In Situ Neutron Depth Profiling of Lithium Transport within Aluminum and Tin

Introduction

Lithium ion (Li-ion) batteries have become the standard power source for notebook computers and other portable electronic devices.  Current graphite anodes in Li-ion batteries offer numerous advantages including good cycle life at relatively low cost.  However, graphite anodes are limited by their theoretical specific capacity of 375 mAh/g for larger storage requirements in some applications such as electric vehicles.  One approach to increasing the energy density of Li-ion batteries is to use higher capacity electrodes.  Among other promising candidates, aluminum (Al) and tin (Sn), with their relatively high theoretical specific capacity of 993 (at LiAl) and 959 (at Li₁₇Sn₄) mAh/g, respectively, may prove to be worthy alternative anode materials.  During the alloying/de-alloying process with lithium (Li), aluminum suffers from volumetric expansions of ~100%, forming a LiAl intermetallic alloy.  Similarly, tin also experience volumetric expansions, the order of ~300%, possibly through the formation of thermodynamically stable intermetallic phases of Li₂Sn₅, LiSn, Li₇Sn₃, Li₅Sn₂, Li₁₃Sn₅, Li₇Sn₂, Li₁₇Sn₄.  This significant volumetric change causes mechanical strain leading to particle pulverization of the active material and believed to be a contributing factor to capacity fade.  Fundamental understanding of the lithiation/de-lithiation and associated processes are necessary in order to address the capacity fade and provide strategies to optimize the chemical and structural parameters of the next generation of Li-ion battery electrodes.

Here, we demonstrate that in situ NDP (in a custom-designed battery coin cell) is an effective tool for the quantification and visualization of Li transport in real time, under battery charge and discharge operations. The obtained spatial and temporal resolution of Li allows for the direct observation of the transient and non-equilibrium nature of the chemical transformations and reactions occurring during the lithiation and de-lithiation of Li storage materials, providing insights in addressing issues related to transport, materials selection, and structural integrity.

Experimental

Electrochemical half-cells were constructed with Al foil (0.016 mm thick, 98.5%, Reynolds) and Sn foil (0.0125 mm thick, 97.4%, Goodfellow) as the working electrode in a disk geometry (12 mm in diameter), metallic lithium (Chemetall Foote Corp) disc (12 mm in diameter) as a combined counter and reference electrode, a Celgard® separator (~ 25 mm), and electrolyte (1 M LiBF₄ or 1 M LiPF₆ in 1:1 ethylene carbonate: dimethyl carbonate from Sigma Aldrich) were assembled in a modified CR2032 coin cells (MTI Corp).  Coin cell components were cleaned in acetone (≥ 99.5%) and methanol (≥ 99.8%) in sequence followed by overnight drying at 55 ^oC under vacuum.  The coin cells were crimped using a manual hydraulic crimping machine (MSK-110, MTI Corp) equipped with a CR2032 die at pressures of 1100 – 1200 psi.  The coin cells were modified with a 9.5 mm diameter hole on one of the casings to allow the placement of a Kapton® film (7.5 mm thickness).  The handling and assembling of materials were performed in an Argon-filled glovebox (mBraun) with continuous detection of H₂O  < 0.5 ppm and O₂< 0.5 ppm.  Electrochemical measurements were performed on a Gamry Potentiostat (Reference 600, Gamry Instruments).  The mass of the electroactive Al and Sn electrode were determined by weighing. 

Results and Discussion

During the initial charging of the battery, lithium is first driven towards and enriched at the surface of the tin followed by propagation throughout the entire tin material.  It was discovered that the highest rate of the lithiation occurred near the surface suggesting that lithiation is not limited by electron transfer.  Our data demonstrated that, in fact, the driving force for lithium transport within the Sn electrode is purely diffusion.  Based on the NDP data, the ambient diffusion coefficient of lithium in Sn is between 0.8 – 2 x10^-7 cm²/second.  Our work also suggests that tin does not lithiated in uniform increments and that significant parasitic side-reactions may have contributed to the decreased coulombic efficiency observed.

Conclusion

The experiments presented here can be replicated for an extensive range of anodes and cathodes, not limited to intermetallic forming materials. This technique can aid in the development of a comprehensive range of materials, architectures for specific purposes (high energy vs. high power applications). For example, this methodology can be used to obtain information concerning Li trapped within localized pockets in the electrode resulting in low coulombic efficiencies; provide understanding of requirements for optimum materials architecture promoting efficient Li transport, which is often the rate limiting step in battery processes, gain insight towards materials properties for high rates of Li transport, and provide a direct experimental verification method for models predicting mass and charge transfer in Li-ion cells, including failure and aging mechanisms.