MIT10_626S11_lec11 - II Equilibrium Thermodynamics Lecture...

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II. Equilibrium Thermodynamics Lecture 11: Reconstitution Electrodes MIT Student (and MZB) “Reconstitution electrodes” undergo phase transformations driven by Faradaic reactions. In equilibrium, the state of charge increases by convert­ ing one immiscible phase into another at a constant voltage, which corre­ sponds to the free energy di±erence between the two phases. Reconstitution electrodes are becoming increasingly common in Li-ion batteries. Typically, there are two immiscible phases, e.g. LiFePO 4 and FePO 4 , with solid solu­ tion behavior possible at the extreme concentrations. In other cases, such as Li x C 6 , the charge/discharge cycle can pass through two or more recon­ stitution steps, involving three or more phases. 1 Two Immiscible Phases 1.1 Lithium iron phosphate cathodes Lithium iron phosphate is a promising cathode material due to safety, low cost and relatively high rate capability and long cycle life, especially in the form of nanoparticles. Unlike most prior Li-ion battery materials, it has a very wide voltage plateau at room temperature, indicating a very strong tendecy to separate into Li rich and Li poor regions. For a given x 1 , as shown in Figure 1, there is a phase separation into a fraction x + - x 1 of x + - x - x - and x 1 - x - of x + . This means that all x , x < x < x + are linear x + - x - - combinations of x - and x + . 1.2 Regular solution model Simplest Model The simplest model is a regular solution of particles and vacancies. First, we look at the homogenous model. The equations 1
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Lecture 11: Reconstitution electrodes 10.626 (2011) Bazant Figure 1: Charge/discharge cycles of Li/Li x FePO 4 near open circuit condi­ tions. [Tarascon et al., Nature (2001).] describing
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MIT10_626S11_lec11 - II Equilibrium Thermodynamics Lecture...

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