Working principle of aqueous lithium battery system

In aqueous electrolytes, the difference between their redox potentials is very large and their combination will establish the general structure of a rechargeable battery system. The assembled aqueous rechargeable lithium battery (ARLB) uses coated metallic lithium as an element. anode and lithium manganate as cathode. The sweep speed of the CV curve is 0.1 mV/s, and there are two pairs of redox peaks, located at 4.14/3.80 and 4.28/3.93 V respectively. From the diagram above, the redox reactions look like this: During charging, there is only one reaction at the anode. Li+ ions are transported from the aqueous electrolyte through the coating layer, thereby reducing the surface area of ​​Li metal and depositing Li metal. Two reactions occur at the cathode: Li+ cations deintercalate from tetrahedral 8a and octahedral 16c sites , which then results in two pairs of redox peaks, similar to the behavior of organi electrolytesques. When discharging, the reverse process occurs. Therefore, there are two pairs of redox peaks in our ARLB CV curve. This shows that our above lithium metal chemical coating for battery, 0.5mol.L-1 Li2SO4/LiMn2O4, can achieve average output voltage of rechargeable batteries with aqueous electrolyte above 3.8V, which is much higher than the theoretical decay voltage of water, i.e. 1.229 V.

Figure 3: (1) Schematic diagram The rechargeable lithium-water battery (ARLB) that We designed uses coated metallic lithium as anode, lithium manganate as cathode and an aqueous solution of 0.5 mol.L-1 Li2SO4 as electrolyte. , and (b) resume ARLB with a scan rate of 0.1 mV s-1. The potential change of Li+ ions in our designed ARLB is shown in Figure 4. Lithium metal has the lowest redox potential, -3.05 V (compared to the hydrogen electrode standard, SHE), and reacts quickly with water to produce hydrogen and LiOH. In addition, the potential of lithium metal is much lower than that of hydrogen release, and hydrogen will be easily produced. However, in our case, the coated metallic lithium is very stable in the aqueous electrolyte and there is no hydrogen evolution. The main reason is that Li+ ions can pass through the potential range of hydrogen evolution through the coating and directly reach lithium metal. This crossing is similar to the potential change between the two sides of the cell membrane24. There is a sharp decrease in the potential of Li+ ions in the coating, from positive to negative. The outer layer of Li+ ions has a higher potential and is very stable. The Li+ ions are not in contact with the water inside the coating and cannot donate electrons to the Li atoms, resulting in the production of hydrogen in the water. At pHowever, water and protons cannot penetrate the inner coating, and they cannot reach a low enough potential to produce hydrogen. As for the LiMn2O4 cathode, it is stable because its potential is underwater, for the release of oxygen and much higher than that for the release of hydrogen.

Figure 4: Schematic diagram of the potential of Li+ ions during the movement of LiMn2O4 between the electrolyte and the coated lithium metal.

The ARLB voltage between 3.7 and 4.25 V is shown in Figure 5.chemical properties. The ARLB curve of constant current charging at a current density of 100 mA g-1 shows two different voltages at 4.04 and 4.18 V. Based on the mass of lithium manganate during the discharge process, two voltage plateaus appear at V of 4.07 and 3.94. , respectively. This is in good agreement with the intercalation and deintercalation of Li+ ions in lithium manganate spinelle, two of the redox peaks observed in the CV curves. About 4.0 V, 0.2 V is higher than the average discharge voltage of these lithium-ion batteries based on LiMn2O4 cathodes and graphite-carbon anodes. Based on discharge and charge voltages, the energy efficiency is over 95%, higher than that of lithium-ion batteries (around 90%) and other battery systems12,22,25. The initial charge and discharge capacity of this battery based on the mass of lithium manganate is 130 and 115 mAh g-1, respectively, and the initial Coulomb efficiency is 88.5%. These values ​​are similar to those of the organic electrolyte7. The capacities are much higher than those of solution 12 based on the new liquid cathode. Of course, in lithium-ion batteries using organic electrolytes, lithium manganate must be doped or coated to ensure good cycle performance26, and its reversible capacity est less than 110 mAh g-1. Here, LiMn2O4 does not require doping or coating16,17 and the specific capacity of ARLB is actually higher than that of lithium manganate in organic electrolytes.

Figure 5: Electrochemical performance of our designed ARLB based on lithium manganate mass at current densities of 100 mA G-13.7 and 4.25 V: (1) Charging curve and galvanostatic discharge in the first cycle and (b) cycling behavior.

Based on the discharge voltage and capacitance of the Li metal anode and LiMn2O4 cathode, the ARLB discharge energy density as a function of the total mass of the material of the electrode is 446 Wh kg-1, much higher than those previously reported. ARLB (30-45 Watts kg-1) 14, 15, 16, 17, 18, 19, 20, 21. Of course, it is higher than for aqueous Li/M+ solutions and other liquid flow batteries3, 4, 5, 9, 12. Based on lithium-ion battery manufacturing technologyion with half the energy density, it can be made almost available7,14, which means that the actual energy density is more than 220 Wh kg-1, which is about 80% higher than that of lithium-ion batteries. the corresponding Li-ion battery for Electric Vehicle (120 Wh kg-1 for C/organic electrolyte/LiMn2O4) 6, 7. This high energy density means that pure electric vehicles can travel between 200 and 400 kilometers on a single charge.

During cycling, the ARLB efficiency in terms of coulombs at a current density of 100 mA g-1 was almost 100% except in the first cycle, which is similar to mass used for LiMn2O4 in lithium-ion. batteries. This high Coulomb efficiency also shows that water is very stable and does not exhibit obvious side reactions.Protons or water. After 30 complete cycles, the discharge capacity remains very stable at around 115 mA G-1, which meansie that no significant decrease in capacity occurs at the 30th cycle. This shows that the cycling performance of this battery chemistry is very excellent, similar to that of LiMn2O4 in conventional ARLBs (see Supporting Information Figure S4A: there is no obvious capacity loss after 200 cycles for this lithium manganate cathode). In the latter case, lithium manganate can retain 93% of its capacity after 10,000 complete cycles, which is better than other batteries under charge16,17. The polymer electrolyte of Li metal can dampen the volume change during the dissolution process of electroless plating to ensure its good contact with the coating. This is also important for achieving excellent cycling performance.

In traditional lithium metal secondary batteries, the use of lithium metal as an anode material is limited, mainly due to thesafety concerns of lithium dendrites, as they form short circuits during repeated charging and discharging processes. In our design, as shown in Figure 1, lithium metal is covered with GPE and LISICON membranes. Li dendrite formation will be inhibited in GPE27 due to its higher viscosity compared to organic liquid electrolytes. Even when lithium dendrites form, they cannot grow through the LISICON film11,12,22. As a result, the safety and cycling performance of Li metal anodes are ensured.

The water electrolyte in this ARLB system has a high heat capacity and can absorb a large amount of heat. During the same charging and discharging process, the system temperature is much lower than that of conventional lithium-ion batteries. In addition, the water or aqueous electrolyte is in direct contact with the metal anode Li and the cathodeLiMn2O4, and the cooling effect will be very effective. Cooling systems, typically required for large capacity battery modules, are not required for applications in electric vehicles. Compared with traditional lithium-ion batteries, safety and reliability are greatly improved.