Zirconium Trisulﬁde as a Promising Cathode Material for Li Primary Thermal Batteries

In this work ZrS 3 has been synthesized by solid state reaction in a sealed quartz tube and investigated as a candidate cathode material in Li thermal batteries. The structure of ZrS 3 before and after cell testing has been studied using powder X-ray diffraction. A new spinel related material, LiZr 2 S 4 , has been identiﬁed as the product of the electrochemical process, which can be indexed to a = 10.452(8) Å cubic unit cell. The electrochemical properties of the batteries were investigated at 500 ◦ C against Li 13 Si 4 by galvanostatic discharge and galvanostatic intermittent titration technique (GITT). In a thermal Li cell at 500 ◦ C a single voltage plateau of 1.70 V at a current density of 11 mA/cm 2 was achieved with capacity of 357 mA h g − 1 . Therefore ZrS 3 material has some promise as a cathode for Li thermal batteries.

Thermal batteries are electrochemical devices that convert chemical energy directly to electrical energy by an electrochemical oxidation-reduction reaction at high temperature utilizing a molten salt electrolyte. The electrolyte is only ionically conductive when is molten and the batteries remain active while the electrolyte stays molten. Like all primary batteries they consist of an anode, an electrolyteseparator and a cathode. The advantage of using thermal batteries is that the electrolyte is solid and poor ionic and electronic conductors at room temperature, which as a result provides stable energy storage for decades. 1 The elevated temperature also enhances the electrochemical kinetics to provide higher power as they can be discharged at higher currents. The most common applications of thermal batteries are those that need a long shelf life but need instantly high power like emergency power systems, such as in pilot ejector seats and sonobuoys.
Until now thermal batteries use a high melting point lithium alloy (Li 13 Si 4 ) as the anode, a halide salt eutectic bound in an insulating porous material as the electrolyte, and a 1 st row transition metal sulfide as the cathode. In this work the Li 13 Si 4 alloy is used as an anode as it has the highest Li content and is the least moisture sensitive of the lithium silicon alloys. The anode Li 13 Si 4 or (x = 3.25 in the formula of Li x Si) is stable and exhibits a potential of 0.157 V against Li metal at 415 • C. During discharge process a phase transition takes place Li 13 Si 4 to Li 7 Si 3 corresponds to 3.66 Li + / mole of Li -Si and the capacity of this alloy is 485 mA h g −1 . 2 The most common electrolyte for use in thermal batteries is the lithium chloride -potassium chloride eutectic (LiCl 44.8 wt%-55.2 wt% KCl) with a melting point of ∼354 • C and this requires a minimum of 35 wt% MgO as separator. 3 MS 2 sulfides where M is Fe, Ni or Co are the most studied cathodes and they have potentials vs Li 13 Si 4 at around 1.70 V for their first electrochemical transition but also have further electrochemical transitions, which eventually ends in complete reduction to the metal. 4 This work investigates ZrS 3 as a possible cathode material for use in Li thermal batteries. The synthesis of ZrS 3 has previously been reported by many authors [5][6][7][8] and the synthetic procedure (via a conventional solid state route) involved multiple firings at temperatures from 600 • C-1000 • C for 1 to 25 days. The work recently had success in synthesizing the sample in shorter time using one firing step.
This material has a pseudo one-dimensional structure ( Figure 1) and Zr atoms occupy the center of a trigonal prism with sulfur atoms at the corners and two of those S atoms bond to each other. Neighboring atoms create an infinite chain along the b-axis of the crystal. The large inter-chain distances results in weak inter-chain interactions. This leads to this family of materials possessing anisotropic material properties. 9 These chains stack to form corrugated layers perpendicular to c-axis. The aim of this work is to synthesize and characterize ZrS 3 and to fabricate into cells in order to test at high temperature vs Li 13 Si 4 using a variety of discharge conditions.

Experimental
ZrS 3 was synthesized by a solid state reaction in sealed quartz tube to prevent oxidation of the starting materials. 0.966 g of zirconium (Aldrich,-100 mesh) and 1.026 g of sulfur (Alfa Aesar,-100 mesh, 99.5%) powders were used. All the powders were weighed out and mixed in a mortar and pestle in air and then sealed into an evacuated (10 −3 mbar) quartz tube before the reactions were carried out inside a tube furnace. The compound was fired at 730 • C for 1 week, with a heating and cooling rate of 1 • C min −1 used.
Room temperature powder X-ray diffraction data was collected using a Panalytical Empyrean diffractometer in Bragg-Bretano geometry with a Ge (220) monochromator and Cu Kα 1 radiation (λ = 1.5406 Å). Data was collected from 5 • to 70 • 2θ for 1 hour with a step size of 0.017 • and a time per step of 0.94 seconds. Variable temperature powder X-ray diffraction data was collected using a Panalytical Empyrean diffractometer with Mo Kα 1,2 radiation, ß-filter and an X'celerator RTMS detector, equipped with an Anton Paar HTK1200N furnace. Data was collected from 4 • to 40 • 2θ for 6 hours with a step size of 0.008 • and a time per step of 600 seconds. The ZrS 3 sample was loaded into a quartz capillary and sealed for variable temperature data collection. WinXPOW software was used for indexing and refining the unit cell parameters. Scanning electron microscopy was carried out using a JEOL JSM-5600 microscope to study the morphology and composition of the cathode powder.
Fabrication of the composite cathode for high temperature electrochemical investigation was made by mixing 75 wt% (0.15 g) ZrS 3 and 25 wt% (0.05 g) Super P Carbon. 45 wt% MgO and 55 wt% LiCl-KCl eutectic (Sigma Aldrich, 99.99%, LiCl 44.8 wt% -55.2 wt% KCl) were ball milled under argon make the separator/electrolyte (0.2 g). The anode was made by mixing 75 wt% (0.15 g) Li 13 Si 4 (Lithium Rockwood) and 25 wt% (0.05 g) LiCl-KCl eutectic. This amount is in excess to keep the discharge on the first anode plateau for the amount of cathode used. The cathode, anode and electrolyte/separator mixtures were individually pressed into pellets of diameter 13 mm at 5 tonnes for 3 minutes. The anode, separator/electrolyte and cathode pellets were held in place using an alumina cup and graphite foil was used as current collectors. The cell assembly was placed in a Swagelok fitting, to allow measurements to be carried out under an inert atmosphere. Cell testing was carried out at 500 • C which is above the melting point of the LiCl-KCl molten salt electrolyte. The electrochemical cells were tested at elevated temperature and were investigated electrochemically by a Maccor battery tester Model 5300 by galvanostatic discharge. Thermal cells were also tested by the galvanostatic intermittent titration technique (GITT). In order to have sufficiently slow potential changes a current density of 7.5 mA/cm 2 was chosen for discharge for the GITT measurements. The experimental capacity was calculated using the Maccor software and then this was converted to x, the moles of lithium ions per moles of formula unit.

Results and Discussion
Cathode material characterization.-Zirconium trisulfide was studied by a powder X-Ray diffraction in both reflection and transmission geometry with the data shown in Figures 2a and 2b. The sample was confirmed to be a single phase and could be indexed to the P2 1 /m unit cell reported by Furuseth et al. Cell parameters were refined using the WinXPOW software and are given in Table I. In order to investigate the stability of ZrS 3 at elevated temperature, XRD data were collected in situ at temperatures in the range from 25 • C to 500 • C as shown in Figure 3a. Throughout this temperature range ZrS 3 is stable and there are no phase transformations or apparent degradation. The volume of the unit cell increases as the temperature increases as shown in Figure 3b.
The morphology of ZrS 3 was investigated by SEM and is presented in Figures 4a and 4b. Figure 4b is at higher magnification than Figure  4a and shows that the crystallites have a uniform morphology, which is consistent with a single phase sample and the crystallites are needles. The shape of the ZrS 3 grains gives a high surface are which could enable lithium ion diffusion in the thermal cell at high temperatures.  EDX analysis confirms the average elemental composition of ZrS 3 with zirconium 25 at% and sulfur 75 at%. The needle-like shape of the ZrS 3 crystals results in a large effect on the diffraction pattern from preferred orientation, and as a result, collecting data in a capillary rather than a pressing into a bulk flat plate sample holder, allowed the collection of data with a much better sampling of intensities. To test the temperature stability of zirconium trisulfide even further, a quantity of ZrS 3 was heated to 700 • C at 6 • C min −1 and was held at this temperature for 1 hour under flowing argon. Then it was cooled to room temperature at 6 • C min −1 . The resulting diffraction pattern is shown in Figure 5. The diffraction data shows that ZrS 3 had decomposed to ZrS 2 . ZrS 2 crystallizes in space group P3m1 and the refined cell parameters were a = b = 3.6631(4) Å, c = 5.8331 (5) Å. This is in agreement with Fotouhi. 10 The mass of ZrS 3 before heating was 0.1597 g and it lost a mass of 0.0435 g after heating to 700 • C. This mass loss of 27% is consistent with the thermal decomposition of ZrS 3 to ZrS 2 and gaseous sulfur at elevated temperatures. 3 was characterized electrochemically by galvanostatic discharge and galvanostatic intermittent titration technique. Galvanostatic discharge curves of the ZrS 3 cathode material at 500 • C are presented in Figure 6a. ZrS 3 was discharged at different current densities (7.5, 11, 15, 19, 23 and 75 mA/cm 2 ) to investigate the performance and the resulting discharge profiles. At both current densities of 7.5 and 11 mA/cm 2 there was a single flat voltage plateau at around 1.70 V and a capacity of 357 mA h g −1 was achieved. 25 wt% Super P Carbon was added to the cathode to increase the electronic conductivity and the value of x number was 2.5. At current densities of 15, 19 and 23 mA/cm 2 it can be observed that the single flat voltage plateau is lower than 1.70 V as the cell resistance might be higher however the capacity is still high (over 300 mA h g −1 ) and the value of x number was around 2.25. At a current density of 75 mA/cm 2 the single plateau is around 1.25 V and a capacity of around 200 mA h g −1 was achieved.

Electrochemical investigation of ZrS 3 at 500 • C.-ZrS
The amount of active anode (0.15 g) corresponds to 73 mA h for the discharge plateau from Li 13 Si 4 to Li 7 Si 3 which is 0.157 V vs Li. The measured capacity of the cathode is 54 mA h which keeps the discharge as being performed against the 0.157 V Li plateau.
GITT testing consists of galvanostatic discharge pulses, each 10 minutes long, followed by 5 minutes of relaxation time, with no current passing through the cell 11,12 as shown in Figure 6b. The potential drops between the pulses and the relaxation until a potential of 1 V is reached. The IR drop is 0.05 V at the beginning of discharge and 0.125 V at the end of the measurement, which indicates that the cell resistance is increasing during the reduction of the cathode, from a resistance of around 6 at the beginning of the measurement to around 16 after the cell discharge, which suggests that it is more difficult to insert lithium ions to the structure of the cathode electrode.
Compared to MS 2 cathodes such as FeS 2 , NiS 2 or CoS 2 , the ZrS 3 has a good voltage profile, a good capacity of 357 mA h g −1 , a good  Table II. Moreover CoS 2 decomposes to Co 3 S 4 at 650 • C, 13 NiS 2 decomposes to NiS at 600 • C 14 and FeS 2 decomposes to FeS at 580 • C. Sulfur gas in all cases is released and reacts exothermally with the anode electrode or dissolved lithium in the chosen molten salt electrolyte. 15,16 In the discharge reactions of NiS 2 and CoS 2 against Li 13 Si 4 no lithiated products are formed. In the case of FeS 2 and ZrS 3 lithiated products such as Li 3 Fe 2 S 4 , Li 2 FeS 2 and LiZr 2 S 4 , respectively are formed. CoS 2 exhibits capacity of 598 mA h g −1 , FeS 2 exhibits capacity of 558 mA h g −1 and NiS 2 exhibits capacity of 545 mA h g −14 as shown in Table II.   XRD data were collected on the cathode after discharge. The diffraction pattern shows peaks from a number of different phases due to the difficulty in separating the cathode from the electrolyte/separator after battery testing. A number of peaks were present in the diffrac-  tion pattern after battery testing, which could not be assigned to any known crystalline phase in the PDF database. These peaks could be indexed to a cubic unit cell, with cell parameters a = 10.452(8) Å and space group Fd3m and suggests a spinel structure. Therefore, a new phase with a cubic structure forms during discharge as shown in Figure 7. The SEM image of the cathode after discharge shows that the crystallites are not needles, indicating a morphology very different to that observed before testing. EDX analysis on individual grains can confirm a new phase with a composition of zirconium 34 at% and sulfur 66 at% as shown in Figure 8.

Conclusions
Zirconium trisulfide has been synthesized by solid state reaction and tested as a cathode in Li thermal batteries. The high temperature discharge behavior is presented in this work. At 500 • C the value of the working voltage plateau was measured for different current densities of 7.5, 11, 15, 19, 23 and 75 mA/cm 2 . The GITT method showed that the cell resistance increased during the discharge. A new cubic phase LiZr 2 S 4 was found after discharge with cell parameter a = 10.452(8) Å. In the past a low temperature Li 0.5 ZrS 2 phase was formed by Li insertion to ZrS 2 with a different polymorph structure. 18 The value of the single voltage plateau at 1.70 V when a current density of both 7.5 and 11 mA/cm 2 was used can define ZrS 3 as promising candidate material for thermal batteries applications. Clearly a full understanding of the chemical composition and crystalline phases present post electrochemical testing is extremely important as it can lead to the discovery of new crystalline phases, whilst also providing insight into the electrochemical processes occurring during discharge.