Low Li-ion Research Articles

The Effects of Low-Temperature Exposures on Li-Ion Battery Performance

Li-ion batteries (LIBs) are the foundation of modern-day electronics, electric vehicles, and renewable energy storage solutions due to their high energy density and Coulombic efficiency and long cycle life. However, using Li-ion batteries at low temperatures, such as in northern climates and space applications, results in drastically reduced performance due to, in part, lower diffusion properties [1,2]. Moreover, recent studies indicate that storing LIBs at low temperatures leads to electrode degradation, which further reduces their performance at normal temperatures [3]. While multiple studies demonstrate reduced LIB performance during operation at low temperatures [4–8], the mechanisms affecting the LIBs during low-temperature storage are not well understood. Therefore, identifying these mechanisms is crucial to developing low temperature-compatible Li-ion batteries. In this work, we study the effects of low-temperature exposures on the performance of Li-ion batteries when they are restored to normal temperature conditions. Single-layer pouch cells with NMC cathodes and graphite anodes are fabricated and thermally cycled between ambient temperature and -60 °C with controlled ramp rates. Electrochemical studies are then performed at ambient conditions, which reveal two performance deterioration contributions: electrolyte effects in the initial stages of low-temperature exposures and loss of active material associated with long-term thermal cycling. Post-mortem analysis was used to identify that cathode particle cracking is the primary electrode degradation mechanism. Acknowledgments The authors gratefully acknowledge the NASA Established Program to Stimulate Competitive Research (EPSCoR) Program for funding this research (NASA Grant Number 80NSSC23M0068). The authors also acknowledge Dr. William West and Dr. Marshall Smart (NASA Jet Propulsion Laboratory) for technical discussion of this work and Dr. Sara Nelson and Ms. Hailey Waller (Iowa NASA EPSCoR) for administrative services.

Poly(Ionic Liquid) Electrolytes at an Extreme Salt Concentration for Solid-State Batteries.

Polymer-in-salt electrolytes were introduced three decades ago as an innovative solution to the challenge of low Li-ion conductivity in solvent-free solid polymer electrolytes. Despite significant progress, the approach still faces considerable challenges, ranging from a fundamental understanding to the development of suitable polymers and salts. A critical issue is maintaining both the stability and high conductivity of molten salts within a polymer matrix, which has constrained their further exploration. This research offers a promising solution by integrating cationic poly(ionic liquids) (polyIL) with a crystallization-resistive salt consisting of asymmetric anions. A stable polymer-in-salt electrolyte with an exceptionally high Li-salt content of up to 90 mol % was achieved, providing a valuable opportunity for the in-depth understanding of these electrolytes at an extremely high salt concentration. This work explicates how increased salt concentration affects coordination structures, glass transitions, ionic conductivity, and the decoupling and coupling of ion transport from structural dynamics in a polymer electrolyte, ultimately enhancing electrolyte performance. These findings provide significant knowledge advancement in the field, guiding the future design of polymer-in-salt electrolytes.

Unlocking fast Li-ion transport in micrometer-sized Mn-based cation-disordered rocksalt cathodes

Unlocking fast Li-ion transport in micrometer-sized Mn-based cation-disordered rocksalt cathodes

Lewis-basic nitrogen-rich covalent organic frameworks enable flexible and stretchable solid-state electrolytes with stable lithium-metal batteries

Lewis-basic nitrogen-rich covalent organic frameworks enable flexible and stretchable solid-state electrolytes with stable lithium-metal batteries

Concerted Formation of Reversibly Precipitated Sulfur Species and Its Importance for Lean Electrolyte Lithium-Sulfur Batteries.

Achieving high energy densities for lithium-sulfur batteries remain elusive. Largely limited by the volume of electrolyte used, lean electrolyte conditions (electrolyte/sulfur mass ratio <3) present enormous challenges that have led to very poor specific capacity and rate performance. Previous studies have identified that the high concentration of polysulfide is responsible for the poor discharge voltage. However, there still lacks sufficient understanding of the processes occurring at lean electrolyte conditions. In this work we uncovered a polysulfide concentration regulating mechanism that operates through the precipitation and redissolution of solid sulfur-based species (reversibly precipitated sulfur species, RPSS). This occurs in a concerted manner in a global sense through the cathode and can be measured using impedance spectroscopy. It was found that the more RPSS that is formed, the higher the energy density of discharge. We propose that high concentration of polysulfide tends to supersaturate, which impeded the formation of RPSS. Employing an electrolyte with low Li ion concentration along with using poorly dissociating lithium salts allowed for more RPSS formation and ultimately enabled discharge at >2.0 V at 0.05 C, at E/S = 2.5, and at room temperature without the use of an engineered cathode.

Effects of different Ti concentrations doping on Li2MnO3 cathode material for lithium-ion batteries via density functional theory

Li2MnO3 is extensively studied for a cathode material in lithium-ion batteries because of its high voltage and specific capacity. Nevertheless, it has the disadvantages due to low conductivity and Li-ion diffusion. To modify its performance, we determine the structure stability and electronic properties of Li2MnO3 cathodes doped with different Ti-ion concentrations using the spin-polarized density functional theory including the Hubbard term (DFT + U). For the calculations, cell parameters, formation energies, band gaps, total density of states, partial density of states and stability voltages are determined. The results highlight that the expansion of the cell volumes by Ti-ion impurities has a positive effect on the diffusion of Li ions in these cathodes. Because of the minor voltage changes, Li2MnO3 cathode doped with a Ti-ion concentration of 0.250 exhibits the highest voltage stability. Overall, these results are effective for the lithium-ion battery application based on Ti-doped Li2MnO3 cathodes.

Inorganic Composition Modulation of Solid Electrolyte Interphase for Fast Charging Lithium Metal Batteries.

The solid electrolyte interphase (SEI) with lithium fluoride (LiF) is critical to the performance of lithium metal batteries (LMBs) due to its high stability and mechanical properties. However, the low Li ion conductivity of LiF impedes the rapid diffusion of Li ions in the SEI, which leads to localized Li ion oversaturation dendritic deposition and hinders the practical applications of LMBs at high-current regions (>3 C). To address this issue, a fluorophosphated SEI rich with fast ion-diffusing inorganic grain boundaries (LiF/Li3P) is introduced. By utilizing a sol electrolyte that contains highly dispersed porous LiF nanoparticles modified with phosphorus-containing functional groups, a fluorophosphated SEI is constructed and the presence of electrochemically active Li within these fast ion-diffusing grain boundaries (GBs-Li) that are non-nucleated is demonstrated, ensuring the stability of the Li || NCM811 cell for over 1000 cycles at fast-charging rates of 5 C (11mA cm-2). Additionally, a practical, long cycling, and intrinsically safe LMB pouch cell with high energy density (400Wh kg-1) is fabricated. The work reveals how SEI components and structure design can enable fast-charging LMBs.

Influence of Zr aggregation on Li-ion conductivity of amorphous solid-state electrolyte Li-La-Zr-O.

In our study, we investigated the influence of the local structure of amorphous Li-La-Zr-O (a-LLZO) on Li-ion conductivity using abinitio molecular dynamics (AIMD). A-LLZO has shown promising properties in inhibiting the growth of lithium dendrites, making it a potential candidate for solid electrolytes in all-solid-state lithium batteries. The low Li-ion conductivity of a-LLZO is currently limiting its practical applications. Our findings revealed that the homogeneous distribution of Zr-O polyhedra within the pristine structure of a-LLZO contributes to enhanced Li-ion conductivity. By reducing the interconnections among Zr-O polyhedra, the AIMD-simulated a-LLZO sample achieved a Li-ion conductivity of 5.78 × 10-4 S/cm at room temperature, which is slightly lower than that of cubic LLZO (c-LLZO) with a Li-ion conductivity of 1.63 × 10-3 S/cm. Furthermore, we discovered that Li-ion conductivity can be influenced by adjusting the elemental ratios within a-LLZO. This suggests that fine-tuning the composition of a-LLZO can potentially further enhance its Li-ion conductivity and optimize its performance as a solid electrolyte in lithium batteries.

Revealing the key role of non-solvating diluents for fast-charging and low temperature Li-ion batteries

Revealing the key role of non-solvating diluents for fast-charging and low temperature Li-ion batteries

Self-Standing Single-Ion Borate Salt-Based Polymer Electrolyte for Lithium Metal Batteries.

Polymer electrolytes (PEs) with excellent flexibility and superior compatibility toward lithium (Li) metal anodes have been deemed as one of the most promising alternatives to liquid electrolytes. However, conventional lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-based dual-ion PEs suffer from a low Li ion transference number and notorious Li dendrite growth. Here, a single-ion conducting polyborate salt without any fluorinated groups, polymeric lithium dihydroxyterephthalic acid borate (PLDPB), is presented for addressing the issues of Li metal batteries. Owing to a nearly immovable bulky anion and the presence of a rigid benzene structure, the PLDPB@poly(ethylene oxide) (PEO) PE exhibits an ultrahigh Li ion transference number (0.94) and excellent mechanical strength, which could significantly restrict the growth of Li dendrites. Postmortem analysis reveals that a fluorine-free solid electrolyte interphase (SEI) enriched with B-O and benzene-containing species is formed on the surface of the Li metal anode, thereby facilitating elimination of excessive parasitic reactions and simultaneously suppressing the formation of Li dendrites. Consequently, the LiFePO4/Li cells with PLDPB@PEO PEs show an improved long-term cycling performance and high capacity retention (90.0%) and Coulombic efficiency (99.9%) after 500 cycles. This work may inspire new ideas to boost the development of single-ion conducting salts for dendrite-free Li metal batteries.

Solvation Structure and Dynamics in Ionic Liquid Electrolytes with Hydrofluoroether for Li-Ion Batteries

The addition of hydrofluoroethers (HFEs) to Li-ion battery electrolytes reduces flammability and enhances the cycling behavior at a wider temperature range. In the case of ionic liquid electrolytes, where the ion concentration is high and Li+ solvation by anions govern the transport properties, HFEs facilitate the separation of ion aggregates. In this study, the addition of HFE to n-methyl-n-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR14][TFSI] with Li-salts of lithium (nonafluorobutane)(trifluoromethanesulfonyl)imide [Li][IM14] and [Li][TFSI] (0 ≤ x Li ≤ 0.3), were studied to understand the compositional changes in phase, the liquid structure and the transport properties. A suppression in the melting point with the addition of HFE was accompanied with improved conductivities at lower temperatures. Li+ was found to primarily solvated by [TFSI] with some contribution by [IM14], as determined from Raman spectroscopy and molecular dynamics (MD) simulations. Further, HFE encouraged florous domain formations as determined by HOESY NMR and even short contacts between HFE and Li+ as a result of tighter anion-solvated Li+ without aggregation. Li+ diffusivity of 1.2610-12 m2/s at 0 °C and x Li = 0.2 was achieved with a high Li+ transference of 0.16, relative to ionic liquid electrolytes at 0 °C. The developed electrolytes demonstrated cyclable Si-Li half cells. This study demonstrates the tunability of ionic liquid electrolytes by asymmetric fluorinated anions and HFE co-solvents for low temperature Li-ion battery applications.

Impact of fluorination on Li+ solvation and dynamics in ionic liquid-hydrofluoroether locally concentrated electrolytes

Impact of fluorination on Li+ solvation and dynamics in ionic liquid-hydrofluoroether locally concentrated electrolytes

Zwitterionic Polyurethane Solid Polymer Electrolytes: A Pathway to High-Performance All-Solid-State Lithium-Ion Batteries

Lithium-ion batteries (LIBs) currently dominate the energy storage market for electronic devices, thanks to their high energy density, high operating voltage, and good cycling performance. However, commercial LIBs employing organic liquid electrolyte and lithium (Li) salts pose significant safety concerns, including flammability and the risk of thermal runaway. Consequently, the development of solid electrolytes is of paramount importance. Among several solid ion conductors, solid polymer electrolytes (SPEs) can offer numerous advantages, including excellent flexibility and interfacial compatibility with electrodes, good processibility, low cost, and light weight characteristics. However, current SPEs face challenges such as poor thermal stability, inferior electrochemical stability, and low Li-ion conductivity at room temperature (~ 10-5 S cm-1 at 25 ℃).In this study, we introduce a multifunctional zwitterionic polyurethane-based solid polymer electrolyte (zPU-SPE) for all-solid-state LIBs (SLBs) that addresses the limitations of conventional SPE materials. We synthesized zPU [i.e., poly((diethanolamine ethyl acetate)-co-poly(tetrahydrofuran)-co-(1,6-diisocyanatohexane))] and demonstrated its capability to host high amounts of LiTFSI without phase separation (up to 90 wt% LiTFSI loading). The Li-ion conductivity of zPU-SPE at 25° C is 7.4 × 10-4 S/cm, nearly 14 times greater than that of poly(ethylene oxide) (PEO) SPE (EO/Li+ = 16). Moreover, the high surface energy of zPU-SPE (487.5 J/m2 vs. 1.39 J/m2 of PEO) minimizes interfacial resistance. The zPU-SPE also exhibits outstanding elasticity, with a tensile break of 1700%, attributable to its dense inter- and intra-molecular hydrogen bonding.We evaluated the SLB battery performance of PEO and zPU-SPEs using a solid-state Li/SPE/LiFePO4 cell, which we cycled at a constant current rate of 1 C at 25 °C. The SLB cell with zPU-SPE demonstrates remarkable cycle stability, retaining 86% capacity after 1,000 cycles and 76% capacity after 2,000 cycles at a 1C rate and room temperature, while maintaining nearly 100% coulombic efficiency. In contrast, the capacity of the SLB cell with PEO rapidly decreases to 30% of its initial capacity and fails after 100 cycles. Given that significant capacity loss in LIBs under cold weather conditions poses a challenge for electric vehicles, we also assessed the temperature-dependent battery performance. The SLB with zPU-SPE retains 93.8% capacity at 0 °C, a substantial improvement over state-of-the-art LIBs (< 80% capacity retention). We attribute the high-capacity retention of zPU-SPE at low temperatures to the excellent chain mobility of the soft segment in the zPU matrix, which has a low glass transition temperature (-45 °C) and a high ion transport rate due to the zwitterionic group. We also conducted atomistic molecular modeling to investigate the dissolution and dissociation of Li salts and the Li-ion transport mechanism within the zPU polymer matrix. Furthermore, we employed small-angle X-ray scattering to examine the structural organization of the zPU-SPE matrix and its correlation with ion conductivity. Our work aims to guide the design of novel polymer electrolytes capable of overcoming the trade-offs associated with ion-conducting polymers (i.e., stability and ion conductivity). Figure 1

A Promising Solid-State Synthesis of LiMn1- yFeyPO4 Cathode for Lithium-ion Batteries.

LiMn1-yFeyPO4 (LMFP) is a significant and cost-effective cathode material for Li-ion batteries, with a higher working voltage than LiFePO4 (LFP) and improved safety features compared to layered oxide cathodes. However, its commercial application faces challenges due to a need for a synthesis process to overcome the low Li-ion diffusion kinetics and complex phase transitions. Herein, a solid-state synthesis process using LFP and nano LiMn0.7Fe0.3PO4 (MF73) is proposed. The larger LFP acts as a structural framework fused with nano-MF73, preserving the morphology and high performance of LFP. These results demonstrate that the solid-state reaction occurs quickly, even at a low sintering temperature of 500 °C, and completes at 700 °C. However, contrary to the expectations, the larger LFP particles disappeared and fused into the nano-MF73 particles, revealing that Fe ions diffuse more easily than Mn ions in the olivine framework. This discovery provides valuable insights into understanding ion diffusion in LMFP. Notably, the obtained LMFP can still deliver an initial capacity of 142.3 mAh g-1, and the phase separation during the electrochemical process is significantly suppressed, resulting in good cycling stability (91.1% capacity retention after 300 cycles). These findings offer a promising approach for synthesizing LMFP with improved performance and stability.

Origin of Li+ Solvation Ability of Electrolyte Solvent: Ring Strain.

Developing new organic solvents to support the use of Li metal anodes in secondary batteries is an area of great interest. In particular, research is actively underway to improve battery performance by introducing fluorine to ether solvents, as these are highly compatible with Li metal anodes because fluorine imparts high oxidative stability and relatively low Li-ion solvation ability. However, theoretical analysis of the solvation ability of organic solvents mostly focuses on the electron-withdrawing capability of fluorine. Herein, we analyze the effect of the structural characteristics of solvents on their Li+ ion solvation ability from a computational chemistry perspective. We reveal that the structural constraints imposed on the oxygen binding sites in solvent molecules vary depending on the structural characteristics of the N-membered ring formed by the interaction between the organic solvent and Li+ ions and the internal ring containing the oxygen binding sites. We demonstrate that the structural strain of the organic solvents has a comparable effect on Li+ solvation ability seen for the electrical properties of fluorine elements. This work emphasizes the importance of understanding the structural characteristics and strain when attempting to understand the interactions between solvents and metal cations and effectively control the solvation ability of solvents.

Asymmetric Trihalogenated Aromatic Lithium Salt Induced Lithium Halide Rich Interface for Stable Cycling of All-Solid-State Lithium Batteries.

Solid polymer electrolytes (SPEs) are the key components for all-solid-state lithium metal batteries with high energy density and intrinsic safety. However, the low lithium ion transference number (t+) of a conventional SPE and its unstable electrolyte/electrode interface cannot guarantee long-term stable operation. Herein, asymmetric trihalogenated aromatic lithium salts, i.e., lithium (3,4,5-trifluorobenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiFFF) and lithium (4-bromo-3,5-difluorobenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiFBF), are synthesized for polymer electrolytes. They exhibit higher t+ values and better compatibility with Li metal than conventional lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). Due to the trihalogenated aromatic anions, LiFFF- and LiFBF-based electrolytes are prone to generate an LiF- and LiBr-rich solid electrolyte interphase (SEI), therefore increasing the stability of the solid electrolyte/anode interface. Particularly, LiFBF could induce a LiF/LiBr hybrid SEI, where LiF shows a high Young's modulus and high surface energy for homogenizing Li ion flux and LiBr exhibits an extremely low Li ion diffusion barrier in the SEI layer. As a result, the Li/Li symmetric cells could remain stable for more than 1200 h without a short circuit and the LiFePO4/Li batteries showed superb electrochemical performance over 1200 cycles at 1 C. This work provides valuable insights from the perspective of lithium salt molecular structures for high-performance all-solid-state lithium metal batteries.

Microwave-Assisted Sol-Gel Synthesis of High-Voltage Ruthenium-Doped LiFePO4 Cathodes for Solid-State Lithium-Ion Batteries

The demand for high voltage and high-capacity energy systems has grown due to increased use of portable electronics, electric vehicles, and grid energy storage systems. LiFePO4 (LFP) shows great promise as a cathode material due to its low cost, structural stability, safety characteristics, and low environmental impact. However, LFP cathodes possess a low Li-ion diffusion coefficient and poor electrical conductivity compared to oxide-based cathodes that are used currently. To make LFP competitive with oxide-based cathodes, LFP was modified to improve its characteristics. Doping of materials is a highly effective method for improving electrochemical characteristics in LFP cathodes. Ruthenium-doping of iron sites within LFP has shown positive effects in the promotion of Li-ion diffusion, reduction in band gap energies, and improved electrical conductivity. Previous studies have shown that ruthenium-doping at 0.01 moles has an increased specific capacity of 162 mAh g-1, better cycling characteristics, and reduced resistance compared to pristine LFP.1 Ruthenium doping levels above 0.02 moles have shown a decrease in specific capacity, cycling characteristics, and an increased resistance due to the formation of ruthenium oxide that disrupts the crystal lattice of LFP. In this study, LiFe1-xRuxPO4 (x= 0.01, 0.0075, and 0.005) and pristine LFP were prepared via microwave-assisted sol-gel synthesis to explore the effects of lower levels of ruthenium-doping on LFP electrochemical characteristics. Microwave synthesis was employed for reduced sintering time, beneficial effects on microstructure, and decreased cost. Samples were characterized via scanning electron microscopy, in-situ/ex-situ X-ray diffraction, energy dispersive X-ray spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry.The authors acknowledge financial support from the NSF IUCRC program for the “Center for solid-state electric power storage” (#2052631) and support from the South Dakota Board of Regents for the “Governor’s Research Center (GRC) for electrochemical energy storage”.1.) Gao, Y.; Xiong, K.; Zhang, H.; Zhu, B. Effect of RU Doping on the Properties of Lifepo4/c Cathode Materials for Lithium-Ion Batteries. ACS Omega 2021, 6 (22), 14122–14129.

(Invited) Modeling the Charge Transfer Reactions at Li/SEI/Electrolyte Interfaces in Lithium-Ion Batteries

Two kinds of charge transfer reactions are critical for the performance and life of lithium battery: the desired ion transfer reaction occurring during each charge/discharge cycle, , and the undesired electron transfer reactions leading to the parasitic chemical decomposition of the electrolyte and solid electrolyte interphase (SEI) formation/growth. The heterogeneous multi-component nature of SEI dominates its ionic and electronic transport properties and controls these two charge transfer reactions. Density Functional Theory (DFT)-informed multiscale modeling has been providing valuable insights under the scarcity of quantitative experiments. For example, the LiF/Li2CO3 interface was demonstrated to increase the ionic conductivity of mixed SEI nanocomposite by forming an ionic space charge region near the interface and promoting more Li-ion interstitials in Li2CO3, although LiF itself has low Li-ion conducting carriers and conductivity. To form a LiF-rich SEI layer, the electrolyte compositions need to be designed. Since the SEI formation occurs on the charged surface, the electric double layer (EDL) structure near the charged surfaces needs to be incorporated into the modeling. Here interactive classical molecular dynamics (MD), DFT, and data statistical analysis were combined to illustrate the effect of EDL on SEI formation in two essential electrolytes, the carbonate-based electrolyte for Li-ion batteries and the ether-based electrolyte for batteries with Li-metal anodes. It was found the effectiveness of adding fluoroethylene carbonate (FEC) to form the beneficial F-containing SEI component (e.g., LiF) varies with the electrolyte and temperature, because of the interplay of ion-solvent interactions with the surface charge. These integrated modeling provided quantitative guidance for electrolyte/SEI/Li-metal interface design.

Li-intercalation chemistry of few-layer borophene with ultrahigh capacity and fast kinetics

Li-intercalation chemistry of few-layer borophene with ultrahigh capacity and fast kinetics

Refined Grain Enhancing Lithium-Ion Diffusion of LiFePO4 via Air Oxidation

LiFePO4 is a type of cathode material with good safety and long service life. However, the problems of the low Li ion diffusion rate and low electron conductivity limit the application of LiFePO4 in the field of electric vehicles. In this paper, FePO4 with different grain sizes was prepared via the air oxidation precipitation method and then sintered to prepare LiFePO4. The refined grain can shorten the diffusion distance of Li+, accelerate the diffusion of Li+, and improve the diffusion coefficient of Li+. The results show that LiFePO4 with a smaller grain size has better electrochemical performance. The discharge capacity of the first cycle is 151.3 mAh g−1 at 1 C, and the capacity retention rate is 95.04% after 230 cycles. Its rate performance is more outstanding, not only at 0.2 C, where the discharge capacity is as high as 155 mAh g−1, but also at 10 C, the capacity fade is less, and it can still reach 131 mAh g−1. The air oxidation precipitation method reduces the production cost, shortens the production process, and prepares FePO4 with small grains, which provides a reference for further improving the properties of precursors and LiFePO4.