Oxygen-Pressure Protocol Breaking Cycle Limit of Continuously Reversible Lithium-Oxygen Batteries

Abstract

Lithium-oxygen (Li-O2) battery is favored among “beyond lithium-ion” technologies for sustainability because of its exceptional energy density. Major impediments are the poor cycle stability and grievous capacity degradation at high current densities. We address these issues by a “killing two birds with one stone” O2-pressure protocol. It first resolves efficient O2 mass transport at high rates. The accelerated reaction kinetics optimizes the composition and growth pathway of discharge products. This protocol secondly achieves protection of Li anodes via densifying corrosion layers on them. Consequently, the battery delivers both ultrahigh discharge capacity (> 9,000 mAh g−1) at 3,000 mA g−1 and excellent cycling stability. Under a dual-strategy effect of high-pressure O2 and artificial protection layers, the battery actualizes over 11-fold increase in cycle life of 5,170 h (2,585 cycles). The strategy opens avenues for advancing Li-O2 batteries towards practical application and confers the extension to other gas-based batteries.

1. Introduction

In the quest for sustainable energy solutions, redox chemistries based on oxygen (O2) are gaining prominence because they diminish reliance on limited transition metal elements and promise high energy density (≈3,500 Wh kg−1) when paired with lithium (Li) anode [1–6]. Typical Li-O2 battery (LOB) chemistry involves reversible formation and decomposition of Li2O2 at the cathode, wherein sluggish reaction kinetics and severe corrosion of Li anode result in inferior cycle life and rate capability [7]. Despite the efficient cathode materials [8–12], the corresponding conversion generally exhibits limited capacities and poor cycle stability at high current densities, which are mainly caused by the “altitude sickness” of cathode and poor charge transfer between cathode and insulating discharge products. Incomplete conversion of Li2O2 accelerates continuous accumulation of it and passivates catalysis sites, further diminishing the limited capacity and impeding cycling stability. Because O2 solubility in organic electrolytes is far lower than the concentration of Li ions [13], O2 supply is the rate limiting factor of forming discharge products (2Li+ + O2 + 2e− → Li2O2). In addition, large charge potentials are generally required to oxidize the insulating Li2O2 deposits. Therefore, the formation of Li-vacancy-type Li2O2 (Li2-xO2) with high conductivity and poor crystallinity are expected during circular operation of LOBs. O2-rich environment is effective to address this concern. Because O2 solubility in electrolyte generally increases with external O2 pressure, high O2 pressure should be an efficient strategy to intrinsically accelerate reaction kinetics especially at high current densities, extend the cycle life and enhance capacity retention of LOBs.

Li anode corrosion is another severe problem to degrade the cycle life of LOBs. The formation of fluffy corrosion layer can’t prevent the shuttle effect of corrosion sources and rapid corrosion of unprotected Li anode. Much work has been conducted to protect the Li metal [14–24], mainly including inorganic layers, organic layers, and inorganic–organic hybrid layers. For inorganic layers, they generally present high mechanical strength and Li+ conductivity but relatively poor toughness. During the repeated dissolution and deposition of Li+, they easily crack due to volume change of anodes. It results in the loss of their protective effect. In contrast, organic protection layers generally have good flexibility, but they present poor Li+ conductivity and rigidity. The effect of effective Li+ transport and inhibiting the growth of Li dendrites is inferior compared to inorganic layers. Combining of their respective advantages, inorganic–organic hybrid layers are