A Novel Carbon Supported Tellurium Electrode for Lithium and Sodium Secondary Batteries

Recently, Se was reported as a promising novel positive electrode active material from several research groups (1-3). Se has better conductivity (˜10^-3 S cm^-1) than S and several studies on Li-Se/carbon composite cell showed important characteristics such as a very limited redox shuttle effect, and much higher activity. It is noteworthy that Se/carbon composites are also used as Na⁺-ion intercalation/de-intercalation electrode at room temperature. As the Se shows quite different electrochemical properties from S, Te can be also a candidate for a cathode active material (4, 5).

We recently reported that Li-Te@CMK-3 battery showed very high C-rate capability with excellent durability (6). The electrochemical reactions during charge-discharge tests are complete oxidation and reduction of Te by following reaction: Te + 2Li⁺ + 2e^- ↔ Li₂Te. The theoretical capacity is 420 mA h g^-1 and redox potential is 1.80 V vs. Li⁺/Li. Compared with Se, higher electroconductivity of Te (5 S cm^-1) and higer density (6.24 g cm^-2) would be advantageous for high rate charge-discharge test.

In this contribution, we show that our novel Te@CMK-3 composite electrode also works for Na intercalation/de-intercalation. The expected chemical reaction is the complete oxidation and reduction of Te by following reaction: Te + 2Na⁺ + 2e^- ↔ Na₂Te  (theoretical potential is 1.57 V vs. Na⁺/Na). Considering this relatively lower working potential, this electrode can be possibly used for negative electrode of Na secondary battery. CMK-3 was synthesized using SBA-15 as substrate based on the previous report (7). Te@CMK-3 composite was synthesized by melt diffusing strategy. The confinement of Te was confirmed by SEM, XRD, and BET measurements. A Na-Te@CMK-3 cell showed plateau between 1.0-1.5 V vs. Na⁺/Na during the discharge test. The Na-Te@CMK-3 cell also showed very high C-rate capability with good durability. Almost no capacity degradation was observed after 500 cycles of charge-discharge test at 1C-rate. The coulombic efficiency of every single cycle was close to 100 %, which means sodiation/desodiation of Te is quite reversible. The specific discharge capacity at every C-rate was approximately 76 % (0.2C), 68 % (0.5C), 61 % (1C), and 52 % (2C) of theoretical specific capacity. Here, the C-rate of 0.5C, 1C, 2C are corresponding to 210 mA g^-1, 420 mA g^-1, 840 mA g^-1, and 1680 mA g^-1, respectively. We firstly demonstrated that Te@CMK-3 composite electrode worked as a Na-ion intercalation/de-intercalation electrode and preliminary studies showed superior rate capability and cyclability. This work is crucial for developing high power density output Na secondary batteries.

References

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