Lithium ion batteries (LIBs) based on intercalation/de-intercalation chemistries have dominated the battery markets of portable electronics and electric vehicles due to its high energy/power density. However, they are expensive to scale up for large-scale electric gird application because of the low abundance and uneven distribution of raw cathode materials in the Earth crust. So, it is highly desirable to develop novel battery chemistries with Earth-abundant elements for next-generation batteries. Here we propose novel metal battery systems in Li⁺-containing non-aqueous electrolytes, in which the redox reaction (M/Mⁿ⁺) of common metals, for instance, stainless steel (iron), nickel and copper, can be applied as cathode chemistries to store energy. When the metals are paired with a Li-based anode, a high output voltage can be obtained.

Such novel metal battery systems equipped with metal cathodes hold remarkable promises in cost, capacity and energy density. First, because the metals such as copper and steel are highly abundance in Earth crust with mature mass-production technologies, these metal batteries could be extremely competitive in cost. Second, as the metals can undergo multi-electron transfer, high theoretical capacity can be expected. For example, with three-electron transfer (Fe/Fe³⁺), stainless steel cathodes can theoretically deliver a high specific capacity of ~1436 mAh g^-1, which is far beyond that of conventional intercalation cathodes. Finally, since the redox potentials of metals are high (e.g., ~ 3.4 V vs. Li/Li⁺ for Cu/Cu⁺ reaction), metal batteries in theory can also give high energy density. Herein, the progress of our work based on metal batteries will be introduced comprehensively; the roadblocks and perspective future work with respect to the scientific investigation and practical application will also be discussed.

The oxidation of metal cathodes fall into two categories: depending on the type of transition metals, we can either have dissolution of cations into electrolyte (e.g., Cu²⁺, Ni²⁺ and Fe²⁺) or precipitation of metal compound such as AgCl on the surface of the metal after charging. To simplify the system, we decide to focus on the latter case first and employ Ag as cathode material to demonstrate the feasibility of our idea. A 2.8 V Ag-Li cell in 0.05 M LiCl 1 M LiPF₆ ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1 was demonstrated. X-ray diffraction confirms the formation of AgCl during charging. We show that the capacity of the electrode increases with decreasing particle size and enhanced electrical conductivity – a capacity of ~ 200 mAh g^-1 can be achieved with nano-Ag supported by CNT (Ag-CNT). Interestingly, Ag/AgCl electrode also shows excellent discharge rate performance, suggesting facile removal of Cl- from AgCl during the process. Cycle performance of the battery is highly influenced by the type of solvent used in the electrolyte. Better cycle stability with smaller amount of AgCl dissolution is achieved with dimethyl carbonate (DMC)-based electrolyte.

The application of Ag cathode demonstrates the feasibility of metal battery systems but with a high cost for cathode materials; so, we next turn to inexpensive mass-produced stainless-steel materials, which possess a major component of iron. Unlike Ag/AgCl solid state redox reaction, the oxidation of iron cathode during charging would release soluble Fe²⁺ into electrolytes, the migration of Fe²⁺ to anode surface would lead to adverse self-discharge, lowing the reversibility of metal batteries. So, an ion-selective membrane is required in the electrolyte to stop the cross-over of Fe²⁺. Here we demonstrate a highly reversible ~2.5 V stainless steel/Li battery separated with anion exchange membrane (AEM) in 1 M LiPF₆ EC/DEC (v = 1:1) with saturated LiCl (0.05 M). During cycling, there is stripping/plating of iron at cathode, accompanied with plating/stripping of Li at anode. The AEM can allow passage of anions to balance the charge and prevents cross-over of cations, while Cl- dissolved in the electrolyte can effectively trigger the dissolution of Fe²⁺ during charging. As the two-electron transfer and high output voltage, this stainless steel-Li system shows high theoretical energy that are competitive to current energy storage technologies.

Besides stainless-steel cathodes, we also investigated the use of copper as cathode with Al as anode for storing energy. As we know, Cu and Al foils are commonly used in lithium-ion batteries as current collectors. They are inactive and do not participate in the charge-discharge reactions but take up resources and space within a cell. Herein, we demonstrate a proof of concept to turn the Cu and Al foils into active materials by constructing a Cu-Al full cell, with Cu undergoing stripping/deposition reactions at the cathode and Al alloying with lithium at the anode. A piece of AEM was also inserted in between to prevent cation cross-over. Stable cycle performance is possible with the use of a highly concentrated electrolyte - a 3 V cell exhibits excellent cycle stability for more than 200 cycles in 6 M LiTFSI DMC electrolyte. Our Cu-Al battery can give a volumetric energy density of the range of 79-156 Wh L^-1, comparable to that of state-of-the-art all-vanadium redox flow batteries. The use of inexpensive Cu and Al as active materials can also potentially reduce the cost of energy storage.

The abovementioned 3 V Cu-Al batteries are shows promising properties for large-scale energy storage owing to the low cost and excellent scalability of the two metal electrodes. However, the wide usage of expensive AEM (about USD 500-700 per square meter) between two electrodes incurs substantial cost and lowers the scalability. We further show that inexpensive polypropylene membrane can act as an effective alternative to AEM to address this issue. We demonstrate that the reversibility of the Cu-Al battery depends strongly on interaction of the Cu ions with the electrolyte solvent and subsequently the affinity of the solvated Cu ion with the membrane separator. Specifically, a series of common carbonate-based electrolyte solvents were investigated via molecular dynamics and contact angle measurements to understand the interaction between the solvents and a polypropylene (PP) membrane, as well as that between cations and solvent. Amongst different solvents, fluoroethylene carbonate (FEC) was shown to drastically enhance the Coulombic efficiency to 97%, compare to that of 27% with dimethyl carbonate. We further demonstrate remarkable cyclability of a 3 V Cu-Al battery with 3 M LiTFSI FEC and PP membrane up to 1000 cycles. This finding opens new opportunities for the development of low-cost, high performance Cu-Al systems for stationary application.

Overall, metal battery systems with inexpensive and highly scalable battery components, including cathodes, anodes, and the membranes are potential low-cost energy storage for further applications. In addition, with multi-electron transfer and high redox potential of transition metals, the battery promises high energy density. The novel battery chemistry is still in its early state. Further breakthroughs, for example, cost-effective electrolyte is still needed to realize its practical application.