Nanodiamond electrolyte

Nanodiamond particles used here were produced by a commercial detonation method at a low cost, then carboxylated and subsequently modified by covalent linking of ODA37, 38. They have a crystal size of ~5 nm and high crystallinity (Fig. 1c and Supplementary Fig. 1). The interplanar crystal spacing in lattice-fringe transmission electron microscopy (TEM) images was measured to be ~ 0.21 nm, which corresponds to diamond (111) planes (0.206 nm, PDF#65-0537). An EC/DEC electrolyte with dispersed modified nanodiamond particles and a saturation concentration of 0.82 mg mL−1 was prepared. Compared to the original colorless and transparent EC/DEC electrolyte, the solution became light-yellow after nanodiamond particle addition (Fig. 1d). Aggregation of nanodiamond particles cannot be fully avoided in the electrolyte and nanodiamond clusters with a size of ~530 nm were measured by dynamic light scattering (Fig. 1e). This size of nanodiamond clusters was largely reduced compared to the pristine commercial nanodiamond39. The solution color and size distribution of nanodiamond clusters in the electrolyte did not change after 2 months, indicating the stability of the nanodiamond dispersion in the electrolyte (Supplementary Fig. 2). The nanodiamond particles are able to adsorb Li ions onto their surfaces (Fig. 1f) and co-deposit onto the Cu foil with Li metal, thus acting as the nucleation seeds that guide Li ion deposition. Furthermore, the charged nanodiamond particles do not aggregate inside the electrochemical cells during the practical charging/discharging processes.

Li plating morphology

Figure 2 illustrates the role of nanodiamond additives in Li ion deposition behavior. Stripping of Li ions from the Li foil to plate on the Cu foil was conducted in an electrolytic bath with or without nanodiamond additives (Fig. 1a). The Cu foil surface before plating was clean and even (Supplementary Fig. 3). The morphologies of Li deposits after the first Li ion plating process (discharging at 0.5 mA cm−2 for 6 h) were shown in Fig. 2a–d and f–i. Without nanodiamond in the electrolyte, an uneven morphology is clearly shown, as many bumps were observed on the surface of deposited Li films (Fig. 2b–d). After introducing nanodiamond into the electrolyte, uniform deposition of Li metal on Cu foil was achieved, as indicated by a bright metallic luster caused by the uniform and dense metal surface (Fig. 2g–i). Since no 500-nm diamond cluster could be observed on the Li surface or incorporated into Li, we assume that only a small number of individual nanometer-sized nanodiamond particles were captured by the growing Li film, if any at all.

Fig. 2 Morphology of Li deposits after galvanostatic plating. Schematic illustration describing Li ion plating behavior in the LiPF 6 -EC/DEC electrolyte a without and f with nanodiamond additives. SEM images of b–e Li deposits in LiPF 6 -EC/DEC electrolyte without nanodiamond additive. SEM images of g–j Li deposits in LiPF 6 -EC/DEC electrolyte with nanodiamond additives. b–d and g–i Li plating after one time at 0.5 mA cm−2 and with plating time of 6 h. e, j Li plating after three cycles at 0.5 mA cm−2 and with each step time of 6 h. The insets in e, j are the optical images of the corresponding Li deposits. The scale bars in b and g, c and h, d and i, e and j are 100, 1, 50, 5 μm, respectively. The scale bars in the insets of b, g, e, and i are 1 μm. The word ‘‘ND’’ in the figure is the abbreviation of ‘‘nanodiamond’’ Full size image

The Li deposits appear to have a columnar structure (Supplementary Fig. 4). In the electrolyte without nanodiamond, the deposited Li columns had an average diameter of 0.7 ~ 0.8 μm (Fig. 2c, Supplementary Fig. 5a, b). Li columns with a small average diameter (0.3 ~ 0.4 μm) were obtained in the nanodiamond-containing electrolyte (Fig. 2h, Supplementary Fig. 5e, f), leading to a dendrite-free morphology40. The colloidal solution remains stable after the nanodiamond electrolyte was stored for 2 months, and can still keep the uniform columnar structure (Supplementary Fig. 6). It is important to mention that the size of those columns is smaller than the size of nanodiamond aggregates in solution, again suggesting that these 500-nm nanodiamond aggregates break apart during Li plating. The difference in the crystal size of Li deposits was analyzed by X-ray diffraction (XRD) (Supplementary Fig. 7). A wider peak of Li (110) of Li deposits in the nanodiamond electrolyte confirms their smaller crystal size, compared to that in the nanodiamond-free electrolyte (Supplementary Note 1). The reduced size of Li deposits in the nanodiamond electrolyte is ascribed to the increased number of nucleation sites that are induced by the nanodiamond particles33. The arrayed morphology of Li deposits can be well maintained when the current density is increased to 1.0 mA cm−2 for 3.0 h (3.0 mAh cm−2) (Supplementary Fig. 8).

After the third Li plating and stripping (charging and discharging at 0.5 mA cm−2 for three cycles with each step time of 6 h), many optically visible particles on Li deposits were observed in the nanodiamond-free electrolyte (Fig. 2e). Imaging via high-magnification scanning electron microscopy (SEM) revealed that these particles were dendritic Li clusters (Fig. 2e). In comparison, the dendrite-free morphology of Li deposits was observed in the nanodiamond-containing electrolyte (Fig. 2j). These results clearly indicate that nanodiamond additives successfully induce smaller crystal sizes of Li deposits, leading to a smooth surface and dendrite-free morphology.

Li morphologies were studied after 1st, 2nd, 5th, 10th, 20th, and 50th cycles at 0.5 mA cm−2 with plating/stripping time of 1 h in each cycle (Supplementary Fig. 9). Although some coarsening appears after 20 and 50 cycles, the primary morphology was maintained after 100 h Li plating/stripping at a relatively high-current density (0.5 mA cm−2). This finding indicates that nanodiamond particles both remain available in the electrolyte and are able to preserve dendrite-free morphology, even after long-term cycling.

X-ray photoelectron spectroscopy (XPS) of Li deposits after Li plating/stripping confirmed that superior long-term stability is induced by the recyclability of nanodiamond during Li plating and stripping (Supplementary Fig. 10). Relative to nanodiamond-free electrolyte, Li in nanodiamond-containing electrolyte displays a new peak, which originates from the co-deposited nanodiamond particles. The co-deposition of nanodiamond particles and Li was also confirmed by the carbon enrichment in the deposited Li layer (Supplementary Fig. 11, Supplementary Note 2 ). After Li stripping, the nanodiamond peak in the XPS spectrum disappears, demonstrating the recyclability of nanodiamond during Li plating and stripping. Additionally supporting their recyclability, nanodiamond particles can not only co-deposit with Li ions, but also strip off from the Cu substrate to render the long-term stability of Li plating morphology.

Diluted (0.41 mg mL−1) and concentrated (4.1 mg mL−1) nanodiamond electrolytes were prepared to investigate the effect of nanodiamond concentration on the Li plating morphology (Supplementary Fig. 12). In the 0.41 mg mL−1 nanodiamond electrolyte, Li deposits were less uniform, containing clearly smooth regions and regions with a few bumps (the size of Li deposits is ca. 2.6 μm) (Supplementary Fig. 13). This can be ascribed to an insufficient number of nucleation sites. Even in the smooth regions, the size of Li crystals varied considerably, ranging from several hundred nanometers to a few microns with an average size of 0.6 ~ 0.7 μm (Supplementary Figs 5c, d and 13c). An interesting phenomenon is that the size of Li in bumpy regions was always larger than that in the smooth regions. This emphasizes the importance of reducing the size of Li crystals by providing a large number of nucleation sites. In the 4.1 mg mL−1 nanodiamond electrolyte, Li deposits had an average size of 0.9 ~ 1.2 μm (Supplementary Fig. 5g, h and 14), which is more uniform than those in the 0.41 mg mL−1 nanodiamond electrolyte. However, the smoothness in the 4.1 mg mL−1 nanodiamond electrolyte was worse than that in the 0.82 mg mL−1 nanodiamond electrolyte. This is because the dispersion contains a large amount of nanodiamond aggregates, and is consequently oversaturated and unstable. The observed concentration dependence highlights the importance of a high concentration and a uniform dispersion of nanodiamond particles in the electrolyte, which may be improved by modifying the functionalization of nanodiamond and controlling the size of the aggregates.

Interaction between Li ions and nanodiamond

To better understand the nanodiamond-guided Li plating behavior, first-principle calculations were performed. We first calculated the surface energies of several low index facets for nanodiamond and Cu to find the most stable and dominating surfaces (Fig. 3a). The results indicate that nanodiamond (110) and Cu (111) are the dominating surfaces for each crystal with the lowest surface energies of 5.76 and 1.62 J m−2, respectively. Therefore, nanodiamond (110) and Cu (111) were chosen as the base surfaces for the following discussions of binding energy and diffusion energy barriers.

Fig. 3 First-principle calculations to describe Li ion plating behavior on nanodiamond surface. a Surface energies of low index facets for nanodiamond and Cu. b Differences of charge density for Li on nanodiamond (110) and Cu (111) surfaces. The turquoise and yellow regions indicate depletion and accumulation of electrons, respectively. c Diffusion barrier of Li on different surfaces. Except for nanodiamond, the diffusion barriers of other materials are cited from the ref. 41. d The most stable adsorption sites and diffusion paths for Li on nanodiamond (110) surface. The word ‘‘ND’’ in the figure is the abbreviation of ‘‘nanodiamond’’ Full size image

The binding energies of Li on nanodiamond (110) and Cu (111) surfaces were calculated to be 3.51 and 2.58 eV, respectively (Fig. 3b). The large charge transfer between Li and nanodiamond (110) surface contributes to its high-binding energy. The nearly 1 eV higher binding energy for nanodiamond and Li ions results in a stronger preferential adsorption of Li ions on the nanodiamond surface rather than onto the Cu surface during Li plating. After adsorption, Li ions can either aggregate into a large dendrite, or distribute uniformly and form dendrite-free Li deposits. To investigate Li ion diffusion behavior, the diffusion barrier of Li on nanodiamond was calculated and compared with that on other materials published in literature (Fig. 3c, d)41. Compared with these materials, nanodiamond has the lowest Li diffusion energy barrier. This indicates that at the interface of the cathode (Cu foil) and electrolyte, Li ions are inclined to adsorb onto the nanodiamond surface and weaken aggregation and can easily diffuse and distribute uniformly to produce a dendrite-free morphology42, 43.

Electrochemical cycling performance

The long-term electrochemical cycling stability of Li electrodes was explored by testing symmetrical Li | Li cells. As shown in Fig. 4a, b, symmetrical Li | Li electrodes have stable cycling in the nanodiamond electrolyte for 200 and 150 h (tests were stopped at that point) at 1.0 and 2.0 mA cm−2, respectively, exhibiting stable Li metal deposition, though with a little increase in the polarization (100 mV at 1 mA cm−2 to 120 mV at 2 mA cm−2). In comparison, symmetrical Li | Li electrodes in the nanodiamond-free electrolyte have obvious fluctuations in voltages caused by the ever-changing and increasing interfaces of the Li metal and electrolyte. At a high-current rate (2.0 mA cm−2), the polarization of the nanodiamond-free electrolyte is much larger than that of the nanodiamond electrolyte, due to Li dendrite growth and dead Li. Additionally, electrochemical impedance (EIS) spectroscopy for the Li | Li cells at 1.0 mA cm−2 was conducted after 10, 30, 40, 50, 60, 70, and 80 cycles (Supplementary Fig. 15). After ten cycles, cells in the nanodiamond electrolyte exhibited a stable Li ion diffusion resistance of ~ 84 Ω, while the impedances of nanodiamond-free electrolytes fluctuated largely, ranging between 219 and 97 Ω, thus demonstrating a stable interfacial impedance induced by the nanodiamond-containing electrolyte.

Fig. 4 Long-term electrochemical cycling stability. Charge-discharge curves of symmetrical Li | Li cells at a 1 mA cm−2 and b 2 mA cm−2. Each charge and discharge time is set as 12 min. c Voltage-time curves to calculate the average Coulombic efficiency of Li | Cu cells at 0.5 mA cm−2. d The enlarged view of c from 5 ~ 15 h. The morphology of a Li deposit cycled at 0.5 mA cm−2 in the electrolyte e without and f with the nanodiamond additive. The scale bars in e and f are 10 μm. The word ‘‘ND’’ in the figure is the abbreviation of ‘‘nanodiamond’’ Full size image

When decreasing the nanodiamond concentration from 0.82 to 0.41 mg mL−1, the cells also retained good stability after voltage was applied (Supplementary Fig. 16a). The 0.41 mg mL−1 nanodiamond electrolyte showed much better performance than the nanodiamond-free electrolyte. While the 0.82 mg mL−1 electrolyte showed similar cycling stability, it exhibited less increase in the voltage polarization with extended cycles. These results clearly demonstrate the role of nanodiamond in stabilizing Li metal to achieve a stable long-term cycling performance.

The Coulombic efficiency during Li plating/stripping was probed in a Li | Cu cell according to Aurbach et al.44 (Fig. 4c). During 12 cycles, the average Coulombic efficiency of cells in the nanodiamond-containing electrolyte (0.82 mg mL−1) was 96%, which is much higher than that in the nanodiamond-free electrolyte (88%). The higher Coulombic efficiency in the nanodiamond-containing electrolyte indicates a higher Li utilization during Li plating/stripping. When the cycling time and number were extended to 100 cycles (200 h), a stable performance was maintained with an average Coulombic efficiency of 96% (Supplementary Fig. 17). For the reduced nanodiamond concentration of 0.41 mg mL−1, an average Coulombic efficiency of 95% was achieved (Supplementary Fig. 16b), which is only slightly smaller than the 0.82 mg mL−1 nanodiamond electrolyte (96%), but still much higher than that of the nanodiamond-free electrolyte (88%). While close to 100% efficiency is expected in commercial batteries, we assume that a lower efficiency in our experiment does not result from the electrolyte breakdown. It rather comes from some Li staying adsorbed on nanodiamond particles due to the strong binding energy between nanodiamond and Li. Still, the nanodiamond additive also renders a reduced polarization of 19 mV during Li plating/stripping, while it is 29 mV for nanodiamond-free electrolyte (Fig. 4d). In the nanodiamond-containing electrolyte, the electrode starts to plate Li at ~ −15 mV, and strips Li at ~ 27 mV. However, in the nanodiamond-free electrolyte, the plating and stripping processes start at approximately −21 and 48 mV, respectively. Thus, nanodiamond particles in the electrolyte effectively promote Li nucleation and dissolution.

SEM imaging of cycled electrodes revealed a large density of dendrites in the nanodiamond-free electrolyte (Fig. 4e), especially compared to the electrode in the nanodiamond-containing electrolyte. These dendrites lead to an unstable Li metal/electrolyte interface, the formation of dead Li, and poor long-term cycling performance. In the nanodiamond-containing electrolyte, the electrode shows a dendrite-free morphology (Fig. 4f). There are two key differences between the Li plating morphologies in the electroplating bath and the coin cells. Firstly, the plating morphology in the coin cells was flattened due to pressure, demonstrating the importance of the electroplating bath in investigating the original Li plating morphology. Secondly, the Li crystal size in the coin cells after many cycles grew larger than that in the electroplating bath. The increased crystal size can be attributed to the aggregation of nanodiamond particles during long-term cycling. Therefore, it is important to produce well dispersed and aggregate-free nanodiamond particles in the electrolyte.

Li | LiFePO 4 (LFP) full cells were assembled to test the viability of nanodiamond-containing electrolyte in the practical batteries (Supplementary Fig. 18). After activation at 0.1 C (1.0 C = 180 mA g−1), testing of the LFP battery with nanodiamond electrolyte indicated a very stable cycling at 1.0 C with a capacity decay of 4.9% after 130 cycles, while the cell with nanodiamond-free electrolyte exhibited a larger capacity decay of 14.2% (Supplementary Fig. 18a). As these cells have the same cathode, the capacity decay can be attributed to the anode depletion. The superior cycling stability of the LFP battery with nanodiamond-containing electrolyte is ascribed to dendrite-free Li deposits and a stable electrode-electrolyte interface on the Li metal anode.

The morphologies of the LFP cathode, Celgard 2400 separator, and Li foils were investigated after 5 and 20 cycles. The LFP morphologies were remarkably similar, with conductive agents (acetylene black) connecting LFP particles (Supplementary Fig. 19). Relative to the pristine separator, the separators after tests maintained their porous structure (Supplementary Fig. 20). There were no obvious nanodiamond particles on the LFP and separator surface, demonstrating that nanodiamond particles were not be absorbed in noticeable accounts onto the high-surface-area LFP cathode and the separator, hence keeping the stable nanodiamond concentration in the electrolyte. The cycled Li morphologies after 5 and 20 cycles (Supplementary Fig. 21) were similar to that in the Li | Cu coin-cell (Fig. 4f). After many cycles, the nanodiamond electrolyte was still able to maintain the arrayed and dendrite-free Li deposits in the Li | LFP cells. Therefore, nanodiamond particles can effectively maintain a stable Li-electrolyte interface and dendrite-free Li morphology in commercial electrolytes, while having little adverse effects on the cathode and separators.

The role of nanodiamond concentration on the full-cell cycling performance was also investigated (Supplementary Fig. 18b). Similar to Li | Li and Li | Cu cells, the Li | LFP cells of both 0.41 and 0.82 mg mL−1 nanodiamond electrolyte indicated a good cycling stability (capacity decay rate: 4.4%), but the cycling capacity of the 0.41 mg mL−1 nanodiamond-electrolyte was 10 mAh g−1 lower than that of the 0.82 mg mL−1 nanodiamond electrolyte. These results indicate that both nanodiamond concentrations can lead to dendrite-free Li deposits and a stable Li-electrolyte interface, but the smaller nanodiamond concentration will not provide sufficient number of nucleation sites for uniform Li plating.