The high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) spinel is a promising candidate for a positive electrode in lithium ion batteries, but LNMO/graphite full-cells display severe capacity fading issues due to Mn dissolution. In this study, the dissolution behaviors of Mn and Ni were examined systematically under various conditions such as state of charge (SOC), temperature, storage time, and crystal structure of LNMO. In addition, surfaces of calendar-or cycle-aged LNMO and graphite electrodes were analyzed by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), or time-of-flight secondary ion mass spectrometry (TOF-SIMS). The chemical composition of aged electrolyte was determined by gas chromatography (GC) analysis after storage of LNMO electrodes under different conditions. It was found that the amounts of dissolved Mn and Ni and diethyl ether, a decomposition product of diethyl carbonate (DEC) in electrolyte, increased with SOC, temperature, and storage time. The decomposition of electrolyte can be explained, in part, by the self-discharge behavior of LNMO, which promotes electrolyte oxidation. Additional HF is believed to be generated during the formation of diethyl ether (via dehydration reaction from EtOH, another decomposition product of DEC), which accelerates Mn and Ni dissolution from LNMO. In addition, various reaction products that form as a result of Mn and Ni dissolution, such as LiF, MnF 2 , NiF 2 , and polymerized organic species, were found on the surface of LNMO electrodes, which will increase battery-cell impedance.

The mechanism of Li(+) transport through the solid electrolyte interphase (SEI), a passivating film on electrode surfaces, has never been clearly elucidated despite its overwhelming importance to Li-ion battery operation and lifetime. The present paper develops a multiscale theoretical methodology to reveal the mechanism of Li(+) transport in a SEI film. The methodology incorporates the boundary conditions of the first direct diffusion measurements on a model SEI consisting of porous (outer) organic and dense (inner) inorganic layers (similar to typical SEI films). New experimental evidence confirms that the inner layer in the ∼20 nm thick model SEI is primarily crystalline Li(2)CO(3). Using density functional theory, we first determined that the dominant diffusion carrier in Li(2)CO(3) below the voltage range of SEI formation is excess interstitial Li(+). This diffuses via a knock-off mechanism to maintain higher O-coordination, rather than direct-hopping through empty spaces in the Li(2)CO(3) lattice. Mesoscale diffusion equations were then formulated upon a new two-layer/two-mechanism model: pore diffusion in the outer layer and knock-off diffusion in the inner layer. This diffusion model predicted the unusual isotope ratio (6)Li(+)/(7)Li(+) profile measured by TOF-SIMS, which increases from the SEI/electrolyte surface and peaks at a depth of 5 nm, and then gradually decreases within the dense layer. With no fitting parameters, our approach is applicable to model general transport properties, such as ionic conductivity, for SEI films on the surface of other electrodes, from the atomic scale to the mesoscale, as well as aging phenomenon.

The solid electrolyte interphase (SEI), a passivation layer formed on electrodes, is critical to battery performance and durability. The inorganic components in SEI, including lithium carbonate (Li2CO3) and lithium fluoride (LiF), provide both mechanical and chemical protection, meanwhile control lithium ion transport. Although both Li2CO3 and LiF have relatively low ionic conductivity, we found, surprisingly, that the contact between Li2CO3 and LiF can promote space charge accumulation along their interfaces, which generates a higher ionic carrier concentration and significantly improves lithium ion transport and reduces electron leakage. The synergetic effect of the two inorganic components leads to high current efficiency and long cycle stability.

Highly active, stable, and cheap Pt-free catalysts for the hydrogen evolution reaction (HER) are facing increasing demand as a result of their potential use in future energy-conversion systems. However, the development of HER electrocatalysts with Pt-like or even superior activity, in particular ones that can function under alkaline conditions, remains a significant challenge. Here, the synthesis of a novel carbon-loaded ruthenium nanoparticle electrocatalyst (Ru@CQDs) for the HER, using carbon quantum dots (CQDs), is reported. Electrochemical tests reveal that, even under extremely alkaline conditions (1 m KOH), the as-formed Ru@CQDs exhibits excellent catalytic behavior with an onset overpotential of 0 mV, a Tafel slope of 47 mV decade , and good durability. Most importantly, it only requires an overpotential of 10 mV to achieve the current density of 10 mA cm . Such catalytic characteristics are superior to the current commercial Pt/C and most noble metals, non-noble metals, and nonmetallic catalysts under basic conditions. These findings open a new field for the application of CQDs and add to the growing family of metal@CQDs with high HER performance.

It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous Li(x)Si phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li(15)Si(4) or Li(22)Si(5). Presently, the detailed atomistic mechanism of this phase transformation and the degradation process in nanostructured Si are not fully understood. Here, we report the phase transformation characteristic and microstructural evolution of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during the charge/discharge process using in situ transmission electron microscopy and density function theory molecular dynamic calculation. We found the crystallization of Li(15)Si(4) from amorphous Li(x)Si is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is observed during the early stages of cyclic charge/discharge. Reversible volume expansion/contraction upon charge/discharge is fully accommodated along the radial direction. However, with progressive cycling, damage in the form of surface roughness was gradually accumulated on the coating layer, which is believed to be the mechanism for the eventual capacity fade of the composite anode during long-term charge/discharge cycling.

Ac hallenging but pressing task to design and synthesize novel, efficient, and robust pH-universal hydrogen evolution reaction (HER) electrocatalysts for scalable and sustainable hydrogen production through electrochemical water splitting.H erein, we report af acile method to prepare an efficient and robust Ru-M (M = Ni, Mn, Cu) bimetal nanoparticle and carbon quantum dot hybrid (RuM/CQDs) for pH-universal HER. The RuNi/CQDs catalysts exhibit outstanding HER performance at all pH levels.The unexpected low overpotentials of 13, 58, and 18 mV shown by RuNi/ CQDs allowacurrent density of 10 mA cm À2 in 1m KOH, 0.5 m H 2 SO 4 ,a nd 1m PBS,r espectively,f or Ru loading at 5.93 mgRu cm À2 .T his performance is among the best catalytic activities reported for any platinum-free electrocatalyst. Theoretical studies reveal that Ni doping results in am oderate weakening of the hydrogen bonding energy of nearby surface Ru atoms,w hich playsac ritical role in improving the HER activity.

Intensive studies of an advanced energy material are reported and lithium polyacrylate (LiPAA) is proven to be a surprisingly unique, multifunctional binder for high‐voltage Li‐ion batteries. The absence of effective passivation at the interface of high‐voltage cathodes in Li‐ion batteries may negatively affect their electrochemical performance, due to detrimental phenomena such as electrolyte solution oxidation and dissolution of transition metal cations. A strategy is introduced to build a stable cathode–electrolyte solution interphase for LiNi0.5Mn1.5O4 (LNMO) spinel high‐voltage cathodes during the electrode fabrication process by simply using LiPAA as the cathode binder. LiPAA is a superb binder due to unique adhesion, cohesion, and wetting properties. It forms a uniform thin passivating film on LNMO and conducting carbon particles in composite cathodes and also compensates Li‐ion loss in full Li‐ion batteries by acting as an extra Li source. It is shown that these positive roles of LiPAA lead to a significant improvement in the electrochemical performance (e.g., cycle life, cell impedance, and rate capability) of LNMO/graphite battery prototypes, compared with that obtained using traditional polyvinylidene fluoride (PVdF) binder for LNMO cathodes. In addition, replacing PVdF with LiPAA binder for LNMO cathodes offers better adhesion, lower cost, and clear environmental advantages.

* These authors contributed equally to this work.The thousandfold increase in data-collection speed enabled by aberration-corrected optics allows us to overcome an electron microscopy paradox -how to obtain atomic-resolution chemical structure in individual nanoparticles, yet record a statistically significant sample from an inhomogeneous population. This allowed us to map hundreds of Pt-Co nanoparticles to show atomic-scale elemental distributions across different stages of the catalyst aging in a proton-exchange-membrane fuel cell, and relate Pt-shell thickness to treatment, particle size, surface orientation, and ordering. 11/28/11 8:49 PM 2 Bulk and reciprocal space measurements provide accurate ensemble averages of nanoparticle systems, yet in doing so lose the connections between microscopic degrees of freedom when integrating over the myriad of different particles in any representative sample.Out of necessity, nanoscale chemical imaging to date has relied on a handful of spectra collected from a few selected particles, as it often takes a few hours to record a spectral map of a single nanoparticle. However, nanoparticle systems-especially during electrocatalysis-are heterogeneous and have multiple competing processes running in parallel. Thus, identifying and quantifying dominant mechanisms requires statistics on scores to hundreds of particles in order to reliably connect the microstructure to the bulk properties. With the development of aberrationcorrected scanning transmission electron microscopy (STEM) 1, 2 and efficient electron energy loss spectra (EELS) collection systems, elemental concentrations and chemical bonding information can now be collected roughly a thousand times faster than on a conventional microscope, allowing rapid and reliable 2-D mapping of chemical distributions at atomic resolution 3 . While much of the focus of aberration correction has been on producing increasingly small sub-Angstrom electron beams, here we instead stop at an atomic-sized beam and increase the usable beam current. This enabled us to collect over one million EELS spectra and map out the concentrations of all atomic species in hundreds of Pt-Co nanoparticles used as fuel cell electrocataysts. We can thus quantify and correlate internal ordering, facet termination and surface structure-nanoparticle by nanoparticle-to identify the dominant degradation chemistries that limit the catalyst's efficiency. These measurements that would have taken years to record, and thus be too slow to provide feedback in a rapidly evolving field, were now collected in sessions of a few hours to days. ; a reduction in surface area for the remaining material as the average particle size increases; and a reduction in the specific activity from the remaining surface area as the particle composition and structure are altered 18,23 . While the first two issues are better understood and common to both Pt and Pt-M alloys, our focus here is on the final two issues which are determined by the less-well understood microscopic underpinnings and ...