English

• 1. INTRODUCTION

  Since its appearance on the market in 1991, the lithium-ion battery (LIB) has been the major power source driving the rapid digitalization of our daily life^[1-3]. Three scientists, John B. Goodenough, M. Stanley Whittingham and Akira Yoshino, won the 2019 Nobel Prize in chemistry for their outstanding contributions on lithium-ion batteries (LIBs). Established on their studies^[5-7], the first generation of LIBs composed of graphite anodes and Li transition-metal oxide cathodes has been well developed over the past 30 years and dominates the current battery markets. However, these first-generation electrodes are all classified as intercalation materials, which can only provide limited space for the intercalation reaction of lithium ions and further limit the energy density below ~300 Wh/kg in practical use^{[8, 9]}. To sustain the energy density demand significantly beyond 300 Wh/kg for electric vehicles^{[10, 11]}, the next generation of LIBs call for a paradigm shift from intercalation chemistry to alloying chemistry^[12].

  2. .PROGRESS

  Silicon, as an alloying material with lithium, is among the most prominent anode materials for next-generation batteries. With Li ions alloying with silicon atoms, one silicon atom can host up to 4.4 Liions by alloying chemistry compared to each carbon just hosting 1/6 Li ions in graphite anodes by intercalation chemistry (Fig. 1a, 1b)^[13-15]. This results in ultra-high theoretical capacity of silicon anode (4200 mAh·g^-1), over 10 times higher than that of graphite anode (375 mAh·g^-1)^[13-17]. However, the increased capacity in alloying chemistry is accompanied with the substantial volumetric change. With > 300% volume expansion during lithiation, silicon anode experiences cracks and even pulveriza-tion during cycling, leading to repeated SEI formation and loss of electronic contact with both conductive materials and current collectors^{[17, 18]}. Over the past decade, there have been significant efforts on Si material design to improve its cyclability.

  Figure 1

  

  Figure 1.  Schemes of the intercalation chemistry and alloying chemistry. The structural transformations and voltage profiles of intercalation graphite (a) and alloying silicon (b)^[15]

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  Currently, three classes of solutions focusing on the structural designs have been explored to address the cyclability issue of silicon anode: 1. Designs of nanostructured silicon anodes; 2. Development of advanced binders for silicon anodes; 3. Prelithiation of silicon anode to compensate the initial lithium loss. Our group alone has had 12 generations of nanostructure designs of silicon anodes^{[13, 19, 20]}. Starting from shrinking silicon to nano-scale, pulverization of Si is avoided to be benefited from particle size below critical fracture size (Fig. 2a)^{[16, 21]}. Then, nano-scale Si is encapsulated by a series of mechanical strong layers to stabilize the Si interface and reduce SEI repeating formation (Fig. 2b), moving from insulated SiO₂^[22] to conductive amorphous carbon^[23], and graphene cage^[24]. Meanwhile, internal void space is designed into the Si nanostructure to accommodate the volume change and further avoid SEI breaking at the interface (Fig. 2c)^{[25, 26]}. Besides the nanostructure design, advanced binders are also developed to strongly bind Si particles together rather than relying on the weak van der Waals forces provided by the conventional poly-(vinylidene fluoride) binder (Fig. 2d, 2e), including conducting hydrogel framework^[27] and self-healing coating^[28]. Prelithiation, aimed at compensating the lithium loss in the first cycle caused by SEI formation, is important for the development of Si anodes (Fig. 2f, 2g). Two prelithiation approaches of electrochemical shorting^{[29, 30]} and Li_xX prelithiation reagents^{[31, 32]} are developed and successfully improve the 1^st cycle CE from conventional 70~80% to 100%.

  Figure 2

  

  Figure 2.  Designs of nanostructured silicon: silicon size reduced to nano-scale (a)^[16], silicon particle encapsulation (b)^[24], and internal void space in designed structures (c)^[25]. Advanced binders: hydrogel framework (d)^[27] and self-healing coating (e)^[28]. Prelithiation approaches: electrochemical method (f)^[29] and Li_xX prelithiation reagents (g)^[32]

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  3. .CONCLUSION

  In summary, to overcome the energy density limit of intercalation chemistries, silicon anodes, with ultra-high theoretical capacity benefited from alloying mechanism, show outstanding potential to impel LIBs from intercalation to alloying chemistry. Though silicon anodes suffer from poor cyclability due to drastic volume expansion, tremendous efforts have been devoted to the issue and three classes of structure designs have been explored to com-prehensively improve the cyclability from different aspects. With cycle performance improvement to hundreds or even a thousand stable cycles, silicon opens up a bright future of next-generation high-energy density batteries and more future efforts are expected in low-cost and environmentally friendly large-scale production.

  
  
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  Figure 1  Schemes of the intercalation chemistry and alloying chemistry. The structural transformations and voltage profiles of intercalation graphite (a) and alloying silicon (b)^[15]

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Designs of nanostructured silicon: silicon size reduced to nano-scale (a)^[16], silicon particle encapsulation (b)^[24], and internal void space in designed structures (c)^[25]. Advanced binders: hydrogel framework (d)^[27] and self-healing coating (e)^[28]. Prelithiation approaches: electrochemical method (f)^[29] and Li_xX prelithiation reagents (g)^[32]

  下载: 全尺寸图片 幻灯片
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