New Anti-Fluorite Transition Metal Nitrides As High Capacity Li-Ion Intercalation Anodes

More than 20 years after the commercialization of Li-ion batteries, graphite (372 mAh g^-1) [1] remains the most widespread negative electrode material for commercial applications. Whilst graphite is typically the “best” anode material for standard battery applications, certain device requirements of a battery make it a less obvious choice and as a result efforts are currently underway to find alternatives that show either higher capacity, such as silicon (3572 mAh g^-1) [2], or higher rate capability, such as Li₄Ti₅O₁₂ (175 mAh g^-1) [3]. The latter offers the additional benefit of enhanced safety derived from its activity at potentials ~ 1.5 V vs. Li⁺/Li. This feature removes the danger of lithium plating at high current densities, known to be a significant problem with current battery technology.

Metal oxides have been heavily investigated over the past 10 years and no stand-out candidate has appeared in the search for materials which can reversibly react with lithium at sufficiently low voltages to be used as anodes. In more recent years, the search was extended to the study of less well known transition metal pnictides, with most research efforts directed toward phosphides and nitrides. Lithium transition metal nitrides and oxynitrides have been reported to show interesting performance, making them candidates for the negative electrode [4–9]. Among them, anti-fluorite-type Li₇MnN₄ and Li_7.9MnN_3.2O_1.6 show specific gravimetric capacities in excess of 300 mAh g^-1 with excellent retention, most likely due to the small volume changes undergone during lithium extraction/insertion [4, 9, 10]. As might be expected in the fluorite type crystal structure, significant lithium mobility through their frameworks was proven to exist, even at room temperature, and was enhanced as lithium was removed from the structure [9,10].  This effect is presumably due to the formation of Li vacancies, which can be easily introduced with appropriate compositional design of the crystal lattice.

Following on from these results, here we will discuss recent investigations into defect engineered varieties of Li₇MnN₄and demonstrate the resultant properties of such defects on the anode performance.

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