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The synthesis and processing of garnet-type conductors is usually done at temperatures above 1100 °C to reach the high Li-ion conducting cubic phase resulting in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode-electrolyte assembly. Here, we tackle this problem and report a novel low-temperature aqueous synthesis route to stabilize the cubic phase of Li 6.1 Al 0.3 La 3 Zr 2 O 12 (c-Al-LLZO) at a temperature as low as 600 °C , while obtaining nano-crystallites at around 100 - 200 nm. Moreover, a dual-reaction mechanism was proposed to describe the process of c-Al-LLZO crystallization. The as-synthesized nanoscale c-Al-LLZO particles facilitate the densification of the solid-state electrolyte (SSE) pellets with less endowed grain boundaries (exhibiting a high relative density of 97.8 %). The ionic conductivity reaches 0.42 mS cm−1 at 21 °C and the low activation energy (half of the published values) is 0.17 eV. The results presented here show that symmetric cells with Li metal exhibit excellent stability at the current density of 0.2 mA/cm2 and 0.5 mA/cm2, and an improved critical current density (CCD) of 2.16 mA/cm2. This work provides a feasible method of low-temperature crystallization of nanoscale cubic garnet SSEs with good performance for the fabrication of stable SSLBs. [Display omitted] • Direct crystallization of cubic garnet (LLZO) solid-state electrolytes at 600 °C. • A new variety of near nanoscale cubic garnet solid electrolyte. • Excellent densification and high lithium-ion conductivity. • Symmetric cells with high CCD and Li metal plating-stripping stability. [ABSTRACT FROM AUTHOR]

Li2FeSiO4 (LFS) is a sustainable Li-ion cathode material composed of earth-abundant elements with potentially high energy density but suffers from limited reversible storage capacity (less than one Li), low intrinsic conductivities, and cycling instability. In search of deeper understanding of the structural chemistry of LFS towards overcoming some of these challenges, the viability of Fe-site doping by Co and other dopants in Pmn21 LFS is investigated using both first-principles calculations and experimental testing. Computational results suggest that the formation of Li2Fe1−xCoxSiO4 is energetically favorable, predictions confirmed by successful hydrothermal synthesis. Substitution of Co in LFS is revealed to have a dual effect; firstly catalyzing faster electrochemically induced phase transformation to the stable inverse-Pmn21 phase; and secondly enabling the formation of a low-resistant, uniform and stable fluorine-rich cathode-electrolyte interphase (CEI) layer that inhibits detrimental reactions between the cathode and electrolyte. As a result, Co-substituted LFS displays enhanced reversibility with 95% capacity retention after 50 cycles as compared to 80% of the undoped LFS. [ABSTRACT FROM AUTHOR]

In this work, a relatively inactive cathode material, βII orthorhombic Li2FeSiO4 with the Pmn21 space group, is shown to undergo crystal structural changes when subjected to nanosizing by high-energy milling that have a pronounced activation impact on Li-ion intercalation electrochemistry. In particular, we present evidence of mechanically induced structural annealing including lattice expansion, reduction of anti-site defects and preferred crystal orientation, opposite of what expected typically from nanosizing. As a result of this type of unreported structural activation for orthosilicates, a threefold increase of the accessible discharge (=intercalation) capacity of the high-energy milled Pmn21 material is attained, much higher than what would be expected solely from surface area increase due to nanosizing and surface area increase. Such mechanochemically induced structural annealing activates Li-ion diffusion (one order of magnitude enhancement in DLi) into less accessible sites via the combined effects of anti-site defect removal and modulation of the Fe–O coordination adjustment. Structural activation of intercalation materials via controlled mechanochemistry opens new possibilities in nanoengineering high energy density Li-ion storage electrodes. [ABSTRACT FROM AUTHOR]

Integration of solar‐energy harvesting and storage functions has attracted significant research attention, as it holds promise for ultimate development of light‐chargeable devices. In this context, a functional nanocomposite anode that not only permits electrochemical energy storage through Li‐ion photo‐intercalation, but also exhibits potential for photoelectrochromic applications, was investigated. The nanocomposite is made of the Li‐ion intercalation compound WO3, thinly coated with TiO2 and sensitized by the photoactive semiconductor CdS. During light exposure, the photoelectrons from CdS are transported to the WO3/electrolyte interface, where Li‐ion intercalation takes place. Photoelectron transport is facilitated by the interfacial TiO2 layer. The WO3 was shown to be functional in multiple photocharge–discharge cycles, but the CdS suffers from degradation and photocorrosion. Hence, the selection of compatible semiconductors and protective coating strategies should be pursued to overcome these issues. [ABSTRACT FROM AUTHOR]

Zero-strain and long-term stability of nanoscale lithium titanate (LTO) anode materials make possible the fabrication of exceptionally stable lithium ion batteries. But one issue must be considered that of nanostructure-induced relaxation in 2D LTO nanosheets which profoundly modifies their Li storage properties and structural stability. Excessively intercalated Li ions at both 8 a and 16 c sites trigger nucleation of the relaxed LTO structure in the near-surface region, which impedes Li-ion diffusion and causes the increasing polarization of LTO nanosheet electrodes. Nuclei of relaxed LTO then undergo isotropic growth along the 3D Li-ion pathways in LTO to completely convert near-surface regions into relaxed LTO. With increasing population of trapped Li ions, the enhanced conductivity due to Ti4+/Ti3+ reduction gradually eliminates the raised polarization. In the meantime, spontaneous electrolyte/LTO reduction to form the solid electrolyte interphase starts playing a major role in capacity loss once the transformation of near-surface region into relaxed LTO becomes saturated. Elucidation of these fundamental intercalation-induced surface structure transformations contribute greatly into the design of highly performing 2D nanoscaled LTO and other electrode materials. [ABSTRACT FROM AUTHOR]

Lithium iron orthosilicate (Li 2 FeSiO 4 ) is an important alternative cathode for next generation Li-ion batteries due to its high theoretical capacity (330 mA h/g). However, its development has faced great challenges arising from significant structural complexity, including the disordered arrangement/orientation of Fe/Si tetrahedra, polytypes and its poorly understood Li storage and transport properties. In this context, ab-initio calculations are employed to investigate the phase stability and Li diffusion profiles of both monoclinic ( P 2 1 ) and orthorhombic ( Pmn 2 1 ) Li 2 FeSiO 4 orthosilicates. The calculations demonstrate that formation of Li Fe antisites can induce a metastability competition between both phases, with neither dominating across nearly the entire discharging profile from Li 2 FeSiO 4 through to LiFeSiO 4 . Furthermore, structural instability is shown to be a serious concern at discharge concentrations below LiFeSiO 4 (1 Li extraction) due to the shared occupation of Li donated electrons with oxygen 2p orbitals – rather than the hypothesized transition to a tetravalent Fe 4+ state. This finding is further supported by diffusion calculations that have determined a high activation energy barrier towards fast charging and rapid phase transitions. In summary, these theoretical results provide critical and timely insight into the structural dynamics of lithium iron orthosilicate, in pursuit of high energy density cathodes. [ABSTRACT FROM AUTHOR]

The reaction between an uncharged Li2FeSiO4 (LFS) cathode and a LiPF6-EC/DMC electrolyte is revealed by in situ XANES in coin cells. This study shows clear evidence of delithiation and iron oxidation in LFS prior to cycling. Subsequent cycling appears to partially restore the original lithiation level, an observation that needs to be taken into consideration in future LFS development work. [ABSTRACT FROM AUTHOR]

Nanostructured lithium metal orthosilicate materials hold a lot of promise as next generation cathodes but their full potential realization is hampered by complex crystal and electrochemical behavior. In this work Li2FeSiO4 crystals are synthesized using organic-assisted precipitation method. By varying the annealing temperature different structures are obtained, namely the monoclinic phase at 400°C, the orthorhombic phase at 900°C, and a mixed phase at 700°C. The three Li2FeSiO4 crystal phases exhibit totally different charge/discharge profiles upon delithiation/lithiation. Thus the 400°C monoclinic nanocrystals exhibit initially one Li extraction via typical solid solution reaction, while the 900°C orthorhombic crystals are characterized by unacceptably high cell polarization. In the meantime the mixed phase Li2FeSiO4 crystals reveal a mixed cycling profile. We have found that the monoclinic nanocrystals undergo phase transition to orthorhombic structure resulting in significant progressive deterioration of the material's Li storage capability. By contrast, we discovered when the monoclinic nanocrystals are cycled initially at higher rate (C/20) and subsequently subjected to low rate (C/50) cycling the material's intercalation performance is stabilized. The discovered rate-dependent electrochemically-induced phase transition and stabilization of lithium metal silicate structure provides a novel and potentially rewarding avenue towards the development of high capacity Li-ion cathodes. [ABSTRACT FROM AUTHOR]