Energy storage materials represent one of the most important research topics today and the focus is on solid-state rechargeable batteries, as a result of their relative safe use and long-life expectancy. Presently, batteries based on lithium (Li) receive most attention because its low molar mass ensures a correspondingly high energy density. Although the demand for more efficient batteries in mobile technologies steadily grows, approximately 8–11 % per year ,1 the state-of-the-art is almost 15 years old concerning the battery cathode.
At present, advanced Li-battery cathode materials like olivine type Li1−xFePO4, delafossite structured Li1-xTMO2 (TM = Co, Mn), and spinel Li1+xMn2O4 are used. All three have high charge capacity per weight, but they differ in other aspects: Li1-xFePO4 is cheap and non-hazardous but the Li+ migration is hampered by Li‒Fe disorder, which decreases battery life-times and (dis-)charge rates. Li1-xCoO2 can be (dis-)charged at high rates but is very expensive due to limited resources of environmentally unfriendly cobalt. Li1+xMn2O4 is cheap and works at high charging rates but the working voltage causes irreversible deterioration of battery parts.
Here, we show a novel series of solid-state crystalline compounds that fulfil all requirements for improved lithium-ion batteries.
Through single-step solid-state reactions, a series of novel bichalcogenides with the general composition (Li2Fe)ChO (Ch = S, Se) are successfully synthesized from Li2O, Fe, and Ch under inert condition. In their cubic anti-perovskite crystal structures, Fe and Li are completely disordered on a common crystallographic site. The single-step synthesis results in highly crystalline materials of at least 95 % purity as estimated from X-ray powder diffraction data.
Figure 1: Crystal structure of the cubic anti-perovskites (Li2Fe)ChO (Ch = S, Se).
Under inert conditions, an as-prepared powder of (Li2Fe)ChO melts congruently at 1020 °C for Ch = Se and at 980 °C for Ch = S. X-ray powder diffraction data confirm that both compounds can be obtained directly from melt. Thus, these new compounds are advantageous for manufacturing highly pure materials in large amounts by single crystal growth, similar to the making of pure silicon by Czochralski or by Bridgman-Stockbarger method.
Figure 2: Ten charge/discharge curves of a (Li2Fe)SeO – graphite battery.
Both compounds (Li2Fe)ChO (Ch = S, Se), were tested as cathode materials against graphite anodes (single cells). They perform outstandingly at very high charge rates (270 mA/g, 80 cycles) and, at a charge rate of 30 mA/g, exhibit charge capacities of about 120 mAh/g, which corresponds to energy capacities of about 230 Wh/kg at 1.9 V working voltage. Compared to highly optimized Li1‒xCoO2 cathode materials, these novel anti-perovskites are easily produced at cost reductions by up to 95 % and, yet, possess a relative specific charge capacity of 75 %.
The size of the atomic lattice (unit cell parameter a) does not change significantly for a charged (delithiated) material, which is a good indication of advantageous, negligible lattice strain (volume change) for (dis-)charge processes and minimizes risks for breaking of battery metal caskets in complete battery set ups. Furthermore, the anti-perovskites can be (dis-)charged without destroying anodes or electrolytes. This is advantageous in comparison with high-voltage cathodes, like Li1+xMn2O4, where a thermal run-away, due to over-voltage, decomposes the fluorine-based electrolytes, which are discussed as potential hazards for users and environment. Nevertheless, the working voltage of anti-perovskite batteries can be designed by selecting another anode materials instead of graphite; this might increase the battery voltage and, thus, offer even higher energy densities.
In conclusion, our novel anti-perovskites can be readily produced, are environmentally friendly, and perform well enough as Li-battery cathodes to compete with Li1‒xCoO2, whilst material costs are reduced by about 95 %. (Li2Fe)ChO (Ch = S, Se) can be charged at high rates. Hence, at viable costs and with less environmental issues, the cubic anti-perovskite compounds can be used preferably in larger energy storage stations but, with further engineering, could power vehicles as well.
EP patent application filed in December 2016.
PCT patent application will be filed in December 2017.
1Fröhlich, P. et al., Angew. Chem. Int. Ed., 2017, 56, 2544–2580, DOI: 10.1002/anie.201605417