Session Q20: Invited Session: Robust Energy Storage with Engineered Si

Silicon Nanowire Anodes: Materials and Composites

Silicon nanowires (SiNWs) have the potential to perform as anodes for lithium-ion batteries with a much higher energy density than graphite. Previously, we have shown that reversible capacities $>$3,000 mAh/g can be obtained by using an electrode geometry consisting of SiNWs grown on metallic current collector substrates using the CVD-based vapor-liquid-solid (VLS) method. These electrodes consisted of SiNWs directly attached and vertically oriented off of the current collector. SiNWs can be synthesized in large quantities using the supercritical-fluid-solid (SFLS) method. Slurries were prepared composed of silicon nanowires synthesized using the SFLS method mixed with amorphous carbon or carbon anotubes and binder and coated onto Cu foil. Recent results regarding the cycling behavior of the SiNWs using different experimental conditions will be presented. The performance of these composite electrodes will also be compared with our previous work using the VLS SiNWs to determine how the electrode architecture affects the electrochemical performance.   

Strain-tolerant High Capacity Silicon Anodes via Directed Lithium Ion Transport for High Energy Density Lithium-ion Batteries

Energy storage is an essential component of modern technology, with applications including public infrastructure, transportation systems, and consumer electronics. Lithium-ion batteries are the preeminent form of energy storage when high energy / moderate power densities are required. Improvements to lithium-ion battery energy / power density through the adoption of silicon anodes--with approximately an order of magnitude greater gravimetric capacity than traditional carbon-based anodes--have been limited by $\sim$300\% strains during electrochemical lithium insertion which result in short operational lifetimes. In two different systems we demonstrated improvements to silicon-based anode performance via directed lithium ion transport. The first system demonstrated a crystallographic-dependent anisotropic electrochemical lithium insertion in single-crystalline silicon anode microstructures. Exploiting this anisotropy, we highlight model silicon anode architectures that limit the maximum strain during electrochemical lithium insertion. This self-strain-limiting is a result of selecting a specific microstructure design such that during lithiation the anisotropic evolution of strain, above a given threshold, blocks further lithium intercalation. Exemplary design rules have achieved self-strain-limited charging capacities ranging from 677 mAhg$^{-1}$ to 2833 mAhg$^{-1}$. A second system with variably encapsulated silicon-based anodes demonstrated greater than 98\% of their initial capacity after 130+ cycles. This anode also can operate stably at high energy/power densities. A lithium-ion battery with this anode was able to continuously (dis)charge in 10 minutes, corresponding to a power / energy density of $\sim$1460 W/kg and $\sim$243 Wh/kg--up to 780\% greater power density and 220\% higher energy density than conventional lithium-ion batteries. Anodes were also demonstrated with areal capacities of 12.7 mAh/cm$^2$, two orders of magnitude greater than traditional thin-film silicon anodes.\\[4pt] In collaboration with Michael W. Cason and Ralph G. Nuzzo.   

Understanding the Degradation of Silicon Electrodes for Lithium-Ion Batteries Using Acoustic Emission and Fracture Mechanics

Silicon is a promising anode material for lithium-ion battery application due to its high specific capacity, low cost, and abundance. However, when silicon is lithiated at room temperature, it can undergo a volume expansion in excess of 280{\%}, which leads to an extensive fracturing. This is thought to be a primary cause of the rapid decay in cell capacity routinely observed. We have developed a special cell design which allows us to monitor acoustic emissions stemming from mechanical events in the cell and allow for detailed structural analysis using X-ray diffraction with an internal standard. The combined result from acoustic emissions and X-ray diffraction allow for a first of its kind detailed look at how silicon anodes degrade and together with presented theories of fracture mechanics enable a material engineering approach to optimize its long term behavior. In collaboration with Kevin Rhodes and Sergiy Kalnaus.\\[4pt] Parts of this research were performed at the High Temperature Materials Laboratory, a national user facility sponsored by the same office.   

Reversible Cycling of Silicon and Silicon Alloys

Lithium ion batteries typically use a graphite negative electrode. Silicon can store more lithium than any other element and has long been considered as an attractive replacement for graphite. The theoretical lithium storage capacity of silicon is nearly ten times higher than graphite volumetrically and three times higher gravimetrically. The equilibrium Si-Li binary system is well known. Completely new phase behaviors are observed at room temperature. This includes the formation of a new phase, Li15Si4, which is the highest lithium containing phase at room temperature [1]. The formation of Li15Si4 is accompanied by a 280 percent volume expansion of silicon. During de-alloying this phase contracts, forming amorphous silicon. The volume expansion of alloys can cause intra-particle fracture and inter-particle disconnection; leading to loss of cycle life. To overcome issues with volume expansion requires a detailed knowledge of Li-Si phase behavior, careful design of the composition and nanostructure of the alloy and the microstructure of the negative electrode [2]. In this presentation the phase behavior of the Li-Si system will be described. Using this knowledge alone, strategies can be developed so that silicon can be reversibly cycled in a battery hundreds of times. Further increases in energy density and efficiency can be gained by alloying silicon with other elements, while controlling microstructure [2]. Coupled with negative electrode design strategies, practical negative electrodes for lithium ion cells can be developed based on bulk materials, with significant energy density improvement over conventional electrodes. \\[4pt] [1] M.N. Obrovac and L.J. Krause, J. Electrochem. Soc., 154 (2007) A103. \\[0pt] [2] M.N. Obrovac, Leif Christensen, Dinh Ba Le, and J.R. Dahn, J. Electrochem. Soc., 154 (2007) A849   

Pair Distribution Function Analysis and Solid State NMR Studies of Silicon Electrodes for Lithium Ion Batteries: Understanding the (De)lithiation Mechanisms

The crystalline-to-amorphous phase transition that occurs on electrochemical Li insertion into crystalline Si, during the first discharge, hinders attempts to link the structure with electrochemical performance. We apply a combination of local structure probes, in situ and ex situ 7Li nuclear magnetic resonance (NMR) studies, and pair distribution function (PDF) analysis of X-ray data to investigate the changes in short-range order that occur during the initial charge and discharge cycles. The distinct electrochemical profiles observed subsequent to the first discharge have been shown to be associated with the formation of distinct amorphous lithiated silicide structures. The first process seen on the second discharge is associated with the lithiation of the amorphous Si, forming small clusters. These clusters are broken in the second process to form isolated silicon anions. The (de)lithiation model helps explain the hysteresis and the steps in the electrochemical profile observed during the lithiation and delithiation of silicon. At deep discharge states a highly reactive lithium excess Li15Si4 phase is detected by in situ NMR which undergoes a self-discharge process in electrolyte.