In summation, this thesis provides a panoramic view of the prevailing challenges and potential solutions associated with achieving unparalleled energy and power densities in GDIBs where graphite reigns supreme as the cathode material of choice. By comprehensively tackling these challenges and integrating the recommended strategies, there lies a promising path ahead for the evolution of advanced dual-ion batteries. Such advancements could redefine benchmarks in en-ergy storage, heralding an era of more efficient and versatile applications.

Anion intercalation in the graphite positive electrode of a dual-ion battery requires high potential (> 4.3 V vs Li+/Li), which aggravates parasitic reactions involving electrolyte decomposition and Al corrosion, manifesting in poor coulombic efficiency, cycle life, and quick self-discharge. This study aims to investigate the stability of anion-intercalated graphite electrodes in a 4 M solution of lithium bis(fluorosulfonyl)imide (LiFSI) in ethyl methyl carbonate (EMC) using both in-situ and ex-situ Raman spectroscopy. The concentrated electrolyte is essential as it limits parasitic reactions at the cathode-electrolyte interface (CEI) occurring in parallel to anion intercalation. Using electrochemical methods including cyclic voltammetry, and post-mortem electron microscopy it was confirmed that the Al current collector is largely stable at potentials as high as 5.2 V in the electrolyte under consideration; no dissolved Al species were detected using EDX characterization. Results from the cyclic voltammetry study also indicate that parasitic reactions can be mitigated when the cut-off potential is limited to 5.0 V leading to higher coulombic efficiency (CE = 94 %) and more stable discharge capacity (85.17 mAh g-1). However, extending the potential to 5.1 and 5.2 V results in the discharge capacity increasing by almost 20 mAh g-1, though at the expense of the coulombic efficiency, which decreases from 94 to 76 %. Upon raising the cut-off potential to 5.3 V, the CE significantly decreased (20.62 %) as a result of extensive solvent decomposition ultimately leading to much quicker capacity fading.  Based on SEM images taken after 50 cycles, graphite particles did not sustain any structural or morphological change during cycling regardless of the cut-off potentials applied. Further tests were conducted on Li-graphite DIBs using galvanostatic methods in the range from 3 to 5 V, and at different specific currents (20, 50, and 100 mA g-1). Though the cells exhibited good performance in terms of capacity retention, and cycle life at all currents, the coulombic efficiency tended to decrease as the test currents were lowered. This observation confirms the presence of parasitic reactions which are only visible when the experimental timescale is sufficiently long. At 50 and 100 mA g-1, the CE reached > 98 % which further verifies the kinetic aspect of electrolyte decomposition reactions. It is evident that self-discharge sustained in the course of open-circuit potential (OCP) relaxation of the fully charged cell can reveal the stability of the electrolyte and the anion-intercalated graphite. Raman spectroscopy measurements conducted in-situ and ex-situ on graphite electrodes charged and discharged to a series of potential cut-offs reveal the existence of self-discharge leading to extraction of anions from the graphite particles. This was demonstrated through the spectral appearance of E2g2(i) band next to E2g2(b) band at a fully intercalated state, as opposed to the in-situ spectrum, which only showed one intercalated band (E2g2(b)). It can be concluded that concentrated electrolytes (such as 4 M LiFSI in EMC) only provide kinetic stability and are unable to entirely inhibit parasitic reactions at the interface. This further highlights the need for electrolyte additives that can create a more stable interfacial passivation layer on the positive electrode so that more reversible anion intercalation can be attained.

Graphite-based dual-ion batteries (GDIBs) represent a promising battery concept for large-scale energy storage on account of low cost, high working voltage, and sustainability. The electrolyte concentration plays a critical role in determining the energy density and cycle life of GDIBs. However, the concentrated electrolytes show low Lithium ions (Li+) transport kinetics, reducing their intercalation and solid electrolyte interface (SEI) formation abilities. Moreover, the GDIBs in the high cut-off voltage suffer from electrolyte degradation, and corrosion of the current collector. Herein, we report a highly concentrated electrolyte formulation based on hybrid lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) salts with a super-wide electrochemical stability window (6 V) and the ability to form SEI and passivation layer on graphite anode and current collector, respectively. By regulating the concentrated LiFSI electrolyte with LiPF6 and solvent additive, the coulombic efficiency of the graphite cathode can be further improved to ∼98%. As a result, GDIB pouch cell exhibits a capacity of 21 mAh g−1 (cell level) at 50 mA g−1, and 98.2% capacity retention after 300 cycles. The resultant battery offers an energy density of 90.3 Wh kg−1, along with a high energy efficiency of 87% and average discharge voltage of 4.3 V.

Constructing dual-ion energy storage devices using anion-intercalation graphite cathodes offers the unique opportunity to simultaneously achieve high energy density and output power density. However, a critical challenge remains in the lack of proper anodes that match with graphite cathodes, particularly in sustainable electrolyte systems using abundant potassium. Here, a surface grafting approach utilizing multifunctional azobenzene sulfonic acid is reported, which transforms V2C MXene into a high-kinetics K+-intercalation anode (denoted ASA-V2C) for dual-ion energy storage devices. Importantly, the grafted azobenzene sulfonic acid offers extra K+-storage centers and fast K+-hopping sites, while concurrently acting as a buffer between V2C layers to mitigate the structural distortion during K+ intercalation/de-intercalation. These functionalities enable the V2C electrode with significantly enhanced specific capacity (173.9 mAh g−1 vs 121.5 mAh g−1 at 0.05 A g−1), rate capability (43.1% vs 12.0% at 20 A g−1), and cycling stability (80.3% vs 45.2% after 900 cycles at 0.05 A g−1). When coupled with an anion-intercalation graphite cathode, the ASA-V2C anode demonstrates its potential in a dual-ion energy storage device. Notably, the device depicts a maximum energy density of 175 Wh kg−1 and a supercapacitor-comparable power density of 6.5 kW kg−1, outperforming recently reported Li+-, Na+-, and K+-based dual-ion devices.