New perspectives on spatial dynamics of lithiation and lithium plating in graphite/silicon composite anodes
Introduction
Li-ion batteries with high energy density and fast-charging capability, are highly desirable for the portable devices, electric vehicles and intermittent energy sources [1], [2], [3]. However, commercial graphite anodes deliver limited theoretical capacity (∼372 mAh g−1 for LiC6) and suffer from risk of Li plating due to the ultra-low operation potential, consequently limiting negative-to-positive capacity ratio (N/P) and high-rate capability [4,5]. Silicon-graphite composites are regarded as promising candidates as anode materials taking the high capacity of silicon and the excellent electrical conductivity/robustness of carbon into consideration [6,7]. Unfortunately, huge volume expansion and unstable solid-electrolyte interphase (SEI) film remain challenging issues to large-scale application of silicon-graphite anode [[8], [9], [10]]. On the other hand, Li plating in graphite undoubtedly induces more serious risk at large operation currents at higher capacities which are the same operation conditions for silicon. To date, numerous reports mainly devote to relieving volume expansion and improving SEI-film stability upon cycling, while rarely focus on competitive lithiation kinetics and preferential Li-plating in silicon-graphite anode and their derivatives [11], [12], [13]. Li-plating on graphite should be responsible for the sluggish Li+ intercalation kinetics which working potential is close to Li deposition [14], [15], [16]. In addition, various polarizations enhanced under high current density such as ohmic resistance, charge transfer overpotential, and concentration overpotential, surely induce the local electrochemical potential of electrons to overcome the energy barrier for metallic Li plating [17], [18], [19], [20]. Furthermore, temperature-dependent shifts of the equilibrium potential also raise the Li plating complexity and allows local Li plating above 0 V vs. Li/Li+ [21,22]. Moreover, absence of a standard reference for full cells with matching cathodes increases the identification difficulty for plated Li [23,24].
To stabilize Li plating for excellent hybrid Li+/Li metal cells, numerous strategies have been proposed including regulating Li+ solvation structure to reduce charge-transfer resistance [25], [26], [27], developing modified graphite with high diffusion coefficient [28], constructing enhanced electrode with the uniform polarization distribution [13,29]. However, such strategies are not clearly suitable for the improved performances of graphite/silicon anodes. It is reasonably expected that low initial lithiation platform (0.1∼0.2 V) of silicon can also trigger possible Li plating similar to graphite [9,30]. Additionally, the co-lithiation due to the close lithiation potential between silicon and graphite, unavoidably is becoming a concern about Li plating sites whether close to silicon or graphite in silicon/graphite anodes [31]. Furthermore, huge volumetric change and particle fracture of silicon upon cycling deteriorate the local electron electrochemical potential, possibly participating in the competition for the preferential Li plating sites in the subsequent cycles. Therefore, comprehensive understanding on competitive lithiation and Li plating behavior in silicon/graphite could significantly facilitate the improved electrochemical performances, consequently providing important insights into reducing the N/P ratio to boost the energy density of full batteries. Unfortunately, up to date few reports focus on competitive lithiation and Li plating behaviors in silicon or silicon/graphite anodes.
To clarify the influence of the competitive lithiation between silicon and graphite on plating Li, herein, silicon/graphite composites with the same graphite are prepared using commercial silicon and P-type silicon as precursors respectively. Systematical investigations on lithiation/Li plating in silicon/graphite anodes reveal the coexistence of lithiation process between graphite and silicon, and the lithiation of graphite is dominated by the interfacial polarization of silicon. Comparing to commercial silicon, P-type silicon has higher Li+ diffusion coefficient and lower energy barrier, leading to lower concentration polarization and interfacial transfer resistance. Such behavior could delay the lithiation process of graphite and promote the spatial homogeneity of Li plating and SEI-film growth, resulting in reversible Li plating and stable long-term cycling. Our work provides a new perspective on spatial dynamics of lithiation and Li plating in graphite/silicon composite anodes, further offering an enlightenment and reference significance for the anode design to obtain high-performance batteries with low N/P ratios.
Section snippets
Results and discussion
Investigations on Li+ interaction and corresponding Li plating during the lithiation/plating process in silicon/graphite anodes are systematically carried out, in which the compositing anodes are prepared by ball-milling method using graphite (Gr), micro-sized silicon, a small amount of polyvinylpyrrolidone and phenolic resin (grinding aid reagents) as precursors. Typical synthesis processes of silicon/graphite particles are schematically illustrated in Fig. 1a. The obtained products utilizing
Conclusions
For the first time, a new perspective is proposed that the competitive co-lithiation behavior in silicon/graphite is dominated by the interfacial polarization of silicon, which influences the preferential Li-plating sites and SEI-film evolution. The electrochemical potential disturbance originating from consecutive volume variation of silicon upon cycling, could keep Li plating sites changing and deteriorate the battery performances. Taking silicon for example, global
Preparation of Gr/Si@C and Gr/PSi@C micro-sized composites
The micro-silicon particles (μSi, 99% purity, with an average particle size of 1 μm) and the photovoltaic silicon waste (PSi, 99.9999% purity, with an average particle size of 1 μm) are obtained from Shanghai Guanjin Powder Material Corp., Ltd., and Jiangsu Sunport Power Corp., Ltd., respectively. Typically, 2 g commercial graphite, 0.5 g μSi, 0.125 g phenolic resin and 0.5 g polyvinylpyrrolidone (PVP, Mw=58,000) are mixed in 50 mL agate jar and ball-milled for 20 h with 400 rpm min−1 using
CRediT authorship contribution statement
Jianming Tao: Conceptualization, Methodology, Writing – original draft, Writing – review & editing. Liwen Liu: Data curation, Formal analysis, Writing – original draft. Juanjuan Han: Visualization, Investigation. Junjie Peng: Investigation. Yue Chen: Formal analysis, Investigation. Yanmin Yang: Software, Supervision. Hu-rong Yao: Investigation, Supervision. Jiaxin Li: Investigation, Data curation. Zhigao Huang: Writing – review & editing. Yingbin Lin: Funding acquisition, Project
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was jointly supported by the Natural Science Foundations of China (No. 12174057, 22179020), Natural Science Foundation of Fujian Province (Grant No. 2021L3011) and Fujian Natural Science Foundation for Distinguished Young Scholars (Grant No. 2020J06042).
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J. Tao and L. Liu contributed equally to this work.