Elsevier

Journal of Power Sources

Volume 426, 30 June 2019, Pages 116-123
Journal of Power Sources

An affordable manufacturing method to boost the initial Coulombic efficiency of disproportionated SiO lithium-ion battery anodes

https://doi.org/10.1016/j.jpowsour.2019.04.032Get rights and content

Highlights

  • Kilogram-scale synthesis of carbon-coated SiOsingle bondMgSiO3single bondSi superparticles.

  • Improving the initial Coulombic efficiency of disproportionated SiO.

  • Achieving record-high initial Coulombic efficiency for SiO anodes.

Abstract

SiO is regarded as one of the most promising anode materials for next-generation lithium-ion batteries due to its high energy density and low cost. However, practical applications of SiO are seriously limited by its low initial Coulombic efficiency. Herein, we report an affordable manufacturing method to synthesize carbon-coated Csingle bondSiOsingle bondMgSiO3single bondSi composites with a secondary superparticle structure and an outstanding initial Coulombic efficiency. The in situ evolution of the MgSiO3 phase through the reaction between MgO and SiO2 contributes to the improved initial Coulombic efficiency. The formation of MgSiO3 consumes the deleterious SiO2 phase within SiO, and MgSiO3 acts as an electrochemically inactive phase that can effectively buffer volume variations of active materials. These attributes and the unique mesoporous superparticle structure lead to a markedly improved electrochemical performance. The resultant Csingle bondSiOsingle bondMgSiO3single bondSi anode exhibits an initial Coulombic efficiency of 78.3%, which, to our knowledge, is the highest value ever reported for SiO-based anodes without prelithiation. This work offers a new strategy for addressing the intrinsic low initial Coulombic efficiency of SiO, paving the way toward the mass production of SiO-based anodes for real applications.

Introduction

Lithium-ion batteries (LIBs) have been established as one of the most important energy storage technologies and are widely used in portable electronics and electric vehicles (EVs) because of their high energy and power densities as well as long lifespan [[1], [2], [3], [4]]. However, the increasing energy density and cost demands for general public acceptance of EVs have begun to exceed the ultimate capability of current commercial LIB technology [[5], [6], [7], [8]]. Silicon monoxide (SiO) has recently aroused great interest as one of the most promising alternative anode materials for next-generation LIBs due to its appropriate working potential (<0.5 V vs. Li+/Li), high theoretical specific capacity (∼2400 mAh g−1), and enhanced cycling stability compared to Si [[9], [10], [11], [12], [13]]. The mechanism of SiO electrode reaction in LIBs can be described as 4SiO+17.2Li+→3Li4.4Si + Li4SiO4 [9]. Although the real microstructure of SiO has been not yet completely understood, two main models have been proposed to interpret the microstructure of SiO [[14], [15], [16]]. The first is the random-bonding model with a random network of O and Si2+ [14], while the other is the random-mixture (RM) model with two separate phases of Si and SiO2 [[15], [16], [17]]. The latter model is generally accepted to better describe the real structure of SiO. In this model, the mechanism of SiO electrode reaction can be described as a combination of Si+4.4Li+→Li4.4Si and 2SiO2+4Li+→Li4SiO4+Si. In addition, the disproportionation reaction of SiO to form Si and SiO2 (2SiO→Si + SiO2) at temperatures above 800 °C has also been verified and the disproportionated SiO species usually exhibit better electrochemical performance than normal SiO [9,18].

Although SiO has been regarded as one the promising candidates for next-generation LIB anodes, some intrinsic limitations of SiO, in particular its low initial Coulombic efficiency (ICE), dramatically hinder its practical applications [[19], [20], [21]] The intrinsic low ICE, which is mainly caused by the irreversible formation of inactive Li2O and Li4SiO4 due to side reactions between Li ions and SiO2 as well as the formation of the solid electrolyte interphase (SEI), is one of the prevailing challenges that must be overcome to drive SiO-based technology forward [22,23]. In full cells, the low ICE of anode materials can lead to excessive consumption of Li ions from cathode materials in the first cycle, resulting in a significant reduction in energy density. Consequently, there is an urgent demand to improve the ICE of SiO-based anode materials.

In an attempt to solve this challenge, many groups have reported using Li metals to modify SiO through so-called prelithiation approaches [[24], [25], [26], [27], [28]]. Although these methods can effectively compensate for the first-cycle irreversible capacity loss, to the best of our knowledge, the prelithiation technology, which requires specialized equipment, is not yet ready for large-scale manufacturing of LIB electrodes [29]. In addition to the prelithiation technology, a few groups have attempted to address the low ICE by changing the inherent microstructure of SiO. However, most methods can only slightly improve the ICE, such that the overall electrochemical performance remains unsatisfactory. For example, Wang et al. reported the preparation of a SiO/Fe2O3 composite by ball milling, and this composite had a higher ICE value (68%) than bare SiO (59%). However, this ICE value is still far from meeting practical application requirements [30].

Herein, we report the design and synthesis of novel carbon-coated Csingle bondSiOsingle bondMgSiO3single bondSi secondary superparticles comprising densely-packed nanostructured SiO, MgSiO3 and Si. The Csingle bondSiOsingle bondMgSiO3single bondSi superparticles are prepared via processes including mechanical milling, spray drying, heat treatment, and carbon coating. This method is scalable and is beneficial for cost-effective mass production of SiO-based anode materials. MgO is known to be one of the side products generated during the process of preparing Si by Mg thermal reduction reactions and is usually removed by acid etching [[31], [32], [33]]. In this work, we for the first time exploit MgO as an efficient component to consume amorphous SiO2 in SiO by forming MgSiO3. This novel material design for forming Csingle bondSiOsingle bondMgSiO3single bondSi superparticles has several advantages. In particular, MgSiO3 is stable and electrochemically inert during cycling, and its formation is realized by consuming SiO2 resulting from the disproportionation reaction of SiO at high temperatures, thus effectively mitigating the side reactions between Li+ and SiO2. As a consequence, our Csingle bondSiOsingle bondMgSiO3single bondSi superparticles exhibit a high ICE of 78.3%, which represents the highest ICE ever achieved for SiO-based anodes without prelithiation. In addition, the buffering effect of MgSiO3 and the mesoporous structure of Csingle bondSiOsingle bondMgSiO3single bondSi superparticles can effectively accommodate the volume expansion of SiO, leading to enhanced cycling stability. This study offers a novel and cost-effective strategy for addressing the intrinsic low ICE issue of SiO, which is crucial for large-scale applications of SiO-based anodes.

Section snippets

Preparation of Csingle bondSiOsingle bondMgSiO3single bondSi and Csingle bondSiOsingle bondMg2SiO4single bondSi superparticles

SiO, MgO and Si bulk materials were ground into nanometer-sized particles in absolute ethyl alcohol through a ball milling process. The molar ratio of SiO, MgO, and Si was 21:7:10. With this molar ratio, MgO can consume most SiO2 in SiO, and the addition of Si can help the composite achieve a high theoretical capacity of 2172 mAh g−1 (Capacitytheoretical = CapacitySi × PSi weight percentage + CapacitySiO × PSiO weight percentage = 3600 mAh g−1 × 0.19 + 2400 mAh g−1 × 0.62 = 2172 mAh g−1). After

Results and discussion

Fig. 1 illustrates the procedure for fabricating Csingle bondSiOsingle bondMgSiO3single bondSi-1100 superparticles. The precursors, i.e., bulk Si, MgO and SiO, were first broken into nanometer-sized particles by a mechanical milling process. Because the precursor ratio determines the structure of superparticles and the electrochemical performance of the final product, we optimized the precursor weight percentages as 62, 19 and 19% for SiO, MgO and Si (molar ratio = 21:7:10), respectively. With this ratio, the final composite

Conclusions

In summary, a new strategy to address the low ICE issues of SiO anodes has been presented via developing Csingle bondSiOsingle bondMgSiO3single bondSi secondary superparticles containing mesopores, nanosized SiO particles and electrochemically inert MgSiO3. Using the exquisitely developed Csingle bondSiOsingle bondMg2SiO4single bondSi and Csingle bondSiOsingle bondMgSiO3single bondSi superparticles as anodes, half-cells were fabricated to diagnose the battery performance. Among various superparticles, Csingle bondSiOsingle bondMgSiO3single bondSi-1100 exhibited the highest ICE of 78.3% with a high initial capacity of

Acknowledgements

A.D. acknowledges the financial support from NSFC (21872038, 21373052'), MOST (2017YFA0207303), and Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (17JC1400100). D.Y. is thankful to the financial support from NSFC (51573030, 51573028, and 51773042).

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