High-voltage resistant zinc salt electrolyte and use thereof

Abstract

The application belongs to the technical field of electrochemical energy storage, and particularly relates to a high-voltage-resistant zinc salt electrolyte and a dual-ion battery composed of a zinc negative electrode and a graphite positive electrode. The electrolyte is composed of a zinc salt and an organic solvent, and the organic solvent is a dinitrile group-containing solvent. Compared with existing zinc salt electrolytes, the zinc salt electrolyte has very high oxidation stability, can significantly improve the coulombic efficiency and cycle performance of a high-voltage dual-ion battery composed of a zinc negative electrode and a graphite positive electrode, and has a good application prospect in the field of large-scale energy storage.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to a high-voltage-resistant zinc salt electrolyte and a dual-ion battery composed of a zinc anode and a graphite cathode. Background Technology

[0002] Renewable energy power generation is intermittent and significantly affected by seasons, weather, and time. Developing low-cost, large-scale electrochemical energy storage technology is one of the effective solutions to the instability problem of renewable energy power generation. A dual-ion battery (zinc / graphite dual-ion battery) composed of a zinc anode and a graphite cathode is a novel energy storage device. During charging and discharging, zinc ion deposition / dissolution occurs at the zinc anode, while anion insertion / extraction occurs at the graphite cathode. Because both the zinc anode and graphite cathode are low-cost, it has great potential application value in the field of large-scale energy storage.

[0003] For zinc / graphite dual-ion batteries, the potential for anion insertion / extraction reactions at the graphite cathode is relatively high. Therefore, the zinc salt electrolyte not only needs to meet the requirements of high ionic conductivity but also needs to withstand high voltage. Currently, zinc / graphite dual-ion battery electrolytes typically use zinc bis(trifluoromethanesulfonyl)imide, zinc trifluoromethanesulfonate, and zinc hexafluorophosphate as zinc salts and acetonitrile as an organic solvent. Although these zinc salt electrolytes using acetonitrile as a solvent have high ionic conductivity and can ensure the smooth deposition / dissolution of zinc ions at the negative electrode and the smooth intercalation / extraction of anions into / out of graphite at the positive electrode, the increased acidity of acetonitrile under high voltage conditions makes it easier for protons to be removed, leading to subsequent oxidative decomposition reactions. Ultimately, this results in insufficient coulombic efficiency and cycle performance of the battery, making it difficult for zinc / graphite dual-ion batteries to meet the practical requirements of large-scale energy storage devices. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention provides a high-voltage resistant zinc salt electrolyte and a dual-ion battery composed of a zinc negative electrode and a graphite positive electrode.

[0005] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0006] A high-voltage resistant zinc salt electrolyte, comprising a zinc salt and an organic solvent, wherein the organic solvent is a dinitrile-containing solvent.

[0007] The final concentration of zinc salt in the electrolyte is 0.2–4 mol / L, preferably 0.5 mol / L.

[0008] The solvent containing dinitrile groups is a dinitrile solvent or a mixed solvent containing dinitrile solvents.

[0009] The dinitrile solvent is one or a combination of several of the following: glutaronitrile, adiponitrile, heptacyanide, octadionitrile, nonadionitrile, and sebaconitrile.

[0010] The mixed solvent containing dinitrile solvent is a mixture of dinitrile solvent and non-dinitrile solvent, wherein the dinitrile solvent accounts for 50%–90% of the volume of the mixed solvent; the non-dinitrile solvent is one or a mixture of several of acetonitrile, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, and sulfolane.

[0011] The zinc salt is one or a combination of several of the following: zinc bis(trifluoromethanesulfonyl)imide, zinc trifluoromethanesulfonate, zinc hexafluorophosphate, zinc perchlorate, and zinc tetrafluoroborate.

[0012] A dual-ion battery includes a zinc negative electrode, a graphite positive electrode, a separator between the positive and negative electrodes, and an electrolyte, wherein the electrolyte is the high-voltage zinc salt electrolyte.

[0013] The zinc negative electrode is a negative electrode sheet made of one or more of zinc sheets, zinc foil, zinc powder, and zinc alloys.

[0014] The active material of the graphite cathode is one or a combination of natural graphite and artificial graphite.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] The electrolyte of this invention incorporates a dinitrile-containing solvent, which prevents the electrolyte from decomposing even at a high potential of 2.8V (relative to the zinc / zinc ion pair), thereby significantly improving the cycle performance of the zinc / graphite dual-ion battery. Specifically:

[0017] 1. The electrolyte of this invention incorporates a dinitrile solvent, which contains cyano functional groups at both ends. The electron-withdrawing effect of the cyano groups at both ends is partially offset. Therefore, the Lewis acidity of the methyl or methylene hydrogen adjacent to the cyano group is weakened, making it less likely to be removed and trigger subsequent oxidative decomposition reactions. This significantly improves the oxidation stability, solving the problem of insufficient oxidation stability of existing acetonitrile-based zinc salt electrolytes under high voltage. Furthermore, the oxidation stability of the zinc salt electrolyte with added dinitrile solvent is significantly higher than that of carbonate-based zinc salt electrolytes. Figure 1 , Figure 2 , Figure 4 , Figure 5 , Figure 7 , Figure 8 , Figure 10 , Figure 11 ).

[0018] 2. The high-voltage zinc salt electrolyte provided by this invention is suitable for dual-ion batteries composed of a zinc anode and a graphite cathode, and can significantly improve the cycle stability of dual-ion batteries. Figure 3 , Figure 6 , Figure 9 , Figure 12 ). Attached Figure Description

[0019] Figure 1 This refers to the linear scanning voltammetry test of the electrolyte in Example 1 and Comparative Examples 1 and 2 of this invention.

[0020] Figure 2 This refers to the chronoamperometry test of the electrolyte in Embodiment 1 and Comparative Examples 1 and 2 of the present invention.

[0021] Figure 3 This is a test of the charge-discharge cycle performance of the batteries in Embodiment 1 and Comparative Examples 1 and 2 of the present invention.

[0022] Figure 4 This refers to the linear scanning voltammetry test of the electrolyte in Embodiment 2 and Comparative Examples 1 and 2 of the present invention.

[0023] Figure 5 This refers to the chronoamperometry test of the electrolyte in Embodiment 2 and Comparative Examples 1 and 2 of the present invention.

[0024] Figure 6 This is a test of the charge-discharge cycle performance of the batteries in Embodiment 2 and Comparative Examples 1 and 2 of the present invention.

[0025] Figure 7 This refers to the linear scanning voltammetry test of the electrolyte in Example 3 and Comparative Examples 1 and 2 of this invention.

[0026] Figure 8 This refers to the chronoamperometry test of the electrolyte in Embodiment 3 and Comparative Examples 1 and 2 of the present invention.

[0027] Figure 9 This refers to the charge-discharge cycle performance test of the batteries in Embodiment 3 and Comparative Examples 1 and 2 of the present invention.

[0028] Figure 10 This refers to the linear scanning voltammetry test of the electrolyte in Example 4 and Comparative Examples 1 and 2 of this invention.

[0029] Figure 11 This refers to the chronoamperometry test of the electrolyte in Embodiment 4 and Comparative Examples 1 and 2 of the present invention.

[0030] Figure 12 This refers to the charge-discharge cycle performance test of the batteries in Embodiment 4 and Comparative Examples 1 and 2 of the present invention. Detailed Implementation

[0031] The following describes in detail the electrode preparation and single-cell assembly method of the dual-ion battery composed of a zinc anode and a graphite cathode involved in the invention. The preparation and assembly methods described below are for illustrative purposes only and are not intended to limit the scope of the invention.

[0032] This invention relates to battery cycle performance testing. The assembled battery is typically a dual-ion battery consisting of a zinc negative electrode and a graphite positive electrode. The positive electrode uses natural or artificial graphite as the active material, acetylene black as the conductive agent, and polyvinylidene fluoride as the binder. The mass ratio of active material, conductive agent, and binder is 80:10:10. The negative electrode sheet is made of one or more of zinc foil, zinc sheet, zinc powder, or zinc alloy, and glass fiber is used as the separator. Cell assembly is performed in a glove box. Finally, the high-voltage-resistant zinc salt electrolyte of this invention is injected into the cells, and they are encapsulated into coin cells for battery cycle performance testing.

[0033] All raw materials involved in the examples are commercially available products.

[0034] The present invention will be further described in detail below through embodiments.

[0035] Comparative Example 1:

[0036] Electrode fabrication and single-cell assembly of coin-type zinc / graphite dual-ion batteries were carried out according to the method described in "Detailed Implementation".

[0037] The electrolyte used is prepared in the glove box, and its composition and formulation are as follows:

[0038] Solvent: Ethyl methyl carbonate;

[0039] Solute: Zinc bis(trifluoromethanesulfonyl)imide was finally concentrated in the electrolyte at a concentration of 0.2 mol / L.

[0040] 5 mL of the prepared electrolyte was pipetted into a three-electrode electrochemical cell. Zinc foil was used as the counter and reference electrode, and titanium foil as the working electrode. The oxidative decomposition potential and decomposition current of the electrolyte were tested using an electrochemical workstation (see [link to electrochemical cell]). Figures 1-12 ).

[0041] Comparative Example 2:

[0042] Electrode fabrication and single-cell assembly of coin-type zinc / graphite dual-ion batteries were carried out according to the method described in "Detailed Implementation".

[0043] The electrolyte used is prepared in the glove box, and its composition and formulation are as follows:

[0044] Solvent: Acetonitrile;

[0045] Solute: Zinc trifluoromethanesulfonate has a final concentration of 1.5 mol / L in the electrolyte.

[0046] 5 mL of the prepared electrolyte was pipetted into a three-electrode electrochemical cell. Zinc foil was used as the counter and reference electrode, and titanium foil as the working electrode. The oxidative decomposition potential and decomposition current of the electrolyte were tested using an electrochemical workstation (see [link to electrochemical cell]). Figures 1-12 ).

[0047] Example 1:

[0048] Electrode fabrication and single-cell assembly of coin-type zinc / graphite dual-ion batteries were carried out according to the method described in "Detailed Implementation".

[0049] The electrolyte used is prepared in the glove box, and its composition and formulation are as follows:

[0050] Solvent: adiponitrile;

[0051] Zinc salt: Zinc bis(trifluoromethanesulfonyl)imide, with a final concentration of 0.2 mol / L in the electrolyte.

[0052] 5 mL of the prepared electrolyte was pipetted into a three-electrode electrochemical cell. Zinc foil was used as the counter and reference electrode, and titanium foil as the working electrode. The oxidative decomposition potential and decomposition current of the electrolyte were tested using an electrochemical workstation (see [link to electrochemical cell]). Figures 1-3 ).

[0053] Figure 1 The curve (1) shows the oxidative decomposition potential of the electrolyte in Example 1. Figure 1 Curve (2) shows the oxidative decomposition potential test of the electrolyte in Comparative Example 1. Figure 1 The curve (3) is the oxidative decomposition potential test of the electrolyte in Comparative Example 2.

[0054] Figure 2 Curve (1) is the curve showing the change of decomposition current of the electrolyte under a constant voltage of 2.8V over time in Example 1. Figure 2 Curve (2) shows the change of decomposition current of the electrolyte in Comparative Example 1 under a constant voltage of 2.8V over time. Figure 2 Curve (3) is the curve of the decomposition current of the electrolyte in Comparative Example 2 under a constant voltage of 2.8V as a function of time.

[0055] The results show that the oxidative decomposition initiation potential of the electrolyte prepared in Example 1 is significantly higher than that of Comparative Example 1 and Comparative Example 2, and the decomposition current of the electrolyte prepared in Example 1 at a constant voltage of 2.8V is significantly lower than that of Comparative Example 1 and Comparative Example 2. Both of these results indicate that the oxidative stability of the electrolyte prepared in Example 1 is significantly higher than that of Comparative Example 1 and Comparative Example 2.

[0056] Figure 3The curve (1) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Example 1. Figure 3 Curve (2) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 1. Figure 3 Curve (3) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 2. The results indicate that the cycle stability of the zinc / graphite dual-ion battery using the electrolyte prepared in Example 1 is better than that of the zinc / graphite dual-ion batteries using the electrolytes prepared in Comparative Examples 1 and 2. Since the adiponitrile solvent added to the electrolyte contains cyano functional groups at both ends, the electron-withdrawing effect of the cyano groups at both ends is offset to a certain extent. Therefore, the Lewis acidity of the methyl or methylene hydrogen adjacent to the cyano group is weakened, making it difficult to remove and trigger subsequent oxidative decomposition reactions, thus greatly improving the oxidation stability. As can be seen from the attached figure, the oxidation stability of the electrolyte prepared in Example 1 is significantly higher than that of Comparative Examples 1 and 2.

[0057] Example 2:

[0058] Electrode fabrication and single-cell assembly of coin-type zinc / graphite dual-ion batteries were carried out according to the method described in "Detailed Implementation".

[0059] The electrolyte used is prepared in a glove box, and its composition and ratio (volume ratio) are as follows:

[0060] Solvent, by volume ratio: 1 part adiponitrile, 1 part heptaonitrile;

[0061] Solute: Zinc trifluoromethanesulfonate has a final concentration of 1.5 mol / L in the electrolyte.

[0062] 5 mL of the prepared electrolyte was pipetted into a three-electrode electrochemical cell. Zinc foil was used as the counter and reference electrode, and titanium foil as the working electrode. The oxidative decomposition potential and decomposition current of the electrolyte were tested using an electrochemical workstation (see [link to electrochemical cell]). Figures 4-6 ).

[0063] Figure 4 Curve (1) shows the oxidative decomposition potential of the electrolyte in Example 2. Figure 4 Curve (2) shows the oxidative decomposition potential test of the electrolyte in Comparative Example 1. Figure 4 The curve (3) is the oxidative decomposition potential test of the electrolyte in Comparative Example 2.

[0064] Figure 5 Curve (1) is the curve showing the change of decomposition current of the electrolyte under a constant voltage of 2.8V over time in Example 2. Figure 5 Curve (2) shows the change of decomposition current of the electrolyte in Comparative Example 1 under a constant voltage of 2.8V over time. Figure 5Curve (3) is the curve of the decomposition current of the electrolyte in Comparative Example 2 under a constant voltage of 2.8V as a function of time.

[0065] The results show that the oxidative decomposition initiation potential of the electrolyte prepared in Example 2 is significantly higher than that of Comparative Example 1 and Comparative Example 2, and the decomposition current of the electrolyte prepared in Example 2 at a constant voltage of 2.8V is significantly lower than that of Comparative Example 1 and Comparative Example 2. Both of these results indicate that the oxidative stability of the electrolyte prepared in Example 2 is significantly higher than that of Comparative Example 1 and Comparative Example 2.

[0066] Figure 6 The curve (1) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Example 2. Figure 6 Curve (2) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 1. Figure 6 Curve (3) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 2. The results indicate that the cycle stability of the zinc / graphite dual-ion battery using the electrolyte prepared in Example 2 is better than that of the zinc / graphite dual-ion batteries using the electrolytes prepared in Comparative Examples 1 and 2. Since the adiponitrile and heptanitrile solvents added to the electrolyte contain cyano functional groups at both ends, the electron-withdrawing effect of the cyano groups at both ends is offset to a certain extent. Therefore, the Lewis acidity of the methyl or methylene hydrogen adjacent to the cyano group is weakened, making it difficult to remove and trigger subsequent oxidative decomposition reactions, thus greatly improving the oxidation stability. As can be seen from the attached figure, the oxidation stability of the electrolyte prepared in Example 2 is significantly higher than that of Comparative Examples 1 and 2.

[0067] Example 3:

[0068] Electrode fabrication and single-cell assembly of coin-type zinc / graphite dual-ion batteries were carried out according to the method described in "Detailed Implementation".

[0069] The electrolyte used is prepared in a glove box, and its composition and ratio (volume ratio) are as follows:

[0070] Solvent by volume ratio: 3 parts octanonitrile, 1 part methyl ethyl carbonate;

[0071] Solute: Zinc hexafluorophosphate, with a final concentration of 1 mol / L in the electrolyte.

[0072] 5 mL of the prepared electrolyte was pipetted into a three-electrode electrochemical cell. Zinc foil was used as the counter and reference electrode, and titanium foil as the working electrode. The oxidative decomposition potential and decomposition current of the electrolyte were tested using an electrochemical workstation (see [link to electrochemical cell]). Figures 7-9 ).

[0073] Figure 7 Curve (1) shows the oxidative decomposition potential of the electrolyte in Example 3. Figure 7 Curve (2) shows the oxidative decomposition potential test of the electrolyte in Comparative Example 1. Figure 7 The curve (3) is the oxidative decomposition potential test of the electrolyte in Comparative Example 2.

[0074] Figure 8 Curve (1) shows the change of decomposition current of the electrolyte under a constant voltage of 2.8V over time in Example 3. Figure 8 Curve (2) shows the change of decomposition current of the electrolyte in Comparative Example 1 under a constant voltage of 2.8V over time. Figure 8 Curve (3) is the curve of the decomposition current of the electrolyte in Comparative Example 2 under a constant voltage of 2.8V as a function of time.

[0075] The results show that the oxidative decomposition initiation potential of the electrolyte prepared in Example 3 is significantly higher than that of Comparative Example 1 and Comparative Example 2, and the decomposition current of the electrolyte prepared in Example 3 at a constant voltage of 2.8V is significantly lower than that of Comparative Example 1 and Comparative Example 2. Both of these results indicate that the oxidative stability of the electrolyte prepared in Example 3 is significantly higher than that of Comparative Example 1 and Comparative Example 2.

[0076] Figure 9 The curve (1) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Example 3. Figure 9 Curve (2) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 1. Figure 9 Curve (3) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 2. The results indicate that the cycle stability of the zinc / graphite dual-ion battery using the electrolyte prepared in Example 3 is better than that of the zinc / graphite dual-ion batteries using the electrolytes prepared in Comparative Examples 1 and 2. Since the octanedionitrile solvent added to the electrolyte contains cyano functional groups at both ends, the electron-withdrawing effect of the cyano groups at both ends is offset to a certain extent. Therefore, the Lewis acidity of the methyl or methylene hydrogen adjacent to the cyano group is weakened, making it difficult to remove and trigger subsequent oxidative decomposition reactions, thus greatly improving the oxidation stability. As can be seen from the attached figure, the oxidation stability of the electrolyte prepared in Example 3 is significantly higher than that of Comparative Examples 1 and 2.

[0077] Example 4:

[0078] Electrode fabrication and single-cell assembly of coin-type zinc / graphite dual-ion batteries were carried out according to the method described in "Detailed Implementation".

[0079] The electrolyte used is prepared in a glove box, and its composition and ratio (volume ratio) are as follows:

[0080] Solvent, by volume ratio: 9 parts heptanonitrile, 1 part dimethyl sulfoxide;

[0081] Solute: Zinc bis(trifluoromethanesulfonyl)imide, with a final concentration of 1.5 mol / L in the electrolyte.

[0082] 5 mL of the prepared electrolyte was pipetted into a three-electrode electrochemical cell. Zinc foil was used as the counter and reference electrode, and titanium foil as the working electrode. The oxidative decomposition potential and decomposition current of the electrolyte were tested using an electrochemical workstation (see [link to electrochemical cell]). Figures 10-12 ).

[0083] Figure 10 Curve (1) shows the oxidative decomposition potential of the electrolyte in Example 4. Figure 10 Curve (2) shows the oxidative decomposition potential test of the electrolyte in Comparative Example 1. Figure 10 The curve (3) is the oxidative decomposition potential test of the electrolyte in Comparative Example 2.

[0084] Figure 11 Curve (1) is the curve showing the change of decomposition current of the electrolyte under a constant voltage of 2.8V over time in Example 4. Figure 11 Curve (2) shows the change of decomposition current of the electrolyte in Comparative Example 1 under a constant voltage of 2.8V over time. Figure 11 Curve (3) is the curve of the decomposition current of the electrolyte in Comparative Example 2 under a constant voltage of 2.8V as a function of time.

[0085] The results show that the oxidative decomposition initiation potential of the electrolyte prepared in Example 4 is significantly higher than that of Comparative Example 1 and Comparative Example 2, and the decomposition current of the electrolyte prepared in Example 4 at a constant voltage of 2.8V is significantly lower than that of Comparative Example 1 and Comparative Example 2. Both of these results indicate that the oxidative stability of the electrolyte prepared in Example 4 is significantly higher than that of Comparative Example 1 and Comparative Example 2.

[0086] Figure 12 The curve (1) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Example 4. Figure 12 Curve (2) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 1. Figure 12 Curve (3) shows the charge-discharge cycle performance of the zinc / graphite dual-ion battery prepared in Comparative Example 2. The results indicate that the cycle stability of the zinc / graphite dual-ion battery using the electrolyte prepared in Example 4 is better than that of the zinc / graphite dual-ion batteries using the electrolytes prepared in Comparative Examples 1 and 2. Since the heptanonitrile solvent added to the electrolyte contains cyano functional groups at both ends, the electron-withdrawing effect of the cyano groups at both ends is offset to a certain extent. Therefore, the Lewis acidity of the methyl or methylene hydrogen adjacent to the cyano group is weakened, making it difficult to remove and trigger subsequent oxidative decomposition reactions, thus greatly improving the oxidation stability. As can be seen from the attached figure, the oxidation stability of the electrolyte prepared in Example 4 is significantly higher than that of Comparative Examples 1 and 2.

[0087] As can be seen from the above embodiments, the addition of dinitrile-based solvents to the electrolyte of the present invention enables the electrolyte to remain essentially undecomposed even at a high potential of 2.8V (relative to the zinc / zinc ion pair), thereby greatly improving the cycle performance of the zinc / graphite dual-ion battery. At the same time, if the dinitrile-based solvent is a mixed solvent, as long as the dinitrile group reaches more than half of the system, the oxidation stability of the zinc salt electrolyte can be significantly improved.

Claims

1. A dual-ion battery, comprising a zinc negative electrode, a graphite positive electrode, a separator between the positive and negative electrodes, and an electrolyte, characterized in that, The electrolyte is composed of zinc salt and organic solvent, wherein the organic solvent is a solvent containing a dinitrile group; the solvent containing a dinitrile group is a dinitrile solvent or a mixed solvent containing a dinitrile solvent; the dinitrile solvent is one or a combination of several selected from glutaronitrile, adiponitrile, heptanilide, octanilide, anonadionitrile, and sebaonitrile; the mixed solvent containing a dinitrile solvent is a mixture of a dinitrile solvent and a non-dinitrile solvent, wherein the dinitrile solvent accounts for 50% to 90% of the volume of the mixed solvent; the non-dinitrile solvent is one or a mixture of several selected from acetonitrile, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, and sulfolane.

2. The dual-ion battery according to claim 1, characterized in that, The final concentration of zinc salt in the electrolyte is 0.2~4 mol / L.

3. The dual-ion battery according to claim 1, characterized in that: The zinc salt is one or a combination of several of the following: zinc bis(trifluoromethanesulfonyl)imide, zinc trifluoromethanesulfonate, zinc hexafluorophosphate, zinc perchlorate, and zinc tetrafluoroborate.

4. The dual-ion battery according to claim 1, characterized in that: The zinc negative electrode is a negative electrode sheet made of one or more of zinc sheets, zinc foil, zinc powder, and zinc alloys.

5. The dual-ion battery according to claim 1, characterized in that: The active material of the graphite cathode is one or a combination of natural graphite and artificial graphite.

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