# Spontaneous Lithiophilic and Lithium-Ion Conductive Functional Layer Formation Enabled by Solution-Casted Zinc Nitride for Highly Stable Lithium Metal Electrode in Carbonate Electrolyte

Authors: Ki Jae Kim; Han Jun Leem; Jisang Yu; Hyun-seung Kim
Journal: International Journal of Energy Research
 (2023)

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## Abstract

To improve the reversibility of the Li metal negative electrodes, the simultaneous formation of individually functionalized zinc metal and lithium nitride during the spontaneous conversion reaction of zinc nitride particle is conducted by casting of zinc nitride particles on the lithium electrode. Lithiophilic zinc metal reduces the nucleation polarization of the lithium electrode, whereas the highly ionic conductive and electronically insulating Li3N decreases the concentration polarization from the facile ion conduction and suppression of further electrolyte decomposition during Li plating and stripping. Both these sophisticated characteristics resulted in an improvement in the deposition morphology of lithium electrode, thereby enhancing the reversibility of lithium electrode. Consequently, the cycleability improvement of Li/LiNi0.8Co0.1Mn0.1O2 is achieved through the application of zinc nitride surface-treated lithium electrode.


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## Body

## 1. Introduction

Lithium metal electrode-based secondary battery systems are regarded suitable for use as high energy post-lithium-ion batteries owing to their sophisticated high-specific capacity (3861 mAh g-1) and low-working voltage (0.0 V vs. Li/Li+) [1–7]. However, the limited reversibility of lithium electrode in the liquid electrolytes hinders the commercialization of lithium metal batteries (LMBs) [8–10] as it degrades the surface morphology of the negative electrode, resulting in internal shortage of cell. Further, the subsequent electrolyte decomposition on a newly deposited lithium surface results in the sudden death of the LMB system via electrolyte dry-out. Thus, the morphological control of the lithium electrode is crucial for enhancing cycleability of LMBs [2, 11–14]. Certain promising approaches to overcome the reversibility problem of lithium electrode are electrolyte compositional engineering [9, 15–17], surface treatment of the lithium electrode [18], and the separator modification [19–21]. Recently, the surface modification of lithium electrode was highlighted as a mitigation strategy, because the degradation mechanism targeted surface control was achieved through the pretreatment of lithium electrode [18]. The most crucial characteristic of the functional layer on the lithium electrode is the controlling ability of the overpotential developed during Li deposition and stripping, which is largely determined by the kinetics of the lithium nuclei formation and the Li-ion conduction during continuous cycling [22, 23]. Recent studies have indicated that the formation kinetics of the lithium nuclei can be manipulated using lithium-miscible metal species, such as zinc, silver, and gold metals [24–26]. Metal species that can be readily alloyed with the lithium-ions to yield formed lithium alloy can aid in the decrease of nucleation overpotential during galvanostatic cycling. Meanwhile, the highly Li-ion conductive and electronically insulating interlayer also plays a crucial role in reducing the side reaction derived from the cathodic decomposition of the electrolyte solution [24, 27]. The solid electrolyte interphase (SEI) layer functions as per the aforementioned role on the Li metal; however, the chemical composition derived from the typical electrolyte solution is not appropriate for the Li electrode. Thus, the introduction of a highly ionic, conductive, and electronically insulating layer is crucial [15, 17, 24, 27].In this study, the introduction of lithiophilic seeds with electronically insulating and highly ionic-conductive functional layer is performed simultaneously on lithium electrode. To achieve this primary objective, the lithium metal surface was modified using zinc nitride. As the negative Gibbs free energy value is obtained by the decomposition of Zn2N3 to Zn0 and Li3N compounds, the spontaneous conversion reaction of the zinc nitride [28, 29] to the metallic zinc and lithium nitride occurred on the lithium electrode, which enhanced the developed overpotentials. Furthermore, the lithiation of the zinc metal generated lithiophilic seeds, which reduced the nucleation overpotential of Li deposits. Moreover, the Li3N compound is the typical chemical composition for electronically insulating and ionic conductive compounds, which is adequate for the reversible Li metal cycling. Consequently, the reversible lithium electrode was obtained through the modification of the Li surface via solution-processed zinc nitride surface modification.


## 2. Experimental

### 2.1. Surface Modification of Lithium Metal Electrode

The solution-processed casting of lithium electrode (Cu-attached, 100μm, Honjo Metal) was conducted using 0.1 wt.% zinc nitride (Alfa Aesar, 99%, metals basis) suspended 0.2 M LiFSI (NIPPON SHOUKUBAI) in dimethyl carbonate (DMC, Sigma-Aldrich) solution. The dip-coating of the suspension on the lithium electrode was performed to introduce a uniform coating layer on the lithium surface. Subsequently, the surface-modified lithium electrode was dried at 25°C overnight.


### 2.2. Coin Cell Fabrication and the Electrochemical Characterization

A 2032-coin cell was fabricated to characterize the surface-modified lithium electrode. The symmetric lithium cell was prepared using the lithium/separator/lithium stack. The electrolyte used was 1.0 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) with 10 wt.% of fluoroethylene carbonate (FEC) additive (Dongwha Electrolyte, battery grade). The time-dependent electrochemical impedance spectroscopy (BioLogics) was recorded at the Li-symmetric cell with the frequency range of 5.0 MHz to 100 MHz at open circuit voltage (OCV) state. In addition, galvanostatic cycling of the Li-symmetric cell (Maccor) at a current density of 2 mA cm-2 was performed with 1.0 h during both lithium deposition and stripping period to evaluate reversibility of lithium electrode. Further, the Li/LiNi0.8Co0.1Mn0.1O2 (NCM811, BTR) cell was fabricated to measure cycleability. The charge and discharge cycling was performed after single 0.1 C constant current-constant voltage (CC-CV, 0.05 C current cut-off) formation cycling within a voltage window of 2.5 to 4.3 V (vs. Li/Li+). The cycling of the Li/NCM811 cell was conducted under 0.2 C CC-CV (0.05 C current cut-off) condition.


### 2.3. Scanning Electron Microscopy Characterization

The pristine and cycled lithium electrode was analyzed via scanning electron microscopy (SEM, JEOL, JSM-IT200) in a dry room. Prior to placing the lithium electrode in the vacuum chamber, liquid nitrogen treatment was conducted to densify the electrode morphology.


### 2.4. X-Ray Photoelectron Spectroscopy Measurement

Surface chemical composition was identified via X-ray photoelectron spectroscopy (XPS, Scientific K-alpha, Thermo Fisher) in a dry room. The cycled lithium electrode was rinsed using diethyl carbonate (DEC) solvent to remove the residual lithium salt on the lithium electrode.


## 2.1. Surface Modification of Lithium Metal Electrode

The solution-processed casting of lithium electrode (Cu-attached, 100μm, Honjo Metal) was conducted using 0.1 wt.% zinc nitride (Alfa Aesar, 99%, metals basis) suspended 0.2 M LiFSI (NIPPON SHOUKUBAI) in dimethyl carbonate (DMC, Sigma-Aldrich) solution. The dip-coating of the suspension on the lithium electrode was performed to introduce a uniform coating layer on the lithium surface. Subsequently, the surface-modified lithium electrode was dried at 25°C overnight.


## 2.2. Coin Cell Fabrication and the Electrochemical Characterization

A 2032-coin cell was fabricated to characterize the surface-modified lithium electrode. The symmetric lithium cell was prepared using the lithium/separator/lithium stack. The electrolyte used was 1.0 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) with 10 wt.% of fluoroethylene carbonate (FEC) additive (Dongwha Electrolyte, battery grade). The time-dependent electrochemical impedance spectroscopy (BioLogics) was recorded at the Li-symmetric cell with the frequency range of 5.0 MHz to 100 MHz at open circuit voltage (OCV) state. In addition, galvanostatic cycling of the Li-symmetric cell (Maccor) at a current density of 2 mA cm-2 was performed with 1.0 h during both lithium deposition and stripping period to evaluate reversibility of lithium electrode. Further, the Li/LiNi0.8Co0.1Mn0.1O2 (NCM811, BTR) cell was fabricated to measure cycleability. The charge and discharge cycling was performed after single 0.1 C constant current-constant voltage (CC-CV, 0.05 C current cut-off) formation cycling within a voltage window of 2.5 to 4.3 V (vs. Li/Li+). The cycling of the Li/NCM811 cell was conducted under 0.2 C CC-CV (0.05 C current cut-off) condition.


## 2.3. Scanning Electron Microscopy Characterization

The pristine and cycled lithium electrode was analyzed via scanning electron microscopy (SEM, JEOL, JSM-IT200) in a dry room. Prior to placing the lithium electrode in the vacuum chamber, liquid nitrogen treatment was conducted to densify the electrode morphology.


## 2.4. X-Ray Photoelectron Spectroscopy Measurement

Surface chemical composition was identified via X-ray photoelectron spectroscopy (XPS, Scientific K-alpha, Thermo Fisher) in a dry room. The cycled lithium electrode was rinsed using diethyl carbonate (DEC) solvent to remove the residual lithium salt on the lithium electrode.


## 3. Results and Discussion

The conceptual description of the effect of introducing the zinc nitride on the lithium metal surface is shown at Figure1(a). After contacting of lithium and zinc nitride, a spontaneous lithiation reaction occurred between the lithium electrode and casted zinc nitride particle on the surface of lithium, because the redox potential of the conversion reaction of zinc nitride to lithium nitride and metallic zinc is higher than that of the lithium metal electrode, which means the Gibbs free energy of the aforementioned reaction is negative. The resultant components from the conversion act as a bifunctional layer, which enhances the reversibility of the lithium electrode. The highly ionic conductive and the electronically insulating lithium nitride compound reduces the cathodic side reaction on the lithium electrode; thus, additional SEI formation during the cycle can be mitigated [15, 17, 27, 30, 31]. Moreover, the Li3N compounds also function as the appropriate SEI composition to reduce the nucleation overpotential of the Li electrode. Hence, the morphological control of the lithium electrode is possible owing to the Li3N introduction. The simultaneously formed metallic zinc can be alloyed with the lithium-ions, which facilitate the possibility of the formation of the Li-Zn phase [32]. Because Zn0 is miscible with the lithium-ions, the lithiophilic seeds can be formed on lithium electrode when the lithium electrode was coated with zinc nitride. The lithiophilic composition introduction reduces the nucleation overpotential of lithium electrode, which improves the deposition morphology, thereby enhancing the reversibility of the lithium electrode. Consequently, the functional compound can be bifunctionally introduced through the casting of the zinc nitride particle on the lithium electrode. Figure 1(b) shows the surface characterization of the zinc nitride casted lithium electrode. The surface of lithium electrode was modified with the embedded zinc-based particles, and subsequent Zn signal is obtained from the energy dispersive spectrometry (EDS) map. The EDS spectrum of the modified lithium surface indicates that the zinc and nitrogen Kα signal was observed on the top-most surface of lithium electrode. (Figure S1) Figure 1(c) depicts the surface chemical composition of the modified Li electrode. The Li 1s narrow-scanned XPS spectrum demonstrates the formation of Li-Zn bonding [33], and the simultaneously formed Li3N compound [17, 31] is demonstrated on the surface of Li electrode. Thus, the improvement of reversibility and kinetics of the Li electrode is expected from the surface modification of Li electrode by lithiophilic and highly ionic-conductive species.Figure 1
(a) Schematics of the effect of introducing zinc nitride on the lithium electrode surface, (b) surface SEM image of Li electrode after modification (inset: EDS map of Zn), and (c) Li 1s XPS spectrum of modified Li electrode.
(a)(b)(c)The introduction effects of bifunctional Li3N and lithiated Zn on Li surface was evaluated by the chemical corrosion inhibition ability during storage through time-dependent electrochemical impedance spectroscopy (EIS) measurements (Figure 2(a)). During storage of lithium electrode with carbonate electrolyte, the spontaneous growth of the SEI film and subsequent corrosion of the Li metal is occurred from insufficient electronic insulating property of SEI on Li. Thus, the growth of interphasial resistance is observed during Li/Li symmetric cell storage test [34]. A continuous increase of the resistance was observed at the pristine electrode during 48 h storage, whereas a well-retained resistance profile was demonstrated at surface-modified lithium electrode. Previous study demonstrates the passivation properties of the SEI which greatly influences the chemical corrosion of Li electrode, and thus, the suppression of the chemical corrosion of the lithium electrode was mainly attributed to the electronically insulating Li3N composition formation on the lithium electrode, which decreased the cathodic decomposition of the electrolyte. Hence, the protection of the lithium electrode owing to the functional layer introduction was defined based on the time-dependent chemical corrosion test.Figure 2
(a) Storage-time-dependent Nyquist plots obtained from the pristine and surface-modified Li/Li symmetric cell. (Inset: magnified Nyquist plots of surface-modified Li) Time vs. voltage curves at (b) 0-250 h, (c) 0-0.1 h, (d) 135-145 h, and (e) 220-230 h obtained from pristine and surface-modified Li/Cu/Li symmetric cells, respectively.
(a)(b)(c)(d)(e)Because the highly ionic-conductive Li3N layer introduction can improve the Li-ion transfer in the SEI layer on Li electrode, the concentration polarization developed during Li deposition can be decreased. Moreover, the electric insulating property of the Li3N compound reduced continuous reduction of electrolyte on Li electrode, resulting in the decrease of further formation of the SEI layer on the Li electrode, which increases the resistance of Li electrode. Hence, the continuous resistance growth and Li-ion transfer resistance can be decreased by the Li3N layer. Furthermore, the Li-alloyed zinc compound reduces the nucleation polarization during the Li cycling by lithiophilic property [35]. Hence, the galvanostatic cycling was performed with pristine and surface-modified Li electrode-comprised symmetric cell, because the improvement of initial polarization can enhance the reversibility of the lithium electrode. Figure 2(b) depicts the continuous galvanostatic cycling results obtained from the two electrodes. The pristine Li-comprised cell exhibited a sharp voltage overshoot after 150 h of cycling, which is typically demonstrated by the dendritic lithium-induced internal shortage of symmetric cell [36]. In contrast, the surface-modified lithium electrode exhibited stable polarization curves for over 200 h of cycling time at current density of 2.0 mA cm-2. Hence, as expected, the spontaneously formed bifunctional layer improved the reversibility of the lithium electrode by controlling the polarization development during Li cycling.The mitigation effects of the overpotential development during galvanostatic cycling of the Li symmetric were further evaluated. Figure2(c) shows the initially developed polarization from the pristine and surface-modified Li electrodes. The overall polarization was alleviated owing to the formation of both Li-Zn and Li3N phase on the Li surface. The lithiophilic seed formation on the lithium electrode reduced the polarization during Li deposition, which originated from the improved lithium affinity by Li-Zn modification. Consequently, the nucleation polarization decreased to approximately 0.1 V during initial galvanostatic cycling. Furthermore, owing to the highly ionic conductive nature of the Li3N compound, the concentration polarization during initial cycling period of Li electrode was decreased after nuclei formation. Thus, the reversible Li electrode cycling could be realized by controlling initial polarization during Li cycling. Figures 2(d) and 2(e) show the magnified polarization curves for the 135 to 145 h and 220 to 240 h cycling periods. Following a 135 h galvanostatic cycling period, the polarization at pristine Li electrode was observed to increase, and a voltage overshoot occurred. Meanwhile, the surface-modified Li electrode showed stable cycling curves even after 220 h (Figure 2(e)).Figure3 shows ex situ morphological analysis results of the cycled-Li electrodes. The overall polarization during galvanostatic cycling of Li-symmetric cell was manipulated via the introduction of the zinc nitride coating layer; hence, a smooth deposition morphology was expected from the surface. A highly dendritic and porous deposition morphology was developed at the pristine electrode even during initial cycling period (5th cycle), and the dendritically grown Li was also observed after 50th cycle. The dendritic deposition morphology was attributed to the high polarization during galvanostatic cycling at pristine lithium electrode. Meanwhile, the deposition morphology of surface-modified lithium electrode after repeated deposition and stripping of Li was rather maintained as a smooth morphology, owing to the controlled polarization by lithiophilic zinc and Li3N compounds. Morphological changes in lithium electrodes can change the subsequent cathodic decomposition of the electrolyte on lithium surface as the dendritic growth of Li deposits increases the surface area of the lithium electrode. Consequently, the decomposition of the electrolyte increases, resulting in thick SEI formation. The oxygen-based compound is generally found within the SEI compounds, and thus, the O Kα mapping was performed with two electrodes. High intensity of oxygen signal was observed with pristine electrode, whereas the surface-modified lithium electrode exhibited a limited oxygen signal. Hence, the decomposition of the electrolyte on the dendritically grown Li with pristine electrode is severe, whereas the surface-modified Li electrode shows decreased electrolyte consumption from the rather dense Li deposits formation.Figure 3
Postmortem SEM image and O Kα EDS map of 5th and 50th cycled (a) pristine Li electrode and (b) surface-treated Li electrode.
(a)(b)The decrease in the electrolyte decomposition on the lithium electrode after 50th cycle was further verified by performing a postmortem XPS experiment with two electrodes (Figure 4). The spectrum was compared after 1.0 s of Ar-etching to remove the residual Li salt on surface. The C 1s spectrum indicates the formation of typical lithium carbonate, O=C-O, and O=C-O, and C-O bonds after cycling are observed for both electrodes, which is commonly derived from the reductive decomposition of carbonate electrolytes [37, 38]. Thus, the direct comparison of evolved amounts of SEI compound was possible with pristine and surface-treated Li electrodes. Intense peaks of SEI components were developed at pristine electrode owing to the increased cathodic decomposition of electrolyte by dendritic grown lithium deposits; however, the smooth grown lithium electrode attributed from the bifunctional layer reduced SEI formation on lithium electrode during cycle. Initial nucleation and concentration polarization of the Li electrode is due to the formation of bifunctional surface modifier, and the deposition morphology change of Li ensued. The morphological change of Li deposits reduces the additional electrolyte decomposition after deposition of fresh Li, and hence, the polarization growth during cycling is decreased. Hence, the control of the polarization during cycle altered the morphology of lithium deposits, which decreased the side reaction on the newly formed lithium-electrolyte interface. Thus, the SEI formation on lithium surface was reduced, resulting in improved cycleability of Li electrode.Figure 4
C 1s XPS spectra of 50th cycled (a) pristine lithium and (b) surface-treated lithium electrodes.
(a)(b)The improvement of reversibility of lithium electrode can improve the cycleability of the lithium metal electrode, because the continuous electrolyte consumption from the dendritically grown Li electrode is decreased from the bifunctional layer. Thus, the modified and pristine Li electrode-comprised Li/NCM811 cell was evaluated with 3.0 mAh cm-2 areal capacity of NCM811 electrode (Figure 5). Furthermore, the relatively lean electrolyte (4.0 μLmgNCM−1) was injected in the Li/NCM811 cell to evaluate the reversibility of Li electrode. The cycleability of the surface-modified Li electrode-comprised cell was found to be stable over 300 cycles, whereas the pristine lithium-comprised cell demonstrated a gradually decreasing lithiation capacity of the positive electrode after the 150th cycle (Figure 5(a)). Further, a sudden drop in the lithiation capacity was observed at the 300th cycle in case of the pristine Li/NCM811 cell. In contrast, the surface-modified Li-comprised cell demonstrated stable cycleability. The improvement in the lithium kinetics by surface modification resulted in the enhancement of the rate performance of Li/NCM811 cell. Further, a C-rate dependent discharge capacity retention test was performed. In case of the surface-modified Li electrode, the discharge capacity was well-retained with high C-rates. However, the pristine Li-comprised cell demonstrated low-discharge capacity retention even at 2.0 C-rates (Figure 5(b)). After evaluating power performance of Li/NCM811 cell, the discharge capacity was recovered as high as initial capacity of the Li/NCM811 cell. Hence, the improvement in the Li metal electrode kinetics because of the surface modification resulted in the enhancement of both the cycle and rate performances of the Li/NCM811 cell.Figure 5
(a) Cycleability of Li/NCM811 cell at 0.3 C (0.9 mA cm-2) with 2.5–4.3 V (vs. Li/Li+) of voltage range and (b) charge-discharge rate performance of Li/NCM811 cell with pristine and surface-modified Li, respectively.
(a)(b)


## 4. Conclusions

This study entailed surface treatment of the lithium electrode using zinc nitride-dispersed solution. The spontaneous reaction of solution-casted zinc nitride particles on lithium electrode generated zinc metal and lithium nitride on the surface, which improved the reversibility of the lithium electrode. The improvement in the lithium metal cycleability was mainly because of the decrease in the polarization during plating and stripping of lithium electrode. Further, the decrease in the nucleation polarization was attributed to the lithophilic zinc improving the initial polarization during lithium cycling. Moreover, the development of concentration polarization was successfully suppressed owing to highly ionic conductive and electronically insulating properties of Li3N compounds, which ensured facile lithium conduction and the prevention of further growth of SEI layer on lithium electrode. Furthermore, owing to bifunctional effects of the spontaneously formed compounds, the reversible lithium electrode cycling is achieved, which improved the cycleability of the Li/NCM811 cells.



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*Source: 9526791-2023-02-11.xml*