Magnesium batteries have long been pursued as potentially high-energy and safe alternatives to Li-ion batteries; however, fast chargedischarge capability, one of the most desired properties for advanced batteries, remains elusive for this technology. Here, we develop a next generation Mg battery prototype, delivering a specific energy of up to 566 Wh kg 1 and an ultrahigh specific power of up to 30.4 kW kg 1 , which is close to two orders of magnitude higher than state-of-the-art Mg battery. This is achieved by coupling a kinetically fast organic cathode material, operating under bond cleavage-free solidliquid reaction, and an electrolyte capable of providing dendrite-free Mg depositionstripping at a record current density of 20 mA cm 2 .One Sentence Summary: Ultrahigh power, an unprecedented quality of Mg battery, is unveiled and demonstrated using a prototype combining a quinone-based cathode and a second to none Mg(CB 11 H 12 ) 2 electrolyte that enables ultrahigh rate cycling of dendrite-free Mg anode.

In typical chloride-containing electrolytes, storage of MgCl + is dominant in organic cathodes. The negative impact of the MgCl-storage chemistry on the specific energy was elucidated through cell tests with controlled amounts of electrolyte. With the right combination of organic cathodes and chloride-free electrolytes, storage of Mg 2+ in organic electrodes can be realized. The Mg-storage chemistry has also enabled the first Mg battery that operates under lean electrolyte conditions, which has important implications for the practicality of high-energy organic Mg batteries.

All-solid-state sodium batteries (ASSSBs) with nonflammable electrolytes and ubiquitous sodium resource are a promising solution to the safety and cost concerns for lithium-ion batteries. However, the intrinsic mismatch between low anodic decomposition potential of superionic sulfide electrolytes and high operating potentials of sodium-ion cathodes leads to a volatile cathode-electrolyte interface and undesirable cell performance. Here we report a high-capacity organic cathode, Na C O , that is chemically and electrochemically compatible with sulfide electrolytes. A bulk-type ASSSB shows high specific capacity (184 mAh g ) and one of the highest specific energies (395 Wh kg ) among intercalation compound-based ASSSBs. The capacity retentions of 76 % after 100 cycles at 0.1 C and 70 % after 400 cycles at 0.2 C represent the record stability for ASSSBs. Additionally, Na C O functions as a capable anode material, enabling a symmetric all-organic ASSSB with Na C O as both cathode and anode materials.

wileyonlinelibrary.comionic radius as that of Li + (0.9 Å), its high charge density and polarity usually lead to a low solid-diffusion rate of Mg 2+ in most of the crystal structures. [ 3 ] An exception is the Chevrel phase (e.g. Mo 6 S 8 ), where octahedral Mo 6 clusters enable a fast and effi cient achievement of local electroneutrality upon Mg 2+ insertion. [ 3 ] However, its energy density is very limited in view of a theoretical capacity of 122 mAh g −1 (corresponding to two Mg 2+ insertion into one Mo 6 S 8 host) and a moderate voltage (1.1-1.2 V vs Mg 2+ /Mg).In order to extend the choice of cathode and circumvent the sluggish Mg 2+ transport in host lattices, recently a concept of dual-salt polyvalent metal storage was proposed, where e.g., Li + instead of Mg 2+ is inserted into cathode from the Li-ion reservoir of dual-salt electrolyte during discharge and at metallic Mg anode Mg 2+ is preferentially electrodeposited over Li + during charge. [ 4 ] It indicates that many Li-insertable materials can become the candidates of cathode as long as their reaction voltages do not exceed the electrochemical window of present Mg-based electrolyte systems. Furthermore Lidendrite problem appears to be avoidable in this hybrid Mg/ Li battery (MLB) design. Some sulfi des and oxides (e.g., TiS 2 , TiO 2 , and Li 4 Ti 5 O 12 ) have been attempted as cathodes in view of their moderate voltages (<1.5 V) matching well with two typical dual-salt electrolytes, i.e., all-phenyl complex (APC) coupled with lithium chloride (LiCl) and magnesium borohydride (Mg(BH 4 ) 2 ) with lithium borohydride (LiBH 4 ). [5][6][7][8][9][10] Their good rate and cycling performances benefi t from faster Li + insertion at cathode and safer Mg 2+ deposition at anode. So far, resorting to higher voltage Li-ion cathodes (e.g., LiFePO 4 ) seems to be unsuccessful due to quick electrolyte degradation. [ 4 ] Therefore, in this work, we propose to utilize Li-driven conversion reaction instead of insertion one in order to signifi cantly improve the capacity performance of Mg-based batteries for the fi rst time.Here, two typical resource-abundant sulfi des FeS 2 and FeS are investigated as compatible conversion electrodes in Mgbased batteries. Although FeS 2 has been assigned as primary and secondary Li battery cathodes for many years, its advantage of high capacity (a theoretical value of 894 mAh g −1 referring to four electron transfer based on Fe 2+ /Fe 0 and S 2 2− /S 2− ) is seriously counteracted by the dissolution of polysulfi de intermediate products as well as Li-dendrite growth, which cause a quick capacity fading during cycling as in Li/S systems. [ 11,12 ] Therefore, the similar strategies for Li/S batteries can be copied to Li/FeS 2 ones, including cathode coating (e.g., by PAN), utilization of polysulfi de-dissolution repressible electrolytes Mg batteries as the most typical multivalent batteries are attracting increasing attention because of resource abundance, high volumetric energy density, and smooth plating/stripping of Mg anodes. Howeve...