Phosphorus: An Alternative for High Capacity Li-Ion Battery Anodes

Greater gravimetric and volumetric electrode capacities could result in battery technologies finding greater use in transportation, industrial equipment, electronics, and electric grid applications. Currently, the volumetric capacities of graphite electrodes used in commercial Li-ion cells approach 500 mAh/cm³, while the volumetric capacities of commercial cathodes approach 700 mAh/cm³. Thus, there is a pressing need to develop high capacity anodes to match or exceed the volumetric capacities of cathodes. Tin (Sn) and silicon (Si) – based anodes have been heavily investigated in recent years, while many other materials have received significantly less attention (1). Phosphorus (P) had attracted very little interest until recent years, perhaps due to its difficulty in handling, but has since been shown to offer high gravimetric capacity and rate capability (2). Furthermore, the theoretical volumetric capacity of P is higher than that of Si (2266 mAh/cm³ for Li₃P vs. 2190 mAh/cm³ for Li₁₅Si₄) (3,4), and Li₃P is known to have high Li⁺conductivity (5).

Here we report on our early investigations of P-based anodes produced using various polymeric binders. The choice of binder material is key to the development of well performing electrodes (6,7).  A broad range of binders were tested and analyzed for their impact on the stability and rate performance of phosphorous-carbon composites. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to analyze both the structure and chemistry of the anode materials before and after cycling. Atomic force microscopy (AFM) was used to test mechanical properties of polymer binders (such as swelling and stiffness) in electrolyte solvent solution. Cyclic voltammetry (CV) at different rates was used to test both the electrochemical stability of the polymers and performance of P-based anodes, while electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge cycling at different current densities were additionally used to test the electrochemical performance of the final electrodes in half cells. Our results indicate significant promise for P-based anodes in select applications.

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