Lignin-based composite polymer electrolytes for potassium batteries

The transition towards renewable energy sources, determined by the problems related to the exploitation of fossil fuels, requires the use of energy storage systems, which allow to overcome the intermittence and the unpredictability of renewable sources. Among the electrochemical systems, the most studied and used one is the lithium-ion technology; due to several interesting features, these batteries are employed for many applications, such as portable electronic devices and electric vehicles. Due to the low availability and uneven distribution of lithium, which considerably increase its cost, it is necessary to find alternative elements: the most promising candidate is potassium. This alkali metal is much more abundant and evenly distributed on the Earth crust; moreover, it allows to obtain performances close to those of lithium, thanks to its standard reduction potential of -2.93 V vs. SHE (lithium: -3.04 V vs. SHE). A fundamental target for potassium-ion batteries (KIBs) development concerns the electrolyte: it is necessary to find cheap and eco-friendly materials, which can simultaneously increase the battery safety and its performances. In this thesis work, some innovative gel-composite polymer electrolytes (GCPEs) were analyzed and tested; they consist of a cross-linked and interpenetrated polymer matrix, which includes polycaprolactone diol di-methacrylate (PCLDMA), polyethylene glycol (PEG) and ureido-pyrimidinone methacrylate (UPy-MA); the latter provides self-healing properties, through the formation of multiple hydrogen bonds in the membrane damaged areas. Bretax nanolignin is present in the matrix as a composite filler and the membranes are swelled in the organic liquid electrolyte KPF6 0.8 M in 1:1 EC:DEC, obtaining the gel form. The five examined types differ in the nanolignin content, which varies between 3 and 20% by weight, referred to the mass of PCLDMA. The dry membranes were analyzed by differential scanning calorimetry and their electrolyte uptake ratio was evaluated. The performed electrochemical tests include linear sweep voltammetry, interfacial stability tests, ionic conductivity measurements, plating and stripping tests, galvanostatic cycling and self-healing tests. The configurations used for the analyses are the coin cell and the EL-cell, assembled in a glove box; for the galvanostatic cycling, a metallic potassium anode and a Super P carbon cathode were used. The results of the performed tests display an overall improvement in the GCPEs electrochemical performances with the increase in the nanolignin content. A higher lignin concentration allows to reduce the crystallinity of the polymeric matrix, to improve its mechanical properties and to increase the electrolyte uptake ratio; as a consequence, the GCPE shows an increased ionic conductivity, the formation of more stable SEI layers and better cycling performances.