Na–K Co‐deposition in Liquid‐Alloy Batteries Revealed by Operando Visualization

Despite the great prospects of alkali metal batteries, safety concerns associated with dendrite growth still limit their commercial applications. An attractive alternative is to use the room temperature liquid sodium–potassium alloy as the anode, which inherently prevents dendrite growth. Currently, Na–K alloy anodes allow only Na+ or K+ to be cycled, depending on the choice of electrolyte and ion selectivity of the cathode, which results in reduced energy density of the battery. Herein, the possibility of concurrent use of both Na+ and K+ ions in Na–K alloy anodes is explored, and the working mechanism by operando optical microscopy is investigated. It is found that the type of deposited metal is dictated by both salt and solvent in electrolyte. Impressively, Na–K co‐deposition is observed in KFSI‐DME electrolyte for the first time, which strongly influences the dendrite morphology and evolution. Furthermore, the current density also has a great impact on the deposition pattern, which allows dendrite‐free Na/K deposition on the liquid‐alloy anode. These findings enrich our understanding of the intricate electrochemical behaviors of Na–K binary electroactive alloy systems and offer guidance for the sufficient use of Na and K while avoiding dendrite formation in such liquid‐alloy batteries.

[26][27] In particular, considering the coexistence of Na and K in liquid metal anode, the ion selectivity of cathode and electrolytes that allow only one type of alkali ions to be cycled will lead to reduced energy density of the battery.Therefore, a deeper understanding of the working mechanism in Na-K alloy-based batteries is very necessary, which is also beneficial for fully exploiting the potential of the Na-K alloy anode.
Herein, we report the operando optical observations to investigate the deposition behaviors of the Na-K alloy in an actual liquid working environment by assembling the Na-K alloy-based half and symmetric cells with transparent windows.The impact of different parameters such as salts, solvents, and current densities is taken into consideration.Comparison studies indicate that the choice of solvent can significantly alter the deposition mode and dendrite growth pattern.Remarkably, we observed the co-deposition of Na and K for the first time in a DME-based electrolyte containing only K salt, which is in stark contrast to the single-metal (Na or K) deposition observed in EC/DEC-based electrolyte.Moreover, the current density is the most critical factor in determining the metal deposition morphology, which dominates the rates of K-deposition, Na-deposition, and their dissolution into the liquid Na-K electrode in DME-based electrolyte.

Result and Discussion
As illustrated in Figure 1a-c, the growth of dendrites was observed using the home-built visualization cell set-up in conjunction with an optical microscope (OM) at room temperature.The liquid Na-K alloy has high surface tension, which was housed into a copper foam using the vacuum infiltration method to ensure adequate electrical contact (Figure 1d).The prepared Na-K alloy@Cu electrode was then clamped (with two copper sheets) at one side of the cell and used as the working electrode (see the schematic in Figure S1, Supporting Information).Another Cu foil or Na-K alloy@Cu electrode served as the counter electrode to assemble the Cu||Na-K half cell or Na-K||Na-K symmetric cell (Figure 1e,f ).There was no separator membrane between the working and counter electrodes, which were separated by a distance of about 2 mm to avoid the mechanical influence on dendrite growth from the counter electrode.
First, we conducted a comparative study of metal deposition behaviors on the Cu electrode in Cu||Na-K half cells using Na þbased and K þ -based electrolytes, respectively.A Na þ -based electrolyte (1 M NaClO 4 in ethyl carbonate (EC)/diethyl carbonate (DEC)) was chosen for the operando deposition test at a constant current density of 2 mA cm À2 .As shown in Figure 2a, a tiny dendritic structure emerged on the surface of the Cu electrode, and it continued to grow and evolve into a mossy morphology as the deposition time increased (Movie S1, Supporting Information).The deposition species was then taken out from the cell for energy-dispersive spectrometer (EDS) analysis.The EDS spectrum showed that the dendrite was primarily composed of Na, with almost no K (Figure 2b), indicating a Na-dominating deposition process.Figure 2c,d shows the dendrite growth in a K þ -based (0.8 M KPF 6 in EC/DEC) electrolyte.Similar to the phenomenon observed in the Na þ -based electrolyte, mossy-like clusters were also formed on the Cu electrode, but with smaller sizes and a discrete distribution (Movie S2, Supporting Information).These dendrites were identified as K dendrites as the EDS spectrum of the deposits showed a predominant K signal and a negligible Na signal.The above control experiments confirm the single-metal (Na or K) deposition mode in EC/DEC-based electrolytes, and the deposited species is solely dominated by the type of salt in the EC/DEC solvent. [22,23]n order to probe the effect of solvent on the deposition processes, a 1 M potassium bis(fluorosulfonyl)imide (KFSI) in dimethoxyethane (DME) electrolyte was used in Cu||Na-K half cells.As depicted in Figure 3a,b, the dendrite evolution in KFSI-DME electrolyte exhibited a radial tip-growing pattern (Movie S3, Supporting Information) and reached the maximum size at 566 s (Figure 3c).Subsequently, the outer branched structures shrank inward to form a smoother surface, and the bush-like dendrites evolved into a spherical core-shell morphology (566-2257 s, Figure 3d-f, where the dashed line indicates the core-shell boundary).With continued deposition, small branched dendrites regrew on the smooth surface of the sphere, followed by another round of dendrite shrinkage (2257-3600 s, Figure 3g-k).At this stage, the size of the spherical dendrite remained nearly constant, but the interior became denser, resulting in a clear contrast difference between the core and shell.This dendrite growth process was substantially different from that in EC/DEC-based electrolyte, highlighting the significant impact of the solvent on dendritic growth behavior.Interestingly, besides the prominent K peaks, the EDS spectrum showed a small but discernible Na peaks in the deposition from the DME-based electrolyte (Figure 3l), which is also in contrast to the EDS results with EC/EDC-based electrolyte (Figure 2b,d).Similar EDS results have also been reported in a prior study, yet the origin of the Na peak has not been explained in detail. [21]ased on the dendrite shrinkage and the appearance of Na in EDS spectrum presented above, it is reasonable to infer that the observed unique dendrite growth behavior in KFSI-DME electrolyte is most likely caused by Na-K co-deposition.The Na species in KFSI-DME electrolyte should be derived from the dissolution  of the Na-K alloy at the anode side, [28] where a SEI layer that contains both Na and K ion channels can form at the anode-electrolyte interface. [29]Specifically, the high oxidization tendency of Na will enable a fraction of Na þ to escape from the Na-K alloy anode into the solvent through this SEI layer, allowing the system to reach equilibrium.It is worth noting that the Gibbs free energy for Na þ reduction is smaller than that of K þ , which means that Na þ reduction is energetically more favorable than K þ at the cathode side. [23]As a result, when the Na þ ions migrate from the anode side to the counter electrode, they can partially participate in the construction of SEI on K-dendrite surface and form Na ion channels, which allow the remaining Na þ ions to be reduced to form Na metal at the interface between the SEI and the deposited K metal.According to the Na-K alloy phase diagram, the Na-K alloy remains entirely a liquid state at room temperature (RT) within a range of 9.2 to 58.2 wt% Na, and a liquid (Na-K alloy) and solid solution (Na or K) will coexist outside this range.Our EDS results demonstrated that the Na content in the deposits is 3.17 wt%, which means that the liquid Na-K alloy should be present in the deposit, in coexistence with solid K (as shown in Figure S2, Supporting Information).Under the surface tension of liquid metal, the branched dendrites shrink spontaneously and flow inward, eventually forming a spherical morphology (to reduce the surface energy).Meanwhile, the formation of liquid Na-K alloy also results in a clear contrast difference between the inner liquid and outer dendrite (as seen in Figure 3f,k).
To confirm the proposed hypothesis, the deposits after Cu|| Na-K coin half-cell tests were analyzed using X-ray photoelectron spectroscopy (XPS) to examine the composition of SEI. Figure S3, Supporting Information displays the XPS full spectrum of SEI formed in NaClO 4 -EC/DEC, KPF 6 -EC/DEC, and KFSI-DME electrolytes, and the corresponding high-resolution XPS spectra are presented in Figure 4. Curve fittings indicate that the SEI formed in NaClO 4 -EC/DEC mainly consists of NaCl and carbonate according to the Na 1s, Cl 2p, C 1s, and O 1s binding energies (Figure 4a-d). [30]In the case of KPF 6 -EC/DEC, the SEI contains K-and C-based decomposition products, including K 2p 1/2 , K 2p 3/2 , C═O, C─O, C─C, and C─H peaks (Figure 4e-h). [31]It is worth noting that there was no sign of Na detected in the SEI formed in the KPF 6 -EC/DEC electrolyte, and no K found with the NaClO 4 -EC/DEC electrolyte either.This indicates that the composition of the SEI formed in the EC/DEC-based electrolyte is solely determined by the cations present in the electrolyte salt.In contrast, the SEI in KFSI-DME electrolyte shows a significant presence of NaF on the surface (Figure 4i,l), which is remarkably different from that in NaClO 4 -EC/DEC or KPF 6 -EC/DEC electrolyte.This finding is consistent with the EDS results mentioned above and provides strong evidence that Na-K co-deposition can occur in KFSI-DME electrolyte, as the SEI includes the pathways for both Na and K ions.
Next, we further compared the evolutions of dendrites in DME and EC/DEC solvents with same K salts.As shown in Figure 5a, b, it was found that K dendrites in KFSI-EC/DEC electrolyte showed a whisker-like morphology and tended to fracture when they reached a certain length.Consequently, the broken pieces of K, also known as "dead K", gradually sank into the electrolyte and became invisible (i.e., out of focus of the microscope) (Movie S4, Supporting Information), resulting in the irreversible loss of active K. Differently, the dendrites in the KFSI-DME electrolyte exhibited a shrinking trend instead of fracturing (Figure 5d,e, Movie S5, Supporting Information).Such different dendrite evolutions can be attributed to the different mechanical stability of the SEIs and the different phases of the metals formed by mono-and co-deposition. [32,33]First, as previously reported, the SEI in EC/DEC is less uniform and dense than that in DME, making it more vulnerable and easily destroyed by stress buildup, leading to the dendrite fracture (Figure 5c). [34,35]On the contrary, the SEI in DME remains stable under high stress and acts as a protective shell, thus avoiding dendrite fracture.Second, as discussed above, Na þ can be deposited on the K dendrite to form liquid Na-K alloy in KFSI-DME electrolyte, and the fluidity of this liquid can effectively mitigate its mechanical attack (due to the volume increase) on the SEI film, preserving the integrity of the SEI though it was deformed during the shrinkage of the liquid alloy (Figure 5f ).
In addition to the type of solvent and salt of the liquid electrolyte, the current density also plays a significant role in determining dendrite growth.We then used the Na-K||Na-K symmetric cell to examine the metal deposition behavior on Na-K alloy electrode (it remained a liquid for its Na/K ratio of 1:1) under different current densities.To ensure that both Na and K are able to deposit, the KFSI-DME electrolyte was applied in the symmetric cells.As shown in Figure 6a, at a low current density (0.5 mA cm À2 ), the Na-K alloy electrode experienced obvious volume expansion, resulting in a crack on its surface.As the deposition proceeded, the width of crack increased (as seen in  Remarkably, no dendrite formation was observed on the surface of the Na-K alloy even after prolonged deposition, indicating that the deposits were fully integrated into the liquid alloy.When the deposition was performed at a moderate current density of 2 mA cm À2 , the formation of dendritic structures could be clearly observed (Figure 6c).More impressively, when dendrites reached to a certain size, they stopped growing, dissolved back into the alloy, and even vanished with further deposition (as seen in Dendrite-II in Figure 6c, Movie S7 in Supporting Information).This phenomenon suggests that dendrite-free deposition can be achieved under such a circumstance, which can be related to Na-K co-deposition as mentioned EARLIER and will be discussed in more detail later.We further conducted the deposition at a high current density of 10 mA cm À2 .As shown in Figure 6e-f and Movie S8 in Supporting Information, the dendrite structures were also formed on the electrode surface and grew fast in a bush-like tip-growing pattern with a high rate of %7 μm s À1 , similar to the dendrite growth on Cu electrode in Cu||Na-K half cell (Figure 3).In addition, close examination shows that dendrite shrinkage also locally occurred at the tips of the dendrite branches, but it is easily neglected during such rapid growth of K dendrites (Figure S4, Supporting Information).Additionally, symmetric coin cells were assembled using Na-K alloy and K electrodes to further investigate the effect of current density on metal deposition by rate tests.As illustrated in Figure 6g, at a low current density (1 mA cm À2 ), the charge/discharge curves of Na-K//KFSI-DME//Na-K symmetric cell exhibit a flat plating/stripping plateau with minimal voltage hysteresis, indicating a stable SEI layer and dendrite-free feature.The charge/discharge curve of K//KFSI-DME//K symmetric cell is comparable to that of Na-K//KFSI-DME//Na-K symmetric cell, but it shows a greater voltage hysteresis at each current density.As the current density increases, the voltage hysteresis becomes more significant, particularly at high current density (8 mA cm À2 ), suggesting the formation of an unstable SEI and the subsequent growth of dendrites.This finding is consistent with the results of the aforementioned operando optical observations.Notably, the Na-K//KFSI-DME//Na-K symmetric cell continues to cycle stably when the current density is reset to 1 mA cm À2 , while the K//KFSI-DME//K symmetric cell fails quickly.To highlight the advantages of the Na-K alloy electrode, we also tested the cycling stability of these two types of symmetric cells.As shown in Figure 6h, the Na-K//KFSI-DME//Na-K symmetric cell demonstrated stable cycling over 100 h with a smaller voltage hysteresis at 2 mA cm À2 and 1 mAh cm À2 , which can be attributed to the stable SEI layer and dendrite-free deposition.In contrast, the K//KFSI-DME//K symmetric cell exhibited relatively large voltage hysteresis and severe fluctuations after only 4 h.Moreover, we conducted a comparison between the deposition morphology on a Cu electrode using K and Na-K alloy electrodes in KFSI-DME electrolyte.The Cu electrode was subjected to a constant current of 1 mA cm À2 for 2 h.As shown in Figure S5, Supporting Information, the Cu electrode using the K electrode displayed a loose structure composed of particles.In contrast, the Cu electrode using the Na-K alloy electrode exhibited a much smoother and more uniform deposition morphology, which can be attributed to the presence of the Na-K liquid alloy and its fluidity nature.This result further confirms the superiority of the Na-K alloy electrode in KFSI-DME electrolyte, which can be brought out by co-deposition.
As illustrated in Figure 7, the differences in the above dendrite growth behaviors can be explained by the rate competition of K-deposition (V K-de ), Na-deposition (V Na-de ), and their dissolution into the liquid Na-K electrode (V di ) at different current densities.At low current densities, both fluxes of Na and K in electrolyte are limited, resulting in a relatively low V Na-de and V K-de .As a result, the deposited Na or K beneath the SEI can quickly dissolve into the bulk Na-K alloy, keeping the mass ratio of Na and K at the alloy surface within the range of the liquid phase, which prevents dendrite formation (Figure 7a 1 ,a 2 ).However, with continuous addition of Na/K, the liquid alloy naturally expands, leading to stress accumulation and crack formation at the weak points of the SEI layer (Figure 7a 3 ).The freshly exposed alloy continues to react with the electrolyte to form a new SEI layer, which repeatedly breaks and repairs, thus resulting in the evident contrast change in Figure 6a.
It can be expected that in a K þ -based electrolyte, K þ is the dominating cation carrier, which would increase more significantly than Na þ as the current density increases.Therefore, at a moderate current density (2 mA cm À2 ), the V K- de increases much faster and will exceed the limited V di into the liquid Na-K alloy.This results in a rapid increase in the concentration of K on the surface of the liquid alloy.Once the concentration of K in certain regions becomes higher than the maximum solubility of K in liquid alloy, solid K deposits will nucleate and grow, evolving into dendritic clusters in the initial stage (Figure 5c and 6b 1 -b 2 ).With the continued depletion of K þ ions, K deposition would slow down due to the decreased K þ concentration in the electrolyte around the growing dendrite tips.Meanwhile, the local concentration of Na þ increased and its deposition rate at dendrite tips can even surpass that of K.That is because of the lower Gibbs free energy required for Na plating, as well as the electrostatic shield effect of Na þ that can further prevent K deposition (Figure 6b 2 ). [36]As a result, the deposited Na reacted with K dendrites to form a liquid-alloy phase at the tips, causing the shrinkage of K dendrites.Moreover, the K dissolution (at the base of the dendrite clusters) into the liquid Na-K electrode still continued, further leading to the size shrinking and even vanishing of the clusters (Figure 6c,b 4 ).
At a high current density, the rate of K deposition will greatly exceed the rate of K dissolving into the bulk Na-K alloy (V K-de >> V di ), and therefore the concentration of K rises dramatically on the alloy surface, leading to the fast growth of solid K dendrites (Figure 7c 1 ,c 2 ).Although the tip shrinkage (due to Na-K co-deposition) and root dissolution (into the liquid alloy) of the dendrites still occur, their rates are quite limited compared with the fast K dendrite growth (Figure 7c 3 ).Consequently, Na-K co-deposition can hardly make an appreciable influence on K dendrites growth at a high current density.This suggest that to realize dendrite-free deposition at high current densities, higher level of Na-K co-deposition is required, which can be achieved by, e.g., increasing the proportion of Na salt in the K-based electrolyte.

Conclusion
In summary, operando optical observations have been conducted on the metal deposition and dendrite growth in Na-K alloy-based half and symmetric batteries.It is revealed that the salt in electrolyte is a critical factor in determining the type of metal deposition in EC/DEC-based electrolytes.The solvent also plays a significant role on metal deposition, including altering the dendrite morphology and mono-/co-deposition mode.In particular, Na-K co-deposition can occur in ether solvent (such as DME), which leads to the formation of liquid Na-K alloy and shrinkage of the dendrites.Furthermore, the magnitude of current density directly determines the dendrites morphology evolution.Specifically, low current densities enabled dendrite-free deposition; moderate current densities can cause the shrinking and even vanishing of the early-formed dendrites by Na-K co-deposition; high current densities, however, failed to achieve dendrite-free deposition.Our work demonstrates the feasibility of concurrent use of both Na and K in Na-K alloy anodes, while preventing dendrite formation by controlling the current density and carefully selecting the electrolyte's salts and solvents.

Experimental Section
Preparation of Na-K Alloy Electrode: The liquid Na-K alloy was obtained by mixing Na (5 g) and K (5 g) in a glass bottle and shaking it vigorously for 2 min.The Na-K alloy was then placed onto the copper foam using vacuum infiltration (Figure 1d).
Operando Optical Microscopy Characterization: Operando optical microscopy experiments were conducted on a NIKON LV100ND metallographic microscope (Figure 1b).Optical images were captured using a digital camera and then transferred to a computer for analysis (Figure 1c).After constructing the visualization cell (Figure 1a and Schematic in Figure S1, Supporting Information), the electrode material was secured between the vertical sections of two copper sheets within an Ar-filled glove box.The vertical section of outer copper sheet-2 was cut into a pit to expose the electrode material's surface, with a width of 1 cm and a vertical height of 0.2 cm.Based on these dimensions, the electrode area was estimated to be 0.2 cm 2 .No separator was used in the visualization cell, and the electrodes were held 2 mm apart from each other, without any compressive forces acting on the electrode surfaces during electrodeposition.The visualization cell was then filled with a sufficient amount of electrolyte to immerse the electrode material entirely before being sealed with transparent glass and glue to avoid contact with air.The completed cell was removed from the glove box and placed on an OM stage, which allowed for real-time bright-and dark-field imaging.An observation window made of transparent glass on top of the cell was used to monitor the cross-section of the electrode material.Electrochemical measurements were carried out at room temperature using an automatic galvanostatic discharge-charge system by varying constant currents.
Material Characterization: The morphology and structural characterizations were performed by SEM (Zeiss SIGMA) and TEM (FEI Talos F200s, 200 kV).X-ray photoelectron spectra (XPS) were acquired on an EscaLab Xi spectrometer using Al K Alpha radiation.The visualization cells were disassembled in an Ar-filled glove box, and the deposits were transferred into the SEM or TEM through a vacuum device to prevent any contact with air.
Electrochemical Measurement: To fabricate the Na-K alloy electrode, 0.6 mL of Na-K alloy was mixed with 100 mg of Super P and stirred for a few minutes until a uniform liquid alloy was obtained.This mixture was coated on Super P particles to form a thick and sticky fluid, which was then painted onto a Cu current collector.All cells were assembled using 2025type coin cells and a glass-fiber separator in an Ar-filled glovebox with O 2 and H 2 O concentrations of less than 0.01 ppm.Electrolytes used in the experiments included 1 M NaClO 4 -EC/DEC, 0.8 M KPF 6 -EC/DEC, and 1 M KFSI-DME, with each cell containing approximately 160 μL of electrolyte.All electrochemical measurements were performed using a NEWARE battery test system at room temperature (25 AE 2 °C).To analyze the composition and micro-morphology of sedimentary products, Na-K alloy electrodes were used as counter electrodes in 1 M NaClO 4 -EC/DEC, 0.8 M KPF 6 -EC/DEC, and 1 M KFSI-DME electrolytes, while bare Cu foils were used as working electrodes.The SEI formed on the copper foil was analyzed by XPS.In addition, Na-K alloy and K were used as counter electrodes, 1 M KFSI in DME was employed as the electrolyte, and bare Cu foils were used as working electrodes to observe the microstructure of the metal deposited on the copper foil by SEM.For symmetric cell tests, Na-K alloy and K were used as electrodes, and 1 M KFSI in DME was employed as the electrolyte.A stripping/plating procedure was performed at different current densities.

Figure 1 .
Figure 1.a-c) Photos of operando OM set-up for observing dendrite growth.d) Photos of Na-K alloy and Na-K alloy@Cu foam.e-f ) Schematics depicting Cu||Na-K half cell and Na-K||Na-K symmetric cell.

Figure 2 .
Figure 2. Metal deposition behaviors on Cu electrode in Cu||Na-K half cells with different electrolytes.Time-resolved operando OM images and the corresponding EDS spectrum of dendrites in a,b) 1 M NaClO 4 -EC/DEC electrolyte and c,d) 0.8 M KPF 6 -EC/DEC electrolyte, respectively.

Figure 3 .
Figure 3. Metal deposition behaviors Cu electrode in Cu||Na-K half cell with 1 M KFSI-DME electrolyte.a-k) Time-resolved operando OM images.l) The corresponding EDS spectrum of dendrites.

Figure 5 .
Figure 5. Evolutions of dendrites in different K þ -based electrolytes.Time-resolved OM images and a-c) schematics of dendrite evolutions in KFSI-EC/DEC electrolyte and d-f ) KFSI-DME electrolyte.

Figure
Figure6band Movie S6 in Supporting Information).Remarkably, no dendrite formation was observed on the surface of the Na-K alloy even after prolonged deposition, indicating that the deposits were fully integrated into the liquid alloy.When the deposition was performed at a moderate current density of 2 mA cm À2 , the formation of dendritic structures could be clearly observed (Figure6c).More impressively, when dendrites reached to a certain size, they stopped growing, dissolved back into the alloy, and even vanished with further deposition (as seen in Dendrite-II in Figure6c, Movie S7 in Supporting Information).This phenomenon suggests that dendrite-free deposition can be achieved under such a circumstance, which

Figure 6 .
Figure 6.Metal deposition behaviors on the Na-K alloy electrode in Na-K||Na-K symmetric cells at different current densities.a) Time-resolved operando OM images showing crack propagation at the current density of 0.5 mA cm À2 .b) Width of crack plotted as a function of time.c) Time-resolved operando OM images showing the dendrites growth and shrinkage at the current density of 2 mA cm À2 .d) Width of dendrites plotted as a function of time.e) Time-resolved operando OM images showing the dendrites growth at the current density of 10 mA cm À2 .f ) Length of dendrites plotted as a function of time.g) Rate performance of the symmetric cells with the Na-K alloy and K electrodes.h) Galvanostatic cycling of Na-K//KFSI-DME//Na-K and K//KFSI-DME//K symmetric cells at 2 mA cm À2 /1 mAh cm À2 .