XRD pattern is shown in Fig. 1, in which DG-CC, EG-CC and DW-CC show the same diffraction peaks. All the diffraction peaks are matched to the standard peaks of CoCO3 (JCPDS No.11–0692), and none impurity phase is observed in these samples. TGA curves (Fig. S1, Supporting Information) demonstrated the thermal behaviors of the CoCO3 samples. The curves first undergo a slow decline, corresponding to the evaporation of water in these materials. In the period of 250–400 ℃, all of the CoCO3 samples lost ~ 33.0% mass, which is consistent with Eq. (3).
6CoCO3 + O2 → 2Co3O4 + 6CO2 (3)
XPS spectra of DG-CC, EG-CC and DW-CC are displayed in Fig. S2 (Supporting Information). Because DG-CC, EG-CC and DW-CC are the same phase, DG-CC was selected as a typical sample for XPS analysis. In the survey XPS spectrum (Fig. S2a), the peaks of O 1 s, C 1 s, Co 2p are clearly observed, revealing the coexistence of O, C, and Co elements. The Co 2p1/2 and Co 2p3/2 peaks located at 797.9 and 782.5 eV with two prominent shake-up satellite peaks at 802.7 and 787.4 eV (Fig. S2d), respectively. A spin-orbital splitting of 15.4 eV between the Co 2p1/2 and Co 2p3/2 peaks is the characteristic feature which indicates the presence of Co2+.
Figure 3. Schematic illustration of the as-synthesized DG-CC, EG-CC and DW-CC
Cyclic voltammetry (CV) is often used to observe the electrochemical reaction potential and speculate the species change during the reaction. CV curves of the first five cycles of DG-CC, EG-CC and DW-CC are shown in Fig. 4(a-c), in which the three kinds of cobalt carbonate have basically similar peaks. In the first cycle, a sharp anode peak at 0.6–0.7 V are observed for the three CoCO3 samples, corresponding to the formation of a solid electrolyte interface (SEI) and the reduction of Co2+ (Eq. (1)). Then, the reduction peak splits into two main peaks centered at around 1.0 and 0.8 V in the following cycles, which is attributed to the reduction of Co2+ to Co0 and C4+ to lower valence C in CO32− (Eq. (2)), respectively [17]. Two main anodic peaks of the three CoCO3 samples are observed at around 1.2 and 1.9 V, which is attribute to the oxidation of low-valence C (Cx−) and Co0, respectively. It can be noted that there is a small oxidation peak in the CV curves of the three CoCO3 materials at ~ 2.6 V during charging process (Fig. S5, supporting information), corresponding to the reversible oxidation Co(II)/Co(III), which is coincide with the previous reports [15]. It can be noted that the peaks corresponding to Eq. (2) is relatively inconspicuous in the curves of DW-CC implying the reaction is not sufficient.
To further evaluate the lithium storage properties of the DG-CC, EG-CC and DW-CC, cycling tests of the three CoCO3 samples are carried out. Discharge-charge profiles in Fig. 4(d) revealed that the initial discharge/charge capacities at 0.1 A/g for DG-CC, EG-CC and DW-CC are 1478.6/1054.6, 1748.0/1168.5 and 1512.9/1116.9 mAh/g, respectively, and the corresponding Coulombic efficiencies (CEs) are 71.32%, 66.84% and 73.82%. The initial irreversible capacity loss is mainly attributed to the formation of the SEI layer. According to the initial discharge capacity, it can be inferred that 4.7–5.2 Li per CoCO3 was released in the first cycles. In terms of capacity, this value indicates that the traditional conversion reaction is not the only one providing capacity. This result is consistent with the analysis in CV curves. As shown in Fig. 4(e), cobalt carbonates are measured at 0.1 A/g during 200 cycles. In the first 20 cycles, the specific capacity of the three materials decrease rapidly due to the structural adjustment due to volume expansion, and the charge capacities for DG-CC, EG-CC and DW-CC are 904.3, 861.5, 612.1 mAh/g, respectively. Then, the curves tend to be stable and the value of CEs are close to 100% in the following cycle. In the latter half of the cycle process, the specific capacity of the three materials began to rise which may due to the growth of a polymeric gel-like film originating from kinetic activation of the electrode [32]. After 200 cycles, the discharge/charge capacities for DG-CC, EG-CC and DW-CC achieved 960.0/957.2, 857.5/851.9 and 829.6/821.6 mAh/g, respectively. Rate performance of DG-CC, EG-CC and DW-CC is shown in Fig. 4(f). The sample DG-CC delivers the highest capacity with average specific capacity of 1014, 841, 711, 595, 476 mAh/g at current densities of 0.1, 0.2, 0.5, 1, 2 A/g, respectively. When the current density returns to 0.1 A/g, the specific capacity of DG-CC returns to 889 mAh/g. Finally, the sample DG-CC is cycled at a current density of 0.5 A/g, in which the cell maintains at about 660 mAh/g. While for the EG-CC and DW-CC, they deliver lower specific capacities than DG-CC with the increased current densities, demonstrating a gradient relationship (DG-CC > EG-CC > DW-CC). the optimal particle size and high specific surface area, which is conducive to the sample DG-CC participating in the electrochemical reactions.
Most important, the three CoCO3 samples also showed excellent cycle stabilities at a large current density. After the activation for initial 3 cycles at 0.1 A/g and then tested at 1 A/g, the specific capacity goes through the process of rising in the beginning and then generally stable (Fig. 4g). The specific discharge/charge capacity of DG-CC, EG-CC and DW-CC is 692.6/690.7, 383.1/382.8, 295.6/295.0 mAh/g after 1000 cycles, respectively. Generally, DG-CC shows the best cycle performance, which is attributed to the highest specific capacity among the three samples. Based on the stable reversible capacity of the 20th cycle, the capacity retention of DG-CC is 92.45%, indicating that the material has a very stable capacity performance. Comparison of our CoCO3 samples and other previous anode materials in which cobalt carbonate is the main component are summarized in Fig. 4(h) and Table S1 (Supporting Information). The reference points of green, orange, purple and blue in Fig. 4(h) correspond to cobalt carbonates of pure, compounding, doped, and both doped and compounding, respectively. The ability of DG-CC to withstand repeated charge and discharge under 1 A/g is better than that of most cobalt carbonate materials, and the capacity of DG-CC at 1 A/g even exceeds the samples compounding with other conductive carbon or doped materials. At the same time, the initial Coulombic efficiency of the DG-CC (71.32%) prepared in this study is also at the upstream level. All of the above results show the superiority of DG-CC as an anode material for lithium-ion battery.
To further analysis the performance difference of the three materials in the 1000 cycles, differential charge versus voltage plots and their corresponding charging curves are conducted to describe the voltage platform and potential change of electrochemical reaction at 1 A/g. The corresponding dQ/dV curves of the three samples are shown in Fig. 5(a). Within the 20th and 1000th cycle, three peaks at ~ 1.3, ~ 2.0 and ~ 2.5 V reveal a three-step electrochemical reaction which is consistent with the CV curves in Fig. 4a-c. Therefore, the voltage can be divided into three regions. Region 1 from 0 to 1.63 V corresponds to the oxidation of Cx−. Region 2 from 1.63 to 2.26 V corresponds to the oxidation of Co0, and region 3 from 2.26 to 3 V corresponds to the transition of Co2+/Co3+ according to the result of CV curves. Based on the charge profiles in Fig. 5(b), different voltage ranges correspond to different capacity contributions. The concrete capacity contribution in different regions of the three materials in different cycles is summarized in Table S2 (Supporting Information) and further expressed as Fig. 5(c). At the beginning of the cycle, there is little difference between DG-CC and EG-CC in performance, but the difference is obvious when reach 1000th cycle. For DG-CC, capacity contributed from three regions is similar between the 20th and 1000th, respectively. However, the capacity provided by these three reactions for EG-CC and DW-CC are greatly reduced compared with DG-CC, suggests that DG-CC has stronger structural stability. Except for the absolute value of capacity, Fig. 5(d) further shows the contribution ratio statistics of different regions. Compared with the case at the 20th cycle, it is obvious that the main reason for capacity declination at the 1000th cycle is the decrease of capacity contribution of regions 1 and 2. The decrease in reversibility may be due to the structural instability caused by the repeated volume changes during cycling. Theoretically, Eqs. (1) and (2) involve more lithium ions, while the transition of Co2+/Co3+ in region 3 only corresponds to 1 equivalent lithium ion. Therefore, the volume effect is more serious in region 1 and 2. What’s more, once the previous reaction becomes inadequate, the remaining reactants in the subsequent cycle will reduce. This kind of chain effect makes the capacity continue to decline. However, DG-CC shows better stability than EG-CC and DW-CC, which may indicate its structural superiority. The particle size and pore size brought by different solvents should be the key to affect the electrochemical performance.
To further confirm the above conjecture, the morphology of the three kinds of electrodes before and after 1000 cycles is displayed in Fig. 6. The typical particles are selected in the macro morphology as the specimens for analysis which are shown in the upper-right corners of each images. Particles in fresh electrodes of DG-CC, EG-CC and DW-CC distribute uniformly across the whole film surfaces (Fig. 6a-c). The smaller nanoparticles distributed between these CoCO3 particles correspond to the conductive agent. After 1000 discharge-charge cycles, apparently, pulverization is observed on EG-CC and DW-CC particles (Fig. 6e-f). As is known to all, conversion-type anode materials suffer from relatively large volume change (< 200%) during repeated cycles [5]. Nevertheless, the particles of DG-CC maintain the integrity of morphology (Fig. 6d), which is attributed to the proper sizes of both primary particles and pore sizes of DG-CC. Small primary particles is beneficial to distribute the strain evenly in all regions of the material. Meanwhile, as shown in Fig. 7, appropriate pore size can accommodate these volume changes, so that the secondary particles will not collapse and effectively reduce the loss of active materials. The structure advantage of DG-CC prevents it from falling into the chain effect mentioned above during a long cycle process, which induced by the initial solvent control.
The superiority of morphology of DG-CC is also reflected in the impedance information. Nyquist plots of freshly assembled cells with the active substances of DG-CC, EG-CC and DW-CC are shown in Fig. 8(a), and the data is fitted by the equivalent circuit exhibited in Fig. 8(b). Rs represents the ohmic resistance of the battery. Rct and Zw (Warburg impedances) describe the resistance of charge transfer and mass transfer, respectively. The specific values of these parameters obtained by fitting are set out in Table S3 (Supporting Information). It is observed that from the Fig. 8(b) that there is almost no difference in the Rs of the cells made by the three CoCO3 samples before cycle. However, DG-CC has the lowest Rct, while DW-CC has the opposite, which further proofs the faster reaction kinetics of DG-CC. Applied by impedance analysis, the change of DG-CC electrode during the cycles was studied. Nyquist plots of DG-CC after the 30th, 90th, 120th and 200th cycle at 1 A/g are displayed in Fig. 8(c). The variation of impedance is summarized in Fig. 8(d) and Table S4 (Supporting Information). From the beginning to the 200th cycle, both RSEI (SEI layer resistances) and Rct has an upward trend in the early stage which may be due to the structural instability caused by volume change. Nevertheless, RSEI and Rct decline rapidly after 90 cycles suggest that the negative effect of electrode structure change disappeared and thus brought about enhanced charge-transport kinetics [8, 33]. It's not difficult to recognize that this result is consistent with the phenomenon that the specific capacity decreases first and then increases during the cycle process (Fig. 4g).