Insights into the Storage Mechanism of Lithium and Sodium in Phosphorus-Doped Graphite

The increasing energy demands of the 21^st century will require fundamental improvements to existing battery technology. One strategy is the improvement of the specific capacity of currently used carbonaceous anodes in Li- and next-generation Na-ion batteries. Phosphorus shows promise as an alternative anode material, with its ability to reversibly form lithium- or sodium-phosphorus compounds during electrochemical cycling, resulting in a potential capacity 6 times greater than that of standard graphite or hard carbons.^1,2 However, the conductivity, stability and safety of elemental P as an anode prevents the technology from reaching real-world applications.³ Whilst previous attempts have been made to utilize carbon-phosphorus composites (i.e., with separate carbon and phosphorus domains),^4,5,6 the incorporation of phosphorus into the graphitic lattice and subsequent use of these materials in electrochemical applications has not been explored. Our single-precursor, single-step pyrolysis synthesis allows the production of stable phosphorus-doped carbon materials. Interstitial incorporation of phosphorus into the turbostratic graphitic lattice gives greater stability than pure phosphorus and improved capacity in Li-ion batteries compared to an undoped graphite. The Na insertion chemistry is similarly explored. Moderate control of the phosphorus content, which is also present in a variety of allotropes, provides some control over capacity and stability. Via a thorough structural, compositional and electrochemical analysis, including the use of <i>ex situ</i> solid-state NMR, the insertion mechanisms of these materials may be explored. Identification of the preferential sites used for ion storage within phosphorus-doped graphite gives an improved understanding of the further alterations needed to develop it as a future anode material.