Electrode and Interface Design for High-Performance Lithium/Sodium-Selenium Batteries

Abstract

Lithium/sodium-selenium (Li/Na-Se) batteries are promising energy storage systems due to their relatively low cost and high volumetric energy density of Se cathode. Rechargeable lithium-ion batteries (LIBs) play a critical role in portable devices, electric vehicles and large-scale stationary energy storage, due to their relatively high energy density and low cost among available technologies [1]. Sodium-selenium (Na-Se) batteries have also drawn much attention, because of the large abundance and low cost of sodium and decent energy density of Se [2]. As a cathode material, Se provides a moderate gravimetric capacity of 678 mAh g−1, a high volumetric capacity of 3270 mAh cm−3, and possesses a high electronic conductivity of 1 ×10 −3 S m−1 [3, 4]. These advantages of Se have stimulated growing research in developing Li-Se and Na-Se batteries over the past years. However, the practical application of these batteries has been hindered by the polyselenides dissolution (or shuttle effect phenomena), which could cause low Coulombic efficiency and poor cycling lifetime. Microporous carbon (MPC) has been fabricated through a facile carbonization method from polyvinylidene fluoride (PVDF) and used without any further activation as selenium host to produce MPC/Se composite employed as cathode in rechargeable lithium/sodium-selenium (Li/Na-Se) batteries and the electrochemical properties of the prepared cell have been investigated in Li-Se batteries. (figures 1 and 4).Basically, in linear carbonate-containing electrolytes, the metal readily decomposes at the anode side, causing capacity drop over cycling. To address this problem, electrolyte design should accompany manifested cathode structure. Hence, fluoroethylene carbonate (FEC) has been used as an additive in the electrolyte solution for stabilizing the capacity in Na-Se batteries. Our investigation showed that exceptional cell stability could be reached by using the FEC in carbonate-based electrolyte. In lithium/sodium-selenium batteries, diethyl carbonate (DEC) electrolyte decomposes in contact with metal, producing some radicals and metal alkyl carbonates. These radicals start a chain reaction, causing the destruction of over cycles. By adding the FEC to the electrolyte solution, this side reaction will be prohibited. [5] FEC readily decomposes on the surface of the metal prior to any reduction of linear carbonates, making the protective and stable surface film on the metal. This layer makes the capacity much more stable for the cell contains FEC in the electrolyte. For the cell without FEC in the electrolyte, the capacity starts to fade from initial cycles and continue fading during the cycling. In sodium/selenium batteries, at the current rate of 0.1 C, the cell contains no FEC in the electrolyte, showed a capacity of 317 and 245 mAh g-1 after 100 and 200 cycles, respectively. However, the cell with an electrolyte solution containing 3% FEC, showing excellent cyclability by way of delivering 382 and 350 mAh g-1 capacity at those above-mentioned cycles, respectively (figures 2 and 5). Another important finding was that the use of FEC additive could prevent the corrosion of metal anode from soluble polyselenides, and preserve the structural integrity of metal anode, contributing to the enhanced cycling stability and specific capacity of Na-Se batteries.The utilization of nanoscale Al2O3 surface coating by atomic layer deposition (ALD) to protect a MPC/Se cathode and reduce polyselenide dissolution was also investigated in another stage. It was found that Al2O3 surface coating effectively suppressed the polyselenide dissolution from the MPC/Se cathode, thus reducing the loss of Se active material and improving the overall performance of Na-Se batteries. Compared to the pristine MPC/Se, Al2O3-coated MPC/Se cathode exhibited improved discharge capacity, cycling stability, and rate capability in Na-Se batteries (figures 3 and 6). Keywords: Fluoroethylene carbonate; lithium/sodium−selenium batteries; ALD coating; carbonate-based electrolyte; microporous carbon