Revitalization of Diluent Amide-Based Electrolyte for Building High-Voltage Lithium-Metal Batteries.

Hitherto, highly concentrated electrolyte is the overarching strategy for revitalizing the usage of amide - in lithium-metal batteries (LMBs), which simultaneously mitigates the reactivity of amide toward Li and regulates uniform Li deposition via forming anion-solvated coordinate structure. However, it is undeniable that this would bring the cost burden for practical electrolyte preparation, which stimulates further electrolyte design toward tailoring anion-abundant Li+ solvation structure in stable amide electrolytes under a low salt content. Herein, a distinct method is conceived to design anions-enriched Li+ solvation structure in dilute amide-electrolyte (1 m Li-salt concentration) with the aid of integrating perfluoropolyethers (PFPE-MC) with anion-solvating ability and B/F-involved additives. The optimized electrolyte based on N,N-Dimethyltrifluoroacetamide (FDMAC) exhibits outstanding compatibility with Li and NCM622 cathode, facilitates uniform Li deposition along with robust solid electrolyte interphase (SEI) formation. Accordingly, both the lab-level LMB coin cell and practical pouch cell based on this dilute FDMAC electrolyte deliver remarkable performances with improved capacity and cyclability. This work pioneers the feasibility of diluted amide as electrolyte in LMB, and provides an innovative strategy for highly stable Li deposition via manipulating solvation structure within diluent electrolyte, impelling the electrolyte engineering development for practical high-energy LMBs.

Steering Sulfur Reduction Pathways via Cisplatin Enables High Performance in Lithium-Sulfur Batteries.

The sulfur reduction reaction (SRR) is an attractive 16-electron transfer process that endows Li-S batteries with a theoretical capacity of 1,672 mAh g-1. However, the slow kinetics and complex pathways of the SRR cause the shuttling of soluble polysulfides (PSs), thus fast capacity fading. Here, we report using cisplatin (cis-Pt) as a novel mediator to improve the SRR kinetics and a molecular probe to identify the SRR pathways. We show that cis-Pt with a reductive Pt2+ center can directly slice the S-S bonds of PSs, leading to enhanced charge transfer kinetics, guided SRR pathways, and depth conversion of PSs to Li2S. With cis-Pt added, Li-S coin cells deliver a maximum specific capacity of 1,437 mAh g-1 and a capacity decay of 0.017% per cycle after 1000 cycles, while a pouch cell with a practical electrolyte-sulfur ratio (2.5 µl mg-1) exhibits a high energy density of 318.8 Wh kg-1. Our mechanistic studies reveal that cis-Pt steers the cathodic SRR pathways by generating redox active cis-Pt/PSs complexes, enabling the replacement of the sluggish SRR with a faster redox cycling of Pt4+/Pt2+ pairs. These findings provide insights into the rational design of functional mediators for tackling the cathodic challenges inside Li-S batteries.

White Latex: Appealing "Green" Alternative for PVdF in Electrode Manufacturing for Sustainable Li-Ion Batteries.

Nowadays, poly(vinylidene fluoride) (PVdF) has been dominantly utilized as a polymeric binder in commercialized Li-ion batteries. However, standardized PVdF-based electrode manufacturing seems cost-intensive and environmentally hazardous, which relies on the usage of toxic N-methyl-2-pyrrolidone (NMP) as a dispersant. In view of cost control and environmental awareness, switching to a water-processable green binder, as a substitute for PVdF, has been imperative with realistic significance. Herein, commercially available white latex (WL), containing poly(vinyl acetate) as a staple ingredient, was directly used as an alternative aqueous binder for PVdF in the fabrication of graphite/Li4Ti5O12-based lithium-ion anodes. WL exhibits robust adhesion of the electrode coating to the current collector; meanwhile, the restricted electrolyte swelling of the binder is verified by in situ electrochemical dilatometry. Outperforming PVdF, WL endows graphite with extensive surface coverage by the binding agent, dramatically reducing irreversible decomposition of the electrolyte (SEI formation) on graphite. Consequently, the WL-based graphite anode delivers the highest initial coulombic efficiency (CE) of 92% and remarkable long cyclic stability with a high capacity retention of 332.7 mAh/g, compared to the PVdF- and carboxymethyl cellulose (CMC)-based ones. Moreover, WL is also compatible with Li4Ti5O12, endowing it with more stable cycling behavior than that of the counterparts prepared with both PVdF and even CMC. Our described WL represents an appealing "green" alternative for PVdF in manufacturing sustainable and ecofriendly energy storage devices.

Interplay among Hexafluorophosphate, Difluoro(oxalato)borate Anions and Ethylene Carbonate during Their Insertion into a Graphite Electrode.

Ethylene carbonate solutions dissolving mixed lithium salts composed of both difluoro(oxalato)borate (DFOB-) and hexafluorophosphate (PF6-) anions are introduced into Li/graphite cells. The anions' intercalation procedures into the graphite positive electrode from these solutions are explored by charge/discharge, cyclic voltammetry, and impedance spectroscopic tests in combination with electrochemical in situ characterization including X-ray diffraction (XRD) and Raman spectroscopy. Furthermore, these solutions are characterized by ionic conductivity together with nuclear magnetic resonance measurements. The properties of the solutions are linked to the capacity values delivered by Li/graphite cells.

Mechanistic origin of low polarization in aprotic Na-O2 batteries.

Research interest in aprotic sodium-air (Na-O2) batteries is growing because of their considerably high theoretical specific energy and potentially better reversibility than lithium-air (Li-O2) batteries. While Li2O2 has been unequivocally identified as the major discharge product in Li-O2 batteries containing relatively stable electrolytes, a multitude of discharge products, including NaO2, Na2O2 and Na2O2·2H2O, have been reported for Na-O2 batteries and the corresponding cathodic electrochemistry remains incompletely understood. Herein, we provide molecular-level insights into the key mechanistic differences between Na-O2 and Li-O2 batteries based on gold electrodes in strictly dry, aprotic dimethyl sulfoxide electrolytes through a combination of in situ spectroelectrochemistry and density functional theory based modeling. While like Li-O2 batteries, the formation of oxygen reduction products (i.e., O2-, NaO2 and Na2O2) in Na-O2 batteries depends critically on the electrode potential, two factors lead to a better reversibility of Na-O2 electrochemistry, and are therefore highly beneficial to a viable rechargeable metal-air battery design: (i) only O2- and NaO2, and no Na2O2, form down to as low as ∼1.5 V vs. Na/Na+ during discharge; (ii) solid NaO2 is quite soluble and its formation and oxidation can proceed through micro-reversible EC (a chemical reaction of the product after the electron transfer) and CE (a chemical reaction preceding the electron transfer) processes, respectively, with O2- as the key intermediate.

Reversibility of Noble Metal-Catalyzed Aprotic Li-O2 Batteries.

The aprotic Li-O2 battery has attracted a great deal of interest because, theoretically, it can store far more energy than today's batteries. Toward unlocking the energy capabilities of this neotype energy storage system, noble metal-catalyzed high surface area carbon materials have been widely used as the O2 cathodes, and some of them exhibit excellent electrochemical performances in terms of round-trip efficiency and cycle life. However, whether these outstanding electrochemical performances are backed by the reversible formation/decomposition of Li2O2, i.e., the desired Li-O2 electrochemistry, remains unclear due to a lack of quantitative assays for the Li-O2 cells. Here, noble metal (Ru and Pd)-catalyzed carbon nanotube (CNT) fabrics, prepared by magnetron sputtering, have been used as the O2 cathode in aprotic Li-O2 batteries. The catalyzed Li-O2 cells exhibited considerably high round-trip efficiency and prolonged cycle life, which could match or even surpass some of the best literature results. However, a combined analysis using differential electrochemical mass spectrometry and Fourier transform infrared spectroscopy, revealed that these catalyzed Li-O2 cells (particularly those based on Pd-CNT cathodes) did not work according to the desired Li-O2 electrochemistry. Instead the presence of noble metal catalysts impaired the cells' reversibility, as evidenced by the decreased O2 recovery efficiency (the ratio of the amount of O2 evolved during recharge/that consumed in the preceding discharge) coupled with increased CO2 evolution during charging. The results reported here provide new insights into the O2 electrochemistry in the aprotic Li-O2 batteries containing noble metal catalysts and exemplified the importance of the quantitative assays for the Li-O2 reactions in the course of pursuing truly rechargeable Li-O2 batteries.

Difluoro(oxalato)borate's Role in the Intercalation Behavior of Mixed Anions from Sulfolane.

Some previous studies have found the synergistic effect of mixed anions in dual-ion batteries (DIBs). In this work, both lithium tetrafluoroborate (LiBF4 ) and lithium difluoro(oxalato)borate (LiDFOB) were dissolved in sulfolane (SL) solvent, and the resultant solutions were applied for DIBs. The storage behavior of mixed anions in natural graphite positive electrodes was investigated. In-situ X-ray diffraction measurements revealed both anions could participate in the formation of graphite intercalation compounds. Their evolution followed with the decreasing tendency of discharge capacities as the content of LiDFOB increased in solutions. The exchange of anions was discussed.

Structurally integrated asymmetric polymer electrolyte with stable Janus interface properties for high-voltage lithium metal batteries.

Solid-state polymer electrolytes are outstanding candidates for next-generation lithium metal batteries in the realm of high specific energy densities, high safeties and tight contact with electrodes. However, their applications are still hindered by the limitations that no single polymer is electrochemically stable with the oxidizing high-voltage cathode and the highly reductive Li anode, simultaneously. Herein, a bilayer asymmetric polymer electrolyte (SL-SPE) without accessional interface resistance that using poly (ethylene glycol) diacrylate (PEGDA) as a "bridge" to connect the sulfonyl (OS = O)-contained oxidation-tolerated layer and polyether-derived reduction-tolerated layer (SPE), is proposed and synthesized by sequential two-step UV polymerizations. SL-SPE can provide widened electrochemical stability window up to 5 V, while simultaneously deploying a stable Janus interface property. Eventually, the superior high-voltage (4.4 V) cycling durability can be displayed in LiNi0.6Co0.2Mn0.2O2|SL-SPE|Li batteries. This finding provides a bran-new idea for designing multifunctional polymer electrolytes in the application of solid-state batteries.

Tackling the Interfacial Issues of Spinel LiNi0.5Mn1.5O4 by Room-Temperature Spontaneous Dediazonation Reaction.

The detrimental interfacial side reactions, inducing electrolyte decomposition and transition-metal dissolution, are regarded as "arch-criminal" for the utilization of spinel LiNi0.5Mn1.5O4 (LNMO) in high-power Li-ion batteries (LIBs). To conquer this issue, herein, we construct a thin polyphenyl film onto the surface of LNMO via the spontaneous dediazonation of C6H5N2+BF4- at room temperature. This conductive film facilitates the Li+ transport within cathode and at LNMO|electrolyte interface while reinforcing the compatibility of LNMO against electrolyte by efficiently suppressing the electrolyte decomposition catalyzed by LNMO and even the transition-metal dissolution. Consequently, polyphenyl-grafted LNMO exhibits improved electrochemical performances, e.g., the considerable discharge capacity of 136.7 mAh g-1 at low current density (0.1C), excellent rate capability, and long-term cyclability with a reversible capacity of 107.4 mAh g-1 along with high capacity retention of ∼85% after cycling 500 times, that are superior to those of the pristine LNMO counterpart. All these results demonstrate that our strategy is instrumental in solving the interface issues with respect to the spinel LNMO cathode, impelling the development of LNMO-based batteries with high energy density.

Unveiling the Complex Effects of H2O on Discharge-Recharge Behaviors of Aprotic Lithium-O2 Batteries.

The addition of H2O, even trace amount, in aprotic Li-O2 batteries has a remarkable impact on achieving high capacity by triggering solution mechanism, and even reducing charge overpotential. However, the critical role of H2O in promoting solution mechanism still lacks persuasive spectroscopic evidence, moreover, the origin of low polarization remains incompletely understood. Herein, by in situ spectroscopic identification of reaction intermediates, we directly verify that H2O additive is able to alter oxygen reduction reaction (ORR) pathway subjected to solution-mediated growth mechanism of Li2O2. In addition, ingress of H2O also induces to form partial LiOH, resulting in reduced charging polarization due to its higher conductivity; however, LiOH could not contribute to O2 evolution upon recharge. These original results unveil the complex effects of H2O on cycling the aprotic Li-O2 batteries, which are instructive for the mechanism study of aprotic Li-O2 batteries with protic additives or soluble catalysts.

Unlocking the energy capabilities of micron-sized LiFePO4.

Utilization of LiFePO4 as a cathode material for Li-ion batteries often requires size nanonization coupled with calcination-based carbon coating to improve its electrochemical performance, which, however, is usually at the expense of tap density and may be environmentally problematic. Here we report the utilization of micron-sized LiFePO4, which has a higher tap density than its nano-sized siblings, by forming a conducting polymer coating on its surface with a greener diazonium chemistry. Specifically, micron-sized LiFePO4 particles have been uniformly coated with a thin polyphenylene film via the spontaneous reaction between LiFePO4 and an aromatic diazonium salt of benzenediazonium tetrafluoroborate. The coated micron-sized LiFePO4, compared with its pristine counterpart, has shown improved electrical conductivity, high rate capability and excellent cyclability when used as a 'carbon additive free' cathode material for rechargeable Li-ion batteries. The bonding mechanism of polyphenylene to LiFePO4/FePO4 has been understood with density functional theory calculations.