Anode Hosts for Lithium Batteries: Revisiting Tin and Aluminum
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Anode Hosts for Lithium Batteries: Revisiting Tin and Aluminum Quan Fan, Peter Zavalij, and M. Stanley Whittingham* Department of Chemistry, State University of New York at Binghamton Binghamton, New York 13902 *[email protected] ABSTRACT Pure tin reacts readily with four lithium atoms, and so is a prime candidate as the host for the anode of lithium batteries. Tin foil and an expanded tin grid (microporous tin) have a capacity of >600 mAh/g over more than 10 deep reaction cycles, indicating the inherent reversibility of tin anode. The microporous tin showed superior chemical capacity retention. Different phases are observed during the intercalation of lithium. Capacity loss was observed after 10 cycles though, consistent with the significant increase of the cell impedance. For comparison aluminum expanded grids were also examined as hosts, where LiAl is formed. Capacities approaching 1 Ah/g were obtained. LiBOB (lithium bis(oxalato)borate) was also studied as the electrolyte salt for comparison with the reactive and high cost LiPF6 salt. INTRODUCTION Commercial lithium ion batteries are using graphitic carbon as anode; it has a relatively low capacity of ~370 mAh/g when forming LiC6, and can pose safety problems at high charge rates. In the past few years, there has been extensive research on metals that alloy with lithium. Aluminum was first studied and it turned out that it operates well in ether electrolytes [1], but not in carbonate electrolytes. Tin is another attractive candidate for use as anode, since it can react readily with Li to give Li4.4Sn below 0.8 V, giving a theoretical capacity of 990 mAh/g, which is 2.6 times of graphite. Yet the problem exists that the capacity of metal tin dropped rather quickly after tens of cycles. It has been generally thought that the tin particle size grows with the forming of LixSn, and that the large volume expansion and shrinkage occurring during the cycling destroy the reversibility [2]. Yang et al. did a detailed investigation on the mechanism of this capacity fade [3]. An in-situ AFM study by Beaulieu and co-workers also confirmed the breaking of the material [4]. To avoid these problems tin oxides were proposed by Idota and co-workers [5], where the lithium oxide acted as a soft matrix to ameliorate the expansion of the nano-sized tin. The mechanism of operation of these tin oxides have been extensively studied [6-11], and it is claimed that lithium can be provided from a source other than the cathode during the first charging of the anode to overcome the large irreversible capacity loss. By using X-ray diffraction and differential capacity data, Dahn et al. argued that aggregation of the nano-sized tin contributes to the quick fading for tin based anode, and that this aggregation could be largely reduced in the oxide leading to enhanced capacity retention [6]. Such electrodes have high reactivity with the electrolyte due to the high surface area, and require an increased SEI layer, as well as conductive diluents and binders, and under such conditions
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