Mechanistic Insights into the De-/Lithiation of Iron-Doped Zinc Oxide: From Fundamental Understanding to Practical Considerations
Authors: Asenbauer Jakob , Eisenmann Tobias , Birrozzi Adele , Hoefling Alexander , Binder Joachim R. , Indris Sylvio , Kuenzel Matthias , Tübke Jens , Passerini Stefano , Bresser Dominic 
Published: December 2020

Owing to their unique combination of high energy and power density, lithium-ion batteries are now the state-of-the-art energy storage technology for powering small consumer electronics and increasingly also for large-scale applications like electric vehicles.[1] Yet, especially for the latter, there is a growing need for batteries that can provide not only high energy densities, but also the possibility to be rapidly recharged.[2] This is challenging for the currently used graphite anodes, since its low lithiation potential (~0.1 V vs. Li/Li+) in combination with the sluggish lithium transport across the solid electrolyte interphase (SEI) and within the graphite structure can lead to lithium plating and dendrite formation during fast charging, particularly at low temperatures.[3]

To overcome this issue, various alternative anodes are being investigated, following, e.g., a conversion or an alloying mechanism.[4] While these alternatives frequently show higher capacities and rate capabilities, conversion materials still suffer from a significant voltage hysteresis, resulting in low energy efficiencies, and alloying materials suffer from extensive volume variations, leading to rapid capacity fading and low coulombic efficiencies. Conversion/alloying-materials (CAMs), as relatively new material class, combine the conversion and alloying mechanism in one single material.[5] In CAMs, such as Zn0.9Fe0.1O, nanograins of an alloying element and a percolating conductive network of transition metal nanoparticles are formed in situ by the initial (reversible) conversion reaction. This metallic nano-network enables fast de-/lithiation kinetics and renders them a promising candidate for high-power applications. Nevertheless, there is still a lack of knowledge about how to potentially tackle the remaining obstacles, i.e., the achievement of sufficiently high energy efficiencies and the volume variations occurring upon cycling.
Herein, we report our findings towards an in-depth understanding of the de-/lithiation of (carbon-coated) Zn0.9Fe0.1O. Combining in situ microcalorimetry, in situ XRD, ex situ
7Li NMR, and ex situ
57Fe Mössbauer spectroscopy allowed us to propose a refined mechanism for the de-/lithiation reaction. Moreover, in situ dilatometry and ex situ cross-sectional SEM analysis reveal that the continuous volume variation at the electrode level is, in fact, in the range of 10%. This is much lower than theoretically predicted when considering bulk densities only – even if cycled within a 3-V potential window. Based on these results we highlight the beneficial effect of a limited operational voltage window, which we finally confirm for Zn0.9Fe0.1O/LiNi0.5Mn1.5O4 full-cells, providing an excellent energy efficiency of >93%, accompanied by an energy and power density of 284 Wh kg-1 and 1105 W kg-1, respectively.
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