A general prelithiation approach for group IV elements and corresponding oxides
Graphical abstract
Introduction
The emerging electric vehicles market has stimulated intensive research on low-cost lithium-ion (Li-ion) batteries with high energy density and long cycle life [1], [2], [3]. Currently, existing Li-ion batteries are produced in the discharged state with Li pre-stored in the cathode due to the air stability of the lithiated cathode materials [4], [5]. During battery operation, organic electrolytes are not stable and decompose to form an SEI layer on the surface of electrodes at the working potential of the anode [6], [7]. This irreversible formation of SEI consumes Li from cathode materials and occurs mainly during the first lithiation process, resulting in an appreciable loss of active cathode materials and low 1st cycle Coulombic efficiency (CE) [8], [9]. The 1st cycle CE of commercial graphite electrodes is typically around 90% [10], [11], whereas 1st cycle CEs for high-capacity alloy anodes including Si, Ge and Sn are usually lower than 80% due to the increased surface area of the nanostructured electrodes utilized to improve cycling performance of alloy anodes [12], [13], [14]. These low 1st cycle CEs impose a significant hurdle in constructing full-cells for practical applications because the Li consumption at the anode side necessitates an excess amount of cathode active material solely to compensate for the first cycle losses, leading to a reduced total energy density [15]. Moreover, cathode capacity loading is limited by coating thickness due to kinetic factors [16].
Prelithiation is a common way to improve the 1st cycle CE of Li-ion batteries. Cathode prelithiation was previously achieved by the addition of Li-rich compounds such as Li2NiO2 [17], Li6CoO4 [18], metal/Li2O composites [19], and metal/LiF composites [20]. However, prelithiation capacities of these materials are generally lower than 1000 mAh/g. Li2O and Li3N powders exhibit much higher specific capacities (>1400 mAh/g), but the use of these additives is accompanied with the evolution of O2 and N2 gas, respectively [21], [22], [23]. In terms of anode prelithiation, Si nanowires and Sn-C electrode were electrochemically lithiated by directly shorting with Li metal [24], [25]. Choi's group further improved this electrochemical lithiation approach in a controlled manner by shorting silicon monoxide with Li metal in the presence of an optimized circuit resistance and simultaneously monitoring the voltage between both electrodes [26]. Microscale stabilized lithium metal powder (SLMP, FMC Lithium Corp) [27], [28], [29]. and a polymer protected Li metal layer [30] have been also effectively employed to compensate for the first cycle irreversible capacity loss of various anode materials including graphite and Si. Recently, we demonstrated that metallurgically synthesized LixSi nanoparticles (NPs) can serve as a high-capacity prelithiation reagent to effectively increase the 1st cycle CE of anode materials [31]. Li2O and artificial-SEI coatings have been utilized to increase the stability of LixSi NPs in air with low humidity level (<10% relative humidity (RH)) [32]. Furthermore, metallurgically lithiated SiOx developed a unique composite structure of homogeneously dispersed reactive LixSi nanodomains embedded in a robust Li2O matrix which was found to further improve ambient-air stability (~ 40% RH) [33]. Group IV elements such as Ge and Sn also have relatively high specific capacities (1640 mAh/g for Ge, and 993 mAh/g for Sn) and similar volumetric capacities to Si (2574 mAh/cm3 for Si, 2275 mAh/cm3 for Ge, and 2111 mAh/cm3 for Sn), making them also suitable for pre-storing Li [15], [34], [35], [36]. As such, the strategy of embedding active lithium alloys in a robust Li2O matrix is also attractive for other group IV elements and is predicted to stabilize the lithiated group IV alloys.
Herein, we developed a one-pot metallurgical process to synthesize Li22Z5 alloys (Z = Ge and Sn) and Li22Z5-Li2O composites by using Z and ZO2 as the source materials, respectively. Both Li22Z5 alloys and Li22Z5-Li2O composites are reactive enough to prelithiate various anode materials such as graphite and Sn, thereby achieving high 1st cycle CEs of>100%. Among all lithiated group IV alloys, LixGe NPs exhibits the best stability under ambient-air conditions, consistent with the simulation results showing that Ge atoms in the cubic Li22Ge5 crystal have the strongest bonding with Li atoms. Li22Z5-Li2O composites further enhanced the air stability over their corresponding Li22Z5 alloys thanks to the strong binding between O atoms in Li2O and Li atoms in Li22Z5.
Section snippets
Synthesis of LixZ alloys and LixZ-Li2O composites
Ge and GeO2 microparticles (Sigma Aldrich) were first ground to obtain fine powders by planetary ball milling operated at a grinding speed of 400 rpm for 24 h. SnO2 nanoclusters were prepared via a bio-inspired hydrothermal method in the presence of tris(hydroxymethyl) aminomethane (THAM, Sigma Aldrich) [37]. 0.27 g of Na2SnO3·3H2O (Sigma Aldrich) and 0.2 g of THAM were first dissolved into 35 ml of distilled H2O, and then transferred into a 40 mL Teflon-lined stainless-steel autoclave. The autoclave
Density functional theory simulation
To study the stability of different LixZ alloys (Z = Si, Ge and Sn) in ambient-air conditions, we performed density functional theory (DFT) simulations to calculate the interaction between Z atoms and Li atoms in the cubic Li22Z5 crystal. The binding energy (Eb) of Li with Si, Ge, and Sn was obtained using CASTEP simulation package in the framework of DFT. Although Li alloyed with group IV elements to form the Li-richest alloys with the same formula, the crystal structures are slightly
Conclusions
In conclusion, we have developed a general prelithiation approach to obtain Li22Z5 alloys (Z = Ge and Sn) and Li22Z5-Li2O composites using Z and ZO2 as starting materials, respectively. This approach is general applicable to Z and oxides with complex nanostructures. Because of their high capacity and low chemical potential, both Li22Z5 alloys and Li22Z5-Li2O composites are reactive enough to prelithiate graphite and alloy-type anode materials. Among all lithiated group IV alloys, LixGe alloy
Acknowledgements
We acknowledge the support from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, Battery Materials Research Program of the U.S. Department of Energy.
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These authors contributed equally.