with SIBs, potassium-ion batteries (KIBs) deliver a higher working voltage due to the lower redox potential of K + /K (−2.92 V vs SHE) than that of Na + /Na (−2.71 V vs SHE). [14] In addition, K + has been demonstrated to be reversibly intercalated/ deintercalated into graphite anode, while very limited amounts of Na + could. [14] However, as the research on KIBs is still at the infancy stage, the mechanisms at an atomic scale and interfaces are unclear. In addition, owing to the large size of K + that would cause sluggish kinetics, only a few of cathode materials (Prussian blue and its analogues) [16] and anode materials (graphite, [14] Sn 4 P 3 /C) [13] were investigated. Therefore, developing suitable electrode materials with good performance as well as comprehensively investigating the mechanism are of great importance.On the other hand, another new type of battery called dual-ion battery (DIB) [5][6][7][8][9][10][11] also arouses scientists' concern, which generally consists of dual graphitic carbon electrodes. Owing to the intrinsic redox amphotericity of graphitic carbon materials, both cations (Li + ) and anions (PF 6 − , BF 4 − , TFSI − , etc.) are intercalated/deintercalated into graphite anode and graphite cathode, respectively, during the charging/discharging process in the dual-graphite DIB. Apparently, the dual-graphite DIBs show advantages in terms of low cost, good safety, and environmental friendliness. In addition, the high working voltage (mainly above 4.5 V) [5] of the DIB caused by high anion intercalation potential is also beneficial for high energy density. Extensive researches on DIBs have been focused on exploiting high-capacity cathode materials, [6c] alternative anode materials, [6a,7-11] as well as suitable electrolyte compositions, [5] and great progresses on their development have been made.In this work, on the purpose of combining both advantages of KIBs and DIBs, we first report a dual-carbon battery (DCB) based on a potassium-ion electrolyte (1 m KPF 6 in carbonate solvent), using expanded graphite (EG) as cathode material and mesocarbon microbead (MCMB) as anode. The working mechanism of the as-prepared K-ion-based dual-carbon battery (named as K-DCB) was investigated, which was further demonstrated to deliver a reversible discharge capacity of 61 mA h g −1 at 1 C (1 C corresponding to 100 mA g −1 ) current rate and also show good cycling performance for 100 cycles with negligible capacity decay. Moreover, the battery works reversibly and stably over a wide voltage window of 3.0-5.2 V with medium discharge voltage of 4.5 V, the highest value among the reported KIBs. [12][13][14][15][16] Figure 1a schematically shows the working mechanism of the K-DCB configuration utilizing an EG cathode and MCMB Although potassium-ion batteries (KIBs) have been considered to be promising alternatives to conventional lithium-ion batteries due to large abundance and low cost of potassium resources, their development still stays at the infancy stage due to the lack of appropriate cathode and anod...

Aurbach et al. [5] indicated that calcium deposition was impossible in organic solutions at room temperature. Molten salt cells were demonstrated to exhibit good reversibility at high operating temperature (550-700 °C), [6] which is difficult to meet with the mainstream applications operated under room temperature. Palacín et al. [3a] achieved the deposition of calcium at 75-100 °C and proved its reversibility for over 30 cycles. Recently, using Ca(BH 4 ) 2 in tetrahydrofuran as electrolyte, Ca ions were realized plating and stripping at room temperature for 50 cycles. [7] Up till now, because of the lack of an appropriate combination of suitable electrode materials and electrolytes, [8] it is difficult to construct calcium-based energy storage devices based on the conventional rocking-chairtype mechanism.Multiion reaction mechanism is a possible approach to break Ca-technology barrier, in which anions and cations react with cathode and anode, respectively, during charge process (Figure 1). In general, supercapacitors (SCs) exhibit superior rate and cycling properties [9] and dual-ion batteries (DIBs) have displayed high capacity and working voltage. [10] Meanwhile, SCs have to stop storing charge when the potential of either electrode reaches the threshold of electrolyte, [11] which prevents SCs from fully utilizing the voltage window of electrolyte, resulting in relatively low capacity and voltage. As for DIBs, the anion intercalation/deintercalation on cathode side will destabilize the structure for the large radius of anions (TFSI − , PF − 6 , etc.), thus degrading the cycling and rate performance. [12] Recently, Tang et al. developed a reversible calcium-based DIB at room-temperature, [13] but the cycling stability (350 cycles) and rate performance (the capacity remained 55% when the current density increased from 0.1 to 0.4 A g −1 ) were far from practical applications.Herein, we reported a novel calcium-ion hybrid energy storage device (Ca-HSC) at room temperature via combining the features of SCs and DIBs (Figure 1c), in which capacitor component cathode and battery component anode served as a complementary to each other while maintaining their unique advantages. [14] In detail, the stable and low anodic potential of this device caused by Faradaic reaction allowed the cathode potential to be tested up until the upper limit of electrolyte, enabling large capacity and high voltage. Meanwhile, the non-Faradaic reaction at the cathode brought fast kinetics performance and long cycling life. After optimization, this Ca-based Ca-ion based devices are promising candidates for next-generation energy storage with high performance and low cost, thanks to its multielectrons, superior kinetics, as well as abundance (2500 times lithium). Because of the lack of an appropriate combination of suitable electrode materials and electrolytes, it is unsuccessful to attain a satisfactory performance on complete Ca-ion energy storage devices. Here, the multiion reaction strategy is defined to construct a complete Ca-ion ener...

The growing demand for advanced lithium-ion batteries calls for the continued development of high-performance positive electrode materials. Polyoxyanion compounds are receiving considerable interest as alternative cathodes to conventional oxides due to their advantages in cost, safety and environmental friendliness. However, polyanionic cathodes reported so far rely heavily upon transition-metal redox reactions for lithium transfer. Here we show a polyanionic insertion material, Li 2 Fe(C 2 O 4 ) 2 , in which in addition to iron redox activity, the oxalate group itself also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution. The current study gives oxalate a role as a family of cathode materials and suggests a direction for the identification and design of electrode materials with polyanionic frameworks.