Abstract
A zinc iodine flow battery includes a positive end plate, a positive current collector, a negative current collector, a positive electrode with a flow frame, a membrane, a negative electrode with a flow frame, a negative end plate. The negative electrolyte is circulated between the negative storage tank and the negative cavity by pump. The negative pipe is provided with a branch pipe for the positive electrolyte circulation. The porous membrane between the positive and negative electrodes can realize the conduction of supporting electrolyte and prevent the diffusion of I3− to the negative electrolyte. In a duel-flow battery system, same electrolyte serves as both the positive electrolyte and the negative electrolyte, which is a mixed aqueous solution containing iodized and zinc salt. The membrane is porous membrane does not contain ion exchange group. Both the positive and negative electrolyte are neutral solutions.
Claims
1. The zinc iodine flow battery, which related a zinc iodine single flow battery or a zinc iodine dual-flow battery; The single flow battery comprises a negative electrolyte storage tank, a zinc iodine single flow battery includes a single battery or a cell stack that assembled with more than two single battery circuits in series; The single battery comprises a positive end plate, a positive current collector, a positive electrode with a flow frame, a membrane, a negative electrode with a flow frame, a negative current collector and a negative end plate; the electrolyte in the negative electrolyte storage tank could be circuited between the negative cavity (the cavity between the membrane and the negative current collector is called the negative cavity; the negative cavity is provided with a negative inlet and a negative outlet) and the electrolyte storage tank by pump; the negative electrolyte storage tank is connected with the negative inlet and outlet through the negative inlet and outlet of pipeline respectively; at the same time, the negative inlet and outlet of pipe are respectively provided with a branch for the circulation of the positive electrolyte; (the cavity between the membrane and the positive current collector is called a positive cavity; the positive cavity is provided with a positive inlet and a positive outlet); the branch on the negative inlet of pipeline is connected with the positive inlet on the positive cavity, and the branch on the negative outlet of pipeline is connected with the positive outlet on the positive cavity; The dual-flow battery comprises a single battery or a stack that assembled with more than two single battery circuits in series; the single battery comprises a positive end plate, a current collector, a positive electrode with a flow frame, a membrane, a negative electrode with a flow frame, a current collector, and a negative end plate; the positive electrolyte in the positive electrolyte storage tank flows through the positive electrode by the pipeline and pump; the negative electrolyte in the negative electrolyte tank could flows through the negative electrode by the pipeline and pump; the positive electrolyte and the negative electrolyte are the same, which are the mixture aqueous solutions of iodine and zinc salt; the membrane is a porous membrane without ion exchange groups or composite membrane.
2. The zinc-iodine flow battery according to claim 1, wherein the membranes of the single-flow battery and the dual-flow battery are both porous membranes without ion exchange groups or composite membranes.
3. The zinc-iodine flow battery according to claim 1, wherein the iodine salt is one or more of CaI2, MgI2, KI, and NaI, and the molar concentration of the iodized salt in the electrolyte is 2 to 8 mol/L; the zinc salt is one or more of ZnNO3, ZnBr2, ZnSO4, and ZnCl2, and the molar concentration of the zinc salt in the electrolytic solution is 1 to 4 mol/L; the molar ratio of iodine and zinc in the electrolytic solution is preferably between 2:1, wherein zinc salt is preferably ZnBr2, and iodine salt is preferably KI.
4. The zinc-iodine flow battery according to claim 1, wherein the electrolyte solution contains a supporting electrolyte, and the supporting electrolyte in a single-flow battery is one or more of KCl, KBr, and NaCl; the electrolyte of dual-flow battery is one or more of KCl, K.sub.2SO.sub.4, KBr; the concentration is 1 to 2 mol/L and the supporting electrolyte is preferably KCl.
5. The zinc-iodine flow battery according to claim 1, wherein the membrane is a porous membrane without ion-exchange groups, including one or more of polyethersulfone (PES), polyethylene (PE), polypropylene (PP), polysulfone (PS), polyetherimide (PEI), and polyvinylidene fluoride (PVDF); the membrane is a porous membrane, and the thickness of a single-flow battery is 100 to 1000 μm; the thickness of membrane employed for the dual-flow battery is 150-1000 μm, preferably 500-1000 μm; the porous membrane material is preferably PE, PP, pore size is 1-10 nm, and porosity is 20%-70%.
6. The zinc-iodine flow battery according to claim 1, wherein the composite membrane is a porous membrane without ion-exchange groups coated with a dense polymer layer on the surface and the polymer layer material is one or more of polybenzimidazole (PBI), Nafion resins and polytetrafluoroethylene (PTFE), preferably Nafion resin, and the thickness of the coating is 1-10 μm.
7. The zinc-iodine flow battery according to claim 1, characterized in that during charging the I.sup.− in the positive electrolyte undergoes an oxidation reaction to generate one or more of I.sub.3.sup.− and I.sub.2, preferably I.sub.2; on the negative electrode Zn.sup.2+ undergoes a reduction reaction to form Zn; during discharge, the I.sub.3.sup.− or I.sub.2 undergoes a reduction reaction to generate I.sup.− on the positive electrode, and the Zn undergoes an oxidation reaction to generate Zn.sup.2+.
8. The zinc-iodine flow battery according to claim 1, wherein the electrode material is one of carbon felt, graphite plate, metal plate, or carbon cloth, preferably carbon felt.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is the structural diagram of the zinc iodine single flow battery of the invention. Among them, 1 refers to positive and negative bipolar plates; 2 refers to positive and negative current collectors; 3 refers to positive and negative flow frames; 4 is the membrane; 5 refers to positive electrolyte inlet and outlet valves; 6 is the electrolyte storage tank; 7 is pump.
[0014] FIG. 2 shows the single battery cycle performance of the zinc iodine single flow battery according to example 1; the positive and negative electrolytes are ZnBr.sub.2: 4 M, KI: 8 M, KCl: 1M, and the porous membrane with the thickness of 900 μm.
[0015] FIG. 3 shows the energy density of the zinc iodine single flow battery according to example 1; the positive and negative electrolyte is ZnBr.sub.2: 4 M, KI: 8 M, KCl: 1 M, and the porous membrane with the thickness of 900 μm.
[0016] FIG. 4 shows the cycle performance of the zinc iodine single flow battery according to example 3; the positive and negative electrolyte is ZnBr.sub.2: 4 M, KI: 8 M, KCl: 1 M, and the porous membrane thickness: 500 μm.
[0017] FIG. 5 shows the cycle performance of the zinc iodine single flow battery according to example 5; the positive and negative electrolyte is ZnCl.sub.2: 4 M, KI: 8 M, KCl: 1 M, and the porous membrane thickness: 900 μm.
[0018] FIG. 6 shows the cycle performance of the zinc iodine single flow battery according to Example 7; the positive and negative electrolyte is ZnBr.sub.2: 4 M, Nat 8 M, KCl: 1 M, and the porous membrane thickness: 900 μm.
[0019] FIG. 7 shows the energy density diagram of the zinc iodine single flow battery according to Example 7; the positive and negative electrolyte is ZnBr.sub.2: 4 M, Nat 8 M, KCl: 1 M, and the porous membrane thickness: 900 μm.
[0020] FIG. 8 shows the cycle performance of the zinc iodine single flow battery according to comparative example 2; the positive and negative electrolyte is ZnI.sub.2: 4 M, porous membrane thickness: 900 μm.
[0021] FIG. 9 shows the cycle performance of the zinc iodine single flow battery according to comparative example 3; the positive and negative electrolytes are ZnBr.sub.2: 4 M, Nat 8 M, KCl: 1 M, Nafion 115 film thickness: 125 μm.
[0022] FIG. 10 shows the cycle performance of the zinc iodine single flow battery according to comparative example 5; the positive and negative electrolyte is ZnBr.sub.2: 4 M, Nat 8 M, KCl: 1 M, and the porous film thickness is 65 μm.
[0023] FIG. 11 is the structural diagram of zinc iodine dual-flow battery using porous membrane: 1 refers to the positive and negative pumps; 2 refers to the positive and negative electrolyte storage tank; 3 refers to the positive and negative end plates; 4 refers to the positive and negative current collectors; 5 refers to the positive and negative flow frames; 6 is the membrane.
[0024] FIG. 12 shows the single battery cycle performance diagram of the zinc iodine dual-flow battery according to example 1; the positive and negative electrolyte is ZnBr.sub.2: 2.5 M, KI: 5 M, KCl: 1 M, and the porous membrane thickness is 900 μm.
[0025] FIG. 13 shows the single battery cycle performance diagram of the zinc iodine dual-flow battery according to example 2; the positive and negative electrolyte is ZnBr.sub.2: 3 M, KI: 6 M, KCl: 1M, the porous membrane thickness is 900 μm.
[0026] FIG. 14 shows the energy density diagram of the single cell of the zinc iodine dual-flow battery according to example 1; the positive and negative electrolyte is ZnBr.sub.2: 2.5 M, KI: 5 M, KCl: 1 M: 1m, the porous membrane thickness is 900 μm.
[0027] FIG. 15 shows the energy density diagram of the single cell of the zinc iodine dual-flow battery according to example 2; the positive and negative electrolyte is ZnBr.sub.2: 3 M, KI: 6 M, KCl: 1M, the porous membrane thickness is 900 μm.
[0028] FIG. 16 shows the single battery cycle performance diagram of the zinc iodine dual-flow battery according to example 3; the positive and negative electrolyte is ZnBr.sub.2: 2m, KI: 4 M, KCl: 1 M, the porous membrane thickness is 900 μm.
[0029] FIG. 17 shows the single cell cycle performance diagram of the zinc iodine dual-flow battery according to example 4; the positive and negative electrolyte is ZnBr.sub.2: 1 M, KI: 2 M, KCl: 1 M, the porous membrane thickness is 900 μm.
[0030] FIG. 18 shows the single battery cycle performance diagram of the zinc iodine dual-flow battery according to example 6; the positive and negative electrolyte is ZnBr.sub.2: 3 M, KI: 6 M, KCl: 1 M, and the thickness of the porous membrane is 500 μm.
[0031] FIG. 19 shows the single battery cycle performance of the zinc iodine dual-flow battery according to example 12; the positive and negative electrolyte is ZnSO.sub.4: 3 M, KI: 6 M, KCl: 1 M, and the thickness of the porous membrane is 900 μm.
[0032] FIG. 20 single battery cycle performance diagram of zinc iodine dual-flow battery according to example 14; positive and negative electrolyte is ZnBr.sub.2: 3 M, KI: 6 M, and the thickness of porous membrane is 900 μm.
[0033] FIG. 21 demonstrates the ratio performance diagram of zinc iodine dual-flow battery according to example 4; the structure of the single cell battery includes successively: positive end plate, positive current collector, positive flow frame, membrane, negative flow frame and negative end plate. The composition of electrolyte in the battery is 2 M KI, 1 M ZnBr.sub.2, and 2 M KCl, flow rate is 10 ml/min, charging current density is 60-140 mA/cm.sup.2, the battery is terminated by the capacity and voltage double cut-off: the charging cut-off time is 45 minutes and the voltage is 1.5 V, discharging cut-off voltage is 0.1 V.
[0034] FIG. 22 is a temperature dependent performance diagram of the zinc iodine dual-flow battery assembled in example 4. Battery temperature dependent performance test: the structure of the single battery is as follows: positive end plate, positive current collector, positive flow frame, membrane, negative flow frame and negative end plate. The composition of electrolyte in the battery is 2 M KI, 1 M ZnBr.sub.2, and 2 M KCl, flow rate is 10 ml/min, charging current density is 80 mA/cm.sup.2, the battery is terminated by the capacity and voltage double cut-off: the charging cut-off time is 45 minutes and the voltage is 1.5 V, discharging cut-off voltage is 0.1 V, temperature range is 10° C.-65° C.
[0035] FIG. 23 demonstrates the voltage curve of a single zinc-iodine dual-flow battery according to example 2. The structure of the single battery is as follows: positive end plate, positive current collector, positive flow frame, membrane, negative flow frame and negative end plate. The composition of electrolyte in the battery is 6 M KI, 3 M ZnBr.sub.2, and 1 M KCl flow rate is 10 ml/min, charging current is 80 mA/cm.sup.2, the battery is terminated by the capacity and voltage double cut-off: the charging cut-off time is 45 minutes and the voltage is 1.5 V, discharging cut-off voltage is 0.1 V. Charge for 1 hour until the battery is short circuited, then reduce the charging time to 45 mins to continue the battery cycling.
[0036] FIG. 24 demonstrates a voltage curve diagram of a zinc-iodine dual-flow battery stack according to example 2. The structure of the stack is: a positive electrode end plate, a current collector, nine batteries each comprises a positive electrode with flow frame, a membrane, a negative electrode with a flow frame, and finally a current collector and a negative electrode end plate connected in series. The electrolyte composition of the battery is 6 M KI, 3 M ZnBr.sub.2, and 1 M KCl with a flow rate of 10 mL/min. The charging current density was 80 mA/cm.sup.2 and the charge cut-off voltage is 13 V with a discharge cut-off voltage of 1 V. Charge for 1 h until the battery is short-circuited, then reduce the charging time to 45 mins to continuously evaluated the battery.
[0037] FIG. 25 is the cyclic performance diagram of the zinc iodine dual-flow battery stack according to example 2; the stack assembled with nine single battery connected in series.
[0038] FIG. 26 shows the cycle performance of a single cell zinc iodine dual-flow battery according to comparative example 1; the positive and negative electrolyte are ZnBr.sub.2: 2.5 M, KI: 5 M, KCl: 1 M Nafion 115 membrane with the thickness of 125 μm.
[0039] FIG. 27 shows the cycle performance of a single zinc iodine dual-flow battery according to comparative example 4; the positive and negative electrolyte is ZnI.sub.2: 3 M, and the thickness of the porous membrane is 900 μm.
[0040] FIG. 28 shows the cycle performance of a single zinc iodine dual-flow battery according to comparative example 5; the positive and negative electrolyte is ZnI.sub.2: 3 M, KI: 5 M, KCl: 1M, porous membrane with the thickness of 65 μm
[0041] FIG. 29 is the cycle performance diagram of a single cell zinc iodine single flow battery according to preferred example 1; the positive and negative electrolyte is ZnBr.sub.2: 4 M, KI: 8M, KCl: 1 M, the composite membrane is PE porous membrane substrate with 7 μm Nafion resin coating.
[0042] FIG. 30 is the energy density diagram of zinc iodine single flow battery according to preferred example 1; the positive and negative electrolyte is ZnBr.sub.2: 4 M, KI: 8 M, KCl: 1 M, the composite membrane is PE porous substrate with 7 μm Nafion resin coating.
[0043] FIG. 31 shows the cycle performance diagram of the zinc iodine single flow battery according to preferred example 2; the positive and negative electrolyte is ZnBr.sub.2: 4 M, KI: 8 M, KCl: 1 M, the composite membrane is PE porous substrate with 7 μm Nafion resin coating.
EMBODIMENTS
[0044] The evaluation of zinc iodine dual-flow battery and single flow battery: the structure of the single battery include, sequentially, positive electrode plate, current collector, carbon felt positive electrode with flow frame, membrane, carbon felt negative electrode with a flow frame, and negative end plate. The flow rate of the electrolyte in the battery was 10 mL/min, the charging current density was 80 mA/cm.sup.2, the battery was terminated by the capacity and voltage double cut-off: the charging time was 45 minutes and the voltage was 1.5 V, discharging cut-off voltage was 0.1 V.
TABLE-US-00001 Electrolyte Composition Examples (mol/L) membrane Thickness (μm) CE VE EE 1 8M KI, 4M ZnBr.sub.2, 1M PE 900 96% 80% 77% KCl 2 6M KI, 3M ZnBr.sub.2, 1M PE 900 96% 81% 78% KCl 3 8M KI, 4M ZnBr.sub.2, 1M PE 500 91% 80% 73% KCl 4 6M KI, 3M ZnBr.sub.2, 1M PE 500 91% 81% 74% KCl 5 8M KI, 4M ZnCl.sub.2, 1M PE 900 92% 71% 65% KCl 6 6M KI, 3M ZnCl.sub.2, 1M PE 900 93% 70% 65% KCl 7 8M NaI, 4M ZnBr.sub.2, 1M PE 900 88% 78% 68% KCl 8 6M NaI, 3M ZnBr.sub.2, 1M PE 900 90% 78% 70% KCl 9 8M KI, 4M ZnBr.sub.2 PE 900 96% 78% 75% 10 6M KI, 3M ZnBr.sub.2 PE 900 97% 78% 76% Preferred 8M KI, 4M ZnBr.sub.2, 1M Composite 900 97% (85%) (82%) example 1 KCl membrane Preferred 8M KI, 4M ZnBr.sub.2, 1M Composite 500 96% (86%) (81%) example 2 KCl membrane
TABLE-US-00002 Comparative Electrolyte Composition example (mol/L) membrane Thickness (μm) CE VE EE 1 ZnI.sub.2 3M PE 900 90% 81% 73% 2 ZnI.sub.2 4M PE 900 89% 78% 69% 3 8M KI, 4M ZnBr.sub.2, 1M KCl Nafion 115 125 99% 70% 69% 4 6M KI, 3M ZnBr.sub.2, 1M KCl Nafion 212 50 99% 68% 67% 5 KI 5M, ZnBr.sub.2 2.5M, 1M KCl PE 65 74% 88% 65%
[0045] FIGS. 2 to 3 are graphs of cycle performance and energy density of the battery under the most preferred conditions. With KI/ZnBr.sub.2 as the electrolyte, the battery assembled with porous membrane achieved excellent cycle stability. Meanwhile, the application of porous membrane greatly improved the ion conductivity, the working current density of the battery can reach 80 mA/cm.sup.2 with the high power density. At the same time, the concentration of KI in the electrolyte can reach about 8 M and the energy density is greater than 90 Wh/L.
[0046] Compared with the most preferred example, the battery in FIG. 4 employs a much thinner porous membrane (500 μm), and the coulombic efficiency of the battery decreases due to the increase of electrolyte crossover. The electrolyte in FIG. 5 employed ZnCl.sub.2 rather than ZnBr.sub.2, the performance is greatly reduced and the stability is deteriorated. This is due to the instability of the electrolyte; during charging, the I.sub.2 desposition formed in the positive electrode and ZnCl.sub.2 in the negative electrolyte would hydrolyze and precipitate. In FIG. 6, when NaI was substituted with KI, the battery efficiency decreased. In particular, the voltage efficiency drop is mainly caused by the decrease of the electrolyte conductivity, which further decreased the energy density of the battery in FIG. 7.
[0047] FIGS. 8-10 are comparative experiments. FIG. 8 employed ZnI.sub.2 as the electrolyte of the battery. The decrease of efficiency was mainly due to the low ion conductivity of the ZnI.sub.2 solution. Further, the battery performance is unstable due to the precipitation of electrolyte. FIG. 9 employed Nafion 115 membrane for the battery assembly. During the charge and discharge process, serious membrane fouling occurred on the membrane surface, which intensified the battery polarization and decreased the battery performance. FIG. 10 used a much thinner porous membrane, the cross-contamination of electrolyte was greatly intensified, and the efficiency of the battery, especially the coulomb efficiency, was severely reduced.
[0048] A preferred example employed a Nafion-coated composite membrane as the membrane. FIG. 29 shows the performance of a battery that used composite membrane with the thickness of 900 μm. The electrolyte was a mixed solution of KI and ZnBr.sub.2. Due to the Donnan exclusion of Nafion coating, the columbic efficiency of the battery was greatly improved. In addition, the battery used a thinner composite membrane (500 μm), and the coulombic efficiency of the battery slightly decreased.
[0049] The evaluation of zinc-iodine dual-flow battery and single flow battery: the structure of a single battery contains, sequentially: a positive electrode plate, a current collector, a carbon felt positive electrode with a flow frame, a membrane, and a battery with a flow frame, a carbon felt negative electrode with a flow frame, and a negative end plate. The flow rate of the electrolyte in the battery was 10 mL/min, the battery was terminated by the capacity and voltage double cut-off: the charging cut-off time was 45 minutes and the voltage was 1.5 V, discharging cut-off voltage was 0.1 V
TABLE-US-00003 Examples Electrolyte (mol/L) membrane Thickness (μm) CE VE EE 1 KI 5M, ZnBr.sub.2 2.5M, 1M KCl PE 900 94% 85% 80% 2 KI 6M, ZnBr.sub.2 3M, 1M KCl PE 900 94% 85% 80% 3 KI 4M, ZnBr.sub.2 2M, 2M KCl PE 900 94% 85% 80% 4 KI 2M, ZnBr.sub.2 1M, 2M KCl PE 900 94% 86% 81% 5 KI 5M, ZnBr.sub.2 2.5M, 1M KCl PE 500 87% 86% 75% 6 KI 6M, ZnBr.sub.2 3M, 1M KCl PE 500 86% 86% 74% 7 NaI 5M, ZnBr.sub.2 2.5M, 1M KCl PE 900 94% 83% 78% 8 NaI 6M, ZnBr.sub.2 3M, 1M KCl PE 900 94% 82% 77% 9 KI 5M, ZnCl.sub.2 2.5M, 1M KCl PE 900 91% 82% 75% 10 KI 6M, ZnCl.sub.2 3M, 1M KCl PE 900 90% 81% 72% 11 KI 5M, ZnSO.sub.4 2.5M, 1M KCl PE 900 76% 81% 61% 12 KI 6M, ZnSO.sub.4 3M, 1M KCl PE 900 75% 80% 60% 13 KI 5M, ZnBr.sub.2 2.5M PE 900 95% 83% 79% 14 KI 6M, ZnBr.sub.2 3M PE 900 95% 83% 79%
TABLE-US-00004 Comparative example Electrolyte (mol/L) membrane Thickness (μm) CE VE EE 1 KI 5M, ZnBr.sub.2 2.5M, 1M Nafion 115 125 99% 81% 80% KCl 2 KI 5M, ZnBr.sub.2 2.5M, 1M Nafion 212 50 98% 83% 81% KCl 3 ZnI.sub.2 2.5M PE 900 99% 71% 70% 4 ZnI.sub.2 3M PE 900 98% 70% 68% 5 KI 5M, ZnBr.sub.2 2.5M, 1M KCl PE 65 74% 88% 65%
[0050] FIG. 11-17 show zinc iodine dual-flow batteries that employed ZnBr.sub.2 and KI as the active substance, KCl as the supporting electrolyte with a 900 μm porous membrane. The battery can continuously run stably for more than 1000 cycles at 80 mA/cm.sup.2. Above all, the energy efficiency is greater than 80% with the energy density above 80 Wh/L. The advantages of the above system include: the introduction of Bf in ZnBr.sub.2 can form a complex agent of I.sub.2Br.sup.−, thereby inhibiting the precipitation of I.sub.2; the replacement of traditional ZnI.sub.2 with KI can avoid the formation of zinc oxide and hydroxide during the charge and discharge process. The employment of porous membranes benefit the conduction of neutral ions, which improves the operating current density and power density of the battery. In addition, the absence of ion exchange groups in the membrane can greatly reduce the membrane fouling issue and improve the cycle stability of the battery.
[0051] Compared with the most preferred example: FIG. 18 employs a thinner porous membrane which resulted in a drop of performance especially the coulomb efficiency. This is mainly due to the employment of a thinner membrane that lead to much more serious cross-contamination. In FIG. 19, ZnSO.sub.4 replaced ZnBr.sub.2 and the voltage efficiency of the battery was greatly reduced, which indicates that the sulfate ion affected the electrochemical kinetic of electrolyte; in FIG. 20, when the supporting electrolyte was removed, the voltage efficiency of the battery was reduced slightly. FIGS. 21 to 25 demonstrate that, under preferred conditions, the battery displayed excellent rate performance and temperature dependent performance; in addition, the porous membrane could eliminate the zinc dendrites formed on the negative electrode as the pore structure was filled with oxidized I.sub.3.sup.−, which could react with the zinc dendrite. Therefore, the single battery and the battery stack can self-recovered after a micro short circuit occur, which greatly improves the stability of the battery. Most importantly, the battery stack can continuously run stably for more than 300 cycles at 80 mA/cm.sup.2.
[0052] Compared with the preferred example: Nafion 115 membrane was used for the battery in FIG. 26. Due to the poor conductivity of the membrane, the voltage efficiency of the battery was lower than that of optimal example, however, the employment of Nafion 115 membrane greatly reduced the crossover issue and greatly improved the coulombic efficiency of the battery. However, the performance of the battery deteriorated sharply after 15 cycles, which was due to the serious membrane fouling of the Nafion 115 membrane caused by I.sub.2 and Zn dendrite, the membrane resistance increased greatly and the polarization was intensified. FIG. 27 used ZnI.sub.2 as the electrolyte and the battery performance was severely degraded, which was caused by the instability of the positive and negative electrolytes. The positive electrolyte would form I.sub.2 precipitation during the charging process and the negative electrolyte would form zinc oxide and hydroxide. FIG. 28 used a much thinner porous membrane, the cross-contamination of the electrolyte was intensified and the coulombic efficiency of the battery was greatly reduced.
