High lithium transference number, t Li + , electrolytes are desired for use in both lithium-ion and lithium metal rechargeable battery technologies. Historically, low t Li + electrolytes have hindered device performance by allowing ion concentration gradients within the cell, leading to high internal resistances that ultimately limit cell lifetime, charging rates, and energy density. Herein, we report on the synthesis and electrochemical features of electrolytes based on nanoparticle salts designed to provide high t Li + . The salts are created by cofunctionalization of metal oxide nanoparticles with neutral organic ligands and tethered lithium salts. When dispersed in a conducting fluid such as tetraglyme, they spontaneously form a charged, nanoporous network of particles at moderate nanoparticle loadings. Modification of the tethered anion chemistry from −SO 3 − to −SO 3 BF 3 − is shown to enhance ionic conductivity of the electrolytes by facilitating ion pair dissociation. At a particle volume fraction of 0.15, the electrolyte exists as a self-supported, nanoporous gel with an optimum ionic conductivity of 10 −4 S/cm at room temperature. Galvanostatic polarization measurements on symmetric lithium metal cells containing the electrolyte show that the cell short circuit time, t SC , is inversely proportional to the square of the applied current density t SC ∼ J −2 , consistent with previously predicted results for traditional polymer-in-salt electrolytes with low t Li + . Our findings suggest that electrolytes with t Li + ≈ 1 and good ion-pair dissociation delay lithium dendrite nucleation and may lead to improved lithium plating in rechargeable batteries with metallic lithium anodes.

Oligomer-suspended SiO 2 -polyethylene glycol nanoparticles are studied as porous media electrolytes. At SiO 2 volume fractions, φ, bracketing a critical value φ y ≈ 0.29, the suspensions jam and their mechanical modulus increase by more than seven orders.For φ > φ y , the mean pore diameter is close to the anion size, yet the ionic conductivity remains surprisingly high and can be understood, at all φ, using a simple effective medium model proposed by Maxwell. SiO 2 -polyethylene glycol hybrid electrolytes are also reported to manifest attractive electrochemical stability windows (0.3-6.3V) and to reach a steady-state interfacial impedance when in contact with metallic lithium. IntroductionLithium ions are the active charge carrying species in the most energy dense secondary batteries of today, those used in electronics and hybrid electric transportation. Currently commercialized lithiated anode materials such as LiC 6 and Li 4 Ti 5 O 12 have relatively low theoretical energy capacities (360 mAh/g and 175 mAh/g, respectively). Advanced secondary battery systems employing electrodes such as LiCoPO 4 , 1 lithium, 2-5 or sulfur 6 require electrolytes with specific properties such as wide electrochemical stability windows, high mechanical strength, and/or inertness or non-solvency towards the electrode materials and their intercalation products.Next-generation lithium ion batteries should also employ electrolytes that are nonflammable, non-volatile, non-leakable, and non-toxic, making them safer both in use and after disposal. In pursuit of such materials, several classes of electrolytes have been studied as replacements for conventional liquid electrolytes: polymers, [7][8][9][10][11] polymer composites, [12][13][14][15] hybrids, 16-18 gels, 19,20 ionic liquids, 21 and ceramics. 22In many cases, mechanical integrity of the electrolyte comes at a cost: namely, a large loss in ionic conductivity, which places undesirable limits on the charge/discharge rate of the cell. Liquid and particulate plasticizers have been used with some success in circumventing this constraint in composite and gel polymer electrolytes. 4,15,19,23 With a mechanically strong framework in place such as a polymer or ceramic, the liquid plasticizer serves as a freely diffusing ionic conduction medium that provides ionic conductivities near that of a pure liquid electrolyte. If a liquid plasticizer with good thermal and electrochemical properties is utilized, safety concerns are reduced. In the case of particulate plasticizers, of either nano-or micron-scale, the particles have been shown to decrease crystallization of the surrounding matrix, thereby enhancing segmental motion of the host polymer and increasing conduction. While nanoparticles have been shown most successful in this area, in typical polymer composites particle aggregation occurs; this reduces the effectiveness of the individual particles in inhibiting crystallization but allows for formation of a percolated particulate network that aids in bulk mechanical strength.Recently, we r...