A novel in-situ polymerization strategy for gel polymer electrolyte (GPE) is proposed to enhance the performance of silicon (Si)||nickel-rich lithium-ion batteries (NMC811). The Si-based anode and NMC811 cathode suffer from mechanical instability, cathode collapse, and severe leakage current due to the high-voltage cycling and elevated temperatures. To address these issues, a gel polymer electrolyte is introduced to reinforce the mechanical integrity of the Si anode and chelate with transition metal cations to stabilize the interfacial properties. The conformal encapsulation of the Si anode and NMC811 cathode with the GPE enables a 2.7 Ah pouch-format cell to achieve a high energy density of 325.9 Wh kg⁻¹, 88.7% capacity retention for 2000 cycles, self-extinguish property, and wide temperature tolerance. The proposed strategy provides a leap toward secure Li batteries. The increasing demand for electric vehicles (EV), unmanned aerial vehicles (UAV), and high-end electronics necessitates electrochemical cells that surpass the capabilities of current lithium-ion batteries (LIBs). The high-capacity silicon anode with nickel-rich NMC cathode can achieve enhanced energy densities of above 300 Wh kg⁻¹. However, the Si-based anode||NMC model with conventional carbonate electrolytes suffers from electrode structure collapse and irreversible Li⁺ depletion. Additionally, the flammable organic solvents pose severe safety risks like liquid leakage, thermal runaway, and fire. The reliable operation of the energy-dense battery system requires both the multiscale interfacial stability and controlled electro-chemo-mechanics of the electrodes, especially in harsh operating conditions. The Si-based anode||NMC full cell model is characterized by high-capacity lithiation capability, appropriate equilibrium voltage, and low cost. However, the huge volume expansion upon deep lithiation leads to electrical contact loss and gradual deactivation of the alloy particles. Dimensional engineering or architectural innovation of the 3D Si structures has achieved performance progress at low mass loadings. With enhanced Si loading, the as-formed Si-based anode||NMC model suffers from interfacial reactions and Li-Si intermediates irreversibly trapping active Li⁺, aggravating cation depletion from the cathode. To stabilize the interfacial electrochemical process, additives like FEC, DFEC, or FEMC have been introduced to derive LiF species on the Si anode surface. However, the continuous consumption of these additives prohibits the long-term viability of the electrodes, and the intrinsic safety concerns of organic electrolyte weeping and fire sensitivity remain unresolved. Gel polymer electrolytes (GPEs) exploit the polymer matrix to immobilize solvent molecules for enhanced stability. Various polymer matrices, including PMMA and PVDF-HFP, have been proposed as the cation carrierA novel in-situ polymerization strategy for gel polymer electrolyte (GPE) is proposed to enhance the performance of silicon (Si)||nickel-rich lithium-ion batteries (NMC811). The Si-based anode and NMC811 cathode suffer from mechanical instability, cathode collapse, and severe leakage current due to the high-voltage cycling and elevated temperatures. To address these issues, a gel polymer electrolyte is introduced to reinforce the mechanical integrity of the Si anode and chelate with transition metal cations to stabilize the interfacial properties. The conformal encapsulation of the Si anode and NMC811 cathode with the GPE enables a 2.7 Ah pouch-format cell to achieve a high energy density of 325.9 Wh kg⁻¹, 88.7% capacity retention for 2000 cycles, self-extinguish property, and wide temperature tolerance. The proposed strategy provides a leap toward secure Li batteries. The increasing demand for electric vehicles (EV), unmanned aerial vehicles (UAV), and high-end electronics necessitates electrochemical cells that surpass the capabilities of current lithium-ion batteries (LIBs). The high-capacity silicon anode with nickel-rich NMC cathode can achieve enhanced energy densities of above 300 Wh kg⁻¹. However, the Si-based anode||NMC model with conventional carbonate electrolytes suffers from electrode structure collapse and irreversible Li⁺ depletion. Additionally, the flammable organic solvents pose severe safety risks like liquid leakage, thermal runaway, and fire. The reliable operation of the energy-dense battery system requires both the multiscale interfacial stability and controlled electro-chemo-mechanics of the electrodes, especially in harsh operating conditions. The Si-based anode||NMC full cell model is characterized by high-capacity lithiation capability, appropriate equilibrium voltage, and low cost. However, the huge volume expansion upon deep lithiation leads to electrical contact loss and gradual deactivation of the alloy particles. Dimensional engineering or architectural innovation of the 3D Si structures has achieved performance progress at low mass loadings. With enhanced Si loading, the as-formed Si-based anode||NMC model suffers from interfacial reactions and Li-Si intermediates irreversibly trapping active Li⁺, aggravating cation depletion from the cathode. To stabilize the interfacial electrochemical process, additives like FEC, DFEC, or FEMC have been introduced to derive LiF species on the Si anode surface. However, the continuous consumption of these additives prohibits the long-term viability of the electrodes, and the intrinsic safety concerns of organic electrolyte weeping and fire sensitivity remain unresolved. Gel polymer electrolytes (GPEs) exploit the polymer matrix to immobilize solvent molecules for enhanced stability. Various polymer matrices, including PMMA and PVDF-HFP, have been proposed as the cation carrier