Silicon is a very promising candidate to replace graphite as the anode in Li-ion batteries because of its very high theoretical capacity. It has not yet made its way into commercial cells because of severe problems with the charge and discharge cycling of the material. It seems that amorphous silicon and amorphous silicon-containing alloys exhibit much improved cycling performance. Therefore, it is desirable to fully understand the reaction of Li with a-Si. To this end, an in situ X-ray diffraction study of the reaction of lithium with a-Si has been performed. The results confirm that a new crystalline <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">Li</mml:mi><mml:mrow><mml:mn>15</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">Si</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:math>Li15Si4 phase is formed below 30 mV vs. <mml:math display="inline"><mml:mi mathvariant="normal">Li</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mi mathvariant="normal">Li</mml:mi><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>Li/Li+ as first reported by Obrovac and Christensen in an article published in Electrochemical and Solid-State Letters. However, the crystalline phase only forms for films of a-Si above a critical thickness of about 2 μm. © 2004 The Electrochemical Society. All rights reserved.

Many intermetallic materials deliver poor capacity retention when cycled vs. Li. Many authors have attributed this poor capacity retention to large volume expansions of the active material. Here we report the volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si (a-Si), and <mml:math display="inline"><mml:mi mathvariant="normal">a</mml:mi><mml:mo>­</mml:mo><mml:msub><mml:mi mathvariant="normal">Si</mml:mi><mml:mrow><mml:mn>0.64</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">Sn</mml:mi><mml:mrow><mml:mn>0.36</mml:mn></mml:mrow></mml:msub></mml:math>a­Si0.64Sn0.36 as they reversibly react with Li measured by in situ atomic force microscopy (AFM). Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes. The crystalline materials do not. We attribute this difference to the homogeneous expansion and contraction that occurs in the amorphous materials. Inhomogeneous expansion occurs in the crystalline materials due to the presence of coexisting phases with different Li concentrations. Thin films of a-Si and <mml:math display="inline"><mml:mi mathvariant="normal">a</mml:mi><mml:mo>­</mml:mo><mml:msub><mml:mi mathvariant="normal">Si</mml:mi><mml:mrow><mml:mn>0.64</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">Sn</mml:mi><mml:mrow><mml:mn>0.36</mml:mn></mml:mrow></mml:msub></mml:math>a­Si0.64Sn0.36 show good capacity retention with cycle number. © 2003 The Electrochemical Society. All rights reserved.

We present a wide range of testing results on an excellent moderate-energy-density lithium-ion pouch cell chemistry to serve as benchmarks for academics and companies developing advanced lithium-ion and other “beyond lithium-ion” cell chemistries to (hopefully) exceed. These results are far superior to those that have been used by researchers modelling cell failure mechanisms and as such, these results are more representative of modern Li-ion cells and should be adopted by modellers. Up to three years of testing has been completed for some of the tests. Tests include long-term charge-discharge cycling at 20, 40 and 55°C, long-term storage at 20, 40 and 55°C, and high precision coulometry at 40°C. Several different electrolytes are considered in this LiNi_0.5Mn_0.3Co_0.2O₂/graphite chemistry, including those that can promote fast charging. The reasons for cell performance degradation and impedance growth are examined using several methods. We conclude that cells of this type should be able to power an electric vehicle for over 1.6 million kilometers (1 million miles) and last at least two decades in grid energy storage. The authors acknowledge that other cell format-dependent loss, if any, (e.g. cylindrical vs. pouch) may not be captured in these experiments.