Increasing concerns regarding the sustainability of lithium sources, due to their limited availability and consequent expected price increase, have raised awareness of the importance of developing alternative energy-storage candidates that can sustain the ever-growing energy demand. Furthermore, limitations on the availability of the transition metals used in the manufacturing of cathode materials, together with questionable mining practices, are driving development towards more sustainable elements. Given the uniformly high abundance and cost-effectiveness of sodium, as well as its very suitable redox potential (close to that of lithium), sodium-ion battery technology offers tremendous potential to be a counterpart to lithium-ion batteries (LIBs) in different application scenarios, such as stationary energy storage and low-cost vehicles. This potential is reflected by the major investments that are being made by industry in a wide variety of markets and in diverse material combinations. Despite the associated advantages of being a drop-in replacement for LIBs, there are remarkable differences in the physicochemical properties between sodium and lithium that give rise to different behaviours, for example, different coordination preferences in compounds, desolvation energies, or solubility of the solid–electrolyte interphase inorganic salt components. This demands a more detailed study of the underlying physical and chemical processes occurring in sodium-ion batteries and allows great scope for groundbreaking advances in the field, from lab-scale to scale-up. This roadmap provides an extensive review by experts in academia and industry of the current state of the art in 2021 and the different research directions and strategies currently underway to improve the performance of sodium-ion batteries. The aim is to provide an opinion with respect to the current challenges and opportunities, from the fundamental properties to the practical applications of this technology.

Reversibility of transition metal migration in layered oxides as cathodes for sodium ion batteries at the expense of polarization increase.

Solid-state batteries with a Li metal anode and polymer–ceramic electrolytes hold the promise to boost safety and energy density, provided that stable conductive interfaces are achieved. Here, by focusing on the composite electrolyte system LLZO–PEO­(LiTFSI), i.e., Li-ion conductive doped-Li7La3Zr2O12 (LLZO) garnet fillers embedded within the Li-ion conductive poly­(ethylene oxide) matrix, we report on the impact that electrolyte processing has on its mesostructure and ultimately on full cell kinetics. Two distinct solvent-based routes are used to prepare composite membranes with a fixed composition of 10 vol % LLZO but with different filler distributions and particle sizes (18 vs 1 μm). Both membranes have a similar Li-ion conductivity of ∼0.4 mS·cm–1 at 70 °C as the bulk polymer matrix drives the macroscopic Li-ion transport. However, when assembled with Li metal/LiFePO4 (LFP) electrodes, the resultant cells show distinct performances. The capacity is enhanced, from 139 to 150 mAh·g–1 at C/10 and from 60 to 97 mAh·g–1 at C/2, when the 1-μm-size LLZO fillers are homogeneously distributed within the membrane. In addition, by minimizing the electrolyte thickness, the capacity can be further enhanced to 168 mAh·g–1 at C/10, which is nearly the theoretical capacity of LFP (170 mAh·g–1). A set of electrochemical and structural characterization techniques, such as galvanostatic cycling, cyclic voltammetry, electrochemical impedance spectroscopy, potentiostatic intermittent titration, and scanning electron microscopy, allow one to identify the electrolyte–Li metal interface stability as the dominating source of the rate capability behavior of the full cells. LLZO fillers with a high surface area and even distribution within the PEO matrix are the key to allow fast and reversible Li-ion flux by minimizing Li concentration gradients. These characteristics are crucial to maximize battery kinetics and capacity utilization. Finally, the design rules of solid-state batteries containing composite electrolytes are proposed to fulfill the industry requirements for practical applications.

Here we report the synthesis via ceramic methods of the high-performance Mn-rich Na2.4Al0.4Mn2.6O7 oxygen-redox cathode material for Na-ion batteries which we use as a testbed material to study the effects...