In the field of energy storage, many researchers have broadly investigated advanced electrode materials for rechargeable lithium-ion batteries (LIBs), which are widely used in portable electronic devices, electric vehicles and energy storage systems [1-5]. Graphite has been mainly used so far as an active anode material in commercialized LIBs. The available capacity of the graphite is about 360 mAh g⁻¹, which is close to its theoretical capacity (372 mAh g⁻¹). To improve the energy density of LIBs, identifying an alternative anode material with high capacity is mandatory [6,7].Recently, alloying/de-alloying and conversion-based materials have been actively investigated due to their high theoretical capacity compared to graphite [8,9]. For example, Si, Sn, Sb and Ge materials exhibit the alloying/de-alloying reaction (M + xLi⁺ + xe⁻ ↔ Li_xM, M = Si, Sn, Sb and Ge) with high theoretical capacities of 4200, 994, 660 and 1600 mAh g⁻¹, respectively [10-12]. However, these alloying/de-alloying materials undergo large volume change, which results in rapid capacity decline during cycling. On the other hand, the conversion-based anode materials, such as Co₃O₄, Fe₃O₄, Fe₂O₃, Mn₂O₃ and MoO₃, could deliver relatively high reversible capacities ranging from 500 to 1000 mAh g⁻¹ through their multiple oxidation states (M_zO_x + 2xLi⁺ + 2xe⁻ ↔ zM + xLi₂O, M = Co, Ni and Fe) [13-17]. Recently, the mixed-metal oxides such as Co₂GeO₄, Co₂SnO₄ and Co₂SiO₄ have been reported as the anode materials for high performance LIBs [18-21]. These mixed-metal oxides exhibited high capacity as well as good cycling stability. In this work, we synthesized various types of M₂GeO₄ (M = Co, Fe and Ni) materials and investigated their electrochemical performance. Here, germanium and metal elements (Co, Fe and Ni) are electrochemically active via conversion and the alloying/de-alloying reaction, respectively. To further enhance the cycling performance of Fe₂GeO₄ material, carbon coating was performed by using glucosamine as a carbon source. The carbon-coated Fe₂GeO₄ material (Fe₂GeO₄@C) exhibited high initial discharge capacity, good cycling stability and enhanced rate capability. The electrochemical characteristics of M₂GeO₄ and Fe₂GeO₄@C will be presented and discussed in detail.

Metal germanates were synthesized by the facile hydrothermal method. Fig. 1 shows the XRD patterns of the synthesized M₂GeO₄ particles. The XRD pattern of Co₂GeO₄ in Fig. 1(a) shows the crystalline peaks at 30.7, 36.1, 43.6, 58.2 and 63.8°, which correspond to (220), (311), (400), (511) and (440) lattice planes, respectively. All the crystalline peaks are consistent with the standard pattern (JCPDS-73-0237), indicating pure phase formation of Co₂GeO₄. The XRD patterns shown in Fig. 1(b) and (c) also matched well with the cubic spinel crystal structure of Fe₂GeO₄ (JCPDS-25-0359) and Ni₂GeO₄ (JCPDS-80-1673), respectively. The Ni₂GeO₄ particles exhibited sharp peaks compared to other M₂GeO₄ materials due to phase formation at a higher temperature.

Fig. 2 shows the FE-SEM images of M₂GeO₄ particles. The Co₂GeO₄ particles shown in Fig. 2(a) exhibit a triangular cube shape of 0.8 to 1.2 µm in size. The triangular cube-shape particles were formed during an Ostwald ripening process, which implies that dissolution and re-crystallization occurred at a high temperature during the hydrothermal process [22,23]. The Fe₂GeO₄ particles shown in Fig. 2(b) have a uniform cube shape with an average particle size of 500 nm. The Ni₂GeO₄ particles show a uniform spherical shape with a particle size of less than 200 nm without any agglomeration.

Cyclic voltammetry of the M₂GeO₄ electrodes was performed in the potential range of 0.01-3.0 V versus Li/Li⁺ at a scan rate of 0.1 mV s⁻¹, and the resulting cyclic voltammograms are shown in Fig. 3. The Co₂GeO₄ electrode shows two reduction peaks at 0.78 and 0.37 V in the first cycle, which can be assigned to the disintegration of Co₂GeO₄ crystal structure into its individual components such as Co, Ge, and the formation of Li₂O, as given in equation (1) and the electrolyte decomposition [24,25]. The reduction peak at 0.02 V was associated with the alloying reaction of lithium with Ge to form the Li_4.4Ge phase, as presented in equation (2) [26]. The small and broad oxidation peaks at 0.52 and 1.38 V during anodic scan could be attributed to the dealloying reaction of Li_4.4Ge into Ge followed by the formation of GeO₂ [24]. The broad peak at a higher potential between 1.80 and 2.50 V elucidates the multiple oxidation reaction of Co⁰ to Co²⁺ and Co²⁺ to Co³⁺ [27]. After the first cycle, the reversible reactions take place according to the electrochemical reactions given in equation (2) to (4). The redox peaks observed at 0.02/0.52, 0.82/1.38 and 1.26-1.53/1.97-2.22 V correspond to the alloying/de-alloying reaction of Ge, and the conversion reaction of GeO₂ and cobalt oxide, respectively [24]. In Fig. 3(b) and (c), the cyclic voltammograms of Fe₂GeO₄ and Ni₂GeO₄ exhibited a reduction peak at 0.61 V and 0.35 V, which can be ascribed to the reduction of Fe₂GeO₄ and Ni₂GeO₄ according to electrochemical reaction (1). After the first cycle, they showed similar oxidation and reduction peaks as in Co₂GeO₄. Among the three materials, Fe₂GeO₄ showed the highest oxidation and reduction peaks, indicating its high reversible capacity during the electrochemical reaction.

Cyclic voltammograms of M₂GeO₄ electrodes were obtained at different scan rates from 0.1 to 1.0 mV s⁻¹, as given in Fig. 4. The oxidation and reduction peaks for all the electrodes were found to be shifted to higher (oxidation) and lower (reduction) potentials due to the increase in electrode polarization [27]. Fig. 4(d) shows the peak current versus the square root of scan rate, which demonstrates the linear relationship between them, indicating the charge storage mechanism pertaining to diffusion controlled process [28,29]. The diffusion coefficient of Li⁺ ions in the electrode can be calculated from the slope of the line given in Fig. 4(d) using the Randles-Servick equation,

where i_p is the anodic peak current (mA), n is the charge transfer number (n = 1), A is the electrode area (1.53 cm²), D_Li+ is the diffusion coefficient of Li⁺ ion, C is the concentration of electrolyte (1 M) and v is the scan rate (mV s⁻¹). The calculated diffusion coefficients of Co₂GeO₄, Fe₂GeO₄ and Ni₂GeO₄ are 6.3×10⁻⁸, 12.7×10⁻⁸ and 3.6×10⁻⁸ cm² s⁻¹, respectively. The Fe₂GeO₄ electrode has the highest diffusion coefficient, which implies the fastest lithium-ion diffusion in the Fe₂GeO₄ electrode.

Fig. 5(a)-(c) show the charge and discharge curves of M₂GeO₄ electrodes at a constant current density of 100 mA g⁻¹. In the first cycle, the discharge capacities of Co₂GeO₄, Fe₂GeO₄ and Ni₂GeO₄ are 1046.6, 1127.8 and 1067.2 mAh g⁻¹ with Coulombic efficiencies of 72.9, 76.3 and 74.1%, respectively. The low Coulombic efficiency in the first cycle is related to the decomposition of electrolyte to form a solid electrolyte interphase (SEI) layer on the electrode surface. The Coulombic efficiency was steadily increased and reached 99.2% at the 50th cycle. Fig. 5(d) shows the discharge capacities of the M₂GeO₄ electrodes as a function of cycle number at a constant current density of 100 mA g⁻¹. The discharge capacities of the M₂GeO₄ electrodes decreased gradually with cycling, which results from the large volume change during the alloying/de-alloying and conversion reaction, which can lead to cracks on the electrode as well as the peeling of the electrode material from the current collector. Among the M₂GeO₄ electrodes investigated, the Fe₂GeO₄ electrode exhibited the best cycling stability. The Fe₂GeO₄ electrode showed a discharge capacity of 740.3 mAh g⁻¹ at the 50th cycle, which corresponds to 65.6% of its initial discharge capacity.

To improve the electrochemical performance of the Fe₂GeO₄ particles, they were coated with carbon using glucosamine as a carbon source through the hydrothermal method. The TGA results depicted in Fig. 6(a) reveals that 5.2 wt.% of carbon is coated on the surface of pristine Fe₂GeO₄ particles. Fig. 6(b) presents the Raman spectrum of the carbon-coated Fe₂GeO₄ particles. It clearly shows two characteristic peaks at 1318 and 1589 cm⁻¹, which can be ascribed to the sp³-hybridized disorder carbon (D-band) and the sp²-hybridized ordered graphitic carbon (G-band), respectively [30]. The calculated I_D/I_G ratio is about 1.14, indicating the presence of amorphous carbon on the surface of Fe₂GeO₄. Fig. 6(c) shows the TEM image of the carbon-coated Fe₂GeO₄ particle, which reveals that Fe₂GeO₄ is non-uniformly coated by the carbon layer with a thickness of 2.0 to 8.0 nm. The flexible nature of the amorphous carbon layer on the Fe₂GeO₄ particle is expected to act as an efficient buffer in volume changes during the repeated cycling. The electronic conductivity of the Fe₂GeO₄@C measured using the four-point probe method was 1.30×10⁻⁵ S cm⁻¹, which is much higher than that of pristine Fe₂GeO₄ (1.49×10⁻⁸ S cm⁻¹). The higher electronic conductivity of Fe₂GeO₄@C is expected to enhance the rate capability of the electrode material.

The cycling performance of the Fe₂GeO₄@C electrode was evaluated at a constant current density of 100 mA g⁻¹, with the results shown in Fig. 7(a). In the first cycle, the Fe₂GeO₄@C electrode delivered a high discharge capacity of 1144.9 mAh g⁻¹ with a Coulombic efficiency of 79.8%. The Coulombic efficiency increased steadily with cycling and reached around 99.6% at the 100th cycle. The Fe₂GeO₄@C electrode exhibited a discharge capacity of 904.2 mAh g⁻¹ with a capacity retention of 79.0% at the 100th cycle. Clearly, the Fe₂GeO₄@C electrode showed better electrochemical performance than the pristine Fe₂GeO₄ electrode, because the carbon coating of Fe₂GeO₄ can provide many advantages such as, (i) it forms the protective layer that suppresses the irreversible reaction between the Fe₂GeO₄ electrode and the liquid electrolyte, (ii) the flexible nature of amorphous carbon mitigates the mechanical stress due to the volume change during charge and discharge cycles, and (iii) the carbon coating enhances the electron transport in the electrode. The rate capabilities of pristine and carbon coated-Fe₂GeO₄ electrodes were evaluated at different current densities. The electrodes were charged to 0.01 V at a constant current density of 100 mA g⁻¹, and then discharged to 3.0 V at different current rates. The discharge capacities of the Fe₂GeO₄ and Fe₂GeO₄@C electrodes at current densities increasing from 100 to 1600 mA g⁻¹ every five cycles are shown in Fig. 7(b). The effect of carbon coating on the rate performance of the Fe₂GeO₄ electrode was noticeable as the current density was increased. At 1600 mA g⁻¹, the Fe₂GeO₄@C electrode delivered a discharge capacity of 581.3 mAh g⁻¹, corresponding to 50.7% of the initial discharge capacity at 100 mA g⁻¹. The carbon layer could provide a continuous electronic pathway between the Fe₂GeO₄ particles, which resulted in rate capability improvement. The Fe₂GeO₄@C electrode also retained high reversible capacity when current density was returned to 100 mA g⁻¹, which assured good cycling stability of the Fe₂GeO₄@C electrode material.

We successfully synthesized M₂GeO₄ (M = Co, Fe and Ni) particles by the hydrothermal method and their electrochemical characteristics were investigated. Among the three types of electrodes, the Fe₂GeO₄ electrode exhibited the highest reversible capacity of 1127.8 mAh g⁻¹. Its cycling stability and rate capability were improved by carbon coating. The carbon-coated Fe₂GeO₄ electrode delivered a high initial discharge capacity of 1144.9 mAh g⁻¹ with a capacity retention of 79.8% after 100 cycles. It also retained a relatively high discharge capacity of 581.3 mAh g⁻¹ at a high current density of 1600 mA g⁻¹, which was superior to that of the pristine Fe₂GeO₄ electrode. These results indicate that the carboncoated Fe₂GeO₄ is expected to be a promising anode material for improving the energy density of lithium-ion batteries.