We present a new method for predicting the thermodynamics of thermal degradation of charged cathode materials for rechargeable Li batteries and demonstrate it on three cathode materials, Li x NiO2, Li x CoO2, and Li x Mn2O4. The decomposition of Li x NiO2 is a two-step process:  the first step is a kinetically controlled exothermic conversion in which the layered structure transforms to the stable spinel structure. The second step is an endothermic decomposition of the spinel into a rocksalt phase, accompanied by the loss of oxygen. The heat generation for the overall reaction from the layered to the rocksalt structure is exothermic when x < 0.5 and endothermic when x > 0.5. From the calculated phase diagram, a similar mechanism is expected for Li x CoO2, but the high migration barrier for Co may inhibit the layered-to-spinel transformation and lead to decomposition into LiCoO2 and Co3O4. For the stable spinel LiMn2O4, high temperature is needed to provide enough thermodynamic driving force for its endothermic decomposition reaction. The fully charged λ-Mn2O4 transforms kinetically into the stable phase β-MnO2 first and then decomposes at elevated temperature into the lower-valence oxides, α-Mn2O3 and Mn3O4. The calculated decomposition heat for the three systems is in good agreement with experiments. When present, the electrolyte can act as a sink for the oxygen released from the cathode. Although oxygen release from the cathode is generally endothermic, its combustion with the electrolyte leads to a highly exothermic reaction.

Using first-principles calculations within the generalized gradient approximation ͑GGA͒ + U framework, we investigate several surface properties of olivine structure LiFePO 4. Calculated surface energies and surface redox potentials are found to be very anisotropic. Low-energy surfaces are in the ͓1 0 0͔, ͓0 1 0͔, ͓0 1 1͔, ͓1 0 1͔, and ͓2 0 1͔ orientations of the orthorhombic structure. We find that the coordination loss of Fe atoms on the surface is energetically more unfavorable than for Li, and generally a low-energy surface has fewer Fe-O bonds affected by the surface cut. Conversely, undercoordinated Li on the surface are somehow beneficial to reduce the energy of a surface except for the twofold coordinated Li. With the calculated surface energies, we provide the thermodynamic equilibrium shape of the LiFePO 4 crystal through a Wulff construction. The two low-energy surfaces ͑0 1 0͒ and ͑2 0 1͒ dominate in the Wulff shape and make up almost 85% of the surface area. Similar calculations for FePO 4 indicate a very low energy for the ͑0 1 0͒ surface of FePO 4. This result suggests that surface chemistry can induce a change in the aspect ratio of the Wulff shape. Surface redox potentials for the extraction and insertion of Li from various surfaces are also investigated in this work. The Li redox potential for the ͑0 1 0͒ surface is calculated to be 2.95 V, which is significantly lower than the bulk value of 3.55 V. For several other surfaces the Li extraction potential is above the bulk potential. We also develop a simple model that can be used to predict surface energies based on the change in the coordination of Fe and Li.

Using first-principles calculations, we investigate the surface energies, equilibrium morphology, and surface redox potentials for LiMnPO 4 in the olivine structure. Low-energy surfaces are found in the ͓100͔, ͓010͔, ͓011͔, ͓101͔, ͓201͔, and ͓301͔ directions of the orthorhombic structure. With the calculated surface energies, we provide the thermodynamic equilibrium shape for the LiMnPO 4 crystal through a Wulff construction. The dominating surfaces in the Wulff shape are ͑010͒, ͑011͒, and ͑201͒. Most of the surfaces in the Wulff shape have lower Li extraction potentials than the bulk, except for the ͑100͒ and ͑011͒ surfaces.

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