Rose E. Ruther

Unrivaled combination of surface area and pore volume in micelle-templated carbon for supercapacitor energy storage

We created Immense Surface Area Carbons (ISACs) by a novel heat treatment that stabilized the micelle structure in a biological based precursor prior to high temperature combined activation – pyrolysis. While displaying a morphology akin to that of commercial activated carbon, ISACs contain an unparalleled combination of electrochemically active surface area and pore volume (up to 4051 m2 g−1, total pore volume 2.60 cm3 g−1, 76% small mesopores). The carbons also possess the benefit of being quite pure (combined O and N: 2.6–4.1 at%), thus allowing for a capacitive response that is primarily EDLC. Tested at commercial mass loadings (∼10 mg cm−2) ISACs demonstrate exceptional specific capacitance values throughout the entire relevant current density regime, with superior rate capability primarily due to the large fraction of mesopores. In the optimized ISAC, the specific capacitance (Cg) is 540 F g−1 at 0.2 A g−1, 409 F g−1 at 1 A g−1 and 226 F g−1 at a very high current density of 300 A g−1 (∼0.15 second charge time). At intermediate and high currents, such capacitance values have not been previously reported for any carbon. Tested with a stable 1.8 V window in a 1 M Li2SO4 electrolyte, a symmetric supercapacitor cell yields a flat energy–power profile that is fully competitive with those of organic electrolyte systems: 29 W h kg−1 at 442 W kg−1 and 17 W h kg−1 at 3940 W kg−1. The cyclability of symmetric ISAC cells is also exceptional due to the minimization of faradaic reactions on the carbon surface, with 80% capacitance retention over 100 000 cycles in 1 M Li2SO4 and 75 000 cycles in 6 M KOH.

High Areal Capacity Si/LiCoO2 Batteries from Electrospun Composite Fiber Mats

Freestanding nanofiber mat Li-ion battery anodes containing Si nanoparticles, carbon black, and poly(acrylic acid) (Si/C/PAA) are prepared using electrospinning. The mats are compacted to a high fiber volume fraction (≈0.85), and interfiber contacts are welded by exposing the mat to methanol vapor. A compacted+welded fiber mat anode containing 40 wt % Si exhibits high capacities of 1484 mA h g-1 (3500 mA h g-1Si ) at 0.1 C and 489 mA h g-1 at 1 C and good cycling stability (e.g., 73 % capacity retention over 50 cycles). Post-mortem analysis of the fiber mats shows that the overall electrode structure is preserved during cycling. Whereas many nanostructured Si anodes are hindered by their low active material loadings and densities, thick, densely packed Si/C/PAA fiber mat anodes reported here have high areal and volumetric capacities (e.g., 4.5 mA h cm-2 and 750 mA h cm-3 , respectively). A full cell containing an electrospun Si/C/PAA anode and electrospun LiCoO2 -based cathode has a high specific energy density of 270 Wh kg-1 . The excellent performance of the electrospun Si/C/PAA fiber mat anodes is attributed to the: i) PAA binder, which interacts with the SiOx surface of Si nanoparticles and ii) high material loading, high fiber volume fraction, and welded interfiber contacts of the electrospun mats.

Chemical Evolution in Silicon–Graphite Composite Anodes Investigated by Vibrational Spectroscopy

Silicon-graphite composites are under development for the next generation of high-capacity lithium-ion anodes, and vibrational spectroscopy is a powerful tool to identify the different mechanisms that contribute to performance loss. With alloy anodes, the underlying causes of cell failure are significantly different in half-cells with lithium metal counter electrodes compared to full cells with standard cathodes. However, most studies which take advantage of vibrational spectroscopy have only examined half-cells. In this work, a combination of FTIR and Raman spectroscopy describes several factors that lead to degradation in full pouch cells with LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes. The spectroscopic signatures evolve after longer term cycling compared to the initial formation cycles. Several side-reactions that consume lithium ions have clear FTIR signatures, and comparison to a library of reference compounds facilitates identification. Raman microspectroscopy combined with mapping shows that the composite anodes are not homogeneous but segregate into graphite-rich and silicon-rich phases. Lithiation does not proceed uniformly either. A basis analysis of Raman maps identifies electrochemically inactive regions of the anodes. The spectroscopic results presented here emphasize the importance of improving electrode processing and SEI stability to enable practical composite anodes with high silicon loadings.

Applying multiscale imaging and spectroscopic techniques for studying capacity and cycle life degradation in high energy density lithium-ion cells (Conference Presentation)

High capacity redox active materials are the building blocks for batteries and modern electrochemical conversion devices. Last couple of decades has witnessed tremendous progress in the area of rechargeable batteries for transportation, grid storage and consumer applications.1 The talk will provide an overview of the current R&D status of advanced batteries for electric vehicles followed by deep dive analysis of lithium-ion battery electrodes using X-ray synchrotron, micro-Raman and neutron spectroscopic and imaging methods. Specifically, the talk will cover recent work related to applying X-ray transmission imaging combined with near edge absorption spectroscopy (XANES) to study the evolution of chemical oxidation state of the transition metal (TM) cations accompanied by changes in the particle morphology for a number of lithium-ion cathode systems such as lithium-manganese rich NMC cathodes (LMR-NMC) and high capacity Li2Cu0.5Ni0.5O2 cathodes.2-4 Ex-situ and in-situ Raman and neutron imaging methods for studying micron scale inhomogeneties associated with high capacity battery electrodes such as silicon-graphite will also be presented.5-7 1. M. S. Whittingham, Chem. Rev. 104, 4271 (2004) 2. H. Dixit, J. Nanda et al, ACS Nano, 8 (12) 12710 (2014) 3. R. Ruther, J. Nanda et al. Chem. Mater. 29, 2997 (2017) 4. F. Yang, Y. Liu, J. Nanda et al. Nano Letts. 14, 4334, (2014) 5. J. Nanda, H. Bilheux, et al, J. Phys. Chem. C, 116, 8401 (2012) 6. R. Ruther, J. Nanda et al J. Phys. Chem. C 119, 18022 (2015) 7. H. Zhou, H. Bilheux, J, Nanda et al ACS Energy Letters, 1, 981 (2016)

Electrolyte Solvation Structure at Solid–Liquid Interface Probed by Nanogap Surface-Enhanced Raman Spectroscopy

Understanding the fundamental factors that drive ion solvation structure and transport is key to design high-performance, stable battery electrolytes. Reversible ion solvation and desolvation are critical to the interfacial charge-transfer process across the solid-liquid interface as well as the resulting stability of the solid electrolyte interphase. Herein, we report the study of Li+ salt solvation structure in aprotic solution in the immediate vicinity (∼20 nm) of the solid electrode-liquid interface using surface-enhanced Raman spectroscopy (SERS) from a gold nanoparticle (Au NP) monolayer. The plasmonic coupling between Au NPs produces strong electromagnetic field enhancement in the gap region, leading to a 5 orders of magnitude increase in Raman intensity for electrolyte components and their mixtures namely, lithium hexafluorophosphate, fluoroethylene carbonate, ethylene carbonate, and diethyl carbonate. Further, we estimate and compare the lithium-ion solvation number derived from SERS, standard Raman spectroscopy, and Fourier transform infrared spectroscopy experiments to monitor and ascertain the changes in the solvation shell diameter in the confined nanogap region where there is maximum enhancement of the electric field. Our findings provide a multimodal spectroscopic approach to gain fundamental insights into the molecular structure of the electrolyte at the solid-liquid interface.