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Progress toward a commercially viable silicon anode for lithium-ion batteries has been impeded by silicon’s rapid capacity fade caused by large volumetric expansion and unstable solid-electrolyte interphases. This study focuses on developing unique coating chemistries to stabilize the surface of silicon (Si) electrodes via molecular layer deposition (MLD), as well as to accommodate volume changes during electrochemical reactions. A new reaction precursor – an aromatic organic diol, hydroquinone – combined with trimethylaluminum, has led to a robust, elastic, conductive surface coating composed of aluminum dioxybenzene. We studied the chemical and physical properties of this surface coating using X-ray absorption spectroscopy, electrochemical impedance, and nanoindentation. The flexibility of the coating enables the accommodation of volumetric changes and maintenance of the mechanical integrity of the Si electrodes. By applying this robust and conductive trimethylaluminum-hydroquinone coating, we demonstrate a Si anode that is reversible and capable of high performance and high rate, achieving over 200 cycles with capacities of nearly 1500 mAh g−1. This research elucidates the significance of surface modification for high-energy battery materials with large volume changes, and also provides a platform for a new design of electrode surface coatings, with the aim of achieving durable, high energy density lithium-ion batteries.

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.

Greatly improved cycling performance has been demonstrated with conformally coated lithium-ion electrodes by atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques. This paper reports the impact of coating on the electrode additives towards mitigating undesired parasitic reactions during cycling. The ALD and MLD coatings with conformality and atomic scale thickness control effectively stabilize the surface of the electrode components, and the current collector, resulting in the increase of coulombic efficiency throughout cycling. The organic fragment integrated into the recently developed MLD process allows the coating to possess excellent mechanical properties and enhanced ionic conductivity, which significantly reduces cell polarizations throughout cycling. This work validates the importance of ALD and MLD as surface modifiers and further demonstrates their versatility and compatibility with lithium-ion battery technology.

Atomic layer deposition (ALD) of LiF and lithium ion conducting (AlF3)(LiF)x alloys was developed using trimethylaluminum, lithium hexamethyldisilazide (LiHMDS) and hydrogen fluoride derived from HF-pyridine solution. ALD of LiF was studied using in situ quartz crystal microbalance (QCM) and in situ quadrupole mass spectrometer (QMS) at reaction temperatures between 125°C and 250°C. A mass gain per cycle of 12 ng/(cm2 cycle) was obtained from QCM measurements at 150°C and decreased at higher temperatures. QMS detected FSi(CH3)3 as a reaction byproduct instead of HMDS at 150°C. LiF ALD showed self-limiting behavior. Ex situ measurements using X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE) showed a growth rate of 0.5-0.6 Å/cycle, in good agreement with the in situ QCM measurements.ALD of lithium ion conducting (AlF3)(LiF)x alloys was also demonstrated using in situ QCM and in situ QMS at reaction temperatures at 150°C A mass gain per sequence of 22 ng/(cm2 cycle) was obtained from QCM measurements at 150°C. Ex situ measurements using XRR and SE showed a linear growth rate of 0.9 Å/sequence, in good agreement with the in situ QCM measurements. Stoichiometry between AlF3 and LiF by QCM experiment was calculated to 1:2.8. XPS showed LiF film consist of lithium and fluorine. XPS also showed (AlF3)(LiF)x alloy consists of aluminum, lithium and fluorine. Carbon, oxygen, and nitrogen impurities were both below the detection limit of XPS. Grazing incidence X-ray diffraction (GIXRD) observed that LiF and (AlF3)(LiF)x alloy film have crystalline structures. Inductively coupled plasma mass spectrometry (ICP-MS) and ionic chromatography revealed atomic ratio of Li:F=1:1.1 and Al:Li:F=1:2.7: 5.4 for (AlF3)(LiF)x alloy film. These atomic ratios were consistent with the calculation from QCM experiments. Finally, lithium ion conductivity (AlF3)(LiF)x alloy film was measured as σ = 7.5 × 10-6 S/cm.

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.

A tunable hierarchical porous framework is fabricated to house the volumetric changes outputted by Si. The nSi@cPAN/cPAN electrodes only expand by 14.3% at full initial lithiation and remain within 23% expansion from its uncycled state after 20 cycles with remarkable cycling stability and high coulombic efficiencies in excess of 99.5%.

We present the fabrication of lithographically defined carbon patterns and nanoribbons using a common carbon precursor, polyacrylonitrile (PAN). This method is based on nanoimprint lithography and has been demonstrated to be reliable and capable of nanofabrication over a large surface area at low cost, compared with current carbon-patterning techniques. Most interestingly, the deformation profile of the PAN during the imprinting process resulted in a distribution of aligned PAN chains within the patterns, which led to a similar anisotropic correlation of the carbon crystallites in the carbonized structures.

D. Molina Piper, T. A. Yersak, S.-B. Son, Prof. S.-H. Lee Department of Mechanical Engineering University of Colorado at Boulder, 80309 USA E-mail: sehee.lee@colorado.edu D. Molina Piper, Dr. C. Ban, Dr. A. C. DillonNational Renewable Energy Laboratory 1617 Cole Boulevard, 80401 USA S.-B. Son, S. C. Kim, C. S. Kang, Prof. K. H. OhDepartment of Material Science and Engineering Seoul National University, 151-742 Korea

The first-ever demonstration of stabilized Si/lithium-manganese-rich full cells, capable of retaining >90% energy over early cycling and >90% capacity over more than 750 cycles at the 1C rate (100% depth-of-discharge), is made through the utilization of a modified ionic-liquid electrolyte capable of forming a favorable cathode-electrolyte interface.

In this study, we report on the effect that an externally applied compressive stress has on the electrochemical performance of Sianodes.Usingthecompressionofanall-solid-statecellasaconvenientformatforsimulatingvolumeconfinement,theelectrochemicalperformance of Si anodes as a function of externally applied compressive stress has been systematically investigated. We verify thatthe extent of Si lithiation is limited by confining the free volume expansion of nano-Si particles. Volume confinement of Si particlesis manifested as an overpotentialand results in a stable anode forlithium-ion batteries. These results are foundationaland lead to thebest understanding to date of the complex electrochemomechanics of a Si-based anode.© 2012 The Electrochemical Society. [DOI: 10.1149/2.064301jes] All rights reserved.Manuscript submitted September 12, 2012; revised manuscript received October 22, 2012. Published November 7, 2012.

Doped Si nanoparticles (SiNPs) with conformal carbon coating and cyclized-polyacrylonitrile (PAN) network displayed capacities of 3500 and 3000 mAh g(-1) at C/20 and C/10, respectively. At 1 C, the electrode preserves a specific discharge capacity of ∼1500 mAh g(-1) for at least 60 cycles without decay. Al2O3 atomic layer deposition (ALD) helps improve the initial Coulombic efficiency (CE) to 85%. The dual coating of conformal carbon and cyclized-PAN help alleviate volume change and facilitate charge transfer. Ultra-thin Al2O3 ALD layers help form a stable solid electrolyte interphase interface.

Optimized performance of silicon-ionic- liquid lithium-ion batteries through the implementation of a new electrode-microgeometry. The incorporation of 1D silicon nanowires into the cyclized-polyacrylonitrile-based electrode-architecture allows for greatly improved active material utilization, higher rate capabilities, and reduced interfacial reactions.

The automotive segment of the global lithium-ion battery market is slated for exceedingly high growth in the near future, stoking a demand for next-generation electrode materials capable of delivering the 400 Wh/kg benchmark at low-costs. While most of the recent research efforts to enable such materials focus on complex material modifications and nano-architectured electrode design, developing alternative electrolyte chemistries presents another avenue towards creating the next generation Li-ion battery. Previous work at the University of Colorado Boulder demonstrated the viability of this approach. By pairing a simple, scalable, yet robust Si electrode architecture with an imide-based room temperature ionic liquid (RTIL) electrolyte, this work enabled a Si/L333 full-cell capable of long-term cycling (>1000 cycles) at a charge-discharge rate of 1C.1 Recently an initial feasibility study has revealed the impressive compatibility of the same imide-based RTIL electrolyte with lithium-manganese-rich (LMR) layered oxides. The newly developed LMR material had the potential to revolutionize the transportation industry, especially if the material could be paired with a high capacity anode material such as silicon (Si). The resulting full-cell was proposed to truly enable the electric vehicle (EV), driving down battery costs to less than $200/kWh while supplying double the drive range of state-of-the-art Li-ion technology. The material, formulated as xLi2MnO3Ÿ(1-x)LiMO2 or Li[LixM1-x]O2 (M = Ni, Mn, Co), is known as the lithium-manganese-rich (LMR) oxide. The beauty of the LMR material lies in the activation process undergone at >4.4 V vs. Li/Li+ during initial charging, resulting in an unprecedentedly high operating voltage and capacities of ~260 mAh g-1.2-8 Despite the potential for massive technological impact, worldwide research has struggled to enable the LMR material. Early work impressively laid the foundation for widespread efforts targeting this material and its signature drawback: the gradual lowering of cell operating voltage over cycling life as the originally layered crystal structure transforms to a spinel phase, accompanied by oxygen evolution during activation of the Li2MnO3 component and transition metal dissolution. In this work, we have focused our efforts on the electrode-electrolyte interactions known to accelerate phase change in the LMR system. Leveraging the understandings of LMR interfacial behavior built by decades of research, we employ a unique electrolyte composition to form a cathode-electrolyte interface (CEI) that allows for the improved long-term voltage stability of the LMR cathode. Our novel CEI is formed in situ through the oxidative decomposition of a room temperature ionic liquid (RTIL) electrolyte doped with a sacrificial fluorinated salt additive. For the first time, we demonstrate an LMR system capable of 1000 high capacity cycles with minimal voltage decay, shedding light on the importance of the LMR CEI and elucidating the complex interplay between the electrolyte and the atomic scale transformations of an unstable crystal lattice. In this talk, these results will be presented in more detail. D. Molina Piper, D. et al. Nat. Commun. 2015, 6, 6230. H. Yu, H. Zhou, J. Phys. Chem. Lett. 2013, 4, 1268. M.M. Thackeray, S.-H Kang, C.S. Johnson, J.T. Vaughey, R. Benedek, S.A. Hackney, J. Mater. Chem. 2007, 17, 3112. G.G. Amatucci, N. Pereira, T. Zheng, J.M. Tarascon, J. Electrochem. Soc. 2001, 148, A171. S.H. Kang, K. Amine, J. Power Sources 2005, 146, 654. M.H. Roussow, M.M. Thackeray, Mater. Res. Bull. 1991, 26, 463. P. Kalyani, S. Chitra, T. Mohan, S. Gopukumar, J. Power Sources 1999, 80, 103. J. Croy, J.S. Park, F. Dogan, C.S. Johnson, B. Key, M. Balasubramanian, Chem. Mater. 2014, 26, 7091.

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.

3,4 ] Despite Si’s inherent advantages, progress towards a commer-cially viable Si anode has been impeded by Si’s rapid capacity fade, poor rate capability, and low coulombic effi ciency (CE). Si exhibits volume changes of ~300% upon lithium alloying and de-alloying, leading to material degradation and presenting a major problem for electrochemical performance. Even though pulverization of the Si particles themselves due to volume changes can be mitigated by integrating particles smaller than 150 nm

Silicon has attracted much attention as a promising Li-ion anode material, due to its high theoretical capacity and natural abundance. However, progress towards a commercially viable Si anode has been impeded by the rapid capacity fade of silicon caused by large volumetric expansion and unstable interface. Surface modifications, which chemically or physically change the surface of electrode components, have been applied to improve the interfacial chemistry, conductivity, and mechanical integration in Si-based electrodes.[1-3] Unlike other electrode materials, Si particles are covered by an insulating oxide layer, but also suffer from the morphological changes during Li cycling. Therefore, besides the chemical stability, a functional coating also requires seamless coverage with the control of thickness and elasticity to address the volumetric changes of Si anodes. This paper will focus on the development of conformal, ultrathin coatings with desirable elastic properties and good conductivity by using molecular layer deposition (MLD). Based on the sequential, self-limiting surface reactions, MLD method allows for the integrating the organic fragments into metal oxide matrix, leading to the formation of hybrid organic-inorganic materials. The thin, conformal, and flexible MLD coating is able to penetrate the electrode's porous structure and covalently bind to available surfaces. This creates a strong, flexible network within the electrode that binds the materials and ensures sufficient contact area throughout cycling. Progress towards synthesis of elastic and conductive coating for Si anodes has been achieved by using MLD reactions between trimethylaluminum (TMA), glycerol.[4] The improvements in the electrochemical performance have been demonstrated for the coated Si anode with the polymeric aluminum alkoxide (alucone) coatings.[4] The chemical and physical properties of this surface coating are studied by using X-ray absorption spectroscopy and nanoindentation. In-situ characterization was applied to understand the impact of coating on structure, morphology and surface chemistry of electrode materials.[5] Due to its unique mechanical properties, the MLD alucone coating proves to be robust and resilient enough to accommodate the extreme volumetric changes of the Si nanocomposite electrodes, helping maintain an intimately linked conductive network and allowing for faster ionic and electronic conduction. This work elucidates the significance of elastic, conductive, ultrathin, and conformal coatings for battery materials with large volume changes, while providing a platform for the development of advanced battery materials. Reference: D. M. Piper, T. A. Yersak, S-B. Son, S. C. Kim, C. S. Kang, K. H. Oh, C. Ban, A. C. Dillon, and S.H. Lee, Adv. Energy Mater. 3 (6) 697 2013 S.-B. Son, B. Kappes and C. Ban, Isr. J. Chem. Mar. 2015, doi: 10.1002/ijch.201400173 D. M. Piper, S-B. Son, J. J. Travis, Y. Lee, S. S. Han, S. C. Kim, K. H. Oh, S. George, S.H. Lee, C. Ban, J. Power Sources, 2014, doi:10.1016/j.jpowsour.2014.11.032 D. M. Piper, J. J. Travis, M. Young, S-B. Son, S. C. Kim, K. H. Oh, S. George, C. Ban, S.H. Lee, Adv. Mater. 26 (10) 1596 2013 Y. He; D. Piper; M. Gu; J. Travis; S. George; S. Lee; A. Genc; L. Pullan; J. Liu; S. Mao; J. Zhang; C. Ban; C. Wang, ACS Nano, 2014 doi: 10.1021/nn505523c.

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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  → Fe + 2Li 2 S) provides a specifi c capacity of 894 mAh g 1 , whereas, the very best LiMO 2 intercalation cathodes can only provide 200 mAh g 1 . [ 2‐4 ] For these reasons Energizer popularized the FeS 2 /Li chemistry as a primary battery, [ 5 ] but a secondary FeS 2 /