Amorphous Si Anode Research Articles

Silicon as the Anode Material for Multivalent-Ion Batteries: A First-Principles Dynamics Study.

Due to its huge capacity, Si is a promising anode material for practical applications in lithium-ion batteries. Here, using first-principles calculations, we study the applicability of the amorphous Si anode in multivalent-ion batteries, which are of great interest as candidates for post-lithium-ion batteries. Of the multivalent Mg2+, Ca2+, Zn2+, and Al3+ ions, only Mg2+ and Ca2+ are able to form Mg2.3Si and Ca2.5Si by alloying with Si, delivering very high capacities of 4390 and 4771 mA h g-1, respectively. Mg2.3Si has an 8% smaller capacity than Ca2.5Si, but its volume expansion ratio and ion diffusivity are ∼200% smaller and 3 orders of magnitude higher than those of Ca2.5Si, respectively. The capacity, volume expansion, and ion diffusion of Mg2.3Si are excellently high, moderately small, and fairly fast, respectively, when compared to those of Li3.7Si, Na0.75Si, and K1.1Si. The high performance of Mg2.3Si can be understood in terms of the coordination numbers of Si and the atomic size of Mg. This work suggests that, as a carrier ion for the amorphous Si anode, Mg2+ is the most competitive among the multivalent ions and is at least as good as monovalent ions.

The lithiation process and Li diffusion in amorphous [formula omitted] and Si from first-principles

Silicon is considered the next-generation, high-capacity anode for Li-ion energy storage applications, however, despite significant effort, there are still uncertainties regarding the bulk Si and surface SiO2 structural and chemical evolution as it undergoes lithiation and amorphization. In this paper, we present first-principles calculations of the evolution of the amorphous Si anode, including its oxide surface layer, as a function of Li concentration. We benchmark our methodology by comparing the results for the Si bulk to existing experimental evidence of local structure evolution, ionic diffusivity as well as electrochemical activity. Recognizing the important role of the surface Si oxide (either native or artificially grown), we undertake the same calculations for amorphous SiO2, analyzing its potential impact on the activity of Si anode materials. Derived voltage curves for the amorphous phases compare well to experimental results, highlighting that SiO2 lithiates at approximately 0.7 V higher than Si in the low Li concentration regime, which provides an important electrochemical fingerprint. The combined evidence suggests that i) the inherent diffusivity of amorphous Si is high (in the order 10−9cm2s−1 - 10−7cm2s−1), ii) SiO2 is thermodynamically driven to lithiate, such that Li–O local environments are increasingly favored as compared to Si–O bonding, iii) the ionic diffusivity of Li in LiySiO2 is initially two orders of magnitude lower than that of LiySi at low Li concentrations but increases rapidly with increasing Li content and iv) the final lithiation product of SiO2 is Li2O and highly lithiated silicides. Hence, this work suggests that - excluding explicit interactions with the electrolyte - the SiO2 surface layer presents a kinetic impediment for the lithiation of Si and a sink for Li inventory, resulting in non-reversible capacity loss through strong local Li–O bond formation.

Fluidized bed reaction towards crystalline embedded amorphous Si anode with much enhanced cycling stability

A facile and large-scale fluidized bed reaction route was introduced for the first time to prepare crystalline embedded amorphous silicon nanoparticles with an average size of 50 nm as anode materials for lithium-ion batteries. By increasing the operating potential to control the electrochemically active degree, the resulting sample showed excellent cycle stability with a high capacity retention of 94.7% after 200 cycles at 1 A g-1 in the voltage range of 0.12-2.00 V.

In operando X-ray diffraction strain measurement in Ni3Sn2 – Coated inverse opal nanoscaffold anodes for Li-ion batteries

Volume changes associated with the (de)lithiation of a nanostructured Ni3Sn2 coated nickel inverse opal scaffold anode create mismatch stresses and strains between the Ni3Sn2 anode material and its mechanically supporting Ni scaffold. Using in operando synchrotron x-ray diffraction measurements, elastic strains in the Ni scaffold are determined during cyclic (dis)charging of the Ni3Sn2 anode. These strains are characterized using both the center position of the Ni diffraction peaks, to quantify the average strain, and the peak breadth, which describes the distribution of strain in the measured volume. Upon lithiation (half-cell discharging) or delithiation (half-cell charging), compressive strains and peak breadth linearly increase or decrease, respectively, with charge. The evolution of the average strains and peak breadths suggests that some irreversible plastic deformation and/or delamination occurs during cycling, which can result in capacity fade in the anode. The strain behavior associated with cycling of the Ni3Sn2 anode is similar to that observed in recent studies on a Ni inverse-opal supported amorphous Si anode and demonstrates that the (de)lithiation-induced deformation and damage mechanisms are likely equivalent in both anodes, even though the magnitude of mismatch strain in the Ni3Sn2 is lower due to the lower (de)lithiation-induced contraction/expansion.

An Amorphous Si Anode: A High Capacity Anode for All-Solid-State Lithium Battery

Lithium ion batteries (LIBs) are widely used as a versatile power supply in various portable devices because of their high energy densities. On the other hand, further enlargement of their capacity is still necessary to apply them in electric vehicles in the future. One way for the enlargement is employment of high capacity electrodes. Si is one of the most promising potential candidates for the anodes in next-generation LIBs, because its theoretical capacity is 4200 mAh/g for the alloying reaction to Li4.4Si, and the value is more than 10 times higher compared to that of the graphite anodes.1 However, the Si has to undergo severe volume change during the electrode reaction to take advantage of the exceptionally high lithium storage capacity; this change leads to some drawbacks including rapid capacity fading.2 To circumvent this difficulty, many efforts have been focused on nanostructured Si in the last decade.3 On the other hand, even a bulky FeS-doped a-Si film has been reported to exhibit excellent anode performance in a solid electrolyte recently.4 In the present work, we prepared a-Si films without FeS and investigated the electrode properties, because the effects of FeS on the electrode performance in the previous study were not clear. a-Si films were deposited by RF-magnetron sputtering onto stainless steel substrates (10 mm diameter) using CFS-4EP-LL (Shibaura Mechatronics). High-purity (99.999%) Si target was purchased from Kojundo Chemical Laboratory. In the cell assembling, In-Li alloy was used as a counter electrode. 70Li2S-30P2S5 glass ceramic5was selected as the solid electrolyte. The electrode performance of the films were investigated in the all-solid-state cell by using multichannel potentio-galvanostats, PS-08 (Toho Technical Research) and VSP (Biologic).Fig. 1(a) shows discharge curves for a 300-nm-thick a-Si film at various current densities. The discharge capacity exceeds 2400 mAh/g even at the high discharge current density of 10 mA/cm2. In Fig. 1(b), the plateau voltages are plotted as a function of the current density, revealing linear relationship with current density. The slope derived from the linear relationship is 107 µÙ, which is almost the same as the resistance of the electrolyte layer in the solid-state cell. Therefore, it can be concluded that the overvoltage is predominantly originated from iR drop in the electrolyte layer and the kinetics of the electrode reaction is very fast. On the other hand, the large volumetric change in Si anodes was reported to promote growth of SEI layer, which raises the interfacial resistance to the order of 103 Ω cm2.6The low electrode resistance observed in this study suggests absence of SEI layer in this all-solid-state electrochemical system, which is considered to have positive effects on other electrode properties. They will be also discussed in the meeting. AcknowledgementsFinancial support by the NEDO Japan and the Toyota Motor Corp. for a project entitled “Applied and Practical LiB Development for Automobile and Multiple Application” is gratefully acknowledged. We also thank the MANA Foundry and the NIMS Molecule & Material Synthesis Platform in “Nanotechnology Platform Project” operated by the MEXT Japan for the use of research facilities. References 1 T.D. Hatchard and J.R. Dahn, J. Electrochem. Soc., 151 (2004) A838.2 J.W. Kim, J.H. Ryu, K.T. Lee and S.M. Oh, J. Power Sources,147 (2005) 227.3 H. Wu and Y. Choi, Nano Today, 7 (2012) 414.4 R.B. Cervera, N. Suzuki, T. Ohnishi, M. Osada, K. Mitsuishi, T. Kambara and K. Takada, Energy Environ. Sci., in press.5 F. Mizuno, A. Hayashi, K. Tadanaga and M. Tatsumisago, Adv. Mater.,17 (2005) 918.6 L. Chen K. Wang, X. Xie and J. Xie, J. Power Sources, 174 (2007) 538.

3D Amorphous Silicon on Nanopillar Copper Electrodes as Anodes for High-Rate Lithium-Ion Batteries

We present an amorphous Si anode deposited on a Cu nanopillar current collector, fabricated using a thermal roll-to-roll process followed by electroformation and LPCVD, for application in high-rate Li-ion batteries. Cu nanopillar current collectors with diameters of 250 and 500 nm were patterned periodically with 1 μm pitch and 2 μm height to optimize the diameters of the pillars for better electrochemical performance. Void spaces between Cu nanopillars allowed not only greater effective control of the strain caused by the Si expansion during lithiation than that allowed by a nonpatterned electrode but also significantly improved cycle performance even at 20 C measured after the same rate test: After 100 cycles at 0.5 C, the patterned electrodes with 250 and 500 nm diameter nanopillars showed high capacity retentions of 86% and 84%, respectively. These electrodes retained discharge capacities of 1057 and 780 mAh/g even at 20 C, respectively.

Diffusion of Lithium in Bulk Amorphous Silicon: A Theoretical Study

The rate performance of lithium-ion secondary batteries depends critically on the kinetic transport of Li within the anode material. Here we use first-principles theoretical calculations to study the diffusion of Li in the low-concentration limit, using model electrodes of crystalline and four-fold coordinated bulk amorphous silicon. We identify Li diffusion pathways that have relatively low energy barriers ( 1 nm) distances depends on the atomic-scale features of the silicon host. We find that both the energy barriers for diffusion and the topology of the atomic structure control the diffusion. We estimate the diffusion rate in amorphous Si anode to be comparable to the rate in crystalline Si anodes. These findings shed light on the wide range of reported experimental results for Li diffusion in Si anodes.

Decoupling Electrochemical Reaction and Diffusion Processes in Ionically-Conductive Solids on the Nanometer Scale

We have developed a scanning probe microscopy approach to explore voltage-controlled ion dynamics in ionically conductive solids and decouple transport and local electrochemical reactivity on the nanometer scale. Electrochemical strain microscopy allows detection of bias-induced ionic motion through the dynamic (0.1-1 MHz) local strain. Spectroscopic modes based on low-frequency (∼1 Hz) voltage sweeps allow local ion dynamics to be probed locally. The bias dependence of the hysteretic strain response accessed through first-order reversal curve (FORC) measurements demonstrates that the process is activated at a certain critical voltage and is linear above this voltage everywhere on the surface. This suggests that FORC spectroscopic ESM data separates local electrochemical reaction and transport processes. The relevant parameters such as critical voltage and effective mobility can be extracted for each location and correlated with the microstructure. The evolution of these behaviors with the charging of the amorphous Si anode in a thin-film Li-ion battery is explored. A broad applicability of this method to other ionically conductive systems is predicted.