Article pubs.acs.org/accounts
Nanostructured Electrolytes for Stable Lithium Electrodeposition in Secondary Batteries Zhengyuan Tu,†,⊥ Pooja Nath,‡,⊥ Yingying Lu,§ Mukul D. Tikekar,∥ and Lynden A. Archer*,†,‡ †
Department of Materials Science and Engineering, ‡School of Chemical and Biomolecular Engineering, and ∥Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853, United States § College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China CONSPECTUS: Secondary batteries based on lithium are the most important energy storage technology for contemporary portable devices. The lithium ion battery (LIB) in widespread commercial use today is a compromise technology. It compromises high energy, high power, and design flexibility for long cell operating lifetimes and safety. Materials science, transport phenomena, and electrochemistry in the electrodes and electrolyte that constitute such batteries are areas of active study worldwide because significant improvements in storage capacity and cell lifetime are required to meet new demands, including the electrification of transportation and for powering emerging autonomous aircraft and robotics technologies. By replacing the carbonaceous host material used as the anode in an LIB with metallic lithium, rechargeable lithium metal batteries (LMBs) with higher storage capacity and compatibility with low-cost, high-energy, unlithiated cathodes such as sulfur, manganese dioxide, carbon dioxide, and oxygen become possible. Large-scale, commercial deployment of LMBs are today limited by safety concerns associated with unstable electrodeposition and lithium dendrite formation during cell recharge. LMBs are also limited by low cell operating lifetimes due to parasitic chemical reactions between the electrode and electrolyte. These concerns are greater in rechargeable batteries that utilize other, more earth abundant metals such as sodium and to some extent even aluminum. Inspired by early theoretical works, various strategies have been proposed for alleviating dendrite proliferation in LMBs. A commonly held view among these early studies is that a high modulus, solid-state electrolyte that facilitates fast ion transport, is nonflammable, and presents a strong-enough physical barrier to dendrite growth is a requirement for any commercial LMB. Unfortunately, poor room-temperature ionic conductivity, challenging processing, and the high cost of ceramic electrolytes that meet the modulus and stability requirements have to date proven to be insurmountable obstacles to progress. In this Account, we first review recent advances in continuum theory for dendrite growth and proliferation during metal electrodeposition. We show that the range of options for designing electrolytes and separators that stabilize electrodeposition is now substantially broader than one might imagine from previous literature accounts. In particular, separators designed at the nanoscale to constrain ion transport on length scales below a theory-defined cutoff, and structured electrolytes in which a fraction of anions are permanently immobilized to nanoparticles, to a polymer network or ceramic membrane are considered particularly promising for their ability to stabilize electrodeposition of lithium metal without compromising ionic conductivity or room temperature battery operation. We also review recent progress in designing surface passivation films for metallic lithium that facilitate fast deposition of lithium at the electrolyte/electrode interface and at the same time protect the lithium from parasitic side reactions with liquid electrolytes. A promising finding from both theory and experiment is that simple film-forming halide salt additives in a conventional liquid electrolyte can substantially extend the lifetime and safety of LMBs.
1. INTRODUCTION
battery (LMB), for long cell operating lifetimes and safety. Specifically, by replacing metallic lithium in the anode of the LMB with an intercalating graphitic carbon host, the LIB offers a nearly 10-fold reduction in anode storage capacity (from 3800 mAh g−1 to 360 mAh g−1 for the fully lithiated host, LiC6), and limits cathode choices to lithiated materials. These penalties are accrued in exchange for the lower propensity of LIBs to
The case for advanced electrochemical energy storage technologies that offer improvements in storage capacity, power, lifetime, and cost has been articulated in several recent reviews.1−3 Among the various options, rechargeable lithium ion battery (LIB) technology is regarded as among the most promising due to its relatively high capacity, low internal resistance, and manufacturability. First commercialized in the 1990s,4 rechargeable LIBs are understood to be a compromise technologythey compromise the high energy, high power, and design flexibility of their predecessor, the lithium metal © XXXX American Chemical Society
Received: September 17, 2015
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DOI: 10.1021/acs.accounts.5b00427 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 1. (a) Schematic of dendritic lithium growth in a lithium metal battery configuration. (b) SEM image of typical dendrite morphology on a lithium electrode.
Figure 2. (a) An illustration of the electrolyte with immobilized anions. The blue spheres indicate the anions immobilized by tethering to a solid matrix. The green and red spheres indicate the mobile ions. (b) Electric field at the metal electrode against a fraction of fixed anions for various values of transport overpotential from down to up as 0.5, 1, 2, 3, 5, 10, 20 (violet to red, respectively). (c) Growth rate versus wavelength of perturbations with varying fractions of immobilized anions. (d) Growth rate versus wavelength of perturbations with varying separator modulus. Adapted with permission from ref 11. Copyright 2014 The Electrochemical Society.
spontaneously form unstable metallic electrodeposits or dendrites during the recharge cycle. The formation mechanisms and growth modes of dendritic electrodeposition of metals, including Ag, Al, Cu, Li, Na, Ni, Pb, and Zn, have been known and actively studied since the discovery of electroplating processes in the 1800s.5−8 Dendritic electrodeposits are important in LMBs because, during the cell recharge, lithium is reduced at the anode and once formed, Li dendrites may catalyze cell failure by multiple processes. First, they continuously roughen the lithium metal surface during each recharge cycle, which promotes parasitic surface reactions with most electrolyte solvents. Over multiple cycles of discharge and charge, these reactions consume the electrolyte as well as the anode material, ultimately causing the internal resistance of the cell to diverge. Second, dendrites once nucleated may proliferate and grow throughout the interelectrode space. The growing dendrites accelerate cell failure either when they break to produce electrically isolated or orphaned
lithium metal in the cell or when they completely bridge the anode and cathode, which causes cell failure by internal short circuits. In extreme cases, the two failure mechanisms can occur in tandem, causing thermal runaway and catastrophic cell failure (Figure 1). This latter failure mode has understandably received the greatest attention and is responsible for the most important commercial barriers to LMBs. Because of the small voltage difference (
