Lithium metal batteries for high energy density: Fundamental electrochemistry and challenges

The dependence on portable devices and electrical vehicles has triggered the awareness on the energy storage systems with ever-growing energy density. Lithium metal batteries (LMBs) has revived and attracted considerable attention due to its high volumetric (2046 mAh cm⁻³), gravimetric specific capacity (3862 mAh g⁻¹) and the lowest reduction potential (−3.04 V vs. SHE.). However, during the electrochemical process of lithium anode, the growth of lithium dendrite constitutes the biggest stumbling block on the road to LMBs application. The undesirable dendrite not only limit the Coulombic efficiency (CE) of LMBs, but also cause thermal runaway and other safety issues due to short-circuits. Understanding the mechanisms of lithium nucleation and dendrite growth provides insights to solve these problems. Herein, we summarize the electrochemical models that inherently describe the lithium nucleation and dendrite growth, such as the thermodynamic, electrodeposition kinetics, internal stress, and interface transmission models. Essential parameters of temperature, current density, internal stress and interfacial Li⁺ flux are focused. To improve the LMBs performance, state-of-the-art optimization procedures have been developed and systematically illustrated with the intrinsic regulation principles for better lithium anode stability, including electrolyte optimization, artificial interface layers, three-dimensional hosts, external field, etc. Towards practical applications of LMBs, the current development of pouch cell LMBs have been further introduced with different assembly systems and fading mechanism. However, challenges and obstacles still exist for the development of LMBs, such as in-depth understanding and in-situ observation of dendrite growth, the surface protection under extreme condition and the self-healing of solid electrolyte interface.