Tin Networked Electrode Providing Enhanced Volumetric Capacity and Pressureless Operation for All-Solid-State Li-Ion Batteries

Abstract

Pure tin (Sn) metal nano-powder is investigated as a high capacity negative electrode for rechargeable all-solid-state Li-ion batteries. Sn is used to form a fully dense network intertwining with solid electrolyte negating necessary conductive additive. Galvanostatic cycling of the Sn composite electrode delivers a reversible capacity 800 mAh g −1 of Sn with a constant coulombic efficiency over 99.2%. We report on the effect of pressure and rate upon the delithiation mechanics, drawing correlations between Sn volume increase factors and stress accumulation over the course of Sn-Li phase transformations. Due to the fabricated electrode microstructure, we are able to operate the cell at ambient pressure conditions – the next step toward commercialization of the solid-state battery. We believe that this initial work provides new opportunities to study the electrochemical expansion of Sn with the inclusion of rigid electrolyte particles. In 1997, Fuji announced a tin (Sn)-based amorphous composite oxide material for commercial Li-ion batteries. 1 Ensuing anode de-ployments include the Sony developed Sn-based compound (Sn-Co-C) in 2005. 2 Although these were the first commercial deployments of Sn-based anodes, Sn has been extensively studied for decades as a candidate in rechargeable Li-ion batteries because of its sub-stantial lithium storage capabilities and quicker charging times. 3 De-spite the theoretical capacity of Sn being lower than the currently spotlighted silicon (Si) anode, Sn has exceptionally appealing fea-tures: high gravimetric and volumetric capacity (959 mAh g −1 and 2,476 mAh mL −1 for 4.25 Li-ions), 4 excellent electrical conductivity (9.17 × 10 6 S m −1), and room temperature Li-ion diffusivity (5.9 × 10 −7 cm 2 s −1 of Li 4.4 Sn). 5 The commercialized Sn-Co-C anode by Sony provides a significant capacity advantage over the currently uti-lized graphite anode material (372 mAh g −1). However, wide use of the Sn-Co-C anode has been limited due to the high cost and environ-mental concerns about cobalt. Iron and nickel have been introduced as replacements for cobalt forming amorphous Sn-Fe and Sn-Ni with similar electrochemical properties to the Sn-Co alloy. 6–10 Despite the low cost and high capacity of the Sn-Fe and Sn-Ni anodes, poor cy-cling stability and coulombic efficiency (CE) hinder their practical use in Li-ion batteries. These drawbacks mainly result from the notorious volume change of Sn (∼255% when 4.25 Li-ion inserted), 4,11 leading to a loss of electric contact, pulverization, and cracking. 12 Therefore, controlling the microstructure of the expandable active material dur-ing lithiation/delithiation processes is a key point to realize a high energy-dense Li-ion battery using Sn-based anode materials. Recently, Molina Piper et al. reported the effect of compressive stress on the electrochemical performance of a Si anode in an all-solid-state Li-ion cell, which similarly suffers from pulverization due to immense volume changes. 13 By applying external compressive stress to the silicon/solid-state electrolyte (SSE) composite anode, free volume expansion of Si as well as solid-solid interfaces between the active material, SSE, and conductive additive were effectively controlled, thereby significantly reducing capacity fade. Generally, interfacial impedance in solid-state cells is much higher than that of conventional liquid electrolyte cell because the junction is limited to the small contact area between SSE and active material particles.