Due to continuous demand of energy storage device for portable electronic devices, such as smart mobile phone, smart watch and wisdom cloth, the advanced flexible/wearable energy storage technologies with low cost and high safety are required. Among various energy storage technologies, electrochemical energy storage employing rechargeable batteries is one of the most effective approaches. Currently, the electrochemical energy-storage device landscape is being dominated by Li-ion battery continuously. However, there are various safety issues of flammable organic electrolyte and obstacles of constrained lithium resource and high cost to hinder Li-ion battery application in large-scale energy storage systems. Despite various limitation for the Li battery, it plays a vital role in our daily life as power source for potable electronics and shows potential for electrified vehicles, due to their high energy density and high specific power. The phenomenon is attributed to non-alternative battery system, which possess equivalent energy as power source to share the market. In such circumstance, exploiting alternative electrochemical energy storage technologies with intrinsic safety and excellent electrochemical performance is urgently desired in terms of boosting the practical applications of portable electronics.

Rechargeable aqueous Zn-ion batteries have been investigated as an alternative battery chemistry for grid-scale electrochemical energy-storage technologies, due to a high-capacity Zn metal anode (819 mAh·g^-1); high ionic conductivity (up to 1 S·cm^-1), low redox potential (-0.76 V vs. standard hydrogen electrode (SHE)), intrinsic safety, environmental friendliness; earth-abundant materials. However, the strong electrostatic interaction with host materials originating from divalent chemistry, leads to low output voltage, poor reversibility and especially sluggish kinetics. The Prussian blue analogues (PBAs), possessing a three-dimensional (3 D) open framework with large interstitial sites, is considered as a promising host material for reversible Zn-ion intercalation/deintercalation with fast charge/discharge properties and high operational voltage, which is ascribed to its ideal crystal structure and desirable electrochemical properties. However, the specific capacity delivered in aqueous battery systems is limited to ~ 60 mAh·g^-1 due to only single electrochemically activated species of transition metal ions. Moreover, most batteries suffer from poor cyclic stability (<1000 cycles), lowly operating voltage (~1.2 V) and unsatisfactory rate capability (<1 A·g^-1). To address this challenge, a two-species redox reaction of Co(II)/Co(III) and Fe(II)/Fe(III) incorporated in cobalt hexacyanoferrate (CoFe(CN)₆) is proposed as a breakthrough to achieve jointly high-capacity and high-voltage aqueous Zn-ion battery. The Zn/CoFe(CN)₆ battery provides a highly operational voltage plateau of 1.75 V (vs. metallic Zn) and an high capacity of 173.4 mAh·g^-1 at current density of 0.3 A·g^-1, taking advantage of the two-species redox reaction of Co(II)/Co(III) and Fe(II)/Fe(III) couples. Even under extremely fast charge/discharge rate of 6 A·g^-1, the battery delivers a sufficiently high discharge capacity of 109.5 mAh·g^-1 with its three-dimensional opened structure framework. This is the highest capacity delivered among all the batteries using PBAs cathode up to now. One further step, a sol-gel transition strategy for hydrogel electrolyte is developed to construct high-performance flexible cable-type battery. With the strategy, the active materials can adequately contact with electrolyte, resulting in improved electrochemical performance (~18.73% capacity increase) and mechanical robustness of the solid-state device.

While the widely reported Zn chemistries operate in and benefit from the adopted aqueous electrolytes, distressingly, Zn anode also persistently suffers from several deep-seated issues resulting from the aqueous electrolyte. One is parasitic hydrogen evolution reaction (HER) through water splitting, eventually leading to cell volume expansion and relatively low coulombic efficiency. The other one is the dendrite growth during Zn anode plating/stripping process. Therefore, we firstly demonstrate that an ionic liquid-based Zn salt electrolyte is an effective route to solve both side-reaction of HER and Zn dendrite growth. The developed electrolyte enables hydrogen-free, dendrite-free Zn plating/stripping over 1500 h cycle (3000 cycles) at 2mA·cm^-2 with nearly 100% coulombic efficiency. Meantime, the oxygen induced corrosion and passivation are also be effectively suppressed. These features bring Zn-ion batteries unprecedented long lifespan over 40000 cycles at 4 A·g^-1 and high voltage of 2.05 V with a cobalt hexacyanoferrate cathode. Furthermore, a 28.6 µm-thick solid polymer electrolyte of a poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) film filled with polyethylene oxide (PEO)/ionic liquid-based Zn salt is constructed to build an all-solid-state Zn-ion battery. The all-solid-state Zn-ion batteries shows excellent cycling performance of 30000 cycles at 2 A·g^-1 at room temperature and withstand high temperature up to 70 ^oC, low temperature to -20 ^oC, as well as abuse test of bending deformation up to 150^o for 100 cycles and 8 times cutting.

Additionally, Mg-ion battery is regarded as promising candidate for grid-scale energy storage due to it low electrochemical potential (-2.37 V (vs. SHE),) and high energy density (3833 mAh·cm^-3) of Mg metal. However, all-solid-state full Mg-ion batteries have not yet been reported due to the limited availability of suitable electrodes and electrolyte. We for the first time, developed an all-solid-sate Mg-ion battery consisting of an cobalt hexacyanoferrate cathode, an organic 3,4,9,10-perylenetetracarboxylic diimide anode and a solid polymer electrolyte of poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) film filled with polyethylene oxide (PEO)/ amide-based molten salt. The all-solid-sate Mg-ion batteries can operate well at rates of 0.1-3A·g^-1 and shows superior cyclic stability of 5000 cycles at current density of 2 A·g^-1 at room temperature and withstand high temperature up to 120 ^oC, low temperature to -20 ^oC, as well as sewability of 180 sewing tests. This is the first demonstration of an all-solid-state Mg-ion battery, which could pave the way for practical application of Mg-ion batteries for grid-scale energy storage and wearable application.

In summary, various metal-ion batteries including Zn-ion batteries and Mg-ion batteries with high voltage, high capacity, high flexibility, excellent environmental adaption, extreme safety, long-cycling life, and outstanding energy output capability are developed and studied in depth in this thesis, targeting at obtaining both high-voltage and high-capacity metal-ion batteries together with excellent flexibility. It is believed that these studies and strategies presented in the thesis can shed light on the design of high-voltage and high-capacity batteries, as well as open up new perspectives to develop reliable flexible energy storage systems for future wearable electronic.