Distorted surface regions (5-6 nm) with an unusual layered-like structure on LiMnO cathode material were directly observed after it was cycled (3-4.9 V), indicating a possible spinel-to-layered structural transformation. Formation of these distorted regions severely degrades LiMnO cathode capacity. As we attempt to get a better understanding of the exact crystal structure of the distorted regions, the structural transformation pathways and the origins of the distortion are made difficult by the regions' nanoscopic size. Inspired by the reduction of Mn to Mn in surface electronic structures that might be associated with oxygen loss during cycling, we further investigated the atomic-level surface structure of LiMnO by heat-treatments between 600 and 900 °C in various atmospheres, finding similar surface spinel-to-layered structural transformation only for LiMnO heat-treated in argon atmosphere for a few minutes (or more). Controllable and measurable oxygen loss during heat-treatments result in Mn for charge compensation. The ions then undergo a disproportionation reaction, driving the spinel-to-layered transformation by way of an intermediate LiMnO-like structure. The distortion of the surface regions can be extended to the whole bulk by heat-treatment for 300-600 min, ultimately enabling us to identify the bulk-level structure as layered LiMnO (C2/m). This work demonstrates the critical role of Mn in controlling the kinetics of the structural transformation in spinel LiMnO and suggests heat-treatment in argon as a convenient method to control the surface oxygen loss and consequently reconstruct the atomic-level surface structure.

Detailed investigation of the influence of surface modification using a typical oxide (TiO2) on the electrochemical cycling performance of LiNi0.5Mn1.5O4 at room temperature (25 °C) and elevated temperature (55 °C) is reported.

Batteries with lithium metal anodes are promising because of lithium’s high energy density. However, the growth of Li dendrites on the surface of the Li electrode in a liquid electrolyte during cycling reduces the safety and cycle performance of batteries, hindering their commercial application. In this work, we observe for the first time a smooth and dendrite-free Li deposition with a vertically grown, self-aligned, and highly compact columnar structure formed during cycling in a mixed carbonate–ether electrolyte. The stable microsized (∼10 μm in diameter and ∼20 μm in length) Li deposits are aligned in arrays on the surface of the Li electrode. The columnar Li deposits still exhibit a dendrite-free morphology and a compact structure after 200 cycles at a current density of 1 mA/cm2 and a 1.5 mAh/cm2 cycling capacity in a mixed carbonate–ether electrolyte. This work shows an optimiztic outlook for Li batteries with liquid electrolytes.

of triboelectrification and electrostatic induction. [11][12][13] Different materials, device architectures, and hybridized nanogenerators have been reported to realize higher power density and output power, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] indicating potential applications for robotics, physiology, entertainment, biomimetics, blue energy harvesters, [31,32] etc. To obtain higher power density, periodically pyramid, square, and hemisphere surface structures were introduced using top-down lithography techniques, [33][34][35] which, unfortunately, imposed a cost barrier for practical applications. On the other hand, under a given power density, it is an important research topic to fabricate large-area structure to realize higher output power. However, a larger system will inevitably result in heavier and bulkier footprint of the generator, which is not suitable for many wearable or implantable applications. Therefore, it is a grand challenge to develop smaller TENGs with higher power generation performance when the compactness and portability are required.

The high-voltage spinel LiNi 0.5 Mn 1.5 O 4 cathode material suffers from the rapid degradation of electrochemical cycling performance at elevated temperatures, which prevents its successful commercialization. Herein, we show that coating the surface of this material with Ta 2 O 5 , which has high resistance against hydrofluoric acid (HF) attack, is an effective way to improve its electrochemical cycling performance. A Ta 2 O 5 -coated LiNi 0.5 Mn 1.5 O 4 half-cell shows a capacity retention of ∼93% and a Coulombic efficiency of ∼98% after 100 cycles at 55 °C, compared to the corresponding values of ∼76% and ∼95% measured for the bare LiNi 0.5 Mn 1.5 O 4 half-cell. The detailed structural analysis of the Ta 2 O 5 -coated LiNi 0.5 Mn 1.5 O 4 shows that a small amount of Ta 5+ ions diffuse into the 16c site on the cathode surface during the coating process, as directly observed by Cs corrected scanning transmission electron microscopy. The modification of the LiNi 0.5 Mn 1.5 O 4 surface with Ta 5+ , together with the residual Ta 2 O 5 coating, stabilizes the surface structure during cycling, leading to reduced Ni and Mn dissolution as well as formation of the solid electrolyte interface (SEI). In contrast, LiNi 0.5 Mn 1.5 O 4 coated with HF scavengers, such as Al 2 O 3 , shows only limited improvement in cycling performance after prolonged cycling at 55 °C, due to the consumption of the surface coating by reaction with HF, which leaves LiNi 0.5 Mn 1.5 MnO 4 unprotected against HF attack. KEYWORDS: lithium ion batteries, LiNi 0.5 Mn 1.5 O 4 , coating, Ta 2 O 5 , scanning transmission electron microscopy (STEM)

Hydrothermal carbonization (HTC) of lawn grass was carried out at 200 °C and 240 °C for 30 to 180 min. The chemical, energetic, and structural characteristics of HTC solid residues were investigated. Results from HTC experiments indicate that solid mass yield of all solid residues was 31 to 50%. The hydrogen/carbon (H/C) and oxygen/carbon (O/C) atomic ratios of all solid residues were 1.17 to 1.64 and 0.45 to 0.65, respectively. The higher heating value (HHV) increased up to 20.54 MJ/kg with increasing HTC residence time at 240 °C for 180 min. Both XRD patterns and FTIR spectra show that differences occur with samples treated as compared to the raw material. Solid hydrochar exhibited higher ordered structure characteristics and was mainly derived from amorphous components degradation when the residence time was increased from 30 to 180 min at 200 °C, while hydrochar formed from cellulose components degradation with increased residence time at 240 °C. According to the results studied, it was found that prolonged residence time was favorable to the formation of hydrochar from lawn grass.

Sodium-metal batteries with conventional organic liquid electrolytes have disadvantages including dendrite deposition and safety concern. In this work, we report a low-flammable electrolyte (NaPF6-FRE) consisting of 1 M NaPF6 in 1,2-dimethoxyethane (DME), fluoroethylene carbonate (FEC), and 1,1,1,3,3,3-hexafluoroisopropylmethyl ether (HFPM) (2:1:2, in volume ratio). The symmetric Na and Na||Cu cells with a 1 M NaPF6-DME electrolyte absorbed in a porous separator, such as the porous glass-fiber, show very poor cycling performance. In addition, the cell with a Na3V2(PO4)3 (NVP) cathode and 1 M NaPF6-DME electrolyte shows low Coulombic efficiency. FEC was added into the NaPF6-DME-based electrolyte to reduce the irreversible capacity of the NVP cathode and improve the Coulombic efficiency of the cell. However, the high reactivity of FEC with the Na electrode leads to formation of an unstable solid electrolyte interphase (SEI) and large interfacial resistance, and HFPM was further added to stabilize the Na electrode surface by forming a new fluorine-containing organic layer. The new prepared low-flammable electrolyte (NaPF6-FRE) with 1 M NaPF6 in DME, FEC, and HFPM (2:1:2, in volume ratio) shows a wide electrochemical window of 5.2 V. The Na symmetric cells with this low-flammable electrolyte show superior cycling performance for 800 h with a stable voltage profile at 0.5 mA cm–2, 0.5 mA h cm–2 and 1 mA cm–2, 1 mA h cm–2, respectively. The NVP||Na cells show an excellent capacity retention of 94% after 2000 cycles and superior Coulombic efficiency of 99.9% on average at 5 C.

Stabilization of the atomic-level surface structure of LiMnO with Al ions is shown to be significant in the improvement of cycling performance, particularly at a high temperature (55 °C) and high voltage (5.1 V). Detailed analysis by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy, etc. reveals that Al ions diffuse into the spinel to form a layered Li(Al,Mn)O structure in the outmost surface where Al concentration is the highest. Other Al ions diffuse into the 8a sites of spinel to form a (MnAl)O structure and the 16d sites of spinel to form Li(MnAl)O. These complicated surface structures, in particular the layered Li(Al,Mn)O, are present at the surface throughout cycling and effectively stabilize the surface structure by preventing dissolution of Mn ions and mitigating cathode-electrolyte reactions. With the Al ions surface modification, a stable cycle performance (∼78% capacity retention after 150 cycles) and high Coulombic efficiency (∼99%) are achieved at 55 °C. More surprisingly, the surface-stabilized LiMnO can be cycled up to 5.1 V without significant degradation, in contrast to the fast capacity degradation found in the unmodified case. Our findings demonstrate the critical role of ions coated on the surface in modifying the structural evolution of the surface of spinel electrode particles and thus will stimulate future efforts to optimize the surface properties of battery electrodes.