Erik J. Berg

Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries

Peering into cathode layered oxides The quest for better rechargeable batteries means finding ways to pack more energy into a smaller mass or volume. Lithium layered oxides are a promising class of materials that could double storage capacities. However, the design of safe and long-lasting batteries requires an understanding of the physical and chemical changes that occur during redox processes. McCalla et al. used a combination of experiments and calculations to understand the formation of O-O dimers, which are key to improving the properties of these cathode materials. Science, this issue p. 1516 A model lithium layered oxide is used to probe the enhanced charge storage capacity of this family of materials. Lithium-ion (Li-ion) batteries that rely on cationic redox reactions are the primary energy source for portable electronics. One pathway toward greater energy density is through the use of Li-rich layered oxides. The capacity of this class of materials (>270 milliampere hours per gram) has been shown to be nested in anionic redox reactions, which are thought to form peroxo-like species. However, the oxygen-oxygen (O-O) bonding pattern has not been observed in previous studies, nor has there been a satisfactory explanation for the irreversible changes that occur during first delithiation. By using Li2IrO3 as a model compound, we visualize the O-O dimers via transmission electron microscopy and neutron diffraction. Our findings establish the fundamental relation between the anionic redox process and the evolution of the O-O bonding in layered oxides.

StatCache: a probabilistic approach to efficient and accurate data locality analysis

The widening memory gap reduces performance of applications with poor data locality. Therefore, there is a need for methods to analyze data locality and help application optimization. In this paper we present StatCache, a novel sampling-based method for performing data-locality analysis on realistic workloads. StatCache is based on a probabilistic model of the cache, rather than a functional cache simulator. It uses statistics from a single run to accurately estimate miss ratios of fully-associative caches of arbitrary sizes and generate working-set graphs. We evaluate StatCache using the SPEC CPU2000 benchmarks and show that StatCache gives accurate results with a sampling rate as low as 10/sup -4/. We also provide a proof-of-concept implementation, and discuss potentially very fast implementation alternatives.

Fast data-locality profiling of native execution

Performance tools based on hardware counters can efficiently profile the cache behavior of an application and help software developers improve its cache utilization. Simulator-based tools can potentially provide more insights and flexibility and model many different cache configurations, but have the drawback of large run-time overhead.We present StatCache, a performance tool based on a statistical cache model. It has a small run-time overhead while providing much of the flexibility of simulator-based tools. A monitor process running in the background collects sparse memory access statistics about the analyzed application running natively on a host computer. Generic locality information is derived and presented in a code-centric and/or data-centric view.We evaluate the accuracy and performance of the tool using ten SPEC CPU2000 benchmarks. We also exemplify how the flexibility of the tool can be used to better understand the characteristics of cache-related performance problems.

Ageing phenomena in high-voltage aqueous supercapacitors investigated by in situ gas analysis

High-voltage aqueous electrolyte based supercapacitors (U > 1.23 V) attract significant attention for next-generation high power, low cost and environmentally friendly energy storage applications. Cell ageing is however markedly pronounced at elevated voltages and results in accelerated overall performance fade and increased safety concerns. Online electrochemical mass spectrometry, combined with cell pressure analysis, is for the first time shown to provide a powerful means for in situ investigation of degradation mechanisms in aqueous electrolyte/carbon based supercapacitors. The activated carbon electrodes possess high specific surface area and oxygen-based surface functionalities (mainly phenol, lactone and anhydride groups), which are oxidized already at a cell voltage of 0.6 V to provoke the evolution of minor amounts of CO and CO2. Noticeable water decomposition starts at a high voltage of 1.6 V with the evolution of H2 on the negative electrode and carbon corrosion on the positive electrode with the generation of predominantly CO. In this paper we also report that short-term cycling leads to partly reversible gas evolution/consumption side-reactions giving negligible capacitance. On the other hand, long-term cycling causes irreversible side-reactions, deteriorates the electrochemical performance, and increases the internal pressure of the cell. Repeated cycling (U < 2 V) is confirmed as a more harmful technique for the electrode integrity compared to the voltage holding in a floating test. In situ gas analysis is shown to provide valuable insights into the electrochemical cell ageing aspects, such as the nature and potential onsets of side-reactions, hence paving the way for fundamental understanding and mitigating the performance and safety loss of high-energy aqueous supercapacitors.

Understanding the roles of anionic redox and oxygen release during electrochemical cycling of lithium-rich layered Li4FeSbO6.

Li-rich oxides continue to be of immense interest as potential next generation Li-ion battery positive electrodes, and yet the role of oxygen during cycling is still poorly understood. Here, the complex electrochemical behavior of Li4FeSbO6 materials is studied thoroughly with a variety of methods. Herein, we show that oxygen release occurs at a distinct voltage plateau from the peroxo/superoxo formation making this material ideal for revealing new aspects of oxygen redox processes in Li-rich oxides. Moreover, we directly demonstrate the limited reversibility of the oxygenated species (O2(n-); n = 1, 2, 3) for the first time. We also find that during charge to 4.2 V iron is oxidized from +3 to an unusual +4 state with the concomitant formation of oxygenated species. Upon further charge to 5.0 V, an oxygen release process associated with the reduction of iron +4 to +3 is present, indicative of the reductive coupling mechanism between oxygen and metals previously reported. Thus, in full state of charge, lithium removal is fully compensated by oxygen only, as the iron and antimony are both very close to their pristine states. Besides, this charging step results in complex phase transformations that are ultimately destructive to the crystallinity of the material. Such findings again demonstrate the vital importance of fully understanding the behavior of oxygen in such systems. The consequences of these new aspects of the electrochemical behavior of lithium-rich oxides are discussed in detail.

A statistical multiprocessor cache model

The introduction of general-purpose microprocessors running multiple threads will put a focus on methods and tools helping a programmer to write efficient parallel applications. Such a tool should be fast enough to meet a software developers need for short turn-around time, but also be accurate and flexible enough to provide trend-correct and intuitive feedback. This paper presents a novel sample-based method for analyzing the data locality of a multithreaded application. Very sparse data is collected during a single execution of the studied application. The architectural-independent information collected during the execution is fed to a mathematical memory-system model for predicting the cache miss ratio. The sparse data can be used to characterize the applications data locality with respect to almost any possible memory system, such as complicated multiprocessor multilevel cache hierarchies. Any combination of cache size, cache-line size and degree of sharing can be modeled. Each modeled design point takes only a fraction of a second to evaluate, even though the application from which the sampled data was collected may have executed for hours. This makes the tool not just usable for software developers, but also for hardware developers who need to evaluate a huge memory-system design space. The accuracy of the method is evaluated using a large number of commercial and technical multi-threaded applications. The result produced by the algorithm is shown to be consistent with results from a traditional (and much slower) architecture simulation.

SIP: Performance Tuning through Source Code Interdependence

The gap between CPU peak performance and achieved application performance widens as CPU complexity, as well as the gap between CPU cycle time and DRAM access time, increases. While advanced compilers can perform many optimizations to better utilize the cache system, the application programmer is still required to do some of the optimizations needed for efficient execution. Therefore, profiling should be performed on optimized binary code and performance problems reported to the programmer in an intuitive way. Existing performance tools do not have adequate functionality to address these needs. Here we introduce source interdependence profiling, SIP, as a paradigm to collect and present performance data to the programmer. SIP identifies the performance problems that remain after the compiler optimization and gives intuitive hints at the source-code level as to how they can be avoided. Instead of just collecting information about the events directly caused by each source-code statement, SIP also presents data about events from some interdependent statements of source code.A first SIP prototype tool has been implemented. It supports both C and Fortran programs. We describe how the tool was used to improve the performance of the SPEC CPU2000 183.equake application by 59 percent.

Self-discharge reactions in energy storage devices based on polypyrrole-cellulose composite electrodes

Abstract The self-discharge behavior of organic electrodes and symmetric devices for sustainable energy storage, composed of electrodes containing a thin layer of polypyrrole coated onto a high surface area cellulose matrix, has been studied for the first time using different electrode sizes and electrolytes. Experimental data from open circuit measurements of the individual electrode potentials of charged symmetrical two-electrode energy storage devices as a function of time were evaluated based on three different self-discharge models. This evaluation clearly showed that the self-discharge process of the positive electrode is governed by a previously undetected activation-controlled faradaic reaction while the self-discharge of the negative electrode is due to diffusion controlled oxidation involving oxygen dissolved in the electrolyte. Potentiostatic three-electrode measurements and spectroelectrochemical experiments also showed that protons as well as maleimide were released from positively polarized polypyrrole electrodes. These new findings clearly show that the self-discharge of the cells originate from two different types of reactions on the positive and negative electrodes and that the main contribution to the self-discharge of the cells comes from an activation controlled reaction involving the positive electrode. These results provide an improved understanding of polypyrrole based devices and also yield new possibilities for the development of stable conducting polymer system for energy storage applications.

Toward high-performance Li(NixCoyMnz)O2 cathodes: facile fabrication of an artificial polymeric interphase using functional polyacrylates

Targeting the long-term performance retention of high-energy-density batteries, a facile yet effective surface coating approach for high-voltage composite cathodes was developed. Ultra-thin polyacrylate (PAA) surface coatings with an estimated thickness of less than 5 nm were integrated onto Li-rich Li(NixCoyMnz)O2 cathodes through a solution-casting method. Obvious improvements of specific charge – around 20 mA h g−1 at the 1st cycle and up to 50 mA h g−1 at the 100th cycle – as well as enhanced capacity retention were achieved post the integration of a lithium polyacrylate (LiPAA) surface coating. A fundamental understanding of the role of PAA coatings was systematically achieved by X-ray photoelectron spectroscopy (XPS), electron microscopy (TEM, SEM/EDX), Fourier transform infrared spectroscopy (FTIR) and online electrochemical mass spectrometry (OEMS). Physically adhesive coatings on the active sites of cathodes were observed, which might contribute to impeded surface reconstruction of active materials during cycling. Furthermore, in OEMS analysis reduced CO2 evolution was detected from coated electrodes, indicating that the LiPAA coating could interrupt electrolyte decomposition reactions and enhance the cathode/electrolyte interfacial stability at high anodic potentials. This functional polymer coating acts as a stable artificial polymeric interphase to mitigate electrode and electrolyte decomposition during long-term cycling.

In situ and Operando Raman Spectroscopy of Layered Transition Metal Oxides for Li-ion Battery Cathodes

In situ and operando Raman spectroscopy is proposed to provide unique means for deeper fundamental understanding and further development of layered transition metal LiMO2 (M=Ni,Co,Mn) oxides suitable for Li-ion battery applications. We compare several spectro-electrochemical cell designs and suggest key experimental parameters for obtaining optimum electrochemical performance and spectral quality. Studies of the most practically relevant LiMO2 compositions are exemplified with particular focus on two experimental approaches: (1) lateral and axial Raman mapping of the electrode’s (near-) surface to monitor inhomogeneous electrode reactions and (2) time-dependent single-particle spectra during cycling to analyze the LixMO2 lattice dynamics as a function of lithium content. Raman Spectroscopy is claimed to provide a unique real-time probe of the M-O bonds, which are at the heart of the electrochemistry of LiMO2 oxides and govern their stability. We highlight the need for further fundamental understanding of the relationships between the spectroscopic response and oxide lattice structure with particular emphasis on the development of a theoretical framework linking the position and intensity of the Raman bands to the local LixMO2 lattice configuration. The use of complementary experimental techniques and model systems for validation also deserve further attention. Several novel LiMO2 compositions are currently being explored, especially containing dopings and coatings, and Raman Spectroscopy could offer a highly dynamic and convenient tool to guide the formulation of high specific charge and long cycle life LiMO2 oxides for next-generation Li-ion battery cathodes.