Phase Changes in Electrochemically Lithiated Silicon at Elevated Temperature

Abstract

as well as the metastable crystalline Li 15 Si 4 alloy were prepared electrochemically. The phase changes in these samples at elevated temperature were explored using differential scanning calorimetry and X-ray diffraction. The amorphous Li x Si phase present in the Li 1 Si and Li 2 Si samples transformed first, near 190°C, to Li 7 Si 3 , instead of the expected Li 12 Si 7 based on the Li-Si phase diagram. The amorphous component in the Li 3 Si sample apparently has a higher average lithium content and transformed to both Li 15 Si 4 and Li 7 Si 3 near 190°C. At higher temperatures all the samples transformed into the thermodynamically stable phases expected based on their average compositions and the Li-Si phase diagram. The metastable crystalline Li 15 Si 4 apparently transforms into thermodynamically stable phases in one step.Silicon-based negative electrode materials for lithium-ion batteries are attractive due to their high theoretical capacity. Furthermore, it has been recently shown that silicon-based electrode materials show less reactivity with solvent or electrolyte at elevated temperature than graphite-based electrode materials, which strongly suggests that silicon-based negative electrodes will contribute to safer Li-ion cells. 1,2 The launch of a 14430-sized lithium-ion battery using a tin-based negative electrode by Sony suggests that silicon or tinbased alloys will begin displacing graphite as the dominant negative electrode material in Li-ion batteries. 3 The Li x Si phases that form as lithium reacts electrochemically with silicon at room temperature are different from those given in the equilibrium Li-Si phase diagram. Limthongkul et al. showed that an amorphous Li-Si alloy forms when silicon is electrochemically lithiated by an electrochemically driven solid-state amorphization process. 4,5 Obrovac and Christensen 6 showed that amorphous Li x Si ͑designated a-Li x Si here͒ suddenly crystallizes below 50 mV ͑vs Li/Li + ͒ to form a new lithium-silicon phase, identified as Li 15 Si 4 . Delithiation of the Li 15 Si 4 phase results in the formation of amorphous silicon. Cycling Li x Si electrodes above 50 mV avoids the formation of crystallized phases completely and results in better cycling performance. 6 Hatchard and Dahn showed that amorphous Li x Si forms even when amorphous silicon is used as the starting electrode material, and that the crystalline Li 15 Si 4 phase is formed below 30 mV vs Li/Li + . 7 Both amorphous Li x Si and crystalline Li 15 Si 4 are not thermodynamically stable phases according to the Li-Si phase diagram. 8 These metastable phases should transform to some mixture of thermodynamically stable phases when the temperature is elevated. The goal of this work has been to study these transformations.Here, the phase changes that occur in Li x Si, prepared by electrochemical methods at room temperature, during subsequent heating are explored using differential scanning calorimetry ͑DSC͒ and X-ray diffraction ͑XRD͒. It is our opinion that these results will be useful for workers ex...

Microstructural evolution induced by micro-cracking during fast lithiation of single-crystalline silicon

“…Once several micro-cracks form, the local stress field near the crack becomes more complex and a network of interconnecting cracks can develop. [5,10,12,22,39]. However, this result conflicts with our TEM study discussed previously in Fig.…”

“…Generally, graphite-based materials are used for anodes due to their low cost and stable performance, but they have a limited capacity of 372 mAh/g [7]. Silicon has drawn considerable attention as one of the most promising anode materials due to its huge theoretical capacity of ~4200 mAh/g (Li 22 Si 5 ) [8][9][10][11]. Associated with this large capacity, insertion of lithium into silicon causes large volumetric expansion in the range of 300% to 400% [11][12][13][14].…”

“…Although silicon transforms to several distinct silicon-lithium crystalline phases at elevated temperature [10], it is well known that electrochemical lithiation of silicon at room temperature results in an amorphous phase of lithiated silicon [5,12,[22][23][24]. Obrovac et al [5] suggested that the lithiation of crystalline silicon is a two-phase reaction in which a reaction front separates the growing amorphous lithiated phase from pristine crystalline silicon.…”

Microstructure Study of Electrochemically Driven LixSi

“…LiC 6 , [ 5 ] therefore many research efforts have focused on fi nding alternative anode materials. Silicon has the greatest potential for next generation LIBs as it has the highest known theoretical capacity of 4200 mAh g − 1 (Li 22 Si 5 ) at room temperature.…”

“…Silicon has the greatest potential for next generation LIBs as it has the highest known theoretical capacity of 4200 mAh g − 1 (Li 22 Si 5 ) at room temperature. [6][7][8] The major drawback to silicon is the volume expansion upon Li insertion in the range of 300% to 400% occurring due to the formation of Li x Si alloys. [9][10][11][12][13][14] Stress associated with the large volume changes have been cited as the cause of cracking and pulverization of Si electrodes that lead to a loss of electrical contact and capacity fade during cycling.…”