Plant-Based Biopolymer Gel Electrolytes for Flexible Solid-State Supercapacitors: Progress, Limitations, and Prospects of Atomic Layer Deposition for Interfacial Engineering

Plant-based biopolymer gel electrolytes (BGEs) are regarded as sustainable alternatives to liquid electrolytes for flexible solid-state supercapacitors, offering biodegradability and competitive electrochemical performance. This review evaluates plant-based BGEs through bulk ionic conductivity enhancement strategies and interfacial engineering, proposing the hypothesis that the electrode–electrolyte boundary, rather than bulk ionic conductivity, may represent the fundamental remaining performance limitation in the higher-performing BGE classes evaluated. Biopolymer classes, including cellulose, alginate, lignin, starch, pectin, and carrageenan, are critically reviewed, covering structural chemistry, fabrication strategies, and electrochemical performance in flexible, symmetric, asymmetric, and quasi-solid-state supercapacitors. Ionic conductivities ranging from 1.8 to 345 mS cm–1, gravimetric specific capacitances slightly above 400 F g–1, and gravimetric energy densities approaching 45 Wh kg–1 suggest that bulk ionic conductivity may no longer be the sole performance limitation for cellulose-, lignin-, alginate-, and carrageenan-based BGEs, where conductivities consistently exceed 10 mS cm–1, though pectin- and soy protein-based BGEs still require improvements in bulk ionic conductivity. An observed disparity between electrolyte bulk ionic conductivity and device-level performance indicates that interfacial resistance and cycling-induced degradation of the electrode/electrolyte contact remain practical challenges. Atomic layer deposition (ALD) is introduced as a precision interfacial engineering tool, with evidence from ALD-modified polymer electrolytes in lithium battery research demonstrating its potential transferability to BGE-based flexible solid-state supercapacitors. Plasma-enhanced ALD is identified as a candidate processing route for direct deposition on thermally sensitive biopolymer substrates, and priority research directions are outlined.