Ni-rich, cathode active materials such as Li[Ni0.8Mn0.1Co0.1]O2 (NMC811) suffer from capacity fade, especially at higher upper cut-off voltages. There is an increasing interest on dopants such as Mo or Ta as a promising dopants for high-Ni materials. In this study four cathode active materials with Li[(Ni0.78Mn0.11Co0.11)1-xMx]O2 (NMC811, x=0, 0.005, 0.007, 0.013; M=Ta) were synthesized via a batch co-precipitation synthesis followed by calcination. To increase the probability of homogeneous distribution of the dopant, the highly porous hydroxide precursor was impregnated with Ta-ethoxide and afterwards calcined. The focus of this study is the effect of Ta on the crystal structure and its influence on stability at high-voltage operation. Using Rietveld method, trends for several unit cell parameters, such as c-parameter, crystallite size, microstrain, transition metal-oxygen bond lengths) were found. Doping NMC811 with 0.5 mol% Ta leads to <3% lower initial capacity (205 mAh g-1 at C/10), >5% higher capacity retention (>85%) and >7% higher accumulated energy output over 100 cycles at 1 C (66.3 kWh kg-1). This performance enhancement is attributed to the high transition metal – oxygen bond dissociation energy, which is thought to significantly suppress surface phase transformations, and hence, reduce the vulnerability towards material degradatio

Doping of Ni-rich LNMC cathode materials with tungsten via a solid-state route has been shown to stabilize the materials against structural degradation at high voltages during electrochemical cycling. Here we use a wet-chemical doping method to homogeneously introduce 0.5 mol% tungsten into dense NMC811 (Li[(Ni0.8Mn0.1Co0.1)]O2), followed by a detailed structural and electrochemical characterization. A homogeneous distribution of W in the materials was evidenced by elemental mapping and TOF-SIMS. The increase in bond-dissociation energy to oxygen of the transition metal (TM) site led to compressed slab thickness (TMO6 octahedron width in c-direction), indicating shorter, and thus strengthened, TM-O bonds, as also confirmed by TGA-MS results. Importantly, statistical analysis of electron backscatter diffraction data revealed a W-induced radial alignment of a-b planes of the primary particles. The cycling stability of the doped material was more than 7% higher for the W-doped (92.4%) as compared to the undoped NMC811 (85.3%) material at a higher upper cut-off voltage of 4.5 V vs. Li/Li+. Furthermore, the voltage decay was lower (0.2 V vs. 0.29 V), leading to >2% lower energy loss (5.3% vs. 7.4%) for the W-doped material.

Fuel cell performance and durability are highly dependent on water management, wherefore wettability properties of the cell's gas diffusion layer (GDL) are important. In this work, we implement a method to determine the GDL wetted surface area, which is based on capacitance measurements by cyclic voltammetry with a pH-neutral, aqueous electrolyte, to an automotive size fuel cell failure analysis process and demonstrate its benefit. The electrolyte penetrates large pores of the GDL, wherefore, in contrast to the most conventional methods, also inner parts of the GDL are measured. Tenside concentration of the electrolyte and penetration time, polytetrafluoroethylene treatment of the GDL, and properties of the microporous layer highly influence the capacitance values.Thus, the method is sensitive to different GDL morphologies and surface modifications. Different degradation patterns for samples either artificially chemically and mechanically aged or after real-operation (e.g., prototype vehicle) are detected by the method. For a comprehensive understanding, the obtained results are compared to ex situ degradation analysis data. A comparison to static contact angle measurements, being a state-of-the-art method to determine GDL wettability, reveals a higher sensitivity of the introduced method to detect degradation, in particular, of chemically aged and real-operation aged GDLs.

Detrimental side-reactions of Ni-rich cathode active materials (CAMs) with the electrolyte have historically impeded the extension of the utilized voltage window to higher upper cut-off voltages. Doping and coating approaches are studied widely to further improve these materials and to reduce the intensity of bulk and surface degradation but suffer from poor control of film thickness and homogeneity, leading to partial doping of the bulk. We herein report the singular effect of a tantalum oxide (Ta2O5) thin film on Li[Ni0.8Mn0.1Co0.1]O2, generated by atomic layer deposition, offering the possibility of a high-level homogeneity and thickness control. After chemical analysis using X-ray photoelectron spectroscopy the composition of the deposited thin film is identified as lithium tantalate (LiTaO3). At an early degradation stage, clear improvements directly attributed to the coating, such as suppressed exothermic side-reactions (-51%), reduced released gas amounts (-14.8%) and less charge-transfer resistance growth (2x lower) are observed. However, at an advanced degradation stage, the materials show similar cycle life, as well as similar gassing behavior and an even higher charge-transfer resistance growth as compared to the uncoated material. This study highlights the necessity of bulk stabilization and identifies the effect of surface coatings on Ni-rich CAMs without any doping influence.