To overcome these severe limitations, we develop novel high-entropy oxide (HEO) electrodes, comprising equimolar multi-metal oxide compositions (Mg, Ni, Co, Cu, Zn), stabilized by entropy-driven configurational disorder. The unique structural attributes of HEOs, including uniform cation distribution, enhanced lattice disorder, abundant oxygen vacancies, and multi-valent transition metal states, inherently promote superior ionic conductivity and structural robustness. The principal objective of this study is to develop HEO electrodes, deploy HEO electrodes in CDI and compare with existing electrodes (e.g., activated carbon and MnO₂) in terms of selectivity and cycling selectivity. Comprehensive characterization methodologies are performed for HEO electrodes. These methods include cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface analysis, and ion-adsorption capacity measurements.
Our thorough electrochemical analysis provided compelling evidence for the performance of HEO electrodes. Specifically, CV results indicated a remarkable specific capacitance of approximately 150 F/g, substantially exceeding MnO₂ (100 F/g) and activated carbon electrodes (80 F/g). Deconvolution of CV curves established that capacitive-controlled processes (approximately 68%) dominated ion adsorption in HEO electrodes compared to diffusion-controlled contributions (approximately 32%), attributed to enhanced ion accessibility and rapid kinetics promoted by their open, defect-limited structure. EIS analysis demonstrated a mitigated lithium-specific charge-transfer resistance (~18 Ω) relative to Na⁺ (30 Ω) and Mg²⁺ (45 Ω), indicating preferential lithium-ion affinity. This pronounced selectivity originates from multi-metal oxide surface heterogeneity and oxygen vacancies facilitating exclusive lithium adsorption at energetically favorable sites. The derived ion diffusion resistance (σ) from the Nyquist plots for HEO electrodes was ~ 7.2 Ω·cm², much lower than significantly lower compared to conventional activated carbon (~12 Ω·cm²) and MnO₂ electrodes (~14 Ω·cm²), clearly indicating enhanced ionic diffusion facilitated by abundant accessible lattice channels within the disordered lattice structure of HEO electrodes.
Furthermore, structural characterizations elucidated the fundamental reasons for the stability and cyclability (>300 cycles) of te HEO electrodes. TEM analysis revealed uniform HEO nanoparticles averaging ~18 nm, providing extensive active surface areas confirmed by BET measurements (~125 m²/g). High-resolution TEM and XRD identified stable cubic crystalline phases with distinct d-spacings averaging ~0.27 nm, evidencing robust lattice structures resilient to volumetric stresses during cycling. XRD peak profiles indicated minimal lattice strain after cycling, directly contrasting MnO₂’s characteristic structural degradation associated with spinel-to-layered phase transitions and disproportionation reactions. XPS analysis confirmed the presence of stable oxidation states (Mg²⁺, Ni²⁺/³⁺, Co²⁺/³⁺, Cu²⁺, Zn²⁺) in HEO electrodes, essential for robust electrochemical activity and structural stability. The substantial presence of oxygen vacancies validated by XPS (O 1s peaks at ~531.5 eV) further supported selective lithium adsorption through vacancy-mediated transport, which was absent in traditional activated carbon electrodes and quite low in MnO₂ materials.
Long-term cycling stability tests (>300 cycles) of CDI demonstrated the enhanced cyclability achievable with HEO electrodes, retaining ~96% capacity retention. In contrast activated carbon and MnO₂ electrodes severely degraded and dropped below 60% retention after 100 cycles. SEM images of electrodes post-cycling verified negligible structural damage or particle aggregation on HEO electrodes, indicating their robust structural stability due to entropy-driven thermodynamic stabilization of the multi-metal oxide lattice. Especially, the multi-elemental composition uniformly distributed within HEO structures immensely mitigates local strain and suppresses particle fragmentation. In contrast, carbon electrodes suffer from particle fragmentation and mechanical pulverization due to repetitive ion insertion/extraction cycles. Furthermore, HEO electrodes circumvent structural instability and irreversible phase transformations common in spinel-structured MnO₂ electrodes, ensuring sustained and stable electrochemical performance during prolonged cycling.
In conclusion, this comprehensive investigation demonstrates the transformative potential of HEO electrodes in CDI and fundamentally resolves lithium selectivity and electrode stability challenges prevalent in traditional materials. The detailed electrochemical and structural characterizations elucidate specific mechanistic insights into how entropy-stabilized multicomponent oxides effectively facilitate selective lithium-ion adsorption, structural integrity, and unprecedented cyclability (>300 cycles). This work will yield profound impacts on the AIChE community by advancing understanding of novel electrode material development for mineral recovery, thereby offering rigorous principles and practical guidelines critical for developing next-generation lithium extraction technologies with substantial environmental and economic advantages.