A novel hermetic differential scanning calorimeter (DSC) sample crucible

Li/Si coin cells were assembled using crystalline Si as a negative electrode material. The Si was electrochemically lithiated and delithiated to various states of charge. Differential scanning calorimetry studies at heating rates varying from 1 • C/min to 20 • C/min were performed and analyzed to yield activation energies for crystallization reactions occurring in lithiated Si with increasing temperatures. Activation energies requiring mainly the diffusion of Li atoms were near or below 150 kJ/mol while activation energies requiring the reworking of Si bonds were near or above 300 kJ/mol. Avoiding the formation of Li 15 Si 4 did not significantly impact the DSC signals of delithiated a-Li x Si samples.

Section: Methodsmentioning

confidence: 99%

Activation Energies of Crystallization Events in Electrochemically Lithiated Silicon

Chevrier

Dahn

2011

“…After the Journal of The Electrochemical Society, 150 ͑10͒ A1299-A1304 ͑2003͒ A1300 charging cycle, the cell was removed and DSC sample cells were prepared in welded stainless steel tubes, as described previously. 16 Upon cell disassembly in the glovebox, the sample was removed from the aluminum current collector and transferred to a stainless steel sample tube. The tube was sealed to ensure no weight loss during analysis, inside the Ar glove box, with no additional electrolyte or solvent added.…”

Section: Methodsmentioning

confidence: 99%

Morphology and Safety of Li[Ni[sub x]Co[sub 1−2x]Mn[sub x]]O[sub 2] (0≤x≤1/2)

Jouanneau¹,

MacNeil²,

Lu³

et al. 2003

In recent papers, our laboratory has reported new positive-electrode materials based on the Li͓Ni x Co 1Ϫ2x Mn x ͔O 2 series. These materials show good reversible capacity ͑170 mAh/g between 3.0 and 4.4 V͒ and very little capacity loss over 50 cycles. Most interesting, they appear to be much less reactive with electrolyte at high temperatures than Li y CoO 2 at the same potential. To investigate the reason for the large increase in thermal stability of these materials the morphology and the reactivity with electrolyte was measured for a number of samples in the Li͓Ni x Co 1Ϫ2x Mn x ͔O 2 series synthesized at 900°C. Materials with x Ͼ 0 showed a large decrease in reactivity and, surprisingly, much smaller primary particles and tap density as compared to the x ϭ 0 sample. For samples with x у 0.075 the morphology and reactivity remained roughly the same. The reactivity for x у 0.075 was significantly improved compared to LiCoO 2 . We conclude that viable alternatives to LiCoO 2 that appear to be safer are available in the range 0.075 Ͻ x Ͻ 0.4.The active electrodes of lithium-ion batteries are known to be reactive in the presence of an electrolyte at elevated temperatures. 1-3 Therefore, lithium-ion cells must pass a number of safety tests before they can be shipped and marketed. 4,5 For academic researchers, the production of full-sized lithium-ion cells for safety evaluation is difficult because the manufacturing equipment and materials are not usually available. Commonly in such situations, other thermal analysis techniques, that probe the individual electrode reactions, have been used. A number of researchers have performed experiments on individual electrodes in electrolyte to propose possible reaction mechanisms for the instability of lithium-ion cells at elevated temperatures. 2,3,6-8 These researchers have used differential scanning calorimetry ͑DSC͒ 2,3,7 thermal gravimetric analysis ͑TGA͒ 1 and accelerating rate calorimetry ͑ARC͒ 9,10 to analyze the stability of various components of a lithium-ion cell.We have previously investigated the layered Li͓Ni x Co 1Ϫ2x Mn x ͔O 2 series and have reported the structure and electrochemistry of this possible replacement for LiCoO 2 in lithium ion batteries. 11,12 The research group of Ohzuku has also reported results on the member of the series with x ϭ 1/3. 13 In Ref. 11, the thermal stability of Li͓Ni 0.25 Co 0.5 Mn 0.25 ͔O 2 and Li͓Ni 0.375 Co 0.25 Mn 0.375 ͔O 2 in electrolyte, illustrated by DSC data was shown to be superior to that of LiCoO 2 charged to the same potential. In Ref. 12, the structure and electrochemical properties of the entire Li͓Ni x Co 1Ϫ2x Mn x ͔O 2 (0 Ͻ x Ͻ 1) series was reported. In addition, it was shown that the thermal stability advantage at 4.2 V develops rapidly as x increases. This series affords a unique opportunity for the study of those factors important for increased thermal stability, because x can be tuned continuously in samples prepared by the same method in the same laboratory.Here, we continue our investigation of the Li͓Ni x...

“…The Li x Si electrode powder was rinsed with dimethyl carbonate ͑DMC͒ four times to remove the electrolyte and then dried under vacuum. 9 The lithiated powders were never exposed to air.DSC sample preparation.-Details of DSC sample preparation can be found in Ref. 10.…”

mentioning

confidence: 99%

Phase Changes in Electrochemically Lithiated Silicon at Elevated Temperature

Wang

Dahn

2006

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...