Where E = energy density, c = molar concentration of active compound, z = number of electrons exchanged, F = Faraday constant, U = operation potential. Thus, one major thrust in RFBs research is the development of new active materials which combine high solubility, high redoxactivity and high stability over a wide potential range. Moreover, active materials capable of storing and releasing multiple electrons are highly sought-after to increase RFB energy density. In addition, using identical species in both RFB half-cells is advantageous as it results in simple operation, where membrane crossover or electrochemical interference can be avoided. Historically, aqueous RFBs have been most explored, and a variety of species have been developed, including metal ions, transition metal complexes and organic species. However, aqueous RFBs suffer from several drawback including a small potential window due to water oxidation/water reduction, also, membrane crossing can lead to self-discharging and loss of capacity. In addition, many redox-species used in aqueous RFBs offer only one-electron storage, which further limits specific energy capacity. Last, most aqueous RFBs utilize highly acidic or basic electrolytes to improve the solubility and stability of the redox-active materials, resulting in high maintenance cost and further diminishing the lifespan of the entire system. To overcome these challenges, non-aqueous RFBs (NRFBs) have garnered significant attention. NRFBs utilize polar organic solvents to achieve wide potential operation windows. As an example, the prototype NRFB solvent acetonitrile has a stable potential window of up to 5 V, which enables redox-events which are not accessible in aqueous systems. Also, the use of organic solvents facilitates the use of redox-active species which are insoluble in water, thereby broadening the scope of RFBs. Molecular metal oxides, or polyoxometalates (POMs) have received significant interest as potential multi-electron storage molecules for NRFBs. The promises of POMs clusters lie in their superior structural configuration and excellent redox properties, which facilitate the reversible storage and release of multiple electrons without structure decomposition. Notably, the solubility and redox properties of POMs clusters can be rationally tailored by introduction different counter cations and various atoms or ligands. Beyond electrochemistry, nano-sized POMs clusters offer significant advantages over smaller ionic/molecular charge carriers, as their size and charge limits membrane crossing and prevents self-discharge. Figure The earliest research on RFBs based on POMs was conducted by Sun and colleagues in 1997, who reported an asymmetric aqueous RFB using Keggin POM H3[PMo12O40] as the anolyte active species and Fe 3+/ Fe 2+ as the catholyte active species. Subsequently, Xiang and coworkers reported an aqueous RFB based on H6[CoW12O40], which demonstrated reversible transfer of three electrons and enabled symmetric RFB operation. In recent years, Matson and co-workers have reported seminal studies in the field of NRFBs based on Lindqvist POMs [V6O7(OR)12] (R=CH3, C2H5). The group also showed how structural modifications (e.g., functionalization with heterometals or different alkoxide ligands) affects NRFB performance. Notably, polyoxovanadates (POVs) have attracted widespread attention as RFB active species due to their significantly lower mass (M(V) = 52 g mol -1 ) compared to more traditional molybdenum (M(Mo) = 95 g mol -1 ) and tungsten (M(W) = 180 g mol -1 ) POMs, implying that POV clusters could offer higher energy density per unit mass than heavier metal oxide clusters. To-date, most POM (N)RFBs, only allow the transfer of up to four electrons across different oxidation states, necessitating an asymmetric configuration to handle charge imbalances. Thus, it remains challenging to develop POM-based RFBs that enable multi-electron storage and release in a symmetric system with high operation voltage. Recently, some of us reported the synthesis of mixed-valent polyoxovanadate co-crystals {MV13} (nBu4N)4[MV13O33Cl] (M= VO 2+ or MgOH + ) which demonstrated excellent redox properties: briefly, the {MV13} co-crystals contain 1:1 molar mixtures of the five-electron reduced species [V IV 5V V 9O34Cl] 4-(= {V14}) and the three-electron reduced species [(MgOH)V IV 3V V 11O33Cl] 4-(={MgV13}). Thus, {MV13} can be given as (nBu4N)8[(MgOH)V13O33Cl][V14O34Cl] = (nBu4N)8{MgV13}{V14}, see Figure . {MV13} is soluble in coordinating organic solvents, e.g., acetonitrile (solubility = 10 mM), facilitating the design of multi-electron storage systems. In detail, {MV13} can reversibly store and release up to 16 electrons in the potential range of -2.2 V -1.4 V vs Fc + /Fc. This is in line with previous studies on related vanadates, where increased redox-activity depending on the exact metal substitution was observed. Also, as the two cluster species in {MV13} form in the same reaction solution, there are no chemical incompatibilities between these components. Herein, we demonstrate that acetonitrile solution of {MV13} can be used as electrolyte for symmetric NRFB (as shown in Figure ). Fundamental electrochemical investigations revealed that {MV13} cluster enables reversible storage and release of 14 electrons spanning a wide electrochemical window with remarkable diffusion coefficients and excellent electron transfer rate constants in acetonitrile, ensuring promises for symmetric NRFBs applications.