differ in density and passivating character.1 Not only are the relative amounts of these two types impacted by the discharge protocol, but their morphologies and volume distributions are also affected. This has implications for both primary cells, where undesirable ZnO formation near the separator can lead to cell failure, and secondary cells, where redistribution of active material inhibits cyclability. Previously reported models of Zn anodes poorly describe the behavior seen when using an intermittent discharge protocol meant to simulate typical real-world applications, and therefore require further development.2 Additionally, prior literature reported that ZnO forms passivating shells around undischarged Zn cores, restricting hydroxide transport, and thus resulting in reduced cell performance.3 In this work, we characterize the ZnO morphology and radial distribution for an array of discharge conditions that simulate both intermittent and continuous use. We then use these results to identify the impact of discharge protocol on ZnO formation within alkaline Zn anodes. This work paired high resolution synchrotron tomography with a novel segmentation algorithm to observe phase morphology and quantify radial distributions in situ. The results obtained when using a continuous discharge protocol matched that of previous literature.3 However, the results when using an intermittent discharge protocol were vastly different than that seen with a continuous protocol. The discharge products in a cell simulating intermittent use formed two distinct phases of varying densities, both of which preferentially formed in the inner portion of the anode. This is radically different than ZnO formation in continuously discharged cells, wherein an apparent single ZnO phase is concentrated at the outer portions of the anode near the separator, creating a dense crust around the remaining Zn. This outer crust can be detrimental to cell performance due to poor hydroxide transport from the cathode to the remaining undischarged Zn, suggesting superior cell performance with intermittent use. Acknowledgements This research was supported by funding from Energizer Holdings, Inc. This research also used resources of the Advanced Photon Source beamline 6-BM, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. References [1] I. Arise et al 2013 J. Electrochem. Soc. 160 D66 [2] E. J. Podlaha and H. Y. Cheh 1994 J. Electrochem. Soc. 141 15 [3] Quinn C. Horn and Yang Shao-Horn 2003 J. Electrochem. Soc. 150 A652

In alkaline Zn anodes, the active material is discharged to both aqueous zincate ions and solid ZnO. The mobility of zincate in the electrolyte allows considerable transport of the discharge products, which can form directly on the Zn active material or far from it, effectively relocating material long distances inside the cell. Additionally, ZnO can exist in more than one form, namely type I and type II, which differ in their density and passivating character.1 This leads to complex compositions of discharged primary Zn anodes and can cause cell failure due to redistribution of Zn in rechargeable cells. Models of Zn anodes are typically designed for continuous discharge,2 yet real-world use conditions usually involve intermittent discharge. Therefore, the ability of these models to predict ZnO morphology and distribution for cells used in real-world applications is questionable. It has been previously reported that when alkaline AA cells are discharged, ZnO forms passivating shells around undischarged Zn cores. At high discharge rates, this passivation of Zn increases in severity and hinders cell performance. Additionally, higher discharge rates increase the non-uniformity of ZnO distribution within the anode, preferentially precipitating ZnO near the separator which impedes hydroxide transport from the cathode to the inner portion of the anode.3 In this work, we analyze cylindrical cells discharged using intermittent profiles that approximate real battery use. We then compare the ZnO distribution to that found after continuous discharge. Synchrotron X-ray tomography was used to obtain in situ volumetric reconstructions at a spatial resolution of 3 microns. To observe the spatial distribution and morphology of ZnO formation in partially discharged Zn anodes, multiple AAA cells were discharged using distinct protocols. When using a continuous discharge, the reconstructions verified previous findings that core-shell structure is the primary morphology of ZnO and is non-uniformly distributed towards the separator.3 However, a radically different morphology and spatial distribution of ZnO was observed when using an intermittent discharge protocol. Specifically, when providing an 8 hour rest after each hour of discharge, ZnO forms isolated clumps distributed throughout the anode. Using this intermittent discharge protocol avoids passivating active material and potentially enables a higher Zn utilization by minimizing isolation of active material. References [1] I. Arise et al 2013 J. Electrochem. Soc. 160 D66 [2] E. J. Podlaha and H. Y. Cheh 1994 J. Electrochem. Soc. 141 15 [3] Quinn C. Horn and Yang Shao-Horn 2003 J. Electrochem. Soc. 150 A652