Low‐Temperature Preparation Copper‐Doped Nickel Chloride Cathode for Thermal Battery Overcomes the Energy‐Power Trade‐Off

Nickel chloride (NiCl2) is a typical hexagonal layered semiconductor material with wide application. However, it is mainly restricted by complicated technological process within ultrahigh dehydration temperature. Utilizing copper doping, a sort of high purity and remarkable crystallinity NiCl2 is fabricated using a simple low‐temperature calcination technique. The dehydration temperature is decreased from 600 to 400 °C because the adsorbed copper ions on NiCl2 dihydrate surface can weaken NiO bond strength. Serving for thermal battery cathode, copper‐doped NiCl2 exhibits remarkable discharge ability at 500 mA cm−2, equipped with supernormal power density of 16.27 kW kg−1 and energy density of 717 Wh kg−1 simultaneously. Its energy density is increased by 28% compared to NiCl2. Copper doping optimizes thermodynamics process of discharge reaction and modifies local electronic structure of NiCl2. For copper‐doped NiCl2, the shift of Ni 3d and Cl 3p to lower energy level results in elevated redox potential, and the reduction of bandgap accelerates the carrier mobility, further promoting discharge degree. Utilizing metal ions dopant, this research surmounts the low‐temperature synthesis of NiCl2 and addresses its inferior electrochemical performance, ensuring high energy‐power output. This will expand the application scenarios of NiCl2‐based cathode materials.

target product.It is urgently needed to develop an easymanipulated technique to prepare NiCl 2 , achieving its commercial application in electrode materials.
Herein, we propose a facile two-step dehydration method to synthesize copper-doped NiCl 2 (Ni 1Àx Cu x Cl 2 ) at a lower temperature of 400 °C via incorporating CuCl 2 •2H 2 O as an additive.The obtained Ni 1Àx Cu x Cl 2 with different copper content all exhibit remarkable crystallinity and high purity.It is noteworthy that doping copper can not only surmount the challenge of preparing NiCl 2 at high temperature, but also overcome the energy-power trade-off as cathode materials in thermal battery, realizing the compatibility of high energy-power density output with lower total polarization.The mechanism for low-temperature dehydration and splendid electrochemistry activity has also been discussed from the point of view of atomic, electronic, and thermodynamic level.

Results and Discussion
The fabrication process of copper-doped NiCl 2 is manipulated via a two-step dehydration method (Figure 1) utilizing ethanol as the medium to achieve the uniform mixing of raw materials.The ethanol solution of mixture was investigated via ultraviolet-visible (UV-vis) absorption spectra.The absorption peaks originate from the superposition of absorption peaks for pristine NiCl 2 •6H 2 O and CuCl 2 •2H 2 O ethanol solution (Figure 2a), verifying no chemical reaction takes place during mixture process.Following the initial dehydration process at a temperature of 120 °C, ethanol is vaporized, leaving behind the mixture of NiCl 2 •2H 2 O and CuCl 2 •2H 2 O as residual components (Figure 2b).The target product Ni 1Àx Cu x Cl 2 is obtained after the second step of dehydration at a temperature of 400 °C (Figure 1).
Figure 2c discloses X-Ray diffraction (XRD) patterns of NiCl 2 synthesized at 600 °C and Ni 0.95 Cu 0.05 Cl 2 synthesized at 400 °C.All of diffraction peaks identify with the standard PDF card (PDF#71-2032).Except for the main peak (003), the remaining diffraction peaks of Ni 0.95 Cu 0.05 Cl 2 exhibit a higher relative intensity comparing with that of NiCl 2 , revealing carrying out calcination at the lower temperature can alleviate the preferred orientation growth of NiCl 2 crystal face.A comparison is made among NiCl 2 calcined at different temperature (Figure S2a, Supporting Information).The absence of crystal water in NiCl 2 powder can only be guaranteed at a temperature of 600 °C.To obtain preferable crystallinity, the temperature needs to be further elevated.The highest temperature reaches a maximum of 900 °C. [28,31]Optical images of NiCl 2 product (Figure S3, Supporting Information) depict the generation of NiO at the calcination temperature of 500-600 °C.The generation of byproduct arises from the reaction between NiCl 2 and crystal water at the higher temperature, resulting in a decrease in the purity of powder materials.By using CuCl 2 •2H 2 O as additive, the calcination temperature can be slashed to 400 °C to completely remove all of crystal water in lattice.The samples with different copper molar ratio are all provided with splendid crystallinity and the respective diffraction peaks can be matched well with the standard PDF card (PDF#71-2032), proving the formation of copperdoped NiCl 2 (Figure S2b, Supporting Information).A trace of cuprous chloride (CuCl) exists in Ni 1Àx Cu x Cl 2 (Figure S2c, Supporting Information), and no parasitic product (NiO) appears on the surface layer (Figure S5, Supporting Information).Thus, this unique technique for dehydration at the low temperature is possessed with the advantage of minimal energy consumption, facile manipulation, remarkable crystallinity, and high purity.
Element oxidation state analysis was carried out using X-Ray photoelectron spectroscopy (XPS).XPS survey spectrum indicates the coexistence of Ni, Cl, Cu, and O elements in Ni 0.95 Cu 0.05 Cl 2 materials (Figure S6a and S7a, Supporting Information).In high-resolution XPS spectrum of Ni 2p, peaks located at 856.2 and 873.9 eV can been assigned to 2p 2/3 and 2p 1/2 of Ni 2þ (Figure 2d).Additional peaks are associated with shakeup satellites. [32,33]The double peaks at 199.1 and 200.7 eV can be attributed to 2p 2/3 and 2p 1/2 of Cl À (Figure 2e).The doping of copper has no distinct effect to the shape or location of deconvolution peaks for Ni 2p and Cl 2p, compared with that of NiCl 2 (Figure S7b,c, Supporting Information).High-resolution XPS spectrum of Cu 2p can be deconvoluted into three double peaks (Figure 2f ), corresponding to 2p 2/3 and 2p 1/2 for Cu þ , Cu 2þ , and shakeup satellites (marked as "Sat."). [34]Deconvolution peaks related to Cu þ unveil the existent of a small amount of CuCl in materials.The oxygen element in samples (Figure S6b and S7e, Supporting Information) derive from slight oxidation on the surface during synthesis process.The hexagonal layered structure was still maintained for Ni 0.95 Cu 0.05 Cl 2 , with the substitution of nickel atoms by copper atoms (Figure 2g).The dopant atoms in lattice can tailor the band structure of semiconductor materials, further altering the electronic, chemical, and optical properties. [35,36]Based on the Tauc plots obtained from UV-vis diffuse reflectance spectra, the bandgap is gradually shrunken with the increasement of copper content (Figure 2h,i).
The microscopic process of dehydration for synthesizing  S1, Supporting Information).The interaction exists between NiCl 2 •2H 2 O substrate and dissociative copper ions, which is assessed using adsorption energy.Figure 3a   dissociative copper ions.The original bond length of Ni1─O1 in pristine NiCl 2 •2H 2 O is 2.14 Å.When capturing isolated copper ions at O1 site, the bond length of Ni1─O1 in two adsorption configurations is enlarged to 2.78 and 2.97 Å, respectively.Crystal orbital Hamilton population (COHP) was utilized to analyze Ni1─O1 bonding information in pristine NiCl 2 •2H 2 O and two sorts of adsorption configurations.Negative and positive values of COHP represent bonding and antibonding states of chemical bonds. [37]e integration of COHP (ICOHP) toward energy under Fermilevel, which can be employed as the parameter to evaluate the bond strength, was separately listed in Figure 3b-d.The Ni1─O1 bond in pristine NiCl 2 •2H 2 O demonstrates a robust bonding force with the -ICOHP value of 2.05 eV (Figure 3b).Whereas the -ICOHP values of Ni1─O1 bond in two sorts of adsorption configurations are reduced to 0.37 and 0.29 eV, respectively, testifying the weaker and easier breakage Ni1─O1 bonding when isolated copper ion is adsorbed on O site (Figure 3c,d).Consequently, the residual crystal water can be easily eliminated from the lattice of NiCl 2 at a relatively lower temperature (400 °C) and the formation of copper-doped NiCl 2 is achieved with lattice recombination.
NiCl 2 shows a flake-like shape with uneven boundary (Figure 4a) whereas all of Ni 1Àx Cu x Cl 2 exhibit a rod-like microstructure embedded with some sheets, as shown in scanning electron microscopy (SEM) images (Figure 4b and S9, Supporting Information).Powder sample will be dispersed into ethanol solution and subjected to sufficient ultrasonic treatment prior to transmission electron microscopy (TEM) characterization.Rod-like structure collapses in this process.Thus, the morphology of Ni 0.95 Cu 0.05 Cl 2 disclosed in TEM images is irregular thin sheet structure (Figure 4c).Nitrogen adsorption/desorption isotherms elucidate the larger specific surface area of Ni 0.95 Cu 0.05 Cl 2 (Figure S10a,b, Supporting Information), but this value does not exhibit the distinct enhancement compared with NiCl 2 .High-resolution TEM (HRTEM) image shows distinct lattice stripes penetrate the whole grain (Figure 4d), demonstrating the remarkable crystallinity of sample.Inverse fast Fourier transform (IFFT) pattern displays the hexagonal lattice structure (Figure 4e), with the interplanar spacing of 0.298 nm, corresponding to (101) lattice plane of NiCl 2 (Figure 4f ).The elements nickel, chlorine, and copper are uniformly distributed in synthesized Ni 0.95 Cu 0.05 Cl 2 , as shown in energy dispersive spectrometer (EDS) mapping images (Figure 4g-i).
Robust thermal stability is a required property for the cathode materials of thermal battery.TG curves demonstrate the exceptional thermal stability of NiCl 2 and its stable structure can be maintained until 678 °C (Figure S11, Supporting Information).The inferior thermal stability of Ni 0.95 Cu 0.05 Cl 2 , with the weight loss temperature of 537 °C, is induced by the weak bonding strength of Cu─Cl bond (Figure S12, Supporting Information).Detailed description has been listed in Supporting Information.Nevertheless, Ni 0.95 Cu 0.05 Cl 2 can still qualify as cathode for thermal battery operating at 500 °C.
The electrochemical performance of NiCl 2 and Ni 1Àx Cu x Cl 2 is detected at 500 °C with the current density of 100 mA cm À2 .NiCl 2 displays the inferior voltage stage with the maximum voltage of 2.43 V (Figure 5a).At the initial discharge process (0-50 mAh g À1 ), Ni 1Àx Cu x Cl 2 with higher copper content (0.1-0.2) is provided with the advantage of higher voltage (illustration in Figure 5a).5b).When the copper content approaches 10 at%, the specific capacity is gradually recessionary with the increasement of copper content.EDS mapping of cross section for cathode-separator tablets confirm copper element will migrate from cathode to separator during discharge (Figure S14, Supporting Information).Excessive copper will parasitize into separator when using Ni 1Àx Cu x Cl 2 (x > 0.1) as cathode, which alters the composition of electrolyte and impedes the migration of lithium ions, further results in the decay of specific capacity.Therefore, Ni 0.95 Cu 0.05 Cl 2 can be regarded as the most suitable cathode active materials in a series of Ni 1Àx Cu x Cl 2 .The electrochemistry performance of NiCl 2 and Ni 0.95 Cu 0.05 Cl 2 is systematically investigated at larger current density.The galvanostatic discharge curves reveal the prevailing voltage platform of Ni 0.95 Cu 0.05 Cl 2 (Figure 5c,d), with the voltage peaks of 2.42 and 2.27 V at 200 and 500 mA cm À2 , which are elevated by 3.82% and 3.65%, respectively, compared with that of NiCl 2 .Thus, Ni 0.95 Cu 0.05 Cl 2 is equipped with more favorable specific power, with 16.27 kW kg À1 at 500 mA cm À2 (Figure 5e).Ni 0.95 Cu 0.05 Cl 2 can still maintain high energy export at 500 mA cm À2 , with specific capacity of 330 mAh g À1 (Figure S13, Supporting Information) and specific energy of 717 Wh kg À1 (Figure 5e), which is drastically promoted by 20.88% and 28.26%, respectively.Therefore, Ni 0.95 Cu 0.05 Cl 2 can be considered as high energy-power density cathode materials.
Generally, it is challenging to achieve the combination of high power density and high energy density for cathode materials.In secondary batteries, the charge-discharge processes are realized via the deintercalation and intercalation of lithium ions between cathode and anode.High power output is restricted by the sluggish diffusion kinetics of lithium ions in cathode or anode.For example, the diffusion coefficient of lithium ions in LiPF 6 electrolyte can reach to 10 À6 -10 À5 cm 2 s À1 , [38,39] whereas this value in LiCoO 2 materials is only in the order of magnitude of 10 À11 -10 À13 cm 2 s À1 . [40,41]And high-rate charge-discharge processes can be realized by sacrificing the integrality and orderliness of lattice, leading to the decay of specific capacity and cycling performance.In thermal battery, cathode section consists of active materials and ionic conductive additive.Remarkable matching can be realized for the migration of lithium ions from separator to cathode.NiCl 2 has been considered as ideal high power density cathode materials with outstanding theoretical voltage (2.64 V vs Li) and stable voltage platform.High energy output is limited by its inferior electronical conductivity, resulting in sluggish transition of electrons from cathode section to current collectors.Especially at larger current density, the polarization induced by ohmic polarization will give rise to the distinct voltage drop and the early termination of discharge process (Figure 5d).Once improving the electronical conductivity, preferable discharge depth can be also achieved for NiCl 2 , realizing high energy-power exporting.This modification will drastically expand the application fields of NiCl 2 not only for the flexible individual combat equipment, but also for the long-range heavy weapons, leading to the reformation and upgradation of power source for special equipment.
The pulse test is conducted to quantify the polarization impedance of battery, which is described as following formula where R, V c , V p , I c , and I p denote total polarization, voltage 1 s before pulse discharge, voltage during pulse discharge, constant current, and pulse current, respectively.As shown in Figure 5f, NiCl 2 shows an average total polarization of 0.367 Ω, whereas that of Ni 0.95 Cu 0.05 Cl 2 is cut down to 0.288 Ω, which can be attributed to ameliorative conductivity of Ni 0.95 Cu 0.05 Cl 2 .
To assess the discharge performance of Ni 0.95 Cu 0.05 Cl 2 , the specific energy (Figure 6a), specific capacity (Figure S15a, Supporting Information) and specific power (Figure S15b, Supporting Information) of other transition metal halides were also listed.Obvious contrast suggests its extraordinary electrochemistry activity with the combination of high energy density and high power density.Besides, the evaluation should be extended to the field of secondary batteries.Figure 6b displays the voltage-specific capacity performance of Ni 0.95 Cu 0.05 Cl 2 and other cathode materials used in secondary batteries.It demonstrates that Ni 0.95 Cu 0.05 Cl 2 can be classified as steady voltage and high specific capacity cathode materials, which can continuously output stable energy for equipment.Figure 6c and S16, Supporting Information, exhibit the specific capacity of these cathode materials at different rate and a series of current density.In generally, the elevation of rate or current density will give rise to the decay of specific capacity.However, collocated with molten salt electrolytes with ultrahigh ionic conductivity, [2] Ni 0.95 Cu 0.05 Cl 2 can still achieve undiminished specific capacity at larger rate and larger current density.Rapid lithium-ion transfer cooperates with robust discharge depth, ensuring high energy-power output of battery.
The cathodic reaction of NiCl 2 during high-temperature discharge process is a type of conversion reaction, which can be described as follows [42,43] NiCl The discharge curves of Ni 1Àx Cu x Cl 2 , with single discharge platform, possess accordant feature to that of NiCl 2 .XRD patterns indicate the discharge product of cathode consists of nickel, lithium chloride (LiCl), and copper (Figure S17a,b, Supporting Information).Therefore, the discharge process of Ni 1Àx Cu x Cl 2 can be depicted as follows Associated schematic diagram of battery structure evolution is shown in Figure 7a.The doping of copper can alter the thermodynamic process of discharge reaction for NiCl 2 .According to Nernst equation, the theoretical EMF of battery can be obtained by following formula [11] EMF ¼ ÀΔG=nF (4)   where ΔG, n, and F represent Gibbs free energy change, the number of electrons transferring during reaction process, and Faraday constant.Density function theory (DFT) was executed to analyze Gibbs free energy based on thermodynamics and the details were listed in Supporting Information.The Gibbs free energy change (ΔG) of cathodic reaction for NiCl 2 and Ni 1Àx Cu x Cl 2 can be described as follows Given that Gibbs free energy of simple substance can be considered as zero, EMF of the above two discharge reaction can be written as follows Evidently, the comparison between EMF NiCl 2 ÀLi and EMF Ni 1Àx Cu x Cl 2 ÀLi mainly depends on the values of G NiCl 2 and G Ni 1Àx Cu x Cl 2 .Figure 7b exhibits the analysis about Gibbs free energy of NiCl 2 and Ni 1Àx Cu x Cl 2 at a series of temperature, indicating that Ni 1Àx Cu x Cl 2 is provided with higher Gibbs free energy.Hence, the electrode reaction between copper-doped NiCl 2 and Li can generate a larger Gibbs free energy change (Figure 7c), which is embodied in prominent electromotive force during discharge process.Combining the Gibbs free energy of LiCl (Figure S18a, Supporting Information), the values of 2.62 and 2.75 V are related to the theoretical electromotive force of NiCl 2 -Li battery at ambient temperature and 500 °C (Figure S18b, Supporting Information).And the values for Ni 1Àx Cu x Cl 2 -Li battery are increased to 2.68 and 2.78 V, respectively (Figure S18b, Supporting Information).
The doping of copper can also modulate the local electronic structure of NiCl 2 itself.Differential charge density graphs describe cations and anions in NiCl 2 and Ni 1Àx Cu x Cl 2 are all chemically bonded through ionic bond (Figure 7d,e).The difference is that the charge transfer quantity from Cu atoms to Cl atoms is lower than that from Ni atoms to Cl atoms (Figure 7d,e and S19a-d, Supporting Information).Thus, the doping of copper can alleviate the electronic polarization of NiCl 2 .Total density of state (DOS) confirms that NiCl 2 is a typical semiconductor materials with a broad band gap of 2.48 eV (Figure 7f ), corresponding well with the previous reports. [44]When copper atoms are introduced in the lattice and replace the situation of Ni atoms, Fermi level is situated in valence band and a type of p-type dopant is formed, so that extra holes become current carrier.The Cu 3d orbit and Cl 3p orbit dominate the top energy level of valance band (À0.52-0.08 eV), verifying the hybridization between Cu 3d and Cl 3p devotes to the reduction of band gap, from 2.48 to 1.89 eV.The narrow bandgap can cut down the energy barrier for electronic migration and elevate intrinsic electronic conductivity, further reducing the internal resistance of battery and achieving more sufficient discharge process.It is notable that the shape of Ni 3d and Cl 3p bands do not alter obviously except for shifting to lower energy level.The redox potential of battery derives from local electronic structure and state of electrons participating in redox reaction, which is dependent on the energy gap between the energy level of cations and anions involved in reaction and the energy level of Li þ /Li 0 . [45,46]In consequence, the introduction of copper in NiCl 2 crystal lattice can generate an elevated operating voltage platform for thermal battery (Figure 7g).

Conclusion
In summary, a series of copper-doped NiCl 2 with high purity are successfully synthesized utilizing a two-step dehydration method  [8,9,12,28,29,[47][48][49][50] The bars with shadow represent related values obtained by estimation according to the parameters in references.[53][54][55][56][57][58][59][60][61][62][63] by adding CuCl 2 •2H 2 O.The fabricating temperature is reduced from 600 to 400 °C, which is caused by the spontaneous adsorption of NiCl 2 •2H 2 O toward dissociative copper ions during preparation process, weakening the bonding strength of Ni─O bond.Meanwhile, the doping of copper significantly boosts the hightemperature discharge performance of NiCl 2 .Ni 0.95 Cu 0.05 Cl 2 shows the most predominant discharge performance with high energy density (717 Wh kg À1 ) and considerable power density (16.27 kW kg À1 ), accompanied by lower total polarization.Thermodynamics and electronic structure analysis demonstrate copper-doped NiCl 2 is equipped with elevated voltage window and shrunken bandgap, which devote to high energy-power export via optimizing the voltage platform and discharge depth.This work puts forward a novel strategy for fabricating high energy-power transition metal chloride cathode materials at the lower temperature toward thermal battery and expands the application field of NiCl 2 -based cathode materials, laying the foundation for the transformation and upgrading of special equipment power supply system.

Figure 2 .
Figure 2. Materials characterization.a) UV-vis absorption spectra of NiCl 2 •6H 2 O ethanol solution, CuCl 2 •2H 2 O ethanol solution, and the ethanol solution of two types of hydrate mixtures.b) XRD patterns of product after the first-step dehydration for the fabrication of Ni x Cu 1Àx Cl 2 .c) XRD patterns of NiCl 2 and Ni 0.95 Cu 0.05 Cl 2 .d-f ) High-resolution XPS spectra of Ni 2p, Cl 2p, and Cu 2p for Ni 0.95 Cu 0.05 Cl 2 .g) Crystal structure diagram for Ni 0.95 Cu 0.05 Cl 2 .h,i) UV-vis diffuse reflectance spectra of NiCl 2 and Ni 1Àx Cu x Cl 2 and corresponding Tauc plots derived from UV-vis diffuse reflectance spectra.

Figure 3 .
Figure 3. Dehydration process analysis.a) Schematic diagram for dehydration process after adding CuCl 2 •2H 2 O. b) COHP analysis for Ni1─O1 bond in pristine NiCl 2 •2H 2 O. c,d) COHP analysis for Ni1─O1 bond in two types of adsorption configurations.The dotted lines in b-d) denote the Fermi level.

Figure 4 .
Figure 4. Electron microscope analysis.a,b) SEM images of NiCl 2 , Ni 0.95 Cu 0.05 Cl 2 .c,d) TEM and HRTEM images of Ni 0.95 Cu 0.05 Cl 2 .e) IFFT patterns of the cyan region in (d).f ) The atom arrangement of NiCl 2 (101) lattice plane and line-scanning intensity profile obtained from the line A to B in (e).g-i) EDS mapping images of Ni 0.95 Cu 0.05 Cl 2 .
the voltage platforms of Ni 0.85 Cu 0.15 Cl 2 and Ni 0.8 Cu 0.2 Cl 2 are significantly declined after 50 mAh g À1 and their discharge process is finalized in advance, testifying the generation of drastic polarization in battery.NiCl 2 , Ni 0.97 Cu 0.03 Cl 2 , and Ni 0.95 Cu 0.05 Cl 2 exhibit similar specific capacity (Figure S13, Supporting Information).Benefited from the advantage of voltage platform, Ni 0.95 Cu 0.05 Cl 2 possesses the maximum specific energy of 678 Wh kg À1 (Figure

Figure 5 .
Figure 5. Detection of electrochemical performance for NiCl 2 and Ni 0.95 Cu 0.05 Cl 2 .a,b) Galvanostatic discharge curves and specific energy of NiCl 2 and Ni 1Àx Cu x Cl 2 at 100 mA cm À2 .c,d) Galvanostatic discharge curves of NiCl 2 and Ni 0.95 Cu 0.05 Cl 2 at 200 and 500 mA cm À2 .e) Specific energy and specific power of NiCl 2 and Ni 0.95 Cu 0.05 Cl 2 at 200 and 500 mA cm À2 .f ) Pulse discharge curves and corresponding total polarization for NiCl 2 and Ni 0.95 Cu 0.05 Cl 2 .The cutoff voltage in (a), (c), and (d) is 1.8 V.

Figure 7 .
Figure 7. Discharge process analysis and DFT calculations.a) Structure evolution of Ni 1Àx Cu x Cl 2 during high-temperature discharge.b) Entropy, enthalpy, and Gibbs free energy of NiCl 2 and Ni 1Àx Cu x Cl 2 at different temperature.c) Schematic diagram of Gibbs free energy change (ΔG) during discharge process for NiCl 2 and Ni 1Àx Cu x Cl 2 .d-e) 2D slice plan view of differential charge density for NiCl 2 and Ni 1Àx Cu x Cl 2 .f ) Total DOS and partial DOS for NiCl 2 and copper-doped NiCl 2 .Fermi level has been identified as zero and is marked by dotted lines.g) Schematic diagrams of electronic structure for NiCl 2 (blue region) and copper-doped NiCl 2 (red region).
Ni 1Àx Cu x Cl 2 is further investigated.Thermal gravimetric (TG) curves confirm crystal water in NiCl 2 •6H 2 O is difficult to completely eradicate until the temperature reaches to 550 °C (Figure S8a, Supporting Information).The removal of crystal water in CuCl 2 •2H 2 O is straightforward and can be easily achieved (Figure S8b, Supporting Information).When adding CuCl 2 •2H 2 O to synthesize Ni 1Àx Cu x Cl 2 , the precursor compositions for calcination are NiCl 2 •2H 2 O and CuCl 2 •2H 2 O.The schematic diagram of dehydration process is illustrated in Figure 3a.During heating process, all of crystal water in CuCl 2 •2H 2 O is entirely eliminated.Due to its low melting point feature, the remained product CuCl 2 exhibits a molten state at 400 °C (Table