On the way to a large-scale industrial application of allsolid-state batteries (ASSBs) it is necessary to overcome a number of challenges. An important task is to maximize the utilization of active material in the cathode composite to achieve high capacities. Carbonbased conductive additives are common in cathode composites for conventional lithium-ion batteries based on liquid electrolytes. In allsolid-state batteries, the beneficial effect of carbon additives is often not maintained over a sufficient number of charge/discharge cycles. Thus, ASSB cells often suffer from an increased long-term capacity loss with an enhanced formation of decomposition products. So far, these effects have not been analyzed in depth and are not fully understood because of the complexity of the composite cathode structure. Together with overlap of the occurring degradation paths, this makes a separation of the individual decomposition processes challenging. In this work, we investigate the influence of vapor-grown carbon fibers as carbon-based conductive additives on the degradation of a LiNi 0.6 Co 0.2 Mn 0.2 O 2 /β-Li 3 PS 4 composite cathode. We use a combination of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS) and combine surface and bulk analyses to separate the overlapping decomposition processes from each other. The results show an initially higher capacity by using vapor-grown carbon fibers due to higher utilization of the active material and an additional capacity contribution caused by redox-active decomposition reactions. The observed capacity fading is associated with the formation of sulfates/sulfites, phosphates, and polysulfides, which are detected directly in LiNi 0.6 Co 0.2 Mn 0.2 O 2 /β-Li 3 PS 4 composite cathodes with ToF-SIMS for the first time. Overall, the results extend the knowledge and understanding of degradation phenomena in thiophosphate-based composite cathodes considerably, which is an essential step to develop protection concepts more efficiently on the way to long-term stable ASSBs.
Rechargeable solid-state lithium ion batteries (SSLB) require fast ion conducting solid electrolytes (SEs) to enable high charge and discharge rates. Li 7 P 3 S 11 is a particularly promising lithium solid electrolyte, exhibiting very high room temperature conductivities of up to 17 mS• cm −1 and high ductility, allowing fast ion transport through the bulk and intimate contact to high surface electrodes. Here we present a novel hot-press setup that facilitates the synthesis of solid electrolytes by combining in situ electrochemical impedance spectroscopy (EIS) with simultaneous temperature-and pressure-monitoring. While a high room temperature conductivity in the order of 10 mS•cm −1 is readily achieved for phase pure Li 7 P 3 S 11 with this design, it further enables monitoring of the different steps of crystallization from an amorphous Li 2 S−P 2 S 5 glass to triclinic Li 7 P 3 S 11 . Nucleation, crystallization andat temperatures exceeding 280 °Cdecomposition of the material are analyzed in real time, enabling process optimization. The results are supported ex situ by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and Raman spectroscopy. Proof-of-principle experiments show the promising cycling-and rate capability of Li 0.3 In 0.7 /Li 7 P 3 S 11 /S-composite all-solid-state batteries. It is furthermore presented that discharging below a limit of 1.2 V results in decomposition of the SE/cathode interface.
All-solid-state batteries promise to enable lithium metal anodes and outperform state-of-the-art lithium-ion battery technology. To achieve high battery capacity, utilization of the active material in the cathode must be maximized. Carbon-based conductive additives are known to improve the capacity and rate performance of electrode composites. However, their influence on cathode composites in all-solid-state batteries is yet not fully understood. Here, we study the influence of several carbon additives with different morphologies and surface areas on the performance of an all-solid-state battery cell Li|β-Li 3 PS 4 | Li(Ni 0.6 Co 0.2 Mn 0.2 )O 2 /β-Li 3 PS 4 /carbon. Cycling tests and microstructure-resolved simulations show that higher utilization of the cathode active material can be achieved using fiber-shaped vapor-grown carbon additives, whereas particle-shaped carbons show a minor influence. Unfortunately, carbon additives generally lead to an accelerated capacity loss during cycling and an enhanced formation of solid electrolyte decomposition products. The latter was studied in more detail using cyclic voltammetry, X-ray photoelectron spectroscopy, and cycling experiments. The results show that carbon additives with a small surface area and a fiber-like morphology result in the lowest degree of decomposition. To completely overcome electrolyte degradation caused by the use of carbon additives, a protection concept is developed. A thin alumina coating with a few nanometers thickness was deposited on the carbon fibers by atomic layer deposition, which successfully prevents decomposition reactions, reduces long-term capacity fading, and leads to an enhanced overall all-solid-state battery performance.
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