Capacity profile of composite electrode cycled at various current densities: 0.3, 0.6, 1.0 and 3.0 A g-1. Wu et al. Click to enlarge.

At a charge/discharge current of 1.0 A g^-1, the composite electrode exhibited a relatively stable reversible lithium capacity of 1,600 mAh g^-1 for 1,000 deep cycles based on the weight of only Si.

The team estimated the volumetric capacity of the SiNP-hydrogel electrode to be ~1,080 mAh cm^-3—about double that of a graphite electrode (~620 mAh cm^-3).

Silicon (Si) has attracted a great deal of attention as an alternative anode material for Li-ion batteries lately, primarily due to: (1) its high theoretical gravimetric capacity of ~4,200 mAh g^-1 based on the Si weight, (2) its relatively low discharge potential (~0.5 V versus Li/Li⁺), (3) the natural abundance of elemental Si, and (4) its safety and environmental benignity.

However, Si-based anodes also face significant challenges due to the ~400% volume expansion upon lithium insertion in Si; this can result in fracture and loss of electrical contact, and an unstable solid electrolyte interphase (SEI) growth on the Si surface. Recently, exciting progress has been made through nanostructuring of Si...However, synthesis of Si nanostructures usually involves high temperature chemical vapor deposition or complex chemical reactions and/or templates, and their scalability and compatibility with existing battery manufacturing processes remain a challenge.

More recently, electrodes made from Si nanoparticle (SiNP) slurries have been investigated by several research groups as a potentially manufacturing-compatible route...Building on the previous work on polymer binders, here we demonstrate that a unique feature of our approach for realizing this excellent performance is the use of in-situ polymerization, to form a bi-functional conformal coating that binds to the Si surface and also serves as a continuous three-dimensional (3D) pathway for electronic conduction. The in-situ polymerization fabrication technique is quite different from previous studies, where the direct mixing of Si particles with polymer binders produces less conformal coatings and non-continuous electronic transport pathway.

—Wu et al.

Stanford scientists used a battery fabrication process to make novel silicon/hydrogel electrodes. Silicon nanoparticles were dispersed in a solution of phytic acid and aniline in water, followed by the addition of an oxidizer. The aniline rapidly polymerizes and crosslinks, resulting in a dark green viscous gel. The viscous gel was then bladed onto a copper foil current collector and dried to form a uniform film.

This solution-based synthesis method is compatible with roll-to-roll coating methods, the team noted, making this system readily scalable for large area electrode films.

Using a scanning electron microscope, the scientists discovered that the porous hydrogel matrix is riddled with empty spaces that allow the silicon nanoparticles to expand when lithium is inserted. This matrix also forms a three-dimensional network that creates an electronically conducting pathway during charging and discharging.

It turns out that hydrogel has binding sites that latch onto silicon particles really well and at the same time provide channels for the fast transport of electrons and lithium ions. That makes a very powerful combination.

—Yi Cui

The team attributed the stable performance of the composite electrode to to the unique nanoscale architecture:

• The porous hydrogel matrix has empty space to allow for the large volume expansion of the SiNPs during lithium insertion.

• The highly conductive and continuous 3D framework, as well as the conformal conductive coating surrounding each SiNP, helps provide good electrical connection to the particles.

• Although pulverization of larger particles may still occur during lithiation and battery cycling, the fractured Si pieces are trapped within the interconnected narrow pores of the polymer matrix, maintaining good electrical connectivity among fractured particles.

A simple mixture of hydrogel and silicon proved far less effective than the in situ synthesis polymerization technique.

Hydrogel primarily consists of water, which can cause lithium-ion batteries to ignite—a potential problem that the research team had to address. In the final production phase, the water was removed, Bao said.

Although a number of technical issues remain, Cui expresses optimism about potential commercial applications of the new technique to create electrodes made of silicon and other materials.

By taking advantage of the conductive polymer matrix, which provides fast electronic and ionic transfer channels, as well as free space for Si volume changes, we successfully achieved high capacity and extremely stable electrochemical cycling. The electrode can be continuously deep cycled up to 5,000 times without significant capacity decay. Moreover, the solution synthesis and electrode fabrication process are highly scalable and compatible with existing slurry coating battery manufacturing technology.

This will potentially allow for this high-performance composite electrode to be scaled up for manufacturing the next generation of high-energy Li-ion batteries, which are important for applications in electric vehicles and grid-scale energy storage systems that require both low-cost and reliable battery systems. In addition, our described materials design for silicon-based anodes may be extended to other battery electrode material systems that experience large volume changes and unstable SEI formation during cycling.

—Wu et al.

Former Stanford postdoctoral scholars Hui Wu, now a faculty member at Tsinghua University-Beijing, and Guihua Yu, now a faculty member at the University of Texas-Austin, are co-lead authors of the study. Other authors are Stanford visiting scholar Lijia Pan and graduate students Nan Liu and Matthew McDowell.

The research was supported by the Precourt Institute for Energy at Stanford and the US Department of Energy through the SLAC Laboratory Directed Research and Development Program. Additional funding was provided by the Natural Science Foundation of China, the U.S. National Science Foundation and the Stanford Graduate Fellowships Program in Science and Engineering.

Resources

• Hui Wu, Guihua Yu, Lijia Pan, Nian Liu, Matthew T. McDowell, Zhenan Bao & Yi Cui (2013) Stable Li-ion battery anodes by in-situ polymerization of conducting hydrogel to conformally coat silicon nanoparticles. Nature Communications 4, Article number: 1943 doi: 10.1038/ncomms2941