High Specific Capacity Thermal Battery Cathodes LiCu2O2 and LiCu3O3 Prepared by a Simple Solid Phase Sintering

The thermal battery has been designed to be active at high temperature to satisfy storage life and large capacity for storage and emergency power. The development of thermal battery with high specific energy requires that the cathode has high thermal stability and excellent conductivity. Here, the semiconductor material Li–Cu–O compounds LiCu₂O₂ and LiCu₃O₃ are synthesized by a simple solid-phase sintering technique, which is simpler than the traditional synthesis process. The thermal decomposition temperatures are 680°C and above 900°C, respectively. This work first applies the Li–Cu–O compounds to the thermal battery. With a cutoff voltage of 1.5 V, the specific capacities of LiCu₂O₂ and LiCu₃O₃ are 423 and 332 mA h g⁻¹. Both the decomposition temperature and specific capacity are higher than in the commercial FeS₂ and CoS₂, especially LiCu₂O₂. This work affords an alternative of the cathode materials for high specific capacity thermal battery.

Introduction

The copper-oxide-based material has been investigated as a high-temperature superconductor, LiBs electrode, and catalyst because of its good electrical conductivity, presence of electron holes, and magnetic interactions (Nakamura et al., 2006; Lepple et al., 2017). Of course, copper oxide itself has become the focus of investigations because of its microstructural details, conductivity, and electron structure (Xu et al., 2019). Interest in the system Li–Cu–O also evolved from cathode material for special battery, where a potential usage of Li/CuO thermal battery was considered (Liao et al., 2020). As a primary battery, thermal battery can only be activated when the temperature reached the melting point of eutectic salts electrolyte (Guidotti and Masset, 2006; Masset and Guidotti, 2007; Jeong et al., 2019). This special feature makes the thermal battery have a long shelf-life, excellent mechanical robustness, and being able to supply high output power; it has been used as power sources for guided missiles and proximity fuzes in ordinance devices (Masset and Guidotti, 2007). The cathode materials with poor thermal stability and low specific capacity limit the further development and application of thermal battery.

The Li–Cu–O compounds with a high initial charge capacity are candidates for a new cathode material, such as Li₂CuO₂ and LiCuO₂ (Vitins et al., 2003; Nakamura et al., 2005; Prakash et al., 2005; Arachi et al., 2012; Zhang et al., 2018). Besides, the well-known lithium copper oxides LiCu₂O₂ and LiCu₃O₃ with mixed valent copper are unique in the Li–Cu–O system, which gives rise to high T_c behavior and are semiconductor with ρ_293K of 10⁶ and 0.1 Ω cm (Goshall, 1986; Hibble et al., 1990; Roessli et al., 2001; Bush and Kamentsev, 2004; Zhu et al., 2011; Kamentsev et al., 2013; Lepple et al., 2013; Ivanov et al., 2014; Bush et al., 2018). Additionally, LiCu₂O₂ and LiCu₃O₃ show excellent thermal stability (Hibble et al., 1990; Bush et al., 2004, 2019), which makes them good cathodes for thermal battery. Based on X-ray emission and photoelectron spectra, the valence states of the their Cu atoms are found to be mixed univalent (Cu^I) and divalent (Cu^II) (Lin et al., 1996; Zatsepin et al., 1998).

However, it is not easy to obtain a pure single or polycrystalline LiCu₂O₂ or LiCu₃O₃ phase. CuO, Cu₂O, and Li₂CuO₂ are typical impurities for them (Paszkowicz et al., 2001). Because some Li₂O or Li₂CO₃ are lost during heating, this leads to the formation of CuO and Cu₂O (Hibble et al., 1990). LiCu₂O₂ and LiCu₃O₃ would be oxidized, forming Li₂CuO₂ at lower temperatures of 310–790°C with oxygen (Bush et al., 2004, 2019). Hence, a majority of synthetic methods for LiCu₂O₂ and LiCu₃O₃ choose quenching from 800 to 900°C. In addition, homogenizing and reheating are necessary to ensure the uniformity and purity of the product. In order to simplify synthetic methods, the raw materials are mixed by ball mill before being heat treated in this work. Moreover, to keep the high purity, both the synthesis process of LiCu₂O₂ and the cooling process of LiCu₃O₃ are carried out in Ar. Additionally, the micromorphology of LiCu₂O₂ and LiCu₃O₃ and their electrochemical performance as cathode materials for thermal batteries were studied.

Experimental

Material Synthesis

All starting materials (Table 1) are purchased from commercial sources and used without further purification. Shiny polycrystalline powders of the LiCu₂O₂ and LiCu₃O₃ are prepared by a simple solid-state method.

TABLE 1

Table 1. Chemical reagents used in the experiment.

As shown in the Figure 1, the raw materials for LiCu₂O₂, Li₂CO₃, Cu₂O, and CuO are put into an agate jar with agate balls (ball-to-powder weight ratio of 10:1) in a 1:1:2 ratio, and anhydrous ethanol is applied as a solvent. The raw materials are grounded in a ball mill (Tencan powder QXQM-2) for 6 h at a rotation speed of 400 rpm and dried to absolute at 80°C in an air-drying oven (Jinghong DHG-9070A). Subsequently, the mixtures are heat treated at 700°C for 7 h in a tube furnace under flow argon atmospheres with heating and cooling rates of 10°C min⁻¹. In order to synthesize the LiCu₃O₃, Li₂O·4CuO is mixed in the same way as raw materials for LiCu₂O₂ and then is heat-treated at 880°C for 7 h under flow argon–oxygen mixed atmospheres with heating and cooling rates of 10°C min⁻¹ (cooling process only under argon). Dark brown powders of the LiCu₂O₂ and black powders of the LiCu₃O₃ are ground and keep in air.

FIGURE 1

Figure 1. Synthesis route of LiCu₂O₂ and LiCu₃O₃ powders.

Materials Characterization

The phase structures of Li–Cu–O are determined by X-ray diffraction (XRD) on a Mini Flex 600 from 5° (2θ) to 90° (2θ) with Cu-Kα radiation (λ = 0.154178 nm) and X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher Scientific K-ALPHA. For all XRD measurements, the resolution in the scans is kept at 0.01° with a rate of 10° min⁻¹. Thermogravimetric analysis (TG, Henven HCT-4) is conducted with a heating rate of 10°C min⁻¹ from room temperature to 900°C and a 50 ml min⁻¹ Ar flowrate. The analyses of the microstructures of Li–Cu–O are performed using a scanning electron microscope (SEM, FEI Quanta 200) and a transmission electron microscope (TEM, FEI Tecnai G2 60-300).

Preparation of a Single Thermal Battery Cell and Discharge Test

The single cell was conventionally assembled by lamination. The 0.6 mm Li–B alloy was punched into a disk with a diameter of 17.5 mm as anode. The separator contains 50 wt.% ternary all-lithium eutectic salt electrolyte (LiF–LiCl–LiBr, m.p. = 436°C) and 50 wt.% MgO. Both the cathode (0.15 g Li–Cu–O) and separator were prepared into Φ 17.5 mm by powder tableting process. The cell was assembled in a glove box with the water and oxygen content being <5 ppm.

Discharge performances are evaluated on an IT8511 plus the single-channel programmable electronic load (ITECH). The single cell was sandwiched between two collectors. Before the tests, the temperature-controlled stainless steel cylinder heater was heated from room temperature to 500°C. The discharge current density is 0.1 A cm⁻² with the cutoff voltage of 1.5 V (75% of the peak voltage) at 500°C.

Results and Discussion

The XRD patterns of both powders closely agreed with LiCu₂O₂ (#81-0344) and LiCu₃O₃ (#81-0345), respectively (Figure 2). The as-grown crystals contain only traces of foreign phases (LiCuO, Cu₂O, and CuO). The purity of the quantitative analysis of the powder diffraction patterns are 93.1 wt.% LiCu₂O₂ and 93.7 wt.% LiCu₃O₃, which were calculated by K value method according to Table 2. The reason why impurity phase such as Cu₂O or CuO existed on samples may be the loss of Li₂CO₃ or Li₂O during prolonged heating (Hibble et al., 1990), while the LiCuO can be attributed to the uncomplete reaction during synthesis process. The high-resolution transmission electron microscopy (HRTEM) presented that the main composition is LiCu₂O₂ or LiCu₃O₃ (Figures 2b,c). A few retained phases, such as CuO, and Cu₂O, can also be detected. These results verified that LiCu₂O₂ and LiCu₃O₃ were prepared successfully.

FIGURE 2

Figure 2. (a) The X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy (HRTEM) images of (b) LiCu₂O₂ and (c) LiCu₃O₃.

TABLE 2

Table 2. The information of X-ray diffraction (XRD) patterns of synthesized LiCu₂O₂ and LiCu₃O₃.

The chemical state of LiCu₂O₂ and LiCu₃O₃ was studied using XPS (Figures 3A–F). Their XPS core-level spectra of Cu 2p comprise intense spin-orbit split peaks for 2p 3/2 (934.1 eV) and 2p 1/2 (954.2 eV) accompanied by satellite peaks (939–945 eV and 959–964 eV), which are typical of Cu^II valency (Zatsepin et al., 1998; Momeni and Sedaghati, 2018). Besides, the remaining two main fitted peaks center at 932.4 and 952.5 eV, representing Cu 2p3/2 and Cu 2p1/2 of Cu^I, respectively (Wang et al., 2018; Zhou et al., 2018). The O 1s peaks located at binding energies of 531.1 and 531.6 eV are in good agreement with the reported values of Cu₂O and CuO, respectively (Han et al., 2018). According to the XRD patterns (Figure 2a), the impurity phases of synthesized LiCu₂O₂ and LiCu₃O₃ are Cu₂O and CuO, respectively. Thus, Cu^II in LiCu₂O₂ and Cu^Iin LiCu₃O₃ can only come from themselves. To sum up, the coexistence of Cu^I and Cu^II ions in LiCu₂O₂ and LiCu₃O₃ can be concluded.

FIGURE 3

Figure 3. The X-ray photoelectron spectroscopy (XPS) spectra of (A–C) LiCu₂O₂ and (D–F) LiCu₃O₃.

The LiCu₂O₂ consisted of many uniform distributed clusters with an average size of about 1 μm (Figure 4a), which increased the compaction density of the cathode. The local high magnification showed that these clusters were composed of LiCu₂O₂ nanograins and impurity-phase Cu₂O (Figures 2b, 4b). Both the statistical data and the XRD pattern indicated that the average grain size is about 40 nm, which is beneficial to shortening the migration path of lithium ion, while the roughness surface of clusters could improve the activation sites of the electrochemical reaction and contact area between the cathode and fusional electrolyte. The roughness of the particle might be caused by the composition of nanograins. However, the cluster size of LiCu₃O₃ increased to 2–15 μm, which were composed of 20–200 nm grains (Figures 4c,d). This size distribution range was far larger than LiCu₂O₂. Thus, it could be expected that the discharge performance of LiCu₂O₂ would be more excellent than that of LiCu₃O₃.

FIGURE 4

Figure 4. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of (a,b) LiCu₂O₂ and (c,d) LiCu₃O₃.

Thermal stability is one of the important indicators for thermal battery cathode (Jin et al., 2018; Luo et al., 2020). As shown in Figure 5A, LiCu₂O₂ began to decompose to LiCuO, Cu₂O, and O₂ at around 680°C. The weight loss at 380°C was about 1.81%, which was mainly caused by the decomposition of Li₂CO₃. Due to the lower content and poor crystallinity, the auxiliary material Li₂CO₃ could not be detected in XRD. Since there is no Li₂CO₃ as the reactant, this weight loss peak cannot be observed in the TG curve of LiCu₃O₃ (Figure 5B). Unlike the LiCu₂O₂, the LiCu₃O₃ was kept stable below 900°C with a weight loss <1.0%. In summary, both of them have great thermal stability compared with the conventional thermal battery cathodes, such as FeS₂ and CoS₂ (Masset and Guidotti, 2008). The high temperature stability completely satisfied the temperature requirements of thermal battery during actual discharge.

FIGURE 5

Figure 5. The thermogravimetric analysis (TG) curves of LiCu₂O₂ (A) and LiCu₃O₃ (B).

The LiCu₂O₂ and LiCu₃O₃ presented excellent electrochemical performance (Figure 6). The voltage of LiCu₂O₂ quickly increased to a peak value of 2.06 V once the thermal battery is activated, in which the impurity phase Cu₂O of LiCu₂O₂ might belong. It is below 2–3 s, which is far shorter than in the secondary Li⁺ or Li metal battery during larger current discharge. The relative steady voltage platforms were 1.95 and 1.72 V, which were about 0.4 V lower than the theoretical values (Patat et al., 1991; Lepple et al., 2013) due to the resistance of the actual discharge. The voltage of LiCu₂O₂ gradually decreased along with three steps during discharge. The LiCu₃O₃ also showed similar discharge characterizations. It should be noted that the peak voltage of LiCu₃O₃ reached about 2.12 V. This high voltage spike is shown by the raw material CuO, which was significantly enhanced in LiCu₃O₃ during the initial discharge (Liao et al., 2020). The discharge time is the key factor to the practical application of thermal battery in the field of defense. In this case, the discharge time of LiCu₂O₂ reached 997 s with high cutoff voltage of 1.5 V at large current density of 0.1 A cm⁻². The corresponding specific capacity was 423 mA h g⁻¹. Since the LiCu₃O₃ particle has a large size and ununiform distribution, it showed a lower specific capacity of 332 mA h g⁻¹, but it is still higher than that of commercial FeS₂ of 210 mA h g⁻¹ and CoS₂ of 250 mA h g⁻¹. Both LiCu₂O₂ and LiCu₃O₃ have excellent conductivity and thermal stability.

FIGURE 6

Figure 6. The discharge curves of LiCu₂O₂ (A) and LiCu₃O₃ (B).

After discharge, the cross-section between LiCu₂O₂ and electrolyte showed relatively clear interface and particle characters (Figures 7a,b). The Cu elemental mapping also indicated that few Cu diffused from the cathode into the electrolyte (Figure 7c). This interface for LiCu₃O₃ thermal battery became blurry, and some Cu can be detected into the electrolyte (Figures 7d–f). The thickness of LiCu₃O₃ cathode varies more than that of LiCu₂O₂ cathode. This phenomenon is known in thermal battery as high solubility in molten salt, which degrades the battery's performance (Jin et al., 2017). For this reason, the performance of LiCu₃O₃ thermal battery is worse than that of LiCu₂O₂ thermal battery. It is demonstrated that LiCu₂O₂ could keep the structure at high temperature instead of being molten, forming blurry interface for LiCu₃O₃. To sum up, their discharge performance, especially for LiCu₂O₂ cathode, provides a new idea for improving the specific capacity of CuO and Cu₂O. LiCu₂O₂ and LiCu₃O₃ are suitable for use as cathode materials for thermal battery with long life and high specific capacity.

FIGURE 7

Figure 7. The images of cathode and electrolyte before discharge, cross-sectional SEM image, and corresponding Cu distribution of thermal battery after discharge: (a–c) LiCu₂O₂ and (d–f) LiCu₃O₃.

Conclusions

The thermal battery is a kind of Li metal primary battery. The characteristics of high temperature, large current, and long working life of the cathode material require it to have high temperature stability and high specific capacity. In this paper, high-purity LiCu₂O₂ and LiCu₃O₃ were synthesized by combining simple mechanical ball mill and solid-phase sintering technique. By considering the excellent thermal stability and high specific capacity for both LiCu₂O₂ and LiCu₃O₃, this work developed two new cathode materials for thermal battery (LiCu₂O₂ or LiCu₃O₃|LiF-LiCl-LiBr|LiB). The electrochemical tests showed that both cathodes exhibited excellent specific capacity. Especially for the LiCu₂O₂, the actual specific energy was up to 423 mA h g⁻¹ with cutoff voltage of 1.5 V at 0.1 A cm⁻² and 500°C. It was higher than the commercial thermal battery cathodes such as FeS₂ and CoS₂. This paper indicates that LiCu₂O₂ and LiCu₃O₃ are promising cathode materials with the high specific capacity for thermal battery.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

YW and LF contributed to conception and design of the study. ZL and LF organized the database. YW and XB performed the statistical analysis. YW wrote the first draft of the manuscript. XB, ZL, and LF wrote sections of the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

Funding

The research was carried out with financial support from the Young Teacher Development Program of Hunan University (No. 2015031).

Conflict of Interest

YW and XB were employed by the company The 18th Research Institute of China Electronics Technology Group Corporation.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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