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Articledepartmental bulletin pape

Introduction

MEXT-Supported Program for the Strategic Research Foundation at Private Universities (2014-2018)othe

Li-Ion Transport and Solution Structure in Sulfolane-Based Localized High-Concentration Electrolytes

Localized high-concentration electrolytes (LHCEs), which are mixtures of highly concentrated electrolytes (HCEs) and non-coordinating diluents, have attracted significant interest as promising liquid electrolytes for next-generation Li secondary batteries, owing to their various beneficial properties both in the bulk and at the electrode/electrolyte interface. We previously reported that the large Li+-ion transference number in sulfolane (SL)-based HCEs, attributed to the unique exchange/hopping-like Li+-ion conduction, decreased upon dilution with the non-coordinating hydrofluoroether (HFE) despite the retention of the local Li+-ion coordination structure. Therefore, in this study, we investigated the effects of HFE dilution on the Li+ transference number and the solution structure of SL-based LHCEs via the analysis of dynamic ion correlations and molecular dynamics simulations. The addition of HFE caused nano-segregation in the SL-based LHCEs to afford polar and nonpolar domains and fragmentation of the polar ion-conducting pathway into smaller clusters with increasing HFE content. Analysis of the dynamic ion correlations revealed that the anti-correlated Li+?Li+ motions were more pronounced upon HFE addition, suggesting that the Li+ exchange/hopping conduction is obstructed by the non-ion-conducting HFE-rich domains. Thus, the HFE addition affects the entire solution structure and ion transport without significantly affecting the local Li+-ion coordination structure. Further studies on ion transport in LHCEs would help obtain a design principle for liquid electrolytes with high ionic conductivity and large Li+-ion transference numbers.journal articl

Electrochemical Pretreatment of Solid-Electrolyte Interphase Formation for Enhanced Li4Ti5O12 Anode Performance in a Molten Li-Ca Binary Salt Hydrate Electrolyte

Water-in-salt electrolytes have been widely explored because of their expanded electrochemical stability (>3.0 V). However, the instability of solid-electrolyte interphase (SEI) in aqueous electrolytes leads to their reductive decomposition on the negative electrodes of low-potential anode materials. Here, we demonstrate significant improvement in the cycle performance of a Li4Ti5O12 electrode using a Li-Ca binary salt hydrate (LCH) electrolyte in combination with an optimized electrochemical pretreatment process. Compared with a hydrate-melt electrolyte, the LCH electrolyte provided less water-soluble Ca-based SEI components, and careful pretreatment process enabled the formation of a thicker SEI layer on the Li4Ti5O12 electrode. Protected with the hardly soluble, thick SEI layer, the Li4Ti5O12 electrode effectively mitigated unfavorable side reactions and achieved 95.5 % capacity retention over 50 cycles. These results offer insight into a promising route for stable SEI layer formation for the practical use of low-potential anode materials in aqueous rechargeable lithium-ion batteries.journal articl

Enhancing Li–S Battery Performance with Limiting Li[N(SO2F)2] Content in a Sulfolane-Based Sparingly Solvating Electrolyte

By enhancing the stability of the lithium metal anode and mitigating the formation of lithium dendrites through electrolyte design, it becomes feasible to extend the lifespan of lithium–sulfur (Li–S) batteries. One widely accepted approach involves the utilization of Li[N(SO2F)2] (Li[FSA]), which holds promise in stabilizing the lithium anode by facilitating the formation of an inorganic-dominant solid electrolyte interface (SEI) film. However, the use of Li[FSA] encounters limitations due to inevitable side reactions between lithium polysulfides (LiPSs) and [FSA] anions. In this study, our focus lies in precisely controlling the composition of the SEI film and the morphology of the deposited lithium, as these two critical factors profoundly influence lithium reversibility. Specifically, by subjecting an initial charging process to an elevated temperature, we have achieved a significant enhancement in lithium reversibility. This improvement is accomplished through the employment of a LiPS sparingly solvating electrolyte with a restricted Li[FSA] content. Notably, these optimized conditions have resulted in an enhanced cycling performance in practical Li–S pouch cells. Our findings underscore the potential for improving the cycling performance of Li–S batteries, even when confronted with challenging constraints in electrolyte design.journal articl

Li+ transference number and dynamic ion correlations in glyme-Li salt solvate ionic liquids diluted with molecular solvents

Highly concentrated electrolytes (HCEs) have attracted significant interest as promising liquid electrolytes for next-generation Li secondary batteries, owing to various beneficial properties both in the bulk and at the electrode/electrolyte interface. One particular class of HCEs consists of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and oligoethers that behave like ionic liquids. [Li(G4)][TFSA], which comprises an equimolar mixture of LiTFSA and tetraglyme (G4), is an example. In our previous works, the addition of low-polarity molecular solvents to [Li(G4)][TFSA] was found to effectively enhance the conductivity while retaining the unique Li-ion solvation structure. However, it remains unclear how the diluents affect another key electrolyte parameter-the Li+ transference number-despite its critical importance for achieving the fast charging/discharging of Li secondary batteries. Thus, in this study, the effects of diluents on the extremely low Li+ transference number under anion-blocking conditions in [Li(G4)][TFSA] were elucidated, with a special focus on the polarity of the additional solvents. The concentration dependence of the dynamic ion correlations was further studied in the framework of the concentrated electrolyte theory. The results revealed that a non-coordinating diluent is not involved in the modification of the ion transport mechanism, and therefore the low Li+ transference number is inherited by the diluted electrolytes. In contrast, a coordinating diluent effectively reduces the anti-correlated ion motions of [Li(G4)][TFSA], thereby improving the Li+ transference number. This is the first time that the significant effects of the coordination properties of the diluting solvents on the dynamic ion correlations and Li+ transference numbers have been reported for diluted solvate ionic liquids.journal articl

Impact of Li Ion Transport Properties on Reversibility of Li Metal Electrode in Glyme-Based Electrolytes

The demand for innovative batteries with high specific energy densities has increased. Li-metal batteries employing Li-metal anodes, regarded as the ultimate anodes with a high theoretical capacity, have been extensively studied over the past few decades. However, the poor reversibility and safety concerns regarding Li-metal anodes remain unresolved. The importance of the electrode/electrolyte interface, especially the solid electrolyte interphase (SEI), for achieving reversibility of Li metal anodes has been extensively studied. Herein, we focused on the impact of the Li ion transport properties in oligoether (glyme)-based electrolytes on the deposition/dissolution efficiency of Li metal anodes. Analysis of the low-frequency impedance spectra of Li-plated Cu/Li cells revealed that the diffusion resistance of Li ions (Rdiffusion) may be a dominant contributor to the internal resistance of the cells employing glyme-based electrolytes. A higher Rdiffusion in poor-mass-transport electrolytes with a lower Li ion transference number resulted in larger polarization during Li deposition/dissolution, leading to more pronounced unfavorable side reactions and lower Coulombic efficiency. Rdiffusion rather than interfacial resistance affected the reversibility of the Li metal anode. Enhancing the Li ion mass transport ability of electrolytes is important for achieving highly reversible charge-discharge performance of Li metal anodes at high current densities.journal articl

Farmers' decision making strategies on selecting rootcrop varieties in the Visayas Region, Philippines

application/pdfIn the Philippines, rootcrops play a significant role in providing livelihood and food security among the resource poor people living in fragile upland environment, which comprise about 65% of the total agricultural land in the country. This paper will present the factors which play into farmers’ decision-making strategies for varietal adoption, particularly of sweetpotato and cassava in Visayas Region, Philippines. Relative importance of factors as they are considered in the farmers’ decisionmaking process vary according to user-orientation (i.e. subsistent, semi-commercial and commercial) because such would have varietal trait specifications, scale of operations, and consequently, resource needs. In largely market or industry-oriented systems, the economic factor is observed to be more important. The market requirement of varieties is enough motivation to grow a variety subject to resource constraints. Further, the degree of intensity of factor or constraint could be location specific due to natural endowments (e.g. land quality), farming systems, and social relations. But in the absence of markets or profitable use, adoption of a new variety is a difficult choice. Overall, given the complexity of circumstances that are factored in the farmers’ decision-making strategies to sustainably adopt varieties from the farmers’ own sphere to that of the institutions (i.e. Research and Development policies), any research and development activity that hopes to eventually make the farmers’ lives better through the adoption of technologies should be designed with a greater partnership between the natural-technical and social scientists, the local partners and communities.departmental bulletin pape

On the concentration polarisation in molten Li salts and borate-based Li ionic liquids

Electrolytes that transport only Li ions play a crucial role in improving rapid charge and discharge properties in Li secondary batteries. Single Li-ion conduction can be achieved via liquid materials such as Li ionic liquids containing Li+ as the only cations because solvent-free fused Li salts do not polarise in electrochemical cells, owing to the absence of neutral solvents that allow polarisation in the salt concentration and the inevitably homogeneous density in the cells under anion-blocking conditions. However, we found that borate-based Li ionic liquids induce concentration polarisation in a Li/Li symmetric cell, which results in their transference (transport) numbers under anion-blocking conditions (tabc Li) being well below unity. The electrochemical polarisation of the borate-based Li ionic liquids was attributed to an equilibrium shift caused by exchangeable B-O coordination bonds in the anions to generate Li salts and borate-ester solvents at the electrode/electrolyte interface. By comparing borate-based Li ionic liquids containing different ligands, the B-O bond strength and extent of ligand exchange were found to be directly linked to the tabc Li values. This study confirms that the presence of dynamic exchangeable bonds causes electrochemical polarisation and provides a reference for the rational molecular design of Li ionic liquids aimed at achieving single-ion conducting liquid electrolytes.journal articl

High-concentration LiPF6/sulfone electrolytes: structure, transport properties, and battery application

Non-flammable and oxidatively stable sulfones are promising electrolyte solvents for thermally stable high-voltage Li batteries. In addition, sulfolane-based high-concentration electrolytes (HCEs) show high Li+ ion transference numbers. However, LiPF6 has not yet been investigated as the main salt in sulfone-based HCEs for Li batteries. In this study, we investigated the phase behaviors, solvate structures, and transport properties of binary and ternary mixtures of LiPF6 and the following sulfone solvents: sulfolane (SL), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), and 3-methyl sulfolane (MSL). The stable crystalline solvates Li(SL)4PF6 and Li(DMS)2.5PF6 with high melting points were formed in the LiPF6/SL and LiPF6/DMS mixtures, respectively. In contrast, LiPF6/EMS, LiPF6/MSL, and LiPF6/SL/another sulfone mixtures remained liquids over a wide temperature range. Raman spectroscopy revealed that SL and another sulfone are competitively coordinated to Li+ ions to dissociate LiPF6 in the ternary mixtures. Although the ionic conductivity decreased with increasing LiPF6 concentration due to an increase in viscosity, Li+ ions diffused faster than PF6−via exchanging ligands in the HCE [LiPF6]/[SL]/[DMS] = 1/2/2, resulting in a higher Li ion transference number than that in conventional Li battery electrolytes.journal articl