Optimization of structural expansion and contraction for TiS 2 by controlling the electrochemical window of intercalation/ delithiation †

A reversible layered structure of TiS 2 with relaxation like a spring, can be achieved by controlling the optimized potential range to 0.9–2.8 V ( vs. Li + /Li) to yield a high discharge capacity, long cycling life, and excellent rate performance. The discharge capacity density and Coulombic efficiency for TiS 2 at the 100th cycle at 0.1 mA cm (cid:2) 1 and 0.9–2.8 V are 321 mA h g (cid:2) 1 and B 100%, respectively, which are more effective and better than those over potential ranges of 0.8–2.8 V and 1.0–2.8 V. In response to this, the deeper lithiation of TiS 2 at 0.8 V leads to the formation of irreversible Ti 2 S and results in poor cycling performance. The reduced lithiation of TiS 2 at 1.0 V results in low capacity density. Therefore, the rela-tionship between the lithiation depth, structural stability, and electrochemical characteristics of TiS 2 as an electrode material is clarified. Controlling the optimized potential range for structural stability during discharge/charge is thus an effective way of improving electrochemical performance.


Introduction
][3] TiS 2 as an anode material with a four-electron electrochemical reaction demonstrates a high theoretical capacity density of 956 mA h g À1 . 4,5The interlayer spacing (5.703 Å) of TiS 2 with a layered structure is greater than that (3.4 Å) of graphite. 6,7herefore, TiS 2 exhibits rapid intercalation/deintercalation and diffusion of lithium ions, and good electronic conductivity. 8Therefore, TiS 2 has great potential for application as an electrode material.
In LIBs, the lithiation/delithiation of TiS 2 can be divided into two processes (Equation (S1) and (S2), ESI †). 9 The discharge potential of TiS 2 is higher than 1.4 V, and a small amount of lithium ions intercalate the TiS 2 layers and form Li x TiS 2 (0 o x r 1).In this process, which is a typical solid solution reaction, the electrode materials exhibit good reversibility.As additional lithium ions intercalate into the Li x TiS 2 (1 o x r 4), TiS 2 gradually transforms into Li 2 S and Ti, and the layered structure is destroyed.Excessive lithium intercalation of TiS 2 causes fracturing of the crystal structure and reduces the cycling stability of the material. 10Therefore, the incompatibility of the capacity density and cycling performance of TiS 2 is a key issue.TiS 2 with a special morphology, and its composites with other materials, can be effectively used to improve electrochemical performance, but most of these synthesis processes are complex and difficult. 11,12Therefore, it is extremely important to find a simple and convenient way to achieve produce TiS 2 that exhibits both a high capacity density and long cycling life.
In previous studies on sulfur 13,14 and MnO 2 , 15 it was found that by controlling the discharge/charge potential range of the electrode materials, their cycling lives and capacity densities could be improved.This suggests that it may be possible to achieve increased capacity density and cycling performance by adjusting the lithiation depth of TiS 2 .Obviously, if the critical point of the stable layered structure of TiS 2 during discharge/ charge is found, these goals could be achieved.Therefore, the evolution of the crystal structure during discharge/charge and the relationships between the electrochemical window, capacity density and cycling life need to be studied in depth.
In this paper, based on powder X-ray diffraction (XRD), selected area electron diffraction (SAED), and electrochemical methods, the lithiation/delithiation processes of TiS 2 over various potential ranges were systematically studied.It was found that lithium ions intercalate to generate Ti 2 S after a series of phase transitions (TiS 2 -LiTiS 2 -Ti 2 S).The formation of irreversible Ti 2 S results in TiS 2 being produced over 0.8-2.8V, which promotes poor cycling performance.Therefore, by controlling the potential window to 0.9-2.8V, the formation of Ti 2 S is avoided, and the Coulombic efficiency and cycling life of TiS 2 are improved.This represents a new approach for enhancing the lithium storage performance of layered transition metal sulfides.

Morphology and crystal structure
As shown in Fig. S1 (ESI †), the diffraction peaks of the sample are consistent with hexagonal TiS 2 (PDF#15-0853).However, it is important to note that the intensity ratios of the ( 101) and (001) peaks in this work are quite different from those of the standard diffraction pattern.The (001) peak intensity is the strongest among the diffraction peaks.During Rietveld refinement, the preferred orientation was carefully considered, and this was an important step in improving the fitting and minimizing the difference.This result suggests a preferential orientation of the (001) plane of TiS 2 . 16TiS 2 has a typical layered structure, with each layer composed of two hexagonal closepacked S atoms.Ti is located in the center to form an octahedron.The layers of TiS 2 are connected via weak van der Waals forces with a layer spacing of 5.691 Å. 17 TiS 2 has two active sites (octahedral and tetrahedral) for lithium ion storage. 18,19Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images indicate that TiS 2 comprises oval flakes with a particle size of B300 nm (Fig. S2, ESI †).

Electrochemical performance
2.2.1 Cycling performance.The galvanostatic discharge/ charge (GDC) curves of the samples with various electrochemical windows are shown in Fig. S3a and b (ESI †).The capacity densities for the first and 50th discharges over 0.01-2.8V are 1527 and 19 mA h g À1 , respectively.Based on early studies, the low capacity retention rate is possibly due to the volumetric expansion of crystals during lithium intercalation/deintercalation, which causes the fracturing of crystals. 10The capacity density for TiS 2 after the 50th discharge over a potential range of 1.5-2.8V is 189 mA h g À1 and 88.9% of the first discharge capacity is retained.Obviously, the cycling performance of TiS 2 over 1.5-2.8V is better than that over 0.01-2.8V. TiS 2 with deep lithiation converted to Li 2 S and Ti.Excessive lithiation caused volumetric expansion and even crystal crushing, leading to poor cycling performance.
This work aimed to clarify the threshold of the electrochemical windows at which TiS 2 could maintain its layered structure and exhibit excellent cycling stability.A wide window (e.g., 0.01-2.80V) with high specific capacity and poor cycling stability and a narrow window (e.g., 1.50-2.80V) with low specific capacity and good cycling stability are compared in Fig. S3c (ESI †).The results urged us to think about the threshold of the electrochemical windows in terms of both good cycling stability and high specific capacity.Based on these results, it was necessary for us to conduct full research to identify this threshold.
The ratio of the actual capacity density to the theoretical capacity density of TiS 2 from 1.5-2.8V is 0.79, therefore B0.79 mol lithium ions are intercalated in TiS 2 when it is discharged to 1.5 V.The good cycling performance of TiS 2 over 1.5-2.8V indicates that its crystal structure is reversible and stable during lithiation/delithiation.Comparing the capacity density and cycling performance of TiS 2 over various potential ranges, it was observed that TiS 2 with a narrow potential range exhibits increased cycling performance and decreased capacity density.TiS 2 with a wide potential range exhibits increased capacity density and poor cycling performance.Therefore, it is necessary to find a suitable discharge/charge potential window for TiS 2 to exhibit both a long cycling life and high capacity density.
The cycling characteristics of TiS 2 over various potential ranges are shown in Fig. 1a.TiS 2 over 0.9-2.80V exhibits excellent cycling performance, with discharge capacity densities of the first and 100th cycles of 321 and 213 mA h g À1 , respectively.The Coulombic efficiency at the 100th cycle is close to 100%.The discharge capacity density and Coulombic efficiency after 100 cycles for TiS 2 from 0.8-2.8V are 58 mA h g À1 and 90%, respectively, and the capacity density is far lower than that of the first cycle (405 mA h g À1 ).Obviously, TiS 2 with a potential range of 0.9-2.8V exhibits the best electrochemical performance.This is related to the structural evolution of TiS 2 during lithiation/delithiation.   9 The smaller differences in the redox peak potentials at B2.31/B2.34V and B1.71/B1.86V show that the reversibilities of the discharge/charge reactions over 0.9-2.8V and 1.0-2.8V are better than those over 0.8-2.8V.The currents of the redox peaks also support this conclusion.The dQ/dV curves over 0.9-2.8V and 1.0-2.8V feature another pair of redox peaks at B1.76/B1.86V, which indicates an additional lithiation/delithiation process for Li x TiS 2 (0 o x r 1).However, there are no corresponding peaks at B1.76/B1.86V in the dQ/dV curves over 0.8-2.8V, which indicates that the TiS 2 crystal undergoes an irreversible phase transition when discharged at 0.8 V. Obviously, this crystal structure and phase change affect the electrochemical behavior during the discharge/charge processes below 0.9 V.
2.2.3 Rate performance.The rate performance of TiS 2 over various potential ranges is shown in Fig. 1c.TiS 2 over 0.9-2.8V exhibits good rate performance, with discharge capacities at 0.05, 0.1, 0.2, 0.5, 1, 2, and 0.1 mA cm À1 of 325, 240, 215, 190, 170, 145, and 250 mA h g À1 , respectively.In contrast, its rate performance over 0.8-2.8V is poor, and its discharge capacity and Coulombic efficiency at 2 mA cm À2 are 85 mA h g À1 and 55%, respectively.Therefore, by controlling the potential range of TiS 2 to 0.9-2.8V, not only does this improve the capacities, but the reversibility of the lithiation/delithiation is also promoted.
2.2.4 Cyclic voltammetry (CV) curves.The first CV curves of TiS 2 over 0.8-2.8V exhibit reduction peaks at 1.19, 1.39, 1.64, 1.75, 2.08, and 2.33 V (Fig. 2a).These reduction peaks correspond to the formation of Li x TiS 2 (0 o x r 1). 9 The oxidation peaks of over 1.5 V correspond to the step-by-step delithiation process.There are no reduction peaks at B1.75 V and an oxidation peak at B1.88 V in the second CV curve, indicating that TiS 2 undergoes an irreversible phase transition after the first cycle.These results from the CV curves over 0.8-2.8V are consistent with those from the dQ/dV curves (Fig. 1b) and GDC curves (Fig. 2b).The first discharge capacity density and the coulombic efficiency of TiS 2 over 0.8-2.8V are 270 mA h g À1 and 63.8%, respectively.The low Coulombic efficiency might be attributed to electrolyte decomposition and the formation of solid electrolyte interface (SEI) films. 9The capacity degradation mainly occurs in the range of 1.5-2.0V.In the following cycle, the plateau of the GDC curve weakens and gradually takes the form of an inclined line.The CV data show a similar trend, and the currents for the redox peaks of CV in the range of 1.5-2.0V decrease with increasing cycle number.
The redox peaks of TiS 2 over 0.9-2.8V can still be observed at B1.75/B1.88V in the following CV curve (Fig. 2c), and their peak currents increase with increasing cycle number.Therefore, the crystal structure of TiS 2 during GDC in the range of 0.9-2.8V is reversible, and the GDC curves also show the same phenomenon (Fig. 2d).The plateau length of the discharge curve elongates with increasing cycle number, and the plateau potential of 1.71 V is higher than the potential that reaches 1.69 V over 0.8-2.8V.The CV and GDC curves in the range of 1.0-2.8V are similar to those in the range of 0.9-2.8V Fig. 2 CV curves at 0.1 mV s À1 and discharge/charge curves at 0.1 mA cm À2 of TiS 2 over (a and b) 0.8-2.8V, (c and d) 0.9-2.8V, and (e and f) 1.0-2.8V.

Materials Advances Paper
Open (Fig. 2e and f), indicating that they show the same lithium storage behavior.

Electrochemical reaction kinetics 2.3.1 Diffusion coefficient.
The lithium ion diffusion coefficient (D Li +) of TiS 2 from the open circuit voltage (OCV) to 0.01 V during the first discharge process was studied by galvanostatic intermittent titration (GITT), where D Li + was calculated using eqn (S3) (ESI †). 20,21As shown in Fig. S4 (ESI †), D Li + shows an increasing trend during the initial discharge stage (1.92-0.9V), and reaches a maximum value at 0.9 V (except for the initial stage) as the lithium ions mainly intercalate within the interlayers of the TiS 2 .The interlayer distance of TiS 2 increases, and the energy barrier for lithium ion diffusion is reduced.The discharge potential is lower than 0.9 V, and additional lithium ions are inserted into the Li x TiS 2 (0 o x r1), and some of the Li x TiS 2 (0 o x r 1) undergoes a conversion reaction.The layered structure is destroyed by the large interfacial stress resulting from the transformation reaction.At this time, lithium ion diffusion requires a higher energy barrier, so D Li + decreases as the depth of discharge (DOD) increases.
2.3.2Impedance evolution.The decay mechanism of TiS 2 over 0.8-2.8V during GDC was explored using EIS with an equivalent circuit diagram (Fig. S5, ESI †), which features solution resistance (R s ), charge transfer resistance (R ct ), and SEI resistance (R SEI ). 22The semicircle in the high-frequency region is dominated by the charge transfer process, and the diameter of the semicircle represents R ct , which indicates that the process is kinetically controlled.The spectrum in the low-frequency region has the appearance of a straight line, which indicates that the process is a diffusion controlled. 23he first discharge/charge curves and EIS curves of TiS 2 at various DODs and depths of charge (DOCs) are shown in Fig. S6 (ESI †).The R ct value gradually decreases during the discharge process and increases during the charge process (Fig. 3a and b), and the R ct is related to the lithium molar content in TiS 2 .The R SEI gradually increases during the early discharge stage, which means that the SEI film is continuously generated.The R SEI value of TiS 2 at a DOD of approximately 50% (B0.8V vs Li + /Li) is at a maximum, which indicates that the SEI film on the electrode surface is unstable.
The EIS curves of TiS 2 from discharge to the cutoff potential (0.8 or 0.9 V) and charge to 2.8 V are shown in Fig. S7 (ESI †).The SEI film over 0.9-2.8V was generated during the first discharge and basically remained stable throughout the cycle process (Fig. 3c and d).The R ct of the regions 0.8-2.8V and 0.9-2.8V gradually decrease with increasing cycle number, which is related to the electrode active material.The exchange current density at 0.9-2.8V is higher than that at 0.8-2.8V, indicating that TiS 2 at 0.9-2.8V shows improved reversibility for a low R ct value and a stable SEI film (Fig. S8, ESI †).
2.3.3Capacity contribution mechanism.The characteristic CV values for TiS 2 at various scan rates were obtained using Equation (S4)-(S7) (ESI †), which were used to determine the capacity contribution mechanism of TiS 2 at 0.8-2.8V and 0.9-2.8V. 24,25 The two CV curves exhibit similar shapes, and View Article Online the current intensities of the redox peaks increase with an increase in the scan rate (Fig. 4).The b values of the reduction and oxidation peaks of TiS 2 at 0.9-2.8V are 0.64 and 0.50, respectively.Therefore, the electrochemical reaction of TiS 2 at 0.9-2.8V is mainly diffusion controlled.Simultaneously, the capacity ratio for the capacitive behavior of TiS 2 increases from 37.5% to 64% upon an increase in the scan rate (Fig. 4a-c).The b values of the reduction and oxidation peaks of TiS 2 at 0.8-2.8V are 0.64/0.75 and 0.65/0.92,respectively, and the corresponding electrochemical reactions are dominated by capacitance and diffusion.The proportion of capacitive behavior increases from 72.8% to 88.7% upon an increase in the scan rate (Fig. 4d-f), which is higher than that at 0.9-2.8V.It is generally believed that capacitive behavior accelerates the kinetics of electrochemical reactions. 26Therefore, the key factor is not the contribution of capacitive behavior, but the integrity of the TiS 2 crystal structure during GDC that led to the excellent rate performance of TiS 2 at 0.9-2.8V.

Lithium storage mechanism
The above results show that TiS 2 at 0.9-2.8V shows improved cycle life and capacity density, and that the poor cycling performance of TiS 2 at 0.8-2.8V is due to the irreversible phase transition of the layered structure (Table S1, ESI †).The XRD patterns of TiS 2 at various DODs and DOCs are shown in Fig. 5a-c.Lattice expansion is observed from the XRD patterns.The c value of the TiS 2 unit cell during discharge at points A (TiS 2 , 2.68 V), B (Li 0.18 TiS 2 , 2.10 V), and C (Li 0.68 TiS 2 , 1.76 V) are 5.69, 5.83 and 6.29 Å, respectively.During the initial discharge process, lithium ions intercalate in the interlayers of TiS 2 , occupying the octahedral position and resulting in a significant increase in the interlayer distance. 27The layered structure shows excellent reversibility for a charge/discharge process, as evidenced by the shift in the (001) peak.The characteristic peak for Ti 2 S is also observed when discharged to point E.During the charge process from point E to point H, the peak intensities of Ti 2 S decrease with increasing DOC, but these peaks do not completely disappear.Therefore, the irreversible Ti 2 S produced by the partial conversion reaction is the reason for the poor cycling performance of TiS 2 at 0.8-2.8V.The layered-structure of the original TiS 2 can be retained by carefully controlling the lithiation depth, e.g., setting a rational electrochemical window of 0.9-2.8V.A further discharge process leads to an irreversible Ti 2 S, therefore resulting in poor cycling stability. 28he SAED results of TiS 2 during GDC are shown in Fig. 5d-i.The original TiS 2 exhibits a typical 1H structure, and the diffraction spots are arranged symmetrically in a hexagon, which indicates that it is single crystalline in nature. 29The interlayer spacing of TiS 2 upon discharge to 0.9 V increases for the formation of Li x TiS 2 , and the layered structure is not destroyed (Fig. 5d).Upon discharge to 0.8 V, the electron diffraction pattern changes from diffraction spots to diffraction rings, which indicates that the material is polycrystalline (Fig. 5i).Simultaneously, new diffraction rings appear with interplanar spacings of 2.15 Å and 1.36 Å, related to the (051) and (062) planes of Ti 2 S, which are consistent with the XRD results.A deep discharge of TiS 2 causes it to undergo a conversion reaction.
Theoretical calculations were employed to further investigate the structural evolution.The geometries of Li x TiS 2 (x = 0-3) were optimized using the SIESTA 4.0 package, and the results are shown in Fig. S10 (ESI †).The expansion in the volume is only 4.5% when one Li + is inserted into the structure, while an approximately 15.5% change occurs on going from LiTiS 2 to Li 2 TiS 2 .This result means that Li 2 TiS 2 is unable to maintain its original structure due to drastic structural changes.These results are consistent with the experiment values.The irreversible Ti 2 S phase was identified by XRD and SAED when 2 Li + ions were inserted into TiS 2 .
Here, we provide a detailed summary of the structural evolution of TiS 2 .The layer spacings of pristine TiS 2 and at 0.9 V are 5.70 Å and 5.81 Å. Upon discharge to 0.8 V, a new phase comprising Ti 2 S is generated, and the layered structure is destroyed.At 0.01 V, Li 2 S and Ti are produced in a conversion reaction.Over the optimized potential window of 0.9-2.8V, TiS 2 retains its layered structure, therefore exhibiting excellent cycling capability.The irreversible Ti 2 S at 0.8-2.8V leads to increased capacity density and reduced cycling performance of TiS 2 .

Conclusions
The layered structure of TiS 2 over an optimized potential range of 0.9-2.8V is retained, and the goals of high capacity density, long cycling life, and excellent rate performance were achieved.The discharge capacity density and Coulombic efficiency of the 100th cycle at 0.1 mA cm À1 are 321 mA h g À1 and B100%, respectively.TiS 2 upon discharge to 0.8 V generates Ti 2 S in a conversion reaction, and the irreversible Ti 2 S leads to poor cycling performance.The discharge capacity of TiS 2 at 0.8-2.8V for the 100th cycle is only 58 mA h g À1 .
Controlling the potential range of TiS 2 during discharge/charge could thus be used to improve the Coulombic efficiency and cycling life of other layered electrode materials, making it an effective and simple method to achieve excellent electrochemical performance of materials by adjusting the depth of lithiation.
Fig. 1 (a) Cycling performance at 0.1 mA cm À2 , (b) dQ/dV curves of the 25th discharge/charge curves at 0.1 mA cm À2 , and (c) the rate performance of TiS 2 across various electrochemical windows.

Fig. 3
Fig. 3 Impedances of TiS 2 at various (a) DODs and (b) DOCs during the first discharge/charge processes and at full states of discharge/charge for various cycles over (c) 0.8-2.8V and (d) 0.9-2.8V.