Si and Ge nanowire (NW) based materials have emerged as viable candidates for next generation rechargeable lithium- ion battery anodes with energy and power densities that are multiples of current graphitic based electrodes. The key advance is the capability of NWs to overcome the well-known pulverization problem that is detrimental to the cycle life and hence viability of their bulk counterparts. NWs also provide good electrical conductivity along their length, have a high interfacial area in contact with the electrolyte, have an optimal short diffusion distance for Li-ion transport, and can be grown directly from current collectors, eliminating the need for binders and conductive additives.

Ge (max. theoretical capacity of 1384 mAh/g) has received less attention than Si (3579 mAh/g), although it has a higher rate of diffusivity of Li at room temperature (400×) and a greater electrical conductivity (10,000×) making it suitable for high power applications. Gold is the most common catalyst for Ge NW synthesis, however as it is expensive and does not reversibly alloy with lithium, alternative more cost-effective catalyst materials that can contribute to the specific capacity of the electrode are desirable.

Improving the cycle life of simple Ge NW arrangements as Li-ion anodes would be very interesting as their ease of processability and scalability, particularly if solution grown, can offer viable alternatives to graphitic materials.

—Kennedy et al.

The low energy, wet-chemical synthesis process begins with a stainless steel substrate with an evaporated layer of Sn on its surface being placed in the vapor zone of a high boiling point solvent. Diphenylgermane is injected into the flask at 430 °C. The Sn (tin) nanoparticle seeds form in situ at this temperature and act as sinks for the Ge that decomposes from the precursor, facilitating high-density NW growth by the vapor−liquid−solid (VLS) mechanism.

Tin also has a high maximum theoretical capacity (994 mAh/g); the researchers found that the Sn seeds at the ends of the NWs reversibly alloy with lithium and contribute to the electrodes’ overall specific capacity.

Taking into account the mass of both the Sn seed and the Ge NW, the team calculated a maximum theoretical specific capacity for the composite anode of 1320 mAh/g. The NWs exhibited an initial discharge capacity of 1103 mAh/g and an average Coulombic efficiency (CE) of 97.0%. The bulk of the capacity fade was during the first 100 cycles; capacity loss beyond 100 cycles dropped by only 0.01% per cycle.

Rate capability, high rate capability and high discharge rate performance of the Sn-seeded Ge NW electrodes cycled in the voltage range of 0.01−1.5 V. (a) The charge and discharge capacities and the C.E. values are shown for an electrode that was charged and discharged at rates of C/10, C/5, C/2, C, 2C, and then back to C/10. (b) The high rate capability of the electrodes is shown. The capacities and the CE values of an electrode charged and discharged at rates of C/2, C, 5C, 10C, 20C, 40C, 60C, and then back to C/2 is presented. (c) The discharge capacities measured for 5 cycles at 6 different discharge rates are shown. The charge rate was kept constant at C/2 for all cycles. (d) Capacity data showing discharge capacities of the material at 20 and 100C discharge rates. The electrodes were charged at a constant rate of 2C. Credit: ACS, Kennedy et al. Click to enlarge.

The team found that the electrolyte additive, vinylene carbonate (VC), plays an important role, facilitating the formation of the stable porous network of nanowires.

Voltage profiles and differential capacity plots revealed that the NWs behave as a composite anode material as both the Ge NWs and the Sn seed reversibly alloy with Li. We believe that the fabrication of Sn seeded Ge NW electrodes via the SVG system is a scalable method and their application as anodes for Li-ion batteries offers a viable alternative to conventional graphite electrodes, as they exhibit comparable stability and higher capacities over extended cycles. Furthermore the excellent high-rate capabilities while discharging suggest that the NWs may also be suited for high power applications that require very high discharge rates such as battery electric vehicles and power tools.

—Kennedy et al.

The research was supported by Science Foundation Ireland (SFI) under the Principal Investigator Program to Dr Kevin Ryan and also by EU funding through the GREENLION Project. The GREENLION project is a large scale collaborative project within the FP7 framework with the goal of manufacturing greener and cheaper lithium-ion batteries for electric vehicle applications.

Resources

• Tadhg Kennedy, Emma Mullane, Hugh Geaney, Michal Osiak, Colm O’Dwyer, and Kevin M. Ryan (2014) “High-Performance Germanium Nanowire-Based Lithium-Ion Battery Anodes Extending over 1000 Cycles Through in Situ Formation of a Continuous Porous Network,” Nano Letters doi: 10.1021/nl403979s