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Battery and energy materials

Which statement about silicon (Si) as an anode material is correct?

  1. AIts theoretical capacity is smaller than that of graphite, but its volume changes by less than 1% on cycling, so it has a long life
  2. BIts theoretical capacity is about ten times that of graphite, but it swells more than threefold on lithiation and degrades easily
  3. CIts theoretical capacity is about the same as graphite, but its metal-like electronic conductivity suits fast charging
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Answer: B. Its theoretical capacity is about ten times that of graphite, but it swells more than threefold on lithiation and degrades easily

Si alloys with lithium, and when it reacts as far as Li₁₅Si₄ at room temperature its theoretical capacity is about 3600 mAh/g per mass of Si, about ten times that of graphite (372 mAh/g). However, its volume swells about three- to fourfold on lithiation, so with repeated cycling the particles crack and become electrically isolated, the electrolyte decomposes on the newly exposed surfaces to re-form the SEI (solid electrolyte interphase), and lithium is gradually lost. In practice, therefore, small amounts of Si or SiO are blended into graphite, or Si is made into nanoparticles, to limit the effect of expansion. Hardly any volume change is a characteristic of lithium titanate (Li₄Ti₅O₁₂); even graphite changes by only about 10%. Si has low electronic conductivity, so the statement that it suits fast charging is also wrong.

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