UMD team develops new nanocomposite sulfur electrode for high-performance all-solid-state Li-S batteries
A team at the University of Maryland have synthesized a mixed conducting nanocomposite sulfur electrode that consists of different nanoparticles with distinct properties of lithium storage capability, mechanical reinforcement, and ionic and electronic conductivities.
As described in a paper published in the ACS journal Nano Letters, the new nanocomposite serves as a mechanically robust and mixed conductive (ionic and electronic conductive) sulfur electrode for all-solid-state lithium–sulfur batteries (ASSLSBs). The team achieved a reversible capacity of 830 mAh/g (71% utilization of Li2S) at 50 mA/g for 60 cycles with a high rate performance at room temperature even at a high loading of Li2S (∼3.6 mg/cm2).
ASSLSBs have a number of conceptual advantages including the the inherent high energy of lithium−sulfur chemistry and the abundance of sulfur; improved safety by using nonflammable inorganic solid electrolyte; and the increase of energy efficiency due to the elimination of polysulfide shuttle.
Practically, however, ASSLSB performance is still far worse than liquid−electrolyte lithium−sulfur batteries in terms of sulfur utilization, cycling, and rate performances, even though sulfide-based solid electrolytes with ionic conductivity comparable to that of liquid electrolytes have been used, the UMD team noted. The poor performance of these ASSLSBs was related to two main challenges.
Poor electronic and ionic conductivities of sulfur and its discharge products. Addressing this require that sulfure, Li2S, be uniformly distributed in a high and balanced ionic/electronic conducting matrix. Despite considerable engineering efforts, however, only limited improvement on enhancing the sulfur utilization and rate performance of these ASSLSBs has been achieved.
… simply increasing the electronic conductivity cannot guarantee a high-performance ASSLSB because the charge/discharge of the electrode also requires the facile transport of lithium ions. As a matter of fact, the enabling of the facile ion transport is more critical in all-solid-state batteries since solid electrolytes are not infiltrative as liquid electrolytes.—Han et al.
Huge stress within the sulfur electrode in the ASSLSBs because of the large volume change (76%) during lithiation/delithiation. Unlike the liquid electrolyte lithium−sulfur battery where the flowable liquid electrolyte infiltrated in the pores of electrodes (porosity ∼30 vol%) can accommodate the volume change, sulfur electrodes in ASSLSB typically consisted of a thick, dense composite of active material, solid electrolyte, and electronic conductive additives.
The large volume changes of the sulfur electrode during lithiation/delithiation are constrained by the rigid solid electrolytes, resulting in a huge strain/stress at the electrode/electrolyte interface, leading to the formation of cracks within the active material.
… the fabrication of a nanocomposite sulfur electrode is the most promising direction to achieve a high-performance ASSLSB based on the simultaneous improvement on the mechanical property, ionic conductivity and electronic conductivity of sulfur cathode. However, it is very challenging to synthesize a nanocomposite with uniform distribution of the three components (carbon, active material, solid electrolyte) at nanoscale.—Han et al.
The UMD team addressed the challenge by dissolving Li2S as the active material; polyvinylpyrrolidone (PVP) as the carbon precursor; and Li6PS5Cl as the solid electrolyte in ethanol, followed by a coprecipitation and high-temperature carbonization process.
The result was Li2S active material and Li6PS5Cl solid electrolyte with a particle size of ∼4 nm uniformly confined in a nanoscale carbon matrix.
|Schematic illustration of the bottom-up synthesis of the mixed conducting Li2S nanocomposite. Credit: ACS, Han et al. Click to enlarge.|
The resulting Li2S−Li6PS5Cl−C nanocomposite exhibited a 3 orders of magnitude increase in ionic conductivity compared to Li2S(from 10−9 S/cm of Li2S to 9.6 × 10−6 S/cm). Electronic conductivity increased from from 10−13 S/cm to 10−5 S/cm.
The simple approach offers a new pathway for large-scale production of the cathode material for high performance ASSLSBs. The result could also provide valuable guidelines to develop other mixed-conducive electrodes for all-solid state lithium batteries.—Han et al.
Fudong Han, Jie Yue, Xiulin Fan, Tao Gao, Chao Luo, Zhaohui Ma, Liumin Suo, and Chunsheng Wang (2016) “High-Performance All-Solid-State Lithium–Sulfur Battery Enabled by a Mixed-Conductive Li2S Nanocomposite” Nano Letters doi: 10.1021/acs.nanolett.6b01754