Silicon is receiving discernable attention as an active material for next generation lithium-ion battery anodes because of its unparalleled gravimetric capacity. However, the large volume change of silicon over charge–discharge cycles weakens its competitiveness in the volumetric energy density and cycle life. Here we report direct graphene growth over silicon nanoparticles without silicon carbide formation. The graphene layers anchored onto the silicon surface accommodate the volume expansion of silicon via a sliding process between adjacent graphene layers. When paired with a commercial lithium cobalt oxide cathode, the silicon carbide-free graphene coating allows the full cell to reach volumetric energy densities of 972 and 700 Wh l−1 at first and 200th cycle, respectively, 1.8 and 1.5 times higher than those of current commercial lithium-ion batteries. This observation suggests that two-dimensional layered structure of graphene and its silicon carbide-free integration with silicon can serve as a prototype in advancing silicon anodes to commercially viable technology.

Developing electrode materials with high-energy densities is important for the development of lithium-ion batteries. Here, we demonstrate a mesoporous molybdenum dioxide material with abnormal lithium-storage sites, which exhibits a discharge capacity of 1,814 mAh g−1 for the first cycle, more than twice its theoretical value, and maintains its initial capacity after 50 cycles. Contrary to previous reports, we find that a mechanism for the high and reversible lithium-storage capacity of the mesoporous molybdenum dioxide electrode is not based on a conversion reaction. Insight into the electrochemical results, obtained by in situ X-ray absorption, scanning transmission electron microscopy analysis combined with electron energy loss spectroscopy and computational modelling indicates that the nanoscale pore engineering of this transition metal oxide enables an unexpected electrochemical mass storage reaction mechanism, and may provide a strategy for the design of cation storage materials for battery systems.

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Li4Ti5O12 with a cubic spinel structure (space group, Fd3(_)m) has a high redox potential at around 1.5 V vs. Li+/Li with a theoretical capacity of 175 mA h g−1.2 The negligible structural difference between pristine Li4Ti5O12 and lithiated Li7Ti5O12 at the two-phase equilibrium junction guarantees an outstanding electrochemical reversibility during the charge/discharge process.3 Furthermore, the high redox potential would prevent not only the lithium metal deposition on the anode at high current conditions but also the formation of the resistive solid electrolyte interphase (SEI) layer, which may lead to an active Li-ion loss and an increase of the cell impedance.4 No SEI formation at the surface of Li4Ti5O12 is a widely accepted argument from the literature point of view. However, in our previous report regarding the electrochemical study of the carbon-free Li4Ti5O12 electrode,5 we noticed the formation and dissolution of the SEI layer through the change in the intensity of Ti 2p XPS core peaks during the charge and discharge process. This fact led us to suspect the stability of Li4Ti5O12 vis-à-vis to the electrolyte in spite of its high redox potential. Despite the interesting properties of Li4Ti5O12, only few literature studies were reported on its reactivity to the electrolyte. Based on a detailed XPS study on the electrolyte/electrode interfaces in LiMn1.6Ni0.4O4/ Li4Ti5O12 system, Dedryvère et al. have reported the formation of organic and inorganic species on the surface of Li4Ti5O12 anode after cycling.6 However, they concluded that those species were first formed at the cathode and then, adsorbed on the surface of Li4Ti5O12 either by diffusion or by migration of organic cationic species. In addition, the lower voltage limit of Li4Ti5O12 anode couldn’t be also guaranteed to be over 1 V in their study because measuring the voltage of Li4Ti5O12 itself is impossible in two-electrode full cell. He et al. also pointed out the formation of SEI film on the Li4Ti5O12 electrode cycled between 2.5 and ~ 0 V vs. Li+/Li., but they mainly focused on the SEI formation occurred below 1 V.7 Moreover, in the aforementioned studies, the results were obtained only at room temperature cycling, and the effects of carbon conducting agent contained in the conventional Li4Ti5O12electrodes were neither considered nor clarified. In this report, for the first time, the Li4Ti5O12/ electrolyte interface is investigated at room and high temperature using the carbon-free Li4Ti5O12 electrode. The new electrode concept5,8 allows us to examine the reactivity of Li4Ti5O12 to the electrolyte and avoid any kind of parasite reaction which may be induced by the high-surface-area carbon conducting additive. Chemical changes at the surface of Li4Ti5O12 were investigated using a step by step X-ray photoelectron spectroscopy (XPS) analysis during charge/discharge cycling. The time-of-flight secondary ion mass spectroscopy (ToF-SIMS) study and scanning electron microscopy (SEM) observation were carried out to examine a quantitative and qualitative change in the surface chemistry and the electrode morphology after cycling, respectively. The differences between the carbon-free and carbon-containing Li4Ti5O12electrodes in terms of stability and cyclability were also discussed. [1] S.S. Zhang, J. Power Sources, 2006, 161, 1385. [2] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc., 1995, 142, 1431; L. Kavan, M. Gratzel, Electrochem. Solid State Lett., 2002, 5, A39. [3] K. Zaghib, M. Simoneua, A. Armand, M. Gauthier, J. Power Sources, 1999, 81-82, 300; G. Armstrong, A. R. Armstrong, J. Canales, P. G. Bruce, Electrochem. Solid-State Lett., 2006, 9, A139; A. N. Jansen, A. J. Kahaian, K. D. Kepler, P. A. Nelson, K. Amine, D. W. Dees, D.R. Vissers, M. M. Thackeray, J. Power Sources, 1999, 81-82, 902; T. Brousse, P. Fragnaud, R. Marchand, D. M. Schleich, O. Bohnke, K. West, J. Power Sources, 1997, 68, 412; T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc., 1995, 142, 1431. [4] J. Christensen, V. Srinivasan, J. Newman, J. Electrochem. Soc., 2006, 153, A560; M. Winter, W. K. Appel, B. Evers, T. Hodal, K. C. Moller, I. Schneider, M. Wachtler, M. R. Wagner, G. H. Wrodnigg, J. O. Besenhard, Monatsch. Chem., 2001, 132, 473. [5] M. S. Song, A. Benayad, Y. M. Choi and K. S. Park, Chem. Commun., 2012, 48, 516. [6] R. Dedryvère, D. Foix, S. Franger, S. Patoux, L. Daniel and D. Gonbeau, J. Phys. Chem. C, 2010, 114, 10999. [7] Y. B. He, F. Ning, B. Li, Q. S. Song, W. Lv, H. Du , D. Zhai, F. Su, Q. H. Yang, F. Kang, J. of Power Sources, 2012, 202, 253. [8] C. J. Kim, N. S. Norberg, C. T. Alexander, R. Kostecki and J. Cabana, Adv. Funct. Mater., 2013, 23, 1214.