The ideal anode material for the next generation battery should meet the following requirements: large reversible capacity, high Coulombic efficiency, safe potential, good rate capability, and stable cycle life. Many studies have been devoted to the development of metal alloy anodes because of the high capacity of such metals as Si, Ge, Sn, and Sb. Although Ge has attracted less attention than Si because of its higher cost, the diffusivity of Li in Ge is 400 times higher than that in Si at room temperature, indicating that Ge may be an attractive electrode material for high-capacity anodes.

Theoretically, Ge can uptake 4.4 Li ions, corresponding to 1623 mAh g⁻¹ specific capacity. However, the huge volume change upon cycling, up to 370%, is accompanied by de/alloying with lithium, which is fatal for cycle life. Multiple approaches have been proposed to minimize such volume strain during charge and discharge, including (i) nanodimensionality such as nanoparticles, films with nanothickness, or nanopores, (ii) 3D structure, like nanowires, nanotubes, or nanofibers, and (iii) mixing of the active material with a buffer component, such as Ge/ carbon, Ge/graphene, or GeO₂.

… Herein, we propose a new metal oxide composite anode material in lithium-ion batteries application, which is prepared by a simple and cheap method. This composite material is a mixture of GeO₂ oxide and Sn−Co−C alloy and is synthesized by mechanical milling of pristine germanium oxide, tin, cobalt, and carbon. Compared with metallic Ge, GeO₂ has several advantages, such as better cycle life, much higher theoretical capacity (1126 mAh g⁻¹), and lower cost. … Our GeO₂−SnCoC composite has the potential of combining the advantageous properties of GeO₂ (high capacity) and Sn−Co−C (long cycle life).

—Liu et al.

To prepare the material, the team blended germanium oxide, tin, cobalt, and surface-modified graphite with a weight ratio of 50 wt % GeO₂/50 wt % Sn_xCo_yC_z (x, y, z atomic ratio: 30/30/40) using a mixing machine to obtain a homogeneous distribution. They then ball-milled the powder for 48 hours.

Rate performance. Credit: ACS, Liu et al. Click to enlarge.

The GeO₂−SnCoC composite electrode shows first-cycle discharge−charge capacities of 1501 and 1200 mAh g⁻¹, corresponding to 80% Coulombic efficiency. By comparison, a GeO₂ electrode delivered discharge-charge capacities of 2209 and 764 mAh g⁻¹ in the first cycle, respectively. First-cycle capacity loss of GeO₂ is as high as 65.4%, which is due to the product of the conversion reaction, Li₂O, being electrochemically inactive and irreversible, thereby causing poor Coulombic efficiency and cycle performance.

The researchers tested the capacity retentions of GeO₂−SnCoC composite, GeO₂, and Sn₃₀Co₃₀C₄₀ anodes in coin cells. A cutoff voltage window of 0.005−2.5 V was used for all of the tests, along with a constant current of 100 mA g⁻¹ in the initial two cycles and 300 mA g⁻¹ in subsequent cycles.

Among their findings were:

• The GeO₂−SnCoC composite electrode exhibits significantly superior specific capacity compared with that of the GeO₂ and SnCoC electrodes.

• The GeO₂ electrode delivers a high initial capacity, over 2000 mAh g⁻¹, but suffers from an extremely rapid degradation with a low Coulombic efficiency (34.6%).

• The GeO₂−SnCoC electrode delivers a reversible capacity of more than 800 mAh g⁻¹ in 100 cycles with a slight fading. The added Sn−Co−C clearly improves not only Coulombic efficiency, but also capacity retention significantly.

• The specific capacity of the GeO−SnCoC composite electrode is 493 mAh g⁻¹ under a high current density (1200 mA g^-1 ). It also exhibits good recoverable capacity after switching back to a low current density (100 mA g⁻¹).

• The ternary Sn₃₀Co₃₀C₄₀ alloy showed little capacity loss in 100 cycles, the capacity is as low as 380 mAh g⁻¹.

Pair distribution function (PDF) measurements indicated the reversible reaction of GeO₂ and SnO₂—the key factor in the improved Coulombic efficiency. The team speculated that the mechanism of the reversible conversion reaction is attributed to the catalytic effect of Co₃Ge₂.

As a result of the testing, the researchers concluded that the GeO₂−SnCoC composite material showed promise for practical use in lithium-ion batteries.

Resources

• Bo Liu, Ali Abouimrane, Mahalingam Balasubramanian, Yang Ren, and Khalil Amine (2014) “GeO₂–SnCoC Composite Anode Material for Lithium-Ion Batteries,” The Journal of Physical Chemistry C 118 (8), 3960-3967 doi: 10.1021/jp411462v