Phosphorus, in several allotropic variations, is emerging as a high capacity anode candidate for lithium and sodium batteries.  The theoretical capacity for Li₃P/Na₃P is 2596 mA·h·g^-1, far exceeding carbon and on par with the best known lithium and sodium battery anodes.  To date, some composites of carbon with red phosphorus^1,2, black phosphorus^3–5, or phosphorene^6–8have been investigated; however, the amorphous or poorly crystalline nature of these composites has hindered the understanding of phosphide intermediates and reaction mechanisms.  Additionally, the nature of the phosphorus–carbon composites is not fully understood.  A more complete chemical and structural picture is required to identify failure mechanisms and improve the lifetime of these promising electrodes.

Solid state nuclear magnetic resonance (NMR) offers unique insight into the complex chemistry of phosphorus composites and lithium phosphides.  Nucleus-specific local atomic structure information is obtained for amorphous red phosphorus and its mechanochemical conversion to black phosphorus, which is relevant to all studies of phosphorus electrodes.  Furthermore, the lithiation mechanism and lithium phosphide reaction intermediates are elucidated via ⁷Li and ³¹P NMR in combination with structure prediction from ab initio random structure searching and atomic species swapping methods.  While Li₃P is a known reaction product – observed via x-ray diffraction – other stable or metastable lithium phosphide intermediates have been hypothesized but not conclusively determined.  We provide experimental and theoretical support for a range of amorphous and well-defined Li_xP_y phases.

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