Judith Alvarado

Exploring Oxygen Activity in the High Energy P2-Type Na0.78Ni0.23Mn0.69O2 Cathode Material for Na-Ion Batteries

Large-scale electric energy storage is fundamental to the use of renewable energy. Recently, research and development efforts on room-temperature sodium-ion batteries (NIBs) have been revitalized, as NIBs are considered promising, low-cost alternatives to the current Li-ion battery technology for large-scale applications. Herein, we introduce a novel layered oxide cathode material, Na0.78Ni0.23Mn0.69O2. This new compound provides a high reversible capacity of 138 mAh g-1 and an average potential of 3.25 V vs Na+/Na with a single smooth voltage profile. Its remarkable rate and cycling performances are attributed to the elimination of the P2-O2 phase transition upon cycling to 4.5 V. The first charge process yields an abnormally excess capacity, which has yet to be observed in other P2 layered oxides. Metal K-edge XANES results show that the major charge compensation at the metal site during Na-ion deintercalation is achieved via the oxidation of nickel (Ni2+) ions, whereas, to a large extent, manganese (Mn) ions remain in their Mn4+ state. Interestingly, electron energy loss spectroscopy (EELS) and soft X-ray absorption spectroscopy (sXAS) results reveal differences in electronic structures in the bulk and at the surface of electrochemically cycled particles. At the surface, transition metal ions (TM ions) are in a lower valence state than in the bulk, and the O K-edge prepeak disappears. On the basis of previous reports on related Li-excess LIB cathodes, it is proposed that part of the charge compensation mechanism during the first cycle takes place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient. We believe that by optimizing and controlling oxygen activity, Na layered oxide materials with higher capacities can be designed.

Improved electrochemical performance of tin-sulfide anodes for sodium-ion batteries

Due to their highly reversible capacity, tin-sulfide-based materials have gained attention as potential anodes for sodium-ion and lithium-ion batteries. Nevertheless, the performance of tin sulfide anodes is much lower than that of tin oxide anodes. The aim of the present investigation is to improve the electrochemical performances of SnS anodes for sodium-ion batteries using conventional organic electrolytes. Three different carbon composite anodes, SnS/reduce graphene oxide (SnS/G), SnS/reduce graphene oxide/hard carbon (SnS/G + C), and SnS/hard carbon (SnS/C), were prepared by hot water bath synthesis followed by mechanical milling. The mechanism of the conversion and alloying reaction was investigated by TEM. The feasibility of SnS anodes was confirmed in a full cell configuration using Na3V2(PO4)2F3 as the cathode.

Improvement of the Cathode Electrolyte Interphase on P2-Na2/3Ni1/3Mn2/3O2 by Atomic Layer Deposition

Atomic layer deposition (ALD) is a commonly used coating technique for lithium ion battery electrodes. Recently, it has been applied to sodium ion battery anode materials. ALD is known to improve the cycling performance, Coulombic efficiency of batteries, and maintain electrode integrity. Here, the electrochemical performance of uncoated P2-Na2/3Ni1/3Mn2/3O2 electrodes is compared to that of ALD-coated Al2O3 P2-Na2/3Ni1/3Mn2/3O2 electrodes. Given that ALD coatings are in the early stage of development for NIB cathode materials, little is known about how ALD coatings, in particular aluminum oxide (Al2O3), affect the electrode-electrolyte interface. Therefore, full characterizations of its effects are presented in this work. For the first time, X-ray photoelectron spectroscopy (XPS) is used to elucidate the cathode electrolyte interphase (CEI) on ALD-coated electrodes. It contains less carbonate species and more inorganic species, which allows for fast Na kinetics, resulting in significant increase in Coulombic efficiency and decrease in cathode impedance. The effectiveness of Al2O3 ALD coating is also surprisingly reflected in the enhanced mechanical stability of the particle which prevents particle exfoliation.

Direct evidence for high Na+ mobility and high voltage structural processes in P2-Nax[LiyNizMn1−y−z]O2 (x, y, z ≤ 1) cathodes from solid-state NMR and DFT calculations

Structural processes occurring upon electrochemical cycling in P2-Nax[LiyNizMn1−y−z]O2 (x, y, z ≤ 1) cathode materials are investigated using 23Na and 7Li solid-state nuclear magnetic resonance (ssNMR). The interpretation of the complex paramagnetic NMR data obtained for various electrochemically-cycled NaxNi1/3Mn2/3O2 and NaxLi0.12Ni0.22Mn0.66O2 samples is assisted by state-of-the-art hybrid Hartree–Fock/density functional theory calculations. Two Na crystallographic environments are present in P2-Nax[LiyNizMn1−y−z]O2 compounds, yet a single 23Na NMR signal is observed with a shift in-between those computed for edge- and face-centered prismatic sites, indicating that Na-ion motion between sites in the P2 layers results in an average signal. This is the first time that experimental and theoretical evidence are provided for fast Na-ion motion (on the timescale of the NMR experiments) in the interlayer space in P2-type NaxTMO2 materials. A full assignment of the 7Li NMR data confirms that Li substitution delays the P2 to O2 phase transformation taking place in NaxNi1/3Mn2/3O2 over the range 1/3 ≥ xNa ≥ 0. 23Na ssNMR data demonstrate that NaxNi1/3Mn2/3O2 samples charged to ≥3.7 V are extremely moisture sensitive once they are removed from the cell, water molecules being readily intercalated within the P2 layers leading to an additional Na signal between 400 and 250 ppm. By contrast, the lithiated material NaxLi0.12Ni0.22Mn0.66O2 shows no sign of hydration until it is charged to ≥4.4 V. Since both TMO2 layer glides and water intercalation become increasingly favorable as more vacancies are present in the Na layers, the higher stability of the Li-doped P2 phase at high voltage can be accounted for by its higher Na content at all stages of cycling.

Electrochemical reaction and surface chemistry for performance enhancement of a Si composite anode using a bis(fluorosulfonyl)imide-based ionic liquid

An ionic liquid (IL) electrolyte with 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI) is applied to a silicon (Si) composite anode for Lithium-ion batteries (LIB). Si is one of the most promising anode materials for LIBs and fluoroethylene carbonate (FEC) has been widely used as an electrolyte additive with Si anodes to enhance electrochemical performance. However, the effect of FEC only lasts for a limited number of cycles. To overcome this issue, a bis(fluorosulfonyl)imide (FSI)-based IL is studied as a potential electrolyte candidate for a Si composite anode. Its effects on the electrochemical performance and the corresponding solid electrolyte interphase (SEI) formation on the Si composite anode are not well understood. This work addresses the correlation between the electrochemical performance and SEI formation to probe the surface chemistry on the Si composite anode. We find that the FSI-based electrolyte provides a stable and reversible capacity in long term cycling tests. This electrolyte has excellent rate capability compared to that of carbonate-based electrolytes. The decomposition products of these electrolytes on Si anodes are investigated by X-ray photoelectron spectroscopy. These results show that the chemical composition on the surface of the Si anode is largely different when using the FSI-based electrolyte than it is when using carbonate type electrolytes. The decomposition products of the IL lead to a large number of inorganic species such as LiOH and Li2O, which yield superior rate capability for the IL electrolyte. The FSI-based IL offers promising applicability for a practical Si composite anode.

Morphological and Chemical Evolution of Silicon Nanocomposite during Cycling

High-capacity materials are required to further the development of Li-ion batteries. As a result, silicon (Si) has been investigated as a promising anode in Li-ion batteries because of its high theoretical capacity (3579 mAh/g), which is 10 times higher than of the commercial graphite anodes (372 mAh/g). However, Si electrodes undergo sever mechanical degradation due to the volume exaptation (300%) upon lithiation and suffers from an unstable solid electrolyte interphase (SEI) layer. The SEI layer occurs at the electrode/electrolyte interface as a result of the electrolyte decomposition. In this work crystalline Si (60 nm particle size) was used to prevent large strain that occurs from the volume expansion. In order to improve the cycling performance of Si composite electrode, fluoroethylene carbonate (FEC) electrolyte additive was added to the electrolyte [1-4]. For the first time, various TEM/STEM EELS techniques demonstrated the effects of FEC on the morphological and chemical evolution of the SEI on Si composite electrode during electrochemical cycling.