Re-Imaging the Lithium-Ion Battery Configuration for Enhanced Thermodynamic Stability and Performance
Authors: Collins Gearoid Anthony 
Published: February 2020

The ever-growing problems of global warming, pollution and the depletion of fossil fuels have propelled Lithium-ion batteries (LiBs) to the forefront of the initiative for clean, renewable and sustainable energy. However, the current state-of-the-art LiB technology is not sufficiently equipped to handle adverse climatic conditions. Typical LiBs consist of a lithiated transition metal oxide cathode and a graphitic anode. Excessively low temperatures mitigate the functionality of the conventional LiCoO2-Graphite cell, rendering it non-operational. Conversely, elevated temperatures accelerate capacity fade through electrolyte degradation. Despite its low-cost and ubiquity, graphite exhibits a meagre gravimetric capacity of 372mAhg-1 along with poor performance under high current densities1. Moreover, at low cycling temperatures, graphitic anodes exhibit poor conductivity, overcharging and metallic lithium plating2. Recently, group-IV semiconducting nanomaterials have appeared as auspicious candidates to replace the archetypal graphitic anodes. Germanium, in particular, has a gravimetric capacity of 1384mAg-1, a fourfold increase to that of graphite. This, in conjunction with the excellent electronic properties of Germanium, highlights the huge potential of Ge-based anodes for high performance LiBs. Moreover, employing nanostructured anodes allows for substantially improved high current performance through larger contact areas and better conductivity through shorter diffusion lengths.

Herein, we present comparative analysis of graphite-based and germanium nanowire-based LiBs operating over a wide temperature range (-40°C to +40°C ) for different electrolyte compositions. Preliminary results have highlighted the substantially superior performance of Germanium nanowire (NW) anodes over graphite anodes, becoming more pronounced at lower temperatures (Fig S1). Ge NWs were synthesised using a simple, solvent-free vapour-solid-solid (VSS) synthetic approach. Ge NWs were grown on a stainless steel current collector with a 2nm coating of Cu. Cyclic voltammetry and galvanostatic cycling of the cells have allowed us to quantify the performance of these cells over a range of temperatures. Moreover, SEM and TEM have allowed us to track changes in the morphology of the anodic material, with cycling. Electrochemical Impedance Spectroscopy (EIS) of LiCoO2-Germanium full cells has revealed that, at low temperatures, the conductivity of the traditional carbonate-based electrolyte is severely diminished; increasing concerns over electrolyte freezing and cell degradation. Conversely, galvanostatic cycling reveals substantial capacity fade for high temperatures (>40°C), indicative of electrolyte decomposition.

Enhancing the thermal stability of LiBs is critical in advancing the commercial viability of LiBs operating at elevated temperatures. The high temperature performance of LiBs is limited by the instability and the susceptibility of the electrolyte to degradation at high temperatures. The traditional LiPF6 electrolyte becomes rapidly unstable at elevated temperatures (>40°C )3, decomposing into toxic by-products like POF3 and HF. Recently, mixtures of ionic liquids (ILs) such as FSI, TFSI and Poly(PPDA) have emerged as potential replacements for the traditional carbonated-based electrolyte4. Moreover, investigatory studies by Xu et al.5 found that lithium bis(oxolato)borate (LiBOB) has appeared as a promising candidate to replace the traditional LiPF6 electrolyte salt, owing to increased thermodynamic stability along with a wider electrochemical stability window6. The low-cost of graphite-based LiBs is of little consolation when considering their drastically diminished performance in harsh climates. Electrolytes that can sufficiently function at low temperatures and remain thermodynamically stable at high temperatures is a requisite in developing the optimal LiB configuration for adequate functionality over a wide temperature range. A dual effort of improving both the anode and the electrolyte's functionality, over a wide temperature range, will go a long way in advancing the field of LiBs.

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Figure 1