Jotti Karunawan

The rapid demand for sustainable, high-performance energy storage has accelerated the development of biomass-derived electrode materials for next-generation supercapacitors. However, the intrinsically low electrical conductivity and limited electrochemical activity of many metal oxide electrodes remain major challenges that restrict their practical application. In this work, a novel Fe-doped ZnO/activated carbon composite derived from oil palm empty fruit bunches (Fe-ZnO/AC) was successfully synthesized via a hydrothermal method and employed as an electrode material for asymmetric supercapacitors. ZnO was selected for its low-cost, abundance, nontoxicity, and facile synthesis, while Fe doping was introduced to enhance electrical conductivity, charge-transfer kinetics, and redox activity. Simultaneously, activated carbon derived from empty fruit bunch (EFB) waste provides a conductive, porous carbon matrix that facilitates ion diffusion and electron transport. This study highlights the synergistic integration of Fe-doped ZnO with activated carbon, enabling simultaneous enhancement of conductivity, electrochemical performance, structural stability, and sustainability within a single-electrode system, a strategy that remains rarely explored for asymmetric supercapacitor applications. The Fe-ZnO/AC electrode exhibited a high specific capacitance of 458.2 F g–1 at 0.1 A g–1 in a three-electrode configuration. In an asymmetric two-electrode configuration, the device delivered specific capacitances of 179.7 and 102.3 F g–1 for the coin cell and flexible cell, respectively. The corresponding energy densities reached 35.9 and 20.5 W h kg–1 with power densities of 239.7 and 240.4 W kg–1, respectively. These findings demonstrate the excellent electrochemical performance, stability, and energy storage capability of the Fe-ZnO/AC composite, highlighting its strong potential as a low-cost, sustainable, and high-performance electrode material for next-generation asymmetric supercapacitors.

The electrochemical performance of LiNi0.5Mn1.5O4 (LNMO) is strongly influenced by the interplay between crystal structure and particle morphology, both of which are determined by thermal regulation. This study systematically investigates the influence of the cooling-rate regulation on hierarchical morphology, ordered-to-disordered phase formation, and electrochemical behavior of spherical LNMO synthesized via coprecipitation. The results reveal that the cooling-rate strategy balances the thermodynamic tendency toward crystal and morphological evolution. Slower cooling promotes the transformation from a highly disordered Fd3̅m spinel to a more ordered P4332 phase, thereby reducing Mn3+ formation and rock-salt impurities. In terms of morphology, rapid cooling produces small polyhedral grains and highly porous microspheres, whereas slow cooling yields larger, well-defined primary particles with smoother surfaces. Owing to this structural-morphological trade-off, neither rapid nor slow cooling provided optimal electrochemical performance. Instead, the LNMO sample synthesized at a moderate cooling rate (LNMO C5) achieved the optimal balance among partial cation disorder, controlled Mn3+ content, and a well-defined hierarchical spherical morphology, resulting in the lowest charge-transfer resistance and polarization. Consequently, LNMO C5 delivered a high discharge capacity of 96.36 mAh g–1 at 5C and excellent cycling stability with 97.90% capacity retention after 400 cycles at 0.5C. These findings highlight that cooling-rate regulation is a simple yet effective strategy for tailoring the thermodynamic and kinetics balance of LNMO, providing a reference for the future development and optimization of high-voltage LNMO cathode materials.

Incorporating graphene into Ni-rich cathode materials, such as the LiNi_0.815Co_0.15Al_0.035O₂ (NCA) cathode, has been frequently reported as an effective strategy to improve the performance of lithium-ion batteries. However, the influence of the oxidation degree of graphene on the electrochemical performance of the mixed NCA/graphene cathode remains largely unexplored. In this work, the oxidation degree of graphene was tuned by varying the electrolyte concentration during the electrochemical exfoliation process, and its impact on the performance of NCA/graphene cathodes was evaluated. The results show that the exfoliated graphene (EG) synthesized using 0.3 M (NH₄)₂SO₄ (denoted as EG 0.3) possesses the lowest oxidation degree among all the samples, which leads to the highest electrical conductivity and the lowest moisture content. When incorporated into the NCA cathode, EG 0.3 produced a composite (NCA/EG 0.3) with the best rate capability and cycling stability owing to the low degree of cation mixing and the lowest charge-transfer resistance in the resulting cathode. Electrochemical measurements revealed that the NCA/EG 0.3 cathode delivers the highest specific capacity of 191.32 mA h g⁻¹ at 0.1C, 67.72% capacity retention rate at 5C compared with 0.1C, and good cycling stability (85.13% capacity retention after 100 cycles at 0.2C), which outperformed other composite cathodes. This study demonstrates that the oxidation degree of graphene can govern the cation mixing degree and charge-transfer resistance in Ni-rich cathodes, offering insights into the design of graphene-based additives and protective layers for lithium-ion battery cathodes.

Nickel‐rich cobalt‐free LiNi_0.9Mn_0.05Al_0.05O₂ (NMA955) is considered a promising cathode material to address the scarcity and soaring cost of cobalt. Particle size and elemental composition significantly impact the electrochemical performance of NMA955 cathodes. However, differences in precipitation rates among metal ions coveys a challenge in obtaining cathode materials with the desired particle size and composition via hydroxide co‐precipitation synthesis. Utilizing complexing agents like ammonia offers an effective strategy to tackle these issues. Here, we investigate the optimal ammonia concentration to achieve moderate particle size and precise material composition. Although ammonia only forms complex coordination with transition metals, its concentration also affects the final product's precipitation and composition, including aluminum. This study shows that ammonia serves a dual function in NMA synthesis via hydroxide co‐precipitation, i. e., regulating particle size and adjusting elemental composition. It was found that an ammonia concentration of 1.2 M achieved optimal particle size and composition, resulting in superior electrochemical performance. NMA955 synthesized in 1.2 M ammonia demonstrated a high specific capacity of 188.12 mAh g⁻¹ at 0.1 C, retained 71.16 % of its capacity after 200 cycles at 0.2 C, and delivered 110.30 mAh g⁻¹ at 5 C. These results suggest that tuning ammonia concentration is crucial for producing high‐performance cathode materials.