We are currently in the midst of a race to discover and develop new battery materials capable of providing high energy-density at low cost. By combining a high-performance Si electrode architecture with a room temperature ionic liquid electrolyte, here we demonstrate a highly energy-dense lithium-ion cell with an impressively long cycling life, maintaining over 75% capacity after 500 cycles. Such high performance is enabled by a stable half-cell coulombic efficiency of 99.97%, averaged over the first 200 cycles. Equally as significant, our detailed characterization elucidates the previously convoluted mechanisms of the solid-electrolyte interphase on Si electrodes. We provide a theoretical simulation to model the interface and microstructural-compositional analyses that confirm our theoretical predictions and allow us to visualize the precise location and constitution of various interfacial components. This work provides new science related to the interfacial stability of Si-based materials while granting positive exposure to ionic liquid electrochemistry.

The molecular-layer deposition of a flexible coating onto Si electrodes produces high-capacity Si nanocomposite anodes. Using a reaction cascade based on inorganic trimethylaluminum and organic glycerol precursors, conventional nano-Si electrodes undergo surface modifications, resulting in anodes that can be cycled over 100 times with capacities of nearly 900 mA h g(-1) and Coulombic efficiencies in excess of 99%.

By cyclizing commercially available polyacrylonitrile (PAN), we show that it is possible to conformally coat nanoparticles of Si with a conjugated polymer. We utilize cyclized-PAN both as a binder and conductive additive because of its good mechanical resiliency to accommodate silicon's (Si) large expansion as well as its good ionic and electronic conductivity. By the 150 th cycle, our nano-Si/cyclized-PAN composite anodes exhibit a specifi c charge capacity of nearly 1500 mAh g − 1 with a coulombic efficiency (CE) approaching 100%. Because Si is naturally abundant and has such a high achievable specifi c capacity, [ 1 , 2 ] the next generation of Lithium-ion batteries will inevitably incorporate an advanced Si based anode like that presented here. At room temperature, Si can accommodate 3.75 mole Li per mole of Si (Li 15 Si 4 ) for a theoretical capacity of 3579 mAh g − 1 . [ 3 , 4 ] Despite these advantages, progress towards a commercially viable Si anode has been impeded by Si's rapid capacity fade, poor ionic transport and low CE.Rapid capacity fade is a primary cause for the delay of Si's commercialization. At room temperature, a volume expansion of 300% occurs upon full lithiation to Li 15 Si 4 . [5][6][7] Such a massive volumetric change can result in cracking and pulverization of the Si particles, which then leads to the interruption of electronic transport pathways and the electrochemical isolation of pulverized particles. [ 2 ] To better accommodate the large stresses and strains generated upon lithiation, utilization of nanoparticles, nanowires, and 3D nano-porous structures have been studied. Nano-Si based structures have the added benefi t of reducing the average Li + diffusion length into Si for faster charge and discharge rates. [8][9][10] However, nano-Si structures often suffer from poor CE due to the continual formation of a solid electrolyte interphase (SEI) layer at the large nano-Si/electrolyte interface. Adding to the frustration, many of these clever nano-Si based engineering solutions are rarely appropriate for commercialization because they may require expensive synthesis techniques. [11][12][13][14][15] Yet, recent work has shown that Si-C nano-composites may be promising candidates for viable, inexpensive, stable and effi cient high capacity anodes. Si-C conventional composites are typically prepared by carbonizing precursors [16][17][18] or by mechanically mixing Si with carbon. [ 19 , 20 ] The result are composites of Si particles embedded in carbon matrices. Unfortunately, these carbon matrices cannot accommodate Si's large volumetric changes because of their tendency for brittle failure. Cracking of these carbon matrices during cycling interrupts electronic conduction pathways and exposes fresh Si to the electrolyte which results in further SEI formation. Several studies using electronically conductive polymer based matrices or coatings have demonstrated better results. [21][22][23][24][25] For example, G. Liu et al. developed a cathodically (n-type) doped polymer binder that mai...

In this study, we report on the effect that an externally applied compressive stress has on the electrochemical performance of Si anodes. Using the compression of an all-solid-state cell as a convenient format for simulating volume confinement, the electrochemical performance of Si anodes as a function of externally applied compressive stress has been systematically investigated. We verify that the extent of Si lithiation is limited by confining the free volume expansion of nano-Si particles. Volume confinement of Si particles is manifested as an overpotential and results in a stable anode for lithium-ion batteries. These results are foundational and lead to the best understanding to date of the complex electrochemomechanics of a Si-based anode.

Surface modification of silicon nanoparticles via molecular layer deposition (MLD) has been recently proved to be an effective way for dramatically enhancing the cyclic performance in lithium ion batteries. However, the fundamental mechanism of how this thin layer of coating functions is not known, which is complicated by the inevitable presence of native oxide of several nanometers on the silicon nanoparticle. Using in situ TEM, we probed in detail the structural and chemical evolution of both uncoated and coated silicon particles upon cyclic lithiation/delithation. We discovered that upon initial lithiation, the native oxide layer converts to crystalline Li2O islands, which essentially increases the impedance on the particle, resulting in ineffective lithiation/delithiation and therefore low Coulombic efficiency. In contrast, the alucone MLD-coated particles show extremely fast, thorough, and highly reversible lithiation behaviors, which are clarified to be associated with the mechanical flexibility and fast Li(+)/e(-) conductivity of the alucone coating. Surprisingly, the alucone MLD coating process chemically changes the silicon surface, essentially removing the native oxide layer, and therefore mitigates side reactions and detrimental effects of the native oxide. This study provides a vivid picture of how the MLD coating works to enhance the Coulombic efficiency, preserves capacity, and clarifies the role of the native oxide on silicon nanoparticles during cyclic lithiation and delithiation. More broadly, this work also demonstrates that the effect of the subtle chemical modification of the surface during the coating process may be of equal importance to the coating layer itself.

High-energy-density FeS2 cathodes en-abled by a bis(trifluoromethanesulfonyl)imide (TFSI-) anion-based room temperature ionic liquid (RTIL) electrolyte are demonstrated. A TFSI-based ionic liquid (IL) significantly mitigates polysulfide dissolution, and therefore the parasitic redox shuttle mechanism, that plagues sulfur-based electrode chemistries. FeS2 stabilization with a TFSI(-) -based IL results in one of the highest energy density cathodes, 542 W h kg(-1) (normalized to cathode composite mass), reported to date.

Silicon (Si)-based materials hold promise as the next-generation anodes for high-energy lithium (Li)-ion batteries. Enormous research efforts have been undertaken to mitigate the chemo-mechanical failure due to the large volume changes of Si during lithiation and delithiation cycles. It has been found that nanostructured Si coated with carbon or other functional materials can lead to significantly improved cyclability. However, the underlying mechanism and comparative performance of different coatings remain poorly understood. Herein, using in situ transmission electron microscopy (TEM) through a nanoscale half-cell battery, in combination with chemo-mechanical simulation, we explored the effect of thin (∼5 nm) alucone and Al2O3 coatings on the lithiation kinetics of Si nanowires (SiNWs). We observed that the alucone coating leads to a "V-shaped" lithiation front of the SiNWs, while the Al2O3 coating yields an "H-shaped" lithiation front. These observations indicate that the difference between the Li surface diffusivity and bulk lithiation rate of the coatings dictates lithiation induced morphological evolution in the nanowires. Our experiments also indicate that the reaction rate in the coating layer can be the limiting step for lithiation and therefore critically influences the rate performance of the battery. Further, the failure mechanism of the Al2O3 coated SiNWs was also explored. Our studies shed light on the design of high capacity, high rate and long cycle life Li-ion batteries.

In this study we embed phase pure natural cubic-FeS 2 (pyrite) in a stabilized polyacrylonitrile (PAN) matrix. The PAN matrix confi nes FeS 2 's electroactive species (Fe 0 and S n 2− ) for good reversibility and effi ciency. Additionally, the stabilized PAN matrix can accommodate the 160% volume expansion of FeS 2 upon full discharge because it is not fully carbonized. At room temperature, our PAN-FeS 2 electrode delivers a specifi c capacity of 470 mAh g −1 on its 50th discharge. Using high-resolution transmission electron microscopy (HRTEM) we confi rm that FeS 2 particles are embedded in the PAN matrix and that FeS 2 's mobile electroactive species are confi ned during cycling. We also observe the formation of orthorhombic-FeS 2 at full charge, which validates the results of our previous all-solid-state FeS 2 battery study.The energy density of conventional Li-ion batteries with LiMO 2 (M = transition metal) cathodes and graphitic anodes is approaching a practical upper limit after two decades of optimization. In order to improve the energy density of Li-ion batteries further, new cathodes must be developed with capacities that compare to those of advanced anodes such as Si. [ 1 ] The FeS 2 conversion chemistry is a promising candidate to replace the LiMO 2 intercalation chemistry because FeS 2 is inexpensive, energy dense, and environmentally benign. The four electron reduction of cubic-FeS 2 (pyrite) with lithium (FeS 2 + 4Li + + 4e −