<p>The effect of Nb⁵⁺ doping on high-voltage spinel LNMO (LiNi_0.5Mn_1.5−xNb_0.8xO₄, (x = 0, 0.025, 0.05, 0.075, 0.1)) synthesized via a solution combustion technique is systematically investigated. Structural, morphological, optical and electrochemical properties are analyzed using XRD, FESEM, Raman, UV-DRS, XPS, CV, GCD and EIS. Nb⁵⁺ incorporation effectively reduces lattice strain, enhances particle uniformity and stabilizes the Mn³⁺/Mn⁴⁺ and Ni²⁺/Ni³⁺ redox couples and improves electronic conductivity. Out of all concentrations, LNMNO-2 exhibits superior redox reversibility and low charge-transfer resistance. To evaluate the cost-effective and eco-friendly applicability, an asymmetric device is fabricated using LNMNO-2 as cathode and reduced Graphene Oxide (rGO) as anode (LNMNO-2//rGO) with a 3&#xa0;M KOH - PVA gel electrolyte instead of conventional Li-based electrolyte for its high ionic conductivity, cost effective and non-toxic behavior, offering a safer alternative. The device achieves high specific capacitance of 138 Fg^− 1, an energy density of 55 Whkg^− 1_, a power density of 850 Wkg^− 1 at a current density of 1 Ag^− 1 along with 71% capacitance retention and columbic efficiency of 81.4% subsequent to 3,000 cycles. These results confirms that Nb doping significantly enhances the structural and electrochemical stability of LNMO, enabling high-potential performance even in alkaline electrolytes.</p>