Rate-Dependent Morphology of Li[subscript 2]O[subscript 2] Growth in Li–O[subscript 2] Batteries

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Compact solid discharge products enable energy storage devices with high gravimetric and volu-metric energy densities, but solid deposits on active surfaces can disturb charge transport and induce mechanical stress. In this Letter we develop a nanoscale continuum model for the growth of Li 2 O 2 crystals in lithium-oxygen batteries with organic electrolytes, based on a theory of electrochemical non-equilibrium thermodynamics originally applied to Li-ion batteries. As in the case of lithium insertion in phase-separating LiFePO 4 nanoparticles, the theory predicts a transition from complex to uniform morphologies of Li 2 O 2 with increasing current. Discrete particle growth at low discharge rates becomes suppressed at high rates, resulting in a film of electronically insulating Li 2 O 2 that limits cell performance. We predict that the transition between these surface growth modes occurs at current densities close to the exchange current density of the cathode reaction, consistent with experimental observations. Introduction. Crystallization on active surfaces is essential in many battery and electrodeposition processes. Crystalline reaction products offer the potential for compact and lightweight energy storage, but accommodating such deposits is challenging for electrode design. The wide range of conditions during crystallization causes a multitude of growth morphologies in electrochemical systems. In lead-acid batteries, particle sizes of deposited Pb depend on voltage sweeping rates [1]; in alkaline Zn batteries or Zn-O 2 batteries, electrodeposited ZnO undergoes a transition from film-growth to dendritic-growth as a function of cycling depth [2], influenced by elec-trolyte additives [3]; in metal electrodeposition, dendritic growth depends sensitively on the electrolyte composition and applied current [4–6]; in rechargeable lithium batteries, morphological changes in Li metal anodes during dissolution, plating and dendritic growth [6], are a critical challenge, subject to ongoing modeling efforts [7, 8]. Recent experiments on Li-O 2 batteries with ether-based electrolyte have revealed that the electronically isolating discharge product Li 2 O 2 can deposit in complex toroid-like morphologies [11, 12] or thin films [12, 13]. In contrast, only quad-shaped particles have been observed in sodium-oxygen batteries so far [14]. Li-O 2 batteries are prominent candidates for next-generation batteries that can replace conventional combustion technologies [15–20]. Although the …