Porous Germanium Anode for Li-Ion Batteries

Abstract

Due to the steady growing demand for higher performance electronic devices it is required an increase of the actual specific energy of Lithium Ion Batteries (LIBs)[1]. This can be achieved enhancing the gravimetric capacity of the electrodes, i.e. the amount of charge that can be stored per unit of mass of the active material. Currently a variety of cathode materials are available, being lithium metal oxides the most commonly used, while for the anode the choice is essentially between TiO2 or carbonaceous materials, among which Graphite is by far the most employed. The theoretical gravimetric capacity of Graphite is 372 mAh/g but several materials show higher capacities. In this respect, the highest gravimetric capacities are found in Lithium metal (3862 mAh/g), Silicon (4200 mAh/g) and Germanium (1624 mAh/g). Even though these numbers would suggest Lithium metal as the best possible active material, it has been discarded due to its high reactivity inducing dendrites formation upon charge and discharge processes, which could eventually lead to short circuits. Silicon and Germanium also show improved capacities compared to Graphite but they both suffer huge volume changes (up to 400%) upon cycling, which could lead to the pulverization of the electrodes. Nano-structuring these materials is an approach to overcome this issue, realizing structures which can reversibly accommodate the occurring volume variations. It is easier to obtain a Germanium compliant matrix rather than a Silicon one, as Germanium requires an average pore dimension that is bigger compared to Silicon[2]. Furthermore, Germanium shows a higher electrical conductivity (10,000 times higher than Silicon) and lithium ion diffusion rate (400 times higher than Silicon), which could make it suitable for high performance applications. These are the reasons that led us to test Germanium as an alternative anodic material with respect to Graphite. In this work we present the results of our novel anodic material made of porous Germanium. The Germanium films are directly grown on a metallic current collector through a plasma enhanced chemical vapor deposition technique (PECVD) without any binder, and subsequently nano-structured to realize a porous matrix using a hydrofluoric acid electrochemical attack. Molybdenum or Stainless Steel were used as current collectors without requiring any binder to enhance the adhesion of Germanium, i.e. the mass loading is truly and uniquely represented by active material. Moreover, the PECVD technique permits to obtain a higher purity compared to chemically grown materials via standard reactions in solution. These anodes are able to perform hundreds of charge and discharge cycles at very high C-rates, retaining a stable capacity of more than 950 mAh/g even at currents as high as 5C (considering 1C as 1600 mA/g), which is 2.5 times the theoretical Graphite capacity. 2032 coin-type half-cells were assembled coupling the electrode to pure Lithium metal used as counter and reference electrode. A standard electrolyte solution composed by EC:DMC has been used with different additives (vinylene carbonate-VC and fluoroethylene carbonate-FEC) whose effect on the performances will be shown. The active material layer thickness varied between 1 to 5 microns and the mass loadings ranged from 0.2 to 1.5 mg. Various etching recipes have been tested to obtain different porous morphologies. As an alternative technique to nano-structure the materials, a suitably tuned Ion Implantation technique has been exploited: the results will be shown for comparison, along with results from bulk samples in order to show the effectiveness of the nano-structuration process on the cell performance. The best results were obtained using porous Germanium grown onto Molybdenum substrate adding FEC: in graph 1 is reported the Capacity vs. Cycle Number plot for one of these samples, which has been tested for thousand cycles at high C-rates (mass loading 0.24 mg). [1] G. Zubi, R. Dufo-López, M. Carvalho e G. Pasaoglu, «The lithium-ion battery: State of the art and future perspectives,» Renewable and Sustainable Energy Reviews, vol. 89, pp. 292-308, 2018. [2] Z. Hu, S. Zhang, C. Zhang e G. Cui, «High performance germanium-based anode materials,» Coordination Chemistry Reviews, vol. 326, pp. 34-85, 2016. Figure 1